Cumulative effect of preceding pacing cycle length on ventricular repolarization after a pause.

This study was designed to investigate if the components of the QT interval after a pause are influenced by the preceding pacing cycle length. Ten patients (seven women and three men; age 79 +/- 9 years, means +/- SD) with complete atrioventricular block or sick sinus syndrome whose own heart rate was < 40 beats/min were examined. All patients had already undergone implantation of a permanent pacemaker. Ventricular pacing protocol was performed with simultaneous recording of a 12-lead electrocardiogram. One set of regular stimuli for 30 seconds (S1) with a variable cycle length (1,000, 700, and 400 ms) was followed by a single stimulus (S2) with a fixed coupling interval of 1,500 ms. QT intervals in response to the last S1 (S1-QT) and S2 (S2-QT) were measured. The QT interval was divided into two components, the interval from start of Q wave to the peak of T wave (QaT) and that from the peak to end of T wave (Tae). The S2-QT and S1-QT interval shortened in association with a decrease in the S1S1 interval. The abbreviation of S2-QT interval was not associated with a significant change in the Tae interval. The results demonstrated that the QT interval after a pause shortened by reducing the preceding pacing cycle length. This shortening is probably due to a homogenous abbreviation of action potential duration across the ventricular wall.