REGIONAL CAPTURE OF FIBRILLATING VENTRICULAR MYOCARDIUM - EVIDENCE OFAN EXCITABLE GAP

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.