Maintenance of exercise stroke volume during ventricular versus atrial synchronous pacing: role of contractility.

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Abstract
Although atrial synchronous and rate-responsive ventricular pacing have been compared, the importance of maintaining synchronized atrial systole in addition to rate responsiveness has been incompletely defined. That is, the effects of these two pacing modes on cardiac volumes and contractility have not been studied. Accordingly, 16 patients with normal ventricular function were studied while in the upright position and at rest with gated radionuclide ventriculography during both atrial synchronous and ventricular pacing. Twelve of these patients were also studied during low-level upright exercise (300 kilopond-meters). Rest and exercise ventricular pacing heart rates were matched to those recorded with synchronous pacing. Ventricular volumes were determined with a counts-based method. The ejection fraction and peak systolic pressure/end-systolic volume ratio were used as measures of contractility. At rest there were no significant differences in either volumes or contractility between the two pacing modes. However, during exercise to identical heart rates, blood pressures, and workloads, although stroke volume was the same during exercise with atrial synchronous and ventricular pacing (78 .+-. 13 vs 75 .+-. 12 ml), end-diastolic and end-systolic volumes were lower with ventricular pacing than with atrial synchronous pacing (end-diastolic volume 101 .+-. 13 vs 113 .+-. 16 ml, p < .001; end-systolic volume 26 .+-. 4 vs 35 .+-. 7 ml, p < .001). Stroke volume during ventricular paced exercise was maintained at atrial synchronous pacing levels by means of increased contractility (ejection fraction of 74 .+-. 4% during ventricular pacing vs 69 .+-. 5% during atrial synchronous pacing, p = .002; peak systolic pressure/end-systolic volume ratio of 6.51 .+-. 1 during ventricular pacing vs 4.85 .+-. 1 during atrial synchronous pacing, p < .001). Thus, during upright bicycle exercise to workloads corresponding to those of usual daily activities, rate-responsive atrial synchronous pacing results in enhanced ventricular filling and spares contractile reserve when compared with rate-responsive ventricular pacing.