This application also contains subject matter that is disclosed in U.S. Pat. No. 6,994,675, issued Feb. 7, 2006 and entitled “Non-Invasive Measurement of Supra-Systolic Signals,” U.S. patent application Ser. No. 11/358,283 filed Feb. 21, 2006 and entitled “System And Method For Non-Invasive Cardiovascular Assessment From Supra-Systolic Signals Obtained From A Wideband External Pulse Transducer In A Blood Pressure Cuff,” Publication No. US 2006/0224070 A1, (“the '070 publication”), and U.S. patent application Ser. No. 11/495,279, filed Jul. 28, 2006. The subject matter of this patent and these two applications are incorporated herein by reference.
It has been known that the peripheral arterial pressure waveform carries information useful for evaluating a person's health and cardiovascular status and performance. The signals recorded from a blood pressure cuff are termed “supra-systolic” signals if the cuff pressure is above the subject's systolic blood pressure. In addition, signals can be recorded when the cuff pressure is below systolic pressure. In all cases, the signals result from pressure energy transmissions and are dependent upon the subject's physiology.
When the heart pumps, a pressure gradient is generated within the cardiovascular system. This results in pulse pressure waves traveling peripherally from the heart through the arteries. Like any wave, they reflect back off a surface or other change in impedance. Arterial pulse waves reflect back from both the peripheral circulation and from the distal aorta when it becomes less compliant. See Murgo, J. P. et al., “Aortic Input Impedence In Normal Man; Relationship to Pressure Waveforms,” Circulation 62, No. 1 (1980), and Latham, R. D. et al., “Regional Wave Travel and Reflections Along the Human Aorta,” Circulation 72, No. 6 (1985). These reflection waves are identifiable in arterial pressure tracings, but the exact timing and magnitude of the waves are difficult to discern. Nevertheless, they have been the basis of several commercial systems to assess reflectance waves. These systems measure arterial contours using applanation tonometry from the radial artery.
If a low frequency sensor is placed over the brachial artery beneath a blood pressure cuff and the cuff is inflated above systole, supra-systolic signals can be recorded. See Blank, S. G. et al., “Wideband External Pulse Recording During Cuff Deflation,” Circulation 77, No. 6 (1988); Hirai, T. et al., “Stiffness of Systemic Arteries in Patients with Myocardial Infarction,” Circulation 80: 78-86 (1989); and Denby, L. et al., “Analysis of the Wideband External Pulse,” Statistics in Medicine, John Wiley & Sons (1994). An idealized supra-systolic signal for one heart beat is shown in FIG. 1 of the '070 published application. These signals contain frequency components of less than 20 Hertz, which are non-audible. Supra-systolic low frequency signals provide clear definition of three distinct waves: an incident wave corresponding to the pulse wave and two subsequent waves. Blank (Blank, et al., Association of the Auscultatory Gap, Ann. Intern. Med. 124 (10): 877-883 (May 1996)) proposed that the second wave emanated from the periphery and the relative amplitude of this wave to the incident wave (K1R) was a measure of peripheral vascular resistance (PVR). He proposed a constant such that PVR could be measured from the ratio of the incident to the first reflectance wave. See also, U.S. Pat. No. 5,913,826, which is incorporated herein by reference in its entirety.
The second supra-systolic wave is, in fact, a reflectance wave from the distal abdominal aorta—most likely originating from the bifurcation of the aorta and not from the peripheral circulation as proposed by Blank. This has been verified in human experiments (Murgo, Westerhof et al. 1980; Latham, Westerhof et al. 1985) and in studies using pulse wave velocity (PWV) measurements. The relative amplitude of the first reflectance wave is now believed to be a measure of the stiffness, compliance, or elasticity of the abdominal aorta rather than peripheral resistance.
The third wave occurs at the beginning of diastole and is believed to be a reflection wave from the peripheral circulation. As such, it is a measure of peripheral vasoconstriction with superimposed secondary reflections. Supra-systolic signals can be utilized to measure compliance by relating the amplitude of the first wave (incident or SS1) to the amplitude of the second (aortic reflection or SS2) wave. The degree of vasoconstriction can be assessed by measuring the amplitude of the diastolic or third wave (SS3 wave) and relating it to the SS1 wave. Amplitudes, areas under the curves, or other values calculated from the waves can be utilized. Data has been analyzed by measuring amplitudes, ratios of amplitudes and time delays between waves.
