Implantable cardioverter defibrillator having a smaller displacement volume

What is claimed is:

1. An implantable cardioverter defibrillator for subcutaneous positioning within a human patient comprising:

a sealed housing structure constructed of a biocompatible material and having a displacement volume of less than about 90 cc;

one or more connector port means, each connector port means disposed in a wall of said housing structure for providing electrical connections between an interior space of said housing structure and a corresponding electrode lead within said human patient;

circuit means disposed within said interior space of said housing structure and operably connected to said connector port means for sensing cardiac signals received from one or more of said electrode leads and, in response to the detection of an arrhythmia in said cardiac signals, controlling delivery of one or more high energy electrical cardioversion/defibrillation countershocks of at least 0.5 Joules to the myocardium of said human patient;

capacitor means disposed within said interior space of said housing structure and operably connected to said circuit means for storing electrical energy to generate said electrical cardioversion/defibrillation countershocks and having an effective capacitance value of less than 120 .mu.F; and

battery means disposed within said interior space of said housing structure and operably connected to said circuit means and said capacitor means for providing electrical energy to said circuit means and said capacitor means,

wherein said battery means and said capacitor means are selected such that a maximum electrical charge energy stored by said capacitor means for each of said electrical cardioversion/defibrillation countershocks is less than about 30 J and said implantable cardioverter defibrillator is capable of delivering at least five of said electrical cardioversion/defibrillation countershocks in a four minute period.

2. The implantable cardioverter defibrillator of claim 1 wherein said circuit means controls the delivery of said electrical cardioversion/ defibrillation countershocks such that a pulse duration of a monophasic one of said cardioversion/defibrillation countershocks, or of a first phase of a multiphasic one of said cardioversion/defibrillation countershock is less than about 6 milliseconds.

3. The implantable cardioverter defibrillator of claim 2 wherein the duration of each of said electrical cardioversion/defibrillation countershocks is determined by said circuit means to be the sum of:

a first value derived from a first predetermined percentage of an RC time constant, with R being a myocardial tissue resistance value and C being said effective capacitance value of said capacitor means; and

a second value derived from a second predetermined percentage of a cardioversion chronaxie, d.sub.c, value.

4. The implantable cardioverter defibrillator of claim 3 wherein said first and second predetermined percentages are between 0.5 and 0.65.

5. The implantable cardioverter defibrillator of claim 4 wherein said first and second predetermined percentages are both 0.58.

6. The implantable cardioverter defibrillator of claim 3 wherein said first value is determined by comparing an output voltage of said electrical cardioversion/defibrillation countershock with said first predetermined percentage and said second value is determined by providing for a fixed time period equal to said second value.

7. The implantable cardioverter defibrillator of claim 2 wherein at least one of said electrical cardioversion/defibrillation countershocks is a biphasic pulse having a positive phase and negative phase.

8. The implantable cardioverter defibrillator of claim 7 wherein the duration of the positive phase and negative phase are equal and is determined by said circuit means to be the sum of:

a first value derived from a first predetermined percentage of an RC time constant, with R being a myocardial tissue resistance value and C being said effective capacitance value of said capacitor means; and

a second value derived from a second predetermined percentage of a cardioversion chronaxie, d.sub.c, value.

9. The implantable cardioverter defibrillator of claim 1 wherein said battery means is capable of charging said capacitor means to said maximum charge amount in less than about 10 seconds.

10. The implantable cardioverter defibrillator of claim 1 wherein said battery means has an estimated life of five years and a total storage capacity of less than about 1.0 Amp-hours.

11. The implantable cardioverter defibrillator of claim 1 wherein an optimum capacitance value (C) for said effective capacitance value of said capacitor means is determined by the simultaneous solution of the equations:

E.sub.c =0.5CV.sub.d.sup.2

C=0.8d.sub.c /R

where E.sub.c is said maximum charge amount, V.sub.d is a maximum voltage for each of said electrical cardioversion/defibrillation countershocks, d.sub.c is a cardioversion chronaxie value and R is a myocardial tissue resistance value.

12. The implantable cardioverter defibrillator of claim 1 wherein said battery means comprises:

a first battery means for providing electrical power to said circuit means to monitor said cardiac signals; and

a second battery means, separate from said first battery means and having different energy storage characteristics, for providing electrical power to charge said capacitor means to generate said electrical cardioversion/defibrillation countershocks.

