Apparatus and method for generation of varying waveforms in arrhythmia control system

Claims

What is claimed is:

1. An implantable device for providing therapy to a patient's inadequately functioning heart, comprising a defibrillation electrode lead system, means for detecting fibrillation of the heart, circuit means including charge storing means for applying electrical therapy to the heart via said electrode lead system, means responsive to the detection of a fibrillation condition for charging said charge storing means to an appropriate energy level for delivering defibrillation therapy to the patient's heart, and means coupled to said circuit means for selectively providing to said defibrillation electrode lead system pulses having at least one of a plurality of different defibrillation waveforms.

2. An implantable device for providing therapy to a patient's inadequately functioning heart, comprising a cardioversion electrode lead system, means for detecting arrhythmias of the heart, circuit means including charge storing means for applying cardioversion therapy to the heart via said electrode lead system, means responsive to the detection of an arrhythmia condition for charging said charge storing means to an appropriate energy level for delivering cardioversion therapy to the patient's heart, and means coupled to said circuit means for selectively providing to said cardioversion electrode lead system pulses having at least one of a plurality of different cardioversion waveforms.

3. An implantable device for providing therapy to a patient's inadequately functioning heart, comprising a pacing electrode lead system, means for detecting a bradycardia condition of the heart, circuit means for applying bradycardia pacing therapy to the heart via said electrode lead system, means responsive to a detected bradycardia condition for supplying power to said circuit means at an appropriate energy level for delivering said bradycardia pacing therapy to the patient's heart, and means coupled to said circuit means for selectively providing to said pacing electrode lead system pulses having at least one of a plurality of different pacing pulse waveforms.

4. An implantable device for providing therapy to an inadequately functioning heart, comprising a pacing electrode lead system, means for detecting a tachycardia condition of the heart, circuit means for applying antitachycardia pacing therapy to the heart via said electrode lead system, means responsive to a detected tachycardia condition for supplying power to said circuit means at an appropriate energy level for delivering said antitachycardia pacing therapy to the patient's heart, and means coupled to said circuit means for selectively providing to said pacing electrode lead system pulses having one or more of a plurality of different pacing pulse waveforms.

5. An implantable device according to any one of claims 1-4, wherein said means for providing a plurality of pulses having any one of a plurality of different waveforms comprises means for generating a selectable train of spaced pulses of electrical energy, and means in series with said train of spaced pulses for smoothing said train of spaced pulses into a discrete single pulse having a continuous waveform.

6. An implantable device according to claim 5, wherein said means for generating a selectable train of spaced pulses of electrical energy includes a plurality of switches in said circuit means.

7. An implantable device according to claim 6, wherein said smoothing means comprise a filter.

8. An implantable device according to claim 7, wherein said continuous waveform is unipolar.

9. An implantable device according to claim 7, wherein said continuous waveform is multipolar.

10. An implantable device according to claim 7, wherein said continuous waveform is monophasic.

11. An implantable device according to claim 7, wherein said continuous waveform is multiphasic.

12. An implantable device for providing therapy to an inadequately functioning heart, comprising a defibrillation electrode lead system, means for detecting fibrillation of the heart, circuit means including charge storage means for applying electrical therapy to the heart via said electrode lead system, means for charging said charge storing means to an appropriate energy level for delivering said defibrillation therapy to the patient's heart, and arrhythmia induction means including means for providing in succession to said defibrillation electrode lead system a plurality of pulses having any one or more of a plurality of different micro-shock waveforms.

13. An implantable device for providing therapy to an inadequately functioning heart, comprising a cardioversion lead system, means for detecting arrhythmias of the heart, circuit means including charge storing means for applying cardioversion therapy to the heart via said electrode lead system, means responsive to the detection of an arrhythmia condition for charging said charge storing means to an appropriate energy level for delivering cardioversion therapy to the patient's heart, and arrhythmia induction means including means for providing in succession to said cardioversion electrode lead system a plurality of pulses having any or more of a plurality different micro-shock waveforms.

14. An implantable device for providing therapy to an inadequately functioning heart, comprising a pacing electrode lead system, means for detecting arrhythmias of the heart, circuit means including charge storing means for applying pacing therapy to the heart via said electrode lead system, means responsive to the detection of an arrhythmia condition for charging said charge storing means to an appropriate energy level for delivering pacing therapy to the patient's heart, and arrhythmia induction means including means for providing in succession to said electrode lead system a plurality of pulses having any one or more of a plurality of different pacing waveforms.

