Monitoring oxygen delivery to the oxygenator of a heart lung machine (HLM) is typically accomplished with an O₂ analyzer connected to the gas inflow line. It is assumed when the FiO₂ is greater than 21% that oxygen is being delivered to the oxygenator. However, this assumption is imperfect because the connection of the inflow line to the oxygenator is downstream from the O₂ analyzer. FiO₂ monitoring will not alert the perfusionist if the inflow line is not actually connected to the oxygenator. Measuring the fraction of expired oxygen (FEO₂) is a more reliable way of monitoring O₂ delivery.

An O₂ analyzer was placed on the scavenging line that is attached to the exhaust port of oxygenator (FEO₂).

Whenever the FiO₂ is greater than 21%, and the inflow line is properly connected, the FEO₂ exiting the oxygenator is greater than 21%. The FEO₂ falls to 21% when the inflow line is not functioning.

Monitoring the FEO₂ is a more reliable way to verify O₂ delivery to an oxygenator. An alarm can be set on the FEO₂ monitor to alert the perfusionist if the FEO₂ falls below a predetermined threshold so any issue with O₂ delivery will always be recognized.