Effect of i.v. and oral tiapamil in the treatment of paroxysmal supraventricular tachycardia.

Eleven patients, with recurrent paroxysmal supraventricular tachycardias (PSVT) underwent programmed electrical stimulation of the heart to evaluate the efficacy of intravenous (i.v.) and oral tiapamil in these arrhythmias. The tachycardia circuit was confined to the atrio-ventricular (AV) node in 8 cases (group 1), and involved an overt or concealed accessory pathway for retrograde conduction in the 3 others (group 2). An i.v. bolus containing 2 mg kg-1 body weight of tiapamil converted 6 of the 8 cases in group 1 from PSVT to normal sinus rhythm (NSR) within a few seconds, but failed to do so in any of the three patients in group 2. Both the oral and i.v. preparations significantly lengthened the shortest cycle length maintaining 1:1 AV conduction. I.v. administration also lengthened the corresponding value for 1:1 VA retrograde conduction. The oral and i.v. preparations were equally effective in preventing induction of PSVT in 6/8 patients in group 1, but failed in group 2. The efficacy of oral tiapamil in preventing such induction was also accurately predicted from the effect of the i.v. tiapamil on conversion to NSR. After the first week, the six successfully treated patients continued taking 1.2 to 1.5 g day-1 tiapamil for 9.7 +/- 4 months. Complete suppression of their symptoms was observed in 5 cases.