Patent Application
20080146894
Not Applicable.
1. Field of Invention
The present invention relates to medical devices, which provide a means to sense physiological signals from nerves and muscles in humans. Specifically, the current invention relates to sensing physiological signals in their pristine form while avoiding irrelevant noise sources.
2. Description of the Related Art
In an exemplary case of electrical sensing and amplifying of physiological signals, the amplifier has competing electromagnetic signal sources that may cause deterioration of signal quality performance. Established methods use common mode rejecting amplifier designs, which reference the leads of a signal pair to a reference and a real or virtual ground. When the signals have amplitudes in the range of a few tens of mV, the performance of such solutions is good, as the operating voltage range is many orders of magnitude greater than the supplied signal. On the other hand, for biological signals encountered in an implanted system, the traditional techniques with an external ground are not optimal as the relatively smaller magnitude of the biological signals can be easily overwhelmed by noise.
In a conventional data acquisition system, the input bandwidth must be limited to avoid aliasing. Aliasing is the result of not having sufficient data samples available to distinguish a component with frequency content F from one with n×2 F. However, aliasing would become an issue only if sufficient energy is contained in higher frequencies. According to Shannon/Nyquist theorem, the sample frequency must be at least twice the lowest frequency component contained in the signal at the lowest amplitude of the dynamic range of the system.
The frequency range for ECG signals has traditionally included the line frequencies, 50 Hz and 60 Hz. In a traditional system, with an input pair and a common ground in an office, home or industrial environment, there is likely considerable line frequency content in the input signal, at the input amplifier and or sampling location. One solution would include a notch filter for 50 Hz and 60 Hz, or one broad band enough to filter out the band from 45-65 Hz. The notch filtering will introduce a non-linear effect from at least 22.5 Hz to 130 Hz resulting in system sensitivity reduction. Even a high Q filter will not avoid this issue. The other common line frequency for aviation and marine equipment is 400 Hz. However, this is generally high enough not to affect ECG signals. If there is no meaningful information contained in the filtered out band, there will not be any adverse issues with the filtering approach. In practical applications, that is almost never the case. Since important information is contained in those frequency bands, there is a need for a technique that includes the entire band from 10 Hz to 200 Hz so pristine biological signals can be acquired.
A further problem afflicting present-day devices relate to the rejection of amplitude modulated or burst electromagnetic fields. One source of burst line frequency noise is faulty, or poorly designed, appliances where the patient is in the close proximity of or in contact with a line frequency AC powered device. The patient actually is part of a direct or induced electrical pathway to ground. In contrast, to sense detection in the presence of continuous additive line frequency interference, the operation of the sensing circuit during amplitude modulated or burst electromagnetic interference (EMI) is probably more important to patient safety. Burst line frequency noise is a potentially dangerous situation for pacemaker-dependent patients because burst noise may inhibit stimulus generation in a cardiac control device. The potential hazard of continuous line frequency noise, in comparison to burst noise, is less precarious because continuous line noise will cause the device to pace asynchronously with respect to a spontaneous cardiac rate, but the device will still support the patient.
Yet another problem with prior art techniques is the usage of digital or active analog filtering in the front end circuit that is directly connected to electrodes. This exposes the internal circuits to the full noise amplitude and has the risk of running out of “dynamic range.” For example, if the amplifier output hits the rails (ground or supply), it is no longer linear, or amplifying. For example, given a normal signal range of 1-10 mV, gain=200, noise burst 100 mV and supply rail 5V, the output amplitude of the true signal is 200 mV to 1.0V and the noise signal in the output is 20V, which is well beyond the supply rail. The amplifier may simply peg at the rail, or oscillate between ground and 5V, without linear relation to the input signal.
In view of the foregoing discussion, there is a need for a system that can amplify biological signals from muscles and/or nerves without concomitantly amplifying the noise.
In accordance with one aspect, an apparatus for sensing biological signals from an animal is provided. The apparatus can include at least one set of electrodes that is configured to be implanted in the animal and disposed at a first set of locations to sense biological signals from the first set of locations, a set of insulated conductors connected to the at least one set of electrodes, the set of insulated conductors formed in a configuration adapted to be substantially immune to electromagnetic interference, a network of filters connected to the set of insulated conductors, the network of filters configured to filter the sensed biological signals, an amplifier connected to the network of filters, the amplifier including an internal voltage reference and the amplifier configured to amplify the filtered biological signals to provide an amplified differential signal, an energy source powering at least the amplifier, the energy source configured to be substantially free of an externally grounded energy supply external to the animal, and a signal analysis module configured to receive the amplified differential signal and to analyze the amplified differential signal to determine at least one physiological parameter.
