Cardiology

Is This an Important Leak? Tips on Recognizing and Evaluating Murmurs in the Horse

By Paul Basilio

The equine heart has a great capacity to compensate. Even in the face of considerable cardiovascular disease the horse will find a way to keep plugging away.

While that is physiologically impressive, it does pose quite a challenge for a cardiovascular examination in a horse presenting with a murmur and a slight decrease in performance. To help clinicians interpret cardiac findings, Katharyn Jean Mitchell, BVSc, DVCS, DVM, PhD, DAVCIM (LAIM), recently offered helpful tips at the 67th AAEP Conference in Nashville.

The initial exam

“Our physical exam really tells us what's leaking,” said Dr. Mitchell, an assistant professor of large animal medicine at the Cornell University College of Medicine. “We don't need the ultrasound to know what's leaking. We need the ultrasound to tell us the consequences of that leak.”

Luckily, a good history, including signalment, offers some clues.

“If I’m examining an Arabian or Friesian horse, I have loud bells going off in my head if I hear a murmur,” she said. “If I hear a murmur in a Standardbred or a Thoroughbred, the bells might be a little quieter.”

To start narrowing down potential pathology, she emphasized the need for a proper history to put the murmur into perspective. What is the horse’s job, and how has it changed recently? Is there a general lack of energy, or does the horse need an extended rest period after exercise? When was the horse last acting “normal”?

Color flow mapping image of the mitral valve and left atrium, showing severe mitral regurgitation and left atrial enlargement with atrial fibrillation.

Courtesy of Dr. Katharyn Jean Mitchell

Color flow mapping image of the left ventricular outflow tract showing severe aortic regurgitation.

Courtesy of Dr. Katharyn Jean Mitchell

For the physical exam, Dr. Mitchell explained that veterinarians need good equipment, a quiet environment, and to take a quick walk around to the right side of the horse.

“Always go around to the right side and have a listen to both sides of the thorax,” she said. “You’d be surprised how many horses have complex heart disease that is missed because nobody walked around to the right side.”

Instead of just a quick listen to the heart rate, it is also important to take note of the heart sounds and the rhythm. The pulse is also a vital part of the exam, especially if a murmur is present.

“If you feel the pulse and hear the noise at the same time, that’s systole,” she said. “If you feel the pulse and then hear the noise, that’s diastole.”

Mitral valve regurgitation

In the case of mitral valve regurgitation, the murmur is less important than the associated heart remodeling.

“We rarely see acute mitral regurgitation,” Dr. Mitchell said. “Sometimes we’ll see it in a racehorse that ruptures a chordae acutely or develops a flail leaflet, or with some sort of ischemic disease. Acute dysfunction of the papillary muscles is rare.”

Chronic mitral regurgitation, in which the left atrium gets larger, is much more common. Over time, the pressure increases a small amount, but the overall volume of the atrium increases to a great extent, causing it to enlarge.

“A big atrium is a substrate for arrhythmias,” she explained. “Most of the consequences of heart disease that we worry about are the arrhythmias that are caused by the heart disease, not the heart disease itself.”

ACVIM consensus recommendations suggest that a grade 3/6 or louder left-side systolic murmur should be evaluated on echocardiography.

Dr. Mitchell said that she also considers further evaluation in any horse with a high resting heart rate, a prolonged recovery after exercise, or any horse that develops any kind of arrhythmia. If a murmur and an arrhythmia are both present, that is much more concerning.

Aortic regurgitation

The left ventricle is the area of concern with aortic regurgitation. Like mitral regurgitation, chronic aortic regurgitation is most common in horses as they get older.

“The left ventricle gets bigger with time, it compensates, it remodels, and it develops eccentric hypertrophy, where the chamber gets bigger and the wall gets thicker,” she explained. “That is an amazing substrate for an arrhythmia, and that’s where our concerns are for these horses.”

If a horse develops an important arrhythmia during exercise, the horse could collapse or even have sudden cardiac death.

In a healthy horse, pulse pressure—the difference between systolic pressure and diastolic pressure—is typically around 40 mmHg.

In horses with severe regurgitation, the aortic valve is leaking, so the diastolic pressure is falling. That leads to a bigger ventricle with a larger stroke volume that’s pushing the larger amount of blood through the same size aorta. This causes systolic pressure to go up.

Color flow mapping image of the right ventricular outflow tract, showing severe tricuspid regurgitation.

Courtesy of Dr. Katharyn Jean Mitchell

“You end up with a bigger difference between systolic pressure and diastolic pressure, and we can feel that in a bounding pulse, also called a water hammer pulse,” Dr. Mitchell said. “If you feel that means the horse has important aortic regurgitation and that left ventricle is big, that is something you need to worry about.”

ACVIM consensus guidelines recommend echocardiography in horses that have grade 3 or louder diastolic murmurs, but Dr. Mitchell also recommends performing a scan in horses with a grade 2 murmur plus a strong pulse.

“I’ve seen many horses that have had a quiet diastolic murmur that was missed,” she said. “But if they have a strong pulse, they have an important amount of regurgitation. So, add that to your exam.”

She also recommends further evaluation in horses with a history of decreased performance, collapse, weakness, exercise intolerance, or one that requires long recovery time after exercise.

Tricuspid regurgitation

Depending on the population of horses you see, tricuspid regurgitation either happens a lot or not often at all.

Outcomes depend on the type of horse. Race-horses often have tricuspid regurgitation but tolerate it well, but the condition may be more important in pleasure horses or low-level performance horses.

“I am particularly worried about horses that have evidence of respiratory disease and tricuspid regurgitation,” Dr. Mitchell said.

History and owner expectations often come into play in determining her concern for the horse and the risk tolerance she is willing to accept. Depending on what the horse’s job is, the experience of the rider, and its lifestyle and environment, the risk assessment may change.

“I’m also going to look at what kind of disease is present,” she said. “Are there atrial consequences, or are we talking about ventricular consequences? I will be more risk averse if something dangerous has happened with the horse in the past. If the horse has a history of being ‘wobbly’ or the horse is weak after exercise, I am more averse to risk in those cases.”

BREAKING DOWN THE MURMURS

If a murmur is soft, early systolic and localized cranially over the great vessels on the left side, it’s more likely to be physiological and not worrisome. According to the ACVIM consensus guidelines for equine athletes with cardiovascular abnormalities, important murmurs tend to be grade 3/6 or louder.

“When we talk about causes of the murmurs, we are lucky because it’s simple in the horse compared with small animals and humans,” Dr. Katharyn Jean Mitchell said. “If you hear a systolic murmur loudest on the left side, that is mitral regurgitation. If you hear a systolic murmur loudest on the right, it’s either going to be tricuspid regurgitation or potentially a ventricular septal defect.”

Diastolic murmurs are even easier. Regardless of the side of the murmur, a diastolic murmur is almost always going to be aortic valve regurgitation. An aortic cardiac fistula is possible with a right-sided diastolic murmur, but those are more likely to be combined systolic/diastolic murmurs.