Increased ventricular fibrillation threshold with severe myocardial ischemia.

The ventricular fibrillation threshold (VFT) is a measure of myocardial electrical vulnerability to exogenous electrical stimulation. Previous studies have shown that the VFT is inversely related to ischemia. We studied the relation of the VFT to myocardial blood flow (MBF) during ischemia produced by interruption of blood flow to the left anterior descending coronary artery, and severe ischemia produced by retrograde bleeding an ischemic segment of myocardium. The VFT with severe ischemia (8.6 +/- 1.3 mA; MBF 0.024 +/- 0.01 ml/min/gm tissue), although lower than nonischemic control values (14.2 +/- 2.0 mA; 0.76 +/- 0.05; both p less than 0.05), was higher than that obtained with moderate ischemia (4.6 +/- 0.9 mA; 0.15 +/- 0.02 ml/min/gm; both, p less than 0.05). Thus the relationship between the VFT and MBF is nonlinear. Interventions which cause the VFT to rise may do so by worsening rather than improving regional MBF.