There had been no prospective, randomized studies that compared the accuracy of EPS versus HM in guiding long-term drug therapy for ventricular tachycardia or ventricular fibrillation. Success had been reported using both techniques. Using a rigorous ECG monitoring protocol in patients, a less than five percent per year incidence of sudden death had been reported. Several investigators reported that the results of electropharmacologic testing were predictive of clinical response. One of the largest studies, by Mason and Winkle, reported that, in 51 patients with recurrent ventricular tachycardia who were treated with drugs predicted to be effective based on the results of electropharmacologic testing, ventricular tachycardia did not recur in 68 percent at 18 months of follow-up. In contrast, ventricular tachycardia did not recur in only 11 percent of patients treated with drugs predicted to be ineffective.