Surgery: light – thermal – and electrical application – Light – thermal – and electrical application – Electrical therapeutic systems
Reexamination Certificate
1999-07-16
2003-03-25
Evanisko, George R. (Department: 3762)
Surgery: light, thermal, and electrical application
Light, thermal, and electrical application
Electrical therapeutic systems
C607S074000
Reexamination Certificate
active
06539255
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to internal and external defibrillation pulses and, more particularly, to exponentially decaying defibrillation pulse waveforms that do not utilize truncation.
BACKGROUND OF THE INVENTION
Devices for defibrillating the heart have been known for some time now. Implantable defibrillators are well accepted by the medical community as effective tools to combat ventricular fibrillation for an identified segment of the population. A substantial amount of research in fibrillation and the therapy of defibrillation has been done. Much of the most recent research has concentrated on understanding the effects that a defibrillation shock pulse has on fibrillation and the ability to terminate such a condition.
In general, defibrillation shock pulses are delivered through use of a monophasic waveform or, alternatively, a biphasic waveform. A monophasic waveform is typically a single phase, capacitive-discharge, time-truncated, waveform with exponential decay. A biphasic waveform is defined to comprise two monophasic waveforms that are separated by time and that are of opposite polarity. The first phase is designated &PHgr;
1
and the second phase is designated &PHgr;
2
. The delivery of &PHgr;
1
is completed before the delivery of &PHgr;
2
is begun.
After extensive testing, it has been determined that biphasic waveforms are more efficacious than monophasic waveforms. There is a wide debate regarding the exact reasons for the increased efficacy of biphasic waveforms over that of monophasic waveforms. One hypothesis holds that &PHgr;
1
defibrillates the heart and &PHgr;
2
performs a stabilizing action that keeps the heart from refibrillating.
Biphasic defibrillation waveforms are now the standard of care in clinical user for defibrillation with implantable cardioverter-defibrillators (ICDs), due to the superior performance demonstrated over that of comparable monophasic waveforms. To better understand these significantly different outcomes, ICD research has developed cardiac cell response models. Waveform design criteria have been derived from these models and have been applied to monophasic and biphasic waveforms to optimize their parameters. These model-based design criteria have produced significant improvements over previously used waveforms.
In a two paper set, Blair developed a model for the optimal design of a monophasic waveform when used for general bodily electrical stimulation. (1) Blair, H. A., “On the Intensity-time Relations for Stimulation by Electric Currents.” I.J. Gen. Physio. 1932; 15:709-729. (2) Blair, H. A., “On the Intensity-time Relations for Stimulation by Electric Currents II.” I.J. Gen. Physiol. 1932; 15:731-755. Blair proposed and demonstrated that the optimal duration of a monophasic waveform is equal to the point in time at which the cell response to the stimulus is maximal. Duplicating Blair's model, Walcott extended Blair's analysis to defibrillation, where they obtained supporting experimental results. Walcott , et al., “Choosing the Optimal Monophasic and Biphasic waveforms for Ventricular Defibrillation.” J.Cardiovasc. Electrophysio. 1995; 6:737-750.
Independently, Kroll developed a biphasic model for the optimal design of &PHgr;
2
for a biphasic defibrillation waveform as applied internally. Kroll, M. W., “A Minimal Model of the Single Capacitor Biphasic Defibrillation Waveform.” PACE 1994; 17:1782-1792. Kroll proposed that the &PHgr;
2
stabilizing action removed the charge deposited by &PHgr;
1
from those cells not stimulated by &PHgr;
1
. This has come to be known as “charge burping.” Kroll supported his hypothesis with retrospective analysis of studies by Dixon, et al., Tang, et al., and Freese, et al., regarding single capacitor, biphasic waveform studies. See, Dixon, et al., “Improved Defibrillation Thresholds with Large Contoured Epicardial Electrodes and Biphasic Waveforms.” Circulation 1987; 76:1176-1184; Tang et al., “Ventricular Defibrillation Using Biphasic Waveforms: The Importance of Phasic Duration.” J. Am. Coll. Cardio. 1989; 13:207-214; and Freese, S. A. et al., “Strength Duration and Probability of Success Curves for Defibrillation with Biphasic Waveforms.” Circulation 1990; 82:2128-2141. Again, the Walcott group retrospectively evaluated their extension of Blair's model to &PHgr;
2
using the Tange and Freese data sets. Their finding further supported Kroll's hypothesis regarding biphasic defibrillation waveforms as applied to internal defibrillation. For further discussions on the development of the models, reference may be made to PCT publications WO 95/32020 and WO 95/09673 and to U.S. Pat. No. 5,431,686. U.S. Pat. No. 5,431,686 is hereby incorporated by reference.
The “charge burping” hypothesis may be used to develop equations that describe the time course of a cell's membrane potential during a biphasic shock pulse. At the end of &PHgr;
1
, those cells that were not stimulated by &PHgr;
1
have a residual charge due to the action of &PHgr;
1
, on the cell. The “charge burping” model hypothesizes that an optimal duration for &PHgr;
2
is that duration which removes as much of the &PHgr;
1
residual charge from the cell as possible. Ideally, these unstimulated cells are set back to “relative ground.” The “charge burping” model proposed by Kroll is based on the circuit model shown in
FIG. 1B
, which is adapted from the general model of a defibrillator in FIG.
1
A. in
FIG. 1B
, R
H
represents the resistance of the heart, the pair C
M
and R
M
represent membrane series capacitance and resistance of a single cell. C
1
represents the &PHgr;
1
and &PHgr;
2
capacitor set. The node V
S
represents the voltage between the internal electrodes, while V
M
denotes the voltage across the cell membrane.
It should be noted that the “charge burping” model may also account for removing residual cell membrane potential at the end of a &PHgr;
1
that is independent of a &PHgr;
2
pulse, i.e., &PHgr;
2
is delivered by a set of capacitors separate from the set of capacitors used to deliver &PHgr;
1
. This “charge burping” model is constructed by adding a second set of capacitors, as illustrated in FIG.
2
. In this figure, in addition to those elements described with reference to
FIG. 1B
, C
2
represents the &PHgr;
2
capacitor set that is separate from C
1
.
Contrary to the internal defibrillators/defibrillator circuit models described above, external defibrillators can not deliver electrical shock pulses directly to the heart. Rather, external defibrillators must send electrical pulses to the patient's heart through electrodes that are applied to the patient's torso. External defibrillators are useful in any situation where there may be an unanticipated need to provide electrotherapy to a patient on short notice. The advantage of external defibrillators is that they may be used on a patient as needed, then subsequently moved to be used on another patient.
While the moveability of the external defibrillator is indeed a useful advantage, that moveability presents at least two problems not found with internal defibrillators. First, the transthoracic defibrillation problem which results, as explained earlier, from the fact that the external electrodes traditionally deliver their electrotherapeutic pulses to the patients heart by first passing through the patient's chest. Second, the patient variability problem which results from the fact that external electrodes and defibrillators must be able to be used on patient's having a variety of physiological differences. To accommodate that variety, external defibrillators have traditionally operated according to pulse amplitude and duration parameters.
The internal defibrillator models described above, do not fully address the transthoracic defibrillation problem or the patient variability problem. In fact, these two limitations to external defibrillators are not fully appreciated by those in the art. For example, prior art disclosures of the use of truncated exponential monophasic or biphasic shock
Brewer James E.
Tchou Patrick J.
Cardiac Science Inc.
Evanisko George R.
Patterson Thuente Skaar & Christensen P.A.
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