Stepped truncated damped sinusoidal defibrillation waveform

Abstract

A method and apparatus for delivering a stepped truncated damped sinusoidal external defibrillation waveform which, when applied through a plurality of electrodes positioned on a patient's torso will produce a desired response in the patient's cardiac cell membranes is provided. Further, the method and apparatus sufficiently approximates a constant current defibrillation shock pulse. The method includes the steps monitoring a patient-dependent electrical parameter and determining a duration based on the parameter determined. First and second charge storage components are then charged. A first truncating switch is then closed to discharge the first charge storage component. After a predetermined delay, a second truncating switch is closed to discharge the second charge storage component. Then, after the duration period that was calculated has expired the switches are opened to truncate the waveform. The computation of discharge duration is made as a function of the desired cardiac membrane response function, a patient model and a defibrillator circuit model.

Description

FIELD OF THE INVENTION

This invention relates generally to an electrotherapy method and apparatus for delivering an electrical pulse to a patient's heart. In particular, this invention relates to a method and apparatus for creating a truncated damped sinusoidal electrical waveform delivered by an external defibrillator based on theory and practice as described herein.

BACKGROUND OF THE INVENTION

Devices for defibrillating a heart have been known for sometime now. Implantable defibrillators are well accepted by the medical community as effective tools to combat fibrillation for an identified segment of the population. A substantial amount of research in fibrillation and the therapy of defibrillation has been done. Much of the most recent research has concentrated on understanding the effects that a defibrillation shock pulse has on fibrillation to terminate such a condition.

A monophasic waveform is defined to be a single phase, capacitive-discharge, time-truncated, waveform with exponential decay. A biphasic waveform is defined to comprise two monophasic waveforms, separated by time and of opposite polarity. The first phase is designated .phi..sub.1 and the second phase is designated .phi..sub.2. The delivery of .phi..sub.1 is completed before the delivery of .phi..sub.2 is begun.

After extensive testing, it has been determined that biphasic waveforms are more efficacious than monophasic waveforms. There is a wide debate regarding the reasons for the increased efficacy of biphasic waveforms over that of a monophasic waveforms. One hypothesis holds that .phi..sub.1 defibrillates the heart and .phi..sub.2 performs a stabilizing action that keeps the heart from refibrillating.

Biphasic defibrillation waveforms are now the standard of care in clinical use for defibrillation with implantable cardioverter-defibrillators (ICDs), due to the superior performance demonstrated over that of comparable monophasic waveforms. To better understand these significantly different outcomes, ICD research has developed cardiac cell response models to defibrillation. Waveform design criteria have been derived from these first principles and have been applied to monophasic and biphasic waveforms to optimize their parameters. These principles-based design criteria have produced significant improvements over the current art of waveforms.

In a two paper set, Blair developed a model for the optimal design of a monophasic waveform when used for electrical stimulation. (1) Blair, H. A., "On the Intensity-time relations for stimulation by electric currents." I. J. Gen. Physiol. 1932; 15: 709-729. (2) Blair, H. A., "On the Intensity-time Relations for stimulation by electric currents." II. J. Gen. Physiol. 1932; 15: 731-755. Blair proposed and demonstrated that the optimal duration of a monophasic waveform is equal to the point in time at which the cell response to the stimulus is maximal. Duplicating Blair's model, Walcott extended Blair's analysis to defibrillation, where they obtained supporting experimental results. Walcott, et al., "Choosing the optimal monophasic and biphasic waveforms for ventricular defibrillation." J. Cardiovasc Electrophysiol. 1995; 6: 737-750.

Independently, Kroll developed a biphasic model for the optimal design of .phi..sub.2 for a biphasic defibrillation waveform. Kroll, M. W., "A minimal model of the single capacitor biphasic defibrillation waveform." PACE 1994; 17:1782-1792. Kroll proposed that the .phi..sub.2 stabilizing action removed the charge deposited by .phi..sub.1 from those cells not stimulated by .phi..sub.1. This has come to be known as "charge burping". Kroll supported his hypothesis with retrospective analysis of studies by Dixon, et al., Tang, et al., and Freese, et al. regarding single capacitor, biphasic waveform studies. Dixon, et al., "Improved defibrillation thresholds with large contoured epicardial electrodes and biphasic waveforms." Circulation 1987; 76:1176-1184; Tang, et al. "Ventricular defibrillation using biphasic waveforms: The Importance of Phasic duration." J. Am. Coll. Cardiol. 1989; 13:207-214; and Feeser, S. A., et al. "Strength-duration and probability of success curves for defibrillation with biphasic waveforms." Circulation 1990; 82: 2128-2141. Again, the Walcott group retrospectively evaluated their extension of Blair's model to .phi..sub.2 using the Tang and Feeser data sets. Their findings further supported Kroll's hypothesis regarding biphasic defibrillation waveforms. For further discussions on the development of these models, reference may be made to PCT publications WO 95/32020 and WO 95/09673 and to U.S. Pat. No. 5,431,686.

The charge burping hypothesis can be used to develop equations that describe the time course of a cell's membrane potential during a biphasic shock pulse. At the end of .phi..sub.1, those cells that were not stimulated by .phi..sub.1 have a residual charge due to the action of .phi..sub.1 on the cell. The charge burping model hypothesizes that an optimal pulse duration for .phi..sub.2 is that duration that removes as much of the .phi..sub.1 residual charge from the cell as possible. Ideally, these unstimulated cells are set back to "relative ground." The charge burping model proposed by Kroll is based on the circuit model shown in FIG. 2b which is adapted from the general model of a defibrillator illustrated in FIG. 2a.

The charge burping model also accounts for removing the residual cell membrane potential at the end of a .phi..sub.1 pulse that is independent of a .phi..sub.2. That is, .phi..sub.2 is delivered by a set of capacitors separate from the set of capacitors used to deliver .phi..sub.1. This charge burping model is constructed by adding a second set of capacitors, as illustrated in FIG. 3. In this figure, C.sub.1 represents the .phi..sub.1 capacitor set, C.sub.2 represents the .phi..sub.2 capacitor set R.sub.H represents the resistance of the heart, and the pair C.sub.M and R.sub.M represent membrane series capacitance and resistance of a single cell. The node V.sub.S represents the voltage between the electrodes, while V.sub.M denotes the voltage across the cell membrane.

External defibrillators send electrical pulses to the patient's heart through electrodes applied to the patient's torso. External defibrillators are useful in any situation where there may be an unanticipated need to provide electrotherapy to a patient on short notice. The advantage of external defibrillators is that they may be used on a patient as needed, then subsequently moved to be used with another patient.

However, this important advantage has two fundamental limitations. First, external defibrillators do not have direct contact with the patient's heart. External defibrillators have traditionally delivered their electrotherapeutic pulses to the patient's heart from the surface of the patient's chest. This is known as the transthoracic defibrillation problem. Second, external defibrillators must be able to be used on patients having a variety of physiological differences. External defibrillators have traditionally operated according to pulse amplitude and duration parameters that can be effective in all patients. This is known as the patient variability problem.

The prior art described above effectively models implantable defibrillators, however it does not fully addressed the transthoracic defibrillation problem nor the patient variability problem. In fact, these two limitations to external defibrillators are not fully appreciated by those in the art. For example, prior art disclosures of the use of truncated monophasic or biphasic shock pulses in implantable or external defibrillators have provided little guidance for the design of an external defibrillator that will successfully defibrillate across a large, heterogeneous population of patients. In particular, an implantable defibrillator and an external defibrillator can deliver a shock pulse of similar form, and yet the actual implementation of the waveform delivery system is radically different.

In the past five years, new research in ICD therapy has developed and demonstrated defibrillation models that provide waveform design rules from first principles. These defibrillation models and their associated design rules for the development of defibrillation waveforms and their characteristics were first developed by Kroll and Irnich for monophasic waveforms using effective and rheobase current concepts. (1) Kroll, M. W., "A minimal model of the monophasic defibrillation pulse." PACE 1993; 15: 769. (2) Irnich, W., "Optimal truncation of defibrillation pulses." PACE 1995; 18: 673. Subsequently, Kroll, Walcott, Cleland and others developed the passive cardiac cell membrane response model for monophasic and biphasic waveforms, herein called the cell response model. (1) Kroll, M. W., "A minimal model of the single capacitor biphasic defibrillation waveform." PACE 1994; 17: 1782. (2) Walcott, G. P., Walker, R. G., Cates. A. W., Krassowska, W., Smith, W. M., Ideker RE. "Choosing the optimal monophasic and biphasic waveforms for ventricular defibrillation." J Cardiovasc Electrophysiol 1995; 6:737; and Cleland B. G. "A conceptual basis for defibrillation waveforms." PACE 1996; 19:1186.

A significant increase in the understanding of waveform design has occurred and substantial improvements have been made by using these newly developed design principles. Block et al. has recently written a comprehensive survey of the new principles-based theories and their impact on optimizing internal defibrillation through improved waveforms. Block M, Breithardt G., "Optimizing defibrillation through improved waveforms." PACE 1995; 18:526.

There have not been significant developments in external defibrillation waveforms beyond the two basic monophasic waveforms: the damped sine or the truncated exponential. To date, their design for transthoracic defibrillation has been based almost entirely on empirically derived data. It seems that the design of monophasic and biphasic waveforms for external defibrillation has not yet been generally influenced by the important developments in ICD research.

