SHORT-PULSE CARDIOVERSION

Abstract: An apparatus for producing a cardioversion waveform of less than about 5 Joules to be delivered to two or more implanted electrodes in a human patient in response to a sensing of a myocardial arrhythmia and a method for operating the apparatus, produces a cardioversion waveform that has an optimized pulse duration shorter than the pulse duration of a defibrillation waveform. The apparatus includes capacitor means, high voltage charging circuit means for charging the capacitor means, and switch means for selectively controlling a time-truncated discharge of the capacitor means through the electrodes. The optimum pulse duration is set automatically by the apparatus based on an optimization of an effective current of the electrical pulse. In one embodiment, the predetermined optimum pulse duration is optimized for an amount of electrical energy required from a battery system that is necessary to store the preselected amount of electrical energy in the capacitor means such that an overall displacement volume of the battery system is reduced. In another embodiment, the predetermined optimum pulse duration is optimized for an amount the preselected amount of electrical energy in the capacitor means that is delivered to the electrodes such that the predetermined pulse duration of the cardioversion waveform essentially matches an average cardioversion chronaxie value of the myocardium of the human patient, whereby an overall pain level of the cardioversion waveform is reduced.

Description: SHORT-PULSE CARDIOVERSION BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to cardioversion therapies. More specifically, the present invention relates to tachycardia-termination using a cardioversion waveform delivered to two or more implanted electrodes, typically by the use of an implantable cardioverter-defibrillator (ICD). 2. Description of the Background Art Implantable cardioverter-defibrillator (ICD) systems now on the market and in use for clinical studies typically employ capacitors of 120 to 180 microfarads to deliver a defibrillation countershock. In order to minimize the complexity of the ICD system, current ICDs use the same capacitor to deliver both the defibrillation countershock and a cardioversion pulse. For similar reasons, the same pulse duration for defibrillation countershocks is also used for cardioversion pulses. Because the energy employed for cardioversion pulses is some four to twenty times less than for defibrillation countershocks (1-5 joules versus 20-40 joules), the voltages used to deliver cardioversion pulses from the same capacitor are correspondingly smaller than the voltages for defibrillation countershocks. Unfortunately, low voltages in the later portions of a stimulation pulse are less effective in terminating tachycardia, and may even induce acceleration or fibrillation. It is worthwhile to provide some background on ICDs. Defibrillation, or causing the cessation of chaotic and uncoordinated contraction of the ventricular myocardium by application of an electrical direct current and voltage, in its most primitive form goes back to the last century. [J. L. Provost and F. Battle, "Sur Quelques Effets Des Descharges Electriques sir I Couer Des Mammifers, "Comptes Rendus Hebdomadaires Des Seances t L 'Acadmie Des Sciences, Vol. 129, p. 1267, 1899.] Because of the large currents required for defibrillation, large-area electrodes are usually employed. [A. C. Gotten and J. Satterfϊeld, "Factors Concerned in Defibrillation of the Heart, Particularly through the Unopened Chest," Am. J. of Physiology, Vol. 167, p. 81, 1951.] For reasons of simplicity and compactness, capacitor-discharge systems are almost universally used in defibrillation. The discharge of a capacitor C through a resistance R results in a curve of voltage versus time, and hence, of current versus time as well, that is a declining exponential function, with a characteristic time constant given by the product RC. It has also been recognized for some time that the long - duration, low-amplitude "tail" of such a capacitor discharge pulse can be detrimental. [J. C. Skidder, G. A. Rahmoeller, and H. Stickily, "Transthoracic Ventricular Defibrillation with Triangular and Trapezoidal Waveforms," Circ. Res., Vol. 19, p. 689, October 1966; W. A. Tacker, et al., "Optimum Current Duration for Capacitor-discharge Defibrillation of Canine Ventricles, " /. Applied Physiology, Vol 27, p. 480, October, 1969.] Although the exact reason for this detrimental effect is not known, plausible speculations exist, with one possibility being that field heterogeneities ease arrhythmias in significantly large regions of the heart. [P. S Chen, et al., "The Potential Gradient Field Created by Epicardial Defibrillation Electrodes in Dogs", Circulation, Vol. 74, p. 626, September 1986.] A convenient way to eliminate the low-amplitude "tail" of a capacitor discharge is by switching, which is to say, simply opening the capacitor-load circuit after a predetermined time, or else when voltage has fallen to a particular value. For this reason, the time-truncated capacitor discharge has been extensively used after its effectiveness was first demonstrated. [J. C. Skidder, et