Alternating sinus rhythm and intermittent sinoatrial block induced by propranolol

Alternating sinus rhythm and intermittent sinoatrial (S-A) block was observed in a 57-year-old woman, under treatment for angina with 80 mg propranolol daily. The electrocardiogram showed alternation of long and short P-P intervals and occasional pauses. These pauses were always preceded by the short P-P intervals and were usually followed by one or two P-P intervals of 0.92-0.95 s representing the basic sinus cycle. Following these basic sinus cycles, alternating rhythm started with the longer P-P interval. The long P-P intervals ranged between 1.04-1.12 s and the short P-P intervals between 0.80-0.84 s, respectively. The duration of the pauses were equal or almost equal to one short plus one long P-P interval or to twice the basic sinus cycle. In one recording a short period of regular sinus rhythm with intermittent 2/1 S-A block was observed. This short period of sinus rhythm was interrupted by sudden prolongation of the P-P interval starting the alternative rhythm. There were small changes in the shape of the P waves and P-R intervals. S-A conduction through two pathways, the first with 2/1 block the second having 0.12-0.14 s longer conduction time and with occasional 2/1 block was proposed for the explanation of the alternating P-P interval and other electrocardiographic features seen. Atropine 1 mg given intravenously resulted in shortening of all P-P intervals without changing the rhythm. The abnormal rhythm disappeared with the withdrawal of propranolol and when the drug was restarted a 2/1 S-A block was seen. This was accepted as evidence for propranolol being the cause of this conduction disorder.