13. The implantable cardioverter defibrillator of claim 12 wherein said second battery means is a rechargeable battery and further include charging circuitry for charging said rechargeable battery from said first battery means or from an external RF power source.

14. The implantable cardioverter defibrillator of claim 1 wherein said battery means comprises:

a low power output primary battery;

a high power output intermediate power intensifying battery; and

switch means connected to said primary battery and said intermediate power intensifying battery for permitting said intermediate power intensifying battery to rapidly charge said capacitor means,

such that said capacitor means discharges in a first pulse an electrical charge derived from said primary battery and discharges one or more subsequent pulses of electrical charge derived from said intermediate power intensifying battery to permit the implantable cardioverter defibrillator to deliver multiple closely spaced cardioversion/defibrillation countershocks.

15. The implantable cardioverter defibrillator of claim 1 wherein said displacement value of said housing structure is greater than about 40cc and less than about 60cc.

16. The implantable cardioverter defibrillator of claim 1 wherein the total weight of said implantable cardioverter defibrillator is less than about 120 grams.

17. The implantable cardioverter defibrillator of claim 1 wherein said housing structure has a length to width to thickness ratio of approximately 5 to 3 to 1.

18. An implantable cardioverter defibrillator for subcutaneous positioning within the pectoral region a human patient comprising:

a sealed housing structure constructed of a biocompatible material and having a displacement volume of less than about 90cc;

one or more connector port means, each connector port means disposed in a wall of said housing structure for providing electrical connections between an interior space of said housing structure and a corresponding electrode lead within said human patient;

circuit means disposed within said interior space of said housing structure and operably connected to said connector port means for sensing cardiac signals received from one or more of said electrode leads and, in response to the detection of an arrhythmia in said cardiac signals, controlling delivery of one or more electrical cardioversion/defibrillation countershocks of at least 0.5 Joules to the myocardium of said human patient;

capacitor means disposed within said interior space of said housing structure and operably connected to said circuit means for storing electrical energy to generate said electrical cardioversion/defibrillation countershocks; and

battery means disposed within said interior space of said housing structure and operably connected to said circuit means and said capacitor means for providing electrical energy to said circuit means and said capacitor means,

wherein said battery means and said capacitor means are selected to maximize a physiologically effective current (I.sub.pe) for a maximum electrical charge energy (E.sub.c) stored by said capacitor means.

19. The implantable cardioverter defibrillator of claim 18 wherein said physiologically effective current (I.sub.pe) is determined by solution of the equation:

I.sub.pe =(I.sub.ave *d)/(d.sub.c +d)

where I.sub.ave is an average current of said electrical cardioversion/defibrillation countershocks, d is a duration of said electrical cardioversion/defibrillation countershocks, and d.sub.c is a cardioversion chronaxie value.

20. An implantable cardioverter defibrillator for subcutaneous positioning within the pectoral region of a human patient comprising:

a sealed housing structure constructed of a biocompatible material and having a displacement volume of less than about 90 cc and including one or more connector ports disposed in a wall of said structure for providing electrical connections between an interior space of said structure and electrode leads in said patient;

circuit means within said interior space for sensing cardiac signals received from said electrode leads and, in response to the detection of an arrhythmia in said cardiac signals, controlling delivery of one or more electrical cardioversion/defibrillation countershocks of at least 0.5 Joules to said patient;

capacitor means within said interior space for storing electrical energy to generate said electrical cardioversion/defibrillation countershocks and having an effective capacitance of less than about 120 .mu.F; and

battery means within said interior space for providing electrical energy to said circuit means and said capacitor means and capable of charging said capacitor means in less than about 10 seconds to a maximum electrical charge energy of less than about 30 Joules.

21. An implantable cardioverter defibrillator for subcutaneous positioning within a pectoral region of a human patient comprising:

a sealed housing structure constructed of a biocompatible material and having a displacement volume of less than about 90 cc;

one or more connector ports, each connector port disposed in a wall of the housing structure for providing electrical connections between an interior space of the housing structure and a corresponding electrical lead that is implanted within the human patient; and

circuit means disposed within the interior space of the housing structure and operably connected to the connector ports and responsive to a cardiac signal received from the human patient via one or more of the electrical leads for detecting an arrhythmia in the cardiac signal and, in response, controlling delivery of one or more high energy electrical cardioversion/defibrillation countershocks of at least 0.5 joules to the human patient.

22. The implantable cardioverter defibrillator of claim 21 wherein the displacement value of the housing structure is greater than about 40 cc and less than about 60 cc.

