The recirculation fraction represents the proportion of oxygenated blood going to the patient that does not reach the patient's circulation but rather returns to the ECMO device. It is a kind of shunt.
Parameters:
(1) oxygen saturation of the patient's blood entering the oxygenator of the ECMO device
(2) oxygen saturation of the patient's blood leaving the oxygenator
(3) oxygen saturation of the patient's mixed venous blood
recirculation fraction =
= ((oxygen saturation of blood before the oxygenator) - (oxygen saturation of mixed venous blood)) / ((oxygen saturation after the oxygenator) - (oxygen saturation of mixed venous blood))
effective flow =
= (total flow) * (1 - (recirculation fraction))
Interpretation:
• The average recirculation fraction is about 0.30 (30%).
• The recirculation fraction increases in a linear fashion as pump flow increases (in mL per minute).
• At lower pump flows effective oxygen delivery is relatively high, but this plateaus and then drops as the pump flow rate and recirculation fraction increases. At high flow rates the recirculation fraction is often less than at lower flow rates.
Factor
Lower Recirculation
Higher Recirculation
cardiac output
high
low
pump flow
low
high
catheter
inflow and outflow separated
inflow and outflow close together
right atrial size
large right atrium
small right atrium
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