Activity‐Sensing, Rate‐Responsive Pacing: Improvement in Myocardial Performance with Exercise

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Abstract
A sensor that detects body activity by low frequency sonic impulses has been incorporated in a pacemaker so that body activity may be translated to an increased pacing rate in response to exercise. The pacemaker is designed for patients who may benefit from an increased cardiac output mediated by an increased heart rate during exercise. Following permanent pacemaker implantation, six patients (mean age 69 years) entered a single blind, randomized, crossover trial for comparison of activity‐sensing, rate‐responsive pacing (A) to fixed rate demand pacing (D). Ventricular function was assessed by gated radionuclide ventriculography at rest and at exercise, while exercise capacity was assessed by treadmill performance, along with measurements of oxygen consumption and carbon dioxide production. Total treadmill duration and maximum oxygen consumption were similar in the two pacing modes (A = 284 ± 244 s, 13.4 ± 3.4 ml O2/min/kg; D = 256 ± 250 s, 11.7 ± 3.7 ml O2/min/kg). Anaerobic threshold, however, was significantly improved (A = 266 ± 199 s, (p<.05), 13.0 ± 2.2 ml O2/min/kg (p < .01); D = 231 ± 208 s, 10.8 ± 2.3 ml O2/min/kg). This improvement was associated with an enhanced exercise cardiac output (A = 4.51 ± 1.3 L/min/m2; D = 3.61 ± .84 L/min/m2; p < .05) which was mediated by an increased heart rate (A = 95 ± 12 bpm; D = 68 ± 2 bpm; p < .01), maintenance of stroke volume (A = 47 ± 10 ml/m2; D = 54 ± 14 ml/m2) and absence of cardiac dilatation (EDVI A = 109 ± 25 ml/m2; D = 120 ± 32 ml/m2). Holter monitoring revealed rare instances of unexpected heart rate increases secondary to external activation of the sensor with no significant adverse effects being encountered. We conclude that activity‐sensing, rate‐responsive pacing offers the potential for increased exercise tolerance secondary to improved cardiac function.