Recently there has been reported research on the development and validation of a biphasic truncated exponential waveform in which it was compared clinically to a damped sine waveform. For additional background, reference may be made to U.S. Pat. Nos. 5,593,427, 5,601,612 and 5,607,454. See also: Gliner B. E., Lyster T. E., Dillon S. M., Bardy G. H., "Transthoracic defibrillation of swine with monophasic and biphasic waveforms." Circulation 1995; 92:1634-1643; Bardy G. H., Gliner B. E., Kudenchuk P. J., Poole J. E., Dolack G. L., Jones G. K., Anderson J., Troutman C., Johnson G.; "Truncated biphasic pulses for transthoracic defibrillation." Circulation 1995; 91:1768-1774; and Bardy G. H. et al, "For the Transthoracic Investigators. Multicenter comparison of truncated biphasic shocks and standard damped sine wave monophasic shocks for transthoracic ventricular defibrillation." Circulation 1996; 94:2507-2514. Although the research determined a usable biphasic waveform, there was no new theoretical understanding determined for external waveform design. It appears that external waveform research may develop a "rules-of-thumb by trial and error" design approach much like that established in the early stages of theoretical ICD research. The noted limitations of the transthoracic biphasic waveform may be due in part to a lack of principles-based design rules to determine its waveform characteristics.

Monophasic defibrillation waveforms remain the standard of care in clinical use for transthoracic defibrillation. Waveform design has not yet been influenced by the important gains made in ICD research. The limitations of present transthoracic waveforms may be due in part to a lack of application of these design principles to determine optimal waveform characteristics. To overcome these limitations, design principles and design rules based on cell response have recently been developed for external defibrillation waveforms. The transthoracic model incorporates elements into a cell response model that extends it to external defibrillation.

Damped sine waves have been used and are well known to those skilled in the art of defibrillators for some time now. Known circuits for developing damped sine waveforms typically have a very large leading edge voltage which is damped by the inductor. Due to rapid rise time, the known damped sine waveform implementations do not track the cell membrane response. By incorporating a larger inductor (25 mH-500 mH) and by truncating each phase of the delivery of the damped sine waveform at appropriate times defined by design rules based on a desired cardiac cell response, damped sine waveforms can better track cell membrane response, thereby providing a more effective defibrillation shock pulse.

It is known that constant current pulses, such as square waves or rectangular waves are the most effective waveforms for defibrillation. Schuder J. C. et al., "Transthoracic Ventricular Defibrillation of 100 Kilogram Calves with Critically Damped Sinusoidal Shocks." AAMI 21st Annual Meeting, Apr. 12-16, 1986. However, generally a constant current waveform has proven costly and size prohibitive.

There is a continued need for an apparatus and method for accurately delivering an external defibrillator waveform to efficiently and effectively provide a desired response in the patient cardiac cell membrane. Additionally, there is a need for a method and apparatus for approximating a constant current waveform.

SUMMARY OF THE INVENTION

The present invention relates to an external defibrillation method and apparatus that addresses the limitations in the prior art. The present invention incorporates three singular practices that distinguish the practice of designing external defibrillators from the practice of designing implantable defibrillators. These practices are 1) designing multiphasic transthoracic shock pulse waveforms from principles based on cardiac electrophysiology, 2) designing multiphasic transthoracic shock pulse waveforms in which each phase of the waveform can be designed without implementation limitations placed on its charging and delivery means by such means for prior waveform phases, and 3) designing multiphasic transthoracic shock pulse waveforms to operate across a wide range of parameters determined by a large, heterogeneous population of patients.

In particular, the present invention provides for a method and apparatus for delivering a stepped truncated damped sinusoidal external defibrillation waveform which, when applied through a plurality of electrodes positioned on a patient's torso will produce a desired response in the patient's cardiac cell membrane. The method includes the steps of monitoring a patient-dependent electrical parameter and determining a duration based on the patient dependent parameter determined. A first and second charge storage component are then charged. A first truncating switch is then closed to discharge the first charge storage component. A short time later a second truncating switch is then closed switching in the second charge storage component and an inductor. Then, after the duration period that was calculated has expired both the first and second switches are opened to truncate the waveform. The computation of the discharge duration is made as a function of a desired cardiac membrane response function, a patient model and a defibrillator circuit model.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a and 1b are perspective views of an AED according to the present invention.

FIG. 2a is a very simplified defibrillator model.

FIG. 2b is a known monophasic defibrillation model.

FIG. 3 is a known biphasic defibrillation model.

FIG. 4 represents a monophasic or biphasic capacitive-discharge external defibrillation model according to the present invention.

FIG. 5a represents a monophasic capacitor-inductor external defibrillator model according to the present invention.

FIG. 5b represents an alternative embodiment of a biphasic capacitor-inductor external defibrillator model according to the present invention.

FIG. 6 is a simplified schematic illustration of an external defibrillation sinusoidal pulse generating circuit.

FIG. 7 is an illustration of a single capacitor truncated damped sinusoidal biphasic defibrillation pulse.

FIGS. 8a, 8b and 8c are illustrations of single capacitor damped sinusoidal waveforms and associated cell membrane responses.

FIG. 9 illustrates a flow chart for the determination and use of the truncation time for .phi..sub.1 of a damped sine wave shock pulse.

FIG. 10 illustrates a flow chart for the determination and use of the truncation time for .phi..sub.2 of the damped sine wave shock pulse.

FIG. 11 is a simplified stepped capacitor truncated damped sinusoidal biphasic circuit according to the present invention.

FIG. 12 is an illustration of a stepped truncated damped sine wave produced by a circuit as in FIG. 11, in which the times between closing switches 76 and 78 or between closing switches 92 and 94 are short.

FIG. 13 is an illustration of a stepped truncated damped sine wave produced by a circuit as in FIG. 11, in which the times between closing switches 76 and 78 or between closing switches 92 and 94 are long.

FIG. 14 is an illustration of a biphasic stepped capacitor truncated damped sinusoidal waveform and its associated cell membrane response.

FIGS. 15a, 15b, 15c, 15d, 15e, and 15f illustrate a plurality of waveforms and their associated cell responses with a variety of parameters.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides a method and apparatus for delivering a stepped truncated damped sinusoidal external defibrillation waveform which, when applied through a plurality of electrodes positioned on a patient's torso will provide a desired response in the patient's cardiac cell membrane. The present invention further provides an external defibrillator method and apparatus for approximating a constant current shock pulse. To better understand the present invention, a discussion of the development of an appropriate model is first needed.

Description of External Defibrillation Model

An automated external defibrillator (AED) is illustrated in FIGS. 1a and 1b. FIG. 1a illustrates an AED 10, including a plastic case 12 with a carrying handle 14. A lid 16 is provided which covers an electrode compartment 18. An electrode connector 20, a speaker 22 and a diagnostic panel (not shown) are located on case 12 within electrode compartment 18. FIG. 1b illustrates AED 10 having a pair of electrodes 24 connected thereto. Electrodes 24 can be pre-connected to connector 20 and stored in compartment 18.

The operation of AED 10 is described briefly below. A rescue mode of AED 10 is initiated when lid 16 is opened to access electrodes 24. The opening of lid 16 is detected by AED 10 to effectively turn on the device. AED 10 then quickly runs a short test routine. After electrodes 24 have been placed on the patient, AED 10 senses patient specific parameters, such as voltage, current, charge or other measurable parameters of the patient. The patient specific parameters are then utilized in the design of optimal waveforms as will be described below.

If a shockable condition is detected through electrodes 24, a plurality of capacitors inside of AED 10 are charged from an energy source, typically a detachable battery pack. Based upon the patient specific parameters sensed, the duration and other characteristics of a discharge waveform are then calculated. The energy stored in AED 10 is then discharged to the patient through electrodes 24.

For a more detailed description of the physical structure of AED 10 or the process involved in sensing, charging, shocking and testing, reference should be made to applicants co-pending application Ser. No. 08/512,441, filed Aug. 8, 1995 entitled AUTOMATED EXTERNAL DEFIBRILLATOR WITH SELF-TEST SYSTEM which is assigned to the assignee of the present invention, the disclosure of which is herein incorporated by reference.

It is not assumed that both phases of a biphasic waveform are delivered using the same set of capacitors or that both phases of a biphasic waveform are delivered using the capacitor set in the same electrical configuration, although such an embodiment is considered within the spirit and scope of the present invention.

Transthoracic defibrillation is generally performed by placing electrodes on the apex and anterior positions of the chest wall. With this electrode arrangement, nearly all current passing through the heart is conducted by the lungs and the equipotential surfaces pass through the myocardium normal to the electrode axis. The transthoracic charge burping model is used to develop design equations that describe the time course of a cell's membrane potential during a transthoracic biphasic shock pulse. These equations are then used to create equations that describe the design of monophasic and biphasic shock pulses for trans chest defibrillation to optimize the design of .phi..sub.1 for defibrillating and the design of .phi..sub.2 for stabilizing. These optimizing shock pulse design equations are called design rules.

