AJP - Heart and Circulatory Physiology, Vol 253, Issue 6 1418-H1424, Copyright © 1987 by American Physiological Society
Improved cardiac cell excitation with symmetrical biphasic defibrillator waveforms
J. L. Jones, R. E. Jones and G. Balasky
Department of Physiology and Biophysics, School of Medicine, Case Western Reserve University, Cleveland, Ohio 44106.
According to the most commonly accepted hypothesis, ventricular
defibrillation is produced by exciting cells in a critical mass of the
ventricle. For monophasic defibrillator waveforms, this hypothesis
correctly predicts a direct correlation between defibrillation threshold in
the transthoracic calf model and excitation threshold for extracellular
field stimulation in the cultured cell model. To further test the
hypothesis, we determined whether symmetrical biphasic waveforms, which
reduce defibrillation threshold in the calf to approximately 65% of that of
the corresponding monophasic waveform (14), decrease excitation threshold
in the cultured cell model. Experiments were performed on 100- to
250-microns aggregates from 10- to 12-day-old chick embryos. Excitation
threshold strength-duration curves obtained at extracellular potassium (Ko)
= 6.5 mM and pacing interval of 1,000 ms showed a significant reduction for
symmetrical biphasic rectangular waveforms, when compared with the
corresponding monophasic waveforms for durations greater than 3 ms. At the
rheobase, the threshold ratio between the biphasic and monophasic waveforms
was 0.63 (SE = 0.02). Transmembrane potentials during stimulation showed
that excitation takes place during the second portion of the biphasic
waveform for intensities that are subthreshold for the monophasic waveform.
The relative effectiveness of the biphasic waveform (5-ms duration)
increases under "fibrillation conditions" of short pacing interval (300 ms)
and high extracellular potassium (10.5 mM). These results show that
symmetrical biphasic waveforms decrease excitation threshold in the
cultured cell model and that the degree of threshold reduction is dependent
on Ko and beat rate.
