Electrocardiography in Horses

A cardiac exam always starts with a detailed history and a thorough clinical exam. An imperative aspect of the clinical examination is a skillfully performed auscultation which allows the gathering of important information about the cardiac cycle and blood flow (Patteson, 1996). Further diagnostic tests, such as bio - chemistry, echocardiography and electrocardiography, can be carried out to obtain additional information about the exact cause and importance of cardiac disease.

Echocardiography allows the imaging of cardiac structures, the determination of chamber size and provides information about the cardiac function (Patteson, 1996). Electrocardiography is the ultimate tool for the diag nosis and classification of dysrhythmias. An electrocardiogram (ECG) can be recorded for brief (ambulatory) or prolonged (e.g. 24-hour or “Holter” recording) periods, at rest and during exercise. ECG recording during exercise used to be reserved to specialized centers because of the expensive equip - ment. Over the last years, relatively cheap, battery-powered recorders have become available to the equine practitioner for ECG recording under field conditions. In order to correctly diagnose a dys - rhythmia, a good quality recording and a thorough knowledge of ECG interpretation are mandatory.

This paper consists of two parts. The first part explains how to make a good quality ECG recording in resting and exercising horses. The second part describes how to interpret the ECG and reach a final diagnosis.

Figure 1. Normal sinus rhythm with P wave, QRS complex and T wave. The arrow indicates Ta wave, the line bar indicates 1 second.

Principles of electrophysiology

When a cardiac cell depolarizes or repolarizes, different currents flow across the cell membrane at various time points resulting in a potential difference and an external electrical field. When enough cells de- or repolarize at the same time, the electrical field will be strong enough to be recorded from the body surface. The ECG register changes in the electrical field around the heart via electrodes, usually placed on the skin. The sum of potential differences of all individual cells can be represented by a mean electrical axis (MEA), which indicates the average direction of electrical activity in the heart. The recorded potential difference is largest when the electrodes are placed parallel to the MEA. The combination of two electrodes, one negative and one positive, between which a potential difference is measured, is called a “lead”. If the direction of the overall depolarizing electrical activity is headed towards the positive electrode, the deflection on the ECG will be upward (positive). If the electrical activity points away from the positive electrode, the deflection will be downward (negative) (Patteson, 1996).