THE HORSE’S HEART:
PULSE RATE AND HEART SOUNDS

by HEATHER SMITH THOMAS

Heart rate is one of the vital signs (along with respiration rate, temperature, etc.) that can tell the horse owner or veterinarian many things about the animal’s health or fitness. Listening to the heart itself can give additional clues.

PULSE RATE: Pulse rate can be detected with the fingers over any major artery (such as under the jawbone, behind the knee), or along the jugular groove, but a stethoscope over the heart (girth area, left side of the horse) can give you heart rate and heart sounds.

Pulse rate in a newborn foal is erratic–30 to 90 beats per minute during the first five minutes after birth, 60 to 200 per minute during the first hour, 70 to 130 for the first 48 hours. Foals up to yearling age have a normal resting pulse rate of 70 to 90, yearlings 40 to 60, while an adult horse’s resting rate is between 30 and 40. The smaller the animal, the faster the heart beat. Ponies have a higher pulse rate than horses, and an athletic horse has a lower resting rate than an unconditioned horse.

The equine heart pumps a tremendous amount of blood when the horse exerts, to supply oxygen to working muscles. When working hard, the heart can greatly multiply its rate and output, jumping quickly from resting rate to as high as 240. The adult horse’s heart pumps about a quart of blood with each beat. When exerting hard, the heart pushes blood through at 60 gallons per minute.

The heart is actually two side-by-side pumps with a solid wall between them, and each pump has two chambers. The right side pump sends oxygen-depleted blood from the muscles to the lungs, under low pressure, and the left side forcefully pumps oxygen-rich blood under high pressure back out to the muscles.

The horse has a strong heart which becomes even stronger in response to the demands on it (athletic conditioning). Fitness conditioning enables the heart to beat faster for longer periods without tiring, and can increase the amount of blood pumped each beat by as much as 40 percent. The fit heart not only delivers more oxygen to the muscles but also recovers faster after exertion, slowing down quickly. The fit horse has a lower resting rate than the average horse; his athletic heart is usually larger and more efficient–more volume of blood pumped per beat. It doesn’t have to beat as often to pump the necessary blood. Some horses are born with more efficient hearts, and have more speed and stamina. Heart efficiency can also improve with athletic conditioning.

LISTENING TO THE HEART: To check the heart’s workings, press a stethoscope firmly against the chest wall in the girth area, left side–a little above the elbow, in the armpit area. Practice when the horse is calm and resting, with no distracting noises. Listen to the beat and sort it from the background sounds of respiratory swishes and intestinal gurgles.

The normally heard rhythm is Lub-Dub pause. The first sound is dull, deep, long and loud and the second sound is sharper and shorter. In horses it’s not uncommon to hear four sounds. The loud LUB is heard as valves between top and bottom chambers close and the ventricles (bottom) contract, with blood rushing out. The sharper DUB occurs as valves in the arteries out of the ventricles snap shut and the heart starts to relax and fill with blood from the top chambers. The soft boo (third sound) occurs with the rapid filling of the ventricles during the heart’s relaxation and is heard as a dull thudding sound immediately after the DUB. This third sound can be heard in most athletically fit horses and is more audible at heart rates slightly higher than resting rate. The fourth sound (du) occurs with contraction of the top chambers and closing of its valves, coming immediately before the blood once again rushes into the arteries with the next LUB. The sequence is thus du, LUB, DUB, boo. In some horses the softer sounds are inaudible even with a stethoscope. When counting heartbeats, use a watch with a second hand, counting the number of LUB-DUBs during a 15 second period (each LUB-DUB as one unit). Multiply by four to arrive at beats per minute for resting heat rate.

Heart rate, rhythm and sounds can give clues as to health and fitness. Familiarize yourself with each horse’s resting rate and heart sounds. It can vary from horse to horse. Then you’ll have a better idea of what is normal for each horse and can recognize abnormalities if they occur.

Rapid heart rate can be a sign of fever, pain, blood loss, shock, anemia, colic, dehydration or congestive heart failure–any condition in which the heart is working harder to try to meet the need. But nervousness, excitement or exertion can increase heart rate, so the horse’s emotional state and recent activity level must be taken into consideration when checking pulse. A too-low rate can also mean trouble.

IRREGULARITIES: The heart beats with a very consistent rhythm, the regularity controlled by its pacemaker–a nerve center at the top of the heart muscle that sends signals to trigger contractions of the muscle. The most common abnormality in horses’ heart rhythm is an occasional skipped beat when the horse is at rest. This is often called athletic heart since it occurs most frequently in fit horses with efficient hearts. When the horse is resting and the heart is beating at its slowest, it may still be pumping more blood per minute than the horse needs, so it skips a beat now and then (at regular intervals, such as every fifth beat) to balance out the blood flow and body’s needs. The first chambers contract as normal, but the wave of contraction does not continue to the second chambers, thus reducing blood flow out of the heart.

This irregularity disappears when the horse starts exercising and heart rate increases. This type of rhythm disruption is normal; dropped beats are common in fit horses. Even a random electrocardiogram sampling of horses will show dropped beats in about 18 percent, and a longer monitoring period would show nearly half of all horses’ hearts skip a few beats in a 24 hour period.

