A patient has the following ABG: pH 7.46, PaCO2 30 mmHg, HCO3 21 mEq/L. Na 138, Cl 105. What is the primary acid-base disorder and calculated anion gap?

• A Respiratory alkalosis with inappropriate metabolic compensation; AG = 15
• B Mixed respiratory alkalosis and metabolic acidosis; AG = 12
• C Respiratory alkalosis with appropriate metabolic compensation; AG = 12 ✓
• D Primary metabolic alkalosis with respiratory compensation; AG = 12

Explanation

AG = Na − (Cl + HCO3) = 138 − (105 + 21) = 12 mEq/L (normal). The primary disorder is respiratory alkalosis (low PaCO2, elevated pH). For chronic respiratory alkalosis, expected HCO3 = 24 − 5 × (40 − 30)/10 = 24 − 5 = 19 mEq/L (range 17–21). Measured HCO3 of 21 is at the upper limit of expected compensation. With a normal AG there is no hidden metabolic acidosis, making this pure respiratory alkalosis with appropriate renal compensation.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.