Determinants of electrocardiographic and spatial vectorcardiographic descriptors of ventricular repolarization in normal subjects

Abstract

T link between the dispersion of ventricular recovery times and arrhythmias has previously been demonstrated. QT dispersion has been used to quantify the dispersion of ventricular refractoriness from the standard 12-lead electrocardiogram. However, not only the accuracy and reproducibility of the “dispersion” indexes, but also the presence of a direct link between the heterogeneity of ventricular repolarization and QT dispersion has been challenged recently. Several studies have now focused on the spatial T-loop morphology features as a more accurate measure of the repolarization heterogeneity. Although the correlation between QT dispersion and the T-loop morphology features has previously been evaluated, there are no adequate data on the determinants of the spatial vectorcardiographic (VCG) descriptors of ventricular repolarization in normal subjects. The objective of the present study was to assess the clinical determinants of the electrocardiographic (ECG) and spatial VCG descriptors of ventricular repolarization in a population of young, healthy men. • • • The study population consisted of 1,394 consecutively recruited Air Force servicemen who had no history of any cardiovascular disease, no risk factors for coronary artery disease apart from smoking, and received no cardiotropic drugs. All servicemen had a normal physical examination and a normal 12-lead surface electrocardiogram in the supine resting position. Patients with left or right bundle branch block, atrioventricular block, ventricular preexcitation, or atrial fibrillation were excluded from the study. The study was approved by the Hellenic Air-Force Major General–Medical Division and by our hospital’s ethics committee. Informed consent was obtained from all participants. All study participants underwent a 12-lead digital electrocardiogram by using previously described techniques. All QT intervals were measured manually using the digitally stored electrocardiograms displayed on a high-resolution computer screen. QT dispersion, defined as the difference between the maximum and the minimum QT intervals in any measurable leads, was also calculated. The maximum QT interval was corrected for heart rate using Bazett’s formula (QTc maximum QT maximum/ RR interval). Intraand interobserver relative errors were determined for all the manually measured ECG indexes in 100 randomly selected study participants. To derive VCG descriptors of ventricular repolarization, orthogonal X, Y, and Z leads were reconstructed from the standard 12-lead electrocardiograms. The maximum (maximum QTxyz) and minimum (minimum QTxyz) QT intervals and their difference (QTxyz dispersion) were also calculated. The projections of the maximum QRS and T vectors on the frontal (xy), horizontal (xz), and right sagittal (yz) planes were automatically calculated by our analysis system. According to previously published equations and by use of the Pythagorean theorem, we calculated the amplitude of the maximum spatial T vector (spatial T amplitude) from the formula: spatial T amplitude [(Txy Txz Tyz)/ 2], and the angle ( °) between the maximum spatial QRS and T vectors (spatial QRS-T angle) from the formula: cos (QRSxTx QRSyTy QRSzTz)/ QRS T . Continuous variables are expressed as mean SEM. Spearman’s correlation coefficients were used to assess the relation between ECG and VCG indexes of ventricular repolarization. To determine the multivariate contribution of other factors to the values of different repolarization indexes, linear regression equations were constructed: Z B0 B1Age B2RR interval, where Z was one of the considered repolarization indexes. For each repolarization index, the statistical significance of the regression coefficients B1 and B2 was evaluated. A p value 0.05 was considered statistically significant. To present the correlation between the VCG descriptors of ventricular repolarization and the RR interval, regression lines were constructed (Figure 1). The clinical, ECG, and VCG characteristics of the study participants are listed in Table 1. QT maximum was significantly dependent on age and the RR interval, QT minimum was dependent on the RR interval, and QT dispersion was poorly, although significantly, associated with the RR interval. Maximum and minimum QTxyz were significantly dependent on age and the RR interval, whereas QTxyz dispersion was not significantly dependent on either of the 2. The spatial T amplitude was significantly dependent on the RR interval and weakly, although significantly, dependent on age. The spatial QRS-T angle was weakly, although significantly, dependent on the RR interval (Table 2). All ECG markers were significantly associated with each other, although there was a minor, although significant, association between the VCG and the ECG indexes (Table 3). This association was From the State Department of Cardiology, Hippokration Hospital, Athens; and The Department of Cardiology, University of Thessaly, Larissa, Greece. Dr. Dilaveris’s address is: 22 Miltiadou Street, 155 61 Holargos, Athens, Greece. E-mail: hrodil@yahoo.com. Manuscript received March 28, 2001; revised manuscript received and accepted June 5, 2001.