Other rhythm irregularities (arrhythmia) are not normal, and could mean a heart problem. The most common significant arrhythmia is atrial fibrillation–in which the upper chambers of the heart vibrate spasmodically instead of rhythmically contracting to pump blood. The heart has a random rhythm, which if extreme, puts it into an ineffective flutter instead of the normal squeeze-relax sequence. The mixed signals (creating chaos in the contractions) are usually due to heart-muscle inflammation. When the horse exercises, the fibrillating heart gets even worse, the horse runs short on oxygen, stopping abruptly or even collapsing.

This type of arrhythmia can sometimes be treated successfully, if the horse has no other complicating heart problems. Using quinidine sulfate, veterinarians are often able to return a heart to normal activities. With treatment, many victims of atrial fibrillation can return to former careers, including racing.

Other variations in heart rhythm sometimes occur, but more rarely. Rule of thumb for determining seriousness of arrhythmias is that those occurring at resting rate (vanishing when the heart works harder) are usually harmless. But extra beats or irregular rhythms at higher heart rates could mean a serious problem.

MURMURS: Horses also sometimes have heart murmurs. Movement of blood through the heart chambers is usually silent, but when there is turbulence in the flow, the extra or peculiar sounds are called murmurs. These are classified according to intensity (loudness), pitch (high squeak or low rumble) and tone (musical or harsh) and the portion of the beat cycle in which they occur. There are many causes of heart murmurs, including illness and birth defects, leading to abnormal structure in the heart.

Noisy turbulence can be caused by a valve that can’t close completely (letting blood leak back), a valve that doesn’t open wide enough, or blood leaking through a hole in the dividing wall between the two pumps. The timing of the murmur in the cycle, and location–left or right side–where it is loudest, can help pinpoint the source. Depending on severity of the condition, a horse with a faulty valve may manage fine except when pushed too hard in strenuous activity.

A murmur caused by a hole between the left (high pressure) side of the heart and the right (low pressure) allows oxygenated blood to leak from the left into the right side and interfere with proper pressure of both pumps. This is the most common congenital heart defect and can produce a noisy murmur. Its seriousness depends on the size of the hole rather than loudness of the murmur.

The loudest murmurs are accompanied by a thrill or vibration against the chest wall, which is obvious to the touch. You can feel it with your hand. Often the murmurs accompanied by a thrill are severe enough to keep a horse from an athletic career, but not always. There are no firm generalizations about what a horse with a murmur can or cannot do.

Sometimes turbulence is produced when blood eddies around an unusual contour or projection in a heart chamber. These are usually nothing to worry about. A low pitched blowing sound following the LUB is an example; this is the sound of rushing blood being ejected from a larger-than-average heart. Athletic horses with big hearts often have this type of murmur. A survey of steeplechasers found that more than 90 percent have this type of murmur.

A less common type occurs sometimes in a newborn foal when a diversionary passageway in the heart fails to close. Before birth, blood is not oxygenated in the lungs, and a natural shunt exists between the right and left atria (tip chambers) and another between the pulmonary vein and aorta. These shunts normally close at birth and force blood into the lungs. Sometimes they don’t completely close, reducing heart efficiency. If the defect is severe, the foal won’t live long. Or he may survive but not do well. If the condition persists, he lacks sufficient size and strength as a yearling or two-year-old for normal training.

The same situation arises when there is an abnormal opening between the two ventricles. In either case, the blood is being shunted in the wrong direction. In trying to pump out both the oxygenated and de-oxygenated blood, the heart is overworked. How long a horse can live with this defect and how much work he can do depends on the size and location of the hole. Some heart defects are not discovered until autopsy.

All of these false routes within the heart change the tempo of blood flow by increasing the quantity of blood in an area. Like a too-full river, blood gushes through the limited passageway, creating the reverberations heard through the stethoscope as a murmur. Murmurs are fairly common. With time, some of these residual shunts close up on their own and the horse goes on to lead a normal life, though it might never become a top performer.

Some systemic bacterial diseases can cause infection of the heart valves, resulting in valves that don’t close properly, making the heart inefficient. A chronic infection (valvular endocarditis) leads to such scarring of the valves that they never close well.

SIGN OF HEART TROUBLE: Some horses can lead normal lives with minor arrhythmias or murmurs, but others have more serious problems that limit athletic performance. In some cases the heart adapts and compensates for structural faults. If there is a leaky valve, the heart may become more muscular to increase the force of its action and offset the inefficiency. But in other instances there is no way to compensate for or overcome the problem and the horse’s performance is seriously impaired.

Muscle, skeletal, and respiratory problems are much more common reasons for poor athletic performance in a horse, but exercise intolerance can also be an indication of heart trouble. The horse is unable to exert because of disrupted blood flow; not enough blood is getting to his tissues to meet the oxygen demand. He tries to drag more oxygen in by breathing harder, but it doesn’t help. The horse with a racing heart and heaving lungs is not a good performer, and the exertion may even cause him to collapse or faint into unconsciousness due to insufficient circulation to the brain.

Other signs of heart insufficiency besides fatigue and labored breathing are cough (because blood may back up in the lungs) or accumulation of fluid in lung tissue. Reduction of blood output by the heart affects the whole body. Gums and mucous membranes may become blue from oxygen starvation. Serum may accumulate in the tissues of abdominal area and chest, and in extreme cases the legs.

If you suspect a heart problem, the horse should be thoroughly checked. Your vet can discover a lot from careful listening with a stethoscope before and after exercise. If abnormal heart sounds are heard, the vet may recommend further diagnostic tests to get a more accurate diagnosis and assessment of the horse’s future.