A 60-year-old man with COPD (PaCO2 55 mmHg chronically) presents with an acute exacerbation. ABG: pH 7.28, PaCO2 78 mmHg, HCO3⁻ 35 mEq/L. Serum Na+ 140, Cl⁻ 98 mEq/L. What is the CORRECT interpretation?
- A Acute respiratory acidosis with normal compensation
- B Chronic respiratory acidosis (adequately compensated) with no acute change
- C Chronic respiratory acidosis with an acute-on-chronic worsening; HCO3⁻ insufficient for the acute PaCO2 rise ✓
- D Mixed respiratory acidosis and metabolic alkalosis
Explanation
The patient's known chronic PaCO2 of 55 mmHg would yield expected HCO3⁻ of ~31 mEq/L (chronic compensation: HCO3⁻ rises 3.5 per 10 mmHg PaCO2 rise above 40). For the new PaCO2 of 78 mmHg (acute rise of 23 mmHg from chronic 55): acutely HCO3⁻ should rise 1 per 10 mmHg, giving ~33–34 mEq/L — the observed 35 mEq/L fits near the chronic compensation for 78 mmHg but not the acute rise alone. pH of 7.28 confirms acidemia despite partial compensation. This is acute-on-chronic respiratory acidosis: the HCO3⁻ reflects prior chronic compensation but pH is significantly acidemic because the acute PaCO2 rise has not been fully buffered. Option D (metabolic alkalosis) would require pH above expected for the PCO2; here pH is too low.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.