A 55-year-old man with diabetic ketoacidosis presents with Kussmaul respirations. ABG shows pH 7.24, PaCO₂ 22 mmHg, HCO₃⁻ 9 mEq/L. A 60-year-old woman with acute pulmonary embolism presents with acute dyspnea. ABG shows pH 7.48, PaCO₂ 28 mmHg, HCO₃⁻ 20 mEq/L. Which finding best discriminates metabolic acidosis with respiratory compensation from primary respiratory alkalosis?

Explanation

## Discriminating Metabolic Acidosis with Respiratory Compensation from Primary Respiratory Alkalosis ### Pathophysiology: Two Different Primary Disorders **Key Point:** In metabolic acidosis, the PRIMARY problem is HCO₃⁻ loss or acid accumulation (markedly low HCO₃⁻). The lungs RESPOND by hyperventilating to blow off CO₂ (low PaCO₂ is secondary/compensatory). The degree of respiratory compensation is PROPORTIONAL to the severity of metabolic acidosis. **Key Point:** In primary respiratory alkalosis, the PRIMARY problem is hyperventilation (low PaCO₂). HCO₃⁻ may be mildly low due to renal compensation (kidneys excrete HCO₃⁻ to partially correct alkalosis), but the HCO₃⁻ drop is DISPROPORTIONATELY SMALL compared to the PaCO₂ drop. ### Comparison Table | Feature | Metabolic Acidosis + Respiratory Compensation (DKA) | Primary Respiratory Alkalosis (PE) | |---------|------------------------------------------------------|------------------------------------| | **HCO₃⁻** | ↓↓ Markedly low (9 mEq/L) — PRIMARY disorder | ↓ Mildly low (20 mEq/L) — secondary compensation | | **PaCO₂** | ↓ Low (22 mmHg) — proportional respiratory response | ↓↓ Markedly low (28 mmHg) — PRIMARY disorder | | **pH** | ↓ Acidemia (7.24) | ↑ Alkalemia (7.48) | | **HCO₃⁻ : PaCO₂ ratio** | Proportional drop (both severely abnormal) | Disproportionate: HCO₃⁻ mildly low, PaCO₂ markedly low | ### Using Winter's Formula to Detect the Primary Disorder **High-Yield:** **Winter's Formula** predicts expected PaCO₂ in metabolic acidosis: $$\text{Expected } PaCO_2 = 1.5 \times [HCO_3^-] + 8 \pm 2$$ **DKA case:** - Expected PaCO₂ = 1.5 × 9 + 8 ± 2 = 13.5 + 8 ± 2 = **19.5 ± 2 = 17.5–21.5 mmHg** - Actual PaCO₂ = 22 mmHg (within expected range → appropriate respiratory compensation) - Interpretation: **Metabolic acidosis with appropriate respiratory compensation** (no concurrent respiratory disorder) **PE case:** - If we apply Winter's formula assuming metabolic acidosis: Expected PaCO₂ = 1.5 × 20 + 8 ± 2 = 38 ± 2 = **36–40 mmHg** - Actual PaCO₂ = 28 mmHg (MUCH lower than expected) - Interpretation: **Primary respiratory alkalosis** (hyperventilation is the primary problem, HCO₃⁻ drop is only compensatory) ### Mnemonic: MARC **Mnemonic:** **MARC** = **M**etabolic Acidosis: **A**bnormal HCO₃⁻ (markedly low), **R**espiratory response (proportional PaCO₂ drop), **C**ompensatory (PaCO₂ is secondary). ### Clinical Pearl **Clinical Pearl:** In DKA, the HCO₃⁻ is often single-digit (9 mEq/L), and the respiratory response is dramatic but still proportional. In PE with respiratory alkalosis, the HCO₃⁻ rarely drops below 18–20 mEq/L because renal compensation limits the drop; the PaCO₂ is the primary abnormality. [cite:Harrison 21e Ch 48]