Arterial blood gas analysis and interpretation in anesthetized patients - Veterinary Medicine
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Arterial blood gas analysis and interpretation in anesthetized patients
Adding blood gas analysis to your clinical toolbox is easier than you may think—especially if you use the principles of interpretation outlined here.



Acid-base disturbances are not primary diseases but are signs of an underlying condition that should be diagnosed and corrected. For example, treating metabolic acidosis with bicarbonate (or with another alkalinizing agent) without treating the underlying cause will only mask the clinical signs and may not improve patient survival. Likewise, treating respiratory acidosis with an alkalinizing agent is contraindicated. The correct treatment for respiratory acidosis is increasing ventilation. The addition of bicarbonate would only worsen the hypercarbia if the ventilation could not compensate.

Some patients have mixed acid-base disturbances. That is, they have two or more simultaneous respiratory or metabolic conditions associated with the primary disturbance (e.g. simultaneous respiratory acidosis and metabolic acidosis). A mixed acid-base disturbance is particularly common during anesthesia when a patient has preexisting metabolic acidosis. When the patient is anesthetized and respiratory depression occurs, the PaCO2 will increase and any respiratory compensation that had occurred will be replaced by respiratory acidosis. Patients may also have two or more metabolic diseases occurring simultaneously. An example would be diabetic ketoacidosis with a superimposed lactic acidosis. Another possible condition is the presence of two or more disorders that may have opposite effects on the pH (offsetting), resulting in a reduction in the magnitude of the primary disturbance.

When a primary metabolic disturbance occurs, it is often worthwhile to analyze the blood gas results with nontraditional methods (i.e. also calculate the anion gap and strong ion difference) and interpret the results in the context of the clinical picture such as previous exposure to toxins, severe dehydration, or electrolyte abnormalities secondary to gastrointestinal disease. Other less traditional methods that are better suited to characterize metabolic disturbances include the strong ion model and simplified strong ion model. These methods may be more appropriate for the acid-base assessment of patients with metabolic disturbances, such as lactic acidosis or renal disease. Metabolic disturbances are usually more long-term and complex than are anesthesia-induced respiratory disturbances.

The effect of plasma constituents. It is important that you understand the influence of plasma constituents on a patient's apparent acid-base balance. Although that cannot be adequately addressed in one article, I hope you appreciate that common protein, bicarbonate, and electrolyte changes can alter a patient's overall acid-base status.9

For example, dilutional acidosis can occur after administration of an alkali-free chloride-containing solution (0.9% sodium chloride solution), which causes volume expansion of the extracellular fluid. Dilutional acidosis occurs because chloride is rapidly absorbed in the kidney and causes a decrease of HCO3 -. A solution containing less chloride (0.45% sodium chloride solution) would reduce this effect.

The opposite occurs when free water is lost and the extracellular fluid constituents are concentrated. In this instance, sodium is often elevated, producing hypernatremic, or contraction, alkalosis. In patients with severe diarrhea, bicarbonate is lost in excess of chloride, leading to hyperchloremic acidosis. This acidosis may be confounded further by lactic acidosis if tissue perfusion is reduced because of severe volume loss and dehydration.

Kurt A. Grimm, DVM, MS, PhD, DACVA, DACVCP
Veterinary Specialist Services
P.O. Box 504
Conifer, CO 80433


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