Many measurement techniques and methods of interpretation have been proposed in the literature. The most well known is the use of the oscillatory part of a cuff pressure signal for the estimation of peripheral blood pressure.
More recently, there has been interest in attempts to use other sources of pulse waves for the evaluation of the cardiovascular status of a subject. The most direct measurement technique involves the applanation tonometry of an artery, for example, the radial. Plethysmographic sensors have also been employed.
In the case of applanation tonometry, the transduction of a good-quality signal requires a high degree of training and skill. Attempts have been made to automate this procedure, however, the cost of doing so is high, and performance still leaves much to be desired in practice.
Plethysmographic sensing (optical, electrical or mechanical) has also been employed. However, these methods do not measure pressure changes directly, and so require significant, error-prone mathematical transformation to estimate the arterial pressure wave. In addition, the measurement site is often at the finger, whereas pressures of interest are generally more proximal, e.g., in the brachial or radial arteries. (Medically speaking, the brachial pressures have the most clinical literature associated with them.)
The use of a wide-band external pulse sensor to record mechanical vibrations has also been proposed. In U.S. Pat. No. 5,913,826, Blank describes the use of such a system to first estimate the arterial pressure waveform, and secondly calculate indices related to cardiovascular status. Sharrock proposes a variation in U.S. Pat. No. 6,994,675 wherein the information is gained from the wide-band external pulse sensor with an occluding cuff inflated to supra-systolic pressures.
As an alternative to the aforementioned methods, it has been proposed that the readily available, oscillometric signal from a pressure cuff be used to infer information about a person's health.
There are two main problems with using a cuff pulse waveform as a basis for deriving information about a person's health. There is generally found instability and irreproducibility in the measurement of a cuff-pulse wave. In particular, the cuff pulse wave changes shape with cuff pressure. Also, the measured cuff pulse wave does not correspond to the intra-arterial pulse wave shape.
The present invention provides a solution to the above problems by providing a waveform that is both stable and repeatable. It is thus useful as the basis for calculating further indices related to the cardiovascular status of the subject. With regard to the second problem, the measurement process is such that there is no need to scale or correct the acquired waveform in order to extract a signal that resembles the intra-arterial pulse wave shape.
In addition to the above features, the invention may be implemented easily without adding significant cost or complexity above the current oscillometric blood pressure methods, while retaining all the benefits of oscillometric measurement methods over applanation tonometry, plethysmography and wide-band external pulse sensors.
It is therefore an object of the invention to provide non-invasive cardiovascular assessment.
It is also an object of the invention to provide non-invasive assessment of aortic compliance by analysis of supra-systolic signals.
It is a further object of the invention to provide a system for assessing cardiovascular status non-invasively, said system comprising an external peripheral pressure cuff, an oscillometric pressure sensor, an analog signal processing circuit including an amplifier, an A/D converter and a digital signal processor.
It is a yet further object of the invention to provide a method for determining a compliance value comprising the steps of measuring a pressure waveform of a peripheral artery with blood flow occluded, measuring the difference in amplitude between a first supra-systolic peak and first supra-systolic trough, measuring the difference in amplitude between a second supra-systolic peak and a second supra-systolic trough, and determining the ratio of the two differences.
It is still another object according to the present invention to provide a non-invasive cardiac monitoring system comprising a blood pressure cuff, a pressure control system for said cuff, a cuff oscillation sensor for measuring pulse waves from the cuff at supra-systolic pressure, and a system for analyzing an output from the cuff oscillation sensor to produce data indicative of cardiac status.
These and other objects of the invention will become more apparent in the description below.
The invention concerns the measurement, processing and utility of cuff oscillation signals recorded from a pressure cuff applied to a body part when the cuff pressure is at supra-systolic pressure.
The present invention therefore provides a system for measuring peripheral arterial signals, e.g. of the brachial artery, using an external peripheral pressure cuff, an oscillometric pressure sensor which transduces the waveform pulse signal to a linear analog signal, an analog signal processing circuit which differentially amplifies the analog signal and further amplifies a comparative signal, an A/D converter and a digital signal processor, to produce a waveform with respect to an inferred original aortic waveform.
