- Email: [email protected]
Determinants of electrocardiographic and spatial vectorcardiographic descriptors of ventricular repolarization in normal subjects
Determinants of Electrocardiographic and Spatial Vectorcardiographic Descriptors of Ventricular Repolarization in Normal Subjects Polychronis Dilaveris, MD, Antonios Pantazis, MD, Elias Gialafos, Filippos Triposkiadis, MD, and John Gialafos, MD he link between the dispersion of ventricular recovery times and arrhythmias has previously been T demonstrated. QT dispersion has been used to quan1
tify the dispersion of ventricular refractoriness from the standard 12-lead electrocardiogram.2 However, not only the accuracy and reproducibility of the “dispersion” indexes,3 but also the presence of a direct link between the heterogeneity of ventricular repolarization and QT dispersion4 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.4 –7 Although the correlation between QT dispersion and the T-loop morphology features has previously been evaluated,8 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.6,9 All QT intervals were measured manually using the digitally stored electrocardiograms displayed on a high-resolution computer screen.6,9 QT dispersion, defined as the difference between the maximum and the minimum QT intervals in any measurable leads, was also calculated.6 The maximum QT interval was corrected for heart rate using Bazett’s formula (QTc maximum ⫽ QT maximum/ 公RR interval). Intra- and interobserver relative errors 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: [email protected]. Manuscript received March 28, 2001; revised manuscript received and accepted June 5, 2001.
912
©2001 by Excerpta Medica, Inc. All rights reserved. The American Journal of Cardiology Vol. 88 October 15, 2001
MD,
were determined for all the manually measured ECG indexes in 100 randomly selected study participants.6,9 To derive VCG descriptors of ventricular repolarization, orthogonal X, Y, and Z leads were reconstructed from the standard 12-lead electrocardiograms.10 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.6 According to previously published equations11 and by use of the Pythagorean theorem, we calculated the amplitude of the maximum spatial T vector (spatial T amplitude) from the formula6: spatial T amplitude ⫽ [(Txy2 ⫹ Txz2 ⫹ Tyz2)/ 2]1/2, and the angle (°) between the maximum spatial QRS and T vectors (spatial QRS-T angle) from the formula6,11: cos ⫽ (QRSxTx ⫹ QRSyTy ⫹ QRSzTz)/ 㛳 QRS 㛳 㛳 T 㛳. 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 0002-9149/01/$–see front matter PII S0002-9149(01)01907-5
dexes. No significant association was found between the VCG indexes and QT dispersion. The pathophysiologic basis of the socalled QT dispersion has recently been reconsidered.4,8 Recent studies concluded that the original concept of portraying QT dispersion as a direct measure of the regional heterogeneity of cardiac repolarization is seriously flawed.4 Therefore, it was suggested that the magnitude of QT dispersion found in normal subjects can be explained by the projection phenomenon and measurement inaccuracy.8,12 Kors et al8 already correlated T-wave loop features with QT dispersion. In this study, no association was found between the VCG indexes of cardiac repolarization and QT dispersion. Nevertheless, QTxyz dispersion, which was calculated from the derived X, Y, and Z leads, had a significant, although very weak, correlation with the same indexes. Hence, we may suggest that in our study population, QT dispersion was rather due to measurement inaccuracy than to projection phenomena compared with QTxyz dispersion. The high intra- and interobserver relative errors reported in this study are in accord with previous publications.12,13 Furthermore, the lower QTxyz dispersion relative to QT dispersion values measured in this