A 32-year-old woman with chronic diarrhea presents with pH 7.32, HCO₃⁻ 14 mEq/L, PaCO₂ 32 mmHg, and normal anion gap. A 28-year-old man with COPD presents with pH 7.32, HCO₃⁻ 32 mEq/L, PaCO₂ 58 mmHg, and normal anion gap. Which feature best distinguishes metabolic acidosis from respiratory acidosis in these two patients?

Explanation

## Distinguishing Metabolic from Respiratory Acidosis ### Primary Disorder Definition **Key Point:** The PRIMARY disorder determines the classification. In metabolic acidosis, HCO₃⁻ is the primary abnormality (decreased). In respiratory acidosis, PaCO₂ is the primary abnormality (increased). ### Comparison Table | Feature | Metabolic Acidosis | Respiratory Acidosis | |---------|-------------------|----------------------| | **Primary Problem** | ↓ HCO₃⁻ | ↑ PaCO₂ | | **pH** | ↓ | ↓ | | **Respiratory Response** | Hyperventilation (↓ PaCO₂) | None (lungs are failing) | | **Anion Gap** | May be normal or elevated | Always normal | | **Cause** | GI/renal HCO₃⁻ loss or acid gain | Hypoventilation (lung/CNS disease) | ### Clinical Correlation **Clinical Pearl:** In the diarrhea case, the PRIMARY problem is loss of HCO₃⁻ (metabolic). The respiratory system compensates by hyperventilating (PaCO₂ = 32, which is LOW). In the COPD case, the PRIMARY problem is inability to eliminate CO₂ (respiratory). There is no respiratory compensation because the lungs are already failing. ### High-Yield Discriminator **High-Yield:** Ask yourself: "Which parameter is ABNORMAL FIRST?" In metabolic acidosis, HCO₃⁻ drops first, then respiratory compensation lowers PaCO₂. In respiratory acidosis, PaCO₂ rises first, and the kidneys slowly increase HCO₃⁻ (metabolic compensation). **Mnemonic:** **MET-AB** = **MET**abolic = **AB**normally low HCO₃⁻; **RES-CO** = **RES**piratory = **CO₂** abnormally high. ### Why pH Alone Is Not the Discriminator **Warning:** Both conditions can present with the SAME pH. You cannot distinguish them by pH value alone — you must look at which parameter (HCO₃⁻ vs. PaCO₂) is the PRIMARY abnormality.