The heart is the most important muscle in the body – without the heart, none of the other muscles would be able to get the fuel that they need – oxygen. The job of the heart is to pump blood that receives oxygen in the lung to the entire body – and certain areas of the body, especially the brain, cannot survive without the oxygen in the blood for more than a few minutes.
The heart has two sides or halves – the left and right.
It is very important that oxygenated and deoxygenated to be kept separate so that the body can get plenty of pure oxygen for fuel. For this reason, the heart has four one-way valves that function to keep the blood flowing in the right direction – which is critical for the survival of any individual.
The right heart has the tricuspid and pulmonic valves. The left heart has the mitral and the aortic valves. Deoxygenated blood flows from the body to the right heart, where it goes into a holding chamber called the right atrium. It then goes through the tricuspid valve and into the pumping chamber called the right ventricle. The blood then flows through the pulmonic valve and into the lungs.
Once the blood circulates through the lungs it becomes oxygenated blood and returns to the heart. Here it goes into the left atrium, through the mitral valve, into the left ventricle, and then out through the aortic valve and to the rest of the body, where it is distributed into body as fuel. It is this series of valves that keep the blood organized and flowing in the right direction.
Valvular heart disease interferes with the normal rate and smoothness of blood flow through the heart. Valvular disease in horses usually leads to congestive heart failure (CHF), and, indeed, is the most common cause of CHF in horses.
If a sufficient amount of blood flows in the wrong direction, the affected individual will be oxygen-deprived. The heart chambers that receive the extra blood (flowing in the wrong direction) will also get overloaded – a leaky valve can produce a futile cycle for the heart. No matter how hard it tries to pump the blood out, more blood leaks back, increasing the work load further. Eventually, the heart becomes deformed (dilated or hypertrophied) and may fail.
Valvular heart disease can be due to problems that were present at birth (congenital disease), or it can be acquired. The most common congenital causes of cardiac disease include stenosis (narrowing) of one of the valves and leaky valves.
Regurgitation is a backward flowing of blood through a defective heart valve, and the most common types of valvular heart disease in the horse include
What to Watch For
Although a common symptom of valvular heart disease in the horse is a heart murmur, it is important to remember that a heart murmur does not necessarily mean that your horse has CHF or even heart disease.
A heart murmur, whether in man, or horse, or any other species, simply tells us that there is turbulent blood flow somewhere in the cardiovascular system. Murmurs denote the release of vibratory energy into the fluid and tissue that surround this disturbance, which is sometimes compared to the sound produced by a violin string. For example, a valve that is vibrating will make a noise like a murmur.
In some cases, the heart murmur is an indication of a malfunctioning heart. In other cases, it has no real health significance at all. Even when there is cardiac disease (that is, there is some anatomical or physiological abnormality associated with the heart), the horse may not show any signs of cardiac failure.
Because horses have very large hearts and exceptionally high blood flow, they are more prone to having audible heart murmurs without having actual heart disease. Nonetheless, cardiac dysfunction is thought to be a very important cause of poor performance in the horse, ranking only behind musculoskeletal problems and respiratory disease as a cause of exercise intolerance.
The first, and most important way to assess the heart in any horse is with a good physical examination, including a thorough auscultation of the heart (listening to the heart with a stethoscope). Your veterinarian will look for signs of heart failure:
Remember, though, before you get too worried, that many normal horses experience mild edema in their legs, called "stocking up" if they remain stationary for too long. For instance, a horse that is hospitalized for other reasons will often stock up in one or more legs, without there being any heart disease. This is because horses need exercise to help drive the blood and accompanying fluid up the legs and back to the heart.
A chemistry profile may show elevated muscle enzymes, which may reflect damage that has been done to the heart muscle. It may also show that there has been damage to the kidneys or liver, because the heart has not been able to pump enough blood to these critical organs.
In some cases, your veterinarian will also choose to perform a blood gas analysis. This test tells us about the level of oxygen and carbon dioxide in the arterial blood. There may be an abnormally low level of oxygen (hypoxia) and high level of carbon dioxide (hypercarbia) in the case of cardiac failure.
At this point, your veterinarian may know quite a bit about your horse's heart, all without having put a stethoscope to your horse's chest. But this is an important part of the exam.
The sounds of S1 and S2 are associated with the large mitral and tricuspid valve movements, S3 is associated with flow of the blood in the ventricles, and S4 is associated with the contraction of the left and right atria. The time between S1 and S2 is considered systole, when the heart is actively pushing blood out into the body. The time between S2 and S1 is considered diastole, when the heart is relaxing, and passively filling with blood.
When there are abnormalities in the valves, we often hear noises in between the normal sounds, and these noises are heart murmurs. Heart murmurs are categorized as
Remember, though, that all a heart murmur really tells you is that there is turbulent blood flow in the heart.