23. The implantable cardioverter defibrillator of claim 21 wherein a total weight of the implantable cardioverter defibrillator is less than about 120 grams.

24. The implantable cardioverter defibrillator of claim 21 wherein the housing structure has a length to width to thickness ratio of approximately 5 to 3 to 1.

25. The implantable cardioverter defibrillator of claim 21 wherein the circuit means comprises:

detection means for receiving the cardiac signal and detecting the arrhythmia;

capacitor means for storing electrical energy to generate the electrical cardioversion/defibrillation countershocks;

battery means for providing electrical energy to the detection means, the control means and the capacitor means; and

control means for controlling storing of the electrical energy in the capacitor means and discharging of the electric energy in the capacitor means as the one or more high energy electrical cardioversion/defibrillation countershocks.

26. The implantable cardioverter defibrillator of claim 25 wherein a maximum electrical charge energy stored by the capacitor means for each electrical cardioversion/defibrillation countershocks is less than about 30 joules.

27. The implantable cardioverter defibrillator of claim 26 wherein the battery means charges the capacitor means to the maximum electrical charge energy in less than about 10 seconds.

28. The implantable cardioverter defibrillator of claim 25 wherein the battery means has a total energy storage capacity of less than 1.0 amp-hours.

29. The implantable cardioverter defibrillator of claim 25 wherein the capacitor means has an effective capacitance value of less than 120 .mu.F.

30. The implantable cardioverter defibrillator of claim 29 wherein the effective capacitance value of the capacitor means is an optimum capacitance value (C) determined by the simultaneous solution of the equations:

E.sub.c =0.5CV.sub.d.sup.2

C=0.8d.sub.c /R

where E.sub.c is said maximum charge amount, V.sub.d is a maximum voltage for each electrical cardioversion/defibrillation countershock, d.sub.c is a cardioversion chronaxie value and R is a myocardial tissue resistance value.

31. The implantable cardioverter defibrillator of claim 25 wherein the control means controls the delivery of the electrical cardioversion/defibrillation countershocks such that a pulse duration of a monophasic one of the cardioversion/defibrillation countershocks, or of a first phase of a multiphasic one of the cardioversion/defibrillation countershocks is less than about 6 milliseconds.

32. The implantable cardioverter defibrillator of claim 31 wherein the duration of each electrical cardioversion/defibrillation countershocks is determined by the control means to be the sum of:

a first value derived from a first predetermined percentage of an RC time constant, with R being a myocardial tissue resistance value and C being an effective capacitance value of the capacitor means; and

a second value derived from a second predetermined percentage of a cardioversion chronaxie, d.sub.c, value.

33. The implantable cardioverter defibrillator of claim 32 wherein the first and second predetermined percentages are between 0.5 and 0.65.

34. The implantable cardioverter defibrillator of claim 33 wherein the first and second predetermined percentages are both 0.58.

35. The implantable cardioverter defibrillator of claim 32 wherein the first value is determined by comparing an output voltage of the electrical cardioversion/defibrillation countershock with a first predetermined percentage and the second value is determined by providing for a fixed time period equal to said second value.

36. The implantable cardioverter defibrillator of claim 32 wherein at least one of the electrical cardioversion/defibrillation countershocks is a biphasic pulse having a positive phase and negative phase.

37. The implantable cardioverter defibrillator of claim 36 wherein a duration of the positive phase and negative phase are equal and is determined by the control means to be the sum of:

a first value derived from a first predetermined percentage of an RC time constant, with R being a myocardial tissue resistance value and C being an effective capacitance value of the capacitor means; and

a second value derived from a second predetermined percentage of a cardioversion chronaxie, d.sub.c, value.

38. The implantable defibrillator of claim 25 wherein a ratio of a physiologically effective current (I.sub.pe) to a maximum stored energy charge (E.sub.c) of the capacitor means is at least about 20%.

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TECHNICAL FIELD

The present invention relates generally to the field of automatic, implantable cardioverters and defibrillators. More particularly, the present invention relates to an implantable cardioverter defibrillator (ICD) that is a capacitor-discharge device having its internal components, including a battery and a capacitor, selected and arranged in such a manner that the ICD has a relatively smaller displacement volume that permits effective subcutaneous implantation of the device in the pectoral region of human patients.

