To evaluate the electrophysiologic changes in the cardiac conduction system that occur during percutaneous mitral or aortic balloon valvotomy, we prospectively studied the conduction system in 19 patients (10 mitral, 8 aortic, and 1 both) undergoing this procedure. A His bundle electrogram was recorded in all patients, and when sinus rhythm was present, the atrioventricular (AV) node effective refractory period was measured. Holter monitoring was performed during and for 24 hours after the procedure. Follow-up electrocardiograms (ECG) were available in 11 patients 2.3 +/- 1.5 months after the procedure. The AV node effective refractory period before (276 +/- 86 msec) and after valvotomy (298 +/- 85 msec) were not significantly different. The maximum His-Purkinje conduction time (HV interval) observed during valvotomy (66 +/- 20 msec) was significantly longer (p less than 0.01) than that measured before (57 +/- 10 msec) or after (60 +/- 18 msec) valvotomy. The mean HV intervals before and after valvotomy were not significantly different. The mean QRS complex duration increased from 95 +/- 28 to 112 +/- 28 msec during valvotomy and remained significantly prolonged (109 +/- 26 msec) 24 hours after the procedure (p less than 0.01). A new intraventricular conduction defect (QRS complex duration greater than 100 msec) or bundle branch block occurred in five of 13 patients who had normal QRS duration before the procedure. The change in HV interval did not correlate with the change in QRS complex duration. In four patients, the newly acquired intraventricular conduction defect was still present on follow-up ECG tracing. Complete heart block was not observed in any patient.(ABSTRACT TRUNCATED AT 250 WORDS)