The systolic pressure of the patient is measured. A cuff is inflated to a determined supra-systolic pressure, such as 15-150 mm Hg above a systolic pressure, preferably about 30 mm Hg above the systolic pressure, measuring with a oscillometric pressure sensor/transducer having sufficient bandwidth to capture detailed waveform information, for example from 0.1 to 1000 Hz, and analyzing the waveform to infer an aortic pressure waveform.
This inferred waveform may then be used for a number of purposes, including analyzing cardiac function, analyzing the central and/or peripheral arterial system, or for analyzing the cardiovascular system as a whole.
Thus, the present invention provides means for extracting useful parameters of central and peripheral cardiovascular system performance, without requiring a direct measurement of waveforms from the heart or aorta.
A reliable system may therefore be provided to acquire supra-systolic signals from patients, a method to analyze the signals, and clinical applications for the signals. The system consists of an oscillometric pressure sensor pneumatically connected to a blood pressure cuff or similar device, placed around a patient's arm. The signals are conditioned and differentially amplified, passed through an analog to digital converter and transferred to a computer or processor for analysis. Analyzed signals will be stored, presented on a screen numerically or graphically. Data can be stored or transmitted to databases or other health care facilities.
A variety of oscillometric pressure sensors can be used. The pressure sensors must be able to sense dynamic pressure signals as low 0 to about 50 kPA (0 to 7.25 psi), have a full scale span of about 40 mV, and be sturdy enough to withstand repeated use under external pressures of about 300 mm Hg. For example, a suitable commercially available piezoresistive pressure sensor is available from Freescale Semiconductor, Inc., as model MPXV2053G, Case 1369-01.
For a full understanding of the present invention, reference should now be made to the following detailed description of the preferred embodiments of the invention as illustrated in the accompanying drawings.
The preferred embodiments of the present invention will now be described with reference to
This invention concerns the measurement, processing and utility of blood pressure cuff oscillation signals recorded from a blood-pressure cuff applied to a body part, when the cuff pressure is above the systolic pressure, i.e., at supra-systolic pressures. The amplitude of such signals is typically much less than the total amplitude of cuff oscillation signals. However, these signals carry information directly related to the intra-arterial pressure waveform. Further, at supra-systolic pressures the artery is always in an occluded state, rendering the system being measured relatively constant. The cuff pulse pressure wave is therefore consistent and reproducible, and comparable between subjects.
With reference to the drawings and in particular
Once cuff 16 is inflated to the desired supra-systolic pressure, pneumatic signals relating to cuff pulse waves are measured through the pneumatic tube by cuff oscillation sensor 18. Cuff oscillation sensor 18 senses the pneumatic pressure waves generated from cuff 16. Commercially available pressure sensors can be used for this purpose. For example, silicon pressure sensors from Freescale Semiconductor, Inc., such as silicon pressure sensors of series MPX2053/MPXV2053G are preferred. These silicon piezoresistive pressure sensors provide a highly accurate and linear output directly related to the pressure applied. The preferred procedure for measuring the cuff pulse waves is further described below.
Thus, cuff oscillation sensor 18 transduces the pneumatic cuff pulse wave signals to an analog electrical signal corresponding to the pneumatic cuff pulse waves, which are then sent to analog signal processing circuit 20, which processes and amplifies the signal, as further described below. The processed analog signal is then converted to a digital signal by A/D converter 22, which digital signal is, in turn, processed by digital signal processor 24. The result is a digital supra-systolic pressure signal that can be recorded on a chart in a manner known to those of skill in the art, and utilized to determine the cardiovascular status of a patient as described, inter alia, in the '070 publication.
