Arterial Blood Gas (ABG) Interpretation

Normal ABG Values

Component Normal Range
pH 7.35–7.45
PaCO2 35–45 mmHg
PaO2 80–100 mmHg
HCO3 22–26 mEq/L
Base Excess −2 to +2 mEq/L
SaO2 95–100%

Step-by-Step Interpretation

  1. Look at pH — Acidosis <7.35; Alkalosis >7.45; Normal 7.35–7.45.
  2. Check PaCO2 — Respiratory component. High PaCO2 (>45) = respiratory acidosis; Low PaCO2 (<35) = respiratory alkalosis.
  3. Check HCO3 — Metabolic component. Low HCO3 (<22) = metabolic acidosis; High HCO3 (>26) = metabolic alkalosis.
  4. Determine primary disorder — Which value (PaCO2 or HCO3) matches the pH direction? That is the primary disorder.
  5. Check for compensation — Is the opposite system trying to correct? Partial = pH still abnormal; Full = pH normal but PaCO2 and HCO3 abnormal.

ABG Disorders

Disorder pH PaCO2 HCO3 Common Causes
Respiratory Acidosis <7.35 >45 Normal or ↑ Hypoventilation, COPD, sedation, airway obstruction, neuromuscular disease
Respiratory Alkalosis >7.45 <35 Normal or ↓ Hyperventilation, anxiety, pain, fever, PE, early sepsis
Metabolic Acidosis <7.35 Normal or ↓ <22 DKA, lactic acidosis, renal failure, diarrhea, toxic ingestions
Metabolic Alkalosis >7.45 Normal or ↑ >26 Vomiting, NG suction, diuretics, hypokalemia, excess bicarbonate
Mixed Varies Varies Varies Two disorders present (e.g., respiratory acidosis + metabolic acidosis)

Compensation

Partial compensation — The opposing system (respiratory or metabolic) is attempting to correct the pH, but pH remains outside normal range. Both PaCO2 and HCO3 will be abnormal.

Full compensation — The opposing system has corrected pH back to normal (7.35–7.45), but PaCO2 and HCO3 remain abnormal. The primary disorder is identified by which value is more severely deranged.

ROME Mnemonic

Respiratory Opposite, Metabolic Equal