Limitations of Rate Response of an Activity- Sensing Rate-Responsive Pacemaker to Different Forms of Activity CHU-PAK LAU, DAVENDRA MEHTA, WILLIAM D. TOFF, ROLLIN J. STOTT, DAVID E. WARD, and A. JOHN CAMM From the Department of Cardiovascular Sciences, St. George's Hospital, London, England LAU, C.-P., ET AL.: Limitations of rate response of an activity-sensing rate-responsive pacemaker to different forms of activity. The responses of an activity-sensing rate-responsive system fActivitrax) to various forms o/physioJogicaJ activity were assessed in 15 individuals who had this pacemaker. Nine ivere patients with complete heart block and atrial arrhythmias; their mean age was 60 years (range, 41-85 years). Six were age-matched healthy volunteers who were exercised with an external Activitrax system attached firmJy to the chest wall. The pacemaker was programmed to achieve a pacing rate of about 100 bpm at the end of the first stage of the Bruce protocol (pacemaker settings; rate = 70-150 bpm; threshold = low to medium; response = 6-9). In the activity-sensing ventricular pacing mode, oil patients achieved a significant increase in treadmill time compared to constant-rate ventricular pacing (mean ± SD, 8.0 ± 3.3 vs 5.4 ± 2.3 minutes; p < 0.01], with a mean maximum pacing rate of 123 ± 18 bpm. Jogging in place produced a prompt increase in pacing rate, with the maximum achieved at the end of the exercise. However, physiological activities such as hand-grip, the Valsalva maneuver and standing resulted in only minimal rate response. Pacing rate after ascending 4 flights of stairs was the same as that achieved after descending the same stairs (100 ± 8 vs 105 ± 4 bpm; p ^ 0.1). All 15 subjects were exercised from resting heart rate for 3 minutes on a treadmill at 1.2 mphand2.5 mph with four gradients at each speed. Although the pacing rate increased with a faster treadmill speed (p < 0.005), it did not respond appropriately to a change in gradient compared to the sinus rate. We conclude that although activity-sensing rate-responsive pacing gives a prompt increase in pacing rate and improves maximum exercise tolerance, further refine- ment is necessary because: (I) physiological activities not associated with significant movement are not detected by this pacing system; (2) detection of vibrations as an indicator of activities does not correlate well with the level of exertion. (PACE, Vol. 11. February 1988] rate-responsive pacing, activity sensing Introduction The activity-sensing rate-responsive pace- maker (Activitrax*) incorporates a piezo-electric crystal bonded to the inside of the pacemaker can.^ Physical activities which produce body vi- brations deform the crystal in a manner similar to Address for reprints: Dr. Chu-pak Lau, Department of Cardio- vascular Sciences, St. George's Hospital, Tooting, London SW17 ORE. England. Received January 2, 1987; revision received February 13, 1987; accepted February 17, 1987. • Medtronic Inc., Minneapolis, USA. that of an accelerometer.^ Vibrations with ampli- tudes above a programmable threshold will be registered as "counts" which are used in an algo- rithm to determine the appropriate pacing rate. In order to detect muscle activities propagated from the extremities, the sensor is highly sensitive and only very low strength signals are ignored. In this way, the "occurrence" of an activity has a greater impact than the "strength" ofthe activity.^ Activ- ity-sensing rate-responsive pacing results in sig- nificant improvements in cardiac output and an- aerobic threshold during graded treadmill exer- cises.^ In the present study, we investigated the response of this pacing system to different forms PACE, Vol. 11 February 1988 141