CORRELATION BETWEEN VBG AND ABG
pH
- Good correlation
- pooled mean difference: +0.035 pH units
pCO2
- good correlation in normocapnia
- non-correlative in severe shock
- 100% sensitive in detecting arterial hypercarbia in COPD exacerbations using cutoff of PaCO2 45 mmHg and laboratory based testing (McCanny et al, 2012), i.e. if VBG PCO2 is normal then hypercapnia ruled out (PaCO2 will be normal), though this conflicts with the meta-analysis by Byrne et al 2014 (see below)
- correlation dissociates in hypercapnia – values correlate poorly with PaCO2 >45mmHg
- Mean difference pCO2 +5.7 mmHg (wide range in 95%CIs among different studies, on the order of +/-20 mmHg)
- A more recent meta-analysis by Byrne et al, 2014 found that the 95% prediction interval of the bias for venous PCO2 was −10.7 mm Hg to +2.4 mm Hg. They note that in some cases the PvCO2 was lower than the PaCO2. The meta-analysis had considerable heterogeneity between studies which limits the reliability of its conclusions.
HCO3
- Good correlation
- Mean difference −1.41 mmol/L (−5.8 to +5.3 mmol/L 95%CI)
Lactate
- Dissociation above 2 mmol/L
- Mean difference 0.08 (-0.27 – 0.42 95%CI)
Base excess
- Good correlation
- Mean difference 0.089 mmol/L (–0.974 to +0.552 95%CI)
PO2
- PO2 values compare poorly
- arterial PO2 is typically 36.9 mm Hg greater than the venous with significant variability (95% confidence interval from 27.2 to 46.6 mm Hg) (Byrne et al, 2014)
- See also: Central venous oxygen saturation (ScvO2) monitoring, mixed venous oxygen saturation (SvO2) and SvO2 vs ScvO2
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