ASSESSMENT OF RATE SMOOTHING IN DUAL-CHAMBER PACEMAKERS

Dual-chamber pacemakers help preserve atrioventricular (AV) synchrony during increasing heart rates1-3 and demonstrate improved hemodynamic parameters compared with those of single-chamber units.4-8 However, abrupt reductions in the ventricular paced rate occur frequently during exercise as the upper rate limit is exceeded. Electronic Wenckebach sequences, and 2:1 and higher degrees of AV block occur as the atrial intervals become progressively shorter than the total atrial refractory period. The ventricular paced rate may decrease to the lower rate limit, fluctuate wildly and often result in severe symptoms, because these rate shifts tend to occur when physiologic stress and metabolic demands on the patient are at a peak.9 A rate-smoothing function controls the allowed heart cycle variation in the ventricles, and thus avoids abrupt rate change. To determine the physiologic and clinical behavior accompanying the use of this function, we investigated its safety and effectiveness during maximal exercise testing, and compared its effects on exercise capacity and the subjective reasons for stress discontinuance.