An FiO2  monitor had been removed from a set-up bypass machine due to failure.
 
 A replacement was not put in place.
 
 Bypass machine was used on patient and air escaped through the  open  port that should have had the FiO2 monitor in place.
 
 The gas pathway had been checked prior to bypass, but it had not been noticed  that the FiO2 monitor was absent.
 
 Upon bypass initiation the venous saturations dropped but not immediately as  there must have still been a small delivery of gas.
 
 Once they dropped and it was noted  that the blood was dark in colour,  bypass was terminated, ventilation was restarted.
 
 The FiO2 monitor was found to be missing and replaced.  Bypass was  re-initiated and the operation was  completed successfully.  

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