Significant variability in the mode of ventricular tachycardia induction and its implications for interpretation of acute drug testing

Fifty-four patients with previous myocardial infarction and sustained ventricular tachycardia on fibrillation underwent two electrophysiologic studies in the drug-free state within 72 hours. Although the concordance of overall ventricular tachycardia induction over the 2 days was good (87% of patients), there was variability in the number of extrastimuii needed to induce sustained ventricular tachycardia on each day in 60% of patients. Of those in whom ventricular tachycardia was inducible on both days, 40% required additional extrastimuli and 20% required fewer extrastimuli. A change by two or more extrastimuli was found in 12% of patients. There was no correlation between the variability observed and multiple clinical and laboratory parameters (including the aggressiveness of the stimulation protocol); however, the direction of the variability (easier or harder to induce) correlated with changes in ventricular refractoriness. Inherent day-to-day variability may affect the reproducibility of electrophysiologic studies and influence the results of serial drug testing.