Variability of spontaneous and exercise-induced ventricular arrhythmias in the absence and presence of treatment with acebutolol or quinidine.

A randomized double-blind crossover trial was undertaken in 20 volunteers to evaluate the effects of acebutolol and sustained release quinidine sulfate. The patients had an average of 10 or more premature ventricular complexes/hour on two 24 hour electrocardiographic recordings, or 10 or more/min during two cycle stress tests, or any number of complex forms of ventricular ectopic activity on either test. The 24 hour recordings yielded greater detection of complex forms than did stress tests and manifested similar relative variability in frequency of ventricular ectopic beats. The extent of variability from hour to hour within the 24 hour monitoring periods tended to have an increasing relation with frequency of ectopic beat activity regardless of the presence or absence of treatment. Within-patient variability among periods with equivalent treatment status also tended to have such an increasing relation with frequency of ectopic beat activity. About 35 percent of the variation was among subjects, 20 percent among months within subjects, 20 percent among days in months and 25 percent among hours in days. Acebutolol, 300 mg three times daily, produced effective beta receptor blockade and was better tolerated than sustained release quinidine sulfate in identical doses and had equal suppressant effects. The results of the variability studies provide guidelines for the design of adequate clinical trials testing suppressant interventions.