Symptomatic sick sinus syndrome due to guanethidine. Case report.

A patient is described who developed symptomatic sinus bradycardia as low as 20 beats per minute and sinus arrest of up to 4.4 seconds while receiving guanethidine, 75 mg daily. The bradycardia resolved following discontinuation of the drug and reappeared upon challenge with it. Intrinsic disease of the sinoatrial and atrioventricular nodes was evidenced 3 weeks following discontinuation of the guanethidine by a borderline abnormally prolonged sinus node recovery time of 1500 msec and a PR interval of 0.28 seconds. Although sinus bradycardia is a known and not infrequent side effect of guanethidine, such an extreme form as seen in our patient appears to be quite rare, and may be related to the pre-existing disease of the conduction system.