Normal Arterial Blood Gas (ABG) Values
Normal arterial blood gas values on room air are: PO2 >90 mmHg, PCO2 <40 mmHg, and pH 7.40. 1
Standard Reference Ranges
The following values represent normal arterial blood gas parameters for adults breathing room air:
- PO2 (Partial Pressure of Oxygen): >90 mmHg 1
- PCO2 (Partial Pressure of Carbon Dioxide): <40 mmHg 1
- pH: 7.40 1
For comparison, normal mixed venous blood values differ significantly: PO2 of 40 mmHg, PCO2 of 50 mmHg, and pH of 7.35. 1
Clinical Context for Interpretation
Oxygen Saturation Targets
For most patients without risk of hypercapnic respiratory failure, target SpO2 should be 94-98%. 1, 2
- Patients at risk for hypercapnic respiratory failure (severe COPD, chest wall disease, neuromuscular disease, severe obesity, cystic fibrosis, bronchiectasis) should target SpO2 of 88-92% 1
- A PO2 >90 mmHg typically corresponds to oxygen saturation >94% 1
Critical Thresholds
A PO2 below 60 mmHg (8 kPa) represents life-threatening hypoxemia requiring immediate intervention. 2
- Hypoxemia with PO2 <60 mmHg demands urgent oxygen supplementation regardless of PCO2 status 2
- Normal oxygen saturation by pulse oximetry does not exclude significant acid-base disturbances or hypercapnia 3
Important Clinical Caveats
When ABG Analysis is Essential
ABG measurement is crucial in critically ill patients because pulse oximetry alone cannot assess ventilation status or acid-base balance. 3
- ABG provides critical information about PaCO2 and pH that pulse oximetry cannot detect 3
- In cardiovascular intensive care, ABG should be obtained from arterial rather than venous sources in patients with shock, hypotension, or on vasopressor therapy 3
- After initiating or changing oxygen therapy, repeat ABG within 30-60 minutes to confirm adequate response without precipitating respiratory acidosis 1, 2
Common Pitfalls to Avoid
- Never assume normal pulse oximetry means normal ABG: Patients can have normal PO2 and SpO2 but abnormal pH or PCO2 3
- Venous blood gas cannot substitute for ABG in critically ill patients: While venous samples may predict some values in stable mechanically ventilated patients, arterial sampling remains the standard for acute assessment 4
- Do not delay oxygen therapy while obtaining ABG: Initiate supplemental oxygen immediately for hypoxemia, then obtain ABG to guide further management 2
Acid-Base Component Reference Values
Beyond oxygenation and ventilation, normal ABG includes:
- Bicarbonate (HCO3-): Typically 22-26 mEq/L (calculated from pH and PCO2 using Henderson-Hasselbalch equation) 5, 6
- Base excess/deficit: -2 to +2 mEq/L 5
Respiratory acidosis is defined as pH <7.35 with PCO2 >6.0 kPa (45 mmHg), indicating inadequate ventilation. 1