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(Circulation Research. 1995;77:430-438.)
© 1995 American Heart Association, Inc.

Articles

Biphasic Defibrillation Waveforms Reduce Shock-Induced Response Duration Dispersion Between Low and High Shock Intensities

Oscar H. Tovar, Janice L. Jones

From the Department of Physiology and Biophysics, Georgetown University, and Cardiac Research Laboratory, Department of Veterans Affairs Medical Center, Washington, DC.

Correspondence to Janice L. Jones, PhD, VA Medical Center 151P, 50 Irving St NW, Washington, DC 20422.

Abstract Mechanisms underlying defibrillation threshold reduction with biphasic waveforms remain unclear. The interaction of local shock-induced voltage gradients, which change with distance from the shocking electrode, and the state of membrane repolarization results in different cellular responses that may influence the success of defibrillation. We used intracellular microelectrodes and S₁S₂ pacing protocols in myocardial cell aggregates to determine the effects of shock intensity and waveform on refractory period responses during simulated fibrillation (3 s of S₁ pacing at 180-ms cycle length). We simulated defibrillation by electric field stimulation S₂ using 8-ms monophasic (MS₂) and 4/4 biphasic (BS₂) waveforms (65% total tilt) delivered at intensities of 1.5, 3, and 5 times S₁ diastolic threshold, or 2 to 7 V/cm. Responses following MS₂ varied with S₂ intensity and coupling interval (P<.001). When averaged over the last 10 ms of the refractory period, MS₂ produced a negligible response (8.8±1.4 ms) at 1.5 times diastolic threshold and a prolonged response (53.0±3.1 ms) at 5 times diastolic threshold (P<.01). In contrast, BS₂ response duration did not change significantly (P=NS) between 1.5 times diastolic threshold (35.1±12.6 ms) and 5 times diastolic threshold (46.2±2.7 ms). Our results suggest that biphasic waveforms not only prolong response duration at low shock intensity but reduce dispersion of refractoriness produced by differing local potential gradients generated by defibrillation shocks compared with monophasic waveforms. Preventing dispersion of refractoriness and prolonging shock-induced responses may improve biphasic waveform efficacy at low shock intensity.

Key Words: defibrillation waveform • refractory period stimulation • biphasic waveforms • myocardial cell aggregates

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