Chronotropic Response of an Activity Detecting Pacemaker Compared with the Normal Sinus Node

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Abstract
We compared the rate responsiveness of an activity‐detecting muitiprogrammable, single chamber pacemaker (Medtronic Acfivitrax) to rate responsiveness of the normal sinus node. This pacemaker changes its basic pacing rate in response to physical activity. The rate responsiveness is programmable by selecting one of three activity thresholds, and one of 10 rate response settings. The study included a group of six normal volunteers and 12 patients implanted with Activitrax to examine the similarity of the pacemaker rate to normal sinus rhythm during acceleration and deceleration. The pacemaker was set to Activity mode, at a basic rate of 60 bpm. In voiunteers, the device was externally secured on the chest wall and tested at two programmed settings. When programmed at a high threshold of activity and high rate response in voiunteers, there was no significant difference in maximum normal sinus rates and pacemaker rates during arm waving, jumping in place, and walking during stress testing. At a medium activity threshold, the only significant difference occurred during submaximal stess testing, when the maximum sinus rate achieved was 141 ± 19 bpm and the maximum pacing rate was 105 ± 8 bpm (p < .02). The pacemaker behaved in a similar manner in patients, successfully simulating the typical fast acceleration and slow deceleration of a normal sinus node in exercise testing. There was no difference in pacer response when implanted in abdominal or in/raclavicular locations. The implanted units have functioned normally over a follow‐up period of nine to 22 months. Activitrax can be programmed to achieve physiologic pacing rates in response to normal daiiy activities with appropriate programming.

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