Clinical meaning
Arterial blood gas interpretation is a foundational clinical competency requiring systematic analysis of acid-base status, oxygenation, and ventilation. The systematic approach: (1) Assess pH to determine acidemia (less than 7.35) or alkalemia (greater than 7.45); (2) Identify the primary disorder by matching the pH direction with the abnormal value (PaCO2 for respiratory, HCO3 for metabolic); (3) Assess compensation -- respiratory compensation for metabolic disorders occurs within hours (hyperventilation for acidosis, hypoventilation for alkalosis), while metabolic compensation for respiratory disorders takes 3-5 days for full renal compensation; (4) Calculate the anion gap (Na - Cl - HCO3, normal 12 plus or minus 4 with albumin correction: for every 1 g/dL decrease in albumin below 4, add 2.5 to the expected anion gap); (5) If anion gap is elevated, calculate the delta-delta ratio (delta AG divided by delta HCO3) to identify concurrent metabolic disorders (ratio less than 1 suggests concurrent non-anion-gap metabolic acidosis, ratio greater than 2 suggests concurrent metabolic alkalosis). Causes of elevated anion gap metabolic acidosis follow the MUDPILES mnemonic: Methanol, Uremia, Diabetic ketoacidosis, Propylene glycol, Iron/Isoniazid, Lactic acidosis, Ethylene glycol, Salicylates. The clinician also assesses oxygenation using the A-a gradient (PAO2 - PaO2, where PAO2 = FiO2 x (Patm - PH2O) - PaCO2/0.8; normal A-a gradient = 2.5 + 0.21 x age) to differentiate hypoventilation (normal A-a gradient) from V/Q mismatch, diffusion impairment, or shunt (elevated A-a gradient).