study has been reported previously.14 An abnormal orientation of the T axis has long been known to provide a global measure of repolarization abnormality,15 and the spatial angle between the direction of the repolarization and depolarization waves (spatial QRS-T angle) has already been used to quantify ventricular repolarFIGURE 1. Scatterplots and regression lines constructed to present the relation ization.6,11 In this study, the spatial VCG between the spatial T amplitude and the RR interval and between the spatial QRS-T angle and the RR interval. descriptors of cardiac repolarization were found to be significantly dependent on heart rate. The spatial T amplitude was also significantly dependent on age. This is in accord with previous studies,16 although only more evident between QTc maximum and the spatial young male subjects were evaluated. The dependence T amplitude. The intraobserver relative errors for QT of these VCG indexes on gender was not evaluated in maximum, QT minimum, QT dispersion, maximum this study. The rate and age dependence of the QT QTxyz, minimum QTxyz, and QTxyz dispersion were intervals has been previously demonstrated,6,9 as have 1.6 ⫾ 0.1%, 4.6 ⫾ 1.6%, 32.6 ⫾ 3%, 1.4 ⫾ 0.1%, 2.1 the significant associations among the different ECG ⫾ 0.5%, and 45.8 ⫾ 4.2%, respectively. The interob- markers of the repolarization duration.6,9 Moreover, server relative errors for the same indexes were 2.7 ⫾ the spatial VCG descriptors of ventricular repolariza0.1%, 5.8 ⫾ 1.5%, 35.3 ⫾ 3%, 1.9 ⫾ 0.1%, 3.1 ⫾ tion showed a minor statistically significant correla0.4%, and 60.2 ⫾ 4.7%, respectively. tion with the ECG indexes of cardiac repolarization. ••• The clinical importance of such very weak associaIn this study, we assessed the determinants of ECG tions between the VCG and ECG indexes is rather and VCG descriptors of ventricular repolarization in limited. Hence, we assume that the spatial VCG marknormal subjects. The spatial T amplitude and the spa- ers of cardiac repolarization may offer unique infortial QRS-T angle were found to be significantly de- mation not obtained from conventional markers dependent on heart rate (Table 2, Figure 1). The VCG rived from the scalar electrocardiogram. indexes of cardiac repolarization showed a very weak, Abnormalities of the T-wave loop morphology although significant, association with the ECG in- seem to be of significant importance.5,17 The hypothBRIEF REPORTS
913
TABLE 1 Clinical and ECG Characteristics of the Study Participants (n ⫽ 1,394) Parameters Mean age (yrs) Heart rate (beats/min) QT maximum (ms) QT minimum (ms) QT dispersion (ms) QTc maximum (ms) Maximum QTxyz (ms) Minimum QTxyz (ms) QTxyz dispersion (ms) Spatial QRS-T angle (degrees) Spatial T amplitude (V)
Mean value ⫾ SEM 23.6 76.3 360.9 323 37.9 403.8 354.6 341.4 13.2 20.4 541.8
⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾
0.09 0.36 0.67 0.92 0.71 0.64 0.68 0.65 0.22 0.29 4.2
95% CI 23.4–23.8 75.5–76.9 359.3–362.1 320.8–324.6 36.5–39.5 402.6–405.1 353.4–356.1 340.2–342.8 12.7–13.6 19.9–21.1 533.5–550.2
appear to exhibit several advantages. They can be measured easily, are not affected by observation biases, and are likely to be less susceptible to noise and problems of definition than conventional ECG indexes of ventricular repolarization. Future studies should examine the ability of these spatial repolarization descriptors to identify high-risk patients for ventricular arrhythmias. In conclusion, the spatial T amplitude and the spatial QRS-T angle are VCG descriptors of ventricular repolarization, and are significantly dependent on heart rate in an homogenous population of young, healthy, male subjects.
See the text for the definition of the individual indexes. CI ⫽ confidence intervals. 1. Han J, Moe GK. Nonuniform recovery of excitability in ventricular muscle.