al., "Transthoracic Ventricular Defibrillation in the Dog with Truncated and Untruncated Exponential Stimuli", IEEE Trans. Biom. Eng., Vol. BME-18, p. 410, November 1971.] Two methods for specifying a time-truncated capacitor-discharge pulse in defibrillation systems have been extensively used. Unfortunately, neither method has involved systematic optimization of the pulse. Some manufacturers, such as Medtronic, Inc. in their PCD® product, simply specify pulse duration, although the physician can choose and adjust the value. A typical value might be a programmable duration being at least 7 ms. Other manufacturers, such as Cardiac Pacemakers, Inc. in their Ventak® product, specify the relative amount of voltage decline at the time of truncation, with a typical value of the decline being 65% of the initial voltage. It has become customary to use the term "tilt" to describe the relative amount of such voltage decline, expressed either as a decimal fraction or a percentage. In algebraic language, tilt = (Vinitial - Vfinal) /Vinitial Eq. 1 As a specific illustration of prior-art practice, one can review an existing ICD system that uses a 140-microfarad capacitor for defibrillation. When used with larger electrodes that typically yield a cardiac electrical resistance of 50 ohms, the system displays an RC time constant of 7 milliseconds. Using the specification of 65% tilt, one obtains a pulse duration of approximately 7 milliseconds, the RC time. Because all current ICD devices specify the same tilt or pulse duration for cardioversion pulses as for defibrillation countershocks, it necessarily follows that, if the initial voltage of a cardioversion pulse is only about 150 volts, compared to 600-750 volts for a defibrillation countershock, then the final voltage of the cardioversion pulse at the time of truncation can be as low as 40 to 50 volts, depending upon the initial cardioversion energy chosen. In particular, for a 1-joule pulse from the Ventak® system, for example, the trailing final voltage is 43 volts. Clinical experience with cardioversion pulses having durations and final voltages in these vicinities have shown an effectiveness of only 50 to 80 percent. While existing cardioversion therapies offer some possibility for the successful treatment of tachycardia arrhythmias without the need to resort to defibrillation countershocks, the problem with the cardioversion therapies as delivered by ICDs is that they are effective in only about 50% of the cases, and in as many as 25% of the cases, the cardioversion waveform may actually cause deterioration of the tachycardia arrhythmia into a condition that requires defibrillation. Accordingly, it would be desirable to provide a cardioversion therapy that has a higher success rate and a lower complication rate than existing cardioversion therapies. SUMMARY OF THE INVENTION In accordance with a first aspect of the invention, an apparatus for producing a cardioversion waveform of less than about 5 Joules to be delivered to two or more implanted electrodes in a human patient in response to a sensing of a myocardial arrhythmia in the patient comprises: capacitor means for storing a preselected amount of electrical energy for the cardioversion waveform; high voltage charging circuit means for charging the capacitor means to the preselected amount of electrical energy; and switch means for selectively controlling a time-truncated discharge of the capacitor means through the two or more electrodes in response to the sensing of the myocardial arrhythmia to produce the cardioversion waveform comprising at least one electrical pulse and having a predetermined optimum pulse duration set automatically by the apparatus based on an optimization of an effective current of the electrical pulse for the preselected amount of electrical energy stored in the capacitor means. In accordance with a second aspect of the present invention, a method for operating an implantable device connected to at least two implanted electrodes located in a human patient to treat a myocardial arrhythmia, the method comprising the device-implemented steps of: a) sensing a myocardial arrhythmia in the human patient; and (b) in response to the myocardial arrhythmia, delivering from electrical energy stored in a capacitor system in the implantable device to the implanted electrodes a time-truncated cardioversion waveform of less than about 5 Joules comprising at least one electrical pulse and having a predetermined optimum pulse duration set automatically by the device based on an optimization of an effective current of the electrical pulse for a preselected amount of electrical energy stored in the capacitor system. In accordance with another aspect of the invention, an implantable cardioverter defibrillator for producing a cardioversion waveform of less than about 5 Joules and a defibrillation waveform of greater than about 5 Joules, both of which are to be delivered to at least two or more implanted electrodes in a human patient in response to a sensing of a myocardial arrhythmia in the patient comprises: capacitor means for storing a first preselected amount of electrical energy for the cardioversion waveform and a second