Why VBG instead of ABG?
- Pain – we all know and warn our patients ABG’s are painful. Patients know it to and delay presentation.
- Number of bloods taken – patients attending with COPD will generally have venous bloods taken at triage. Would you want stabbing again if it could be avoided?
VBG vs ABG
There have been many studies looking at this, these have generally done on healthy volunteers, DKA and COPD patients
- pH – we have been using VBG as equivalent to ABG in the treatment of DKA of years, and studies consistently show a 0.03 difference (Forest plot below – ref. 1).
- CO2 – the mean difference between ABG and VBG has been shown to be approximately 0.5kPa. However, the forest plot is not as consistent as pH (1) a recent paper showed the 95% confidence intervals were 1.4 to -2.9kPa(2). So probably not clinically accurate enough to rely on, but worth considering.
- O2 – the correlation between ABG and VBG is very poor with a mean difference of 4.9kPa (95% CI 3.4-6.3kPa), so is clinically very unreliable.
In ED we performed a study in 2017 looking at ABG vs VBG in our COPD patients. Our results demonstrated that our population reflects the previous study populations. the pH difference was +/- 0.03. It showed using a cut-off of pH 7.38 would not miss any patients with respiratory acidosis.
the algorithm we have instituted reflects the one developed by McKeever(2), with some slight modifications. (We could potentially reduce AGB’s by 40%)
Reasons for ABG
- Drowsy/altered mental state – these patients are far more likely to have significant respiratory acidosis requiring urgent NIV & are less responsive to pain. So delaying an ABG is of no benefit.
- SpO2 <85% – there is some debate over where this cut-off should be. As what value will actually change your management? However, our respiratory colleagues felt that this was a safe cut-off.
- pH <7.38 – this indicates a risk of respiratory acidosis. Hence an ABG is required to start NIV if necessary.
- Deterioration – if your patient is deteriorating despite treatment again we need the accuracy of ABG to guide treatment.
if the your patient doesn’t require an ABG, ensure we are treating the patient appropriately with bronchodilators, steroids, maintain SpO2 88-92%, and review regularly.
We are studying the implementation of this so please document the following in the clinical notes
- Blood gas result – this is good clinical practice anyway (paper frequently gets lost).
- DECAF score – (presence of AF and exercise tolerance would be enough for us to calculate)
- Bryne et al. Peripheral venous and arterial blood gas analysis in adults: are they comparable? A systematic review and meta-analysis. Respirology (2014) 19:168–175
- McKeever et al. Using venous blood gas analysis in the assessment of COPD exacerbations: a prospective cohort study. Thorax (2016) 71:210–215