Circ Res 1964;14:44–60. 2. Day CP, McComb JM, Campbell RW. QT dispersion: an indication of arrhythmic risk in patients with long QT intervals. Br Heart J 1990;63:342–344. TABLE 2 Regression Coefficients and Significance of Various 3. Savelieva I, Yap YG, Gang Y, Guo X, Camm AJ, Malik M. Comparative Independent Variables (age, RR interval) Entered in the Linear reproducibility of QT, QT peak, and T peak-T end intervals and dispersion in Regression Models normal subjects, patients with myocardial infarction, and patients with hypertrophic cardiomyopathy. PACE 1998;21(pt. II):2376–2381. Age RR 4. Malik M, Acar B, Gang Y, Yap YG, Hnatkova K, Camm AJ. QT dispersion does not represent electrocardiographic interlead heterogeneity of ventricular Dependent Variable RC p Value RC p Value repolarization. J Cardiovasc Electrophysiol 2000;11:835–843. 5. Zabel M, Acar B, Klingenheben T, Franz MR, Hohnloser SH, Malik M. QT maximum 0.689 ⬍0.001 0.128 ⬍0.001 Analysis of 12-lead T-wave morphology for risk stratification after myocardial QT minimum 0.388 0.107 0.117 ⬍0.001 infarction. Circulation 2000;102:1252–1257. QT dispersion 0.301 0.159 0.01 0.032 6. Dilaveris P, Gialafos E, Pantazis A, Synetos A, Triposkiadis F, Gialafos J. The Maximum QTxyz 0.603 ⬍0.001 0.127 ⬍0.001 spatial QRS-T angle as a marker of ventricular repolarization in hypertension. J Minimum QTxyz 0.576 ⬍0.001 0.125 ⬍0.001 Hum Hypertens 2001;15:63–70. QTxyz dispersion 0.02 0.683 0.001 0.400 7. Lee KW, Kligfield P, Dower GE, Okin PM. QT dispersion, T-wave projection, Spatial T amplitude ⫺3.027 0.012 0.340 ⬍0.001 and heterogeneity of repolarization in patients with coronary artery disease. Am J Cardiol 2001;87:148–151. Spatial QRS-T angle ⫺0.133 0.130 ⫺0.006 0.002 8. Kors JA, Van Herpen G, Van Bemmel JH. QT dispersion as an attribute of See the text for the definition of the individual indexes. T-loop morphology. Circulation 1999;99:1458–1463. 9. Dilaveris P, Gialafos E, Poloniecki J, Hnatkova K, Richter D, Andrikopoulos RC ⫽ regression coefficient. G, Lazaki E, Gialafos J, Malik M. Changes of the T-wave amplitude and angle: an early marker of altered ventricular repolarization in hypertension. Clin Cardiol 2000;23:600–606. 10. Edenbrandt L, Pahlm O. Vectorcardiogram syntheTABLE 3 Correlation Coefficients Between ECG and VCG Indexes sized from a 12-lead ECG: superiority of the inverse Dower matrix. J Electrocardiol 1988;21:361–367. QT QTc Maximum QTxyz Spatial T Spatial QRS11. Ishizawa K, Ishizawa K, Motomura M, Konishi T, Dispersion Maximum QTxyz Dispersion Amplitude T Angle Wakabayashi A. High reliability rates of spatial pattern analysis by vectorcardiogram in assessing the severity QT maximum 0.252‡ 0.069* 0.908‡ 0.122‡ 0.118‡ ⫺0.106‡ of eccentric left ventricular hypertrophy. Am Heart J QT dispersion 0.092† 0.196‡ 0.162‡ 0.016 0.033 1976;91:50–57. QTc maximum ⫺0.016 0.103‡ ⫺0.314‡ ⫺0.001 12. Lee KW, Kligfield P, Okin PM, Dower GE. Deter‡ ‡ ‡ Maximum QTxyz 0.250 0.096 ⫺0.101 minants of precordial QT dispersion in normal subjects. ‡ QTxyz dispersion ⫺0.118 ⫺0.056* J Electrocardiol 1998;31(suppl):128–133. ‡ Spatial T amplitude ⫺0.139 13. Kautzner J, Yi G, Camm AJ, Malik M. Short- and long-term reproducibility of QT, QTc, and QT disper*p ⬍0.05; †p ⬍0.01; ‡p ⬍0.001. sion measurement in healthy subjects. PACE See the text for the definition of the individual indexes. 1994;17:928–937. 14. Macfarlane PW, McLaughlin SC, Rodger JC. Influence of lead selection and population on automated measurement of QT dispersion. Circulation 1998; 98:2160–2167. esis that all information concerning cardiac repolar- 15. Cooksey JD, Dunn M, Massie E. Clinical Vectorcardiography and Electrocardiography. Chicago: Year Book Medical Publishers, 1977. ization is contained in a single T-wave vector most 16. Liebman J, Lee MH, Rao PS, Mackay W. Quantitation of the normal Frank likely is an oversimplification. The VCG derivation of and McFee-Parungao orthogonal electrocardiogram in the adolescent. Circulation 1973;48:735–752. a common T-wave vector from a sample number of 17. Kors JA, de Bryne MC, Hoes AW, van Herpen G, Hofman A, van Bemmel surface ECG leads may average existing nondipolar JH, Grobbee DE. T axis as an indicator of risk of cardiac events in elderly people. 1998;352:601–604. contents within cardiac repolarization.18 However, the Lancet 18. Franz MR, Zabel M. Electrophysiological basis of QT dispersion spatial VCG descriptors of the T-wave vector loop measurements. Prog Cardiovasc Dis 2000;42:311–324.
