New algorithm for QT interval analysis in 24-hour Holter ECG: performance and applications

Fifty-five patients with recent myocardial infarction and 55 healthy controls, matched for age, sex, race, height, weight, education and job, had an electrocardiogram taken every two months for seven years. Twenty-eight patients and one control had a sudden cardiac death. The QTc (mean of all values recorded) was found prolonged in one control (2%), five of 27 surviving patients (18%) and in 16 of 28 patients who had sudden death (57%). The difference between surviving and sudden death patients is significant (P less than 0.01). It is interesting that the only control with a long QT was the one when died suddenly of myocardial infarction. Among patients with previous myocardial infarction a prolonged QTc constitutes a 2.16 times greater risk for sudden death. We conclude that a constant prolongation of QTc in patients with myocardial infarction may help, with other risk factors, in defining a subgroup at higher risk for sudden death.

Although prolongation of QTc interval has been shown to increase the risk of sudden death after recent myocardial infarction, no data exist on the relationship between sudden death and QTc duration in patients with chronic ischemic heart disease. Furthermore, it is not known whether patients with long QTc intervals (greater than or equal to 440 units) have more prevalent coronary risk factors. Thus 141 nonsurvivors (128 with coronary death and 13 with noncoronary death) representing the follow-up deaths of a cohort of 1157 medically treated patients with ischemic heart disease over a four-year period were compared to 141 consecutive long-term survivors of the same cohort. Thirty-one patients were excluded because of drug interactions, bundle-branch block or atrial fibrillation. QTc duration was calculated on the ECG immediately prior to angiography in 62 patients with sudden death, 36 with intermediate death, 13 with noncoronary death and 140 long-term survivors with chronic ischemic heart disease. In addition, in 64 nonsurvivors (58%) in whom more than one yearly follow-up ECG was available, QTc was calculated in the last ECG preceding death (mean of four months before death). These data were compared to those obtained in 140 long-term survivors at the time of last ECG (mean 48 months after enrollment). At the time of angiography, mean QTc intervals were similar in patients who later died of ischemic heart disease and in long-term survivors (423 +/- 35 vs 421 +/- 25 units). No difference in QTc duration was apparent among nonsurvivors with ischemic heart disease. All study patients were divided into normal and long QTc subgroups.(ABSTRACT TRUNCATED AT 250 WORDS)

To estimate variations in intra- and interindividual measurements of the corrected QT (QTc) interval, duplicates of 50 twelve lead electrocardiograms (100 photocopies, paper speed 50 mm/s) were given to each of nine investigators in random order. The electrocardiograms were recorded from patients with acute myocardial infarction consecutively admitted to a coronary care unit. Patients receiving drug therapy and those manifesting various arrhythmias were included. Two-way analysis of variance was used to evaluate the results from all 900 QTc measurements. Significant differences in these measurements were registered among investigators and were of major importance (p less than 0.001). This finding illustrates the difficulty in comparing mean values from different studies and emphasizes the difficulties in applying limits for a normal QTc interval to data obtained by different observers. Of less but still significant importance was the interaction between the investigator and electrocardiogram (p less than 0.001). Finally, the random error was calculated and proven to be of no importance (less than 0.5 mm) when more than 11 measurements were performed.