Acid-base disorders

Metabolic Acidosis

• Definition: pH < 7.35 due to HCO3⁻ < 24.
• Compensation: Hyperventilation (↓ pCO2).
• Anion Gap (AG): Na⁺ - (Cl⁻ + HCO3⁻). Normal = 8–12.
• High AG Metabolic Acidosis (MUDPILES)
  • Mechanism: An acid ($HA$) is added to the extracellular fluid. The acid dissociates into $H^{+}$ and an unmeasured anion ($A^{-}$).
    • $H^{+}$ binds to and consumes $HC{O}_3^{-}$, decreasing its concentration.
    • The unmeasured anion ($A^{-}$, e.g., lactate, acetoacetate) replaces the lost $HC{O}_3^{-}$ to maintain electrical neutrality.
    • Because $C{l}^{-}$ remains unchanged, the formula $N{a}^{+}-(C{l}^{-}+\downarrow HC{O}_3^{-})$ yields a high AG (> 12 mEq/L).
    • Measured anions are Cl- and HCO3-. Others are all unmeasured.
  • Methanol (Vision loss)
  • Uremia
  • DKA
  • Propylene glycol
  • Iron / INH
  • Lactic acidosis (Shock, Metformin) t
  • Ethylene glycol (Calcium oxalate crystals)
  • Salicylates (Mixed Met Acidosis + Resp Alkalosis)
• Normal AG Metabolic Acidosis (HARDASS)
  • Mechanism: $HC{O}_3^{-}$ is directly lost from the body (via kidneys or GI tract) or neutralized by $HCl$ (saline infusion).
    • To maintain electroneutrality, the kidneys retain $C{l}^{-}$, or $C{l}^{-}$ is directly added to the system.
    • As $HC{O}_3^{-}$ goes down, $C{l}^{-}$ goes up in a 1:1 ratio.
    • Because the sum of $(C{l}^{-}+HC{O}_3^{-})$ remains constant, the formula $N{a}^{+}-(\uparrow C{l}^{-}+\downarrow HC{O}_3^{-})$ yields a normal AG ($\le 12$ mEq/L).
  • Hyperalimentation
  • Acetazolamide
  • RTA (Types 1, 2, 4)
  • Diarrhea (#1 cause, Urine AG is Negative)
  • Aldosterone antagonists
  • Saline infusion
• Key Calculations
  • Winter’s Formula: Checks compensation.
    • Exp pCO2 = 1.5 × [HCO3⁻] + 8 ± 2 c2
  • Urine AG: (Urine Na + Urine K) - Urine Cl.
    • Negative: Diarrhea (Gut loss).
    • Positive: RTA (Renal loss).