Cat with metabolic acidosis and normal PCO₂
A 4 kg cat with chronic kidney disease has an arterial blood gas: pH 7.22, PCO₂ 33 mm Hg, HCO₃⁻ 14 mEq/L. PCO₂ is right at the cat reference midpoint. If you apply the dog rule for expected respiratory compensation, you'd predict PCO₂ around 25 mm Hg. Does the observed PCO₂ of 33 mean this cat has a mixed disorder?
Hint
This is a species-specific physiology question, not a math question. What does DiBartola say about the feline kidney's adaptive response to metabolic acidosis compared with the dog and the human?
Another hint
DiBartola Ch. 12 p. 304 explicitly warns that dog and human compensation formulas should not be extrapolated to cats with metabolic acidosis. The feline kidney lacks the adaptive ammoniagenesis response, and the respiratory compensation response also appears blunted.
Show worked answer
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First the math check: cat reference HCO₃⁻ midpoint ≈ 18. Observed HCO₃⁻ 14, a drop of 4 mEq/L. If we (wrongly) applied the dog rule: expected PCO₂ drop = 0.7 × 4 = 2.8 mm Hg. Expected PCO₂ ≈ 31 − 2.8 ≈ 28 mm Hg. Observed 33 is above this. The dog rule predicts a mismatch.
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But the dog rule should not be applied. DiBartola Ch. 12 p. 304: "the feline kidney apparently is unable to adapt to metabolic acidosis ... cats may not compensate for metabolic acidosis to the same extent (if at all) as do dogs and humans. Thus formulas for dogs or humans should not be extrapolated for use in cats."
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The cat may simply not be hyperventilating in response to the metabolic acidosis. A normal PCO₂ in a cat with metabolic acidosis is NOT by itself evidence of a mixed disorder.
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Clinical interpretation in this cat depends on history, physical exam, and ancillary labs, not on a borrowed compensation formula. CKD is sufficient to explain the metabolic acidosis (impaired renal acid excretion).
No mixed disorder is established by this blood gas. The cat may not be compensating because cats often don't compensate for metabolic acidosis. Dog compensation formulas should not be extrapolated to cats in this setting (DiBartola Ch. 12).