Paradoxical early lengthening and subsequent linear shortening of the QT interval in response to exercise.

The QT responses to exercise during ventricular pacing were evaluated in 12 patients in whom permanent pacemakers were implanted for complete heart block. The initial response of the QT to exercise was a paradoxical lengthening during the first minute of exercise (mean 4.8 ms P < 0.01). Thereafter, the QT was found to shorten in a linear fashion in response to increasing exercise, both in terms of exercise duration (r = 0.787; P < 0.001) and atrial rate (r = 0.712; P < 0.001). The total QT shortening with exercise was small (20 ms for a mean increase in atrial rate of 60 beats.min-1) and displayed substantial inter-individual variability (9-31 ms). These results explain some of the limitations of the QT pacemaker, and provide insights in to the dynamics of the QT response, which may help tailor the programming of these systems to the individual patient.