preselected amount of electrical energy for the defibrillation waveform; high voltage charging circuit means for selectively charging the capacitor means to either the first or second preselected amounts of electrical energy; and switch means for selectively controlling a time- truncated discharge of the capacitor means through the two or more electrodes in response to the sensing of the myocardial arrhythmia to produce either the cardioversion waveform comprising at least one electrical pulse and having a first predetermined pulse duration, or the defibrillation waveform comprising at least one electrical pulse and having a second predetermined pulse duration, where the first predetermined pulse duration of the cardioversion waveform is less than the second predetermined pulse duration of the defibrillation waveform. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 illustrates a preferred-embodiment monophasic pulse for cardioversion using the present invention; Figure 2 illustrates the same preferred-embodiment monophasic pulse of the present invention, but with different specification; Figure 3 illustrates a simplified representation of a circuit for generating the monophasic pulse of Figures 1 and 2; Figure 4 illustrates an example of an embodiment for a biphasic pulse for cardioversion in accordance with the present invention; Figure 5 constitutes a simplified representation of a circuit for generating the biphasic pulse of Figure 4; and, Figure 6 illustrates defibrillation and cardioversion waveforms of the prior art and a cardioversion waveform for one embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The following detailed description, when considered in connection with the accompanying drawings in which like reference numerals des¬ ignate like parts throughout the figures, describes some of the embodiments of the present invention. In essence, the present invention provides a cardioversion therapy that has a higher success rate and a lower complication rate than existing cardioversion therapies by optimizing the pulse duration of the cardioversion waveform such that the pulse duration is shorter than existing pulse durations of cardioversion and defibrillation countershocks, e., the optimized pulse duration is shorter than about 6 milliseconds. In accordance with one embodiment of the present invention, the pulse duration is optimized for an amount of electrical energy required by the high voltage charging circuit means from a battery system that is necessary to store the preselected amount of electrical energy in the capacitor means such that an overall displacement volume of the battery system is reduced. In accordance with another embodiment of the present invention, the pulse duration is optimized for an amount the preselected amount of electrical energy in the capacitor means that is delivered to the electrodes such that the predetermined pulse duration of the cardioversion waveform essentially matches an average cardioversion chronaxie value of the myocardium of the human patient, whereby an overall pain level of the cardioversion waveform is reduced. In addition to the hazard of supplying a low voltage trailing voltage to the heart, the longer cardioversion pulse duration supplied by existing ICDs constitutes a waste of energy. In tachycardia, heart cells that must be reset are in a resting state of polarization (Phase 4), a task requiring less energy than resetting cells that are in an actived state of depolarization (Phases 0-3), as in the case of ventricular fibrillation. It is believed that this phenomenon is responsible for the lower energies typically required by, and hence chosen for, cardioversion. The present invention takes this one step further by providing for a reduction of the pulse duration without sacrificing effectiveness, and eliminating the dangerous low- voltage tail from the cardioversion pulse. One of the principal factors which has been essentially ignored in existing cardioversion therapies is the natural stimulation time constant of the heart itself. A characteristic time associated with far-field stimulation using electrodes relatively remote from the heart is in the neighborhood of 1 millisecond. For purposes of comparison, for a far-field defibrillation countershock the characteristic stimulation time is between 2 to 4 milliseconds, whereas the characteristic stimulation time for a pacing pulse is on the order of 300 microseconds. It will be understood that the characteristic ime is an average time period and can vary slightly between patients and ICD systems, depending upon the myocardial resistance and the electrode size, for example. The elucidation of this characteristic time employs the concept of "chronaxie", that requires some background explanation. The foundation for defining such a characteristic time is a family of mathematical neurophysiological models for tissue stimulation going back to the turn of the century, with the first important such model having been developed by Weiss. [G. Weiss, "Sur I Possibilite t Randall Comparable entire Eux le Appareils Suivant a l'Excitation Electrique," Arch. Ital. t Biol, Vol. 35, p. 413, 1901.] He employed the ballistic-rheotome ■ technique for