Relationship of delayed depolarization to the QT interval after acute myocardial infarction

The relationship between conduction delay, as manifested by a prolonged QRS or late potentials (LP) detected by signal averaging, and QT prolongation was analyzed in six patients who had QTc greater than or equal to 0.42 second within 48 hours of acute myocardial infarction (AMI). Total QRS, LP, QT, and QTc durations were measured on days 2 to 3, 4 to 5, 6 to 7, and 8-9. In each recording period, the QT interval and QTc interval did not correlate with the QRS duration and LP duration (r less than or equal to 0.52 for each comparison). In 19 out of 27 instances, a sequential change in QT or QTc intervals was discordant with changes in QRS duration and/or LP, i.e., temporal changes in QT intervals were not determined by conduction. Thus, QT prolongation after AMI is not primarily due to regional slowing of conduction that results in regional delays in termination of some action potentials. Global prolongation of repolarization would seem to result from dispersion of action potential duration, not onset.