Failure of back-up oxygen supply despite a full oxygen cylinder.

concentration of 21%. In this incident the sampling tube damage in the operating theatre was not detected by visual inspection. We believe that atmospheric oxygen reached the anaesthetic gas analyser unit through the break in the gas sampling tube. The gold standard check that a breathing circuit is not blocked is by the use of capnography to demonstrate that expired air can pass retrogradely into the circuit [3] but this test can only be performed in an awake patient before induction of anaesthesia. We believe that direct measurement of exhaled breath through the gas sampling tube would have detected the faulty gas sampling tube. There is no substitute for regular inspection and proper check of anaesthetic machine and its components. We believe that routine check of sampling tubes in the operating theatre should include visual inspection as well as the direct measurement of exhaled breath and accept a rare theoretical risk of contamination.