Clinical Pearls: VBG vs ABG
Two truths exist in the majority of hospitals across America.
ED staff will deliver a patient to the ICU staff alive…or mostly alive. They may be covered in dirt, with clothes half cut off, and wires tangled in a complex knot, but again … in some state of alive-ness. The ICU staff will receive this pt and overlook the alive factor to hone on in the mess factor.
ED staff will think to themself “I wonder what the blood gas would show” followed by “let me order a VBG.” The ICU staff will then order an ABG.
Hopefully my humor is apparent in these words. Obviously I am a slave to the drama factor, but surely we can all acknowledge the nugget of truth that underlies these stereotypes. There is a legitimate reason why ER providers consistently order a VBG over an ABG and the ICU provider then may or may not believe the VBG choosing to tack on an ABG instead.
Many patients in the hospital require assessment of their acid base balance and oxygenation/ventilation status which is best quantified from serum arterial samples. Additionally, the ABG provides expedited lab results such as hemoglobin, potassium and other electrolytes, and lactic acid. These values are very helpful in making a rapid diagnosis and treatment plan. A venous blood gas seems to be the standard replacement for an arterial blood gas in the emergency department. It does offer some benefits, but has limitations as well which should be acknowledged.
The focus of this article is to describe the pros/cons of using a VBG as a surrogate, the exclusion criteria, and the method of converting a VBG to an ABG for ease of interpretation.
VBG Pros:
Less risk to the patient
Less pain for the patient
Easier and quicker to obtain; could prevent treatment delays
VBG Cons:
Multiple reasons for invalidated results including
Prolonged tourniquet use
Delayed processing of sample or non iced sample
Circulatory shock (MAP <60, CI<1) or significant hypoxia or mechanically ventilated
So you see my concern right? How many ICU patients do you see who do not exhibit shock or hypoxia and are non ventilated? But first, lets talk about the evidence. Many studies have been performed and meta analyses have demonstrated that in the hemodynamically stable patient a VBG can act as a surrogate measurement of acid-base and ventilation status with accuracy.
In the setting of shock or hypoxia however, there have been mixed results regarding the difference between ABG/VBG values. This study https://pubmed.ncbi.nlm.nih.gov/27283009/ indicates there is good correlation. This review https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5997042/ only finds correlation between pH and therefore does not recommend exchanging an ABG with a VBG. There is this systematic review https://pubmed.ncbi.nlm.nih.gov/24383789/ and this 2021 meta analysis https://pubmed.ncbi.nlm.nih.gov/33780397/ submitted to Anesthesia and Analgesia suggesting the two are not comparable in critically ill and perioperative patients.
Why are the results invalidated in these settings? The difference in the values are highly variable, in some instances up to 4x greater b/w ABG and VBG, so where on the spectrum of difference does the reality fall? So you can’t convert a VBG to an ABG without generating a wide range. Ex. typically a VBG PCO2 of 40 will equate to an ABG PCO2 of ~ 35, but in the aforementioned settings it could be anywhere from 35 - 20. A pH on VBG of 7.2 would typically convert to 7.25 but in this instance could be 7.25-7.45. Those numbers reflect the difference between normal and abnormal, so how helpful is this study really? Think of it like this: arterial blood reflects what the tissue is seeing or being delivered after gas exchange in the lungs occurs and then it is pumped out to the body. So if we draw this and the numbers are poor we can confidently say the tissue is seeing poor numbers as well. Venous blood on the other hand has been delivered to to the tissue, O2 was extracted and Co2 was generated and deposited back in. Factors like how much O2 was delivered, how well are the tissues extracting (consider septic shock where demand is very high), how long is blood spending time at the tissue (longer = more CO2 can be dumped into the venous system so the more acidic the venous sample will be) all lead to disparate venous and arterial values.
Low BP/CO -> decreased and slower O2 delivery -> higher CO2 extraction from the tissue/more dumping of CO2 in the venous system.
Respiratory failure + circulatory failure -> poor CO2 clearance in the pulm capillary beds so more CO2 remains in the arterial sample so your VBG may not demonstrate how acidic the ABG could actually be.
Massive circulatory failure + mechanical vent -> poor delivery from heart (bad arterial pH/PCO2) + effective ventilation by a machine = lower venous pCO2 and better pH. (when drawn from a venous sample). But these will be inaccurate because it’s not what the tissue is seeing. The VBG in these patients will give false hope. Need an arterial sample.
Opposing the last bullet point. In a patient in massive circulatory shock who is not ventilating (not on a vent) well the two may be closer to the same values. Both will be pretty bad.
Low BP/CO -> decreased and slower retention of CO2 in the venous system.
Low BP/CO -> reduced gas exchange at the pulmonary arterio-venule capillary bed.
In summary, with regards to ICU patients with low BP/CO/hypoxia +/- mechanical ventilation I do not advocate for use of a VBG in regards to interpreting acid-base, PCO2, or PO2. Now, there are still some pertinents you can get from a VBG if you are in a tight spot with limited options: potassium, hemoglobin are the main ones that come to mind.
For clinically appropriate scenarios this is how you would “convert” a VBG to ABG values:
Venous pH + 0.05 = arterial pH
Venous PCO2 - 5 = arterial PCO2
Above is the proposed algorithm for assessing if your pt’s VBG could correlate with an ABG per Josh Farkas of Pulmcrit.
References
https://www.the-hospitalist.org/hospitalist/article/33587/clinical-guidelines/is-vbg-an-alternative-to-the-troublesome-abg/
https://www.tamingthesru.com/blog/2018/5/20/whats-in-a-blood-gas-vbg-vs-abg
https://emcrit.org/pulmcrit/vbg-abg/
https://pubmed.ncbi.nlm.nih.gov/27283009/
https://pubmed.ncbi.nlm.nih.gov/21143397/
https://pubmed.ncbi.nlm.nih.gov/33780397/
Every Thursday at 3pm I go live on Youtube answering your questions and talking about the topic of the weekly blog. I would love to see you there.