Measuring Arterial Blood Gas in Animals

Jamie M. Burkitt, DVM, DACVECC, assistant professor of clinical surgical and radiological sciences at the University of California, Davis, explains that arterial blood gas (ABG) analysis can be used to monitor pulmonary function. In general, she says, an ABG test can help veterinarians distinguish whether an animal is hypoxemic.

"Arterial blood gas analysis can be used to monitor pulmonary function in dogs-and also in cats although it's much harder sample to get from a cat. But in general, the main thing that I would remember when it comes to measuring an arterial blood gas is that what you want it to do for you is to help you distinguish whether or not an animal truly is hypoxemic. So whether or not they really have low blood oxygen and if they do have low blood oxygen, is it secondary to hypoventilation? So, are they simply not managing a large enough tidal volume or a high enough respiratory rate to get enough fresh gas in or is it due to some type of what we call venous admixture.

Venous admixture is a very broad term that describes a situation in which heart blood moves from the right side of the heart-so the venous side of the circulation-to the left side of the heart-or the arterial side of the circulation-without actually getting oxygenated by the lungs. That can happen if you have a true anatomic shunt from the right side of the heart to the left side of the heart, or through the lungs as well, there can be large anatomic shunts through the lungs that can do this. But that's also true anytime we have a disease process like aspiration pneumonia or congestive heart failure or noncardiogenic pulmonary edema, pleural effusion, and asthma. All these different disease processes basically lead to hypoxemia due to a venous admixture, or blood moving from the right side of the heart to the left side of the heart without being oxygenated.

The thing that a blood gas can do for you is tell you whether that hypoxemia is due to hypoventilation or if instead it's actually due to venous admixture. What it cannot do is tell you which of those causes of venous admixture are actually leading to your hypoxemia. So, you already know from your blood gas that the animal has venous admixture, but you really want to know if it's aspiration pneumonia or congestive heart failure. For that, you need a radiograph or another imaging modality or airway sampling and things along those lines. Arterial blood gas analysis does not tell you the underlying etiology-that's what your imaging is for."