The heart usually has a very regular rhythm (beat) but abnormalities in cardiac rhythm occur occasionally and can have serious consequences for affected horses. The cardiac arrhythmia that occurs most commonly in horses is atrial fibrillation; this is almost always performance limiting particularly in horses that work at speed. Other arrhythmias are detected occasionally in horses but might only occur sporadically throughout the day. Electrocardiography, more often termed ECG or EKG, is used to diagnose abnormalities in the heart’s rhythm. Telemetric Electrocardiography (ECG) allows us to monitor and record the heart’s rhythm remotely and continuously.
A small recording and transmitting unit is attached to the horse, either to a surcingle or to the mane, and this unit receives, records and transmits the heart rhythm detected by small electrodes placed on the horse’s chest. Telemetric ECG means that we can record the heart’s rhythm without needing to tie the horse up or having it connected to a traditional recorder. This makes it possible to record an ECG for an extended length of time; The ECG can be recorded for analysis at a later time or can be watched on a remote computer in real time. Telemetric ECG is especially useful for monitoring the heart’s rhythm during quinidine treatment for horses with atrial fibrillation; this makes the procedure considerably safer and we now consider telemetric ECG monitoring the standard of care when treating these horses.
Heart murmurs are extremely common in horses and many horses will have subtle cardiac murmurs that have no ill effect on the horse or its ability to perform as an athlete. Unfortunately, it is almost impossible to determine the significance of a cardiac murmur by auscultation (listening) with a stethoscope alone. Echocardiography is ultrasonography of the heart and is an essential tool when trying to determine the significance of heart murmurs. Echocardiography is technically difficult to perform but is extremely useful when assessing heart size and function. Echocardiography is also used to detect congenital heart defects which occur occasionally in horses and to assess for the presence of abnormal fluid around the heart (pericardial effusion) which is seen sometimes after viral or bacterial infections.
As with evaluating the respiratory system, the treadmill can be an extremely useful tool for monitoring the heart’s performanceduring a careful controlled exercise test. Performing an exercise test on the treadmill means that we can compare the performance of one horse to another under identical circumstances – something that is difficult to achieve out in the field. Telemetric ECG can be used to observe and record the cardiac rhythm trace whilst the horse is under pressure. This is particularly useful for diagnosing problems that only occur during or after strenuous exercise such as intermittent (paroxysmal) atrial fibrillation or ventricular premature contractions (abnormal heart beats). Echocardiography can be performed immediately after exercise to evaluate for abnormalities in heart muscle function that can be missed if the horse is only evaluated at rest.