Specific Causes of Valvular Heart Disease
Endocarditis refers to an infection of the inner lining of the heart. Although it is not common, it is an important cause of valvular heart disease in the horse.
Scamp, a 12 year old Quarterhorse gelding, lived in a comfortable barn with 10 other horses. They were all well-loved companions, and did a fair amount of traveling to small local shows, hunter paces, and the like. A respiratory virus had gone through the barn a month and a half ago, and everyone had recovered quickly except Scamp. He had had a fever and a cough for 5 days, and his veterinarian, suspecting that he had a secondary bacterial respiratory infection on top of a virus, put him on antibiotics for 7 days.
He seemed to do well, with the fever going down after just a few days of treatment. After that, Scamp did well for a week or two, but then he seemed to deteriorate. He lost his appetite, and began to lose weight. He didn't cough anymore, but he looked a little ragged.
His owner called his veterinarian, Dr. Green, who decided to do a thorough work-up before putting Scamp back on antibiotics. He found pitting edema under his belly and the jugular veins were distended. He also detected a pulsation in his veins and a high fever of 104.2 degrees Fahrenheit.
Dr. Green also found an abnormally high heart rate and a loud systolic murmur with a regurgitant quality – a grade 5 out of a possible 6. He suspected endocarditis, and recommended bloodworm, a blood culture, and a consult with a cardiologist for an echocardiogram. In the meantime, he started Scamp on some very broad spectrum antibiotics.
The complete blood count (CBC) showed a high white cell count, and a high fibrinogen level. The chemistry profile also showed a high globulin level, indicating that Scamp's immune system was working hard at trying to eradicate an infection. The lab also called to say that the blood culture was already starting to grow bacteria – indicating that Scamp had an infection in the blood.
The cardiologist scanned Scamp's heart, and, just as they suspected, found an infection involving the mitral valve. The infection had already caused some valve deformity, and Scamp's owner could appreciate that the valve, which should be smooth and working with clockwork precision, was thickened and lumpy. This is called a vegetation; the valve no longer works to keep the blood flowing in one direction.
Dr. Green started Scamp on some very powerful antibiotics until he received the results of the blood culture and could prescribe something more specific to the causative organism. Hopefully, several months on antibiotic therapy would eliminate the organism.
Scamp was on antibiotics for 8 weeks. During the first 3 weeks, he stayed in the hospital on intravenous antibiotics, then he was able to receive oral antibiotics at home. Scamp remained thin and depressed for quite some time, but after the first three weeks, he gradually began to seem a little more like himself. His heart rate came into the normal range, and he no longer had any fevers. After 8 weeks, his blood work looked normal, and he held his tail high as he trotted around the pasture. It took another 4 months before Scamp had regained the lost weight, and was able to go back to his usual rounds of hunter paces and showing.
What happened to Scamp?
Scamp probably got influenza, like all the other horses. Unlike the others, he developed a bacterial infection on top of the viral infection. The bacteria were resistant to the antibiotic that Scamp was on, and eventually spread to the blood, and then to the heart. This is not a common sequela to influenza, but it is a common history in a horse that does develop endocarditis. Scamp was lucky that his veterinarian was careful and astute, and found the endocarditis quickly.
Buttercup was a 22 year old mare, who had been in the company of the same owner for the past 12 years. Buttercup had never had any major illnesses; nevertheless, she had a "well horse" examination performed by her veterinarian every year.
This year, Dr. Green heard a murmur between in the interval between the second and first heart sounds, which means that it was diastolic, and it had the sound of a cooing dove. He suspected that Buttercup had aortic regurgitation. Although he didn't find any signs of cardiac failure on examination, he ordered an echo done just to be careful.
Dr. Green asked the following questions of the owner:
The answer to all these questions was 'no'. Buttercup was in excellent condition.
When the cardiologist examined Buttercup the following week, she noticed the left side of the heart was slightly enlarged and noticed there was blood flowing backward through the aortic valve. This was responsible for the murmur, and for the mildly enlarged left heart.
The cardiologist reassured Buttercup's owner that the horse did not show signs of cardiac failure, although she definitely had cardiac disease. Aortic regurgitation is the most common valvular problem in older horses. Although it is a degenerative disease, it tends to progress slowly.
Buttercup did not need drugs for her heart, but sometime in the future she would probably need to be on drugs, such as digoxin to help strengthen her heart. He advised the owner to be a little more careful with Buttercup, but he advised the following:
Oilcan Harry was a 2-year old Standardbred who was not performing as well as his owner would have liked. He had a lifetime mark of 1:56 – good, but not good enough. Because his breeding was good enough that he had the potential to be a faster horse, his owner brought him into the Sports Medicine Clinic at the nearby veterinary school.
