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British Journal of Anaesthesia, 2001, Vol. 87, No. 2 289-291
© 2001 The Board of Management and Trustees of the British Journal of Anaesthesia

Short Communications

Air flow resistance of three heat and moisture exchanging filter designs under wet conditions: implications for patient safety

N. J. Morgan-Hughes1,3, G. H. Mills1,2 and D. Northwood3

1Department of Anaesthesia and Intensive Care and 2Sheffield University Department of Anaesthesia, Royal Hallamshire Hospital, Glossop Road, Sheffield S10 2JF, UK. 3Department of Anaesthesia and Intensive Care, Doncaster Royal Infirmary, Armthorpe Road, Doncaster DN2 5LT, UK*Corresponding author. Present address: Department of Anaesthesia and Intensive Care, Northern General Hospital, Herries Road, Sheffield S5 7AU, UK

{dagger}Presented in part at the European Society of Intensive Care Medicine Annual Congress, Rome, Italy, October 2000.

Heat and moisture exchanging filters (HMEFs) can be blocked by secretions. We have studied HMEF performance under wet conditions to see which particular design features predispose to this complication. Dar Hygrobac-S (composite felt filter and cellulose exchanger), Dar Hygroster (composite pleated ceramic membrane and cellulose exchanger) and Pall BB22-15 (pleated ceramic membrane) HMEFs were tested. Saline retention, saline concealment, and changes in air flow resistance when wet were assessed. The cellulose exchanger in the composite Hygrobac-S and Hygroster retained saline, producing a ‘tampon’ effect, associated with bi-directional air flow resistances in excess of the international standard of a 5 cm H2O pressure drop at 60 litre min–1 air flow. Furthermore, high air flow resistances occurred before free saline was apparent within the transparent filter housing. The pleat only BB22-15 showed a significant increase in expiratory air flow resistance, but only after the presence of saline was apparent. These data imply that composite HMEFs with cellulose exchangers are more likely to block or cause excessive work of breathing as a result of occult accumulation of patient secretions than pleat only HMEFs.

Br J Anaesth 2001; 87: 289–91


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D. Turnbull, P. C. Fisher, G. H. Mills, and N. J. Morgan-Hughes
Performance of breathing filters under wet conditions: a laboratory evaluation
Br. J. Anaesth., May 1, 2005; 94(5): 675 - 682.
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