Comparison of 2 methods of measuring the QT interval

BACKGROUND: Prolonged cardiac repolarization is associated with ventricular tachycardia and sudden cardiac death. Repolarization, represented by the QT interval, is usually measured on a 12-lead ECG recording. Measurements of the interval on bedside monitor ECG recordings have not been compared quantitatively with measurements on 12-lead ECG recordings. OBJECTIVE: To determine if QT intervals and QTc values obtained by using monitor recordings are as accurate as those obtained by using 12-lead ECG recordings. METHODS: For each of 50 subjects, 2 ECG recordings were obtained, 1 with a 12-lead ECG and 1 with the bedside monitor, and QT intervals were measured manually. The QT intervals on each type of recording were compared on a lead-by-lead basis, the maximum QT interval and the QTc maximum determined with each method were compared, and the "best single leads" for determining the QTc were ascertained for each method. RESULTS: QT intervals, on a lead-by-lead basis; maximum QT intervals; and QTc maximum values measured on the monitor recordings were consistently longer than those measured on the 12-lead ECG recordings. When the monitor ECG leads I or II and the 12-lead ECG QTc maximum were examined for simple agreement by using 460 milliseconds as a cutoff, agreement was found in 82% to 84% of the sample, and false negatives were 12% and 8%, respectively. CONCLUSION: Recordings from leads I or II on the bedside ECG monitor should be used to measure the QT interval. Once prolonged QT values are detected, recordings obtained with a 12-lead ECG can be used to confirm the analysis.