PubMed1993
To determine if rate adaptation of the atrioventricular (AV) delay (i.e., linearly decreasing the AV interval for increasing sinus rate) improves exercise left ventricular systolic hemodynamics, we performed paired maximal semi-upright bicycle exercise tests (EXTs) on 14 chronotropically competent patients with dual chamber pacemakers. Nine patients with complete AV block (CAVB) and total ventricular pacing dependence during exercise comprised the experimental group. Pacemakers in these patients were programmed randomly to rate adaptive AV delay (AVDR) for one EXT and fixed AV delay (AVDF) for the other EXT. AVDF was 156 msec; AVDR decreased linearly from 156-63 msec from rates of 78-142 beats/min. The other five patients had intact AV conduction and comprised the control group who were exercised in identical fashion while their pacemakers were inhibited throughout exercise to assure reproducibility of hemodynamic measurements between EXTs. Cardiac hemodynamics were calculated using measured Doppler echocardiographic systolic aortic valve flows recorded suprasternally with an independent 2-MHz Doppler transducer during a graded ramp exercise protocol. For analysis, exercise was divided into four phases to compare Doppler measurements at submaximal and maximal levels of exercise: rest, early exercise (1st stage), late exercise (stage preceding peak), and peak. Patients achieved statistically similar heart rates between EXTs at each phase of exercise. Although at lower levels of exercise cardiac hemodynamics did not differ, experimental patients (with CAVB) showed a statistically significant benefit to cardiac output at peak exercise with heart rates of 129 +/- 13 beats/min (AVDR: 9.4 +/- 2.8 L/min; AVDF: 8.2 +/- 2.6 L/min, P = 0.002), stroke volume (AVDR: 74.1 +/- 25.6 mL; AVDF: 64.3 +/- 24.4 mL, P = 0.0003), and aortic ejection time (AVDR: 253.3 +/- 35.7 msec; AVDF: 226.7 +/- 35.0 msec, P = 0.002). Duration of exercise, peak rate pressure product, peak aortic flow velocities, and acceleration times did not differ. In contrast, control group patients (intact AV conduction throughout exercise) showed no statistical differences between any hemodynamic parameters measured at any phase of exercise from the first to second exercise test. These data demonstrate that systolic cardiac hemodynamics measured echocardiographically at the high heart rates achieved with peak exercise are improved with AVDR compared to AVDF in chronotropically competent patients with complete AV block. This is due primarily to improved stroke volume and a longer systolic ejection time with AV delay rate adaptation.