Factors affecting the mean electrical axis of the heart in trained Thoroughbreds

Background: Assuming that the ventricular myocardium of horses is subjected to exercise-induced hypertrophy, we hypothesised that the mean electrical axis (MEA) of the heart would change. Objectives: To define a longitudinal study to detect any changes in the direction of the MEA in Thoroughbred horses using ECG. Study design: ECGs were recorded on each horse in each training group at day 0 (T0), 1 month (T1) and 2 months (T2) of training. Methods: A total of 43 Thoroughbred horses in training in Italy were recruited. The horses were divided into three groups according to age. The ECGs were recorded by positioning the electrodes according to Dubois's method for measuring MEA in the frontal plane. Intervals with artefact-free QRS complexes in both bipolar DI and augmented unipolar aVF leads were selected, and the vector obtained was identified as the MEA. The statistical analysis was performed via generalised linear mixed model (GLMM) and principal component analysis (PCA). Results: A statistically significant effect of time passing between T0 and T2 (p < 0.001) and an interaction between time and sex on the MEA was found (p = 0.04). PCA revealed that the population studied had different patterns, with three horses showing higher variability in the MEA direction. Main limitations: There was no good sex balance in the age groups of the population studied, and there was no control group. The 1-month sampling intervals of ECGs may have been too short. Confirmatory studies are needed. Conclusions: We believe that our results are the first to suggest that training may lead to changes in MEA orientation in horses. Sex and individuality were found to influence MEA orientation and may have contributed to the difficulty in detecting training-dependent changes in MEA to date.