[Shortening the stimulus T time: indicator for physical activity in patients with demand modulated cardiac pacemakers].

Activity-modulated pacemakers detect body vibrations and increase their pacing rate. As the body vibrations are not always related to physical activity, we studied in 20 patients with the activity-modulated pacemaker Legend (Medtronic) whether a second physiological signal, the paced QT duration, can help to verify exercise. At rest, pacing rate was programmed from 60 ppm to 130 ppm in steps of 10 ppm which shortened the QT duration from 467 +/- 32 ms (60 ppm) to 391 +/- 24 ms (130 ppm). During treadmill exercise, pacing rate increased from 62 +/- 1 ppm to 98 +/- 9 ppm and the QT duration decreased from 450 +/- 25 ms to 372 +/- 27 ms. Compared to rest, the exercise-induced QT shortening was significantly shorter for pacing rates > or = 70 ppm. To simulate artificial sensor activation, an electrical razor was put onto the pulse generator. Pacing rate increased to 81 +/- 7 ppm, but the QT duration shortening was similar to the measurement at rest with 80 ppm. In conclusion, the paced QT duration is able to confirm whether body vibrations are related to physical exercise.