Arterial blood gas
Formal Definition
A blood test obtained by arterial puncture (usually radial artery) measuring partial pressures of oxygen (PaO2) and carbon dioxide (PaCO2), blood pH, bicarbonate (HCO3), and oxygen saturation; used to assess ventilation, oxygenation, acid-base status, and to guide ventilator settings; interpreted in the context of the patient's clinical state and FiO2.
How It's Used on the Ward
ABG or gas or arterial blood gas — the blood test that tells you how well the lungs are oxygenating and removing CO2, and whether the patient is acidotic or alkalotic.
Example
"Ventilated patient, ABG: pH 7.31, PaCO2 58, PaO2 82 on FiO2 60%, HCO3 26. Interpretation: acute respiratory acidosis (low pH, high CO2) with metabolic compensation (normal HCO3). On SIMV rate 12, RR 12 — consider increasing rate or decreasing dead space. PaO2/FiO2 ratio 137: moderate ARDS."
Clinical Context
Step-by-step interpretation: (1) pH: acidemia (<7.35) or alkalemia (>7.45); (2) identify primary disorder — if PaCO2 abnormal, respiratory; if HCO3 abnormal, metabolic; (3) check compensation — expected change in HCO3 for respiratory = 1 mEq/L per 10 mmHg change in PaCO2 (acute: 2, chronic: 4-5); (4) anion gap: AG = Na - (Cl + HCO3), normal 8-12; elevated AG metabolic acidosis → calculate delta gap. Common patterns: DKA → high AG metabolic acidosis; COPD exacerbation → respiratory acidosis with metabolic compensation; salicylate → mixed respiratory alkalosis + metabolic acidosis (AG elevated). PaO2/FiO2 ratio: <300 mild ARDS, <200 moderate, <100 severe.