To assess the electrophysiologic effects of acute hemodynamic improvement in patients with left ventricular systolic dysfunction, 12 patients with a left ventricular ejection fraction less than 0.40 and a history of sustained monomorphic ventricular tachycardia were studied. All patients had underlying coronary artery disease. Patients underwent programmed cardiac stimulation in random order during a baseline period and with nitroprusside infusion. Mean pulmonary capillary wedge pressure decreased from 20 +/- 8 mm Hg at baseline study to 8 +/- 3 mm Hg during nitroprusside infusion (p less than 0.0001). Pulmonary artery, right atrial and systemic arterial pressures also decreased with nitroprusside (p less than 0.01). Cardiac output did not change. Left ventricular dimensions, determined by two-dimensional echocardiography, decreased significantly during nitroprusside infusion. The right ventricular effective refractory period, measured during ventricular drive trains at cycle lengths of 400 and 600 ms, were similar during baseline and nitroprusside periods (271 +/- 30 versus 274 +/- 31 ms at 600 ms, and 249 +/- 25 versus 246 +/- 18 ms at 400 ms). In 2 patients no ventricular arrhythmias were induced during either study period; in the other 10, ventricular tachyarrhythmias were induced during both periods. The mean number of extrastimuli required to induce a ventricular tachyarrhythmia was similar during the baseline period (1.8 +/- 0.6) and during nitroprusside infusion (1.9 +/- 0.7). As well, the mean cycle length of ventricular tachycardia induced was similar during the baseline period (347 +/- 61 ms) and during nitroprusside infusion (342 +/- 70 ms).(ABSTRACT TRUNCATED AT 250 WORDS)