ABG Basics
Normal values:
pH: 7.35 to 7.45
PaCO2: 35 to 45 mmHg
HCO3: 22 to 26 mEq/L
PaO2: 80 to 100 mmHg
SaO2: > 95%
Simplified Interpretation:
· Check pH: If pH < 7.4, primary disorder is acidosis, if pH > 7.4 primary disorder is alkalosis
· Check PaCO2:
o In patients with acidosis (pH < 7.4), if pCO2 > 40 acidosis is respiratory and if pCO2 ≤ 40 acidosis is metabolic
o In patients with alkalosis (pH > 7.4), if pCO2 ≥ 40 alkalosis is metabolic and if pCO2 < 40 alkalosis is respiratory
Metabolic acidosis:
· Calculate predicted pCO2 = HCO3 + 15; or decimal digits of pH (eg, if pH is 7.27 then pCO2 should be approx. 27 mmHg)
· Check respiratory compensation:
o If actual pCO2 within +/- 2 of predicted → proper respiratory compensation
o If actual pCO2 > predicted pCO2 → concomitant respiratory acidosis
o If actual pCO2 < predicted pCO2 → concomitant respiratory alkalosis
· Calculate anion gap = Na – (Cl + HCO3)
o Normal anion gap: Diarrhea, RTA
o Elevated anion gap (> ≈18): Lactic acidosis, Ketoacidosis (diabetes, alcohol, starvation), Renal failure
· Calculate corrected bicarb = Anion gap – 12 + HCO3
o If corrected bicarb between 23 to 30: Normal
o If corrected bicarb > 30: concomitant metabolic alkalosis
o If corrected bicarb < 23: concomitant non anion gap metabolic acidosis
Metabolic alkalosis:
· Predicted pCO2 = multiple formulas but unreliable. Typically, 41 to 55 mmHg
· Check respiratory compensation:
o If actual pCO2 between 41 to 55 → proper respiratory compensation
o If actual pCO2 > predicted pCO2 → concomitant respiratory acidosis
o If actual pCO2 < predicted pCO2 → concomitant respiratory alkalosis
· Common causes: Vomiting, Diuretics, Hypokalemia