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
- Look at pH — Acidosis <7.35; Alkalosis >7.45; Normal 7.35–7.45.
- Check PaCO2 — Respiratory component. High PaCO2 (>45) = respiratory acidosis; Low PaCO2 (<35) = respiratory alkalosis.
- Check HCO3 — Metabolic component. Low HCO3 (<22) = metabolic acidosis; High HCO3 (>26) = metabolic alkalosis.
- Determine primary disorder — Which value (PaCO2 or HCO3) matches the pH direction? That is the primary disorder.
- 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
- Respiratory Opposite — In respiratory disorders, pH and PaCO2 move in opposite directions. Acidosis = high PaCO2; Alkalosis = low PaCO2.
- Metabolic Equal — In metabolic disorders, pH and HCO3 move in the same direction. Acidosis = low HCO3; Alkalosis = high HCO3.
References
Public sources for arterial blood gas interpretation and acid–base physiology.
- StatPearls — Arterial Blood Gas (NIH/NCBI Bookshelf)
- StatPearls — Physiology, Acid Base Balance (NIH/NCBI Bookshelf)
- MedlinePlus — Blood Gas Test (U.S. National Library of Medicine)
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