QT interval in patients with unstable angina and non-Q wave myocardial infarction.

BACKGROUND

Non-Q wave myocardial infarction (NQMI) and unstable angina (UAP) have similar clinical presentations and similar ST-T changes on the electrocardiogram. The purpose of this study was to assess whether changes in QT interval might help differentiating between these entities.

METHODS

The QT intervals of 52 patients hospitalized with NQMI were compared to those of 52 patients hospitalized for UAP. All patients had repeated ECG for at least 4 days.

RESULTS

Maximal QTc in patients with NQMI was significantly longer than in patients with UAP (475 vs 439 ms, P < 0.0001). QTc on the admission ECG was 450 ms in patients with NQMI compared to 417 ms in UAP (P < 0.005). QTc > 460 ms was present in 48% patients with NQMI and in 19% of UAP patients. Maximal QT prolongation was observed within 36 hours of admission with return to normal within 96 hours. QT dispersion was within normal range, being longer in patients with NQMI than patients with UAP (55 vs 43 ms, P < 0.003). QT prolongation was not associated with increased frequency of arrhythmia. The cause of QT prolongation in NQMI may be related to the damage of subendocardial layer exposing the M cells layer which markedly prolong action potential duration.

CONCLUSION

Transient QT prolongation is observed in about half of patients with NQMI. These ECG changes may help differentiating between patients with NQMI and UAP already on admission.