Few studies have dealt with the effects of
isoproterenol on ventricular
parasystole. Intravenous
isoproterenol (2 to 4 micrograms/min) was administered to 11 nonmedicated patients with ventricular
parasystole. At the onset of the
drip infusion, 8 patients had continuous
parasystole, 2 had intermittent
parasystole, and 1 patient (in whom intermittent
parasystole was documented 2 to 5 days earlier) showed no manifest parasystolic activity. In all patients, whose control parasystolic cycle length varied between 960 and 2,530 ms,
isoproterenol caused a decrease of the parasystolic cycle lengths ranging from 12 to 36%. Therefore,
isoproterenol produced a consistent increase of the parasystolic rate. In 4 patients, parasystolic activity ceased to be manifest when the concomitantly enhanced (by
isoproterenol) sinus cycle lengths became shorter than 430 ms. This phenomenon reflected a
tachycardia-dependent parasystolic concealment, presumably as a result of interference in the parasystolic-ventricular junction. In every case, the
arrhythmia reappeared at its initial rate upon stopping the
drip infusion. In no patient did parasystolic
ventricular tachycardia develop. In the patient without manifest parasystolic beats,
isoproterenol unmasked the intermittent
parasystole that previously had been intrinsically manifest. The latter effect reflected a true exposure, or unmasking of a latent, rate-independent concealed, parasystolic focus.(ABSTRACT TRUNCATED AT 250 WORDS)