Chronotropic incompetence is one cause of diminished exercise capacity in heart transplant recipients. If reinnervation occurs, it often is late after transplantation and is not always accompanied by functional improvements in peak heart rate and appropriate tachycardia during exercise. To determine the efficacy of rate-responsive pacing on peak heart rate and exercise capacity, the authors studied eight male heart transplant recipients (age 57 ± 12 years; 23 ± 9 months after transplantation) that had either atrial or dual-chambered pacemakers.
All subjects completed two maximal graded exercise tests (GXT) using the Naughton treadmill protocol. During the first GXT, pacemakers were programmed for bradycardia support only and without rate responsiveness (unpaced). After a 14-day regimen of beta blockade with metoprolol to nullify the influence of circulating catecholamines on heart rate, subjects performed the second GXT with pacemakers programmed to respond optimally in the rate-responsive mode (paced).
Peak heart rate (149 versus 129 bpm), peak oxygen uptake (18.9 versus 15.4 mL/kg/min), treadmill time to exhaustion (14.6 versus 12.4 min), and minute ventilation (76.7 versus 66.2 L/min) were significantly increased (P ≤ 0.05) during the paced versus unpaced GXT.
The results of this study demonstrate that chronotropic support of the transplanted heart using a rate-responsive pacemaker, with activity-based sensors programmed for maximal sensitivity, improves both peak heart rate and exercise capacity in heart transplant recipients significantly more than circulating catecholamines alone.