A 52-year-old man with chronic obstructive pulmonary disease (COPD) presents with acute dyspnea. Arterial blood gas shows: pH 7.28, PaCO₂ 72 mmHg, HCO₃⁻ 32 mEq/L, PaO₂ 55 mmHg. Which of the following best describes the primary acid-base disturbance and the expected metabolic compensation?

Explanation

## Analysis of Acid-Base Status in COPD Exacerbation ### Interpretation of Blood Gas Values **Key Point:** pH 7.28 indicates acidemia. The elevated PaCO₂ (72 mmHg) confirms **respiratory acidosis** as the primary disorder. The elevated HCO₃⁻ (32 mEq/L) represents the metabolic compensatory response. ### Determining Whether Compensation Is Appropriate This patient has **chronic COPD**, so we must apply the **chronic respiratory acidosis compensation rule** (Harrison's Principles of Internal Medicine, 21e): $$\text{For every 10 mmHg rise in } PaCO_2 \text{ above 40, } HCO_3^- \text{ rises by } 3.5 \text{ mEq/L (chronic)}$$ In this patient: - PaCO₂ = 72 mmHg → rise = 32 mmHg above normal (40 mmHg) - Expected rise in HCO₃⁻ = (32 ÷ 10) × 3.5 = **11.2 mEq/L** - Expected HCO₃⁻ = 24 + 11.2 = **~35 mEq/L** (range: 24 + 3.5×3.2 ≈ 35) The **actual HCO₃⁻ is 32 mEq/L**, which falls **within the expected range** for chronic respiratory acidosis compensation (typically 24 + 3–4 mEq per 10 mmHg rise). This is **appropriate metabolic compensation** — the kidneys have retained bicarbonate to partially buffer the chronic hypercapnia. ### Why the pH Remains Low Even with appropriate metabolic compensation, the pH of 7.28 remains acidemic because: 1. Metabolic compensation is **partial, not complete** — it attenuates but does not normalize pH. 2. The degree of hypercapnia (PaCO₂ 72) is severe enough that even maximal renal compensation cannot restore pH to normal. 3. This is the expected physiological outcome: compensation reduces the pH change but does not eliminate it. **Clinical Pearl:** "Appropriate compensation" does NOT mean pH is normalized — it means the HCO₃⁻ response is exactly what the body's physiology predicts. A pH that remains low despite appropriate compensation is expected in severe respiratory acidosis. ### Ruling Out Other Options | Option | Assessment | |--------|-----------| | **A (Correct)** | HCO₃⁻ 32 is within expected range for chronic respiratory acidosis → appropriate compensation ✓ | | B | Metabolic alkalosis would require HCO₃⁻ significantly **above** expected (~>38); 32 does not qualify | | C | Mixed respiratory + metabolic acidosis would show HCO₃⁻ **below** expected (~<28); 32 is not low | | D | "Inadequate compensation" implies HCO₃⁻ is lower than predicted; actual HCO₃⁻ of 32 meets or approaches expected values | ### Summary - **Primary disorder:** Respiratory acidosis (↑PaCO₂, ↓pH) - **Compensation:** Metabolic (renal HCO₃⁻ retention, 32 mEq/L) — **appropriate for chronic respiratory acidosis** - **Diagnosis:** Respiratory acidosis with appropriate metabolic compensation **High-Yield:** In chronic respiratory acidosis (as in COPD), the expected HCO₃⁻ rise is ~3.5 mEq/L per 10 mmHg rise in PaCO₂. An HCO₃⁻ within this predicted range confirms appropriate — not inadequate — compensation. [cite: Harrison 21e Ch 48; Davenport Acid-Base Nomogram]