11/13/24, 7\:34 PM Guide | VBG interpretation

VBG interpretation

Table of contents

Introduction

This guide describes the venous blood gas (VBG) test, explains key di
an approach to VBG result interpretation.
Understanding how a VGB diperforming an
ABG and interpreting an ABG.

What is a venous blood gas (VBG)?

A VBG is a venous blood sample drawn into an ABG (heparinised) syringe and then run through a blood gas analyser.
This blood gas machine provides a rapid (results within 1-2 minutes) analysis of key physiological parameters, including\:
pH
pCO *
2
pO *
2
HCO
-
3
Base Excess (BE)
*Of these results, only pO 2 2
and pCO vary signi
The blood gas analyser reports other key values\:
+ + 2+ -
Electrolytes (Na , K , Ca , Cl )
Glucose
Urea
Lactate
These values are generally equivocal between venous and arterial samples.
1
Tip\: when taking a peripheral venous blood sample for VBG analysis, tourniquet time should be minimised where possible, as
prolonged tourniquet time can alter blood biochemistry.

Advantages of VBG analysis

Both ABGs and VBGs can rapidly provide physiological information about the patient, which can be vital when dealing with
acutely unwell patients.
The main advantage of VBG analysis is that collection of venous blood is often a technically easier procedure, and it avoids
the need for arterial sampling, which comes with a range of potential complications, including patient discomfort.
Complications of arterial puncture
Complications of arterial puncture can include\:
Pain\: arterial sampling can be extremely uncomfortable for patients
Failure of procedure\: more technically di
Arterial damage\: vessel laceration/vessel thrombosis/pseudoaneurysm formation
https\://app.geekymedics.com/osce-guides/data-interpretation/vbg-interpretation/ 1/311/13/24, 7\:34 PM Guide | VBG interpretation
Haemorrhage/local haematoma\: more common in those on anticoagulation

Di

The key di
In theory, a diagnosis of respiratory failure can only be made when an arterial sample con
(PaO ) \<8 kPa / \<60 mmHg. The venous oxygen tension (PvO ) cannot be used to equate to the arterial oxygen tension and, as
2 2
such, is not of any clinical relevance.
The correlation of venous pCO 2 2
to arterial pCO is more controversial, and practice around this area may vary between
hospitals and departments. Research shows that the venous pCO 2 2.
tends to be between 3 to 8 mmHg higher than arterial pCO
However, con
1
A practical use of venous blood gas in assessing respiratory function is to exclude type 2 respiratory failure and respiratory
acidosis.
It has been demonstrated that a low-normal venous pCO 2
can exclude type 2 respiratory failure with a 100% negative
predictive value. 2
Venous pH has also been shown to equate very closely to arterial pH in acutely unwell patients, and thus a
VBG can be su
3

When to consider a VBG

Indications for performing a VBG include\:
Rapid measurement of key metabolic values in the acutely unwell patient
Serial measurements to determine response to treatment (e.g. monitoring lactate in sepsis, glucose/pH measurements in
DKA or potassium measurements when treating hyperkalaemia)
To indicate a patient’s respiratory function without the need for arterial puncture\: when combined with pulse oximetry, a
venous pH and pCO2 can give a good indication of respiratory function; however, to accurately assess the patient's
oxygenation status, an ABG will be required

When is an ABG necessary?

If a patient presents acutely unwell with a primary respiratory pathology, an initial ABG is often necessary to provide an
accurate picture of PaO and PaCO .
2 2
Tip\: if a VBG is taken simultaneously, values can be compared, and serial VBG monitoring (e.g. of pCO ) may be acceptable to
2
guide response to treatment and avoid repeated arterial puncture.
There is also evidence that the correlation between VBG and ABG can become unreliable in hypotensive (shocked) patients
1
or those with extreme acid-base derangements. In these patients, arterial sampling will be necessary.
In non-shocked patients without evidence of signi
disturbances and provide rapid biochemical results.

References

1. UpToDate. V e n o u s b l o o d g a s e s a n d o t h e r a l t e r n a t i v e s t o a r t e r i a l b l o o d g a s e s . April 2022. Available from\: [LINK]
2. Kelly, AM. Kerr, D. Middleton, P. V a l i d a t i o n o f v e n o u s p C O 2 t o s c r e e n f o r a r t e r i a l h y p e r c a r b i a i n p a t i e n t s w i t h c h r o n i c o b s t r u c t i v e
a i r w a y s d i s e a s e . May 2005. Available from\: [LINK]
3. Kelly, AM. McAlpine, R. Kyle, E. V e n o u s p H c a n s a f e l y r e p l a c e a r t e r i a l p H i n t h e i n i t i a l e v a l u a t i o n o f p a t i e n t s i n t h e e m e r ge n c y
d e p a r t m e n t . September 2001. Available from\: [LINK]
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Source\: geekymedics.com
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