DKA dog with low PCO₂: simple or mixed?
A 15 kg dog presents in diabetic ketoacidosis. Arterial blood gas: pH 7.10, PCO₂ 15 mm Hg, HCO₃⁻ 8 mEq/L. Apply the dog metabolic acidosis compensation rule and decide whether this is a simple disorder or a mixed disorder.
Hint
First confirm metabolic acidosis is present (it is: low pH, low HCO₃⁻). Then apply the dog rule: PCO₂ should fall 0.7 mm Hg for every 1 mEq/L fall in HCO₃⁻ below the reference midpoint.
Another hint
Baseline HCO₃⁻ ≈ 22, observed is 8, so HCO₃⁻ has fallen by 14. Expected PCO₂ drop = 0.7 × 14 = 9.8. Expected PCO₂ = 37 − 9.8 ≈ 27 mm Hg. Compare to observed PCO₂ of 15.
Show worked answer
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Confirm primary disturbance. pH 7.10 = severe acidemia. HCO₃⁻ 8 (very low) explains the acidosis; PCO₂ 15 is low, consistent with compensation. Primary: metabolic acidosis.
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Apply the dog metabolic acidosis rule. HCO₃⁻ fell from baseline ~22 to observed 8, a drop of 14 mEq/L.
$$\Delta \text{HCO}_3^- = 22 - 8 = 14 \,mEq/L$$ -
Expected PCO₂ drop = 0.7 × 14 = 9.8 mm Hg. Expected PCO₂ from baseline 37 = 37 − 9.8 ≈ 27 mm Hg.
$$\text{expected PCO}_2 = 37 - (0.7 \times 14) \approx 27 \,mm\,Hg$$ -
Observed PCO₂ is 15, well below the expected 27 (±2 → 25–29). PCO₂ is lower than compensation alone should produce.
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Conclusion: mixed disorder. Primary metabolic acidosis from DKA, plus a concurrent respiratory alkalosis (the patient is hyperventilating beyond what compensation requires, which could reflect sepsis, pain, anxiety, or central drive from the ketoacidosis itself).
Mixed disorder: metabolic acidosis (DKA) plus a concurrent respiratory alkalosis. PCO₂ of 15 is below the expected 27 ±2 for simple compensation.