Not All Equine Heart Murmurs Are Created Equal

No one wants to hear their horse has a heart murmur. The diagnosis can raise many questions: Is my horse safe to ride? Is my horse going to die? Is there anything I can do? The answers to those questions usually require a deeper dive into the diagnosis to determine the specific type of heart murmur the horse has and how significant it might be.

Amy Polkes, DVM, Dipl. ACVIM, provides cardiac evaluations through her mobile practice, HV Equine Internal Medicine, which serves Maryland and the surrounding states. She discussed heart murmurs during a recent presentation at the 2020 Northeastern Association of Equine Practitioners Symposium.

Characterizing the Murmur

Most people are familiar with the two-beat “lub-dub” sound a normally functioning heart makes. The beats correspond to valves closing as blood flows through the heart. The first heart sound (lub) happens when the mitral and tricuspid valves close and is referred to as S1. The second heart sound (dub) takes place when the aortic and pulmonic valves close and is referred to as S2.

Listen to a normal equine hearbeat: 

The time between S1 and S2, when blood is being ejected from the ventricles, is called systole. The time between S2 and the next S1, when the ventricles are filling, is known as diastole.

A murmur can be defined as an atypical sound of the heart, such as whooshing or swishing. A systolic murmur is heard between S1 and S2, while a diastolic murmur will be heard after S2 and before the next S1. It is also possible for a murmur to be both systolic and diastolic, in which case is it called a continuous murmur.

“When we’re characterizing a murmur, the most important thing is the timing and recognizing whether it is systolic or diastolic,” Polkes said, “because that is very important in what you’re going to be thinking the cause of the murmur is.”

Listen to an early systolic murmur: 

Listen to an example of a heart murmur: 

Determining the Type of Murmur

In addition to identifying whether the murmur is systolic or diastolic, a veterinarian will determine whether the murmur is functional or pathologic.

“The type, functional versus pathologic, will help us decide whether the murmur is really that significant,” Polkes said. “A functional murmur is a manifestation of normal blood flow—there’s not actually a problem at the valve—versus pathologic, where there is abnormal blood flow and a problem at one of the valves.”

Functional murmurs can be broken down into functional systolic murmurs and functional diastolic murmurs, which veterinarians can differentiate by their sounds and locations. While functional diastolic murmurs are uncommon, functional systolic murmurs can be caused by changes in blood flow that may occur due to things such as colic, fever, exercise, pain or anemia. When the blood flow returns to normal—the horse is no longer colicky, for example—the murmur goes away.

Pathologic murmurs can also be systolic or diastolic, and they can be further differentiated into genetic or acquired murmurs. Again, the location of the murmur will help your veterinarian determine its type. Certain murmurs can more readily be heard on the horse’s right side rather than the left, for example, or at the apex of the heart as opposed to the base.

Murmurs are also graded based on the severity. The grading scale of 1 to 6 allows your veterinarian to set a baseline and track the progression of you horse’s murmur over time.

“A grade 1 is really difficult to hear and probably clinically insignificant,” Polkes said. “A grade 2 is readily heard. It’s soft sounding, but you can consistently hear it. A grade 3 can definitely be heard and may radiate into the chest. Then grade 4 to 6 get louder and louder. The biggest difference is at a grade 5 and 6, you’re going to actually feel a thrill on the chest.”

The Importance of Diagnostics

Depending on the type, characterization of and grade of the murmur, diagnostics should be considered, even if the horse doesn’t have clinical signs. Those diagnostics would include ultrasound of the heart, also known as an echocardiogram, and electrocardiogram to examine the structure, size and function of the heart. This is important to determine the level of concern and if further diagnostics or treatment is warranted.

“Diagnostic testing is essential when the horse is being ridden,” Polkes said. “If your horse was lame, you would have a lameness exam. If your horse has a murmur, you should really have a cardiac evaluation. It’s very important.”

Polkes also recommends yearly cardiac exams, where a veterinarian listens to your horse’s heart on both sides of the body to make sure there aren’t any new murmurs or changes. Regular examinations and record keeping can help you take the best care of your horse’s cardiac health.