Time dependent variability of QT dispersion after acute myocardial infarction and its relation to ventricular fibrillation: a prospective study

Abstract

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.

Figure 1

QT dispersion measurements in various studies. Numbers along the top show group sizes, figures in parentheses indicate number with ventricular fibrillation. Error bars show 95% confidence intervals for the means, except for Tomassoni and colleagues¹⁹ which shows the 25th and 75th centile measurements. VF, ventricular fibrillation.

Figure 2

QTc dispersion scatterplots. Horizontal lines represent mean values for each group and a circle represents each result. VF, ventricular fibrillation.

Figure 3

Examples of profiles of QTc dispersion. Each frame shows the corrected QTc dispersion profile for a single patient over the first eight hours after admission. Patients' data are grouped by resolution or persistence of symptoms and ST segment elevation following thrombolysis. The first group (group A) had resolution of symptoms and ST elevation, whereas the second (group B) did not.