pulse generation, wherein a rifle shot of known velocity is used to cut two wires in sequence, their spacing being set and measured. Cutting the first wire eliminated a short from a dc source, causing current to flow through the tissue under test, and cutting the second wire opened the circuit, termmating the pulse applied. Converting the electrical data into charge delivered by the pulse, Weiss found that the charge Q needed for stimulation was linearly dependent on pulse duration, d. Specifically, Q = kι + k 2 d Eq. 2 Subsequently and similarly, the physiologist L. Lapicque collected substantial amounts of data on the amount of current required for tissue stimulation, using constant-current pulses of various durations. [L. Lapicque, "Definition Experimentelle t l'excitability," Proc. Soc. t Biol., Vol. 77, p. 280, 1909.] Lapicque established an empirical relationship between the current I and the pulse duration d, having the form: I = _Cι + (K 2 / d) Eq. 3 Equation 3 " of Lapique shows that the necessary current and the pulse duration are related by a simple hyperbola, shifted away from the origin by the amount of the constant term Ki. Hence the stimulating current required in a pulse of infinite duration is Ki, a current value Lapicque termed the rheobase. Shortening the pulse required progressively more current, and the pulse duration that required a doubling of current for excitation, or 2Kι, he termed the chronaxie, d c . Substituting 2Kι and d c into Eq. 3 in place of I and d, respectively, yields d c =K 2 / Kι Eq. 4 Lapicque's model described cell stimulation, rather than defibrillation, but Bourland demonstrated that defibrillation thresholds in dogs and ponies followed the Lapicque model, provided average current is used in the exercise. Q. D. Bourland, W. Tacker, and L. A. Jades, "Strength- Duration Curves for Trapezoidal Waveforms of Various Tilts for Transchest Defibrillation in Animals," Med. Instr, Vol. 12, p. 38, 1978.] In a companion paper, the same workers showed that average current, I aVe is a useful and consistent measure of defibrillation effectiveness for time- truncated pulses of a given duration through a substantial range of durations, from 2 to 20 milliseconds; in other words, so long as the exponential "tail" is eliminated, pulse effectiveness is not very dependant upon waveform details. . D. Bourland, W. Tacker, and L. A. Jades, "Comparative Efficacy of Damped Sine Waves and Square Wave Current for Transchest Defibrillation in Animals," Med. Instr., Vol. 12, p. 42, 1978.] The defibrillation chronaxie for the heart is usually between 2 milliseconds and 4 milliseconds, as is borne out by a substantial fund of published data. For cardioversion, the chronaxie is approximately 1 millisecond. For purposes of the present invention, the phrase "pulse duration set automatically by the apparatus" differentiates from those existing ICD devices that can be programmed to a short pulse duration by the physician. The phrase "is less than the second predetermined pulse duration for the defibrillation waveform" differentiates the ICD of the present invention from the existing ICDs, all of which have a fixed cardioversion pulse duration equal to their defibrillation pulse duration. The general purpose of the present invention is to provide a process for optimization of the pulse duration of a cardioversion waveform. In doing so, the present invention builds upon the models of Lapicque and Weiss, and the finding of Bourland. To do this, the present invention defines a "sufficiency ratio," the ratio of Bourland's ruling average current and the current needed for defibrillation according to the Lapicque model for a heart of a given Ki, rheobase current, and a given K 2 , a charge. Algebraically, Sufficiency ratio = I ave / ( i + K 2 /d) Eq. 5 It is simply the ratio of Bourland's available therapeutic current (or the average current I aV e) to the current required according to the Lapicque formulation. Hence for a ratio of unity, the waveform of average current lave and duration d will be able to defibrillate a heart characterized by Ki and K 2 . To review briefly, recall that the current KI and the charge K characterize the sensitivity of particular tissue to electrical stimulation. The smaller these quantities are, the more sensitive is the tissue in question to electrical stimulation. The quotient of these two quantities yields a characteristic time, the chronaxie, which does not by itself indicate sensitivity or insensitivity. Multiplying Eq. 5 through by the rheobase current Ki yields an expression that, of course, has dimensions of amperes; noting from Eq. 4 that Ki d c = K 2 , makes it possible to eliminate the heart-characterizing quantities Ki and K 2 from this expression: D- ave / (l +K 2 / K 1 d) ] = [I aV e/ (l + d c /d) ] Eq. 6 Thus, one has an expression in the two pulse characterizing quantities I aVe and d, and in one heart-characterizing quantity, d c , the chronaxie time. Note that for an infinite pulse duration, this current simply equals the average current I ave , but for a pulse of finite duration, it will be smaller than I ave - This current, therefore, measures the effectiveness of a particular waveform in