BACKGROUND OF THE INVENTION

Existing implantable cardioverter defibrillators (ICDs) are typified by a relatively large size that usually requires implantation of the prosthetic device in the abdominal cavity of a human patient. In order to allow for effective subcutaneous implantation of a prosthetic device in the pectoral region of a human patient, the maximum size of the prosthetic device needs to be less than about 40-90 cc, depending upon the physical size and weight of the patient. Unfortunately, all existing ICDs have total displacement volumes of at least 110 cc or greater. Even though there are numerous advantages to developing an ICD having a displacement volume small enough to permit implantation of the device in the pectoral region of a human patient, to date it has been difficult to develop a practical ICD having a total displacement volume of less than about 100 cc.

For reasons of simplicity and compactness, existing ICDs are universally capacitors-discharge systems that generate high energy cardioversion/defibrillation countershocks by using a low voltage battery to charge a capacitor over a relatively long time period (i.e., seconds) with the required energy for the defibrillation countershock. Once charged, the capacitor is then discharged for a relatively short, truncated time period (i.e., milliseconds) at a relatively high discharge voltage to create the defibrillation countershock that is delivered through implantable electrode leads to the heart muscle of the human patient.

One of the primary reasons why capacitor-discharge ICDs of a smaller volume have not been developed to date relates to the electrical requirements for storing the high energy cardioversion/defibrillation countershocks that are currently used to defibrillate human patients. Cardioversion countershocks have delivered energies of between about 0.5 to 5.0 Joules and are used to correct detected arrhythmias, such as tachycardia, before the onset of fibrillation. Defibrillation countershocks, on the other hand, have delivered energies greater than about 3.0 Joules and are use to correct ventricular fibrillation or an advanced arrhythmia condition that has not responded to cardioversion therapy.

Presently, all capacitor-discharge ICDs are designed such that the capacitor can store a maximum electrical charge energy of at least about 35 Joules. In contrast, implantable pacemakers, which currently have displacement volumes of less than 50 cc, are designed to deliver pacing pulses of no more than about 50 .mu.Joules. The requirement that a capacitor-discharge ICD be capable of storing an electrical charge with enough energy to deliver an electrical pulse almost one million times as large as that of an implantable pacemaker significantly increases the size of the ICD over the size of the pacemaker due to the size of the electrical components necessary to store this amount of electrical charge energy.

The accepted requirement that ICDs be capable of storing a maximum electrical charge energy of at least about 35 Joules arises out of the definition of an appropriate safety margin for the device according to a clinically developed defibrillation success curve as shown in FIG. 11. The defibrillation success curve plots the percentage probability of successful defibrillation for a ventricular fibrillation of about 5-10 seconds versus the energy of a monophasic defibrillation countershock as measured in Joules. The safety margin for a given device for a given patient is presently accepted to be the difference between the maximum electrical charge energy (E.sub.c) stored by the capacitor in that device and the median defibrillation threshold energy (DFT) required for that patient.

Under existing medical practice, each time an ICD is implanted in a human patient, an intraoperative testing procedure is attempted in order to determine the median DFT for that patient for the particular electrode lead combination which has been implanted in the patient. The intraoperative testing procedure involves inducing ventricular fibrillation in the heart and then immediately delivering a defibrillation countershock through the implanted electrode leads of a specified initial threshold energy, for example, 20 Joules for a monophasic countershock. If defibrillation is successful, a recovery period is provided for the patient and the procedure is usually repeated a small number of times using successively lower threshold energies until the defibrillation countershock is not successful or the threshold energy is lower than about 10 Joules. If defibrillation is not successful, subsequent countershocks of 35 Joules or more are immediately delivered to resuscitate the patient. After a recovery period, the procedure is repeated using a higher initial threshold energy, for example, 25 Joules. It is also possible that during the recovery period prior to attempting a higher initial threshold energy, the electrophysiologist may attempt to lower the DFT for that patient by moving or changing the electrode leads.

The intraoperative testing procedure is designed to accomplish a number of objectives, including patient screening and establishing a minimum DFT for that patient. Typically, if more than 30-35 Joules are required for successful defibrillation with a monophasic countershock, the patient is not considered to be a good candidate for an ICD and alternative treatments are used. Otherwise, the lowest energy countershock that results in successful defibrillation is considered to be the median DFT for that patient. The use of the lowest energy possible for a defibrillation countershock is premised on the accepted guideline that a countershock which can defibrillate at a lower energy decreases the likelihood of damage to the myocardial tissue of the heart. For a background on current intraoperative testing procedures, reference is made to M. Block, et al., "Intraoperative Testing for Defibrillator Implantation", Chpt. 3; and J. M. Almendral, et al., "lntraoperative Testing for Defibrillator Implantation", Chpt. 4, Practical Aspects of Staged Therapy Defibrillators, edited by Kappenberger, L. J. and Lindemans, F. W., Futura Publ. Inc., Mount Kisco, N.Y. (1992), pgs. 11-21.