The main series pathway for current is to pass through the chest wall, the lungs, and the heart. Additionally, there are two important shunting pathways in parallel with the current pathway through the heart. These shunting pathways must be taken into consideration. The lungs shunt current around the heart through a parallel pathway. The second shunting pathway is provided by the thoracic cage. The resistivity of the thoracic cage and the skeletal muscle structure is low when compared to lungs. The high resistivity of the lungs and the shunting pathways are characterizing elements of external defibrillation that distinguish the art from intracardiac defibrillation and implantable defibrillation technologies.

Therefore, in the transthoracic defibrillation model illustrated in FIG. 4, there are several resistances in addition to those discussed for the charge burping model above. R.sub.S represents the resistance of the defibrillation system, including the resistance of the defibrillation electrodes. R.sub.CW and R.sub.LS represent the resistances of the chest wall and the lungs, respectively, in series with resistance of the heart, R.sub.H. R.sub.TC and R.sub.LP represent the resistances of the thoracic cage and the lungs, respectively, in parallel with the resistance of the heart.

The design rules for external defibrillation waveforms are determined in three steps. In the first step, the transchest forcing function is determined. The transchest forcing function is the name that is given to the voltage that is applied across each cardiac cell during an external defibrillation shock. In the second step, the design equations for .phi..sub.1 of a shock pulse are determined. The design equations are the equations describing the cell's response to the .phi..sub.1 transchest forcing function, the equation describing the optimal .phi..sub.1 pulse duration, and the equation describing the optimal .phi..sub.1 capacitor. Therefore, step two relates the cell response to the action of a monophasic shock pulse or the first phase of a biphasic shock pulse. This relation is used to determine the optimal design rules and thereby design parameters for the implementation of this phase in an external defibrillator. It will be clear to those in the art that step two is not restricted to capacitor discharge shock pulses and their associated transchest forcing function. Another common implementation of an external defibrillator incorporates a damped sine wave for a shock pulse and can be either a monophasic or biphasic waveform. This type of external defibrillator is modeled by the circuit shown in FIG. 5. In the third step, the design equations for .phi..sub.2 of a shock pulse are determined. The design equations are the equations describing the cell's response to the .phi..sub.2 transchest forcing function, the equation describing the optimal .phi..sub.2 pulse duration and the equation describing the optimal .phi..sub.2 capacitor. These design equations are employed to determine the optimal design rules and thereby design parameters of .phi..sub.2 of a biphasic shock pulse with respect to how the cell responds to the shock pulse. An important element of this invention is to provide shock pulse waveforms that are designed from a cardiac cell response model developed from first principles and that correctly determines the effects of the chest and its components on the ability of a shock pulse to defibrillate.

The transchest forcing function is determined by solving for the voltage found at node V.sub.3 in FIG. 4. The transchest forcing function is derived by solving for V.sub.3 using the following three nodal equations: ##EQU1## Equation 1 can be rewritten as ##EQU2## Rewriting equation 2, we have ##EQU3## By substituting equation 4B for V.sub.1 into equation 4C, we can solve for V.sub.2 as an expression of V.sub.S and V.sub.3 : ##EQU4## Now solving for V.sub.3 as an expression of V.sub.S and V.sub.M, equation 3 may be re-arranged as ##EQU5## Substituting equation 5 for V.sub.2 into equation 7, we can solve for V.sub.3 as an expression of V.sub.S and V.sub.M : ##EQU6## From equation 8 we define .OMEGA..sub.M to be: ##EQU7## From equation 8 we also define .OMEGA..sub.S to be: ##EQU8## are the general transchest transfer function as shown in FIG. 4 or FIGS. 5a and 5b. Equation (14) incapsulates the transchest elements and their association between the forcing function V.sub.S (which models a defibrillation circuit and the shock pulse) and the cell membrance voltage V.sub.M. Therefore, this completes the first step.

The variable V.sub.S may now be replaced with a more specific description of the defibrillation circuitry that implements a shock pulse. For a first example, a monophasic time-truncated, capacitive-discharge circuit may be represented by V.sub.S =V.sub.1 e.sup.-t/.tau..sbsp.1, where V.sub.1 is the leading-edge voltage for the shock pulse and .tau..sub.1 =RC.sub.1, with R determined below.

As shown in FIGS. 5a and 5b, a second example would be a monophasic damped sine wave circuit, represented by ##EQU9## where V.sub.1 is the voltage on the charged capacitor C.sub.1, .tau..sub.C1 =RC.sub.1 and .tau..sub.L1 =L.sub.1 /R. Every step illustrated below may be performed with this and other similar transchest forcing functions which represent defibrillator circuitry.

To proceed with step two, from FIG. 4, nodal analysis provides an equation for V.sub.M : ##EQU10## Rearranging equation 15, we have ##EQU11## Next, substituting equation 14 as an expression for V.sub.3 into equation 16, the cell membrane response is now calculated as follows: ##EQU12## Dividing through by C.sub.M, and setting .tau..sub.M =R.sub.M C.sub.M, then equation 18 becomes ##EQU13##

Equation 19 is a general ordinary differential equation (ODE) that models the effects of any general forcing function V.sub.S that represents a phase of a shock pulse waveform applied across the chest. The general ODE equation 19 models the effects of a general shock pulse phase V.sub.S on the myocardium, determining cardiac cell response to such a shock pulse phase.

In the equations given below:

C.sub.1 equals the capacitance of the first capacitor bank and V.sub.S =V.sub.1 e.sup.-t/.tau..sbsp.1 ; C.sub.2 equals the capacitance of the second capacitor bank and V.sub.S =V.sub.2 e.sup.-t/.tau..sbsp.2 ; R=R.sub.S +R.sub.B, where R.sub.S =System impedance (device and electrodes); R.sub.B =body impedance (thoracic cage, chest wall, lungs (series, parallel), heart).

To determine body impedance, R.sub.B, we see that the series combination of R.sub.H and R.sub.LS yields R.sub.H +R.sub.LS. (FIG. 4). The parallel combination of R.sub.H +R.sub.LS and R.sub.LP yields: ##EQU14## The series combination of equation 20 and R.sub.CW yields: ##EQU15## The parallel combination of equation 21 and R.sub.TC yields: ##EQU16## where R.sub.B is the impedance of the body for this model.

The discharge of a single capacitor is modeled by V.sub.S =V.sub.1 e.sup.-t/.tau..sbsp.1 for an initial C.sub.1 capacitor voltage of V.sub.1. Placing V.sub.S into equation 19 gives: ##EQU17## where .tau..sub.M =R.sub.M C.sub.M represents the time constant of the myocardial cell in the circuit model, and .tau..sub.1, which equals R.sub.S C.sub.1, represents the time constant of .phi..sub.1. Such a standard linear ODE as equation 23 has the form ##EQU18## These linear ODEs have an integration factor that equals e.sup..intg.pdx. The general solution to such equations is:

Y=e.sup.-.intg.pdx �.intg.e.sup..intg.pdx Qdx+c!.

The ODE in equation 23 models the effects of each phase of a time-truncated, capacitor-discharged shock pulse waveform. Equation 23 is a first-order linear ODE, and may be solved using the method of integration factors, to get: ##EQU19## Equation 24 is an expression of cell membrane potential during .phi..sub.1 of a shock pulse. To determine the constant of integration k, the initial value of V.sub.M1 is assumed to be V.sub.M1 (0)=V.sub.G ("cell ground"). Applying this initial condition to equation 24, k is found to be ##EQU20## Assuming .tau..sub.1 =RC.sub.1, where R=R.sub.S +R.sub.B, then the solution to the initial-value problem for .phi..sub.1 is: ##EQU21## Equation 26 describes the residual voltage found on a cell at the end of .phi..sub.1.

Assuming V.sub.G =0 and V.sub.1 =1, the solution for cell response to an external shock pulse is ##EQU22##

We may now determine optimal durations for .phi..sub.1 according to criteria for desired cell response. One such design role or criterion is that the .phi..sub.1 duration is equal to the time required for the external defibrillator shock pulse to bring the cell response to its maximum possible level. To determine this duration, equation 27 is differentiated and the resulting equation 27B is set to zero. Equation 27B is then solved for the time t, which represents shock pulse duration required to maximize cardiac cell response. ##EQU23## Solving for t, the optimal duration d.phi..sub.1 for a monophasic shock pulse or .phi..sub.1 of a biphasic shock pulse is found to be ##EQU24## where "In" represents the logarithm to the base e, the natural logarithm.

For .phi..sub.2, an analysis almost identical to equations 20 through 27 above is derived. The differences are two-fold. First, a biphasic waveform reverses the flow of current through the myocardium during .phi..sub.2. Reversing the flow of current in the circuit model changes the sign on the current. The sign changes on the right hand side of equation 23.

