Dual chamber pacemakers: upper rate behavior.

Upper rate management of a dual chamber pacemaker requires sensing of the atrium and limitation of ventricular response to desired levels. Limitation of ventricular rate response is accomplished by setting atrial channel refractoriness which consists of two separate and continuous intervals, the atrioventricular interval (AV interval) and the atrial refractory interval, after the ventricular pace or sense event (AR interval). A P-wave that falls within the total atrial refractory interval (TARI) remains unsensed and one that falls beyond the TARI is sensed. If the upper rate limit interval (URI) is programmed to equal the TARI, the upper rate limit will occur by development of sudden AV block. If the URI is of greater duration (lower allowed rate) than the TARI, the difference in interval between the two is the Wenckebach interval (WI) and is the duration of a response plateau, when the atrial coupling interval is less than the upper rate interval. All dual chamber timing cycles can be interpreted in terms of atrial refractoriness and upper rate limitation with consideration of the lower rate interval, and the ventricular refractory and blanking intervals.