In accordance with another aspect of the invention, an implanted apparatus for sensing biological signals from an animal is provided. The apparatus can include at least one implanted electrode pair disposed at a first set of locations to sense biological signals. Each electrode pair can be connected to a pair of insulated conductors that are in turn connected to an instrumentation amplifier via a passive network of filters. The insulated conductors can be configured to avoid picking up of EMI noise. The amplifier can amplify the filtered biological signal from each of the electrode pairs to provide an amplified differential signal. The amplifier can have an internal voltage reference. Additionally, an energy source can power the apparatus without being connected to mains or an isolation transformer of medical equipment. A signal analysis module can analyze amplified differential signals to obtain at least one physiological parameter. The apparatus may also include a signal presentation module to display amplified signals and physiological parameters associated with the signal. The energy source may be a battery, an infrared source or a radio frequency source. The electrodes may be located inside a blood vessel, extravascular or transvascular and sense signals from various tissue locations.
Embodiments of the present invention can provide for the elimination of ground and associated noise sources by passive component and instrumentation amplifier design, elimination of DC and very low frequency noises by a high pass filter, elimination of common mode noise by a low pass filter over 500 Hz and a noise filter over 1 kHz. There can be no need for line frequency filtering with the elimination of traditional ground. Another aspect of the invention is the use passive filtering at the front end, before any active components are involved. Electronic circuitry can use light emitting diode (LED) or any other suitable diode as an internal reference with a constant 1.2 V or other suitable voltage. Additionally, the conductors connecting to the electrodes can be paired to avoid the formation of EMI pickup loops.
Although the present invention may be described in the context of cardiac pacing and of implanting a sensing circuit in a vein or artery of the heart, the present apparatus can be employed to sense signals from muscles and/or nerves in other areas of the human body. In addition to cardiac applications, the sensing apparatus can provide brain signal sensing, for treatment of Parkinson's disease or obsessive/compulsive disorder for example. The transvascular electrical sensing also may be applied to signals from muscles, the spine, the gastro/intestinal tract, the pancreas, and the sacral nerve. The apparatus may also be used for sensing in GERD treatment, endotracheal sensing, pelvic floor sensing, sensing for obstructive airway disorder and apnea, and molecular therapy delivery sensing.
An aspect of embodiments of the invention is the use of a plurality of electrode pairs disposed at a first set of location for the signal sensing. It should be understood that each electrode pair in close proximity or farther apart is included in the set. Further, if more electrode pairs are involved, the term set should encompass all such paired locations as well. In the subsequent description, signal sensing and amplification is described at only one electrode pair for the sake of convenience and it should be understood without loss in generality, that the present invention can be configured to sense from a plurality of locations.
A signal amplifier and associated electronics that do not require an external ground is described. The signal amplifier with an internal ground will only see common mode signals especially when the signal pair is either a coaxial or a twisted pair.
First, referring to the
If the main signal leads, providing Va and Vb are contained within a space or volume with noise sources external to that volume as would be the case in an implanted system, the reference or ground lead may be removed with a concomitant performance improvement of the system 50 shown in
Vout 72 is the voltage of the output signal, Va 74 and Vb 76 are the voltages sensed by electrode pair 52, 54, Vsignal 78 is the voltage of signal 80, and Gain is the voltage gain of the instrumentation amplifier 60. In this case, Vout=Gain (Va−Vb), where Va−Vb=Vsignal; and Vout=Gain (Vsignal). It should be noted that the twisted pair is a specific case of the more general helical configuration.
In the
There are other methods of mitigating electromagnetic interference. One of the methods include running wires in close proximity, for example, a mm or less, relative to the wavelength of the EMI field, e.g., 5000 km for a EMI field of 60 Hz, from which immunity is needed. Another method of mitigating the effects of EMI filed is a multifilar helical coil, where the insulated conductors run parallel, but at the same time form helical coils. This option offers additional combinations of tuning dimensions, such as conductor/helix diameter ratio, spacing, pitch, and spacing pattern. Note that the coil does not need to be evenly spaced.
