Venous Blood Gas (VBG) Interpretation

This guide describes the venous blood gas (VBG) test, explains key differences from an arterial blood gas (ABG), and provides an approach to VBG result interpretation.

Understanding how a VGB differs from an ABG is important. For more information on ABGs, see our guides to performing 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
• pCO₂*
• pO₂*
• HCO₃^–
• Base Excess (BE)

*Of these results, only pO₂ and pCO₂ vary significantly between arterial and venous samples.

The blood gas analyser reports other key values:

• Electrolytes (Na⁺, K⁺, Ca²⁺, 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. 

Differences in ABG / VBG analysis

The key difference in VBG analysis compared to ABG analysis is the assessment of respiratory function.

In theory, a diagnosis of respiratory failure can only be made when an arterial sample confirms an arterial oxygen tension (PaO₂) <8 kPa / <60 mmHg. The venous oxygen tension (PvO₂) cannot be used to equate to the arterial oxygen tension and, as such, is not of any clinical relevance.

The correlation of venous pCO₂ to arterial pCO₂ is more controversial, and practice around this area may vary between hospitals and departments. Research shows that the venous pCO₂ tends to be between 3 to 8 mmHg higher than arterial pCO_2. However, confidence intervals are large.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 pCO₂ 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 sufficient to rule out respiratory acidosis.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 PaO₂ and PaCO₂.

Tip: if a VBG is taken simultaneously, values can be compared, and serial VBG monitoring (e.g. of pCO₂) may be acceptable to 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 or those with extreme acid-base derangements.¹ In these patients, arterial sampling will be necessary. 

In non-shocked patients without evidence of significant hypoxia, a VBG is almost always sufficient to investigate metabolic disturbances and provide rapid biochemical results.

Editor

Dr Chris Jefferies

References

1. UpToDate. Venous blood gases and other alternatives to arterial blood gases. April 2022. Available from: [LINK]
2. Kelly, AM. Kerr, D. Middleton, P. Validation of venous pCO2 to screen for arterial hypercarbia in patients with chronic obstructive airways disease. May 2005. Available from: [LINK]
3. Kelly, AM. McAlpine, R. Kyle, E. Venous pH can safely replace arterial pH in the initial evaluation of patients in the emergency department. September 2001. Available from: [LINK]