My last post was an overview of heart sounds and function. Commonly, the heart and lungs are considered as synchronous systems where cardiac function and lung function are referred to frequently as the cardio-pulmonary system. Think of the living cells of your body and that of your horses' needing oxygen to stay alive while having to remove the waste product of metabolism, carbon dioxide. We need a system to pick up the oxygen, take it to the tissue cells, grab the waste, and remove it from the body.

Get to Know Your Lungs
Obviously, we breathe in oxygen from the air into our lungs and transport and exchange it through our blood transport system. The heart pumps oxygenated blood to capillaries in our tissue, then pumps deoxygenated blood back to the lungs, exchanges it again and we exhale the used carbon dioxide from our lungs. That exchange of oxygen and carbon dioxide must be precise to sustain the balance of life. The balance is called the ventilation (lungs) to perfusion (circulation) ratio. If an imbalance of either blood circulatory function or lung air function is present, we will be sick, unable to perform, or could even die.

Lung function has a seemingly simple task: to oxygenate cells efficiently. It is really a complex process that requires blood flow with rate adjustments, changes in blood vessel size and pressures, blood components such as hemoglobin to be the carrier, sensors from the brain and vessels to detect acid and alkaline imbalances, and precise tissue thicknesses between the lung alveoli and the blood capillaries for optimum gas exchange.

When we change the demand or consistency of oxygen access, the cardio-pulmonary systems adjust accordingly. Examples of heart and breathing rate adjustments occur while exercising or during exposure to high altitudes with lower oxygen content. The mechanics of the chest must also have normal compliance. The spring-loaded recoil following the end of inspiration involves stimulation of the intercostal muscles between the ribs and the diaphragm. Of course, we must respect the upper airway that serves as a complex spigot that if abnormal air cannot effectively get into or out of the lungs. The need for air is perpetual and life is dependent upon the constant availability and exchange of air.

Listening to Your Horse's Lungs
It can be difficult to listen to your horse’s lungs when he’s at rest--even with a stethoscope--because his breathing is relatively silent. Knowing how to listen is important. Close your mouth so that you become an "obligate nasal breather" like your horse, who does not breathe through his mouth. Breathe in and you can hear yourself. This is because you are actively using your chest muscles and diaphragm to suck the outside air into your respiratory tract. Now relax and let the air go out. This is pretty silent since it represents passive recoil of your chest that normally doesn't need to push air out. Since you can't tell your horse to "take a deep breath," listening to your horse requires some technique to hear lung sounds. Standing beside your horse's head place your hand over both nostrils until the horse begins to resent it by tossing its head or moving away from you. This happens pretty quickly because the carbon dioxide builds up in the brain telling the horse I've got to breath! When you release your hand from the nostrils, the horse will breathe excessively to compensate for the recent lack of the aeration you caused.

Breathing in will be deeper and faster and breathing out will also become active and more rapid. You can now use your stethoscope to listen to the chest for accentuated lung sounds. Recovery back to normal breathing rate and pattern should occur quickly. If you listen to both sides of the chest you may have to repeat the hand-over-the-nostrils technique. You can also listen over the neck to the trachea or windpipe. In a normal horse, coughing should not occur with the examination procedures. If coughing does occur the upper airways usually generate a "hacking cough," the lungs provide more of a deep cough that can be dry or productive with mucus, and finally a soft cough that usually means chest pain like with fractured ribs or pleurisy.

Remember, the cardio-pulmonary function can additionally be assessed by looking at your horses' oral mucus membranes: bluish indicates too much CO2; bright red indicates too much O2; pale to white means there is not enough blood; purple means toxic. Normal mucus membranes may also be seen in the presence of a disease process because your horse has physiologically compensated. The capillary refill time (CRT) can be used to assess mucus membrane perfusion.

A normal respiratory rate in adult horses at rest is about 8-12 breaths per minute. You can get the rate by listening, watching the thoracic movements or wetting your hand and feeling for air flow cupping your hands near the nostrils (be careful not to stimulate his sense of smell). Resting respiratory rates can increase with high ambient temperatures and humidity elevations. Blowing off heat through the respiratory tract and sweating are normal heat regulatory compensations in the horse and using a cold hose bath can result in immediate relief. However, consistent abnormalities of the lungs should alert you to contact your veterinarian.

Coughing is not always present as a clinical sign but the respiratory rate or pattern of thoracic movements may be consistently abnormal. For example if your horse has heaves expiration becomes an accentuated effort and the abdominal muscles are needed to expel trapped air. This constant effort results in an abdominal "heave line." Pneumonic sounds can include wheezing, crackles, and harsh airways where inspiration and expiration lack a change in the intensity of sounds.

If your horse's vital signs suggest a problem with lung function have your veterinarian confirm a clinical diagnosis. This may entail the veterinarian auscultating (listening) to the lungs, perhaps an endoscopic examination, or an ultrasound evaluation using a field scanner. Chest x-rays are seldom performed although they may provide additional diagnostic information for difficult cases.

If you are routinely checking your horse's vitals, take a little extra time occasionally to listen to the lungs as a reassuring sign of the health of your horse. In the healthy horse there is very little to hear but don't be afraid to listen and call your veterinarian if you suspect a problem.

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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