Evaluation by exercise Doppler echocardiography of maintenance of cardiac output during ventricular pacing with or without chronotropic response.

To examine the effectiveness of activity-initiated rate-responsive pacing, this study assessed the increases in stroke volume and cardiac output during randomized treadmill exercise in rate-responsive and fixed-rate ventricular (VVI) pacing in 10 patients. Stroke volume index and cardiac index were determined by suprasternal Doppler measurements. Compared with the findings during VVI pacing, the rate-responsive pacing was associated with (1) prolongation of exercise duration (8.0 +/- 4.0 vs 7.3 +/- 3.6 minutes, p less than 0.05); (2) greater exercise-induced positive chronotropic response (mean maximal heart rate 127 +/- 12 vs 78 +/- 15 beats/min, p less than 0.001); (3) smaller increase in stroke volume index (38 +/- 10 vs 50 +/- 11 ml/m2, p less than 0.001), and (4) greater increase in cardiac index (4.7 +/- 1.1 vs 3.9 +/- 1.0 liters/min/m2, p less than 0.001). A significant correlation was observed between age and percent increase in stroke volume index during VVI pacing (p less than 0.05). These findings indicate that VVI pacing increased stroke volume more than did rate-responsive pacing, especially in younger patients, but the increase in cardiac output was less than that seen with rate-responsive pacing due to the absence of chronotropic response. Accordingly, an activity-sensing, rate-responsive pacemaker can effectively increase the heart rate, significantly augment cardiac output and extend the duration of exercise.