Relationship between "extension of refractoriness" and
probability of successful defibrillation.
Tovar, Oscar H., and Janice L. Jones.
Department of Physiology and Biophysics, Georgetown University, and
Cardiac Research Laboratory, Department of Veterans Affairs Medical
Center, Washington D.C., 20422
APStracts 3:0372H, 1996.
The "extension of refractoriness" hypothesis, which suggests
that the shock halts fibrillation by extending the refractory period,
has not been directly tested. Defibrillation (five isolated rabbit
hearts; 111 episodes) was attempted using 8 ms pulses (65% tilt)
delivered through epicardial patches. Monophasic action potentials
were recorded in a low current density region (6.3 V/cm at 90%
success). Fifty shocks failed to convert; 61 shocks successfully
defibrillated. Postshock response duration (from shock to
repolarization) was significantly longer for successful type A (with
no postshock activations) defibrillation (102.3+/-7.5 ms) than for
unsuccessful defibrillation (47.6+/-4.3 ms; p<0.0001), for
shocks occurring during the last 40% of the fibrillation action
potential. Probability of success and postshock response duration
both increased with current intensity. However, at each intensity,
response durations for successful defibrillation were significantly
longer than those for unsuccessful defibrillation. A minimum
prolongation of 75 ms was associated with type A defibrillation.
These results suggest that shock-induced response duration correlates
with successful defibrillation, and that a response of 75 ms is
required to completely block fibrillation wavefronts.
Received 26 March 1996; accepted in final form 6 August 1996.
APS Manuscript Number H280-6.
Article publication pending Am. J. Physiol. (Heart Circ. Physiology).
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 19 September 1996