OBJECTIVE To show whether increased QT dispersion on admission predicts ventricular fibrillation after acute myocardial infarction, and to determine the nature of time related changes in QT dispersion.

DESIGN Prospective cohort study.

SETTING Coronary care units of three teaching hospitals in Newcastle-upon-Tyne over an eight month period.

PATIENTS All had acute myocardial infarction according to World Health Organization criteria.

INTERVENTIONS For all patients, QT dispersion (QTd) and Bazett rate corrected QTc dispersion (QTcd) were measured from a high quality 12 lead ECG recorded on admission at a paper speed of 50 mm/s. In a subset, serial ECGs were recorded regularly to show time related changes in QTcd following acute myocardial infarction.

MAIN OUTCOME MEASURES Occurrence of ventricular fibrillation within the first 24 hours after myocardial infarction.

RESULTS Data collected from 201 patients, 12 of whom (6%) developed ventricular fibrillation within 24 hours. Neither QTd nor QTcd differed between those developing ventricular fibrillation and those who did not: QTd mean (SD), 74 (24) ms (95% confidence interval (CI) 59 to 89)v 66 (24) ms (95% CI 62 to 70), respectively; QTcd, 86 (26) ms^0.5 (95% CI 70 to 102)v 77 (29) ms^0.5 (95% CI 72 to 82), respectively. Significant QTcd changes occurred early after myocardial infarction.

CONCLUSIONS Admission QTd and QTcd do not predict ventricular fibrillation after acute myocardial infarction. There are significant changes in QTcd with time, which may account for this measured lack of correlation.