Recent Advances in Diagnosing Cardiac Abnormalities With an EKG During Exercise: A Review

Cardiac disease is a rare primary cause of poor performance in the equine athlete. On the rare occasion that performance is affected by cardiac disease, dysrhythmia is the most common underlying cause. The technique of taking a resting electrocardiogram (EKG) using a base-apex lead configuration is now well established in equine practice; most equine practices are equipped with both the expertise and the technology to perform the task. The method can be valuable in confirming a diagnosis of sustained arrhythmias like atrial fibrillation or in diagnosing the origin of premature beats. However, a resting EKG usually offers little to the client in establishing a prognosis for future use of their horse. The limitations of a resting EKG arise largely because of the enormous cardiac reserve of the horse, which means that performance-limiting cardiac disease or abnormal rhythms during exercise rarely manifest themselves at rest. This is compounded by the fact that alterations in resting cardiac rhythm are common in athletic horses because of normal high parasympathetic drive, yet some of these rhythms can also occur as a result of cardiac pathology. As a result, the effect of a resting arrhythmia on performance or the effect of exercise on an arrhythmia can only be established if an EKG can be taken during strenuous exercise when sympathetic tone and myocardial oxygen demand are increased and parasympathetic influence is reduced. In the past, exercising EKG examination, although well described in the equine literature, was restricted to the specialist cardiologist or the performance laboratory; this was largely because of the very high cost of radiotelemetric EKG equipment. However, the newest technology, based on palm-top and laptop computers or specialized battery-operated devices, has become increasingly affordable for equine veterinarians in Europe. This equipment is rapidly becoming standard for equine practitioners in the United Kingdom and is now available in North America. As a result, it is timely to review the methodology and potential applications of the new generation of EKG devices capable of producing high-quality exercising electrocardiographs in performance horses.