defibrillating a particular heart. For this reason the inventors have named it the effective current or I e so that the defining equation is Ie = I ave / (l + d c /d) Eq. 7 Note further that le would be the same as I ave if one had a zero value of chronaxie time, d c . In this sense, Eq. 7 constitutes a correction from actual average current necessitated by the chronaxie phenomenon. The effective current l e can be expressed in several ways: I_ = Paved / (dc + d)] = [(delivered charge)/(d c + d)] = CV(ϋh τ&ύo)/(d c + d) Eq. 8 Applying this formulation to an existing ICD system employing C = 140 microfarad, tilt = 0.65, Vi = 125 volts, and an inter-electrode resistance of R = 50 ohms (for a pulse duration of about 7 milliseconds) yields an effective current of: l e = [(140 F) (125 V) (0.65) / (1 ms + 7 ms)] = 1.42 A. Eq. 9 It is convenient to illustrate the kind of information obtainable from this analysis by using the hypothetical (and easy to calculate) case of a rectangular current pulse employed for cardioversion. With a current I delivered to the (cardiac) load resistance R, we can write the energy E of the pulse in joules as: E = PRd Eq. 10 so that I = (E / Rd)0-5 Eq. 11 The object, now, is to maximize the effective current I e , which is given by le = lave / σ + dc/d) = (E / Rd) .5 / (l + dc/d) Eq. 12 or, le = (E / R)0-5 [ (1/d )0.5 / (1 + d c /d) ] = (E / R)0.5 [ (d)0.5 / (d c + d) Eq. 13 Differentiating this expression with respect to d and equating the results to zero yields: d = d c Eq. 14 Thus, for a truly rectangular pulse, the optimal duration is simply the chronaxie time. This result has intuitive appeal, because one is matching the pulse duration to the natural time constant of the cardiac system. This result is known in pacing. [J. A. Pearce, e a., "Myocardial Stimulation with Ultrashort Duration Pulses", PACE, Vol. 5, p. 52, Jan.- Feb. 1982], and has been derived for defibrillation as well. It was, in fact, noted by Lapique. Hence, our result using the concept of effective current is consistent with prior knowledge regarding the optimum duration for ideal and hypothetical rectangular pulses. It is instructive to calculate the current in an ideal hypothetical 1- joule rectangular pulse having a duration of 1 millisecond for comparison with the effective current obtained above for an existing truncated capacitor-discharge cardioversion/defibrillation pulse having a duration of 7 milliseconds. From Eq. 10, I = [ (1 J) / (50 ohm) (1 ms) ]0-5 = 4.47 A. Eq. 15 Thus the 1-joule pulse in this hypothetical example yields an effective current over three times greater than that of a 1-joule pulse having a duration of 7 milliseconds, illustrating the virtue of a short pulse in spite of the hypothetical nature of the waveform in the present example. It is important to understand, however, that the characteristics of an ideal hypothetical rectangular pulse are not directly applicable to the performance of a truncated capacitor-discharge ICD. Given that a truncated capacitor-discharge pulse is used in real life, the next task is to optimize such a pulse for cardioversion. A capacitor C will be charged to a voltage Vi and discharged into a cardiac load resistance R, with time truncation to yield a pulse duration d. The effective capacitance value of the capacitor has been chosen with an eye to high- energy defibrillation. This fixed capacitor will be charged to a fixed voltage, resulting in the storage of a fixed energy in the capacitor. Neglecting converter losses, this energy will equal the energy drawn from the battery. That is, because it is not practical to recover energy left in the capacitor after a truncated pulse is delivered, if the goal is to mi-nimize the amount of energy required by the ICD, it is the energy stored in the capacitor initially (rather than the energy delivered) that is relevant in deterrriining the optimum pulse duration of the cardioversion waveform in terms of how to budget battery energy among the pacing function, the cardioversion function with multiple shocks, and the defibrillation function, also with multiple shocks. In contrast, if the goal is to mi-nimize the myocardial damage and, hence, the pain associated with a cardioversion waveform, then it is energy delivered (rather than the energy stored) that is relevant in determining the optimum pulse duration of the cardioversion waveform. With respect to the first goal, the first challenge is to maximize the effective current l e for a given energy stored initially in the capacitor. Because the waveform is a declining exponential function, and given that RC = τ, the system time constant, tilt as a decimal fraction can be written as follows: tilt = 1 - exp (-d/τ) Eq. 16 Combining this expression and Eq. 8 yields: Ie = CV[l - exp (-d/τ)]/(d c + d) Eq. 17 It is clear the I e vanishes at both extremes of d, so the intermediate extremum must be a maximum, defining explicitly the optimum waveform that can be achieved by varying pulse duration with a particular average current. To determine this optimum pulse duration, set dl e (d + d c ) (e ' /t) - ( l -e~ d /-) = cv- dd - = 0 (d =, , d .c 2'), Eq. 18 Hence, Eq. 19 Using the system time constant τ = RC for normalization