Breathing system filters

It is interesting to see the number of hospitals in the three regions surveyed by Drs Heaton, Hall and Fell (Anaesthesia 1998; 53: 407) where microbial filters are routinely used as part of the anaesthetic breathing system. I note also their apparent faith in the efficacy of all such devices. Hedley and Allt-Graham compared the filtration efficiency of heat and moisture exchangers (HME), and found significant differences [1] as did Lee and others when the bacterial retention properties of specifically designed HME with filter (HMEF) were evaluated [2]. More recently, Lloyd and co-workers have in addition shown that some HMEF were unsatisfactory in providing a barrier to the passage of hepatitis C virus [3]. At present, the testing of filters or HMEF for use in breathing systems is dependent on the manufacturer and the method used. There is no national or international standard available and so comparison between devices is difficult. However, it should be mentioned that a European standard is in preparation and one manufacturer has already adopted the proposed test method to provide an indication of the filtration efficiency of their products. It is to be hoped that an agreed standard will soon be approved, making it a requirement for all such devices.