QT dispersion and mortality after myocardial infarction

Dynamics of QT dispersion during myocardial infarction and ischaemia

We studied the dynamics of QT dispersion over the first few days of myocardial infarction and during coronary angioplasty. Ten patients with anterior myocardial infarction and an equal number with inferior infarction had electrocardiograms (ECGs) recorded on admission to hospital (day 1), on the subsequent 2 days (day 2, 3), and prior to discharge (day 6). Ten patients undergoing therapeutic coronary angioplasty were studied; ECGs were recorded prior to, during, and after balloon inflation. Simultaneous 12-lead ECGs were scanned into a personal computer; specially designed software skeletonised and joined each image. The images were then available for user-interactive measurement of QT dispersion. Mean (S.D.) QTc dispersion on day 1 of acute myocardial infarction was 107 (44.8) ms, rose further over the next 48 h, reaching a maximum on day 3 (QTc dispersion, 162.3 (64.8) ms, P < 0.01), and was falling by hospital discharge (QTc dispersion, 117.4 (67.4) ms). There was no difference in QT dispersion measurement during coronary angioplasty. It is unlikely that acute ischaemia plays an important role in the dynamic changes seen in QT dispersion over the first few days of myocardial infarction. These rapid changes in QT dispersion have important implications in the design of any study of QT dispersion after myocardial infarction, and in comparison of studies.

Left ventricular hypertrophy and QT dispersion in hypertension

The interlead variation in QT length on a standard electrocardiograph reflects regional repolarization differences in the heart. To investigate the association between this interlead variation (QT dispersion) and left ventricular hypertrophy, we subjected 100 untreated subjects to 12-lead electrocardiography and echocardiography. Additionally, 24 previously untreated subjects underwent a 6-month treatment study with ramipril and felodipine. In the cross-sectional part of the study, QT dispersion corrected for heart rate (QTc dispersion) was significantly correlated with left ventricular mass index (r = .30, P < .01), systolic pressure (r = .30, P < .01), the ratio of peak flow velocity of the early filling wave to peak flow velocity of the atrial wave (E/A ratio) (r = -.22, P = .02), isovolumic relaxation time (r = .31, P < .01), and age (r = .21, P < .04). In the treatment part of the study, lead-adjusted QTc dispersion decreased from 24 to 19 milliseconds after treatment, and after a subsequent 2 weeks of drug washout remained at 19 milliseconds (P < .01). The changes in left ventricular mass index at these stages were 144, 121, and 124 g/m2 (P < .01). Systolic pressure decreased from 175 to 144 mm Hg and increased again to 164 mm Hg after drug washout (P < .01). The E/A ratio (0.97, 1.02, and 1.02; P = 69) and isovolumic relaxation time (111, 112, and 112; P = .97) remained unchanged through the three assessment points. In conclusion, QT dispersion is increased in association with an increased left ventricular mass index in hypertensive individuals. Antihypertensive therapy with ramipril and felodipine reduced both parameters. If an increased QT dispersion is a predictor of sudden death in this group of individuals, then the importance of its reduction is evident.

Clinical significance of increased OT dispersion in the 12-lead standard ECG for arrhythmia risk prediction in dilated cardiomyopathy

QT dispersion was determined from the 12-lead standard ECGs from 107 patients with idiopathic dilated cardiomyopathy (IDG) and compared to QT dispersion measurements in 100 healthy age and sex matched controls. QT dispersion, rate corrected QT dispersion and adjusted QTc dispersion were significantly greater in patients with IDC compared to controls. During a prospective follow-up of 13 +/- 7 months, arrhythmic events, defined as sustained VT, VF, or sudden death, occurred in 12 (11%) of 107 study patients with IDC. QT dispersion was increased in patients with arrhythmic events compared to patients without arrhythmic events during follow-up (76 +/- 17 vs 60 +/- 26 ms; P = 0.03). Differences in QTc dispersion and adjusted QTc dispersion between patients with and without arrhythmic events, however, failed to reach statistical significance. Thus, although QT dispersion was increased in patients with IDC and arrhythmic events during follow-up, its clinical usefulness for risk stratification appears to be very limited due to the large overlap of QT dispersion among patients with and without arrhythmic events.

