From the Guidelines
Venous Blood Gas (VBG) is not recommended after an Arterial Blood Gas (ABG) to assess oxygenation needs, as ABGs are the gold standard for evaluating oxygenation status. The British Thoracic Society guideline for oxygen use in adults in healthcare and emergency settings, published in Thorax in 2017 1, emphasizes the importance of blood gas analysis in specific clinical situations, such as critically ill patients, unexpected falls in SpO2, and deteriorating oxygen saturation. However, it does not support the routine use of VBG after ABG for oxygenation assessment.
Key Points to Consider
- ABGs directly measure arterial oxygen levels (PaO2) and oxygen saturation (SaO2), reflecting how well oxygen is being delivered to tissues.
- VBGs are drawn from venous blood, which has already passed through tissues where oxygen has been extracted, making VBG oxygen values consistently lower than arterial values and unreliable for oxygenation assessment.
- VBGs can provide useful information about acid-base status, as pH and bicarbonate levels correlate reasonably well between venous and arterial samples.
- In clinical practice, if ongoing assessment of oxygenation is needed after an initial ABG, either another ABG would be performed or non-invasive monitoring like pulse oximetry would be used, as recommended by the guideline 1.
Clinical Decision Making
When deciding on the utility of VBG after ABG, it's crucial to consider the primary concern of the patient's condition. If oxygenation assessment is critical, ABGs or non-invasive monitoring should be prioritized. VBGs are more appropriately used when acid-base status is the primary concern and oxygenation assessment is not critical, or when arterial sampling is difficult or contraindicated. The guideline 1 provides a framework for clinical decision-making, emphasizing the importance of individual patient assessment and the need for urgent clinical reassessment in patients requiring increased concentrations of oxygen.
From the Research
Utility of Venous Blood Gas (VBG) after an Arterial Blood Gas (ABG)
- The utility of VBG as an alternative to ABG has been explored in several studies 2, 3, 4, 5, 6.
- VBG parameters of pH and Pco2 are potentially good surrogates for determining arterial pH and Pco2 in stable patients without severe acid-base disturbances 2.
- Central VBG can be used as a useful screening tool for arterial hypercapnia 2, 3, 4.
- The combination of VBG analysis plus pulse oximetry provided accurate information on acid-base, ventilation, and oxygenation status for undifferentiated critically ill patients in the ED and ICU 5.
Trending of VBG after an ABG
- Studies have shown that VBG can be used to trend changes in pH, Pco2, and oxygenation status over time 3, 4, 5.
- The mean difference between arterial and central venous pH and Pco2 was 0.03 units and 4-6.5 mm Hg, respectively, in hemodynamically stable patients 2.
- An adjustment formula for ABG conversion from central VBG has been derived: (1) arterial pH = venous pH + 0.05 units and (2) arterial Pco2 = venous Pco2 - 5 mm Hg 2, 6.
- The accuracy of VBG for detecting hypercapnia and hypoxemia has been demonstrated to be high, with sensitivity and specificity values ranging from 80-100% 3, 4.
Limitations and Considerations
- The use of VBG as a surrogate for ABG may not be suitable for patients with severe acid-base disturbances or circulatory failure 2, 3.
- Respirator adjustments may be done too often if VBG is used, highlighting the need for careful interpretation of VBG results 3.
- Further studies are needed to fully establish the utility and limitations of VBG in different clinical settings and patient populations 2, 3, 4, 5, 6.