Single-chamber cardiac pacing with activity-initiated chronotropic response: evaluation by cardiopulmonary exercise testing.

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Abstract
In this study, sequential cardiopulmonary exercise testing was used to assess the physiologic benefits of a single-chamber ventricular pacing system that utilizeds a piezoceramic sensor to adjust heart rate by detecting "physical activity. An initial exercise test was conducted with the pacemaker was programmed (based on a randomization table) to either the fixed rate (VVI 70 beats/min) or rate-variable (VVI-Act) mode, and the results were compared with those obtained during a second exercise test in which the pacemaker was programmed to the alternate pacing mode. A 1.5 to 2 hr rest period was permitted between exercise tests, each of which consisted of a symptom-limited constant speed (3.0 mph) Balke protocol with 2 min stages commencing at 0.0% grade with increments of 2.5% at end of each stage. Compared with findings during fixed-rate VVI pacing, VVI-Act pacing was associated with (1) greater exercise-induced positive chronotropic response (mean maximum heart rate VVI-Act 128 .+-. 15.3 beats/min vs VVI 90 .+-. 28.4 beats/min; p < .01), (2) prolongation of exercise duration (VVI-Act 10.2 .+-. 3.8 min vs VVI 7.7 .+-. 2.5 min; p < .01), (3) increased peak oxygen consumption (VVI-Act 1617 .+-. 656 ml O2/min vs VVI 1325 .+-. 451 ml O2/min; p < .01), and (4) onset of anaerobic threshold at a higher oxygen consumption (VVI-Act 1208 .+-. 343 ml O2/min vs. VVI 1064 .+-. 377 ml O2/min: p < .01). Additionally, of 44 comparable exercise stages tested in the two pacing modes, perceived exertion (assessed by a numerical grading system) was lower in 38 of 44 instances during VVI-Act compared with VVI pacing. Thus provision of chronotropic response during exercise by single chamber rate-variable ventricular pacing substantially improved exercise tolerance and therefore may be for patients with bradycardia-related symptoms in whom associated sinoatrial disease or difficulty establishing or maintaining atrial sensing preclude optimum function of conventional atrial-tracking pacing systems.