The second difference is the step taken to incorporate an independent .phi..sub.2 into the charge burping model. Therefore, the .phi..sub.2 ODE incorporates the C.sub.2 capacitor set and their associated leading-edge voltage, V.sub.2, for the .phi..sub.2 portion of the pulse. Then .tau..sub.2 represents the .phi..sub.2 time constant; .tau..sub.2 =RC.sub.2, and V.sub.S =-V.sub.2 e.sup.-t/.tau..sbsp.2. Equation 23 now becomes: ##EQU25##

Equation 29 is again a first-order linear ODE. In a similar manner, its general solution is determined to be: ##EQU26## To determine the constant of integration k, the value of V.sub.M2 at the end of .phi..sub.1 is

V.sub.M2 (0)=V.sub.M1 (d.sub..phi.1)=V.phi..sub.1.phi., (31) where d.sub..phi.1 is the overall time of discharge for .phi..sub.1 and V.sub..phi.1 is the voltage left on the cell at the end of .phi..sub.1. Applying the initial condition to equation 30 and solving for k: ##EQU27## The solution to the initial-value problem for .phi..sub.2 is ##EQU28##

Equation 33 provides a means to calculate the residual membrane potential at the end of .phi..sub.2 for the cells that were not stimulated by .phi..sub.1. Setting Equation 33 equal to zero, we solve for t, there by determining the duration of .phi..sub.2, denoted d.phi..sub.2, such that V.sub.M2 (d.phi..sub.2)=0. By designing .phi..sub.2 with a duration d.phi..sub.2, the biphasic shock pulse removes the residual change placed on a cell by .phi..sub.1. We determine d.phi..sub.2 to be: ##EQU29##

From the equations above an optimal monophasic or biphasic defibrillation waveform may be calculated for an external defibrillator.

As an example, an external defibrillator may be designed as set forth below. Assume a monophasic truncated exponential shock pulse, a 200 .mu.F capacitor, so that .tau..sub.1 =R.multidot.(200 .mu.F). Suppose also that the external defibrillator is designed to apply the maximal cardiac cell response design rule (equation 27C) to determine the duration of the discharge. Suppose further that the human cardiac cell time constant is estimated to be 3.+-.1 ms. Further assume that the external defibrillator energy source comprises five 1000 .mu.F capacitors in series to implement a 200 .mu.F capacitor bank. If each capacitor is charged to 400V, for a total of 2000V for the leading-edge voltage this represents 400 J of stored energy. The transchest elements are estimated at: 82% current through the thoracic cage; 14% through the chest wall and lungs in parallel; and 4% of applied current through the lung in series with the heart. Then the membrane resistance coefficient .OMEGA..sub.M =5.9, and the system resistance coefficient .OMEGA..sub.S =2.3. Then the table below illustrates the application of the design rule as the overall chest resistance ranges from 25.OMEGA. to 200.OMEGA.:

______________________________________R (.OMEGA.) .tau..sub.1 d(.phi.1) (ms) V.sub.final (V) E.sub.delivered (J)______________________________________ 25 5.2 5.05 757 343 50 10.2 6.90 1017 297 75 15.2 8.15 1170 263100 20.2 9.10 1275 238125 25.2 9.90 1350 216150 30.2 10.55 1410 201175 35.2 11.15 1457 186200 40.2 11.65 1497 176______________________________________

Description of a Single Capacitor/Inductor Circuit

Having developed the transthoracic model above, a general description of a single capacitor/inductor circuit will now be given.

FIG. 6 is a simplified schematic illustration of a general external defibrillation pulse generation circuit 30 which is configured for producing truncated damped sinusoidal biphasic (i.e., multiphasic) and/or monophasic defibrillation pulses (without the external modeling parameters of FIGS. 4 and 5). As shown, circuit 30 includes a first pulse component generation circuit 32 and a second pulse component generation circuit 34 which are connected in a parallel arrangement to a pair of electrode terminals 36 and 38. First pulse component generation circuit 32 includes a charge storage device such as capacitor 40, an inductor 42 and a circuit or device represented by switch 44 connected to one another in a series arrangement between terminals 36 and 38. Similarly, second pulse component generation circuit 34 includes a capacitor 46, an inductor 48 and a switch 50 connected to one another in a series arrangement between terminals 36 and 38. Switches 44 and 50 are coupled to and are independently controlled by a pulse controller 52 to initiate and terminate (truncate) defibrillation pulses.

The defibrillation pulses generated by circuit 30 will be applied to the torso of a patient in a conventional manner through a pair of electrodes, such as electrodes 24 from FIG. 2, electrically connected to terminals 36 and 38. In one embodiment capacitors 40 and 46 are in the range of 25 .mu.F-500 .mu.F, and inductors 42 and 48 are in the range of 25 mH-500 mH. Estimated optimal values for capacitors 40 and 46 and inductors 42 and 48 are chosen for AED 10. In one embodiment, capacitor 40 is chosen to be 45 .mu.F, capacitor 46 is chosen to be 180 .mu.F, inductor 42 is chosen to be 250 mH and inductor 48 is chosen to be 250 mH.

FIG. 7 is an illustration of a single capacitor truncated damped sinusoidal biphasic defibrillation pulse which includes a first phase having a positive polarity component 54 and a second phase having a negative polarity pulse component 56. With capacitors 40 and 46 charged to their respective opposite polarity charge potentials, pulse controller 52 causes the circuit element(s) forming switch 44 to switch to a closed state at time t.sub.0 to initiate first phase pulse component 54. At time t.sub.1 switch 44 is switched to an open state to terminate pulse component 54. Although not shown in FIG. 7, pulse component 54 can also be truncated by circuitry controlled by pulse controller 52 which rapidly discharges capacitor 40 and inductor 42. Second phase pulse component 56 is initiated by pulse controller 52 at time t.sub.2 by switching switch 50 to a closed state. At time t.sub.3 switch 50 is switched to an open state to truncate second phase pulse component 56.

It has been determined that one preferred and efficacious waveform has first phase component 54 with a relatively slow onset with respect to a convention damped sinusoidal pulse. In particular, the shape of first phase pulse component 54 can be tailored in an attempt to match the cell membrane response to the first pulse component. These preferred characteristics of first phase component 54 are illustrated generally in FIGS. 8a, 8b and 8c. FIG. 8a illustrates a damped sinusoidal pulse waveform and the associated cell membrane response. This diagram assumes an 80.OMEGA. load for the heart, capacitor 40 is 45 .mu.F and inductor 42 is 250 mH.

The damped sinusoidal pulse waveform shown in FIG. 8b has a slower onset than that of the waveform shown in FIG. 8a, with little or no loss in the onset time of the associated cell membrane response. In this embodiment, an 80.OMEGA. load is again assumed for the heart, capacitor 40 is 90 .mu.F and inductor 42 is 250 mH. The damped sinusoidal pulse waveform shown in FIG. 8c has an even slower onset time than that of the waveform shown in FIG. 8b, and again generates an associated cell membrane response with little or no loss in the onset time with respect to the cell membrane response generated by the waveform shown in FIG. 8a. In this embodiment, an 80.OMEGA. load is again assumed, capacitor 40 is 180 .mu.F and inductor 42 is again 250 mH.

Another preferred and effacious waveform has first phase component 54 which is truncated at a time which closely corresponds to the time that the maximum or peak cell membrane response is achieved (i.e., when the slope of the cell membrane response is zero). First phase components 54 having these characteristics are also illustrated in FIGS. 8a, 8b and 8c. Defibrillation efficacy can thereby be maximized with respect to the peak cell response design rule using relatively low energy and short waveforms.

The ability of circuit 30 to independently generate and shape second phase pulse component 56 also contributes to the efficacy of the waveform. In particular, this characteristic of the circuit enables second phase pulse component 56 to be tailored in voltage, length, shape and other characteristics to maximize its charge burping effects on the cell membrane. For example, second phase component 56 can have a duration which is different than the duration of first phase component 54, and which is truncated at the time that the cell membrane response reaches about zero.

In operation a single capacitor/inductor truncated damped sine wave is developed and applied to a patient that approximates the patient's cell response and that truncates at the peak cell response. In order to develop a waveform that matches (or attempts to match) the cell response, it is first necessary to measure a patient dependent parameter, such as impedance, voltage, current, charge or other measurable parameters of the patient. The values of capacitors 40 and 46, and inductors 42 and 48 are preselected, thus once the patient dependent parameter is determined a first duration of a monophasic or .phi..sub.1, of a biphasic waveform can be determined using the equations developed above for modeling a human chest. In particular, solving for t in equation 36 below yields the optimal duration d.phi. for a monophasic or .phi..sub.1 of a biphasic waveform. Equation 39 below is used to determine the optimal duration for .phi..sub.2 of the biphasic waveform. As can be appreciated from the above, the determination of the optimal duration of .phi..sub.2 is independent from that of .phi..sub.1.

After the optimal durations have been determined, capacitor 40 is charged. For biphasic waveforms, capacitor 42 is also charged. Switch 44 is then closed discharging capacitor 40 to electrical terminals 36 and 38. At the end of the optimally calculated duration d.phi..sub.1, switch 44 is opened to truncate the phase one waveform. Switch 50 is then closed discharging capacitor 42 to electrode terminals 36 and 38. At the end of the optimally calculated duration d.phi..sub.2 switch 50 is opened to truncate the .phi..sub.2 portion of the biphasic waveform.

The design rules for the truncated damped sine waveform are now developed. Equation 19 is now used to solve for V.sub.M by replacing V.sub.S with the defibrillation circuit model for a damped sine waveform, as shown in equation 14B. Doing so provides: ##EQU30## The solution of equation 35 for V.sub.M is found in the same manner as described above for equations 24-27, so that ##EQU31##

To maximize cell response, V.sub.M1 (t) is differentiated, the derivative (dV.sub.M1 (t)/dt) is to zero, and the time t is determined such that (dV.sub.M1 (t)/dt)=0. The time t solution of the differential equation of V.sub.M1 (t) provides the design rule for the duration of .phi..sub.1 of the truncated damped sine waveform. For a predetermined time step T.sub.S (for example 50 ms) a real-time implementation of the truncation process is described in FIG. 9 for .phi..sub.1 of the damped sine wave shock pulse. The time T.sub.1 (old) determined from this process is the time of maximum cell response and therefore the duration of .phi..sub.1.

