A 72-year-old woman with chronic kidney disease stage 4 (eGFR 18 mL/min/1.73m²) and hypertension presents with fatigue and poor appetite for 2 weeks. She is on lisinopril and amlodipine. Vital signs: BP 148/92 mmHg, RR 18/min. Laboratory findings: pH 7.28, PaCO₂ 32 mmHg, HCO₃⁻ 15 mEq/L, Na⁺ 138 mEq/L, K⁺ 6.2 mEq/L, Cl⁻ 108 mEq/L, BUN 68 mg/dL, creatinine 4.2 mg/dL. What is the primary acid-base disturbance?

Explanation

## Systematic Acid-Base Analysis ### Step 1: Identify the Primary Disorder - **pH 7.28** → Acidemia - **HCO₃⁻ 15 mEq/L** → Low → Primary **metabolic acidosis** - **PaCO₂ 32 mmHg** → Low → Respiratory compensation present **Primary disorder: Metabolic acidosis** ✓ ### Step 2: Apply Winter's Formula for Expected Respiratory Compensation $$\text{Expected } PaCO_2 = 1.5 \times [HCO_3^-] + 8 \pm 2$$ $$= 1.5 \times 15 + 8 \pm 2 = 22.5 + 8 \pm 2 = 30.5 \pm 2 \text{ mmHg}$$ **Expected PaCO₂ range: 28.5–32.5 mmHg** **Observed PaCO₂ = 32 mmHg → Falls within the expected range** ✓ ### Step 3: Conclusion on Compensation Because the observed PaCO₂ (32 mmHg) lies within the expected range of 28.5–32.5 mmHg, the respiratory system is compensating **appropriately** for the degree of metabolic acidosis. This is **not** inadequate compensation; a PaCO₂ above 32.5 mmHg would be required to label compensation as inadequate. **Answer: Metabolic acidosis with appropriate respiratory compensation (Option B)** ### Step 4: Calculate Anion Gap $$\text{AG} = Na^+ - (Cl^- + HCO_3^-)$$ $$= 138 - (108 + 15) = 15 \text{ mEq/L}$$ **AG = 15 mEq/L** → **Elevated anion gap metabolic acidosis** (normal < 12 mEq/L), consistent with **uremic acidosis** in CKD stage 4. ### Step 5: Why the Other Options Are Wrong | Option | Reasoning | |--------|-----------| | A – Mixed metabolic and respiratory acidosis | Would require PaCO₂ > 32.5 mmHg (higher than expected); here PaCO₂ is appropriately low | | C – Metabolic acidosis with inadequate respiratory compensation | Incorrect; PaCO₂ 32 mmHg is within the Winter's formula range | | D – Respiratory acidosis with metabolic compensation | pH is acidemic with low HCO₃⁻ as the primary driver; PaCO₂ is low, not high | ## Key Point: **Winter's formula (Expected PaCO₂ = 1.5 × HCO₃⁻ + 8 ± 2)** is the standard tool to assess respiratory compensation in metabolic acidosis. If the observed PaCO₂ equals the expected value, compensation is appropriate — this does NOT mean the pH is normal; it simply means no concurrent respiratory disorder exists. *(Harrison's Principles of Internal Medicine, 21st ed., Chapter on Acid-Base Disorders)* ## High-Yield: - **Appropriate compensation ≠ normal pH** — the pH remains acidemic; compensation merely limits the degree of acidemia. - **Inadequate compensation** = observed PaCO₂ **above** the expected range → concurrent respiratory acidosis. - **Over-compensation** = observed PaCO₂ **below** the expected range → concurrent respiratory alkalosis. ## Clinical Pearl: In CKD, uremic acidosis is the classic cause of **elevated anion gap metabolic acidosis** (accumulation of sulfates, phosphates, urate). The respiratory system typically compensates appropriately unless there is concurrent pulmonary or neuromuscular disease. Always verify compensation with Winter's formula before labeling a mixed disorder.