Cardiac arrhythmias in exercising horses

Cardiac arrhythmias during exercise are most often incidental findings with no effect on performance. However, some arrhythmias may result in reduced performance or even collapse and death in horses. As it is impossible to auscultate the heart during exercise, the arrhythmias can only be diagnosed by use of telemetric ECG recordings. During exercise the effect of myopotential interference is high and artifacts may make the interpretation of ECGs challenging. The most common cardiac arrhythmias diagnosed during exercise are supra-ventricular premature complexes (SVPCs). These are characterized by ectopic premature atrial activation often with changes in P wave morphology. During exercise the P wave is often buried in the preceding T wave and therefore the diagnosis is primary based on shortening of the RR interval.

Isolated premature beats originating from the ventricles are termed ventricular premature beats (VPCs) and are characterized by premature ventricular activation without an associated P wave. Both the QRS complex and the T wave often have an abnormal morphology. If more than three VPCs occur repetitively the term ventricular tachycardia is used. Other arrhythmias such as atrial fibrillation or bradycardic events such as second degree AV blocks may also occur. The immediate post-exercise period is prone to arrhythmia development during cardiac deceleration. The clinical significance of the post-exercise arrhythmias is unknown but most are considered of no clinical significance.

Defining the safety risks to the horse and rider is challenging, but in general complex ventricular arrhythmias are of concern and further assessment of the cardiovascular system should always be conducted.