Anesthetized dog hypoventilating
A dog under isoflurane anesthesia has not been mechanically ventilated. An arterial blood gas drawn 30 minutes into the procedure shows: pH 7.25, PCO₂ 65 mm Hg, HCO₃⁻ 24 mEq/L. What is the primary disturbance, and is the bicarbonate response consistent with a simple disorder?
Hint
Acidemia with high PCO₂ is respiratory acidosis. The next question is acute vs chronic. A dog 30 minutes into anesthesia has had no time for renal compensation, which takes 2–5 days.
Another hint
Apply the acute respiratory acidosis rule: HCO₃⁻ rises 0.15 mEq/L per 1 mm Hg rise in PCO₂. Baseline HCO₃⁻ for dogs is about 22, baseline PCO₂ about 37.
Show worked answer
-
pH 7.25 is acidemic. PCO₂ 65 is markedly elevated; HCO₃⁻ 24 is within reference range but at the upper end. Primary disturbance is respiratory acidosis driven by inhalant-induced hypoventilation.
-
Time course is acute (30 minutes). Renal compensation takes 2–5 days to fully develop, so HCO₃⁻ should rise only modestly via intracellular non-bicarbonate buffer titration.
-
Apply the dog acute respiratory acidosis rule: PCO₂ went up by 65 − 37 = 28 mm Hg. Expected HCO₃⁻ rise = 0.15 × 28 = 4.2 mEq/L. Expected HCO₃⁻ ≈ 22 + 4.2 = 26 mEq/L.
$$\Delta \text{HCO}_3^- = 0.15 \,\tfrac{mEq/L}{\cancel{mm\,Hg}} \times 28 \,\cancel{mm\,Hg} = 4.2 \,mEq/L$$ -
Observed HCO₃⁻ of 24 is within ±2 of the expected 26, consistent with simple acute respiratory acidosis. The fix is mechanical ventilation, not bicarbonate.
Acute respiratory acidosis from inhalant-induced hypoventilation. Expected HCO₃⁻ ≈ 26 mEq/L; observed 24 is within range. Simple disorder. Treat by ventilating, not by giving bicarbonate.