Diurnal variation of QT interval in patients with VVI pacemaker.

In nine patients with chronically implanted ventricular pacemaker (VVI), diurnal variation of the QT interval was studied. The mean age of the patients was 74.2 years and underlying diseases were atrioventricular block (three patients), sick sinus syndrome (four patients) and chronic atrial fibrillation with bradycardia and heart failure (two patients). Eight of the nine patients showed longer QT intervals during sleep than during waking. Only one patient showed no change in QT interval during sleep and during wake time. The mean QT interval of the nine patients at each hour was slightly longer (9 msec) during sleep than that during each awake hour (p less than 0.001). The prolongation of QT interval during sleep may offer a possible mechanism of some nocturnal arrhythmias.