Understanding upper rate responses of DDD pacers.

Understanding upper rate responses should not be a frustrating experience. A thorough working knowledge of DDD pacer timing cycles is recommended before attempting to assimilate this information. Anyone involved in the interpretation and troubleshooting of DDD pacer ECGs should be familiar with these four basic upper rate responses. As shown in Table I, these responses are used to limit the upper ventricular tracking rate in various pacer models either as a single option or in combination. The operation of combined upper rate responses can be difficult to interpret. If the initial acceleration of the atrial rate is available from a Holter monitor or a standard electrocardiographic recording, the fallback response can be easily detected as the transition from pacing at the upper rate limit to the fallback rate. When this transition is not observed and the timing cycle indicates a Wenckebach-type behavior at a ventricular-paced rate of 80 pulses/min, the fallback response can be suspected because programming an upper rate limit of 80 pulses/min would be unusual. Rate smoothing is more difficult to detect when the tachycardia is accelerating. Wenckebach behavior may be observed as the ventricular pacing rate increases according to the smoothing constant. Rate smoothing can be suspected if Wenckebach behavior is observed as the ventricular pacing rate increases from cycle to cycle. Fig. 1 attempts to clarify similar characteristics of the four upper rate responses. It also provides rudimentary differences in characteristics of the four responses and guidelines for interpretation. Furthermore, it should be used as a quick reference to determine the criteria for each response.(ABSTRACT TRUNCATED AT 250 WORDS)