Figure 1.
This is a published formula that is thought to be a way to calculate R. Where R is recirculation (%), SO2preox is the O2 saturation of the blood entering the oxygenator, SO2postox is the O2 saturation of the blood exiting the oxygenator and SVO2 is the mixed venous O2 saturation in the patient.
* (See reference 10 for source)