QT dispersion and components of the QT interval in ischaemia and infarction

Will QT dispersion play a role in clinical decision-making?

(1) Dispersion of QT intervals is the difference between the longest and the shortest QT interval in the ECG. Owing to the relative ease of measurement and the perceived need for new markers of arrhythmogenicity, the method has attracted the interest of clinical investigators but has not reached the level of practical utility. (2) It is postulated that to pass the test of practical utility, the method must meet the following criteria: (a) standardization; (b) establishment of normal values; (c) established sensitivity and/or specificity for diagnosis and/or prognosis; and (d) uniqueness of relevant information. (3) Analysis of the data from the literature suggests that standardization of the method and the range of normal values have not been established, and that the method lacks specificity for separating healthy persons from patients with heart disease. (4) Large values, such as average QT dispersion > 65 msec, have been found predominantly in patients with serious, life-threatening ventricular tachyarrhythmias, and the largest values, i.e., > 110 msec in patients with congenital long QT syndrome. (5) The prognostic value of QT dispersion has been disputed, and the uniqueness of the relevant information has not been tested. (6) It is concluded that the acceptance of QT dispersion as a useful test in practice faces manifold and serious obstacles. It remains to be established whether these obstacles are insurmountable.

Dispersion of the QT interval and autonomic modulation of heart rate in hypertensive men with and without left ventricular hypertrophy

Left ventricular hypertrophy is an independent risk factor for sudden cardiac death in hypertension, but the mechanisms of electrical instability associated with hypertrophy are not well known. We studied dispersion of the QT interval, an index of inhomogeneity of repolarization, and heart rate variability, a measure of cardiac autonomic modulation, in a randomly selected population of 162 men with systemic hypertension and made comparisons between the patients with echocardiographic evidence of left ventricular hypertrophy (left ventricular mass index > or = 131 g/m2, n = 44) and those without hypertrophy (left ventricular mass index < 131 g/m2, n = 118). The heart rate-corrected QT dispersion (67 +/- 37 versus 53 +/- 21 milliseconds, P < .05) and QT apex dispersion (55 +/- 22 versus 44 +/- 16 milliseconds, P < .01) were significantly longer in the patients with left ventricular hypertrophy than in those without hypertrophy. Thirteen of the 44 patients (30%) with hypertrophy versus 7 of the 118 patients (6%) without hypertrophy had an abnormally long QT apex dispersion ( > 70 milliseconds) (P < .001). The time and frequency domain measures of heart rate variability did not differ significantly between the patient groups with and without left ventricular hypertrophy. The measures of heart rate variability were not related to QT dispersion or left ventricular mass index but had a negative correlation with blood pressure values (eg, r = -.30 between the low-frequency component of heart rate variability and systolic pressure, P < .001). Age, body mass index, antihypertensive medication, and the other demographic variables were similar between the groups, but the patients with left ventricular hypertrophy had higher systolic (P < .01) and diastolic (P < .01) pressures compared with the patients without hypertrophy. Left ventricular hypertrophy in hypertensive men is associated with inhomogeneity of the early phase of ventricular repolarization, favoring susceptibility to reentrant ventricular tachyarrhythmias. Abnormalities in cardiac autonomic function, which may trigger a spontaneous onset of arrhythmias, are related to elevated blood pressure but not specifically to left ventricular hypertrophy.

Three-lead measurement of QTc dispersion

INTRODUCTION

QTc dispersion has traditionally been calculated from all 12 leads of a standard electrocardiogram (ECG). It is possible that alternative, quicker methods using fewer than 12 leads could be used to provide the same information.

METHODS AND RESULTS

We have previously shown a difference in QTc dispersion from ECGs recorded at least 1 month after myocardial infarction between patients who subsequently died and long-term survivors. In the current study, we recalculated QTc dispersion in these ECGs using different methods to determine if the observed difference in QTc dispersion measurements between the two groups, as calculated from 12-lead ECGs, persisted when using smaller sets of leads. QTc dispersion was recalculated by four methods: (1) with the two extreme QTc intervals excluded; (2) from the six precordial leads; (3) from the three leads most likely to contribute to QTc dispersion (aVF, V1, V4); and (4) from the three quasi-orthogonal leads (aVF, I, V2). For each of the 270 12-lead ECGs examined, a mean of 9.9 leads (SD 1.5 leads) had a QT interval analyzed; the QT interval could not be accurately measured in the remaining leads. Using the standard 12-lead measurement of QTc dispersion, there was a difference in the fall in QTc dispersion from early to late ECG between the groups: 9.1 (SD 60.8) msec for deaths versus 34.4 (55.2) msec for survivors (P = 0.016). This difference in QTc dispersion between early and late ECGs was maintained using either three-lead method (quasi-orthogonal leads: -2.6 [56.2] msec for deaths vs 26.9 [54.3] msec for survivors [P = 0.003]; "likeliest" leads: 8.6 [64.9] msec vs 29.5 [50.2] msec [P = 0.05]), but not when using the other two methods (precordial leads: 19.1 [55.5] msec vs 22 [50.8] msec [P = 0.76]; extreme leads removed: 9.2 [50.1] msec vs 21.8 [42] msec [P = 0.13]).

CONCLUSION

QTc dispersion calculated from three leads may be as useful a measurement as QTc dispersion calculated from all leads of a standard ECG. Its advantages over the standard measurement are its simplicity and the lack of problems with lead adjustment.

Dispersion of repolarization. Relation to heart rate and repolarization duration

Repolarization duration is highly dependent on heart rate. A major concern when evaluating dispersion of repolarization is the possible influence of heart rate on the magnitude of dispersion. Another consideration relates to a potential relationship between overall duration of repolarization and the magnitude of dispersion, that is, whether patients with longer repolarization duration present with increased or decreased dispersion of repolarization. Therefore, the following relationships were studied in 380 normal subjects, 68 coronary artery disease (CAD) patients, and 41 long QT syndrome (LQTS) patients: the magnitude of dispersion (JTd) versus cycle length (R-R) and dispersion versus repolarization duration (QTc interval). Dispersion of repolarization (JTd), measured as the maximal difference in JT interval duration between precordial leads, was significantly higher in LQTS patients than in normal subjects or CAD patients (120 +/- 72 vs 53 +/- 42 and 48 +/- 22 ms, respectively). In neither normal subjects, CAD patients, or LQTS patients were there significant relationships between the magnitude of dispersion and the R-R interval (r = .094, .158, and .233, respectively; not significant) and between the magnitude of dispersion and QTc duration (r = .0443, -.094, and .126, respectively; not significant). In normal subjects, CAD patients, and LQTS patients, the magnitude of dispersion is not significantly related to heart rate, indicating that there is no need for heart rate adjustment of dispersion parameters. In addition, there is no significant association between the magnitude of dispersion and duration of repolarization.