Treadmill assessment of an activity-modulated pacemaker: the importance of individual programming

Maximum benefit from a rate-modulated pacemaker requires individualized programming of rate response settings. We tested an externally strapped activity-sensing pacemaker (Activitrax-Medtronic 8400) in eight healthy volunteers, to assess the pacing responses of the different rate response and activity threshold settings. Five males and three females, aged 20 to 70 years (mean 40), performed a total of 67 treadmill exercise tests, using a specific protocol designed to assess the activity-sensing unit. The external unit was compared to implanted units in four patients, to validate its accuracy. A reproducible sinus response to the treadmill protocol was observed, against which pacing responses were compared. The activity threshold determines the degree of activity required to elicit a pacing rate response, whereas the rate response setting determines the rate attained. Rates of 140 bpm were rarely achieved, despite vigorous exercise. The sensor responds rapidly to activity, not to physiologic demand; to increase in speed, not grade. Four patients performed repeated limited treadmill tests to determine their optimum program setting, with symptomatic status and the healthy volunteer sinus response as guides. These results, and those from the external Activitrax unit, suggest that LOW 6 and MEDIUM 6-10 settings will prove optimum for most patients.