yields z = d / τ Eq. 20 and α = d c / τ Eq. 21 Using these definitions, 0 = (z + α + 1) [exp (-z)] - 1 Eq. 22 Next multiply through by -ez to obtain the simplified equation whose root is: 0 = [exp (-z)] - z - α - l Eq. 23 Because the equation is transcendental, it cannot be solved in closed form, so define the function on the left-hand side as f(z) and the first approximation for its root as zo- The Newton-Raphson method gives an approximate value for the root as z' = zo -f(z 0 ) / f'(zo) Eq. 24 Experience shows that waveforms with a tilt of about 65% are effective, and this corresponds to d = τ or zo = 1. Hence an appropriate approximate root is z' = l - (e - l - α - l) / (e - l) = (l + α) / (e - l) Eq. 25 Denormalization yields: d = (τ + d c ) / (e - l) Eq. 26 for the approximate optimum value of pulse duration d as a function of chronaxie d c and system time constant. Carrying through the optimization numerically shows that this estimate is valid within 0.2% for typical values of R, C, and d c . Even for extreme values of these system and heart parameters, the approximate value of optimum duration produces a value for the current le that is within 2% of the optimum. Since (e - 1) = 1.72, the optimum pulse duration is approximately (and somewhat larger than) the average of the system's time constant and the heart's characteristic time d c . In other words, the optimum pulse duration is a compromise between the two characteristic times involved. To illustrate the application of this analysis, let us return to the case of an existing ICD system for producing a cardioversion waveform employing C = 140 microfarads, Vi = 125 volts, and an inter-electrode resistance of R = 50 ohms. We now determine the optimal pulse duration d for this case, describing a preferred embodiment of the present invention in the process. From Eq. 26, recalling that d c = 1 millisecond for cardioversion, d = [ (7 ms + l ms)/(e - l) ] = 4.65 ms Eq. 27 For this pulse duration, the tilt amounts to 49%, or 0.49. Hence the effective current from Eq. 8, once more, is l e = [(140 F) (125 V) (0.49) / (1 ms + 4.65 ms)] = 1.52 A. Eq. 28 ' This constitutes an increase over the prior-art value (1.42 ampere) for the same system in spite of the fact that less energy is delivered by the truncated cardioversion pulse with its optimal shorter duration. The optimal pulse duration brings the further benefit of eliminating the lingering low voltage portions of the pulse that can lead to ventricular fibrillation. The optimized pulse as described in this embodiment has a final voltage of V f = 64 V, while for the 7 millisecond cardioversion pulse of an existing ICD, the value is V f = 44 V. The further advantage is that the 64-V value exists at a time less then 5 ms, while the 44-V value of the prior art exists at 7 ms. The delivery of an optimized pulse duration such as this for cardioversion is one of the essences of the present invention. In addition to the advantages just cited, there are further benefits to be gained by maximizing the quotient of effective current by pulse energy. Diminished energy reduces the inevitable damage to myocardial tissue. Even further, it diminishes pain. Thus, an impressive set of betterments flow from the short cardioversion pulse of the present invention. The discussion thus far has addressed the issue of optimization for the case where energy initially stored in the battery is held constant. There is, however, a different problem that is worth addressing. That is, the case where delivered energy is held constant. This implies a higher initial voltage Vi as pulse duration shrinks. But, this further involves a diminished tilt, and hence, a more nearly rectangular pulse. Conceding that increased amounts of energy initially must be delivered from the battery to the capacitor for each such pulse, and that any energy remaining in the capacitor cannot be recovered, and hence is wasted, we nonetheless believe this line of inquiry to be worthwhile. Note first that going from a 7-ms conventional pulse to a 1-ms pulse reduces tilt from about 65% to about 14%, as is illustrated in Figure 6. Because this is an excellent approximation to a rectangular form, we receive the benefit of the kind of pulse connected with the calculation in Eq. 15. Further, as demonstrated in Eq. 14, the optimal duration of a rectangular pulse is simply the chronaxie, d c . Thus, the insights involving the short pulse of Figure 6 yields an optimal duration only slightly larger than the chronaxie, and having an effective current within 1% of that obtained by simply using the chronaxie as the optimum pulse duration. Cardioversion can be accomplished by using a monophasic wave, such as that illustrated in Figures 1 and 2. The first shows a preferred embodiment of the present invention, with the pulse properties specified in terms of initial voltage and pulse duration; the second employs initial voltage and tilt, producing the same result in this case. Figure 1 illustrates the waveform 10 of a preferred embodiment monophasic pulse for cardioversion of the present invention, specified by means of an initial voltage 12 and a duration 14. Figure 2 illustrates the waveform 20 of a preferred