Increased dispersion of ventricular recovery time as a new repolarization abnormality in the Wolff-Parkinson-White syndrome

The aim of our study was to assess whether the presence of ventricular preexcitation affects the spatial distribution of ventricular recovery time. Recent reports support the hypothesis that QT and QTc dispersions (QTd and QTcd) can be reliably adopted as a non-invasive parameter to estimate regional discrepancies of ventricular repolarization. The ECGs of 32 healthy subjects with Wolff-Parkinson-White syndrome and of 29 normal individuals have been analysed using a Digitizer (Calcomp 9000), in order to obtain, for each subject, a mean QRS (M-QRS), QT (M-QTe), QTc (M-QTec), JT (M-JT), JTc (M-JTc) from all the measured intervals of the 12 standard ECG leads. QRS, QT and QTc dispersions (QRSd, QTd, QTcd) were defined as the difference between the maximal and minimal QRS, QTe and Qtec values calculated in the various leads. We attained the following results: patients with WPW syndrome exhibited, with respect to controls, longer M-QRS (P < 0.001) and M-QTec (P < 0.001) values, despite similar M-QTe (P = NS), M-JT (P = NS) and M-JTc (P = NS). QRSd did not differ in the two groups(P = NS), while QTd and QTcd both resulted significantly greater in pre-excited subjects (P < 0.001). In the WPW group, QRSd was not related either to QTd (r = 0.325, P = NS) or to QTcd (r = 0.148, P = NS), while in the controls there was a significant relation between QRSd and both QTd (r = 0.522, P = 0.004) and QTcd (r = 0.379, P = 0.042). Our findings suggest that the presence of ventricular pre-excitation does not determine a prolongation of the mean ventricular recovery time, but increases regional discrepancies of the re-polarization process. This assumption is supported by the observation of greater values of QTd and QTcd associated with a similar QRSd.

QT dispersion and arrhythmic events in idiopathic dilated cardiomyopathy

QT dispersion was measured in the 12-lead standard electrocardiogram in 107 patients with idiopathic dilated cardiomyopathy (IDC) and 100 age- and sex- matched controls without structural heart disease. All 107 study patients with IDC were prospectively followed in order to determine possible associations between QT dispersion and arrhythmic events, i.e., sustained ventricular tachycardia, ventricular fibrillation, or sudden death. QT dispersion, rate-corrected QT dispersion, and adjusted QTc dispersion, which takes account of the number of leads measured, were significantly greater in patients with IDC than in controls. During 13 +/- 7 months follow-up, arrhythmic events occurred in 12 of 107 study patients with IDC (11%). QT dispersion was increased in patients with versus without arrhythmic events during follow-up (76 +/- 17 vs 60 +/- 26 ms; p=0.03). QTc dispersion and adjusted QTc dispersion were not significantly different between patients with and without arrhythmic events (80 +/- 21 vs 75 +/- 35 ms, and 27 +/- 6 vs 24 +/- 10 ms, respectively). Thus, although QT dispersion was increased in patients with IDC and arrhythmic events during follow-up, its usefulness for arrhythmia risk prediction was limited by the large overlap of QT dispersion between patients with and without arrhythmic events.

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.