Once the median DFT for a patient is established, the electrophysiologist will determine a safety margin for a given ICD device usually by subtracting the median DFT from the maximum E.sub.c stored by that device. Alternatively, a different calculation for the safety margin is sometimes determined by estimating that point on the defibrillation success curve where the electrical energy of a defibrillation countershock will insure a 99% success (E.sub.99). Under either definition, the safety margin needs to be large enough to accommodate upward deviations along the defibrillation success curve. Such deviations may be expected, for example, with subsequent rescue defibrillation countershocks delivered later in a treatment after initial cardioversion or defibrillation countershocks of lesser energies were not successful. In these situations clinical data has found that, when delivered after 30 to 40 seconds of ventricular fibrillation, the electrical energy necessary to achieve effective defibrillation may increase 50% or more over the median DFT. As a result, an electrophysiologist usually will require that a given ICD have a first type of safety margin that is typically a factor of at least 2 to 2.5 times the median DFT for that patient before the electrophysiologist will consider implanting the given ICD in that patient. For the alternate E.sub.99 point safety margin, the electrophysiologist will require that a given ICD have a maximum E.sub.c at least 10 Joules above the E.sub.99 point.

Based on current clinical data that the average median DFT is somewhere between 10-20 Joules for a monophasic countershock, the lower limit for the maximum E.sub.c that must be stored by the ICD is accepted to be at least about 35 Joules, and more typically about 39 Joules, in order to generate a maximum defibrillation countershock having an adequate safety margin. The accepted lower limit for the maximum E.sub.c of at least 35 Joules is supported by clinical evaluations, such as Echt, D. S., et al., "Clinical Experience, Complications, and Survival in 70 Patients with the Automatic Implantable Cardioverter/Defibrillator", Circulation, Vol. 71, No. 2:289-296, Feb. 1985. In this article, the authors evaluated data for early AICD devices having maximum E.sub.c energies of 32 Joules stored in a 120 .mu.LF capacitor with a discharge voltage V.sub.d of 750 Volts. In analyzing the clinical data for minimum DFTs, the authors concluded that the 32 Joule device had insufficient energy for effective defibrillation. It should be noted that in the next generation of the particular AICD devices studied, the maximum E.sub.c for the device (the CPI Ventak.RTM.) was increased to 39.4 Joules by increasing the capacitance value of the ICD by using a 140 .mu.F capacitor.

Unfortunately, the requirement that an ICD be capable of storing a maximum E.sub.c of this magnitude effectively dictates that the size of the ICD be greater than about 100 cc. This relationship between the maximum E.sub.c that is required for an ICD and the overall size of the ICD can be understood by examining how an ICD stores the electrical energy necessary to deliver a maximum defibrillation countershock.

The only two components that impact on the ability of a capacitor-discharge ICD to store a maximum E.sub.c are the capacitor and the battery, which together occupy more than 60% of the total displacement volume of existing ICDs. Thus, it will be apparent that the size of a capacitor-discharge ICD is primarily a function of the size of the capacitor and the size of the battery. For a capacitor, the physical size of that capacitor is principally determined by its capacitance and voltage ratings. The higher the capacitance value, the larger the capacitor. Similarly, the physical size of a battery is also principally determined by its total energy storage, as expressed in terms of Amp-hours, for example. Again, the higher the Amp-hours, the larger the battery. With these concepts in mind, it is possible to evaluate how a maximum E.sub.c affects the size of the capacitor and the size of the battery in an ICD.

