Normal pH but abnormal PCO₂ and HCO₃⁻
A 30 kg dog with chronic bronchitis presents for acute illness. Arterial blood gas: pH 7.40, PCO₂ 55 mm Hg, HCO₃⁻ 33 mEq/L. The pH is normal. Does this dog have an acid-base disorder?
Hint
Normal pH does not rule out an acid-base disorder. Two disorders pulling pH in opposite directions can normalize it. Look at PCO₂ and HCO₃⁻: both are abnormal, and they're moved in the same direction. Is that consistent with a single disorder?
Another hint
Try the chronic respiratory acidosis rule for dogs: HCO₃⁻ rises 0.35 mEq/L per 1 mm Hg rise in PCO₂. Compare the observed HCO₃⁻ rise to what that rule would predict. If observed > predicted, there's a metabolic alkalosis on top.
Show worked answer
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pH is 7.40, exactly normal. That does NOT mean no disorder. Compensation never fully normalizes pH; overcompensation does not occur. A normal pH with both PCO₂ and HCO₃⁻ abnormal raises strong suspicion of a counterbalancing mixed disorder.
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PCO₂ 55 is high (reference 31–43); HCO₃⁻ 33 is high (reference 19–26). Both moved in the same direction. A single primary disorder + compensation can't produce both abnormalities pushing pH the SAME way. The two would be in opposite directions if compensation alone were at work.
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Apply the chronic respiratory acidosis rule (chronic bronchitis suggests longstanding hypercapnia): HCO₃⁻ should rise 0.35 mEq/L per 1 mm Hg rise in PCO₂.
$$\Delta \text{PCO}_2 = 55 - 37 = 18 \,mm\,Hg$$ -
Expected HCO₃⁻ rise from the chronic respiratory rule: 0.35 × 18 = 6.3 mEq/L. Expected HCO₃⁻ ≈ 22 + 6.3 ≈ 28 mEq/L.
$$\text{expected HCO}_3^- = 22 + (0.35 \times 18) \approx 28 \,mEq/L$$ -
Observed HCO₃⁻ is 33, well above the predicted 28 (±2 → 26–30). The bicarbonate is higher than chronic respiratory compensation alone explains. There's a concurrent metabolic alkalosis.
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Final interpretation: mixed disorder. Chronic respiratory acidosis (the bronchitis baseline) plus a superimposed metabolic alkalosis. Common causes in this setting: loop diuretic use (furosemide for the bronchitis), vomiting, corticosteroid effects.
Mixed disorder: chronic respiratory acidosis (from the underlying bronchitis) plus a concurrent metabolic alkalosis. Observed HCO₃⁻ of 33 is well above the ~28 predicted by chronic respiratory compensation alone. Normal pH is a clue here, not reassurance.