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Accuracy of ten isoflurane vaporisers in current clinical use

There is a lack of independent data regarding the accuracy of isoflurane vaporisers in current clinical use. We studied the accuracy of 10 isoflurane vaporisers (six Penlon Sigma Delta, four Datex‐Ohmeda Isotec 5) taken from our operating theatres and anaesthetic rooms. Isoflurane output was measured at a range of fresh gas flow rates using both 100% oxygen and a 1:2 oxygen:nitrous oxide mixture as carrier gas. Within a clinical range of fresh gas flow rates (1–6 l.min−1) the worst relative output deviation of each vaporiser was at least ±20%, with under‐delivery of isoflurane being commoner than over‐delivery. Relative output deviation ranged from −62% to +21%, which could lead to a risk of awareness or overdose. Despite the existence of an in‐house calibration schedule, the vaporisers we found in current clinical use under‐ or occasionally over‐delivered isoflurane to a dangerous and unpredictable extent.