Pathophysiology
Clinical meaning
Advanced ABG interpretation requires the ability to identify mixed disorders (two or more primary acid-base disturbances occurring simultaneously) and calculate the anion gap to narrow the differential diagnosis of metabolic acidosis. Anion Gap (AG): The anion gap represents unmeasured anions in the blood. It helps classify metabolic acidosis into two categories. Formula: AG = Na+ - (Cl- + HCO3-) Normal: 8-12 mEq/L (some labs use 12 ± 4 depending on albumin correction) • Elevated Anion Gap Metabolic Acidosis (AGMA): Accumulation of unmeasured acids. Each acid provides an H+ (which consumes HCO3-) and an unmeasured anion (which increases the gap). MUDPILES: Methanol, Uremia, DKA (beta-hydroxybutyrate), Propylene glycol, Isoniazid/Iron, Lactic acidosis (shock, sepsis, ischemia), Ethylene glycol, Salicylates. • Non-Anion Gap (Normal AG) Metabolic Acidosis (NAGMA): Loss of bicarbonate OR gain of chloride (hyperchloremic). Causes: Diarrhea (loss of HCO3-rich intestinal fluid), Renal Tubular Acidosis, early renal failure, excessive NS infusion (dilutional acidosis), acetazolamide, ureteral diversion. Delta-Delta (Δ/Δ) Analysis: Identifying Hidden Disorders: In elevated AG metabolic acidosis, the delta-delta ratio helps identify coexisting metabolic disorders: Δ Anion Gap = Calculated AG - Normal AG...