In the same manner that equation 33 is derived, the cardiac cell response to .phi..sub.2 of a damped sine wave shock pulse is found to be ##EQU32## in accordance with FIG. 5b; where ##EQU33## where d.sub.1 is the duration for .phi..sub.1 of the damped sine wave shock pulse. To determine the .phi..sub.2 design rule, equation 39 is set to zero and the time t is determined such that V.sub.M2 (t)=0. A real-time implementation of .phi..sub.2 truncation process is described in FIG. 10.

Description of the Present Invention

The present invention provides a method and apparatus for delivering a stepped truncated damped sinusoidal waveform that defibrillates the heart based on models of the cell response. The invention truncates the monophasic pulse or .phi..sub.1 of a biphasic pulse at the peak or approximate peak cell response time. The invention truncates the .phi..sub.2 of a biphasic pulse at the time the cell response has indicated that the cell is reset to a natural resting potential. Additionally, the present invention provides a waveform that attempts to approximate a constant current waveform.

FIG. 11 is a schematic illustration of an external defibrillation pulse generation circuit 60 which is configured for producing stepped truncated damped sinusoidal biphasic (i.e. multiphasic) and/or monophasic defibrillation pulses according to the present invention. As shown, circuit 60 includes a first pulse component generation circuit 62 and a second pulse component generation circuit 64 which are connected in a parallel arrangement to a pair of electrode terminals 66 and 68. First pulse component generation circuit 62 includes first and second charge storage device such as capacitors 70 and 72, inductor 74 and first and second circuits or other devices represented by switches 76 and 78 connected to one another in a series arrangement between terminals 66 and 68. A diode 80 is connected at node 82 which is between capacitor 70 and switch 78 and 84 which is between inductor 74 and electrode 68. Similarly, second pulse component generation circuit 64 includes first and second capacitors 86 and 88, inductor 90 and first and second switches 92 and 94, connected to one another in a series arrangement between terminals 66 and 68. A diode 96 is connected at node 98 which is between capacitor 86 and switch 94 and node 100 which is between conductor 90 and electrode 68. Switches 76, 78, 92 and 94 are coupled to and independently controlled by a pulse control circuit 102 to initiate and truncate (terminate) defibrillation pulses. The defibrillation pulses generated by circuit 60 will be applied to the torso of a patient in a conventional manner through a pair of electrodes (as illustrated in FIG. 1) electrically connected to terminal 66 and 68.

FIG. 12 is an illustration of a stepped truncated damped sinusoidal biphasic defibrillation pulse which includes a first phase and positive polarity component 110 and a second phase and negative polarity pulse component 112. To generate the pulse shown in FIG. 12, capacitors 70 and 72 are charged to their desired first polarity charge potentials, while capacitors 86 and 88 are charged to their desired and opposite second polarity charge potentials. At time t.sub..phi. pulse control circuit 102 causes the circuit elements forming switch 76 to switch to a closed state to initiate first phase pulse component 110 with voltage step 114. Very quickly (e.g. less than about 1 ms, and before capacitor 70 discharges to any substantial degree) after switch 76 has been closed, pulse control circuit 102 switches switch 78 to a closed state to initiate the sinusoidal component 116 of first phase pulse component 110. At time t.sub.1 switches 76 and 78 are switched to an open state to terminate pulse component 110. Although not shown in FIG. 11, first phase pulse component 110 can also be truncated by circuitry controlled by pulse control circuit 102 which rapidly discharges capacitors 70 and 72 and inductor 74. A voltage step 118 of second phase pulse component 112 is initiated by pulse control circuit 102 at time t.sub.2 by switching switch 92 to a closed state. Very quickly thereafter (i.e., before capacitor 86 substantially discharges), switch 94 is closed by pulse control circuit 102 to initiate the sinusoidal component 120 of pulse component 112. At time t.sub.3 switches 92 and 94 are switched to an open state to truncate second phase pulse component 112. Defibrillation pulse components 110 and 112 provide an initial step of voltage and current to rapidly initiate the cell membrane response, and are efficacious.

FIG. 13 is an illustration of a two-part stepped truncated damped sinusoidal biphasic defibrillation pulse according to the present invention which can be generated by circuit 60 shown in FIG. 11. As shown, the pulse includes a first phase and positive polarity component 122 and a second phase and negative polarity pulse component 124. To generate the pulse capacitors 70 and 72 are charged to their desired first polarity charge potentials, while capacitors 86 and 88 are charged to their desired and opposite second polarity charge potentials. At time t.sub.0 pulse control circuit 102 causes the circuit elements forming switch 76 to switch to a closed state to initiate first phase pulse component 122 with voltage step 126. The maximum voltage of step 126 will be the charge potential on capacitor 70. Between time t.sub.0 and time t.sub.1 switch 76 remains in its closed state and switch 78 in its open state to provide an exponential decay component 128. At time t.sub.1 pulse control circuit 102 switches switch 78 to a closes state to initiate a sinusoidal component 130 of first phase pulse component 122. A time t.sub.2 switches 76 and 78 are switched to an open state to terminate pulse component 122. Although not shown in FIG. 11, first phase pulse component 122 can also be truncated by circuitry controlled by pulse control circuit 102 which rapidly discharges capacitors 70 and 72 and inductor 74. Voltage step 132 of second phase pulse component 124 is initiated by pulse control circuit 102 at time t.sub.3 by switching switch 92 to a closed state. The maximum voltage of step 132 will be the charge potential on capacitor 86. Between time t.sub.3 and t.sub.4 switch 92 remains in its closed state and switch 94 in its open state to provide an exponential decay component 134. At time t.sub.4 pulse control circuit 102 switches switch 94 to a closed state to initiate a sinusoidal component 136 of pulse component 124. At time t.sub.5 switches 92 and 94 are switched to an open state to truncate second phase pulse component 124. Defibrillation pulses of the type shown in FIG. 13 provide an initial step of voltage and current, as well as a short expotential decay, to rapidly initiate cell membrane response, and are efficacious.

Optimal characteristics for defibrillation pulse waveforms and the component values of circuit 60 for generating the waveforms can be determined using the design methodology described above.

The present invention provides for a higher effective current than does the single capacitor/inductor circuit of FIG. 6. By providing a rapid boost in the delivery of the waveform, the peak cell response is obtained quicker which produces a more efficient waveform. Additionally, the present invention allows for reducing the size of the inductor while still providing an effective and efficient waveform which saves cost, size and weight. The present invention also more closely approximates a constant current waveform, which have been proven to be the most efficacious.

The design rules and methodology for the stepped capacitor damped sine waveform are described below. Equation 19 is used to solve for V.sub.M by replacing V.sub.S with the defibrillation circuit model for a stepped capacitor damped sine waveform, as shown in the first pulse component generation circuit 62 of FIG. 11. The stepped capacitor damped sine wave model is described by: ##EQU34## where C.sub.1 represents capacitor 70, C.sub.2 represents capacitor 72 and L.sub.2 represents inductor 74.

As stated above, the circuit in FIG. 11 operates as set forth below; capacitors 70 and 72 are charged. Switch 76 is then closed for a period of time. At the end of that time period, switch 78 is closed. Therefore, voltage at the time d.sub.1 switch 76 is closed is equal to ##EQU35## where .tau..sub.1 =RC.sub.1. So, at the time that switch 78 is closed, equation 42 is rewritten as ##EQU36## where V.sub.1 is the voltage on C.sub.1 and V.sub.2 is the voltage on C.sub.2.

The solution to equation 43 and thereby the description for the stepped capacitor damped sine wave model is ##EQU37##

The stepped capacitor damped sine wave model operates as a single capacitor discharge for a time from 0 to d.sub.1 in duration. This is the time that switch 76 is closed and switch 78 is open. The cell response during this time is described by equation 27, so that at the moment immediately prior to closing switch 78, the cell response is V.sub.M1 (0)=V.sub.M11 (d.sub.1), where V.sub.M11 designates the cell response to the stepped capacitor damped sine wave after closing switch 76 and prior to closing switch 78, and V.sub.M12 designates the cell response following the closing of switch 78, all during .phi..sub.1 of the waveform (or the only phase, if the waveform is monophasic). Therefore, in general, the .phi..sub.1 cell response to the model at time t.gtoreq.d.sub.1 is:

V.sub.M1 (t)=V.sub.M11 (d.sub.1)e.sup.-t/.tau..sbsp.M +V.sub.M12 (t).

The description of the cell response V.sub.M12 (t) is ##EQU38##

The design rule for the stepped capacitor damped sine wave is to deliver the waveform shock pulse for a time d.sub.2 >d.sub.1 such that the cell response is maximized. This defines the optimal duration for the first phase .phi..sub.1 of the shock pulse. This also defines the optimal duration for a monophasic shock pulse. FIG. 9 describes the determination of d.sub.2 and thereby implementation of the monophasic shock pulse or .phi..sub.1 of a biphasic shock pulse, so that V.sub.M1 (d.sub.2) is the desired cell response.

