Identify the primary disturbance: vomiting/diarrhea dog
A 20 kg dog with 3 days of vomiting and diarrhea has the following arterial blood gas: pH 7.25, PCO₂ 28 mm Hg, HCO₃⁻ 12 mEq/L. What is the primary acid-base disturbance?
Hint
Start with pH. Is the patient acidemic (pH < 7.35) or alkalemic (pH > 7.46)? That tells you the direction. Then look at HCO₃⁻ and PCO₂. Which one moved in the direction that explains the pH?
Another hint
Acidemia + low HCO₃⁻ = metabolic acidosis. Acidemia + high PCO₂ = respiratory acidosis. The low PCO₂ here is the body's attempt to compensate (hyperventilation blowing off CO₂), not the primary process.
Show worked answer
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pH is 7.25, which is below the dog reference range (7.35–7.46). The patient is acidemic.
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HCO₃⁻ is 12 mEq/L (reference 19–26), which is markedly low. A low HCO₃⁻ would push pH down. This explains the acidemia.
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PCO₂ is 28 mm Hg (reference 31–43), which is also low. A low PCO₂ would push pH up, so it cannot be causing the acidemia. It's the compensatory response (hyperventilation).
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Primary disturbance: metabolic acidosis with respiratory compensation. Loss of HCO₃⁻ in diarrheal fluid is the classic mechanism here.
Metabolic acidosis (with appropriate respiratory compensation).