120 rule

The 120 Rule allows the clinician to quickly assess lung function (in room, i.e. air 21% oxygen and at sea level). 

PaCO₂ + PaO₂  = >120

So if PaCO₂ = 60, the PaO₂ would drop to 85 (as above 105-20=85).  85 + 60 = 145, so this patient is suffering from hypoventilation only.  

If, however, the PaCO₂ was 60 and the PaO₂ was 40mmHg =100 – then the patient has admixture and hypoventilation.

Another example is if the PaCO₂ was 40mmHg and the PaO₂ was 60 = 100 – then the patient has admixture and not hypoventilation.