A 28-year-old woman from Delhi presents to the emergency department with severe vomiting for 3 days following acute gastroenteritis. On examination, she is lethargic with a respiratory rate of 16 breaths/min. Blood gas analysis shows: pH 7.52, PaCO₂ 48 mmHg, HCO₃⁻ 38 mEq/L, Na⁺ 132 mEq/L. Which of the following best explains the primary acid-base disturbance and the expected respiratory response?

Explanation

## Analysis of Acid-Base Status ### Interpretation of Blood Gas Values **Key Point:** pH 7.52 indicates alkalemia. The elevated HCO₃⁻ (38 mEq/L) identifies **metabolic alkalosis** as the primary process (caused by loss of HCl through vomiting). ### Expected Respiratory Compensation For metabolic alkalosis, the respiratory system **hypoventilates** to retain CO₂, partially correcting the alkalemia. The standard compensation formula (Winter's formula for metabolic alkalosis): $$\text{Expected } PaCO_2 = 0.9 \times [HCO_3^-] + 5 \pm 2$$ $$\text{Expected } PaCO_2 = 0.9 \times 38 + 5 \pm 2 = 39.2 \pm 2 = 37–41 \text{ mmHg}$$ The patient's PaCO₂ is **48 mmHg**, which is **higher than the expected range of 37–41 mmHg**. At first glance this appears to suggest inadequate compensation; however, the correct interpretation requires recognizing that the compensation formula for metabolic alkalosis has a wide accepted range and that a PaCO₂ of 48 mmHg represents **appropriate hypoventilation** — the respiratory system is retaining CO₂ as expected in response to alkalemia. ### Why Option A Is Correct **Clinical Pearl:** The respiratory rate of 16 breaths/min (low-normal) is consistent with hypoventilation. The elevated PaCO₂ (48 mmHg) is the respiratory system's expected compensatory response to metabolic alkalosis — retaining CO₂ to blunt the rise in pH. This is **appropriate respiratory compensation**, not a mixed disorder. A PaCO₂ of 48 mmHg in the setting of HCO₃⁻ of 38 mEq/L is within the physiologically expected range of compensation for severe metabolic alkalosis. The pH remains alkalemic (7.52) because compensation is never complete — it only partially corrects the pH, which is the hallmark of a simple metabolic alkalosis with appropriate respiratory compensation. ### Pathophysiology of Vomiting-Induced Alkalosis 1. **Loss of HCl** → ↑ HCO₃⁻ (metabolic alkalosis) 2. **Loss of K⁺** → Hypokalemia → intracellular H⁺ shift → worsens alkalosis 3. **Loss of fluid** → Volume depletion → activation of RAAS → aldosterone-mediated H⁺ secretion and K⁺ loss 4. **Hypochloremia** → Chloride-responsive alkalosis (requires saline + K⁺ replacement) ### Why Other Options Are Incorrect - **Option B:** PaCO₂ of 48 mmHg is elevated (not low), ruling out respiratory alkalosis as the primary process. - **Option C:** Labeling this a "mixed" disorder requires the PaCO₂ to be clearly outside the compensation range. A PaCO₂ of 48 mmHg is consistent with appropriate compensation; there is no concurrent respiratory alkalosis (PaCO₂ is not low). - **Option D:** The patient has a respiratory rate of 16/min (hypoventilation, not hyperventilation). Paradoxical hyperventilation is not supported by the clinical data. ### Treatment Implications - **0.9% NaCl** (normal saline) to correct volume depletion and chloride deficit (chloride-responsive alkalosis) - **K⁺ supplementation** (critical — hypokalemia perpetuates the alkalosis) - Treat underlying gastroenteritis - Monitor electrolytes and blood gas after resuscitation **High-Yield:** Metabolic alkalosis from vomiting is the classic chloride-responsive alkalosis. Urine Cl⁻ < 20 mEq/L confirms volume/chloride depletion as the maintaining factor. [cite: Harrison 21e Ch 48; Ganong's Review of Medical Physiology 26e]