15. An implantable device according to any one of claims 12-14, wherein said means for providing a plurality of pulses having any of a plurality of different waveforms comprises means for generating selectable trains of spaced pulses of electrical energy, and means in series with said trains of spaced pulses for smoothing each of said trains of spaced pulses into a discrete pulse having a continuous waveform.

16. An implantable device according to claim 15, wherein said means for generating selectable trains of spaced pulses of electrical energy includes a plurality of switches in said circuit means.

17. An implantable device according to claim 16, wherein said smoothing means comprises a filter.

18. An implantable device according to claim 17, wherein said continuous waveform is unipolar.

19. An implantable device according to claim 17, wherein said continuous waveform is multipolar.

20. An implantable device according to claim 17, wherein said continuous waveform is monophasic.

21. An implantable device according to claim 17, wherein said continuous waveform is multiphasic.

22. An implantable device according to any one of claims 1-14, wherein at least a portion of the following parameters can be manually programmed into the device: waveform; amplitude of each phase of a wave; width of a waveform; waveform polarity; polarity of each phase of a waveform; phase width for each phase in a given waveform; number of waveform phases; number of waveforms in a series; selection of electrodes for the delivery of waveforms, including pacing electrodes and cardioversion/defibrillation electrodes; selection of electrodes for the delivery of phases of waveforms, including pacing electrodes and cardioversion/defibrillation electrodes; selection of electrodes for the delivery of successive waveforms, including pacing electrodes and cardioversion/defibrillation electrodes; measurement of the transcardiac impedance for the determination of the waveform; timing between a trigger from an electrical or haemodynamic sensor and the delivery of a waveform or sequence of waveforms; and timing between successive waveforms in a sequence.

23. An implantable device according to claim 22, wherein at least a portion of the programming of said parameters into the device is performed automatically by the device.

24. An implantable medical device according to any one of claims 1-14, wherein said waveforms are delivered to any one or more chambers of the heart.

25. An implantable device for providing antiarrhythmia therapy to a patient's inadequately functioning heart, comprising: an electrode lead system adapted to be connected to the patient's heart; circuit means including a capacitor for delivering to the electrode lead system antiarrhythmia therapy in the form of electrical energy; means for detecting an arrhythmia condition of the heart; means responsive to the detection of said arrhythmia condition for charging said capacitor to an appropriate energy level for the arrhythmia condition detected; means responsive to the detection of said arrhythmia condition and operative to couple said circuit means to said electrode lead system for discharging a train of spaced pulses of electrical energy from said charged capacitor to said electrode lead system; and means in series with said train of spaced pulses for smoothing said train of spaced pulses into a discrete single pulse having a continuous waveform.

26. An implantable device according to claim 25, wherein said arrhythmia condition is fibrillation, wherein said electrode lead system is a defibrillation electrode lead system, and wherein said capacitor charging means charges said capacitor to an appropriate level for use in defibrillating said fibrillation.

27. An implantable device according to claim 25, wherein said arrhythmia condition is tachycardia, wherein said electrode lead system is a defibrillation electrode lead system, and wherein said capacitor charging means charges said capacitor to an appropriate level for use in cardioverting said tachycardia.

28. An implantable device according to claim 25, wherein said arrhythmia condition is tachycardia, wherein said electrode lead system is a pacing lead system, and wherein said capacitor charging means charges said capacitor to an appropriate level for use in reverting said tachycardia.

29. An implantable device according to claim 25, wherein said arrhythmia condition is bradycardia, wherein said electrode lead system is a pacing lead system, and wherein said capacitor charging means charges said capacitor to an appropriate level for use in bradycardia support pacing.

30. An implantable device for providing antiarrhythmia therapy to a patient's inadequately functioning heart, comprising: an electrode lead system adapted to be connected to the patient's heart; circuit means for delivering to the electrode lead system antiarrhythmia therapy in the form of electrical energy; means for detecting an arrhythmia condition of the heart; means responsive to the detection of said arrhythmia condition for supplying power to said circuit means at an appropriate energy level for the arrhythmia condition detected; means responsive to the detection of said arrhythmia condition and operative to couple said circuit means to said electrode lead system for discharging a train of spaced pulses of electrical energy from said circuit means to said electrode lead system; and means in series with said train of spaced pulses for smoothing said train of spaced pulses into a discrete single pulse having a continuous waveform.

31. An implantable device according to claim 30, wherein said arrhythmia condition is fibrillation, wherein said electrode lead system is a defibrillation electrode lead system, and wherein said capacitor charging means charges said capacitor to an appropriate level for use in defibrillating said fibrillation.

