The case of the two-timing pacemaker.

A 71-year-old man with a permanent transvenous pacemaker had recent history of lightheadedness and was brought to the hospital emergency room complaining of shortness of breath. His blood pressure was 80/50, pulse rate 148. Administration of a pressor drug restored blood pressure and coincidentally the heart rate returned to 70. His condition was caused by formation of a dendritic spur in the substrate of the programming circuit in his implanted pacemaker, with consequent intermittent doubling of the impulse frequency. Another pacemaker was substituted surgically, and he recovered uneventfully.