Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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HEAT AND MOISTURE EX(,~HANGING FILTEI~~
The invention relates to heat and moisture retaining
filters.
In humans, inspired air is filtered by the nasal cavities
and upper respiratory tract. In addition, in most
climates, inspired air contains a proportion of water
vapour and during its passage to the lungs, inspired air
becomes fully saturated with moisture which is taken from
the mucus secreted by the goblet cells of the mucous
membranes which lie in the airways. In certain medical
procedures and also, for example, the supply of air in
enclosed spaces such as aircraft cabins, the moisture
levels in inspired air can be less than optimal for
satisfactory breathing.
For example, procedures such as intubation or tracheostomy
bypass these upper airways and so no filtration or
saturation function is performed on gases inspired from
the ventilating apparatus used in these procedures. The
clinical consequences of inspiring unfiltered and
unsaturated gases are well documented. See for example
the article "Filtration and Humidification" by Lloyd and
Roe in Volume 4, No. 4, of the October/December 1991
Edition of the publication "Problems in Respiratory
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Care". Reference is also made to the article entitled
"Humidification for Ventilated Patients" by Ballard,
Cheeseman, Ripiner and Wells on pages 2-9 of Volume 8
(1992) of the .publication "Intensive and Critical Care
Nursing".
In order to overcome this problem, it is common practice
to incude in the ventilating apparatus a device which both
filters expired breath and heats and humidifies inspired
gases. Such devices are discussed in the two publications
referred to above and in the axticle "A Comparison of the
Filtration Properties of Heat and Moisture Exchangers" by
Hedley and Allt-Graham in Anaesthesia 1992, Volume 47,
pages 414-420, and in the article "An Alternative Strategy
for Infection Control of Anesthesia Breathing Circuits: A
Laboratory Assessment of the Pall HME Filter" by Berry and
Nolte, pages 651-655 of the publication "Anesth.Analg"
1991; 72.
The Lloyd and Roe publication identifies three categories
of heat and moisture exchanging filters. The first
category are called "hygroscopic (first generation)" heat
and moisture exchanging filters. These contain wool, foam
or paper-like materials that are usually impregnated with
hygroscopic chemicals such as lithium chloride or calcium
chloride to absorb chemically water vapour molecules
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present in exhaled breath. The second category are called
"hygroscopic (second generation)" heat and moisture
exchanging filters. These are the same as the Eirst
generation but with the addition of electret Eelt filter
material. Electrets are materials that maintain a
permanent electric polarity and Eorm an electric Eield
around them without an external electric field. Such
materials remove micro-organisms by electrostatic interaction.
An example of this is shown in EP-A1-0011847 (published 11
June 1980).
EP-A2-0265163 (published 27 April 1988) discloses the use of
a layer of hydrophobic filter material and a layer of
hydrophilic foam in a housing. The layers are formed by non-
bonded flat sheets contacted together. The hydrophobic layer
is of polypropylene fibres which are electrostatically charged
to perform viral and bacterial filtration and so acts as
an electret of the second category of filters described
above. The foam-layer is treated to absorb moisture. It
acts as a material similar to the arrangement of
EP-A1-0011847 with its consequent disadvantages.
The third category involves the use of a hydrophobic
membrane that removes micro-organisms by pure filtration
and retains moisture on the surface of the membrane as a
result of the hydrophobicity.
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All three categories of filter operate in broadly the same
way. On expiration, expired water vapour is condensed on
the filter and on inspiration, the inspired gases collect
water vapour (and heat) from the device by evaporation.
Micro-organisms such as bacteria and virus are removed
from the expired and inspired air by the filters in their
respective ways.
The first category of filters finds little current
application. They have low airborne bacterial removal
efficiencies even when impregnated with bactericidal
agents. In addition, because of their mode of action,
they do not achieve maximum heat and moisture exchange
efficiency instantaneously and have a relatively long
acclimatising period before steady state levels are
achieved. In addition, they have relatively large pores
and a relatively large thickness which enable liquids to
soak into the pores and pass throughout the material,
leading to a water-logged state and a consequent increase
of resistance.
The second category of filters offer improved levels of
micro-organism filtration in comparison with first
generation hygroscopic filters. There still exists,
however, the problem of contaminated liquids passing
through the layers due to the relatively large pore
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sizes. In addition, and as discussed in the Lloyd and Roe
reference, the filter efficiencies may not achieve the
99.99770 which has been suggested as the minimum removal
rate to make a filter suitable for~clinical use.
The third category of Filters, utilizing hydrophobic
membranes, have extremely small pores typically with an
alcohol wetted bubble point greater than 710 mm (28 in)
H20. The bubble point is measured by the method of the
American Society of Testing Materials. These prevent the
passage of contaminated liquids at usual ventilation
pressures. These filters also act as a barrier to
water-borne micro-organisms and allow efficiencies greater
than 99.99770 to be achieved. In many cases, hydrophobic
membrane filters have been shown to provide heat and
moisture exchange comparable with normal nasal breathing.
An optimal humidification efficiency occurs almost
instantaneously.
Expired breath contains water not only in the Eorm of
water droplets and globules, but also water in the form of
water vapour. It is possible for such water vapour to
pass through the filter and be lost to the system. This
means that on inspiration, not all expired water is
available for humidification. In general, this is not a
problem. However, a small number of long
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term ventilator patients may require greater
humidiEication than a heat and moisture exchange filter of
the third category can provide. It has been
suggested that this problem can be overcome by
incorporating in the breathing circuit a humidifier. An
alternative attempt to overcome this problem is to combine
the hydrophobic material with a hygroscopic material of
the kind used in the first and second categories, in order
to absorb water
GB-A-2167307 (published 29 May 1986) discloses a heat and
moisture exchange filter comprising alternating hydrophobic
and hydrophilic washers mounted in a housing with the
hydrophilic washers being impregnated with a hygroscopic
material. These are likely to suffer from water-logging.