32. An implantable device according to claim 30, wherein said arrhythmia condition is tachycardia, wherein said electrode lead system is a defibrillation electrode lead system, and wherein said capacitor charging means charges said capacitor to an appropriate level for use in cardioverting said tachycardia.

33. An implantable device according to claim 30, wherein said arrhythmia condition is tachycardia, wherein said electrode lead system is a pacing lead system, and wherein said capacitor charging means charges said capacitor to an appropriate level for use in reverting said tachycardia.

34. An implantable device according to claim 30, wherein said arrhythmia condition is bradycardia, wherein said electrode lead system is a pacing lead system, and wherein said capacitor charging means charges said capacitor to an appropriate level for use in bradycardia support pacing.

35. An implantable device for providing antiarrhythmia therapy to, and inducing arrhythmia in, a patient's inadequately functioning heart, comprising: an electrode lead system adapted to be connected to the patient's heart; circuit means for providing antiarrhythmia therapy to said electrode lead system, said circuit means including means for storing an electrical charge; means coupled to said charge storing means for charging said charge storing means to an appropriate first energy level for use in reverting arrhythmia, and for charging said charge storing means to an appropriate second energy level for selectively inducing arrhythmia in the patient's heart; mean operative to couple said circuit means to said electrode lead system for discharging from said electrical storing means to said electrode lead system a train of spaced pulses of electrical energy at said first energy level for reverting arrhythmia in the patient's heart; means operative to couple said circuit means to said electrode lead system for discharging from said electrical storing means to said electrode lead system a plurality of trains of spaced pulses of electrical energy at said second energy level for inducing arrhythmia in the patient's heart; and means in series with said trains of spaced pulses for smoothing each of said trains of spaced pulses into a discrete pulse having a continuous waveform.

36. An implantable device according to claim 35, wherein said antiarrhythmia therapy comprises defibrillation therapy, wherein said electrode lead system is a defibrillation electrode lead system; wherein said first energy level is an appropriate level for use in defibrillating the patient's heart, and wherein said second energy level is an appropriate level for the smoothing means to form the plurality of trains of spaced pulses discharging from said arrhythmia inducing means into a plurality of successive micro-shocks.

37. A device according to any one of claims 25-36, further including means for varying the spacing between pulses in said train of spaced pulses to thereby vary the continuous waveform of said discrete single pulse.

38. A device according to any one of claims 25-36, further including means for varying the durations of the pulses in said train of spaced pulses to thereby vary the continuous waveform of said discrete single pulse.

39. A device according to anyone of claims 25-36, further including means for varying the polarities of the pulses in said train of spaced pulses to thereby vary the continuous waveform of said discrete single pulse.

40. A device according to anyone of claims 25-36, further including means for varying the spacing between, and the polarities and durations of, the pulses in said train of spaced pulses to thereby vary the continuous waveform of said discrete single pulse.

41. A method for providing antiarrhythmia therapy to a patient's inadequately functioning heart, comprising: providing an implantable device including an arrhythmia detection means, an electrode lead system for delivering antiarrhythmia therapy to the heart, and a circuit having an electrical charge storing means therein for providing antiarrhythmia therapy to the electrode lead system; charging said charge storing means to an appropriate level for providing said antiarrhythmia therapy in response to a detected arrhythmia condition; discharging a train of spaced pulses of electrical energy from the charged electrical charge storing means to the electrode lead system; and smoothing said train of spaced pulses into a discrete single pulse having a continuous waveform prior to its reaching the heart.

42. A method according to claim 41, further including the step of varying the spacing between pulses in said train of spaced pulses to thereby vary the continuous waveform of said discrete single pulse.

43. A method according to claim 41, further including the step of varying the durations of the pulses in said train of spaced pulses to thereby vary the continuous waveform of said discrete single pulse.

44. A method according to claim 41, further including the step of varying the polarities of the pulses in said train of spaced pulses to thereby vary the continuous waveform of said discrete single pulse.

45. A method according to claim 41, further including varying the spacing between, and the polarities and durations of, the pulses in said train of spaced pulses to thereby vary the continuous waveform of said discrete single pulse.

