Catheter ablation of the atrioventricular junction (AVJ) is a widely accepted treatment for drug refractory atrial fibrillation. Unfortunately, there have been some reports of pause dependent ventricular arrhythmias associated with QT interval prolongation, mainly in patients with reduced LV function. The present investigation evaluates the association of LV function with QT dispersion in response to a sudden rate drop. ECGs were' recorded on 20 patients (13 with normal LV function) on the day following AVJ ablation while paced at a range of ventricular rates (40-120 beats/min), and during a sudden drop from 80 to 40 beats/min. The maximum QT interval (QTmax), minimum QT interval (QTmin), and QT interval dispersion (QTdisp) were compared. In both groups, the QTmax and QTmin increased at slower paced heart rates while the QTdisp did not change. In response to a sudden rate drop from 80 to 40 beats/min, the QTmax increased in both groups of LV function (trend), while the QTmin increased in those with normal LV function (24 +/- 22 ms), but not in those with reduced LV function (0 +/- 14 ms; P = 0.01). Consequently, the QTdisp increased significantly in those with reduced LV function (31 +/- 23 ms) but not in normal LV function (-5 +/- 29 ms; P = 0.01). Morphological QTU changes developed following the sudden rate drop in 67% of the reduced LV versus 8% of the normal LV (P = 0.02) function groups. Following AVJ ablation, QTdisp increased during a sudden rate drop in patients with reduced LV function, but not in patients with normal LV function.