Continuous QT Interval Measurements from 24-Hour Electrocardiography and Risk after Myocardial Infarction

QT dynamicity: a prognostic factor for sudden cardiac death in chronic heart failure

INTRODUCTION

The aim of this study was to determine whether impaired adaptation of the QT interval to changes in heart rate predicts sudden death in patients with chronic heart failure (CHF).

METHODS

We prospectively included 175 CHF patients in sinus rhythm. QT dynamicity was evaluated by analyzing 24-h Holter recordings. The linear regression slope of QT interval measured to the apex and to the end of T wave plotted against RR intervals was calculated using a dedicated Holter algorithm.

RESULTS

Mean follow-up was 29.9+/-17.9 months. There were 48 deaths, of which 21 were sudden. The actuarial 3-year mortality rates were 38.4% for overall mortality and 14.1% for sudden death. Of all the parameters, an increased QTe/RR slope (>0.28) was the strongest independent predictor of sudden death (relative risk 3.47, 95% confidence interval 1.43-8.40, p=0.006).

CONCLUSION

Increased 24-h QTe dynamicity is independently predictive of sudden death among patients with heart failure. This simple parameter may help to stratify risk and select patients who may benefit from antiarrhythmic prophylaxis.

QT dynamics and variability

Repolarization dynamics and variability are of increasing interest as Holter-derived parameters reflecting changes in myocardial vulnerability and contributing to increased risk of arrhythmic events and sudden death. Repolarization dynamics is usually defined as phenomenon describing and quantifying QT adaptation to changing heart rate. The analysis of QT-R-R slopes in long ECG recordings is one of the ways to evaluate repolarization dynamics. Increased QT-R-R slopes are frequently observed in patients at risk for cardiac death and arrhythmic events: postinfarction patients, long QT syndrome patients, patients with nonischemic cardiomyopathy as well as in patients taking drugs affecting repolarization. QT variability reflects beat-to-beat changes in repolarization duration and morphology and such changes can be quantified using a number of algorithms currently in various phases of development and validation. Increased QT variability is observed in several conditions with increased risk of arrhythmias. Recent data from MADIT II indicate that increased QT variability is a powerful predictor of arrhythmic events in postinfarction patients with left ventricular dysfunction. More studies are needed to determine further the potential clinical usefulness for diagnosing patients and for risk stratification purposes using both QT dynamics and QT variability methods, and compare these methods with exercise-induced T wave alternans.

QT dynamicity and sudden death after myocardial infarction: results of a long-term follow-up study

INTRODUCTION

The aim of this study was to determine whether impaired adaptation of the QT interval to changes in heart rate predicts sudden death after an acute myocardial infarction.

METHODS AND RESULTS

The Groupe d'Etude du Pronostic de l'Infarctus du Myocarde (GREPI) trial was a prospective multicenter study designed to evaluate the long-term outcome of myocardial infarction. QT dynamicity was evaluated in 265 patients by analyzing 24-hour Holter recordings obtained 9 to 14 days after myocardial infarction. The linear regression slope of QT intervals measured to the apex and to the end of the T wave (QTe) plotted against RR intervals was calculated using a dedicated Holter algorithm. The value of QT/RR in predicting sudden death and total mortality was compared with those of ejection fraction, heart rate variability, and late potentials. Mean follow-up was 81 +/- 27 months. There were 73 deaths, of which 23 were sudden. Of all the parameters, an increased diurnal QTe/RR slope (>0.18) was the strongest independent predictor of sudden death (relative risk 6.07, confidence interval 1.48-24.95, P = 0.01).

CONCLUSION

Increased diurnal QTe dynamicity is independently predictive of sudden death among patients with myocardial infarction. This simple parameter may help to stratify risk and select patients who may benefit from antiarrhythmic prophylaxis.

Circadian rhythm of the corrected QT interval: impact of different heart rate correction models

A reduced circadian pattern in the QTc interval has been repeatedly reported to provide prognostic information in cardiac patients. However, the results of studies in healthy subjects in which different heart rate correction formulas were used are inconsistent regarding the presence and extent of diurnal variations in QTc. This study compared the diurnal variations in QTc obtained with four frequently used heart rate correction models with those based on individually optimized heart rate correction. In 53 subjects (25 men aged 27 +/- 7 years and 28 women aged 27 +/- 9 years) 12-lead digital ECGs were obtained every 30 seconds during 24 hours. The QT interval was measured automatically by six different algorithms provided by a commercially available device. The QT/RR relation was estimated by four common heart rate correction models and by an individually optimized correction model, QTc = QT/RR alpha. In each 24-hour recording, RR, QT, and WTc intervals of separate ECG samples were averaged over 10-minute intervals. Marked differences were found in the extent of the circadian pattern of QTc obtained with different formulas for heart rate correction. Under and overcorrection of the QT interval resulted in significant over- or underestimation of the circadian pattern. Thus, the extent of circadian variation in QTc depends highly on the heart rate correction formula used. To obtain proper insight regarding diurnal variation in QTc prolongation during pharmacologic therapy and/or to assess higher risk due to impaired autonomic regulation of ventricular repolarization, individualized heart rate correction is necessary.

Relation of ventricular repolarization to cardiac cycle length in normal subjects, hypertrophic cardiomyopathy, and patients with myocardial infarction

BACKGROUND

Prolonged QT interval and QT dispersion have been reported to reflect an increased inhomogeneity of ventricular repolarization, which is believed to be responsible for the development of arrhythmic events in patients with long QT syndrome, coronary heart disease, and myocardial infarction, congestive heart failure, and hypertrophic cardiomyopathy (HC).

HYPOTHESIS

This study was undertaken to determine whether an abnormal QT/RR dynamicity may reflect autonomic imbalance and may contribute to arrhythmogenesis in patients with heart disease.

METHODS

The relation between QT, QTpeak (QTp), Tpeak-Tend (TpTe) intervals and cardiac cycle length was assessed in 70 normal subjects, 37 patients with HC, and 48 survivors of myocardial infarction (MI). A set of 10 consecutive electrocardiograms was evaluated automatically in each subject using QT Guard software (Marquette Medical Systems, Milwaukee, Wisc.).

RESULTS

In patients with HC, all intervals were significantly prolonged compared with normals (p < 0.001 for QT and QTp; p < 0.04 for TpTc); in survivors of MI, this was true for the maximum QT and QTp intervals (p < 0.05). A strong linear correlation between QT, QTp, and RR intervals was observed in normals and in patients with MI and HC (r = 0.65-0.59, 0.82-0.77, 0.79-0.74, respectively, p < 0.0001). TpTe interval only showed a weak correlation with heart rate in normals (r = 0.24, p < 0.05) and was rate-independent in both patient groups (p = NS). Compared with normals, the slopes of QT/RR and QTp/RR regression lines were significantly steeper in patients with MI and HC (0.0990-0.0883, 0.1597-0.1551, 0.1653-0.1486, respectively). Regression lines were neither parallel nor identical between normals and patients (T > 1.96, Z > 3.07). There was no difference in steepness for TpTeR/RR lines between groups (0.0110, 0.0076, 0.0163, respectively). TpTe/QTp ratio was similar in normals and in patients with MI and HC (0.30 +/- 0.03, 0.31 +/- 0.07, 0.30 +/- 0.04, respectively), in the absence of any correlation between QTp and TpTe intervals, suggesting disproportional prolongation of both components of QT interval.

CONCLUSION

Compared with normals, a progressive increase in QT and QTp intervals at slower heart rates in patients with MI and HC may indicate an enhanced variability of the early ventricular repolarization and may be one of the mechanisms of arrhythmogenesis.