The maximum electrical charge energy (E.sub.c) of an ICD is usually defined in terms of the capacitance value (C) of the capacitor that stores the charge and the discharge voltage (V.sub.d) at which the electrical charge is delivered as defined by the equation:

E.sub.c =0.5*C*V.sub.d.sup.2 (Eq. 1)

The maximum electrical charge energy (E.sub.c) can also be defined in terms of how the energy is transferred from the battery to the capacitor. In this case, E.sub.c is determined by the charging efficiency (e.sub.c) of the circuitry charging the capacitor, the battery voltage (V.sub.b), the battery current (I.sub.b) and the charging time (tc) as defined by the equation:

E.sub.c =e.sub.c *V.sub.b *I.sub.b *t.sub.c (Eq. 2)

When Eqs. 1 and 2 are used to calculate a maximum E.sub.c to be stored by the device, the capacitance value (C) and the charging time (t.sub.c) end up being the only true variables in these equations because the remaining values are all effectively determined by other constraints. In Eq. 1, for example, the discharge voltage (V.sub.d ) for present ICDs can be no more than about 800 Volts due to voltage breakdown limitations of high power microelectronic switching components. As a result, V d is typically between 650-750 Volts. In Eq. 2, it will be found that, for batteries suitable for use in an ICD, the maximum battery output voltage (V.sub.b) for ICDs is typically less than 6 Volts and, due to internal impedances within these batteries, the maximum battery current (I.sub.b) is about 1 Amp. In addition, the charging efficiencies (e.sub.c) of existing ICDs are presently on the order of about 50%.

When Eqs. 1 and 2 are evaluated for any given maximum E.sub.c, it will be found that there necessarily is a minimum capacitance value (C.sub.min) for the capacitor and a minimum charging time (t.sub.min) required to store that maximum E.sub.c in the capacitor of the ICD. Knowing E.sub.c and V.sub.d, Eq. I can be reworked as follows to solve for C.sub.min : ##EQU1##

Similarly, knowing E.sub.c, V.sub.b, I.sub.b, and e, Eq. 2 can be reworked as follows to solve for t.sub.min : ##EQU2##

In other words, the fact that all ICDs presently use a maximum E.sub.c of at least 35 Joules means that all existing ICDs will require capacitors of greater than 124 .mu.F, and that all existing ICDs which draw 1 Amp of current from the battery will have a charging time of greater than 12 seconds. Because the physical size of the capacitor is directly proportional to the capacitance rating of the capacitor in farads for a fixed voltage, the requirement that the capacitor be at least 124 .mu.F is effectively a minimum size limitation on the capacitor for discharge voltages of less than about 800 Volts. Similarly, the requirement that each charging time for a defibrillation countershock draw at least 12 Ampseconds of current from the battery is also a constructive minimum size limitation on the battery. Thus, it can be seen that the existing requirement for a maximum E.sub.c of at least about 35 Joules effectively dictates the size of both the capacitor and the battery and, consequently, the size of the ICD.

While existing ICDs have been successful in defibrillating human patients, and thereby saving lives, these devices are primarily limited to implantation in the abdominal cavity due to their relatively large size of greater than 110 cc. It has long been recognized that it would be advantageous to reduce the total displacement volume of an ICD sufficiently to allow for subcutaneous implantation of the device in the pectoral region of human patients. This can only be done, however, so long as the device provides for a sufficient safety margin to insure its effectiveness. Accordingly, it would be desirable to provide for an arrangement and configuration of the internal components of a capacitor-discharge ICD such that the total displacement volume of the ICD is reduced, while a sufficient safety margin for the device is retained.

SUMMARY OF THE INVENTION

The present invention is a capacitor-discharge implantable cardioverter defibrillator (ICD) having a relatively smaller displacement volume of less than about 90 cc that permits effective subcutaneous implantation of the device in the pectoral region of human patients. The smaller volume of the ICD of the present invention is achieved by selecting and arranging the internal components of the capacitor-discharge ICD in such a manner that the ICD delivers a maximum defibrillation countershock optimized in terms of a minimum physiologically effective current (I.sub.pe), rather than a minimum defibrillation threshold energy (DFT). One of the important results of optimizing the maximum defibrillation countershock in terms of a minimum effective current I.sub.pe is that there is a significant decrease in the maximum electrical charge energy (E.sub.c) that must be stored by the capacitor of the ICD to less than about 30 Joules, even though a higher safety margin is provided for by the ICD. Due to this decrease in the maximum E.sub.c, as well as corollary decreases in the effective capacitance value required for the capacitor and the net energy storage required of the battery, the overall displacement volume of the ICD of the present invention is reduced to the point where subcutaneous implantation of the device in the pectoral region of human patients is practical.

