Equine Cardiac Arrhythmias – NAVLE Study Guide

Overview and Clinical Importance

Cardiac arrhythmias are disturbances in the normal rhythm, rate, or electrical conduction of the heart and are more common in horses than in any other large animal species. Approximately 25% of horses with no evidence of heart disease may have cardiac arrhythmias detectable during routine examination or with electrocardiography (ECG). Understanding equine arrhythmias is critical for NAVLE success because they directly impact performance assessment, pre-purchase examinations, and anesthetic risk evaluation.

The unique cardiovascular physiology of horses, including their large atrial size and high vagal tone, predisposes them to certain arrhythmias, particularly atrial fibrillation. Additionally, arrhythmias in horses may be brought on by excitement, fever, toxemia, colic, electrolyte imbalance, congenital defects, myocarditis, and valvular heart disease.

Normal Equine Cardiac Conduction

In normal sinus rhythm, the electrical impulse is initiated in the sinoatrial node (SA node) and transmitted across the atrial myocardium, generating the P wave. The impulse is delayed at the atrioventricular node (AV node), creating the PR interval. Rapid conduction then occurs through the His bundle, bundle branches, and Purkinje fiber network, activating the ventricular myocardium and generating the QRS complex. Ventricular repolarization produces the T wave.