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BJA Advance Access published online on February 25, 2005
British Journal of Anaesthesia, doi:10.1093/bja/aei091
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© The Board of Management and Trustees of the British Journal of Anaesthesia 2005. All rights reserved. For Permissions, please e-mail: journal.permissions@oupjournals.org
Accepted December 20, 2004

Laboratory Investigation

Performance of breathing filters under wet conditions: a laboratory evaluation{dagger}

D. Turnbull 1*, P. C. Fisher 2, G. H. Mills 1, and N. J. Morgan-Hughes 3

1 University of Sheffield, Academic Anaesthetic Unit, Sheffield, South Yorkshire, UK
2 Royal Hallamshire Hospital, Anaesthetic Department, Sheffield, South Yorkshire, UK
3 Northern General Hospital, Anaesthetic Department, Sheffield, South Yorkshire, UK

* To whom correspondence should be addressed.
D. Turnbull, E-mail: cahillturnbull{at}doctors.org.uk


  Abstract

Introduction. Heat and moisture exchangers in combination with a bacterial and viral filter (HMEF) are widely used during general anaesthesia. Excess patient secretions occluding the HMEF have been responsible for previous case reports of airway obstruction. A previous study suggested that differences in HMEF design might contribute to filter obstruction under wet conditions.

Methods. We tested 14 types of HMEF under wet conditions to establish which design features contributed to HMEF obstruction. Incremental amounts of saline were added to each filter. The pressure across the filter was measured with an air flow of 60 litre min-1.

Results. We observed that saline added to the filter was often not easily visible to the casual observer. This concealment volume varied between filters. Ceramic hydrophobic pleated-membrane filters did not absorb saline and their resistance did not change. The composite filter where the moisture exchange component was either polyurethane foam or cellulose absorbed saline and contributed to a rise in resistance of 70-480% with the higher value more typical of the cellulose-paper-based HMEF.

Conclusion. The ideal HMEF for use during general anaesthesia should prevent the passage of viral, bacterial and prion material, should provide this filtration performance even under wet conditions, should supplement humidification of the inspired air and anaesthetic gases and should not increase respiratory work. We have identified large variations in HMEF performance under wet conditions. Users should be aware of performance variation in HMEFs and use a filter suited to the intended application.

Keywords: equipment, breathing filters; safety, equipment; ventilation.
{dagger} Presented in part to the European Society of Anaesthesia, Glasgow, May 2003.
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