In one preferred embodiment, the duration d.sub.1 is predetermined and short by comparision to the total length d.sub.2. This d.sub.1 provides the jump start to V.sub.1 that the shock pulse requires in order to simulate a square wave, or constant current, shock pulse. An illustrative example will now be given. It should be noted that the values chosen are simply examples and are not meant to limit the present invention in any way. The duration d.sub.1 is chosen to be less than 1 ms, and in particular, 0.1 ms. This means that switch 76 is closed and switch 78 is open for this period of time. At time d.sub.1 switch 78 is closed until time d.sub.2. Time d.sub.2 is predetermined as described in FIG. 9. The shock pulse and associated cell response are shown as .phi..sub.1 of the waveform in FIG. 14. Equation 27 describes the cell response due to closing switch 76 prior to closing switch 78. This cell response is expressed by V.sub.M11 (d.sub.1) and represents a portion of the total cell response. Therefore, V.sub.M1, over the time 0 to time (d.sub.1 +d.sub.2), expresses the cumulative cell response to a stage or pulse component generation circuit.

Additional waveforms and cell responses are illustrated in the series of FIG. 15. FIGS. 15a, 15b, 15c, 15d, 15e, and 15f illustrate comparisons between a single capacitor exponential decay waveform, a double capacitive exponential decay waveform and a stepped capacitor-inductor waveform and associated cell responses for various parameters. The single capacitor waveform and cell response is designated as "1 cap.", the double capacitor waveform and cell response is designated as "2 cap.", and the stepped capacitor/inducer waveform and all response is designated "2 cap./ind.". As illustrated, the stepped capacitor-inductor waveform has the highest and or most effective cell response of the three.

In the same manner that equation 33 and equation 39 were derived, .phi..sub.2 of a biphasic waveform is developed to implement the charge burping design rule. The .phi..sub.2 of a shock pulse is required to remove the residual charge remaining on a cardiac cell if that cell did not depolarize due to .phi..sub.1 of the shock pulse. As a next step in this process, the cell response to a stepped capacitor damped sine wave implementation of .phi..sub.2 is ##EQU39## where d.phi..sub.1 is the total duration of .phi..sub.1 of the biphasic shock pulse and .tau..sub.C and .tau..sub.M, .OMEGA..sub.S and .OMEGA..sub.M are defined in accordance with defibrillation circuitry that implements the stepped capacitor damped sine wave as .phi..sub.2 of a biphasic shock pulse. FIG. 10 describes the implementation the charge burping design rule, which is operated prior to the discharge of a .phi..sub.2 of a biphasic waveform. The design rule is independent of any implementation of .phi..sub.1 of a biphasic shock pulse. The effects of .phi..sub.1 on the cardiac cell are incapsulated in the V.sub.M1 (t) and V.sub.S (t) elements of equations 48, 49, and 50.

The present invention applies a waveform that defibrillates the heart based on models of cell response, and which truncates a monophasic waveform of .phi..sub.1 of a biphasic waveform at the peak or approximate peak cell response time. As desired, the present invention further applies a second phase of a waveform that further defibrillates the heart based on models of the cell response, and which truncates .phi..sub.2 of a biphasic waveform at the time a cell response is reset to the cell's natural resting state.

It should be stressed and understood that .phi..sub.2 is independent from .phi..sub.1. In order to design an effective .phi..sub.2 waveform, the only thing needed from .phi..sub.1 is to know where the cell response was left when .phi..sub.1 truncated. Phase one may be designed based on the truncated damped sine wave equations given above, while .phi..sub.2 may be designed and implemented utilizing other technology such as single capacitor for discharge technology and vice-versa. The corresponding design rules for a .phi..sub.1 circuitry may be used in conjunction with the design rules for a .phi..sub.2 circuitry, regardless of the specific circuitry used to implement each phase of a monophasic or biphasic shock pulse.

Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit or scope of the present invention.

Claims

1. An external defibrillator for delivering a stepped truncated damped sinusoidal waveform comprising:

first and second electrode connectors only;

first and second charge storage components;

a first inductive component connected to the first charge storage component and the first electrode connector, and directly connected to the second charge storage component;

a first truncating switch connected between the first charge storage component and the second electrode connector;

a second truncating switch connected between the first and second charge storage components, and

pulse control circuitry connected to the first and second truncating switches for independently operating the first and second switches.

2. The defibrillator of claim 1, further comprising:

third and fourth charge storage components;

a second inductive component connected to the third charge storage component and the first electrode connector;

a third truncating switch connected between the third charge storage component and the second electrode connector;

a fourth truncating switch connected between the third and fourth charge storage components; and

wherein the pulse control circuitry is also connected to the third and fourth truncating switches for independently operating the first, second, third and fourth truncating switches.

3. The defibrillator as in claim 2 wherein the third and fourth charge storage components comprise capacitors, each having a capacitance value within the range of 25 .mu.F to 500 .mu.F.

4. The defibrillator of claim 2 and further including:

a first diode connected between the second electrical connector and a first node, the first node defining a junction of the second truncating switch and the first storage component; and

a second diode connected between the second electrical connector and a second node, the second node defining a junction of the fourth truncating switch and the third storage component.

5. The defibrillator as in claim 1 wherein the first and second charge storage components comprise capacitors, each having a capacitance value within the range of 25 .mu.F to 500 .mu.F.

6. The defibrillator of claim 1 and further including:

a first diode connected between the first electrical connector and a first node, the first node defining a junction of the second truncating switch and the first storage component.

7. The defibrillator of claim 1 wherein the first inductive component comprises an inductor having an inductance value within the range of 25 mH to 500 mH.

8. The defibrillator of claim 1 wherein the second inductive component comprises an inductor having an inductance value within the range of 25 mH to 500 mH.

9. A method of generating a stepped truncated damped sinusoidal waveform using a defibrillator having first and second electrode connectors, first and second charge storage components, a first inductive component connected to the first charge storage component and to the first electrode connector a first truncating switch connected between the first charge storage component and the second electrode connector, a second truncating switch connected between the first and second charge storage components and pulse control circuitry connected to the first and second truncating switches, the method comprising:

a) monitoring a first patient dependent parameter;

b) determining a first discharge duration based on the monitored first patient dependent parameter;

c) charging the first and second charge storage components;

d) closing the first truncating switch;

e) closing the second truncating switch after a predetermined delay;

f) opening the first and second truncating switches to truncate the waveform at the expiration of the determined first discharge duration.

10. The method of claim 9, wherein step b) is carried out using a quantitative model of a patient including at least one of a chest component, a heart component and a cardiac cell component.

11. The method of claim 9 wherein the step of determining the first discharge duration is based on a predetermined design rule.

12. The method of claim 11 wherein the predetermined design rule is based on a transthoracic model that represents the time course of cardiac cell membrane potential during a transthoracic shock pulse.

13. The method of claim 9 wherein the first patient-dependent parameter is impedance.

14. The method of claim 9 wherein the first patient-dependent parameter is current.

15. The method of claim 9 wherein the first patient-dependent parameter is voltage.

16. The method of claim 9 wherein the first patient-dependent parameter is charge.

17. The method of claim 9 wherein the step of determining the first discharge duration includes determining a truncation point of the waveform based on a maximum cardiac cell membrane response.

18. The method of claim 17 wherein determining the truncation point further comprises:

iteratively calculating a voltage value of a cardiac cell response function at incrementally increasing time values of a first discharge duration function until the voltage value of the cardiac cell response function reaches a maximum, at which time the waveform is truncated and a maximum time value of the first discharge duration function equals the first discharge duration.

19. The method of claim 18 wherein determining the truncation point further comprises the steps of:

g) setting a prior time value of a first discharge duration function equal to zero and setting a prior voltage value of a cardiac cell response function equal to zero;

h) determining a subsequent time value of the first discharge duration function by adding an incremental time period T.sub.S to the prior time value of the first discharge duration function;

i) determining a subsequent voltage value of the cardiac cell response function at the subsequent time value;

j) comparing the subsequent voltage value to the prior voltage value and upon the subsequent voltage value being greater than the prior voltage value, resetting the prior voltage value equal to the subsequent voltage value and resetting prior time value equal to the subsequent time value; and

k) repeating steps h)-j) until the subsequent voltage value of the cardiac cell response function is no greater than the prior voltage value of the cardiac cell response function at which time the truncation point is established wherein the subsequent time value equals the first discharge duration and the subsequent voltage value represents the maximum cardiac cell membrane response.

20. The method as in claim 9, wherein step e) is carried out with a predetermined delay of less than 1 millisecond.

21. The method as in claim 9, wherein step b) is carried out using a quantitative model of a patient including at least one of a chest component, a heart component and a cardiac cell component.

22. The method of claim 9 and further including third and fourth electrode connectors, third and fourth charge storage components, a second inductive component connected to the third charge storage component and to the third electrode connector, a third truncating switch connected between the third charge storage component and the fourth electrode connector, a fourth truncating switch connected between the third and fourth charge storage components and pulse control circuitry connected to the third and fourth truncating switches, the method comprising:

g) monitoring a second patient dependent parameter;

h) determining a second discharge duration based the monitored second patient dependent parameter;

i) charging the third and fourth charge storage components;

j) closing the third truncating switch;

k) closing the fourth truncating switch after a predetermined delay;

l) opening the third and fourth truncating switches to truncate the waveform at the expiration of the determined second discharge duration.

