Previous investigations have suggested that during ventricular fibrill
ation (VF) pacing stimuli are incapable of evoking propagated ventricu
lar activations. To determine whether regional myocardial capture coul
d be achieved during rapid pacing in VF, extracellular unipolar potent
ials were sampled (2 kHz) and recorded from 506 Ag-AgCl electrodes arr
anged in a rectangular grid (22x23, 1.12-mm spacing) embedded in a pla
que overlying two pacing electrodes in the epicardium of the anterobas
al right ventricle in pentobarbital-anesthetized pigs (25 to 30 kg, n=
6). During separate episodes of electrically induced VF, two bursts of
40 monophasic stimuli (10 mA, 2-millisecond duration) were asynchrono
usly applied to the stimulating electrodes in either a bipolar, unipol
ar anodal, or unipolar cathodal mode. Evidence of regional capture was
provided by (1) animating the first temporal derivative of the extrac
ellular potentials, (2) analyzing interbeat interval patterns, and (3)
employing the Karhunen-Loeve decomposition method to quantify the rep
etitiveness of spatiotemporal patterns of activation. Regional capture
of ventricular myocardium during VF was observed when pacing stimuli
fell late in the local myocardial activation interval and when the pac
ing cycle length was 80% to 115% of the mean subplaque activation cycl
e length. When myocardial activations became phase locked to the pacin
g stimuli, repeatable spatiotemporal patterns of activation followed e
ach stimulus. Poincare sections al the plaque border revealed that dur
ing VF prior to pacing, interbeat intervals were irregular but were dr
iven by pacing to stable fixed values at times corresponding to our qu
alitative declaration of regional capture. A similar correspondence wa
s demonstrated between the time of capture, defined by direct observat
ion of the activation patterns, and a rise in the power contained in t
he first two spatial modes of a Karhunen-Loeve decomposition. These da
ta demonstrate that appropriately timed stimuli produce regional captu
re of fibrillating right ventricular myocardium in the pig and support
the existence of an excitable gap during VF in this model.