In the ED we are often concerned about the pH, CO2 level, and oxygen saturation of patients. How does one determine whether to get a venous blood gas (VBG) or arterial blood gas (ABG)? ABGs are typically seen as the “gold standard” for evaluating pH, PaCO2, and HCO3 but in certain situations VBGs are a better option. Negative aspects of ABGs include increased pain, neve injury, increased risk of bleeding and hematoma, and delays in care while waiting for a specialized individual such as a respiratory therapist to obtain the sample. 

Historically, values obtained via VBG have been criticized for a perceived lack of accuracy in all domains. So the question remains – are values (such as pH, PCO2, and HCO3) truly disparate enough between ABG’s and VBG’s to actually change clinical practice?

• Many studies have been conducted in both ED and ICU populations to assess the correlation of VBG to ABG values
• For most patients, there is good correlation and it is completely appropriate to start with a VBG
• In general, a pH on a VBG tends to be similar to the pH on an ABG (difference of about 0.035 between them).
• In general, CO2 on a VBG will correlate with but not be the same as the CO2 on an ABG (average difference of about 5.7 between the two).
• In general, bicarb on a VBG tends to be similar to the bicarb on an ABG (difference of about 1.4 between the two). 
• PO2 does not correlate between the two

VBGs can be used as a reliable alternative to ABGs in many clinical cases. However there are several situations where an ABG is preferred:

• Critically ill patients/patients in shock
• Patients with decompensated heart failure
• Mixed acid/base disorders
• Patients in who you need a PO2

The above patients will have excessive discordance between the VBG and ABG and the VBG values should not be relied on for medical decision making.

Take Home Points:

• In most of our patients in the ED, a VBG can be used instead of an ABG
• Consider an ABG in patients:
  • who are in shock
  • whose VBG results do not fit with the clinical picture
  • who you need a precise result for clinical decision making
  • who you need a PO2

The benefits of a VBG vs ABG are obvious – decreased pain, complications, and time – so perform a VBG instead of an ABG in the ED when appropriate!