The QT interval in atrial fibrillation.

The electrocardiogram was recorded for 100 seconds in 50 patients with atrial fibrillation to determine the relations between QT intervals and both the mean and instantaneous ventricular rates. The mean ventricular rate was 94 beats per minute with a mean QT interval of 357 ms. The mean QTc, corrected beat by beat with Bazett's formula, was 444 ms--longer than reported for sinus rhythm. Between subjects, the mean QT interval was linearly related to the mean RR interval, with a slope of +21%. Within all 50 recordings there was a statistically significant correlation between QT intervals and immediately preceding RR intervals, with an average slope of +7%. This within subject QT/RR interval slope was greater at faster mean ventricular rates. In atrial fibrillation, as in sinus rhythm, the QT interval is a function of both the mean ventricular rate and the instantaneous ventricular rate, with the mean ventricular rate predominating; a simple correction of QT intervals for heart rate is therefore inadequate. Comparison of uncorrected QT intervals with those of earlier published series of people in sinus rhythm, however, suggested that atrial fibrillation is associated with prolongation of the mean QT interval.