Spontaneous shifts in the sinus node pacemaker complex.

Shifts in sinus node pacemaker complex may occur spontaneously, but occurrence of clinically relevant shifts is very rare. In this report, three patients (2 are siblings) with a history of palpitations and nearly permanent shifts in sinus node pacemaker complex are presented. Often, but not always, the pacemaker shifts followed spontaneous sinoatrial exit blocks. The shifts were probably related to varying vagal tone, since they were eliminated by atropine and exercise. The experience with these patients suggests that sinus pacemaker shifts can be a cause of symptomatic nonrespiratory sinus arrhythmia. A 4-year follow-up period showed no changes in symptoms or in heart rhythm; therefore, a benign course of the disease can be expected.