By using a physiologically effective current (I.sub.pe) to determine what is a safe and effective maximum defibrillation countershock, the present invention takes advantages of the realization that it is the effective current delivered to the heart by the defibrillation countershock, and not the total energy of the defibrillation countershock, that results in effective defibrillation. In other words, the present invention recognizes that all Joules are not created equal and that the cells in the heart muscle will make more effective use of some types of electrical energy and less effective use of other types of electrical energy. The prior art technique of using a minimum DFT energy of the defibrillation countershock to establish safety margins effectively ignores the accepted fact that defibrillation countershock waveforms which differ in shape, tilt and duration, for example, can have significantly different defibrillation threshold energies. In contrast, the effective current I.sub.pe as used by the present invention automatically compensates for any differences in the effectiveness of different waveforms. Consequently, the ICD of the present invention uses a minimum effective current I.sub.pe delivered to the heart muscle, rather than using a minimum DFT energy, as the measure for insuring an adequate safety margin for the device.

To understand how the present invention can use a minimum effective current I.sub.pe to insure an appropriate safety margin for the ICD, it is necessary to recognize that the objective of any defibrillation countershock is to generate an electric field across as much as possible of the heart muscle, the myocardium. This electric field must have a current strong enough to extinguish all cardiac depolarization wavefronts in the myocardium, and the current must be strong enough to prevent the myocardium cells from being restimulated during their vulnerable period. In essence, the present invention recognizes that the electric current generated by the defibrillation countershock must be larger than whatever minimum electric current is required for cell stimulation by at least a sufficiency ratio that will insure successful defibrillation. In this way, the use of an effective current I.sub.pe can be thought of as a correction factor applied to the actual current of the defibrillation countershock in order to compensate for the cellular phenomenon that currents below some minimum value simply do not have any effect on the cells.

It has long been known that in order to stimulate cells, a current applied to those cells must have a value at least equal to a rheobase value of those cells, otherwise the current applied to the cells is not effective in stimulating the cells. G. Wiess, "Sur la Possibilite de Rendre Comparable entre Eux les Appareils Suivant a l'Excitation Electrique", Arch. Ital. deBiol., Vol. 35, p. 41 (1901); and L. Lapicque, "Definition Experimetelle de l'excitabilite", Proc. Soc. deBiol., Vol. 77, p. 280 (1909). Lapicque defined the rheobase value as the stimulating current required for a pulse of infinite duration. From this definition, he further defined a chronaxie value (d.sub.c) to be the duration of a pulse that required a current twice that of the rheobase value. These two works have been combined in the literature to define a strength-duration model for the required average current for neural stimulation known as the Weiss-Lapicque strength-duration curve, an example of which is shown in FIG. 12.

The present invention builds on the Weiss-Lapicque strength-duration model to define a physiologically effective current I.sub.pe as a simple model for the efficiency of a monophasic defibrillation countershock in terms of the actual average current of the defibrillation countershock. The actual average current (I.sub.ave) is given by the amount of electrical charge delivered at the electrode leads divided by the duration of the pulse delivering that charge. The end result of the derivation of a definition of effective current I.sub.pe as taught by the present invention is that the effective current I.sub.pe is given by the charge delivered to the electrode leads divided by the sum of the pulse duration (d) and the chronaxie time constant for the heart (d.sub.c). Expressing the charge delivered to the electrode leads in terms of the actual average current I.sub.ave yields a definition equation as follows:

I.sub.pe =(I.sub.ave *d)/(d+d.sub.c) (Eq. 5)

It can be seen from Eq. 5 that if the chronaxie value d.sub.c were zero, the effective current I.sub.pe would simply be I.sub.ave, the average current of a monophasic defibrillation countershock. In this way, the definition of an effective current I.sub.pe distills the information contained in the Weiss-Lapicque strength duration curve to correct the actual average current I.sub.ave of a monophasic defibrillation countershock in order to compensate for the chronaxie phenomenon of the cells of the myocardium.

When a minimum effective current I.sub.pe is used to select and arrange the internal components of a capacitor-discharge ICD, the end result is a pair of surprising and non-intuitive conclusions.

