Testing Modern Airway FiltersTesting Modern Airway Filters

Tony WilkesTony Wilkes

Medical Device Evaluation Unit

Department of Anaesthetics and Intensive Care Medicine

University of Wales College of Medicine

Cardiff, Wales, UK

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

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

Which breathing system filter to use?

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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

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

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

Types of HMEs and breathing system filters

Glass fibre

Electrostatic: fibrillated

Electrostatic: tribocharged

+ -

a

b

c

d

e

cross-sectionalarea for diffusionalimpaction

gasflow

fibre

streamlines

Mechanisms of particle capture

a: interception d: diffusional impactionb: inertial impaction e: electrostatic attractionc: gravitational settling

Mechanisms of particle capture

0

20

40

60

80

100

0.01 0.10 1.00 10.00

particle diameter ( m)

filt

rati

on

eff

icie

nc

y (

%)

interception

diffusion

combined efficiency

settling

impaction

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

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

Droplet: 3 m Bacteria: 0.8 m

Effect of droplet size

Mechanisms of particle capture

0

20

40

60

80

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0.01 0.10 1.00 10.00

particle diameter ( m)

filt

rati

on

eff

icie

nc

y (

%)

interception

diffusion

combined efficiency

settling

impaction

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)

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

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

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

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%)

( )

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!

Effect of flow

0.001

0.01

0.1

1

10

100

0 10 20 30 40 50 60 70 80

flow (L min-1)

pen

etra

tio

n (

%)

Effect of loading (varying the challenge)

Anaesthesia 2003; 58: 562-7

Electrostatic filters

0

10

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30

40

50

0 1 2 3 4 5 6

challenge

pen

etra

tio

n (

%)

Effect of loading (varying the challenge)

Anaesthesia 2003; 58: 562-7

Pleated hydrophobic filters

0

0.04

0.08

0.12

0.16

0 1 2 3 4 5 6

challenge

pen

etra

tio

n (

%)

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

Moore’s Test Rig

Capture of particles on fibres

Capture of particles on fibres

Penetration

0.001 0.01 0.1 1 10 100

Paed, Elec, Cond

Paed, Elec, Unused

Paed, Pleated, Cond

Paed, Pleated, Unused

Adult, Elec, Cond

Adult, Elec, Unused

Adult, Pleated, Cond

Adult, Pleated, Unused

Penetration (%)

Overall results from MHRA report

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Data from: Anaesthesia 2000; 55: 458-65 and 2002; 57: 162-8.

-6

-5

-4

-3

-2

-1

0

1

-2 -1.5 -1 -0.5 0 0.5 1 1.5

NaCl penetration (log(%))

mic

rob

ial p

en

etr

ati

on

(lo

g(%

))

Bacterial v NaCl PViral v NaCl PBacterial v NaCl EViral v NaCl E

Microbial v NaCl particle challenges

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