Cardiac limitations to athletic performance

Introduction

Cardiac disease is a cause of poor performance in equine athletes. The musculoskeletal and respiratory systems, are the body systems more commonly involved in poor performance in horses participating in any equestrian disciplines, followed by the cardiovascular system.

Mitral Valve Disease

Cardiac auscultation is often the first clue for the detection of valvular, arrhythmic or congenital disease that can affect performance, safety or life expectancy. The most common physical examination findings that alert clinicians about cardiovascular disease are murmurs and irregular rhythms. Careful description of the cardiac auscultation may seem superfluous but is key to the diagnosis of heart disease in horses. Mitral valve disease is. A left sided systolic murmur should be considered mitral regurgitation until proven otherwise. The exception to this rule is the physiologic ejection (or flow) murmur. Auscultation is often sufficient for the differentiation. A physiologic ejection murmur is usually 1-2/6 (sometimes 3/6) early to mid-systolic blowing or coarse crescendo-decrescendo murmur with the point of maximal intensity over the pulmonic or aortic valve areas. Ejection murmurs do not radiate and often vary in intensity with exercise or excitement. Physiologic murmurs are common in horses and this leads to many horse owners to the false conclusion that a cardiac murmur in a horse is never a problem. Mitral regurgitation murmurs are 1-6/6 holo- or mid- to late systolic blowing, coarse, musical or honking band shaped or crescendo murmurs with the point of maximal intensity over the mitral or aortic valve areas. Murmurs of mitral regurgitation may or may not radiate and usually do not vary in intensity unless they are caused by a prolapsed valve. Systolic murmurs that are variable in intensity (with excitement, exercise sedation or pain) are frequent. Physiologic ejection murmurs and murmurs of mitral valve prolapse are the common variable murmurs and the clue to their differentiation is their timing and shape: physiologic ejection murmurs are early to mid-systolic crescendo-decrescendo and mitral valve prolapse are mid to late crescendo.

Aortic Regurgitation

Aortic regurgitation is a common disease of teenage horses. Aortic valve disease is frequently an incidental finding but some horses with aortic regurgitation can develop exercising arrhythmias, exercise intolerance or heart failure. A diastolic murmur in a horse should be considered to be aortic regurgitation until proven otherwise. The presence of a diastolic murmur, and the consequent suspicion of aortic regurgitation, should prompt the clinician to feel the peripheral pulses. A horse with a diastolic murmur and strong or bounding peripheral pulses likely has moderate or severe aortic regurgitation. Moderate to severe aortic regurgitation predisposes horses to ventricular arrhythmias particularly during exercise. Aortic regurgitation and exercise make the perfect arrhythmogenic cocktail; the ventricular enlargement and remodeling, the decreased coronary perfusion caused by the aortic regurgitation, the shortened diastole caused by the increase in heart rate, the increased oxygen demand and the increased sympathetic tone caused by the exercise create ideal conditions for arrhythmias. Horses with moderate or severe aortic regurgitation that continue to exercise should have an exercising electrocardiogram to investigate if exercising arrhythmias are present and an echocardiogram to assess the cardiac structure and function.

Ventricular Septal Defect

Ventricular septal defect (VSD) is the most common congenital heart disease of horses. A VSD can be identified on cardiac auscultation by the presence of a characteristic combination of murmurs. The left to right shunt causes a loud right sided systolic murmur. The second murmur is a systolic crescendo-decrescendo murmur with the point of maximal intensity over the pulmonic valve area that is less loud than the one on the right. This second murmur is caused by the ejection of blood (increased due to the shunt) out of the right ventricle and is called the murmur of ‘relative pulmonic stenosis’. Echocardiograms are needed to determine the prognosis and level of exercise that will be tolerated by a horse with a VSD.

Atrial Fibrillation

Atrial fibrillation is the most common clinically relevant arrhythmia in humans and horses.Atrial fibrillation affects performance in horses that practice high intensity exercise but many horses used for pleasure riding or low intensity equestrian sports can do so while in atrial fibrillation. The decision to convert a horse to normal sinus rhythm is multifactorial. Duration of the arrhythmia, presence of previous episodes, presence of underlying heart disease, economic factors, risk aversion of the owners and the use of the horse are some of these factors. There is ongoing debate about the safety/risk for collapse in horses with AF. If a horse is not going to be converted an echocardiogram and an exercising electrocardiogram are needed to determine if it is safe for the horse to exercise. Current recommendations in horses with sustained atrial fibrillation are that these horses should only be used by informed adult riders and exercise should be limited to a level considered relatively safe based on the exercising ECG. Other cardiac diseases that can cause decreased performance are myocarditis, poor myocardial function, aortocardiac fistulas, complex congenital heart disease, 3rd degree AV block, sick sinus syndrome, severe pericarditis etc.

Exercising Arrhythmias

The field of exercising arrhythmias deserves special mention. The presence of arrhythmias is common in normally and poorly performing horses. The influence of exercising arrhythmias on performance or tolerance to exercise is intuitive and extrapolated from basic physiologic studies documenting the associated cardiovascular changes. However this influence is not proven and in many cases it is not possible to ascertain if arrhythmias are incidental or the cause of poor performance. The fact that a malignant arrhythmia has the potential of causing collapse or sudden cardiac death (SCD) complicates the decision making processes. Collapse and SCD can affect the health of horses, the public perception of welfare during equestrian sports and most importantly the safety of riders. The incidence of malignant arrhythmias causing collapse or SCD is fortunately low but likely 50-100 fold higher in horses than in analogous human athletes.

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