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.