A 58-year-old man with COPD presents to the emergency department with severe dyspnea and altered mental status. His wife reports he has had a productive cough for 3 days. Vital signs: RR 28/min, BP 140/90 mmHg, HR 110/min. Arterial blood gas: pH 7.28, PaCO₂ 68 mmHg, HCO₃⁻ 32 mEq/L, PaO₂ 52 mmHg. Which acid-base disturbance is present?

Explanation

## Acid-Base Analysis ### Step-by-Step Interpretation 1. **Identify the primary pH abnormality:** - pH = 7.28 → **Acidemia** (normal 7.35–7.45) 2. **Determine the primary process:** - PaCO₂ = 68 mmHg (elevated; normal 35–45) → **Primary Respiratory Acidosis** - HCO₃⁻ = 32 mEq/L (elevated; normal 22–26) → Elevated, consistent with renal compensation 3. **Assess appropriateness of compensation (chronic respiratory acidosis):** - This patient has known COPD — a chronic condition. The appropriate formula for **chronic respiratory acidosis** compensation is: - **Expected HCO₃⁻ = 24 + 3.5 × [(PaCO₂ − 40) / 10]** - ΔPaCO₂ = 68 − 40 = 28 mmHg → 28/10 = 2.8 - Expected HCO₃⁻ = 24 + (3.5 × 2.8) = 24 + 9.8 ≈ **33.8 mEq/L** - **Actual HCO₃⁻ = 32 mEq/L** — this falls **within the expected range** for chronic renal compensation 4. **Conclusion:** - The elevated HCO₃⁻ is **appropriate and expected** for a COPD patient with chronic CO₂ retention. There is **no concurrent metabolic alkalosis**. - Diagnosis: **Respiratory acidosis with appropriate metabolic (renal) compensation** ### Clinical Interpretation **Key Point:** In a known COPD patient, an elevated HCO₃⁻ in the setting of elevated PaCO₂ most likely represents **chronic renal compensation**, not a superimposed metabolic alkalosis. The kidneys retain HCO₃⁻ over days to weeks to buffer the chronic hypercapnia. (Harrison's Principles of Internal Medicine, 21st ed., Chapter on Acid-Base Disorders) **High-Yield:** Distinguishing chronic from acute respiratory acidosis: - **Acute:** HCO₃⁻ rises ~1 mEq/L per 10 mmHg rise in PaCO₂ - **Chronic:** HCO₃⁻ rises ~3.5 mEq/L per 10 mmHg rise in PaCO₂ **Clinical Pearl:** The altered mental status and CO₂ narcosis in this patient are due to the acute-on-chronic hypercapnia (PaCO₂ 68 mmHg, PaO₂ 52 mmHg). This is a medical emergency requiring controlled oxygen therapy and consideration of non-invasive ventilation (NIV/BiPAP). ### Why This Is Not the Other Options | Option | Reasoning | Verdict | |---|---|---| | Mixed respiratory + metabolic acidosis | Would require low HCO₃⁻ (< 22); actual HCO₃⁻ is 32 | ✗ | | **Respiratory acidosis with metabolic compensation** | Expected HCO₃⁻ ≈ 34 mEq/L; actual = 32 mEq/L → appropriate | **✓ Correct** | | Metabolic acidosis with respiratory compensation | Primary process would be low HCO₃⁻; here HCO₃⁻ is elevated | ✗ | | Respiratory acidosis + concurrent metabolic alkalosis | Would require HCO₃⁻ significantly above expected (~34); actual = 32 is within range | ✗ | **Mnemonic:** **ROME** — **R**espiratory → **O**pposite (pH low, PaCO₂ high). When HCO₃⁻ is elevated proportionally to the PaCO₂ rise, it represents appropriate compensation — not a second disorder.