Decreased incidence of antiarrhythmic drug efficacy at electrophysiologic study associated with the use of a third extrastimulus.

Drug trials during electrophysiologic study were performed in 166 patients with sustained ventricular tachycardia (VT) or ventricular fibrillation. In the first 21 patients (group I), a maximum of two extrastimuli was used. In 145 subsequent patients (group II), drug efficacy was assessed using three extrastimuli if one or two failed to induce VT. The incidence of acute drug efficacy was 15 of 21 (71%) in group I, but only 44 of 145 (30%) in group II (p less than 0.01). To assess the influence of the third extrastimulus on the differing incidence of acute drug efficacy, we examined the 69 group II patients whose VT was initially induced by one or two extrastimuli. Acutely effective agents were identified for 19 of these 69 patients using up to three extrastimuli to define efficacy. In 24 of te 50 patients for whom no effective drug could be found, VT was induced with one or two extrastimuli during all drug trials. In the remaining 26 patients VT could be induced after drug administration only with use of three extrastimuli. If efficacy had been defined using a maximum of two extrastimuli, these 26 patients would have been considered responsive to drug testing. Overall, 45 to 69 (65%) rather than 19 of 69 (28%) would have had a favorable response (p less than 0.001). Fourteen group I patients and 17 group II patients in whom VT was initially induced by one or two extrastimuli received chronic therapy with agents predicted effective during acute drug trials. At 18 months, the cumulative percent of patients free of arrhythmia recurrence by actuarial analysis was 79 +/- 14% in group I and 80 +/- 14% in group II (NS). Use of three extrastimuli during drug trials is associated with a lower incidence of acute drug efficacy. Limited data suggest that prophylaxis against VT induction by three, rather than two, extrastimuli may not be a stronger predictor of chronic efficacy in patients whose VT is initially induced by one or two extrastimuli. Further study is needed to identify optimal pacing methods for antiarrhythmic drug assessment.