Checking the Vitals: Heart Function and Sound

You are at a concert when suddenly the orchestra stops playing their instruments with the exception of the kettledrums. The kettle drummer hits two beats with a pause. This is repeated over and over as you realize the drumbeats are the heart and rhythm for the orchestra. In your head you can mimic your own heartbeat with valvular doors that slam with distinct thuds and a synchrony of rhythm that is supposed to alert you to any delay in beats or an undeniable rumble of the added percussion that is a sound gone array. These sounds are meaningful, they emanate from your heart, and that of your horse's, as barometers for the physicians, and veterinarians, to rely on for interpretation of your vital signals to your health status.

checking the vital signs in a horse

Dr. Byars listens to a broodmare’s heart.

The heart functions as a magnificent pump. It fills and empties thousands of times a day. There are four chambers, and when divided into two sides, the left and right sides each perform their functions to transport blood to the cells of the body. The left side of the heart pumps oxygenated blood into the arterial circulation through arteries, arterioles, and capillaries where the oxygen is exchanged and the used blood is returned to the venules, veins, and eventually back to the right side of the heart. The blood then travels from the right side of the heart where it is pumped into the lungs to become re-oxygenated, and the exchanged carbon dioxide waste is expelled through the air. The new oxygen-rich blood is then returned to the left side of the heart and again pumped out into the arterial circulation.

The heart pump does this through its chambers, which are separated by valves while adjusting to different volumes and rates. These changes are signals of pressure and oxygen sensors throughout a complex physiological system that feeds from a network of regulators throughout the body and brain.

The heart should function with a rate and rhythm that is consistent with an overall assessment of your horse's health. If there is an inconsistency, it does not always mean that your horse's heart is failing, in fact heart failure in horses is reasonably uncommon. It may mean the heart rate and rhythm is adjusting to other physiological issues such as in stress, exercise, electrolyte abnormalities, shock, toxic insults, and a myriad of circulatory disturbances that may require regulation of heart function.

The heart rate and rhythm is most easily attainable with a stethoscope. Sit in a chair and listen to your own heart by moving the stethoscope over your chest until you get the most distinct "lub-dub," the two sounds that make up a single heart beat followed by a delay. Use a watch or nearby clock with a second hand and count the heart beats over a 15 second interval then multiply by four to determine your heart rate per minute. If extra beats or dropped beats are present you will probably need to count for a full minute. These sounds are the interpretive information provided through your stethoscope to your ears. The EKG or electrocardiogram is the visual tracing device at the hospital or doctor's office used for the same or similar purposes.

Mammals, including yourself and your horse, have four-chambered hearts; a left and right ventricle (lower compartment) and a left and right atrium (upper compartment). The chambers are separated by valves and walls (septums). For simplicity we label the heart by left and right side according to the function. As mentioned the left side carries the oxygenated blood that has returned from the lungs and ejects it out of the aorta during cardiac contraction (systole) and then refills with fresh blood during relaxation (diastole).

Lub, Dub Explained


Systole or contraction phase (blood pumping to the body & lungs):

  • Both the mitral and the tricuspid valves (which allow blood flow from the left atrium to the left ventricle and from the right atrium to the right ventricle respectively) close to prevent backfilling; this closure creates the "lub" sound;
  • Fresh, oxygenated blood is pumped out of the left ventricle, through the aortic valve to the horse's body; and
  • Used, deoxygenated blood is pumped out of the right ventricle through the pulmonary valve to the lungs


Diastole or filling phase (the heart relaxes & blood flows into the heart from the lungs & body).

  • At the end of the contraction (systole) phase both the aortic valve and the pulmonary valve close creating the "dub" sound;
  • Simultaneously the mitral valve (separating the left atrium from the left ventricle) and the tricuspid valve (separating the right atrium from the right ventricle) open;
  • Used, deoxygenated blood from the body enters the right ventricle through the tricuspid valve; and 
  • Fresh, oxygenated blood from the lungs enters the left ventricle through the mitral valve.

A leaking valve will usually be heard as a heart murmur (swishing sound).

Now go listen to your horse. The heart sounds and rhythm should be similar to your own sounds. The heart functions are also the same although the horse has a much more efficient muscle electrical conduction system that assures peak heart muscle performance. Still, when listening the "lub" will be the first heart sound (S1) and will reflect closure of the ventricular valves (A-V) during systole, then S2 as the dub sound from the semilunar valves. These two sounds are the basic ones to be heard although variations may occur such as hearing three or four total sounds between delays. Trot your horse and listen again to listen to normal correction of heart sounds.

Murmurs will be heard as swishing or dampening of either the lub or the dub. The majority of murmurs arise from leaking heart valves and if of low intensity they are likely not pathological. Most abnormal sounding hearts are actually within the normal limits and getting an opinion from your veterinarian is important in the final interpretation of either the rate, rhythm or the presence of a murmur as suspected abnormalities.

Remember, the lub and dub make up one heartbeat with the heart rate of an adult horse being between 32-44 beats per minute.

Let's apply this to a couple of clinical scenarios. Your horse does not eat all of his food. His temperature is normal and the heart rate is 48 per minute while respiration is not elevated. You can alert your veterinarian about the not eating but the possibility of an emergency farm call is unlikely. You monitor your horse's vitals and your veterinarian can visit at both of your conveniences. Same scenario, your horse is not eating, heart rate is 80 per minute and you've checked it twice. The temperature is normal with a slight elevation in respiratory rate. Your veterinarian is on the way. The heart rate is the impact number that changes the response rate and makes you and your veterinarian a team. In actuality that second scenario horse developed a severe diarrhea shortly after the veterinarian arrived and was treated appropriately.

Contact your veterinarian and learn how to assess your horse's vital signs today. Knowing how to identify abnormalities in your horse's heart rate and rhythm will help you and your veterinarian treat him or her when illness strikes.

Reprinted with permission from the Kentucky Horse Council.

About the Author

Doug Byars, DVM, Dipl. ACVIM

Doug Byars, DVM, Dipl. ACVIM, is Director of the medicine clinic at Hagyard-Davidson-McGee equine practice in Lexington, Ky.

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