23. The method of claim 22 wherein in step c), the third and fourth charge storage components are charged with a negative polarity charge.

24. The method of claim 22 wherein the second patient-dependent parameter is charge.

25. The method of claim 22 wherein the second patient-dependent parameter is voltage.

26. The method of claim 22 wherein the second patient-dependent parameter is impedance.

27. The method of claim 22 wherein the second patient-dependent parameter is current.

28. The method of claim 22 wherein the second patient-dependent parameter is independent of the first patient-dependent parameter.

29. The method of claim 22, wherein step h) is carried out independently of step b).

30. The method of claim 22, wherein step h) is carried out using a quantitative model of a patient including at least one of a chest component, a heart component and a cardiac cell membrane component.

31. The method of claim 22 wherein step h) further comprises:

determining a truncation point based on a residual cardiac cell membrane potential.

32. The method of claim 22 wherein the determining the truncation point further comprises:

iteratively calculating a voltage value of the cardiac cell response function at incrementally increasing time values of a second discharge duration function until the voltage value of the cardiac cell response function reaches a minimum, at which time the waveform is truncated and a maximum time value of the second discharge duration function represents the second discharge duration.

33. The method of claim 22 wherein the step of iterative calculation further comprises the steps of:

m) setting a prior value of a second discharge duration function equal to zero and setting a prior voltage value of a cardiac cell response function equal to the voltage value of the cardiac cell response function at the end of the first discharge duration;

n) determining a subsequent time value of the second discharge duration function by adding an incremental time period T.sub.S to the prior time value of the second discharge duration function;

o) determining a subsequent voltage value of the cardiac cell response function based on the subsequent time value and on the voltage value of the cardiac cell response function at the end of the first discharge duration;

p) comparing the subsequent voltage value to the prior voltage value and upon the subsequent voltage value being greater than the prior voltage value, resetting the prior time value equal to the subsequent time value; and

q) repeating steps n)-p) until the subsequent voltage value of the cell response function is equal to zero at which time the truncation point is established, wherein the subsequent time value equals the second discharge duration and identifies the expiration of the waveform.

34. A method of generating a stepped truncated damped sinusoidal waveform using a defibrillator having a first pulse generation circuit and pulse control circuitry connected to the first pulse generation circuit, the method comprising:

a) monitoring a patient dependent parameter;

b) determining a first discharge duration based on the monitored patient dependent parameter;

c) activating the first pulse generation circuit to produce an exponential decay waveform component;

d) further activating the first pulse generation circuit after a predetermined delay to produce a sinusoidal waveform component, the sinusoidal waveform component being added to the exponential waveform component;

e) deactivating the first pulse generation circuit to truncate the waveform at the expiration of the determined first discharge duration.

35. The method as in claim 34, wherein step d) is carried out with a predetermined delay of less than 1 millisecond.

36. The method as in claim 34, wherein step b) is carried out using a quantitative model of a patient including at least one of a chest component, a heart component and a cardiac cell component.

37. The method of claim 34 wherein the first pulse generation circuit further comprises first and second electrode connectors, first and second charge storage components, a first inductive component connected to the first charge storage component and to the first electrode connector, and a first truncating switch connected between the first charge storage component and the second electrode connector, a second truncating switch connected between the first and second charge storage components.

38. The method of claim 34 wherein step d) includes further activating the first pulse generation circuit after the predetermined delay which has a duration sufficiently short to maintain a positive slope of the waveform at all times.

39. The method of claim 34 wherein step d) includes further activating the first pulse generation circuit for the predetermined delay which has a duration sufficiently long to result in the exponential decay waveform component having a negative sloped waveform and to result in the waveform having a positive slope upon the further activation of the sinusoidal waveform component.

40. The method of claim 34 and further comprising step c of activating the first pulse generation circuit to produce the exponential decay waveform component, which has a duration less than a duration of step d of further activating the first pulse generation circuit to produce the sinusoidal waveform component.

41. The method of claim 34 and further comprising the defibrillator having a second pulse generation circuit and pulse control circuitry connected to the second pulse generation circuit, and the method further comprising:

f) determining a second discharge duration based on the residual voltage on cardiac cells that were not depolarized during the first phase shock pulse;

g) activating the second pulse generation circuit to produce an exponential decay waveform component having a negative polarity;

h) further activating the second pulse generation circuit after a predetermined delay to produce a sinusoidal waveform component having a negative polarity, which is added to the exponential decay waveform component;

i) deactivating the second pulse generation circuit to truncate the waveform including both the exponential decay and sinusoidal components at the expiration of the determined second discharge duration.

42. The method of claim 41 wherein the second pulse generation circuit further comprises third and fourth electrode connectors, third and fourth charge storage components, a second inductive component connected to the third charge storage component and to the third electrode connector, a third truncating switch connected between the third charge storage component and the fourth electrode connector, a fourth truncating switch connected between the third and fourth charge storage components.

43. The method of claim 41 wherein the second discharge duration is determined independently from the first discharge duration.

44. The method of claim 41 wherein step h) includes further activating the second pulse generation circuit after the predetermined delay which has a duration sufficiently short to maintain a positive slope of the waveform at all times.

45. The method of claim 41 wherein step h) includes further activating the second pulse generation circuit after the predetermined delay which has a duration of sufficient length to result in the exponential decay waveform component having a negative sloped waveform and to result in the waveform having a positive slope waveform upon the further activation of the sinusoidal waveform component.

46. A method of generating a two-part stepped truncated damped sinusoidal waveform using a defibrillator having a first pulse generation circuit and pulse control circuitry connected to the first pulse generation circuit, the method comprising:

a) monitoring a patient dependent parameter;

b) determining a first discharge duration based on the monitored patient dependent parameter;

c) activating the first pulse generation circuit to produce a first step of the waveform having a negative slope;

d) further activating the first pulse generation circuit after a predetermined delay to produce a second step of the waveform having a positive slope;

e) deactivating the first pulse generation circuit to truncate the waveform at the expiration of the determined first discharge duration.

47. The method of claim 46 wherein a maximum voltage of the first step is equal to a charge on a first charge storage component of the first pulse generation circuit prior to the first activation step.

48. A method of generating a stepped truncated damped sinusoidal shock pulse external defibrillation waveform having a substantially constant current characteristic using a defibrillator having a first pulse generation circuit and pulse control circuitry connected to the first pulse generation circuit, the method comprising:

a) monitoring a patient dependent parameter;

b) determining a first discharge duration based on the monitored patient dependent parameter;

c) activating the first pulse generation circuit to produce a first waveform component;

d) further activating the first pulse generation circuit after a predetermined delay to produce a second waveform component, which is added to the first waveform component;

e) deactivating the first pulse generation circuit to truncate the waveform at the expiration of the determined first discharge duration.

49. The method of claim 48 wherein step c) and step d) further comprise operating the first pulse generation circuit to produce the first and second waveform components having a positive slope at all times up to the truncation point.

50. The method of claim 48 wherein step c) further comprises operating the first pulse generation circuit to produce the first waveform component having a negative slope.

51. The method of claim 48 wherein step d) further comprises operating the first pulse generation circuit to produce the combined first and second waveform component having a positive slope.

52. A method of generating a second phase of a biphasic stepped truncated damped sinusoidal waveform using a defibrillator having a pulse generation circuit and pulse control circuitry connected to the pulse generation circuit, the method comprising:

a) determining a discharge duration of the second phase based on a predetermined charge burping design rule;

b) activating the pulse generation circuit to produce a first waveform component having a negative polarity;

c) further activating the pulse generation circuit after a predetermined delay to produce a second waveform component having a negative polarity, which is added to the first waveform component;

d) deactivating the pulse generation circuit to first and second waveform components at the expiration of the determined discharge duration.

53. The method of claim 52 wherein step b) further comprises producing an exponential decay waveform component.

54. The method of claim 52 wherein step c) further comprises producing a sinusoidal waveform component.

55. The method of claim 52 wherein in step a), the predetermined charge burping design rule implements a cell response to a stepped capacitor damped sine wave, the cell response being represented by the following equation: ##EQU40## where d.phi..sub.1 is the total duration .phi..sub.1 of the biphasic shock pulse, and

.tau..sub.C=RC

.tau..sub.M =R.sub.M C.sub.M, where R.sub.M represents a cardiac cell membrane resistive component and C.sub.M represents a cardiac cell membrane capacitance component, and

.OMEGA..sub.S represents a system impedance .OMEGA..sub.M represents a cardiac cell membrane impedance, and

.tau..sub.L =L/R,

L represents an inductive component of a defibrillation circuit model

C represents a capacitive component of the defibrillation circuit model

R represents impedance.

56. The method of claim 55 wherein the stepped capacitor sine wave model is represented by the following equation: ##EQU41## where V.sub.1 is the voltage on C.sub.1 and V.sub.2 is the voltage on C.sub.2.

57. The method of claim 52 wherein the cardiac cell response to the shock pulse prior to further activation of the pulse generation circuit is represented by the following equation: ##EQU42## where .OMEGA..sub.S represents a system impedance

.OMEGA..sub.M represents a cardiac cell membrane impedance, and

.tau..sub.M=R.sub.M C.sub.M, where R.sub.M represents a cardiac cell membrane resistive component and

C.sub.M represents a cardiac cell membrane capacitance component, and

.tau..sub.C1 =RC1,

C1 represents a capacitive component of the defibrillation circuit model

.tau..sub.L =L/R,

L represents an inductive component of a defibrillation circuit model

R represents impedance.

