Comparison of Isoproterenol and Exercise Tests in Asymptomatic Subjects with Wolff-Parkinson-White Syndrome

In order to evaluate the effects of increases of sympathetic tone in ventricular response during atrial fibrillation and in the relationship between the accessory pathway effective refractory period (ERP) and ventricular rate during atria] fibrillation, 20 male subjects, aged 19 ± 6 years, were studied electrophysiologically in basal conditions, after isoproterenol infusion (2–4 μ/min) and during submaximal bicycle exercise test, at a constant workload equal to that which increases the sinus rate to the same extent (140 beats/min) induced by isoproterenol infusion. Accessory pathway ERP was evaluated at the same driven rate (150 beats/min) in both instances. In the control study as during both tests atrial fibrillation paroxysms were induced by burst stimulation. In control conditions the rate increase from 100 to 150 beats/min induced a reduction of accessory pathway ERP from 266 ± 27 msec to 244 ± 22 msec (P < 0.005). At the same driven rate of 150 beats/min, isoproterenol infusion and exercise test induced a more marked shortening of accessory pathway ERP to 211 ± 28 msec (P < 0.005) and to 214 ± 29 msec (P < 0.005), respectively. Atrial fibrillation paroxysms lasting more than 10 seconds were induced in 20/20 cases in the control study, in 15/20 during isoproterenol infusion and in 13/19 cases during exercise test. The shortest cycle length during atrial fibrillation was reduced from a basal value of 253 ± 72 msec to 204 ± 27 msec (P < 0.05) during isoproterenol infusion and to 236 ± 32 msec (NS) during exercise test. Similar behavior was observed in the mean cycle length during atrial fibrillation (from 372 ± 67 to 291 ± 36 and to 329 ± 70 msec, respectively). In conclusion, isoproterenol and exercise test, when exerting the same effect on sinus rate, induce the same accessory pathway ERP decrease, mainly due to a direct electrophysiological effect, which overcomes that secondary to increase in heart rate. Isoproterenol reduces the shortest and mean RR intervals during atrial fibrillation more than exercise test, probably because other determinants in addition to the accessory pathway ERP may influence heart rate during atrial fibrillation. The increase of sympathetic tone seems to reduce atrial fibrillation electrical inducibility.