[Torsades de pointes initiated by slow ventricular stimulation]

The authors report a case of isolated and probably congenital complete heart block with spontaneous torsades de pointes preceded by an increase in the QT interval. During positioning of a temporary pacing catheter mechanically induced extrasystoles were observed, which seem to favor ventricular extrasystoles and unsustained runs of torsades de pointes. When the basal rhythm showed no extrasystoles, electrical stimulation delivered late in diastole captured the ventricle normally and was then followed in reproducible fashion by episodes of torsades de pointes or minor equivalents. The mechanism of this phenomenon is thought to be related to a concealed phase 4 intramyocardial block. It may have important practical consequences in paced patients predisposed to torsades de pointes. If the pacemaker is programmed at too slow a rate, with stimulation falling late in diastole, torsades de pointes could occur, some attacks of which are known to degenerate to true ventricular fibrillation.