Auricchio A, Auricchio U, Chiariello L. Universita di Roma Tor Vergata. G Ital Cardiol 1994 Feb;24(2):131-6. We have prospectively evaluated the reversibility of the protective effect of propafenone (P) and flecainide (F) on atrial fibrillation (AFib) inducibility in 31 patients with a symptomatic accessory atrioventricular connection during exercise. Each patient underwent repeated transesophageal stimulation and exercise testing after a drug-free week, or after a week of P (900 mg/daily), or F (300 mg/daily) given orally. The end point of the transesophageal stimulation was the induction of AFib. Because no AFib could be induced both at rest and at peak exercise, P or F was considered effective in 5 patients (16%) and 4 patients (13%), respectively. In contrast, drug therapy with P was totally ineffective in 10 patients (32%), while F was completely unable to control AFib induction in 14 patients (45%). A partial efficacy of antiarrhythmic drug therapy, in terms of AFib inducibility only at peak exercise, was achieved in 16 patients (52%) treated with P, and in 13 patients (40%) with F. The comparison of the mean R-R intervals during AFib between patients considered partial responders to drug therapy (AFib inducibility only at peak exercise) and those who completely failed any drug therapy did not show significant differences both at rest or during exercise. No correlation in the plasma concentration of P or F and AFib inducibility or duration was found. Thus, the data of this study shows that the reversible protective effect of IC antiarrhythmic class drugs by exercise seems related to inappropriate control on the substrate for AFib inducibility, and there exists a partially discrepant antiarrhythmic effect between the heart rate control during AFib and the inducibility of AFib.