Repetitive sinoatrial exit block as the major mechanism of drug-provoked long sinus or atrial pause

Prolonged sinus or atrial pause occurred in six patients with paroxysmal supraventricular tachycardia after drug administration. All six patients had normal sinus node function during control electrophyslologic study; the sinus cycle length ranged from 510 to 900 ms (mean 743 ± 141) and the longest sinus node recovery time ranged from 800 to 1,230 ms (mean 1,018 ± 168). A long sinus or atrial pause occurring at the termination of tachycardia or cessation of atrial pacing, ranging from 3,100 to 8,200 ms (mean 6,270 ± 1,674), was provoked by the administration of various drugs. These included an intravenous bolus injection of adenosine triphosphate (5 mg; one patient), intravenous bolus injection of verapamil (5 mg; one patient), a combination of a single oral dose of diltiazem (120 mg) and propranolol (20 to 40 mg; three patients), oral diltiazem (240 mg/day; one patient) and a combination of oral diltiazem (240 mg/day) and propranolol (160 mg/day; one patient). In five patients, low frequency deflections suggestive of sinus node activity with a cycle length between 620 and 3,500 ms were recorded during pauses. These findings suggest that repetitive sinoatrial exit block was responsible for the pause. Sinus slowing with a long arrest suggesting suppression of sinus automaticity was also noted in three of these five patients; the longest sinus arrest in these three patients was 4,160, 4,800 and > 4,910 ms, respectively. The remaining patient with a pause of 6,840 ms had no recordable sinus activity, either reflecting suppression of sinus automaticity or technical failure. In conclusion, this study suggests that both sinoatrial exit block and suppression of sinus automaticity are mechanisms for drug-provoked long sinus or atrial pauses.