Pure atrial-based lower rate timing of dual chamber pacemakers: implications for upper rate limitation.

The release of dual chamber pulse generators with atrial-based lower rate timing has added a new dimension to the complexity of pacemaker function and electrocardiography. This discussion focuses on the concept that in pulse generators with pure atrial-based lower rate timing, preservation of the atrial lower rate interval takes hierarchical precedence over all other timing intervals including the ventricular-based upper rate interval. Under certain circumstances whenever upper rate limitation requires extension of the programmed AV interval, a DDD pulse generator with pure atrial-based lower rate timing can violate its atrial-driven upper rate interval to provide constancy of the lower rate interval. Such behavior also has important implications for upper rate control in devices that function in the DDI (DDIR) or DDDR mode with pure atrial-based lower rate timing.