4.1.6 ECGs recorded during exercise
Many horses have arrhythmias at rest which result from the high vagal tone which is found in this species. These arrhythmias are usually abolished by the fall in vagal tone and increase in sympathetic tone which accompanies excitement or exercise. Auscultation is usually sufficient to detect resolution of the arrhythmia. Only a slight increase in heart rate is usually required, so the rhythm can also be recorded by performing a standard ECG examination once the animal comes to rest.
It is more difficult to detect arrhythmias which occur during high heart rates at peak exercise, or which may occur at exercise more or less frequently than at rest. This is particularly important in the case of atrial premature complexes (APCs), ventricular premature complexes (VPCs), and paroxysmal atrial fibrilTo evaluate these arrhythmias, which are often associated with poor athletic performance, it is very helpful to record an ECG while the animal is actually exercising. To do this the horse can be run on a treadmill and attached to the ECG machine by wires ('hard-wired'). Alternatively, radiotelemetry can be used with the horse exercised in its usual environment, or on a treadmill. With radiotelemetry, there is less chance of wires pulling on the electrodes and causing interference. For outdoor use, a battery-powered machine is useful. The range of the transmitter and receiving unit ideally should be at least 100m.
The main problem with ECGs recorded at exercise is poor electrical contact which results in artefacts and makes interpretation of the ECG trace difficult. This is best avoided by placing the electrodes in small perforated chamois leather pouches sewn on to a girth strap. The leather pouches are soaked in water or saline and the strap is pulled very tight. The best contact is found when the animal starts to sweat. Alternatively, the electrodes can be attached to a shaved area of skin with adhesive. Electrodes are usually placed either side of the chest. Trial and error may be required in order to find the electrode positions which produce the largest QRS complex but least artefact due to movement and poor electrical contact in different individuals.
Radiotelemetry is also useful for monitoring animals with arrhythmias while they are being treated with anti-dysrhythmic drugs, or for the monitoring of any acute-care patient. It is particularly useful for monitoring animals during treatfor atrial fibrillation (AF). If these animals are suddenly excited or are moved for an ECG to be recorded, and are already hypotensive, they have been known to collapse. In addition, because an ECG can be recorded continuously, sigchanges in the cardiac rhythm such as the development of an atrial tachycardia can be seen and appropriate action taken before clinical signs deteriorate.