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

Metabolic compensation in respiratory acidosis and alkalosis