A 68-year-old woman with COPD presents with acute dyspnea and confusion. Arterial blood gas shows pH 7.25, PaCO₂ 68 mmHg, HCO₃⁻ 30 mEq/L, and PaO₂ 52 mmHg. What is the most common cause of respiratory acidosis in COPD exacerbations?

Explanation

## Respiratory Acidosis in COPD Exacerbation **Key Point:** The most common cause of respiratory acidosis in COPD exacerbations is **ventilation-perfusion (V/Q) mismatch combined with hypoventilation** due to airway obstruction, small airway collapse, and respiratory muscle fatigue. ### Pathophysiology of Respiratory Acidosis in COPD ```mermaid flowchart TD A[COPD Exacerbation]:::outcome --> B[Airway Obstruction & Inflammation]:::outcome B --> C[Increased Work of Breathing]:::outcome C --> D[Respiratory Muscle Fatigue]:::outcome D --> E[Hypoventilation]:::action E --> F[CO₂ Retention]:::urgent F --> G[Respiratory Acidosis]:::urgent B --> H[V/Q Mismatch]:::outcome H --> F ``` ### Mechanism Breakdown 1. **Airway obstruction** → small airways collapse during expiration (air trapping, auto-PEEP) 2. **Increased airway resistance** → greater work of breathing 3. **Respiratory muscle fatigue** → inability to maintain minute ventilation 4. **Hypoventilation** → inadequate CO₂ elimination 5. **V/Q mismatch** → perfused but poorly ventilated lung units → worsening hypoxemia and hypercapnia ### ABG Interpretation | Parameter | Value | Interpretation | |-----------|-------|----------------| | pH | 7.25 | Acidemia | | PaCO₂ | 68 mmHg | **Elevated** (normal 35–45) | | HCO₃⁻ | 30 mEq/L | Elevated (metabolic compensation) | | PaO₂ | 52 mmHg | Severe hypoxemia | **Clinical Pearl:** The elevated HCO₃⁻ (30 mEq/L) indicates metabolic compensation for chronic CO₂ retention. This patient likely has chronic COPD with acute exacerbation (acute-on-chronic respiratory acidosis). **High-Yield:** In COPD, **V/Q mismatch is the primary mechanism of hypoxemia**, not hypoventilation alone. Both V/Q mismatch AND hypoventilation cause CO₂ retention and respiratory acidosis. **Warning:** Do NOT confuse: - ~~Metabolic acidosis~~ (HCO₃⁻ is ELEVATED, not low) - ~~Primary pulmonary embolism~~ (less common; would present with acute hypoxemia and hypocapnia initially, not hypercapnia) - ~~Pneumothorax~~ (would be acute and obvious on imaging; not the chronic pattern in COPD) [cite:Harrison 21e Ch 258]