Cardiovascular malfunction is the most common reason for sudden death in horses during and immediately after exercise. It has various causes, and knowing which, if any, affect your horse could save his life. Cris Navas, LV, PhD, Dipl. ACVIM, large animal internist at the University of Pennsylvania’s New Bolton Center Veterinary Hospital, in Kennett Square, elaborated on the cardiovascular-related causes of poor performance in horses at the 2023 Veterinary Meeting and Expo held Jan. 14-17 in Orlando, Florida.

Heart murmurs, congestive heart failure, pulmonary hypertension, atrial fibrillation- many issues can affect your horse’s cardiac health, and science is teaching us when and how these conditions might disrupt your plans for your horse’s athletic career.

Atrial fibrillation

“Atrial fibrillation (A-fib) is the most common clinically- relevant arrhythmia in all mammals- humans and horses included,” said Navas. “Any irregularly irregular heartbeat is considered A-fib until proven otherwise. A-fib does not appear to affect general health or wellness but does limit the performance of equine athletes engaging in high-intensity exercise such as racing, cross-country jumping, and high-level show jumping. Racehorses cannot succeed while suffering from the condition because it decreases their cardiac output, reducing the potential of their cardiopulmonary function.” The two main treatments for A-fib include the drug quinidine and a transvenous electrical cardiac inversion procedure, he added.

Systolic murmur

“A loud left-sided systolic murmur is mitral regurgitation until proven otherwise,” said Navas. “The prognosis is good if the disease is clinically insignificant or mild but affects performance in moderate and severe cases.” The work-up of all heart murmurs begins with a thorough physical examination that includes careful auscultation (listening with a stethoscope). “Unlike the chihuahua whose heart races at 200 beats per minute (bpm), the horse’s 30-40 bpm rate gives us the ability to listen, in detail, to the individual characteristics of each murmur. Remember that excitement, pain, sedation, or drugs like Buscopan all increase blood pressure, making a normally- inaudible murmur suddenly obvious.” Beyond auscultation, an echocardiogram helps veterinarians better understand the relevance of different murmurs.

Pulmonary hypertension

“Pulmonary hypertension in horses occurs secondary to severe heart disease and respiratory diseases, like severe equine asthma,” said Navas. “Not only is athletic performance impaired in these horses, but riding them is outright dangerous because exercise increases the pulmonary pressure from 35-50 mm Hg to 90-100 mm Hg, which can lead to pulmonary artery rupture. Pulmonary hypertension makes the horse unfit for athletic work.”

Aortic regurgitation of blood

“Aortic regurgitation of blood is often an incidental finding in teenage horses,” said Navas. All diastolic heart murmurs are aortic regurgitation until proven otherwise. However, moderate and severe forms of the disease can affect health, performance, and safety. They predispose the horse to ventricular arrhythmias, which can be linked to poor performance and sudden death. These horses need an exercise test- measuring heart function during exercise- if they continue to be ridden, he added.

Ventricular septal defect

The most common congenital heart disease in horses is a ventricular septal defect (VSD), a birth defect in the septum that separates the left from the right ventricle. “VSD can be identified on cardiac auscultation by the presence of a characteristic combination of heart murmurs: left-to-right shunts cause a loud right-sided systolic murmur, and relative pulmonic stenosis causes a left-sided murmur that is less loud than the one on the right and centered over the pulmonic valve area,” Navas explained. “An echocardiogram is needed to establish a prognosis and decide if the horse is fit to exercise. These animals are unlikely to become elite athletes- such as racehorses or eventers- but often can safely and successfully participate in lower-intensity disciplines.”

Will a heart problem impede your equine athlete’s performance goals? The short answer is it depends. A veterinarian with expertise in heart disease can help you define what it depends on. The nature, characteristics, and severity of the condition, and the rider’s goals for the horse will make all the difference between a benign finding and a career-stopping diagnosis.