Once the supra-systolic pressure is determined, oscillometric NIBP measurement module 26 operates to achieve the supra-systolic pressure in cuff 16 for at least one heartbeat. Accordingly, NIBP measurement module 26 comprises pressure generator 14 and a controller for pressure generator 12. Pressure generator 14 can be a pump for providing air pressure to cuff 16, thus generating pneumatic pressure in cuff 16 using the pump and valves of NIBP measurement module 26. When the supra-systolic pressure in cuff 16 is generated and maintained, pulse wave oscillations within the pressure cuff 16 are measured and transduced by pressure sensor 28. For this purpose, the Freescale MPX 2053 Case 1369-01 silicon piezoresistive pressure sensor is preferred. Other types of pressure sensors may also be used. For example, strain sensors may be attached to the outer wall of the cuff to transduce oscillations in the stretch of the cuff in response to intera-arterial pressure changes.
After the pressure sensor 28 transduces the pulse wave oscillation signals to analog electrical signals, the analog signals are processed and amplified to extract the supra-systolic oscillations of interest. As further explained below, this manipulation of the analog signal is conducted in latched amplifier 30 and the analog signal produced is then sent to A/D converter 22 where it is converted into a digital signal for further processing by digital signal processor 24.
In order to achieve acceptable resolution from the digitization, the resolution of A/D converter 22 needs to be increased or analog amplification needs to be used to increase the voltage range seen by A/D converter 22. It is infeasible to amplify the supra-systolic analog signal relative to a 0 V reference, as the signal will quickly saturate at the upper limit of A/D converter 22. For example, in the recordings shown above, the supra-systolic analog signal is at a voltage level of approximately 2.5 V. If amplified directly by a factor of two, the supra-systolic analog signal would be twice as large (approximately 0.02 V amplitude) but sitting at a level of 5 V, which is the limit of the A/D converter 22.
It is therefore necessary to amplify the supra-systolic signal relative to a reference that is near the supra-systolic cuff pressure. In the above example, if we were to amplify the supra-systolic signal relative to a reference voltage of 2.5 V with a gain of 40, the maximum signal level would be 4.58 V (that is, (2.552−2.5)×40+2.5) and the minimum signal level 3.94 V (that is, (2.536−2.5)×40+2.5). This obviously allows much larger amplification without saturation.
Thus it is necessary to first determine the reference voltage level. The actual, “average” supra-systolic signal level is not fixed but dependent on the systolic blood pressure of the patient. The purpose of SCON circuit 56 is to determine an appropriate reference voltage and then apply amplification.
In operation, SCON circuit 56 operates in conjunction with a pressure profile for a cuff inflation cycle, an example of which is illustrated in
The reference pressure (voltage) provided by the SCON circuit 56 (which is called the “latched voltage”) is given as the dotted line. The actual pressure in cuff 16 oscillates with each heart beat around the solid line.
While the cuff pressure remains below a predetermined reset level, the output latched voltage is zero. When the cuff pressure increases above the reset level, the output latched voltage becomes the cuff pressure voltage. As the cuff 16 deflates, the latched voltage remains at the maximum cuff pressure voltage. When the cuff pressure voltage falls below the reset level, the latched voltage then also returns to zero. In this way, the latched voltage provides a reference level corresponding to a maximum pressure. In the preferred embodiment, as the supra-systolic pressure is only ever attained by increasing the pressure in cuff 16, the latched voltage provides a suitable reference to use for differentially amplifying the supra-systolic signal.
Accordingly, the signal produced by pressure sensor 28 is input into differential amplifier 62 within SCON circuit 56, where it is amplified by a factor of 274. Referring to
As described, the above embodiment achieves an equivalent analog to digital conversion resolution of 17.4 bit, using a physical 12-bit converter. This effective resolution is required to extract the supra-systolic information from the pressure voltage signal, where the voltage span of about 5 V corresponds to the entire operating range of the pressure sensor (in this case, from 0 to approximately 300 mmHg).
However, the same effective A/D converter resolution may be achieved using a high-resolution A/D converter, as shown in
It can be seen that the waveforms closely resemble an intra-arterial wave and contains information about incident and reflected pressure waves, the peaks and troughs of which are shown using circled points. See
The preceding preferred embodiments are illustrative of the practice of the invention. It is to be understood, however, that other expedients known to those of skill in the art, or disclosed herein, may be employed without departing from the spirit of the invention or the scope of the claims.
This application claims priority from U.S. provisional application No. 60/936,495, filed Jun. 20, 2007.
Number | Date | Country | |
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60936495 | Jun 2007 | US |