ECG interpretation in the horse

CARDIAC arrhythmias are common in horses, although the majority do not require antiarrhythmic therapy. If an arrhythmia is suspected on cardiac auscultation, electrocardiography will enable a definitive diagnosis to be made. However, in each case, the significance of the electrocardiographic findings must be interpreted in the light of the clinical signs exhibited by the horse. This involves evaluation of the cardiac rate and rhythm and detection of the presence of abnormal complexes. Unlike in small animals, little information can be obtained about enlargement of the cardiac chamber(s) in horses. This is because simultaneous depolarisation of the entire ventricular myocardium via the extensive Purkinje system means that the amplitude of the QRS complex is not related to chamber size. This article describes the key features which may be present on an electrocardiographic trace and discusses how these might be interpreted.