Today we look at the difference between ABG and VBG in the ED clinical setting: When should you order an ABG and when does a VBG suffice?
pH
- Correlates closely
- VBG typically 0.03-0.04 lower than ABG
pCO2
- VBG can be used as a screen for hypercarbia (100% sensitive) for values < 45 mmHg
- ABG should be used to assess pCO2 for patients in shock states and when value is > 45 mmHg
- Mean difference is PvCO2 = PaCO2 + 5.7 mmHg
PO2
- Does not correlate
- ABG should be collected depending on clinical circumstance, ie. ARDS, severe hypoxemia, severe acidosis, poor peripheral perfusion, pulse ox not accurate, etc.
HCO3
- Correlates closely
- Is a calculated value - get a BMP for more accurate result
Lactate
- Correlates closely
- Mean difference is 0.02-0.08
Base Deficit / Base Excess
- Correlates closely
- Difference is not clinically significant
Remember:
- ABGs are not without risk!
- Disadvantages include risk of bleeding/hematoma, pain, nerve injury, digital ischemia, delays in care, risk of pseudoaneurysm and AV fistula, etc.
Takeaways:
- Get an ABG in patients with severe shock, hypoxemia, or when PaO2 is clinically relevant.
- A VBG is sufficient to trend pH, lactate, pCO2 when not hypercarbic, and base deficit.
- If you’re really concerned about HCO3, check a BMP.
- As always, any test you order should be clinically relevant and benefits outweigh the risks.
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