The relationship between QT interval and heart rate during physiological exercise and pacing.

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Abstract
Conventional doctrine states that the QT interval is related to heart rate in an inverse exponential relationship, so that with increasing rate the QT interval shortens. This relationship was studied in a group of patients undergoing physiological exercise stress tests, atrial pacing stress test and, in a further group of patients, with complete heart block undergoing exercise at a fixed ventricular rate controlled by cardiac pacemaker. Examinations of recordings made during physiological exercise do show the expected shortening in QT interval; this shortening is probably due in part to the intrinsic effect of increased rate. Patients who were atrially paced to similar rates and within the same age group showed only a small decrease in measured QT interval, and patients undergoing exercise at fixed ventricular rate showed shortening in QT interval which was related to the independent atrial rate. It appears that the QT interval is governed mainly by extrinsic factors and not intrinsically rate-related. The physiological control of QT interval is being used now to construct a cardiac pacemaker which senses the interval between the delivered stimulus and the evoked T wave, so that the stimulus-evoked T wave interval could be used to set the subsequent escape interval and, subsequently, the overall pacing rate. Physiological control of cardiac pacing rate using conventional unipolar lead systems and independent of atrial activity is possible and currently being investigated.

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