46. A method for inducing fibrillation in and providing defibrillation therapy to a patient's inadequately functioning heart, comprising the steps of: providing an implantable device including a fibrillation detection means, a defibrillation electrode lead system for delivering defibrillation therapy to the patient's heart and a defibrillation circuit having an electrical charge storing means therein for providing defibrillation therapy to the defibrillation electrode lead system; charging said charge storing means to an appropriately high energy level for use in defibrillation therapy in response to a detected tachycardia condition, and to an appropriately low energy level for inducing arrhythmia in the patient's heart at selected other times; discharging from said charge storing means to said defibrillation electrode lead system a train of spaced pulses of electrical energy at said low energy level for inducing arrhythmia in the heart; discharging from said charge storing means to said defibrillation electrode lead system a train of spaced pulses of electrical energy at said high energy level for defibrillating the heart; and smoothing each of said trains of spaced pulses into a discrete single pulse having a continuous waveform prior to its reaching the heart.

47. A method according to claim 46, further including the step of varying the spacing between pulses in said train of spaced pulses to thereby vary the continuous waveform of said discrete single pulse.

48. A method according to claim 46, further including the step of varying the durations of the pulses in said train of spaced pulses to thereby vary the continuous waveform of said discrete single pulse.

49. A method according to claim 46, further including the step of varying the polarities of the pulses in said train of spaced pulses to thereby vary the continuous waveform of said discrete single pulse.

50. A method according to claim 46, further including varying the spacing between, and the polarities and durations of, the pulses in said train of spaced pulses to thereby vary the continuous waveform of said discrete single pulse.

TECHNICAL FIELD

This invention relates to implantable medical devices which monitor the cardiac state of a patient by sensing the patient's intrinsic cardiac rhythm, particularly for the presence of tachyarrhythmias, and which deliver therapy in the form of electrical energy to cardiac tissue in an attempt to revert such tachyarrhythmias and restore the heart to a normal sinus rhythm.

In particular it relates to an apparatus and method for the generation of varying waveforms in an implantable medical device which is capable of both delivering defibrillation therapy to a patient's inadequately functioning heart and performing the induction of fibrillation and other arrhythmias therein. Preferably, the implantable defibrillator also has the capability of delivering bradycardia and antitachycardia pacing therapies when necessary.

PRIOR ART

U.S. Pat. No. 3,857,398 to Rubin describes a combined pacemaker/defibrillator. This device either performs a bradycardia pacing or a defibrillation function depending on the detection of a ventricular tachycardia or a ventricular fibrillation (VT/VF). If a VT/VF is detected, the device is switched to the defibrillating mode. After a period of time to charge the capacitor, a defibrillation shock is delivered to the patient. This device has no provision for antitachycardia pacing and is unable to provide fibrillation/arrhythmia induction either through the pacing circuitry or the defibrillation circuitry. Furthermore, the device disclosed contains no provision for varying the shape of the defibrillation or pacing waveform.

A multiprogrammable, telemetric, implantable defibrillator is disclosed in copending patent application Ser. No. 576,178 to N. L. Gilli et al., entitled "Reconfirmation Prior to Shock for Implantable Defibrillation," filed Aug. 29, 1990. The Gilli et al. device contains a bradycardia support system as well as a high energy shock system to revert ventricular tachycardias to normal sinus rhythm. 0n reconfirmation of the presence of a tachycardia, a shock is delivered to the patient either at a predetermined time or when the desired energy level is reached. This implantable pacemaker/defibrillator does not include an antitachycardia pacing facility. It cannot be used to induce either ventricular tachycardias or ventricular fibrillations by delivering a rapid succession of either pacing pulses through the pacing circuitry or micro-shocks via the defibrillation circuitry. Furthermore, there is no provision in the device for generation of waveforms from the defibrillation circuitry, other than truncated exponential waveforms.

A further development in the field of combined implantable devices is described in U.S. Pat. No. 4,940,054 to R. Grevis et al., entitled "Apparatus and Method for Controlling Multiple Sensitivities in Arrhythmia Control Systems Including Post Therapy Pacing Delay". This device is a microcomputer based arrhythmia control system which is programmable by means of a telemetric link. The device provides single chamber bradycardia support pacing, antitachycardia pacing, and cardioversion or defibrillation shocks for restoring normal sinus rhythm to a patient. This implantable pacemaker/defibrillator device incorporates a facility to induce ventricular fibrillation and ventricular tachycardia. This is for the purpose of testing and evaluating the effectiveness of the programmed therapy. This induction is achieved by rapid pacing pulses via the pacing circuitry. Although capable of delivering multiple defibrillation shocks in succession either manually or automatically, the device is not able to provide more than one type of waveform by means of the defibrillation circuitry, which takes the form of truncated exponential capacitor discharges. Similarly, the pacing circuitry is limited to one type of waveform.

