Effect of ventricular dilatation on defibrillation threshold in the isolated perfused rabbit heart.

Ventricular dilatation has important electrophysiologic effects, but its effect on ventricular defibrillation threshold (DFT) is unknown. A fluid-filled, latex balloon was placed in the left ventricular cavity of 19 isolated rabbit hearts. In each experiment, an undilated volume (equivalent to a left ventricular end-diastolic pressure of approximately 0 mmHg) was compared to a dilated volume achieved by adding 1.0 mL of saline (n = 10) or 5% dextrose (n = 9) to the intracavitary balloon. Left ventricular effective refractory period (ERP) and DFT were determined at each volume. Defibrillation was attempted with a monophasic shock delivered between a patch electrode positioned over the posterior left ventricle (cathode) and a metallic aortic cannula (anode). DFT was determined using a modified "down/up" protocol with 10 V steps. Ventricular dilatation increased the left ventricular end-diastolic pressure from 0 +/- 0.5 mmHg to 35 +/- 3 mmHg (P < 0.001), decreased the average left ventricular ERP 15% (from 116 +/- 3 msec to 99 +/- 3 msec; P < 0.001), and increased the average DFT 30% (from 96 +/- 4 V to 125 +/- 7 V; P < 0.001). In one third of experiments, the dilated DFT was > or = 150% of the DFT at zero volume. The mechanism of the observed increase in DFT is unknown but may be related to the decrease in refractoriness observed with ventricular dilatation. Acute ventricular dilatation in this model increased DFT an average of 30%, an effect not previously described. This observation may have implications for patients with implantable cardioverter defibrillators.