Failure of Quintech Tx pacemaker caused by loss of stimulus-T interval shortening during exercise

A 74 year old man with a Quintech Tx pacemaker experienced a reduction in his exercise capacity because his heart rate failed to increase with effort. This was shown to be caused by the failure of the interval between the delivered stimulus and evoked T wave to decrease during effort where previously it had done so. Although the stimulus-T interval did not change during exercise, it did decrease in response to isoprenaline infusion and in response to pacing the heart at increasing rates. Catecholamine concentrations increased during exercise and the patient had normal thyroid function.