U.S. Pat. No. 4,821,723 describes a variation of the defibrillation waveform, involving a reversal of the phase of the defibrillatory shock during the delivery of the shock to the heart. Though this waveform can be multi-phasic, each phase is still in the form of a truncated exponential.

Some existing defibrillators provide a relative variation of the defibrillation waveform when delivering the defibrillatory shock to multiple electrodes. In this case the shape of each of the resultant waveforms produced by means of a particular electrode configuration, although different relatively from the waveform produced at each of the other electrode configurations, is, however, still based on the truncated exponential capacitor discharge waveform.

The waveshape of the pacing pulse in existing devices is based on capacitor-discharged, truncated, exponential waveform technology, although the voltage droop that occurs tends to be less than that which occurs in the case of a defibrillation pulse, and more closely approaches a square wave.

Thus, existing pacemaker devices, including pacemaker/defibrillators when delivering pacing pulses, are not able to deliver pacing pulses which have selectable or variable waveforms.

A variant on the pacing pulse may be attainable by means of a slow ramp-up in voltage prior to the delivery of the pulse, such that the net delivered charge is zero. Charge balancing, such as this, is a means of minimizing polarization of the electrode-tissue interface. Polarization occurs when there is a charge difference across a boundary of differing electrical impedances. In order to stimulate cells across a polarized boundary, the polarization potential must first be overcome in order to deliver the required stimulus voltage to the cells.

This raises the required voltage for stimulating cells and, as such, increases the size of implantable pacemakers and defibrillators and decreases the longevity of the batteries within such devices. Maintaining a polarization potential across such a boundary also increases the risk of damage to the underlying cells.

An important consideration in increasing patient safety, increasing implantable pacemaker and defibrillator longevity and decreasing the size of the implant, is the ability to decrease the amount of polarization occurring not only at the electrode-tissue interface, but also at the interface of each cell membrane in the current path between the electrodes. In an article by O. Z. Roy and R. W. Wehnert, entitled "A More Efficient Waveform for Cardiac Stimulation", appearing in Medical and Biological Engineering, Vol. 9, pages 495-501 (1971), it was found that "the rising sawtooth produces what seems to be a better matching between the electrode and electrolyte . . ." "This tends to indicate that the polarization effects are lower for a rising sawtooth." Roy and Wehnert were limited in their approach to overcoming the effects of polarization as they only concerned themselves with sawtooth waveforms and their effects on pacing.

However, prior art implantable pacemaker/defibrillators generally have not been able to generate this type of waveform for use in an implantable device.

The relationship between efficient energy transfer and a particular type of waveform is described in a book by L. A. Geddes and L. E. Baker, entitled "Principles of Applied Biomedical Instrumentation", 3rd edition, Wiley-Interscience, 1989, at p. 507. An approximate equivalent circuit of the electrodes and heart tissue set-up as used in internal defibrillation is shown. Of particular interest is the equivalent circuit for living tissue. This circuit incorporates a capacitor in parallel with part of the tissue resistance. From the equation for capacitance I=C (dV/dT), it can be seen that for a rapidly rising voltage across the tissue (that is, a large dV/dT), as in the leading edge of a truncated exponential discharge, most of the current delivered to the tissue is shunted across this capacitance with very little current being delivered to the resistive load of the tissue in parallel with it. As the capacitor charges up, however, dV/dT decreases as does the current I, and more current passes through the resistive load of the tissue. Ultimately, it is desirable for all of the current to pass through the resistive load and none through the capacitive load. To achieve this, a low dV/dT is required. This can be obtained with a slow leading edge to a pulse as in a sawtooth, sine, triangular or similar wave. Hence by decreasing the slope of the leading edge of a pulse, whether for pacing, defibrillation, fibrillation or other arrhythmia induction, or other electrical stimulus of biological tissue, a more efficient energy transfer can take place and a reaction to the stimulus can be achieved with a lesser energy requirement.