First, the optimum capacitance value for the capacitor in a capacitor-discharge ICD is not determined by any stored or delivered energy requirement, but instead is a relatively constant value much smaller than any currently used capacitance values. The use of a minimum effective current I.sub.pe predicts that the optimum capacitance value will be a function of only the chronaxie time constant and the inter-electrode resistance of the electrode leads. This means that a capacitor with a smaller effective capacitance actually delivers a defibrillation countershock with more effective current I.sub.pe than a capacitor having a larger effective capacitance. When the optimum capacitance value is analyzed in-terms of effective current I.sub.pe, it is found that the optimal capacitance value is given by the formula:

C=(0.8* d.sub.c)/R (Eq. 6)

Second, there is no single optimum pulse duration for a defibrillation countershock having an arbitrary capacitance value. Instead, a defibrillation countershock of a shorter duration can provide a more effective current I.sub.pe than a defibrillation countershock of a longer duration. The use of a minimum effective current I.sub.pe predicts that the optimum pulse duration is a compromise between the RC time constant of the capacitor-discharge circuitry and the heart's defibrillation chronaxie time constant, d.sub.c. Thus, the predicted optimum pulse duration is not a constant, but rather is a function of the effective capacitance and other variables. The predicted optimum pulse duration can be most simply, and robustly, expressed as a fixed tilt or exponential decay followed by a fixed time duration extension. When the optimum pulse duration value is analyzed in terms of effective current I.sub.pe , it is found that the optimal pulse duration is given by the formula:

d=((R*C)+d.sub.c)/(e-1) (Eq. 7)

Because the physical size of the capacitor is a function of its capacitance rating, the use of a capacitor with a smaller effective capacitance provides for a significant reduction in the displacement volume of the capacitor. In addition, because less energy is required to charge up a capacitor with a smaller effective capacitance, a battery with a smaller total energy storage, and, hence, a smaller displacement volume, may also be used. Finally, the shortening of the duration of the defibrillation countershock further decreases the energy requirements of both the capacitor and the battery, and also improves the safety margin of the device. In the preferred embodiment, several additional innovations are also used to further enhance the effectiveness of the defibrillation countershock and decrease the energy storage requirements of the ICD.

As a result of all of these improvements in the selection and arrangement of the internal components of the ICD of the present invention, the capacitor in the device only needs to store a maximum E.sub.c of less than about 30 Joules, and preferably less than 27 Joules. The effective capacitance of the capacitor required by the present invention can be less than 120 .mu.F, and preferably less than about 95 .mu.F. By optimizing both the charging time and the countershock duration for the smaller maximum E.sub.c, the size of the battery required by the present invention is reduced because the total energy storage capacity of the device can be less than about 1.0 Amp-hours. In the preferred embodiment, the charging time for each defibrillation countershock is reduced to less than about 10 seconds and the pulse duration of a monophasic defibrillation countershock, or of a first phase of a multiphasic defibrillation countershock, is reduced to less than about 6 milliseconds.

By significantly reducing the displacement volume of both the capacitor and the battery, the overall displacement volume of an ICD in accordance with the present invention can be reduced below 90 cc, and preferably to between 40-60 cc. Because the size requirements for effective pectoral implantation will be distributed across the range from 40-90 cc for the entire population, it is obvious that the smaller the overall displacement of the ICD, the greater the percentage of human patients who can benefit from pectoral implantation of the device. At the displacement volumes provided for by the present invention, subcutaneous implantation of the device in the pectoral region of a human patient can be quite practical and effective.

Accordingly, it is a primary objective of the present invention to provide an implantable cardioverter defibrillator (ICD) having a smaller displacement volume than existing ICDs that permits effective subcutaneous implantation of the ICD in the pectoral region of human patients.

It is another primary objective of the present invention to provide an ICD that delivers a maximum defibrillation countershock optimized in terms of a minimum physiologically effective current (I.sub.pe), rather than a minimum defibrillation threshold (DFT).

It is a further primary objective of the present invention to provide an ICD with a discharge-capacitor that stores a maximum electrical charge energy (E.sub.c) of less than about 30 Joules.

It is a still further primary objective of the present invention to provide an ICD that utilizes a discharge-capacitor having an effective capacitance of less than 120 .mu.F to store the electrical charge for the cardioversion/defibrillation countershock.

It is another objective of the present invention to provide an ICD that delivers a monophasic defibrillation countershock, or a first phase of a multiphasic defibrillation countershock, having a pulse duration of less than about 6 milliseconds.

It is a further objective of the present invention to provide an ICD that has a battery and capacitor selected such that the battery can charge the capacitor to its maximum E.sub.c in less than about 10 seconds.

It is a still further objective of the present invention to provide an ICD with a five year life and a battery having a total storage capacity of less than about 1.0 Amp-hours.

These and other objectives of the present invention will become apparent with reference to the drawings, the detailed description of the preferred embodiment and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a frontal plan view showing the automatic, implantable cardioverter defibrillator of th