Chapter 9: Alveolar O₂ and CO₂ Exchange, Physiological Shunt, and Acid-Base Balance

Chapter 9. Alveolar O₂ and CO₂ Exchange, Physiological Shunt, and Acid-Base Balance

A 38-year-old man is brought to the ER after being found near his home and apparently unable to stand upright. Physical examination reveals lethargy and mid-epigastric abdominal tenderness; skin turgor is poor and his mucous membranes are dry. Blood pressure is 85/60 mmHg while supine. Laboratory studies indicate serum [Na⁺] = 143 mM, [K⁺] = 3.2 mM, [Cl⁻ ] = 101 mM, and total [CO₂] = 11 mM; arterial blood gases while on room air are P_aO₂ = 80 mm Hg, P_aCO₂ = 23 mm Hg, and pH_a = 7.28. Which of the following best describes his current acid-base status?

a. Uncompensated metabolic acidosis

b. Acute metabolic alkalosis

c. Chronic respiratory acidosis

d. Mixed respiratory acidosis and metabolic alkalosis

The most correct answer is a.

The patient’s arterial pH of 7.28 is clearly not alkalotic (answer b), and his P_aCO₂ of 23 mm Hg is inconsistent with a respiratory cause for the reduced pH, either acute or chronic (answers c, d). The total [CO₂] of just 11 mM (normal range = 22-26) supports a metabolic cause of distress, perhaps a perforated ulcer or ruptured appendix that has initiated peritonitis. Given his low blood pressure and pallor, one cannot rule out septic shock at this point (Chap. 28).