ABG interpretation
Formal Definition
The systematic analysis of arterial blood gas values (pH, PaCO2, PaO2, HCO3, base excess) to characterize acid-base disorders (metabolic vs respiratory, acidosis vs alkalosis, with or without compensation) and assess oxygenation and ventilation; a foundational clinical skill for managing critically ill patients.
How It's Used on the Ward
"Walk me through the ABG" or "the gas looks bad" — bedside shorthand for a critical assessment tool that reveals the underlying physiology.
Example
""ABG on 4L NC: pH 7.28, PaCO2 58, PaO2 62, HCO3 26. Respiratory acidosis with early metabolic compensation — patient is tiring out. He's a COPD exacerbation. If he doesn't improve on BiPAP in the next hour, we're intubating.""
Clinical Context
Step-wise ABG interpretation: 1) pH < 7.35 = acidosis, > 7.45 = alkalosis. 2) PaCO2 > 45 = respiratory cause, HCO3 < 22 = metabolic cause. 3) Is compensation adequate (use Winter's formula for metabolic acidosis, expected pH for respiratory disorders)? 4) If metabolic acidosis: calculate anion gap (Na - [Cl + HCO3], normal 8–12). 5) If elevated AG: check delta-delta for mixed disorders. PaO2/FiO2 ratio < 300 = ARDS. A-a gradient = [(FiO2 × 713) − (PaCO2/0.8)] − PaO2; elevated gradient = intrinsic lung disease.