The equine electrocardiogram with standardized body and limb positions.

Eleven-lead electrocardiograms (ECGs) were recorded from 50 Thoroughbred (TB) and 50 Standard bred (SB) horses at rest with 40 tracings being selected from each group for more complete analysis. Incomplete atrioventricular block with dropped beats commonly reported in mature horses was recorded only in the TB (15%), while wandering of the atrial pacemaker (WAP) thought also to be associated with variations in tone of the autonomic nervous system was detected almost equally in both breeds (30%). In some horses (12.5%), the initial component of the P wave (P1) did not vary as seen with WAP but had a mean manifest electric axis in the frontal plane between 0 and -64 degrees with vectors directed craniad and sinistrad. Breed differences were noted for the mean duration of the P-wave and for the PR and QT intervals while no significant differences were present for mean heart rate. Significant differences have also been reported for other ECG values (amplitude and duration) in relation to age, breed and sex. It may be of value when attempting to determine the clinical or research significance of cardiac vectors or other ECG parameters, to establish values for each of the different breeds. Interpretation of ST-segment deviations and changes in the form of the T-wave are complicated by their associations with conditions of excitement or exercise in clinically normal horses as well as in those with organic heart disease. The diagnostic significance of changes in amplitude and direction of the T-wave requires further elucidation.