Testing Modern Airway Filters

1. Testing Modern Airway Filters Tony Wilkes Medical Device Evaluation Unit Department of Anaesthetics and Intensive Care Medicine University of Wales College of Medicine Cardiff, Wales, UK

2. Medicines and Healthcare products Regulatory Agency • Medical Devices Agency (MDA) and Medicines ControlAgency (MCA) combined in April 2003 • New agency called the Medicines and Healthcare products Regulatory Agency (MHRA) • MHRA is an executive agency of the Department of Health • One of the MHRAservices is to evaluatecertain medical devices • Assessment of filters available on UK market

3. Use of breathing system filters 5. A new, single-use bacterial/viral filter and angle piece/catheter mount must be used for each patient. It is important that these are checked for patency and flow, both visually and by ensuring gas flow through the whole assembly when connected to the circuit. CHECKING ANAESTHETIC EQUIPMENT 3 (Draft)Association of Anaesthetists of Great Britain and Ireland, 2003

4. Which breathing system filter to use? ?

5. Types of breathing system filters • Glass fibres (‘Pleated hydrophobic’) • density of fibres is high • high resistance per unit area • pleated to  surface area and therefore to  resistance • pleats  volume (deadspace) of filter housing • filter material repels water

6. Types of breathing system filters • ‘Electrostatic’ • density of fibres is lower • lower resistance to gas flow • flat sheets of material used • lower volume (deadspace) of filter housing • easier to add HME layer to  humidification • filter material also repels water, but ... • electrostatic charge on fibres to  capture

7. Types of breathing system filters • ‘Electrostatic’: 2 types • fibrillated • sheet of polypropylene electrostatically charged • sheet then split into fibres (fibrillation) • tribocharged • 2 different fibres (e.g. modacrylic & polypropylene) • rubbed together during manufacture to electrostatic charge

8. Types of HMEs and breathing system filters

9. Glass fibre

10. Electrostatic: fibrillated

11. Electrostatic: tribocharged

12. Mechanisms of particle capture a: interception d: diffusional impaction b: inertial impaction e: electrostatic attraction c: gravitational settling

13. Mechanisms of particle capture

14. Effect of droplet size • Microbial challenges • CAMR, Porton Down • Bacillus subtilis, 0.96 to 1.25  0.55 to 0.67 m • 80% of particles < 2.1 m • Nelson Laboratories • Staphylococcus aureus, spherical, 0.8 m diameter • mean particle size = 3 m • NaCl challenge • Standard (BS EN 13328-1) • mass median aerodynamic diameter  0.3 m

15. Mass median aerodynamic diameter • Mass median diameter, MMD • Diameter of particle such that 50% of the total mass of particles is contributed by particles with diameter > MMD, and 50% < MMD • Aerodynamic diameter, da • da of a particular particle is the diameter of a spherical particle with a density of 1000 kg m-3 that has the same settling velocity as the particle

16. Effect of droplet size Droplet: 3 mm Bacteria: 0.8 mm

17. Mechanisms of particle capture

18. Increasing filtration performance •  density of fibres, or thickness of wad •  capture of particles •  pressure drop •  filter area •  face velocity (flow per unit area) •  capture of particles •  pressure drop •  deadspace (internal volume)

19. MHRA Evaluation report on filters • 106 filters tested (all included?) • 25 pleated hydrophobic • 23 for adults, 2 for paediatrics • 81 electrostatic • 50 for adults, 31 for paediatrics • not always defined by manufacturer • Penetration (%) • unused • after 3 h simulated use

20. Standards • Breathing system filters (EN 13328-1) • use a challenge aerosol containing NaCl particles with most penetrating particle size • no requirement for minimum level of filtration (test method only) • Breathing system filters (EN 13328-2) • moisture output, pressure drop etc. • no requirements, except for connectors

21. MHRA Evaluation • Penetration measured at • 15 L min-1 for filters intended for paediatric patients • 30 L min-1 for filters intended for adult patients • Challenge to filters • 13 mg m-3 particles over 30 s • 0.1 mg or 0.2 mg for filters intended for paediatric or adult patients, respectively

22. Filtration performance (units) • Penetration (%) 100  mass of particles passing through filter mass of particles in challenge • equivalent to probability of transmission • e.g. 1% = 1 in 100 particles pass through • Efficiency (%) 100 - penetration (e.g. 100 - 0.1 = 99.9%) ( )

23. Filtration performance • Comparing performance • easier using penetration • e.g. compare 0.24% v 0.02% penetration • first lets 12 times as many particles through • cf efficiency • compare 99.76% v 99.98% • not so easy!

24. Effect of flow

25. Effect of loading (varying the challenge) Anaesthesia 2003; 58: 562-7

26. Effect of loading (varying the challenge) Anaesthesia 2003; 58: 562-7

27. Measuring filtration performance • Moore’s test rig • Neutral hydrogen flame photometer • Intensity of light  to mass of sodium • Intensity measured at 589.3 nm • Neutral density filters used to prevent light flooding photometer • Penetration read from a graph depending on • Meter reading • Neutral density filters used

28. Moore’s Test Rig

29. Capture of particles on fibres

30. Capture of particles on fibres

31. Overall results from MHRA report ?

32. Microbial v NaCl particle challenges Data from: Anaesthesia 2000; 55: 458-65 and 2002; 57: 162-8.

33. MHRA Evaluation report • Data on each filter • penetration • pressure drop • internal volume • connectors • moisture output (if measured) • list price • Provide sufficient data to be able to make informed choice