The Coach at has a great podcast on using the VBG in situations where we used to try to get an ABG (which was never fun to do in small children).

From the podcast: the rule of 4’s: (note ABG values are always higher):

VBG pH is 0.04 lower than ABG pH

VBG pCO2 is 4 lower than ABG pCO2

VBG pO2 is approximately 40 lower than ABG pO2

The bottom line is that for most of our clinical concerns, we can use VBG to assess pH and pCO2, and O2 sat to assess oxygenation. VBG may be less reliable in shocky or hypercapneic patients (but end-tidal CO2 will be useful in hypercapneic patients).

When do we really need an ABG? When we want to calculate the Aa gradient, looking for e.g. VQ mismatch, shunt, or a diffusion problem.