The Relationship between Electrical Axis by 12-Lead ECG and Anatomical Axis of the Heart by CMR in Healthy Subjects Short title: Electrical vs anatomical heart axis

Background: The traditional assumption has been that there is a close relationship between the electrical and anatomical axes of the heart. The aim of this study was to test the hypothesis that there is a correlation between the electrical and anatomical axes of the heart, in both the frontal and transverse planes, in healthy subjects. Methods: Ninety-four healthy volunteers (48 males, 46 females; age, 21-82 years) were studied by cardiac magnetic resonance (CMR) and 12-lead ECG. The anatomical axis was determined by CMR and projected onto the frontal and transverse orthogonal planes for comparison with the electrical axis in the corresponding planes. Results: The electrical and anatomical axes were in the same range in the frontal plane (+39 ± 31° and +38 ± 10°, mean ± SD), but in different ranges in the transverse plane (-30 ± 18° and +46 ± 7°, mean ± SD). The partial correlation coefficients between electrical and anatomical axes was r = 0.30 (p < 0.01) and r = 0.14 (p = NS) in the frontal and transverse planes, respectively. Age was more strongly correlated to electrical axis than to anatomical axis in the frontal plane. Conclusions: There is only a weak correlation between electrical and anatomical axes in the frontal plane and no correlation in the transverse plane. The change of electrical axis with increased age is not explained only by change in the anatomical axis. The results suggest that there is no simple relationship between the electrical and anatomical axes of the heart.