914 THE AMERICAN JOURNAL OF CARDIOLOGY姞
VOL. 88
OCTOBER 15, 2001
tify the dispersion of ventricular refractoriness from the standard 12-lead electrocardiogram.2 However, not only the accuracy and reproducibility of the “dispersion” indexes,3 but also the presence of a direct link between the heterogeneity of ventricular repolarization and QT dispersion4 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.4 –7 Although the correlation between QT dispersion and the T-loop morphology features has previously been evaluated,8 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.6,9 All QT intervals were measured manually using the digitally stored electrocardiograms displayed on a high-resolution computer screen.6,9 QT dispersion, defined as the difference between the maximum and the minimum QT intervals in any measurable leads, was also calculated.6 The maximum QT interval was corrected for heart rate using Bazett’s formula (QTc maximum ⫽ QT maximum/ 公RR interval). Intra- and interobserver relative errors 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: [email protected]. Manuscript received March 28, 2001; revised manuscript received and accepted June 5, 2001.
912
©2001 by Excerpta Medica, Inc. All rights reserved. The American Journal of Cardiology Vol. 88 October 15, 2001
MD,
were determined for all the manually measured ECG indexes in 100 randomly selected study participants.6,9 To derive VCG descriptors of ventricular repolarization, orthogonal X, Y, and Z leads were reconstructed from the standard 12-lead electrocardiograms.10 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.6 According to previously published equations11 and by use of the Pythagorean theorem, we calculated the amplitude of the maximum spatial T vector (spatial T amplitude) from the formula6: spatial T amplitude ⫽ [(Txy2 ⫹ Txz2 ⫹ Tyz2)/ 2]1/2, and the angle (°) between the maximum spatial QRS and T vectors (spatial QRS-T angle) from the formula6,11: cos ⫽ (QRSxTx ⫹ QRSyTy ⫹ QRSzTz)/ 㛳 QRS 㛳 㛳 T 㛳. 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 0002-9149/01/$–see front matter PII S0002-9149(01)01907-5
dexes. No significant association was found between the VCG indexes and QT dispersion. The pathophysiologic basis of the socalled QT dispersion has recently been reconsidered.4,8 Recent studies concluded that the original concept of portraying QT dispersion as a direct measure of the regional heterogeneity of cardiac repolarization is seriously flawed.4 Therefore, it was suggested that the magnitude of QT dispersion found in normal subjects can be explained by the projection phenomenon and measurement inaccuracy.8,12 Kors et al8 already correlated T-wave loop features with QT dispersion. In this study, no association was found between the VCG indexes of cardiac repolarization and QT dispersion. Nevertheless, QTxyz dispersion, which was calculated from the derived X, Y, and Z leads, had a significant, although very weak, correlation with the same indexes. Hence, we may suggest that in our study population, QT dispersion was rather due to measurement inaccuracy than to projection phenomena compared with QTxyz dispersion. The high intra- and interobserver relative errors reported in this study are in accord with previous publications.12,13 Furthermore, the lower QTxyz dispersion relative to QT dispersion values measured in this study has been reported previously.14 An abnormal orientation of the T axis has long been known to provide a global measure of repolarization abnormality,15 and the spatial angle between the direction of the repolarization and depolarization waves (spatial QRS-T angle) has already been used to quantify ventricular repolarFIGURE 1. Scatterplots and regression lines constructed to present the relation ization.6,11 In this study, the spatial VCG between the spatial T amplitude and the RR interval and between the spatial QRS-T angle and the RR interval. descriptors