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

Introduction: Ventricular dilatation has important electrophysiologic effects, but its effect on ventricular defibrillation threshold (DFT) is unknown, Methods and Results: 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 mt 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 greater than or equal to 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, Conclusion: 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.