Noise voltage 84 of noise 86 can still be injected within each individual conductor and present an unbalanced noise component to the amplifier 60 where it will be amplified and spoil the original signal. Depending on location and application, the contributions of unbalanced noise must be considered before choosing this method as described next.
There are a few considerations in a practical implementation of the previous circuit. First, there are DC considerations. Second, there is an internal reference consideration. Third, there are filtering considerations. In the following, each of these is described in detail.
At the interface between electrode and tissue, a galvanic system is formed with a DC potential. If there is complete symmetry in this circuit from electrode-1 to electrode-2, then the sum of all the contact potentials will cancel. However, if the materials used are dissimilar, the electrode/tissue and or the electrode/blood interface will yield potentially different galvanic generators that do not cancel. In this case, the input amplifier is presented with the source voltage of interest along with the galvanic voltage difference. This galvanic component is relatively static, but it could potentially be modulated with body or organ movement, as the electrode may wander between touching the vessel wall and the blood pool thereby presenting a varying “DC” voltage. The variance over time is expected to be synchronous with the movement, and thus in the sub 2 Hz range, if respiratory and cardiac movements are included. Another DC issue stems from the amplifier itself, which will require a DC current bias into or out of the input terminals. In MOSFET amplifiers, this “bias current” is very small, but doubles with every ten degree Celsius (10° C.)in temperature rise. Also, this current can have an offset, leaving a differential current that can spoil the balance of a high impedance circuit. This problem can be substantially alleviated by providing a form of AC coupling with the electrodes, and a DC current path for the bias currents.
The AC coupling performs two functions. The first function is DC decoupling from the galvanic voltages, Galv.1136 and Galv.2138, and the second function is to form a high pass filter with a corner frequency of FHP=½π RC, where R=Ra+Rb.
The bias and offset currents are in the order of 10−9 to 10−8 A, and with path resistances of e.g. 100 kOhm, still yield 0.1 to 1.0 mV. Since source voltages are in order of 0.5-10 mV, these bias and offset voltages are not negligible. Therefore, in this embodiment, the amplifier specification selection should be such that these currents are low enough to allow for reasonably high input resistance values in the order of 100 kOhm or better for Ra and Rb (
Careful selection of Ra and Rb will yield an acceptable low bias current offset voltage component (Voffset=Ioffset×Ra, where Ra=Rb), and a proper FHP (high pass filter frequency). The traditional corner frequency range for FHP is in the order of 0.5 Hz to 2.0 Hz, but other values can be selected depending on spectral regions of interest.
A natural feature that helps our proposed implementation is the relatively low impedance of the tissues involved, typically 300 to 120 Ohm between, for example, 5 mm spaced electrodes. Thus, in order to create a net 1 mV across such an impedance, energy density of 0.4 mW/m would be needed with the energy contained from 0-1 kHz.
In order to incorporate a floating AC coupled signal, such as the one shown in
Additional details for the internal reference are provided in
If there is no meaningful information contained in the filtered out band, there will not be any adverse issues with the filtering approach. In practical applications, that is, however, rarely the case. Since important information is contained in those frequency bands, an embodiment is tailored to include the entire band from 10 Hz to 250 Hz. For robustness reasons even a wider range of frequencies (e.g., 2 Hz-500 Hz) can be used. With this consideration, the fast rise time of the sinus node signals containing high frequency content in the 100-250 Hz range can be easily accommodated in their pristine form. Additionally, by including these frequency components, the natural physiological signals can be easily distinguished from background signals, such as noise, voluntary and involuntary muscle movement, etc.
Between the biological environment and the signal amplifier, a network of filters, which for example can comprise at least three filters, is provided to perform various functions. The first of these filters is a high pass filter 416 to substantially block DC and low frequencies up to a prespecified cut-off (e.g., 2 Hz). This high pass filter 416 comprises of passive elements with capacitance and resistance, where resistance may be obtained by a combination of resistors, and source impedance in series. A suitable low pass filter 418 (LPF1) is configured to suppress common mode noise. Low pass filter 418 comprises of passive elements 420, which can comprise capacitance C and resistance R, and their symmetrical counter parts 422 (LPF1′). A second low pass filter 424 (LPF2) is configured to reject high frequency noise signals. Low pass filter 424 filter cam comprise passive elements capacitor and resistors in series. EM broadband ambient noise from appliances and other equipment could swamp the input circuit and consume dynamic range. This needs to be filtered out. A low pass filter LPF2 with a cut-off at 1 kHz frequency can be selected since the EM noise is broad band, but its energy is rather low below 10 kHz and can be effectively filtered out.