of cardiac repolarization were found to be significantly dependent on heart rate. The spatial T amplitude was also significantly dependent on age. This is in accord with previous studies,16 although only more evident between QTc maximum and the spatial young male subjects were evaluated. The dependence T amplitude. The intraobserver relative errors for QT of these VCG indexes on gender was not evaluated in maximum, QT minimum, QT dispersion, maximum this study. The rate and age dependence of the QT QTxyz, minimum QTxyz, and QTxyz dispersion were intervals has been previously demonstrated,6,9 as have 1.6 ⫾ 0.1%, 4.6 ⫾ 1.6%, 32.6 ⫾ 3%, 1.4 ⫾ 0.1%, 2.1 the significant associations among the different ECG ⫾ 0.5%, and 45.8 ⫾ 4.2%, respectively. The interob- markers of the repolarization duration.6,9 Moreover, server relative errors for the same indexes were 2.7 ⫾ the spatial VCG descriptors of ventricular repolariza0.1%, 5.8 ⫾ 1.5%, 35.3 ⫾ 3%, 1.9 ⫾ 0.1%, 3.1 ⫾ tion showed a minor statistically significant correla0.4%, and 60.2 ⫾ 4.7%, respectively. tion with the ECG indexes of cardiac repolarization. ••• The clinical importance of such very weak associaIn this study, we assessed the determinants of ECG tions between the VCG and ECG indexes is rather and VCG descriptors of ventricular repolarization in limited. Hence, we assume that the spatial VCG marknormal subjects. The spatial T amplitude and the spa- ers of cardiac repolarization may offer unique infortial QRS-T angle were found to be significantly de- mation not obtained from conventional markers dependent on heart rate (Table 2, Figure 1). The VCG rived from the scalar electrocardiogram. indexes of cardiac repolarization showed a very weak, Abnormalities of the T-wave loop morphology although significant, association with the ECG in- seem to be of significant importance.5,17 The hypothBRIEF REPORTS
913
TABLE 1 Clinical and ECG Characteristics of the Study Participants (n ⫽ 1,394) Parameters Mean age (yrs) Heart rate (beats/min) QT maximum (ms) QT minimum (ms) QT dispersion (ms) QTc maximum (ms) Maximum QTxyz (ms) Minimum QTxyz (ms) QTxyz dispersion (ms) Spatial QRS-T angle (degrees) Spatial T amplitude (V)
Mean value ⫾ SEM 23.6 76.3 360.9 323 37.9 403.8 354.6 341.4 13.2 20.4 541.8
⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾ ⫾
0.09 0.36 0.67 0.92 0.71 0.64 0.68 0.65 0.22 0.29 4.2
95% CI 23.4–23.8 75.5–76.9 359.3–362.1 320.8–324.6 36.5–39.5 402.6–405.1 353.4–356.1 340.2–342.8 12.7–13.6 19.9–21.1 533.5–550.2
appear to exhibit several advantages. They can be measured easily, are not affected by observation biases, and are likely to be less susceptible to noise and problems of definition than conventional ECG indexes of ventricular repolarization. Future studies should examine the ability of these spatial repolarization descriptors to identify high-risk patients for ventricular arrhythmias. In conclusion, the spatial T amplitude and the spatial QRS-T angle are VCG descriptors of ventricular repolarization, and are significantly dependent on heart rate in an homogenous population of young, healthy, male subjects.
See the text for the definition of the individual indexes. CI ⫽ confidence intervals. 1. Han J, Moe GK. Nonuniform recovery of excitability in ventricular muscle.
Circ Res 1964;14:44–60. 2. Day CP, McComb JM, Campbell RW. QT dispersion: an indication of arrhythmic risk in patients with long QT intervals. Br Heart J 1990;63:342–344. TABLE 2 Regression Coefficients and Significance of Various 3. Savelieva I, Yap YG, Gang Y, Guo X, Camm AJ, Malik M. Comparative Independent Variables (age, RR interval) Entered in the Linear reproducibility of QT, QT peak, and T peak-T end intervals and dispersion in Regression Models normal subjects, patients with myocardial infarction, and patients with hypertrophic cardiomyopathy. PACE 1998;21(pt. II):2376–2381. Age RR 4. Malik M, Acar B, Gang Y, Yap YG, Hnatkova K, Camm AJ. QT dispersion does not represent electrocardiographic interlead heterogeneity of ventricular Dependent Variable RC p Value RC p Value repolarization. J Cardiovasc Electrophysiol 2000;11:835–843. 