58. The method of claim 57 wherein the cardiac cell response to the shock pulse after further activation of the pulse generation circuit is represented by the following equation: ##EQU43## where .OMEGA..sub.S represents a system impedance

.OMEGA..sub.M represents a cardiac cell membrane impedance, and

.tau..sub.M =R.sub.M C.sub.M, where R.sub.M represents a cardiac cell membrane resistive component and

C.sub.M represents a cardiac cell membrane capacitance component, and

.tau..sub.C1 =RC1

C1 represents a capacitive component of the defibrillation circuit model

.tau..sub.L =L/R,

L represents an inductive component of a defibrillation circuit model

R represents impedance.

59. A method for applying electrotherapy to a patient through electrodes adapted to be connected externally to the patient, the electrodes being connected to an energy source, the method comprising the steps of:

a) monitoring a patient dependent parameter;

b) determining a first discharge duration based on the monitored patient dependent parameter;

c) activating the energy source to produce an exponential decay waveform component;

d) further activating the energy source to produce a sinusoidal waveform component after a predetermined delay, the sinusoidal waveform component being added to the exponential waveform component to produce a combined waveform;

e) deactivating the energy source to truncate the combined waveform at the expiration of the determined first discharge duration.

60. The method of claim 59 wherein the predetermined delay is sufficiently short that the combined waveform has a positive slope at all times up to the truncation of the combined waveform.

61. The method of claim 60 wherein the predetermined delay is sufficiently long that the combined waveform has first portion with a negative slope and a second portion with a positive slope up to the truncation of the combined waveform.

62. A method for applying a transchest external defibrillation shock pulse through electrodes positioned on a patient's torso to produce a desired response in the patient's cardiac cell membranes, the method including the steps of:

a) monitoring a patient dependent parameter;

b) determining a first discharge duration of the shock pulse based on the monitored patient dependent parameter;

c) applying a first portion of the shock pulse;

d) further applying a second portion of the shock pulse after a predetermined delay, the second portion being added to the first portion of the shock pulse;

e) terminating application of the shock pulse at the expiration of the determined first discharge duration.

63. The method of claim 62 wherein the predetermined delay has a duration that is sufficiently short that the combined waveform has a positive slope at all times up to the truncation of the combined waveform.

64. The method of claim 62 wherein the predetermined delay has a duration that is sufficiently long that the first portion has a positive value that decreases over time and the combination of the first portion and the second portion has a positive value that increases over time up to the truncation of the combined waveform.

65. A method for operating a device connected to at least two electrodes for placement on the chest on a human patient to treat myocardial arrhythmia in the human patient, the method comprising the device implemented steps of:

sensing a myocardial arrhythmia in the human patient;

in response to the myocardial arrhythmia, delivering from electrical energy stored in a capacitor system in the device to the electrodes, a truncated defibrillation waveform comprising at least a first waveform component and a second waveform component added to the first waveform component after a predetermined delay, wherein the waveform is truncated at the expiration of a discharge duration that is based on a monitored patient-dependent parameter.

66. The method of claim 65 wherein the first waveform component is an exponential decay waveform.

67. The method of claim 65 wherein the second waveform component is a sinusoidal waveform.

68. The method of claim 65 wherein the step of delivering the waveform includes adding the second waveform component after the predetermined delay which has a duration sufficiently short that the defibrillation waveform has a positive slope at all times up to truncation of the waveform.

69. The method of claim 65 wherein the step of delivering the waveform includes adding the second waveform component after the predetermined delay, which has a duration sufficiently long that the defibrillation waveform has a negative slope during the first waveform component and a positive slope upon the second waveform component being added to the first waveform component.

70. An improved external defibrillation system for producing a defibrillation shock pulse to be delivered through at least two electrodes adapted for placement across a chest of a human patient, the system comprising:

means for selectively discharging a first component of the shock pulse and a second component of the shock pulse;

the means for selectively discharging further comprising:

means for applying the first component of the shock pulse wherein a determined discharge duration is based on a patient-dependent parameter measured on the patient between the electrodes; and

means for applying a second component of the shock pulse after a predetermined time delay from application of the first component of the shock pulse so that the second component is added to the first component of the shock pulse;

means for terminating application of the first component and the second component of the shock pulse at the expiration of the determined discharge duration.

71. The defibrillation system of claim 70 wherein the means for applying the first component of the shock pulse further includes means for producing an exponential decay waveform.

72. The defibrillation system of claim 70 wherein the means for applying the second component of the shock pulse further includes means for producing a sinusoidal waveform.

73. The defibrillation system of claim 70 wherein the predetermined delay has a duration sufficiently short that the shock pulse has a positive slope at all times up to truncation of the shock pulse.

74. The method of claim 70 wherein the predetermined delay has a duration sufficiently long that the shock pulse has a negative slope during the first component of the shock pulse and a positive slope upon the second component of the shock pulse being added to the first component of the shock pulse.

75. A method of generating a biphasic stepped truncated damped sinusoidal waveform using a defibrillator having a pulse generation circuit and pulse control circuitry connected to the pulse generation circuit, the method comprising:

a) implementing a first phase of the biphasic waveform including the steps of:

i) monitoring a first patient dependent parameter;

ii) determining a first discharge duration of the first phase of the waveform based on the monitored first patient dependent parameter;

iii) activating the pulse generation circuit to produce an exponential decay waveform component of the first phase of the biphasic waveform;

iv) further activating the pulse generation circuit after a predetermined delay to produce a sinusoidal waveform component of the first phase of the biphasic waveform, the sinusoidal waveform component being added to the exponential waveform component;

v) deactivating the pulse generation circuit to truncate the first phase of the biphasic waveform at the expiration of the determined first discharge duration; and

b) implementing a second phase of the biphasic waveform including the steps of:

i) determining a second discharge duration based on the residual voltage on cardiac cells that were not depolarized during the first phase of the biphasic waveform;

ii) activating the pulse generation circuit to produce an exponential decay waveform component of the second phase of the biphasic waveform, the exponential decay waveform component of the second phase having a negative polarity;

iii) further activating the pulse generation circuit after a predetermined delay to produce a sinusoidal waveform component of the second phase of the biphasic waveform, the sinusoidal waveform component having a negative polarity and being added to the exponential decay waveform component of the second phase of the biphasic waveform; and

iv) deactivating the pulse generation circuit at the expiration of the determined second discharge duration to truncate the biphasic waveform including both the exponential decay and sinusoidal components of the second phase of the biphasic waveform.

76. A defibrillator for delivering a stepped truncated damped sinusoidal waveform comprising:

first and second electrode connectors;

first and second charge storage components;

third and fourth charge storage components;

a first inductive component connected to the first charge storage component and the first electrode connector;

a second inductive component connected to the third charge storage component and the first electrode connector;

a first truncating switch connected between the first charge storage component and the second electrode connector;

a second truncating switch connected between the first and second charge storage components; and

a third truncating switch connected between the third charge storage component and the second electrode connector;

a fourth truncating switch connected between the third and fourth charge storage components;

pulse control circuitry connected to the first, second, third, and fourth truncating switches for independently operating the first, second, third and fourth truncating switches.

77. The defibrillator of claim 76 and further including:

a first diode connected between the second electrical connector and a first node, the first node defining a junction of the second truncating switch and the first storage component; and

a second diode connected between the second electrical connector and a second node, the second node defining a junction of the fourth truncating switch and the third storage component.

78. The defibrillator as in claim 76 wherein the third and fourth charge storage components are capacitors, each having a capacitance value within the range of 25 .mu.F to 500 .mu.F.

79. A defibrillator for delivering a stepped truncated damped sinusoidal waveform comprising:

first and second electrode connectors;

first and second charge storage components comprising capacitors, each having a capacitance value within the range of 25 .mu.F to 500 .mu.F;

a first inductive component connected to the first charge storage component and the first electrode connector;

a first truncating switch connected between the first charge storage component and the second electrode connector;

a second truncating switch connected between the first and second charge storage components; and

pulse control circuitry connected to the first and second truncating switches for independently operating the first and second switches.

80. A defibrillator for delivering a stepped truncated damped sinusoidal waveform comprising:

first and second electrode connectors;

first and second charge storage components;

a first inductive component connected to the first charge storage component and the first electrode connector;

a first truncating switch connected between the first charge storage component and the second electrode connector;

a second truncating switch connected between the first and second charge storage components;

pulse control circuitry connected to the first and second truncating switches for independently operating the first and second switches;

a first diode connected between the first electrical connector and a first node, the first node defining a junction of the second truncating switch and the first storage component.

81. A defibrillator for delivering a stepped truncated damped sinusoidal waveform comprising:

first and second electrode connectors;

first and second charge storage components;

a first inductive component connected to the first charge storage component and the first electrode connector;

a first truncating switch connected between the first charge storage component and the second electrode connector;

a second truncating switch connected between the first and second charge storage components; and

means for closing the first truncating switch at a first predetermined point in time, for closing the second truncating switch after a predetermined time delay following the first predetermined point in time, and for opening the first and second truncating switches after a predetermined duration.