Arterial blood gas interpretation is a foundational skill for anesthesia providers, essential for intraoperative management, ICU care, and perioperative assessment. A systematic approach to acid-base analysis — evaluating pH, primary disorder, compensation, and anion gap — enables rapid identification of metabolic and respiratory derangements and guides appropriate intervention.
Normal values: pH 7.35–7.45, PaCO2 35–45 mmHg, HCO3 22–26 mEq/L, PaO2 80–100 mmHg
Systematic approach: assess pH → identify primary disorder → evaluate compensation → check anion gap
Anion gap = Na - (Cl + HCO3); normal 8–12 mEq/L; elevated in lactic acidosis, ketoacidosis, toxins (MUDPILES)
Winter's formula for expected PaCO2 in metabolic acidosis: PaCO2 = (1.5 × HCO3) + 8 ± 2
P/F ratio (PaO2/FiO2) assesses oxygenation: <300 = ALI, <200 = ARDS, <100 = severe ARDS
Always calculate the delta-delta (change in anion gap / change in bicarbonate) when the anion gap is elevated. A ratio >2 suggests a concurrent metabolic alkalosis, while <1 suggests a concurrent non-anion gap metabolic acidosis. Mixed disorders are commonly missed without this step.
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