From the Guidelines
An elevated partial pressure of oxygen (PO2) in a Venous Blood Gas (VBG) analysis most likely indicates a technical error in sample collection or processing, such as accidental arterial puncture or contamination with room air. This is because venous blood typically has lower oxygen levels, as tissues have extracted oxygen from the blood, with normal values ranging from 40-50 mmHg. The presence of hyperoxia, as described in the British Thoracic Society guideline for oxygen use in adults in healthcare and emergency settings 1, is more relevant to arterial blood gas analysis, where high PO2 levels can indicate hyperoxaemia. However, in the context of a VBG, an elevated PO2 is more likely to be an artifact of the sampling process rather than a true reflection of the patient's oxygenation status. Possible causes of this error include:
- Accidental arterial puncture instead of venous sampling
- Contamination of the sample with room air during collection
- Rarely, an arteriovenous fistula or shunt allowing arterial blood to mix with venous blood. Given the potential for error, if clinical decisions depend on accurate oxygenation assessment, it is recommended to verify the sampling technique and possibly repeat the test or obtain an arterial blood gas (ABG) instead of relying on the elevated VBG PO2 value.
From the Research
Elevated Partial Pressure of Oxygen (PO2) in Venous Blood Gas (VBG) Analysis
- An elevated partial pressure of oxygen (PO2) in a Venous Blood Gas (VBG) analysis may indicate hyperoxia, as seen in studies 2, 3, 4.
- Hyperoxia can occur when a patient is receiving supplemental oxygen therapy, and the PO2 levels in the blood become higher than normal 2.
- The correlation between arterial and venous PO2 values is generally poor, with studies showing low Pearson correlation coefficients 5, 6.
- However, some studies have found that venous PO2 values can be used in conjunction with pulse oximetry to estimate arterial PO2 levels 3, 4.
- Elevated PO2 levels in VBG analysis may not always accurately reflect arterial PO2 levels, and therefore, should be interpreted with caution 5, 6.
Clinical Implications
- Hyperoxia can be toxic to patients, particularly those with certain medical conditions, and therefore, it is essential to monitor PO2 levels closely 2.
- The use of VBG analysis and pulse oximetry can provide a less invasive alternative to arterial blood gas (ABG) analysis for monitoring oxygenation status 3, 4.
- However, the accuracy of VBG analysis and pulse oximetry for detecting hyperoxia and hypoxemia should be validated against ABG analysis 4.