Which of the following acid-base disturbances is characterized by a primary increase in PaCO₂ with a secondary (compensatory) increase in HCO₃⁻?

Explanation

## Respiratory Acidosis: Definition and Compensation **Key Point:** Respiratory acidosis is defined by PRIMARY elevation of PaCO₂ (> 45 mmHg). The kidneys compensate by INCREASING HCO₃⁻ reabsorption. ### Pathophysiology 1. **Primary defect:** Hypoventilation → ↑ PaCO₂ → ↓ pH 2. **Renal compensation:** Increased HCO₃⁻ reabsorption in proximal tubule and collecting duct 3. **Result:** Secondary ↑ HCO₃⁻ (typically 24–30 mEq/L in chronic respiratory acidosis) ### Classification and Compensation Timeline | Type | Onset | Expected HCO₃⁻ Change | Formula | |------|-------|----------------------|----------| | **Acute** | Hours | ↑ 1 mEq/L per 10 mmHg ↑ PaCO₂ | HCO₃⁻ = 24 + (PaCO₂ − 40)/10 | | **Chronic** | Days–weeks | ↑ 3–4 mEq/L per 10 mmHg ↑ PaCO₂ | HCO₃⁻ = 24 + 3.5(PaCO₂ − 40)/10 | ### Example **Acute respiratory acidosis:** PaCO₂ = 60 mmHg - Expected HCO₃⁻ = 24 + (60 − 40)/10 = **26 mEq/L** **Chronic respiratory acidosis:** PaCO₂ = 60 mmHg - Expected HCO₃⁻ = 24 + 3.5(60 − 40)/10 = **24 + 7 = 31 mEq/L** **High-Yield:** The **acute vs. chronic distinction** is critical for NEET PG. Chronic respiratory acidosis shows a MORE robust renal compensation (3–4 mEq/L per 10 mmHg) than acute (1 mEq/L per 10 mmHg). **Mnemonic:** **ROME** — Respiratory Opposite, Metabolic Equal - In respiratory disorders, pH and PaCO₂ move in **opposite** directions - In metabolic disorders, pH and HCO₃⁻ move in the **same** direction **Clinical Pearl:** If HCO₃⁻ is LOWER than expected for the degree of PaCO₂ elevation, suspect concurrent metabolic acidosis (mixed disorder). [cite:Harrison 21e Ch 50]