Venous blood gas interpretation and risks of mortality in veterinary medicine | VETgirl Veterinary Continuing Education Podcasts

In today’s VETgirl online veterinary CE podcast, we review whether or not venous blood gas parameters (such as anion gap, base excess, lactate, etc.) predict survival in veterinary patients. When animals are initially presented to the emergency room, the extent of their condition often cannot be fully assessed without additional diagnostics. Is there a way we can handle questions from pet owners regarding the financial investment in their pet’s medical treatment (such as prognosis and anticipated costs of medical care) based on evidence-based medicine? We strive to look for indicators in our physical examination findings and in our initial diagnostic work up (e.g., preliminary lab work or quick assessment tests) to help bolster our understanding of the patient’s prognosis, but presently veterinary medicine is greatly lacking in these indicators of mortality and disease severity. In the veterinary emergency room, blood gases are a quick and easy piece of lab work that can be obtained relatively quickly at the time of triage for cats and dogs. So, Kohen et al out of University of California at Davis wanted to look at the information that can be obtained off a simple blood gas analysis for possible predictors of mortality. In this retrospective study, they looked at plasma lactate concentrations, pH, base deficit, and anion gap in both cats and dogs, and looked for any correlation of these values to an increased in mortality risk.

First, let’s review two parameters in the venous blood gas that often confuse people: anion gap and base excess. Let’s start with the anion gap. Anion gap is the calculated difference between serum or plasma cations and anions. This equation is used to help try to identify the cause of a metabolic acidosis. A higher than normal anion gap should lead the clinician to look for causes within the patient to have elevated concentrations of anions, the most common being lactate, beta-hydroxybutyrate, acetoacetate (both ketoacids), phosphate and sulfate (associated with kidney injury). If the anion gap is normal, yet the patient is experiencing a metabolic acidosis, this means that the body has lost too much bicarbonate which is being replaced by chloride, and signals the clinician to look for a reason for bicarbonate loss such as gastrointestinal or renal losses. Rarely does a negative anion gap occur, but hypoalbuminemia will decrease the anion gap, and so a corrected equation should be used in the presence of hypoalbuminemia.

Next, let’s look at base excess – this is VETgirl’s go-to measurement on the venous blood gas. The base measurement ignores respiratory causes of acid-base disturbances and instead focuses only on describing the metabolic contribution to the acid-base status. If there is a low amount of base in the body, this is coined “negative base excess” or a “base deficit” and represents a metabolic acidosis. On the opposite spectrum, a large amount of base in the blood creates a “base excess” and represents a metabolic alkalosis. The anion gap can then be used to help identify whether a metabolic acidosis (e.g., base deficit) is caused by addition of too much acid in the blood or too much loss of bicarbonate. If the anion gap is elevated with an existing base deficit, this means there is too much acid in the blood (ketoacids in a DKA). If the anion gap is normal with an existing base deficit, this means there is too much loss of bases (gastrointestinal or renal losses of bicarbonate).

So, back to the study… Kohen et al retrospectively evaluated medical records from dogs and cats presenting to the William R. Pritchard Veterinary Medical Teaching Hospital between January 2010 and December of 2011 that had blood gas analyses performed within 2 hours after initial presentation. Their values were then compared to survival to discharge. A noteworthy limitation of this study is that the authors included euthanasia as a cause of death instead of only analyzing the pets that died as the result of their disease in-hospital. We know that financial limitations often play a role in owners’ decisions on whether to proceed with treatment, so this study’s findings may be skewed by the inclusion of euthanasia as a cause of death.

Patients were said to have hyperlactatemia if their plasma lactate levels were > 2.5mmol/L. Dogs were categorized as having metabolic acidosis if their pH was <7.32 and their base deficit was < -4mmol/L. If they fit these criteria for metabolic acidosis and had a concurrent hyperlactatemia, they were further categorized as having a lactic acidosis. Cats were categorized as having a metabolic acidosis if their pH was <7.34 and their base deficit was <-5 mmol/L. If they fit these criteria and had a concurrent hyperlactatemia, they were further categorized as having a lactic acidosis.

A total of 566 dogs and 185 cats had a venous blood gas analysis performed. Of these, about half of the dogs (53%) and 1/3 of the cats (30%) were hyperlactatemic. Almost half of the hyperlactatemic dogs had a concurrent acidosis and were labeled as having lactic acidosis (42%), while a much smaller percentage (17%) had hyperlactatemia without an acidosis. What was interesting to discover is that in the dogs, lactic acidosis was found to be associated with the highest mortality rate at almost 60%, while those that had a hyperlactatemia with a normal pH had a similar mortality rate to the canines that had normal lactate measurements. These findings suggest that, in dogs, lactate alone was not a good predictor of mortality, but rather the combination of acidosis AND hyperlactatemia was a better predictor of mortality. The most common diseases represented by the canines that had hyperlactatemia included (in descending order of incidence) traumatic injuries and hemorrhage, neoplasia, and gastrointestinal disease.

Of the cats that were included in the study, again the majority of cats had a concurrent lactic acidosis (80%) whereas only 9% of cats had hyperlactatemia with a normal pH. Interestingly, cats with only hyperlactatemia and a normal pH had almost the same incidence of mortality (44.6%) as the cats that had a lactic acidosis (49%). This finding suggests that in cats, lactate alone is a good predictor of mortality with or without a concurrent lactic acidosis. In hyperlactatemic cats, the most common diseases included (in descending order of incidence) urinary tract disease, traumatic injuries and hemorrhage, and gastrointestinal disease.

This study found that, in dogs, venous blood pH was an independent predictor of mortality. The pH was found to be even more important than the anion gap when looking for prognostic information. However, this study used only the traditional anion gap equation that does not correct for hypoalbuminemia:

AG = (Na+K)-(Cl+HCO3)

Albumin is a negative acute phase protein, and so, is often low in our critically ill pets. One potential limitations of this study is that, by not using the corrected anion gap equation, this study may not adequately evaluate the utility of the anion gap as a prognostic indicator in this patient population since the anion gap is heavily influenced by albumin. The pH of feline venous blood was not found to be of significant prognostic value; only the lactate level was of prognostic value. Therefore the acid base information of the feline population did not have any independent prognostic value as it did in canines.

So, what do we take from this VETgirl podcast? Based on this study, in dogs, evaluation of the pH in combination with lactate levels can act as a predictor of mortality with lactic acidosis carrying a mortality risk of a little over a 50%. However, in cats, they found that initial lactate alone was a predictor of mortality and the pH or anion gap did not add any predictive value. There are limitations to this study such as using cases that were euthanized as a cause of death which opens the door for financial considerations to effect the study results. In addition, the corrected anion gap equation for hypoalbuminemia was not utilized to remove the effect of hypoalbuminemia on this measurement and may have misleading results. That said, this study was helpful in that it provides some evidence-based medicine on counseling pet owners about their critically ill patients with venous blood gas disturbances. When in doubt, make sure you are utilizing your venous blood gas appropriately!

AG = anion gap
DKA = diabetic ketoacidosis

1. Kohen CJ, Hopper K, Kass PH, et al. Retrospective evaluation of the prognostic utility of plasma lactate concentration, base deficit, pH, and anion gap in canine and feline emergency patients. JVECC early view 2017; pp1-8.

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