embodiment monophasic pulse for cardioversion of the present invention, specified by means of an initial voltage 12 and a tilt 22. A simplified representation of a circuit for generating such a monophasic pulse is illustrated in Figure 3. A representation 30 of an electrical circuit for generating the monophasic pulse of Figures 1 and 2, including high-voltage charging circuitry 32 for the capacitor 34, and a switch 36 that is closed to deliver the pulse to the cardiac electrodes 38 and 39. Cardioversion, like defibrillation, can be accomplished by means of a multiphasic pulse as well as a monophasic pulse. An example of a multiphasic pulse is the biphasic pulse illustrated in Figure 4, and the corresponding circuit is shown in Figure 5. In the biphasic case, the present method is used to fix the duration of the first phase, and then one usually lets the duration of the second phase equal a fraction of that determined to be optimal for the first. Figure 4 illustrates the waveform 40 of an example of a biphasic pulse for cardioversion of the present invention, including a first phase 42 - and a contiguous second phase 44. Figure 5 illustrates a representation 50 of an electrical circuit for generating the biphasic waveform 40 of Figure 4, including a capacitor 52, the four switches 54, 56, 58, and 60 that are able to reverse polarity in going from the first phase 42 of Figure 4 to the second phase 44 of Figure 4 for purposes of delivering the biphasic waveform 40 of Figure 4 to the cardiac electrodes 62 and 64 positioned on or near the heart 66. Figure 6 illustrates a set 70 of voltage-versus-time waveforms, including the defibrillation waveform 72 of the prior art, the cardioversion waveform 74 of the prior art, and the cardioversion waveform 76 for one preferred embodiment of the present invention. The model that is developed in the present invention is based upon the pioneering neurophysiological models of Lapicque and Weiss. The present model determines mathematically the optimum pulse duration, d, for a truncated capacitor-discharge waveform employed for cardioversion. It comprehends the system time constant, RC, where R is tissue resistance and C is the value of the capacitor being discharged, and also the heart's cardioversion chronaxie, d c defined by Lapicque, which is a characteristic time associated with the stimulation of heart tissue. The present model and analysis find the optimum pulse duration for the case of fixed energy stored to be roughly the average of RC and d c , appreciably smaller than the value employed in the prior art. The optimized-pulse prescription of this invention can be applied to a monophasic waveform, or to the first phase of a biphasic waveform. In the latter case, when it is applied to the first phase, the second phase can be specified to have equal or lesser duration than the first. When energy delivered to the heart is taken to be constant, and the waveform is optimized on that basis, then the pulse duration approximates the chronaxie of the myocardial tissue.

Claims: CLAIMS 1. An apparatus for producing a cardioversion waveform of less than about 5 Joules to be delivered to two or more implanted electrodes in a human patient in response to a sensing of a myocardial arrhythmia in the patient, comprising: capacitor means for storing a preselected amount of electrical energy for the cardioversion waveform; high voltage charging circuit means for charging the capacitor means to the preselected amount of electrical energy; and switch means for selectively controlling a time-truncated discharge of the capacitor means through the two or more electrodes in response to the sensing of the myocardial arrhythmia to produce the cardioversion waveform comprising at least one electrical pulse and having a predetermined optimum pulse duration set automatically by the apparatus based on an optimization of an effective current of the electrical pulse for the preselected amount of electrical energy stored in the capacitor means. 2. A method for operating an implantable device connected to at least two implanted electrodes located in a human patient to treat a myocardial arrhythmia, the method comprising the device-implemented steps of: (a) sensing a myocardial arrhythmia in the human patient; and (b) in response to the myocardial arrhythmia, delivering from electrical energy stored in a capacitor system in the implantable device to the implanted electrodes a time- truncated cardioversion waveform of less than about 5 Joules comprising at least one electrical pulse and having a predetermined optimum pulse duration set automatically by the device based on an optimization of an effective current of the electrical pulse for a preselected amount of electrical energy stored in the capacitor system. 3. The invention of claim 1 and 2 wherein the predetermined optimum pulse duration is between about 0.5 ms and 6.0 ms. 4. The invention of claim 1 and 2 wherein the predetermined optimum pulse duration is optimized for an amount of electrical energy required from a battery system that is necessary to store the preselected amount of electrical energy in the capacitor means such that an overall displacement volume of the battery system is reduced. 