A 62-year-old man with COPD presents to the emergency department with acute shortness of breath. ABG shows: pH 7.28, PaCO₂ 68 mmHg, HCO₃⁻ 32 mEq/L, PaO₂ 55 mmHg. Chest X-ray reveals no acute infiltrate. What is the most appropriate immediate next step in management?

## Interpretation of Acid-Base Status **Key Point:** This patient has acute respiratory acidosis with metabolic compensation (pH 7.28, elevated PaCO₂ 68, elevated HCO₃⁻ 32). ### Expected Metabolic Compensation For acute respiratory acidosis, the expected HCO₃⁻ rise is approximately 1 mEq/L per 10 mmHg increase in PaCO₂ above 40. With PaCO₂ = 68 (28 mmHg above baseline), expected HCO₃⁻ ≈ 24 + 2.8 ≈ 27 mEq/L. The observed HCO₃⁻ of 32 suggests concurrent metabolic alkalosis (likely from chronic COPD with renal compensation). ### Management Algorithm ```mermaid flowchart TD A[Acute Respiratory Acidosis<br/>pH &lt; 7.30, PaCO₂ &gt; 60]:::outcome --> B{Hypoxemia present?}:::decision B -->|Yes| C{Stable airway?}:::decision C -->|Yes| D[Non-invasive ventilation<br/>BiPAP/CPAP + O₂]:::action C -->|No| E[Intubation]:::urgent D --> F{Response in 1-2 hrs?}:::decision F -->|Good| G[Continue NIV]:::action F -->|Poor| H[Escalate to intubation]:::urgent B -->|No| I[Supportive care<br/>Bronchodilators, Steroids]:::action ``` **High-Yield:** Non-invasive ventilation (NIV) is first-line for acute-on-chronic respiratory acidosis in COPD patients with intact airway reflexes and haemodynamic stability. It reduces intubation risk and mortality. **Clinical Pearl:** Sodium bicarbonate is contraindicated in respiratory acidosis because it generates CO₂, worsening the underlying problem. The goal is to improve ventilation, not buffer the acid. ### Why BiPAP is Correct Here - Patient is conscious and cooperative (implied by ability to present) - No mention of altered mental status or inability to protect airway - PaO₂ 55 mmHg is low but correctable with supplemental O₂ - NIV improves CO₂ clearance directly, addressing the primary pathology - Avoids risks of intubation (ventilator-associated pneumonia, prolonged sedation) **Mnemonic:** **NIPPV** = Non-Invasive Positive Pressure Ventilation — **N**o intubation, **I**mproves CO₂, **P**reserves airway reflexes, **P**revents complications, **V**entilates acutely.