In further regard of a device capable of delivering more effective therapy or induction waveforms, the relationship between cycle frequency and effectiveness of electrical defibrillation is considered in an article by H. P. Schwan and C. F. Kay, entitled "The Conductivity of Living Tissues", published in The Annals of the New York Academy of Sciences, Vol. 65, pages 1007-1013 (1956-57). Here it is shown that the ratio of capacitive current flow to resistive current flow through the heart muscle increases for frequencies above 100 Hz. That is, capacitive or shunting current through the heart muscle increases as the frequency component of the signal becomes very high (as occurs in the leading edge of a truncated exponential capacitor discharge). Support for the hypothesis can also be found in a paper by D. Witzel, L. A. Geddes, J. McFarlane and W. Nichols, entitled "The Influence of Cycle Frequency on the Effectiveness of Electrical Defibrillation on the Canine Ventricles", and published in the Cardiovascular Research Centre Bulletin, Vol. 5, at pages 112-118 (1967). In this paper it is shown that defibrillation requires increasingly more energy to become effective when frequencies above 60 Hz are used. This also happens for frequencies below 60 Hz; however, the reason for this is that the duration of a single cycle of defibrillatory shock becomes so long that fibrillation is likely to be reinduced.

A lesser energy requirement to effect a stimulus, as described above, allows a reduction in the size of an implantable stimulus generator as well as an increase in longevity of the device. Reducing the size of such a generator is an important consideration as present implantable cardioverter/defibrillators are somewhat cumbersome and are uncomfortable in certain patient groups. In addition, by delivering the energy required for a stimulus such as defibrillation more efficiently, lesser energies are required and hence less damage to the heart results.

Waveforms with slow leading edges have not previously been used in implantable devices due to the difficulty of generation of these waveforms. In U.S. Pat. No. 4,090,519, a defibrillator circuit for producing a Lown waveform (damped sinusoid) defibrillation pulse is described. It is also stated therein that a large inductor of 50 millihenries is required to create the shape of the required waveform. For this reason, and because size restricts the use of large inductors, sinusoid waveforms have not been used in implantable devices. Waveforms other than truncated exponential have also not been used in implantable devices before due to the energy loss (in the form of heat) which occurs within the output amplifier circuitry. A transistor, regardless of type, dissipates substantial amounts of energy in the form of heat if it is not in either the fully on or fully off state.

Therefore, if a transistor is required to vary the amount of current passing through it, as would normally be required in the generation of a wave such as a sinusoid, a great deal of deliverable energy would be lost in heat from the circuit. For defibrillation purposes for example, where high current levels are required from the device, the loss of energy in the form of heat would not only risk damage to the transistors, but would reduce the effectiveness of the defibrillation shock. To counteract this, even more energy would be required from the device which, in turn, would further shorten the life of the transistors and the batteries. Furthermore, the device would be required to be much larger in order to accommodate larger capacitors and batteries.

U.S. Pat. No. 4,768,512 discloses an alternative method for the generation of waveforms other than truncated exponential, in which the energy available from charged capacitors is chopped and then the resultant waveform is smoothed with a filter in parallel with the load (heart). By varying the duty cycle of the chopper, any waveform can be generated. Furthermore, by chopping the charge on the capacitors, the transistors involved rapidly change from being fully on to fully off, and vice versa. Hence minimal energy is lost in the form of heat in the transistors.

The extra circuitry required for the creation of the arbitrary waveforms described above is small in size and is compensated for by a reduction in capacitor and battery size within an implantable pacemaker or defibrillator, due to the increased efficiency of the device.

The invention disclosed in U.S. Pat. No. 4,768,512 has several major deficiencies with regard to its usefulness, however. Firstly, the waveform generated by this device is only provided for defibrillation purposes and the device neglects any advantages that might result from the use of the waveform for pacing, arrhythmia induction and other biological stimulation. Secondly, the chopping is at a very high frequency which, as stated before, increases the energy requirement for successful defibrillation. Thirdly, this device chops the voltage which is applied from the storage capacitor to the implantable electrodes at a regular duty cycle and at a regular rate. It does not attempt to vary the shape of the voltage wave being delivered to the implantable electrodes. Finally, the chopped pulses are not smoothed and, as a result, are each seen by the heart as being very high in frequency, and consequently very high in threshold. Moreover, the slope of the wave does not compensate for the polarization effect caused by the large amplitude rapid edges of each chopped pulse.

It is also known in the technology that waveforms can be current-based or voltage-based.

Conventional technology for the delivery of pacing and defibrillation pulses is to charge a capacitor (or bank of capacitors) to a specified voltage, disable the charging circuitry, and connect the charged capacitors directly to the heart. The connection of the capacitors to the heart causes the capacitors to discharge their energy into the heart. Due to the relatively low impedance usually associated with the heart and the pacing or defibrillation electrodes, the capacitors discharge their energy into the heart exponentially and with a time constant proportional to a multiple of the value of the capacitance and the impedance of the heart-electrode system. As the discharge is exponential, however, the low energy