QT dispersion(QTD=QTmax-QTmin) on the 12 lead ECG has been known to reflect regional variation of ventricular repolarization, and thus a marker of an increased risk of arrhythmia events. Late potential(LP) on signal averagina ECG(SAECG) is independent risk factor of ventricular arrhythmia following acute myocardial infaction(AMI). However, the relation between LP and QTD as indicator of electrophysiologic instability in AMI remains to be determined.

To determine whether there is a difference in QTD between in parients with AMI during acute phase and in normal control and whether thrombolytic therapy is assiciated with a reduction in QTD, and to determine the relationship between change of QTD and late potential on SAECG, we studied 71 patient with AMI(male 54, female 14, mean age 57yrs) and 23 controls(malw 17, female 6, mean age 58yrs). QT interval was measured on a standard 12 lead ECG in patients with AMI on admission, 2 hours after urokinase IV and 10-14 days post-AMI, and QT dispersion was calculated by difference of maximal and minimal corrected QT interval(QTmax-QTmin). A signal averaged ECG was recorded in 36 patients before discharge and coronary angiogeaphy(CAG) was performed in all patients 10-14 days post-AMI.

QTD is significantly increased in AMI compared to control(78.7±39.5ms vs. 24.6±22.3ms, P < 0.01). In patients who received thrombolytic therapy with urokinase, QTD is decreased from 75.0±34.4ms to 53.9±36.0ms(P < 0.01), whereas there is no significant change in patients who did not received thrombolytic therapy(84.8±47.6ms vs. 78.9±36.2ms, NS). There in no difference in QTD between patients with positive LP(68.4±23.6ms) and those with negative LP(77.8±32.1ms) on admission, those with positive LP(66.6±27.6ms) and those with negative LP(56.0±26.4ms) after 10-14days post-AMI. But magnitude of change of 10-14 days post-AMI QTD in patients with negative LP is larger than those with positive LP(-21.7±33.4ms vs. -1.8±15.2ms, P=0.06).

QTD in acute phase of AMI is significantly reduced by the thrombolytic therapy. Patients with negative late potential tent to have greater QTD reduction within 14 days after AMI. These finding suggest that QT dispersion in patients with AMI can be reduced by early recanalization and its reduction is associated with the presence of late potential.