Heart Murmurs and Valvular Heart Disease in the Horse
Dr. Melissa Mazan
Assessment First, your veterinarian will assess your horse from a distance. She will look to see if your horse shows any signs of breathing difficulty, which can be secondary to cardiac disease.
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:
Your veterinarian will feel your horse's pulses. The pulses are felt over the arteries and should be strong, rhythmical, and they should have a normal rate (28 – 44 beats per minute). Abnormalities that might indicate that there is a problem with the heart include an abnormally high heart rate, weak or overly-strong pulses and skipped beats.
Your veterinarian will also want to assess your horse's veins. In cases of congestive heart failure the veins may be large and distended, because blood is backing up in the heart, and the heart cannot adequately accept the normal load of blood. This is reflected in the veins becoming large and distended. The jugular vein (the large vein that runs the length of your horses neck) is often the first vein to show distention, and may even appear to have a pulse (only arteries have true pulses, but the blood backing up rhythmically from the heart may give the jugular vein the appearance of a pulse).
Your veterinarian will then run his/her hands over the whole of your horse's body, especially the underbelly to look for signs of edema. When a sufficient amount of blood backs up from the failing heart, it eventually forces some of the body's fluid out of the vessels, and into the surrounding tissue. This causes the tissue to become thickened, and if you gently press your finger into an area of edema, you will find that it leaves a dent. This is called pitting edema.
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.
Your veterinarian will carefully examine your horse's mucous membranes – in the male horse, this will be primarily the gums and the vulva in the female horse. Normally, the mucous membranes are a medium to pale pink, they are moist, and the capillary refill time (CRT) is less than 2 seconds. This means that if you gently but firmly press into the mucous membranes, the area will initially blanch, and then will return to its normal color. In the normal horse, this occurs rapidly. If the horse is in cardiac failure, the CRT will often be greater than 2 seconds. In severe cardiac failure, the mucous membranes may appear cyanotic, or blue, because not enough blood is being oxygenated in the lungs.
Your veterinarian will also take your horse's temperature. Your horse may have a fever if he has an infectious cause of cardiac disease.
Your veterinarian may also choose to do baseline bloodwork. If your horse has an infectious cause of cardiac disease, he will often have an elevated white cell count, and an elevated fibrinogen (this is a protein that reflects the presence of inflammation in the body.
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.
If your horse has a fever, your veterinarian may also choose to perform blood cultures to help to determine whether there are bacteria in the blood stream that would indicate an infectious cause of valvular heart disease.
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.
During auscultation, your veterinarian will listen for the normal rate and rhythm, as well as for any murmurs. The normal horse has four auscultable sounds, termed S1, S2, S3, and S4. In most other species, only S1 and S2 are audible, but because the horse's heart is so large, we can hear sounds that we cannot hear in smaller animals, such as humans.
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
Systolic (happening between S1 and S2)
Diastolic (happening between S2 and S1)
Regurgitant (the type of sound that is heard because blood is flowing in the wrong direction through a leaky valve)
Ejection (the type of sound that is heard when blood is being pushed through an opening that is too small).
Remember, though, that all a heart murmur really tells you is that there is turbulent blood flow in the heart.
Your veterinarian will palpate your horse's pulses while listening to the heart. This will help to determine where in the cardiac cycle the murmur is, and whether there are any skipped pulses (that is, you can hear the heart beat, but there is no accompanying beat of the pulse).
Finally, if there is a murmur, your veterinarian will put together the physical examination findings and the history to determine if an echocardiogram is warranted. An echocardiogram is an ultrasonographic picture of the heart that can help to determine where the valvular abnormality exists, and if there is a functional problem with the heart. Many veterinarians can perform this examination in the field. Otherwise, your veterinarian may choose to refer you to a facility where the examination can be performed.
Along with the echocardiogram, your horse will usually have an electrocardiogram (ECG, or EKG) performed as well. In the horse, this primarily tells us if there is a problem with the rhythm of the heart.