5. Zabel M, Acar B, Klingenheben T, Franz MR, Hohnloser SH, Malik M. QT maximum 0.689 ⬍0.001 0.128 ⬍0.001 Analysis of 12-lead T-wave morphology for risk stratification after myocardial QT minimum 0.388 0.107 0.117 ⬍0.001 infarction. Circulation 2000;102:1252–1257. QT dispersion 0.301 0.159 0.01 0.032 6. Dilaveris P, Gialafos E, Pantazis A, Synetos A, Triposkiadis F, Gialafos J. The Maximum QTxyz 0.603 ⬍0.001 0.127 ⬍0.001 spatial QRS-T angle as a marker of ventricular repolarization in hypertension. J Minimum QTxyz 0.576 ⬍0.001 0.125 ⬍0.001 Hum Hypertens 2001;15:63–70. QTxyz dispersion 0.02 0.683 0.001 0.400 7. Lee KW, Kligfield P, Dower GE, Okin PM. QT dispersion, T-wave projection, Spatial T amplitude ⫺3.027 0.012 0.340 ⬍0.001 and heterogeneity of repolarization in patients with coronary artery disease. Am J Cardiol 2001;87:148–151. Spatial QRS-T angle ⫺0.133 0.130 ⫺0.006 0.002 8. Kors JA, Van Herpen G, Van Bemmel JH. QT dispersion as an attribute of See the text for the definition of the individual indexes. T-loop morphology. Circulation 1999;99:1458–1463. 9. Dilaveris P, Gialafos E, Poloniecki J, Hnatkova K, Richter D, Andrikopoulos RC ⫽ regression coefficient. G, Lazaki E, Gialafos J, Malik M. Changes of the T-wave amplitude and angle: an early marker of altered ventricular repolarization in hypertension. Clin Cardiol 2000;23:600–606. 10. Edenbrandt L, Pahlm O. Vectorcardiogram syntheTABLE 3 Correlation Coefficients Between ECG and VCG Indexes sized from a 12-lead ECG: superiority of the inverse Dower matrix. J Electrocardiol 1988;21:361–367. QT QTc Maximum QTxyz Spatial T Spatial QRS11. Ishizawa K, Ishizawa K, Motomura M, Konishi T, Dispersion Maximum QTxyz Dispersion Amplitude T Angle Wakabayashi A. High reliability rates of spatial pattern analysis by vectorcardiogram in assessing the severity QT maximum 0.252‡ 0.069* 0.908‡ 0.122‡ 0.118‡ ⫺0.106‡ of eccentric left ventricular hypertrophy. Am Heart J QT dispersion 0.092† 0.196‡ 0.162‡ 0.016 0.033 1976;91:50–57. QTc maximum ⫺0.016 0.103‡ ⫺0.314‡ ⫺0.001 12. Lee KW, Kligfield P, Okin PM, Dower GE. Deter‡ ‡ ‡ Maximum QTxyz 0.250 0.096 ⫺0.101 minants of precordial QT dispersion in normal subjects. ‡ QTxyz dispersion ⫺0.118 ⫺0.056* J Electrocardiol 1998;31(suppl):128–133. ‡ Spatial T amplitude ⫺0.139 13. Kautzner J, Yi G, Camm AJ, Malik M. Short- and long-term reproducibility of QT, QTc, and QT disper*p ⬍0.05; †p ⬍0.01; ‡p ⬍0.001. sion measurement in healthy subjects. PACE See the text for the definition of the individual indexes. 1994;17:928–937. 14. Macfarlane PW, McLaughlin SC, Rodger JC. Influence of lead selection and population on automated measurement of QT dispersion. Circulation 1998; 98:2160–2167. esis that all information concerning cardiac repolar- 15. Cooksey JD, Dunn M, Massie E. Clinical Vectorcardiography and Electrocardiography. Chicago: Year Book Medical Publishers, 1977. ization is contained in a single T-wave vector most 16. Liebman J, Lee MH, Rao PS, Mackay W. Quantitation of the normal Frank likely is an oversimplification. The VCG derivation of and McFee-Parungao orthogonal electrocardiogram in the adolescent. Circulation 1973;48:735–752. a common T-wave vector from a sample number of 17. Kors JA, de Bryne MC, Hoes AW, van Herpen G, Hofman A, van Bemmel surface ECG leads may average existing nondipolar JH, Grobbee DE. T axis as an indicator of risk of cardiac events in elderly people. 1998;352:601–604. contents within cardiac repolarization.18 However, the Lancet 18. Franz MR, Zabel M. Electrophysiological basis of QT dispersion spatial VCG descriptors of the T-wave vector loop measurements. Prog Cardiovasc Dis 2000;42:311–324.
914 THE AMERICAN JOURNAL OF CARDIOLOGY姞
VOL. 88
OCTOBER 15, 2001