What are the normal partial pressure of oxygen (PO2) and partial pressure of carbon dioxide (PCO2) values in an arterial blood gas (ABG)?

Normal Arterial Blood Gas Values

The normal range for arterial partial pressure of oxygen (PaO2) is 75-100 mmHg (10-13 kPa), and the normal range for arterial partial pressure of carbon dioxide (PaCO2) is 35-45 mmHg (4.7-6.0 kPa). 1

Detailed Normal ABG Parameters

Oxygen Parameters

• PaO2 (Arterial Oxygen Partial Pressure):

  • Normal range: 75-100 mmHg (10-13 kPa) 1
  • Values below 60 mmHg (8 kPa) indicate hypoxemia 2
  • Values above 100 mmHg may indicate hyperoxemia

• SaO2 (Arterial Oxygen Saturation):

  • Normal range: 94-98% 1
  • Values below 90% indicate significant hypoxemia 2

Carbon Dioxide Parameters

• PaCO2 (Arterial Carbon Dioxide Partial Pressure):
  • Normal range: 35-45 mmHg (4.7-6.0 kPa) 1
  • Reference range from multiple studies: 4.6-6.1 kPa (34-46 mmHg) 1
  • Values above 45 mmHg indicate hypercapnia
  • Values below 35 mmHg indicate hypocapnia

Clinical Significance

Hypoxemia Classification

• Mild hypoxemia: PaO2 60-80 mmHg, SaO2 90-94% 2
• Moderate hypoxemia: PaO2 45-60 mmHg, SaO2 80-90% 2
• Severe hypoxemia: PaO2 < 45 mmHg, SaO2 < 80% 2
• Profound hypoxemia: PaO2 < 30 mmHg, SaO2 < 56% 2

Hypercapnia and Respiratory Failure

• PaCO2 > 45 mmHg (6.0 kPa) indicates hypercapnia 1
• Type 1 respiratory failure: PaO2 < 60 mmHg (8 kPa) with normal or low PaCO2 1
• Type 2 respiratory failure: PaCO2 > 45 mmHg (6.0 kPa) regardless of oxygen level 1

Special Considerations

Arterial vs. Venous Blood Gases

• Venous blood gases are not reliable substitutes for arterial measurements of PO2 3
• Venous PCO2 correlates well with arterial PCO2 but is typically 4-5 mmHg higher 4
• Venous pH also correlates well with arterial pH but is slightly lower 4

Clinical Context

• In trauma patients, maintaining PaO2 between 60-100 mmHg and PaCO2 between 35-40 mmHg is recommended 1
• In post-cardiac arrest patients, targeting normocapnia (PaCO2 35-45 mmHg) is suggested 1
• In patients with COPD, some may tolerate lower oxygen levels (SaO2 as low as 80%, PaO2 approximately 45 mmHg) 2

Pitfalls and Caveats

1. Age-related changes: PaO2 decreases with age; the formula 104 - (0.27 × age in years) can estimate the lower limit of normal PaO2
2. Altitude effects: Normal values are lower at higher altitudes due to decreased atmospheric pressure
3. Pulse oximetry limitations: May not accurately reflect PaO2, especially in patients with dark skin pigmentation 1
4. Sampling technique: Improper sampling can lead to air contamination and falsely elevated PaO2 values
5. Temperature effects: ABG values are temperature-dependent; results are standardized to 37°C but may need correction in hypothermic or hyperthermic patients

By understanding normal ABG values and their clinical significance, clinicians can better interpret these results in the context of patient assessment and management.