System 400 further includes an amplifier 426 connected to the network of filters (i.e., filters 416, 418, and 424). Amplifier 426 is connected to a positive power supply 428, is connected to an internal ground 430, and receives an internal reference 432 provided by an internal reference module 434. Amplifier 426 has an output 436 having an output voltage 438.
For ECG signals obtained by direct connection to the cardiac venous vessel wall or muscle tissue, the signal path between the two or more input electrodes should exclude any electromagnetic pickup loop, for example, by twisting the lead and or wire pairs. Therefore, symmetrical layouts are favored.
Absence of a traditional ground is a significant departure from the prior art and it has obviated the need for notch filtering and other kinds of signal degrading processes. Another aspect of the invention as already mentioned is the use passive filtering at the front end, before any active components are involved. As a result, physiological signals devoid of the traditional noise are obtained.
Referring now to
Module 452 can be comprised of system 100 of
Signal analysis module 454 receives the amplified differential signal from module 452. Signal analysis module 454 is configured to analyze the amplified differential signal to determine at least one physiological parameter of the biological signal sensed by module 452. For example, in a cardiac application of the apparatus, the parameter obtained may be heart rate. In general, the parameter extracted from the analysis module may be used to provide a therapy (e.g., stimulation) to a patient. The signal analysis module may include a hysteresis based signal transition detector or a zero-crossing detector. Additional signal processing algorithms for detection and identification of biological signals may be used as part of the signal analysis module.
The recording module 456 can be connected to signal analysis module 454 to record the amplified differential signal and/or the at least one physiological parameter. The recording module 456 can be configured to communicate (e.g., wirelessly via remote telemetry) to present saved and/or live data to a further module (not shown).
Additionally, presentation module 458 can be configured to receive the amplified differential signal and/or the at least one physiological parameter. Presentation module can display amplified signals and physiological parameters associated with the sensed biological signal. The display module may be accessed remotely via telemetry at a readout station (e.g. doctor's office). Alternatively, the signal presentation module may provide a print out of a recording of the signal. In other alternatives, recorded signal may be stored in an electronic form for a later retrieval. The presentation module can be located outside the body and be configured to wirelessly communicate with signal analysis module 454 and/or module 452. Moreover, the presentation module 458 can be configured to wirelessly communicate with the recording module 456, which can be configured to transmit saved data to the presentation module 458.
An energy source for any of modules 452, 454, 456, and 458 can be a battery, an infrared source or a radio frequency source. The energy source is not connected to the mains or via an isolation transformer of medical equipment. This is to avoid connecting to any external grounding as mentioned earlier to avoid introduction of noise.
In summary, embodiments include leads that minimize EMI noise, passive filtering prior to signal amplification with relatively high frequency high pass filter with a cut-off frequency in the range of 20 to 70 Hz combined with a relatively high low pass filter with the cut-off frequency above 300 Hz, an amplifier with an internal voltage reference, and avoidance of connecting the energy source to an external ground.
In one embodiment, at least a pair of electrodes may be located inside a blood vessel and sense signals from various tissue locations. In another embodiment, such as in the case of most nerve stimulators, the electrode location may be extravascular. In another embodiment, the electrode pair is implanted under the skin for detecting biological rhythms. In another embodiment, the electrode pair may be part of sensing pressure in the heart. In general, the invention is applicable to signal amplification beyond nerve and cardiac applications where physical parameters are converted to electrical signals and could be affected by noise. Thus, the invention is applicable for sensing applications of all physiologic data including intravascular, extravascular and transvascular. The applicability of the invention further includes systems which are temporary and are both in the body and extend out of the body, such as temporary pacing leads.
The foregoing description was primarily directed to preferred embodiments of the invention. Although some attention was given to alternatives within the scope of the invention, it is anticipated that one skilled in the art will likely realize additional alternatives that are now apparent from disclosure of embodiments of the invention. Accordingly, the scope of the invention should be determined from the following claims and not limited by the above disclosure.
This application claims the benefit of U.S. Provisional Patent Application No. 60/870,649 filed on Dec. 19, 2006, the disclosure of which is hereby incorporated by reference as if set forth in its entirety herein.
| Number | Date | Country | |
|---|---|---|---|
| 60870649 | Dec 2006 | US |
