© 2003 by European Society of Cardiology
MECHANISMS OF PACING AND DEFIBRILLATION: LETTERS TO THE EDITOR
Kinetics of defibrillation shock-induced response: design implications for the optimal defibrillation waveform
Department of Legal Medicine, University Hospital Frankfurter Strasse 58, 35392 Giessen, Germany
Mowrey and colleagues[1]
investigated the response of the myocardium with respect to defibrillation ‘shocks’. However, their trials are concerned with effective (Fig. 10) or partially ineffective (Fig. 6) voltage pulses within or outside the absolute refractory period. This is remarkable as they assume an equal mechanism for defibrillation and stimulation without expressing it, a thesis which we presented in 1990[2]. If the cell response is equal for stimulation and defibrillation, one has a simple opportunity to test the defibrillation hypotheses, for instance concerning efficient waveforms, by electrostimulation of the heart with higher accuracy and less physiological damage to the heart as must always be expected with defibrillation. Under this assumption we can ascertain that several statements of the authors contradict the stimulation theory or stimulation results:
- Pacing with 10 µs pulse duration is possible without excessive voltage. The cell membrane integrates the applied voltage so that a sharp rise is especially suited to raise the membrane voltage. The membrane response would best be investigated with a step function.
- Regardless of waveform, the pulse is effective if the voltage averaged during pulse duration (mean value) is equal to that of a square wave at threshold level (Weiss' Law[3]). Following the Weiss Law, ascending and descending pulses are of equal effectiveness if their mean value is equal.
- The above threshold definition is valid as long as parts of a pulse are not below rheobase as they do not contribute. Sinusoidal pulses are, therefore, less effective as they possess voltages below rheobase. These findings, which were surely experienced by others too, were published by us as early as 1976[4].
- If the measured membrane time constants vary between 1.6 and 14.2 ms, the message is not clear as to what defibrillator time constant RC should best be chosen. Theoretical considerations suggest that lowest stored energy is reached with an RC=0.8 times chronaxie, whereas lowest delivered energy is gained with RC=1.19 times chronaxie for exponentially decaying pulses[5].
- With 50
assumed and an output capacitance of 150 µF (the 80 µF of the legend of Fig. 2 is not correct for the HVS-02) the time constant is 7.5 ms. Theoretically[5] the corresponding pulse duration should be 4.9 ms, the tail of the exponential discharge beyond is below rheobase in the far field, which in other words, no longer contributes to the charging of the membrane capacitance. Its voltage starts to decrease again. It would be interesting to learn how this limitation in pulse duration would influence the measured membrane time constants.
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[1] Mowrey KA, Cheng Y, Tchou PJ, Efimov IR. Kinetics of defibrillation shock-induced response: design implications for the optimal defibrillation waveform. Europace 2002; 4: 27–39.
[2] Irnich W. The fundamental law of electrostimulation and its application to defibrillation. Pacing Clin Electrophysiol 1990; 13: 1433–1447.[CrossRef][Medline]
[3] Irnich W. Georges Weiss' Fundamental Law of Electrostimulation is 100 years old. Pacing Clin Electrophysiol 2002; 25: 245–248.[Medline]
[4] Irnich W. Electrotherapy of the Heart 1976; Berlin Fachverlag Schiele and Schön.
[5] Irnich W. Optimal truncation of defibrillation pulses. Pacing Clin Electrophysiol 1995; 18: 673–688.[CrossRef][Medline]
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