Is there an abnormal QT interval in sudden cardiac death survivors with a "normal" QTc?

Correcting the QT interval for heart rate may mask ventricular repolarization abnormalities and may lead to a misinterpretation of the physiologic and pathophysiologic findings. In this study the QT/R-R relationship was studied in eight sudden cardiac death (SCD) survivors without overt structural heart disease and compared with eight age- and sex-matched normal subjects. All patients were in a drug-free state. The QT intervals and their preceding R-R intervals were measured on electrocardiogram (ECG) strips from 24-hour Holter recordings at hourly intervals. The differences in mean heart rate, mean QT intervals, and QTc values between SCD survivors and normal subjects were not statistically significant. There was a significant correlation between the QT and R-R intervals in normal subjects (tau = 0.71 +/- 0.13, p < 0.05) and in SCD survivors (tau = 0.79 +/- 0.07, p < 0.05). However, the regression line of the QT interval against the R-R interval was significantly (p < 0.01) altered in SCD survivors compared with normal subjects. These observations suggest that there is abnormal ventricular repolarization (QT interval) despite an apparently normal QTc using Bazett's formula in these patients. Evaluation of the QT/R-R relationship by means of 24-hour ambulatory Holter ECG monitoring may provide a useful clinical tool for the assessment of ventricular repolarization abnormalities.