Download - Testing Modern Airway Filters
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?
?
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
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
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
20
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
?
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