DIFFERENCES IN CORRECTED QT INTERVALS AT MINIMAL AND MAXIMAL HEART-RATE MAY IDENTIFY PATIENTS AT RISK FOR TORSADES-DE-POINTES DURING TREATMENT WITH ANTIARRHYTHMIC DRUGS

The mechanism of torsades de pointes as a proarrhythmic response to an tiarrhythmic drugs is not clear. We hypothesized that the difference i n the corrected QT interval (QT(c), Bazett's formula) with varying aut onomic tone and heart rate during 24-hour ambulatory ECG would help id entify patients at risk. Ten patients with antiarrhythmic drug-induced torsades de pointes were compared with 28 controls. The QT(c) at maxi mal and minimal heart rate during antiarrhythmic drug-free ambulatory ECGs were measured. The mean QT(c) at minimal heart rates for patients was 0.413+/-0.102 seconds and 0.420+/-0.072 seconds for controls (P=0 .715). The mean QT(c) at maximal heart rates for patients was 0.555+/- 0.022 seconds and for controls was 0.439+/-0.011 seconds (P=0.001). Me an QT(c) between minimal and maximal heart rates were significantly di fferent for patients (P=0.015) but were not for controls (P=0.151). Us ing an arbitrary QT(c) difference cutoff of 0.075 seconds, this approa ch identified patients at risk for antiarrhythmic drug-induced torsade s de pointes with a sensitivity of 70% (7 of 10) and a specificity of 89% (P less than or equal to 0.003 by Chi-square analysis with Yates' correction). In conclusion, patients with antiarrhythmic drug-induced torsades de pointes had a greater rise in QT(c) from minimal heart rat e during ambulatory ECG than controls. Further larger prospective tria ls will be required to establish the value of this approach to identif y patients at risk for this type of proarrhythmia.