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