Preparatory Mindset
Acid-base balance is one of the most clinically relevant topics. Every patient has a blood gas. The key is systematic interpretation: check pH first (acidosis or alkalosis), then pCO₂ (respiratory component), then HCO₃⁻ (metabolic component). Use Winter's formula to check compensation. Know the MUDPILES mnemonic for anion gap metabolic acidosis.
Core Concepts
- Acid-base homeostasis is maintained by buffers (HCO₃⁻/CO₂ system), lungs (CO₂ excretion), and kidneys (HCO₃⁻ reabsorption, H⁺ excretion).
- Normal values: pH 7.35-7.45, pCO₂ 35-45 mmHg, HCO₃⁻ 22-26 mEq/L.
- Anion Gap = Na⁺ − (HCO₃⁻ + Cl⁻), normal 8-12.
- Elevated AG = unmeasured anions (lactate, ketones, uremic toxins).
- Winter's formula for metabolic acidosis: expected pCO₂ = 1.5(HCO₃⁻) + 8 ± 2.
- If measured pCO₂ > expected → additional respiratory acidosis; < expected → respiratory alkalosis..
High-Yield Points
- Metabolic acidosis: Anion Gap = Na⁺ − (HCO₃⁻ + Cl⁻); normal 8-12
- MUDPILES: Methanol, Uremia, DKA, Propylene glycol, Iron/Isoniazid, Lactic acidosis, Ethylene glycol, Salicylates
- Winter's formula: expected pCO₂ = 1.5(HCO₃⁻) + 8 ± 2
- Compensation: lungs respond in minutes, kidneys take days
- Anion gap metabolic acidosis → calculate delta-delta for mixed disorders
Topic Summary
Acid-base disorders are classified as respiratory (pCO₂ abnormal) or metabolic (HCO₃⁻ abnormal). Compensation tries to normalize pH. Anion gap helps identify the cause of metabolic acidosis. Winter's formula: expected pCO₂ = 1.5(HCO₃⁻) + 8 ± 2.