Ionic mechanisms of ischemia-related ventricular arrhythmias

The aim of this review is the utmost simplification of the cellular electrophysiologic background of ischemia-related arrhythmias. In the acute and subacute phase of myocardial infarction, arrhythmias can be caused by an abnormal impulse generation, abnormal automaticity or triggered activity caused by early or delayed afterdepolarizations (EAD and DAD), or by abnormalities of impulse conduction (i.e., reentry). This paper addresses therapeutic intervention aimed at preventing the depolarization of "pathologic" slow fibers, counteracting the inward calcium (Ca) influx that takes place through the L-type channels (Ca antagonists), or hyperpolarizing the diastolic membrane action potential, increasing potassium (K) efflux (K-channel openers) in arrhythmias generated by an abnormal automaticity (ectopic tachycardias or accelerated idioventricular rhythms). If the cause enhanced impulse generation is related to triggered activity, and since both EAD and DAD are dependent on calcium currents that can appear during a delayed repolarization, the therapeutic options are to shorten the repolarization phase through K-channel openers or Ca antagonists, or to suppress the inward currents directly responsible for the afterdepolarization with Ca blockers. Magnesium seems to represent a reasonable choice, as it is able to shorten the action potential duration and to function as a Ca antagonist. Abnormalities of impulse conduction (re-entry) account for the remainder of arrhythmias that occur in the acute and subacute phase of ischemia and for most dysrhythmias that develop during the chronic phase. Reentrant circuits due to ischemia are usually Na channel-dependent. Drug choice will depend on the length of the excitable gap: in case of a short gap (ventricular fibrillation, polymorphic ventricular tachycardia, etc.), the refractory period has been identified as the most vulnerable parameter, and therefore a correct therapeutic approach will be based on drugs able to prolong the effective refractory period (K-channel blockers, such as class III antiarrhythmic drugs); on the other hand, for those arrhythmias characterized by a long excitable gap (most of the monomorphic ventricular tachycardias), the most appropriate therapeutic intervention consists of depressing ventricular excit-ability and conduction by use of sodium-channel blockers such as mexiletine and lidocaine. Compared with other class I antiarrhythmic agents, these drugs minimally affect refractoriness and exhibit a use-dependent effect and a voltage dependent action (i.e., more pronounced on the ischemic tissue because of its partial depolarization).

Use of lead adjustment formulas for QT dispersion after myocardial infarction

OBJECTIVE

To determine whether lead adjustment formulas for correcting QT dispersion measurements are appropriate in patients after myocardial infarction.

DESIGN

Retrospective analysis of QTc dispersion measurements in 461 electrocardiograms (ECGs). Data are presented as uncorrected QTc dispersion "adjusted" for a number of measurable leads and coefficient of variation of QTc intervals for ECGs in which between six and 12 leads had a QT interval that could be measured accurately.

PATIENTS

Patients were drawn from the placebo arm of the second Leicester Intravenous Magnesium Intervention Trial. Some 163 patients who subsequently died and an equal number of known survivors had ECGs recorded on day 2 or 3 of acute myocardial infarction. ECGs were also available in 135 of these patients from at least 1 month postinfarct.

RESULTS

The most common lead in which a QT interval measurement was omitted was aVR (n = 176), the least common lead was V3 (n = 13). The longest QTc interval measured was most usually in lead V4 (n = 72) and the shortest in lead V1 (n = 67). As the number of measurable leads decreased there was a small, nonsignificant increase in QTc dispersion from 12 lead to eight lead ECGs (mean (SD) 100 (35.5) v 109.5 (47.9) ms). Lead adjusted QTc dispersion (QTc dispersion/square root of the number of measurable leads) showed a large, significant increase when the number of measurable leads decreased from 12 to eight (28.9 (10.3) v 38.7 (16.1) ms, P < 0.001). A similar trend was seen for coefficient of variation of QTc intervals (standard deviation of QTc intervals/mean QTc interval 64.3 (2.19) v 8.45 (3.94)%, P < 0.001).

CONCLUSIONS

Lead adjustment formulas for QT dispersion are not appropriate in patients with myocardial infarction. Large differences in lead adjusted QTc dispersion are produced, dependent on the number of measurable leads, for very small differences in QTc dispersion. It is recommended that QT dispersion is presented as unadjusted QT and QTc dispersion, stating the mean (SD) of the number of leads in which a QT interval was measured.

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.