5. The invention of claim 1 and 2 wherein the optimum pulse duration is determined by the switch 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 value. 6. The invention of claim 1 and 2 wherein the first and second predetermined percentages are between 0.5 and 0.65. 7. The invention of claim 1 and 2 wherein the first value is determined by comparing an output voltage of the cardioversion waveform with the first predetermined percentage and the second value is determined by providing for a fixed time period equal to the second value. 8. The invention of claim 1 and 2 wherein the time period for the optimum pulse duration is between about 4.0 and 5.0 ms. 9. The invention of claim 1 and 2 wherein the time period for the optimum pulse duration is between about 4.5 and 4.8 ms. 10. The invention of claim 1 and 2 wherein the optimum pulse duration is measured by a percentage tilt decay between an initial voltage value and a final voltage value where the percentage tilt is between about 44% and 54%. 11. The invention of claim 1 and 2 wherein the percentage tilt is between about 48% and 50%. 12. The invention of claim 1 and 2 wherein the time period is determined for a first pulse of a multiple pulse cardioversion waveform. 13. The invention of claim 1 and 2 wherein the optimum pulse duration is a time period that is shorter than a first time constant that is a system time constant, RC, of the cardioverter, where R is an average tissue resistance of the myocardium of the human patient and C is an effective capacitance of the capacitor means, and longer than a second time constant that is an average cardioversion chronaxie value of the myocardium of the human patient. 14. The invention of claim 1 and 2 wherein the predetermined optimum pulse duration is optimized for an amount the preselected amount of electrical energy in the capacitor means that is delivered to the electrodes such that the predetermined pulse duration of the cardioversion waveform essentially matches an average cardioversion chronaxie value of the myocardium of the human patient, whereby an overall pain level of the cardioversion waveform is reduced. 15. The invention of claim 1 and 2 wherein the time period for the optimum pulse duration is between about 0.5 ms and 3.0 ms. 16. The invention of claim 1 and 2 wherein the time period for the optimum pulse duration is between about 0.8 and 1.2 ms. 17. The invention of claim 1 and 2 wherein the optimum pulse duration is measured by a percentage tilt decay between an initial voltage value and a final voltage value where the percentage tilt is between about 5% and 20%. 18. The invention of claim 1 and 2 wherein the percentage tilt is between about 12% and 16%. 19. An implantable cardioverter defibrillator for producing a cardioversion waveform of less than about 5 Joules and a defibrillation waveform of greater than about 5 Joules, both of which are to be delivered to at least two or more implanted electrodes in a human patient in response to a sensing of a myocardial arrhythmia in the patient, comprising: capacitor means for storing a preselected amount of electrical energy for the cardioversion waveform and a second preselected amount of electrical energy for the defibrillation waveform; high voltage charging circuit means for selectively charging the capacitor means to either of the preselected amounts of electrical energy; and switch means for selectively controlling a time-truncated discharge of the capacitor means through the two or more electrodes in response to the sensing of the myocardial arrhythmia to produce either the cardioversion waveform comprising at least one electrical pulse and having a predetermined pulse duration, or the defibrillation waveform comprising at least one electrical pulse and having a second predetermined pulse duration, where the predetermined pulse duration of the cardioversion waveform is less than the second predetermined pulse duration of the defibrillation waveform. 20. A method for operating an implantable cardioverter defibrillator for producing a cardioversion waveform of less than about 5 Joules and a defibrillation waveform of greater than about 5 Joules, both of which are to be delivered to at least two or more implanted electrodes in a human patient in response to a sensing of a myocardial arrhythmia in the patient, the method comprising the device-implemented steps of: (a) sensing a myocardial arrhythmia in the human patient; and (b) in response to the myocardial arrhythmia, selectively -- delivering from electrical energy stored in a capacitor system in the implantable cardioverter defibrillator to the implanted electrodes a time-truncated either the cardioversion waveform comprising at least one electrical pulse and having a predetermined pulse duration, or the defibrillation waveform comprising at least one electrical pulse and having a second predetermined pulse duration, where the predetermined pulse duration of the cardioversion waveform is less than the second predetermined pulse duration of the defibrillation waveform.

Inventors: KROLL MARK W, KROLL, MARK, W.
Applicants: ANGEMED INC, ANGEMED, INC.
Application year: 1993
Main classification: A61N1
IPC codes: A61N1