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Testing Modern Airway Filters

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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 Control Agency (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 MHRA services is to evaluate certain 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 Microbial challenges NaCl challenge Effect of droplet size
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 ) mass median aerodynamic diameter  0.3 m

15 Mass median diameter, MMD
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)  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 ) moisture output, pressure drop etc. no requirements, except for connectors

21 MHRA Evaluation Penetration measured at Challenge to filters
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 = 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 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 Data from: Anaesthesia 2000; 55: 458-65 and 2002; 57: 162-8.
Microbial v NaCl particle challenges Data from: Anaesthesia 2000; 55: and 2002; 57:

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


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