At the clinic, Harry had an extremely thorough work-up. The first step, as always, was a thorough history and physical examination. The clinician, resident, and students peppered the trainer with questions.
It turned out that about six months ago Harry had had a case of the flu, as had most of the other young horses in the barn. Harry had seemed to recover well, except for a cough that had never really gone away. There were two other youngsters in the barn who were still coughing as well.
Harry didn't seemed sick other than that, and after the initial episode of influenza, never had any fevers. His trainer was concerned, however, that Harry never seemed to have a very good appetite. He was fed 8 lbs of a sweet feed mix twice daily, and two flakes of timothy/alfalfa hay in the morning and at night – and he seldom finished it all.
On physical examination, Harry seemed to be in good general health, although on the thin side. One of the senior students noticed a systolic murmur with a regurgitant quality – a grade two out of a possible six. Dr. Smith ordered an echocardiograph, which showed that the cause of the murmur was tricuspid regurgitation, meaning that every time Harry's right heart contracted, a little of the blood washed back into the right atrium, instead of progressing all the way into the right ventricle.
The echo suggested that the heart was doing just fine – the dimensions were normal, and the ejection fraction, a measure of the strength of the hearts contractions, was normal.
Concerned about the cough that Harry had had once in a while, Dr. Smith returned to the history. They performed a few more tests – CBC and chemistry profile looked absolutely normal. However, when they listened to Harry's lungs with a rebreathing bag, Harry coughed and coughed. At this time the lungs didn't sound normal. Rather, they could hear a few wheezes when Harry breathed out.
Dr. Smith decided to go a little further, and performed tests to look for small airway inflammatory disease (SAID) – and they found that Harry indeed had all the findings to confirm SAID.
As it turned out, the tricuspid regurgitation was just an incidental finding and not the cause of Harry's poor performance. Many young horses have a small amount of tricuspid regurgitation; in fact, it is more common in young racehorses – probably because their hearts are pumping so much blood.
Harry got the flu just like all the other youngsters in the barn, but his airways remained reactive to stimuli such as dust and molds, and he developed a non-infectious inflammation in the small airways, which limited his ability to get oxygen when he was working at very high speeds.
Dr. Smith decided to treat Harry for SAID. After two months of treatment, Harry was no longer coughing. After 4 months, the trainer called to say that Harry was faster than he had ever been. He probably would never be world class, but the trainer was happy that Harry was performing at the level he was bred for.
Pulmonic stenosisPumpkin was a 6-month old Morgan filly who just didn't seem to be growing well. She wasn't weaned yet, but shared a large pasture with five other mares and foals – all roughly the same age as Pumpkin. Her dam and sire were not overly large themselves, so Pumpkin's owner, Mrs. Bright, wasn't too worried in the first few months when Pumpkin seemed smaller than the other foals. Lately, it seemed that not only was Pumpkin smaller than the other foals, but she also couldn't keep up with them.
When Dr. Green came out, of course she had many questions for Mrs. Bright.
Pumpkin had always seemed a little smaller, but just seemed to stop growing at about 3 months of age. She was dewormed every four weeks with all the others, and Dr. Green herself had vaccinated Pumpkin and the others just last month. Pumpkin seemed to have an excellent appetite – in fact, she did little else but eat.
Dr. Green examined Pumpkin carefully and found an unmistakable heart murmur – a grade 6 out of a possible 6, meaning that it was palpable just by laying a hand over the area of her heart. It was a systolic murmur, ejection type, clearer on the right side, which meant that it could be pulmonic stenosis – an abnormally narrow pulmonic valve – or possibly a ventral septal defect, or VSD – an abnormal opening between the left and right ventricles. Pumpkin most likely had a congenital heart defect, which was the cause of her stunted growth.
The next day, the cardiologist scanned Pumpkin's heart and found her heart to be abnormally large and thick. It looked as though it has been working too hard.
The defect had been making Pumpkin's heart work much too hard – and Pumpkin's heart could no longer deliver enough oxygen to the rest of her body, which made her weak and tired. Pumpkin would probably have a shortened life span. She'd be able to be a pet – but she would never again be a pleasure horse. I
n general, congenital cardiac defects are rare in horses – and pulmonic stenosis is one of the rarest defects. Some congenital heart defects, such as small ventricular septal defects, which are more common, often result in no exercise intolerance at all – indeed, some very good racehorses have small VSDs. A congenital valvular defect that is significant usually causes signs such as those seen in Pumpkin –stunted growth, exercise intolerance, and general failure to thrive. At this time, there is not much that veterinary medicine can do for horses such as Pumpkin. Hopefully, in the future, we may be able to do more.