According to a first aspect of the invention, there is
provided a heat and moisture exchange Filter comprising a
housing having a first part for connection to a supply of
breathable gas and an expiratory line and a second part
for connection to a person inhaling and exhaling the gas,
the housing containing a sheet of hydrophilic medium and a
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sheet of hydrophobic filter medium arranged in series in a
flow path between the first and second parts, the
hydrophilic media being the closer to the first part in
said flow path and the hydrophobic medium having an
alcohol bubble point of greater than 710 mm (28 in) H20
for removing micro-organisms.
In all three categories of filter, it is essential that
the filter media do not produce such a substantial
pressure drop as to make inspiration and expiration of air
difficult. This is not usually a problem with filters of
the first and second categories, because the pore size of
the filter media are large enough not to produce a
pressure drop sufficient to cause a problem. The filters
of the third category may, however, suffer from this
problem.
Preferably the sheets of filter media are pleated. This
gives a greater area of media and thus reduces the
pressure drop.
According to a second aspect of the invention, there is
provided a method of manufacturing a heat and moisture
exchanging filter comprising taking a sheet of hydrophobic
medium having an alcohol bubble point of greater than 710
mm (28 in) H20 and having two opposed surfaces, taking a
sheet of hydrophilic medium having two opposed surfaces,
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and connecting one surface of the hydrophobic medium to
one surface of the hydrophilic medium.
The following is a more detailed description of some
embodiments of the invention, by way of example, reference
being made to the accompanying drawings in which:-
Figure 1 is a schematic end view of heat and moisture
exchange filter formed from pleated filter media;
Figure 2 is a schematic view of a first configuration of a
hydrophilic medium of the filter of Figure 1, and
Figure 3 is a schematic view of a second configuration of
a hydrophilic medium of the filter of Figure 1.
Figure '9 is a schematic view of an artificial patient for
use in testing the heat and moisture exchanger efficiency
of a filter,
Figure S shows the artificial patient of Figure 4
connected to a ventilator in a first configuration for
setting out prior to tests,
Figure 6 shows the artificial patient of Figure 9
connected to a ventilator in a second configuration for
testing heat and moisture exchanger efficiency.
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Figure 7 is a schematic diagramm of equipment used for
determining the efficiency of a filter by aerosol
challenge.
Referring to Figure 1, the filter comprises a sheet of
hydrophobic medium 10 pleated with a sheet of hydrophilic
material 11. The two layers may be separate, or connected
together by being laminated together or bonded together by
any convenient method. The media are enclosed in a
housing 12 having two ports 13,14 leading to respective
opposite sides of the media. As seen in Figures 1 and 6,
the pleated media fill the housing 12 so that the pleats
on one side of the media are closely adjacent one port 13
and the pleats on the other side of the media are closely
adjacent the other port 14.
The hydrophobic medium 10 is preferably of resin bonded
ceramic fibres and removes micro-organisms by direct
mechanical interception and so has an alcohol wetted
bubble point in excess of 710 mm (28 in) H20. This is
measured by the American Society of Testing Materials
method for such a test. The hydrophilic medium is
preferably a cellulose material. The material should be
non-particle shedding.
Preferably, the filter has a pressure drop of not greater
than 3.0 cm H20 at an air flow of 60 1/min and an
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aerosol bacterial removal efficiencyy when measured by
AEROSOL CHALLENGE TEST described below, of >99.999%.
The hydrophilic, medium may, as seen in Figure 2, be a
continuous sheet of material. As seen in Figure 3,
however, it could be discontinuous with a series of rows
of parallel spaced slits being provided through the
medium. The slits, where provided, ensure that the
maximum pressure drop across the device is controlled by
the hydrophobic medium. The slits allow flow through the
hydrophilic medium should it 'wet out' (i.e. become
saturated with water)
The device is used at the "patient end" of open breathing
systems of the kind used mainly in intensive care units.
Such systems are used by patients undergoing long term
ventilation and by patients who, by the nature of their
clinical condition, require extra humidification while
being ventilated.
Such systems comprise a ventilator, a tube connecting the
ventilator to one port of the device on the side of the
hydrophilic media 11, and a tube connecting the other port
on the side of the hydrophobic media to the patient
inhaling and exhaling gas from the ventilator. A valve
system is provided which allows exhaled breath to vent via
an expiratory line after passing through the device.
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In use, the hydrophobic medium 10 does not wet out with
patient fluids, offers low air flow resistance and also
offers high efficiency of removal of micro-organisms by
mechanical interception.
The hydrophilic medium 11 acts in the following way.
Water which passes through the hydrophobic medium, almost
entirely in the form of water vapour, is captured by the
hydrophilic medium, by virtue of its hydrophilic nature.
This moisture then spreads over the entire area of the
hydrophilic medium 11. In this way, inhaled gases pass
first through the hydrophilic medium 11 where they pick-up
this moisture before picking up additional moisture from
the hydrophobic medium in the normal way. This has the
advantage, therefore, of increasing the humidification
levels, so avoiding the need for the use of an additional
humidifier.
In addition, it overcomes a further problem of the
hydrophobic medium. This is the fact that since water
will not spread evenly over a hydrophobic medium, there
can be areas of the medium which are uncovered by water.
This can provide a preferential passage for inhaled gases
through the hydrophobic medium, during which passage
little or no humidification takes place. Since moisture
spreads evenly through the hydrophilic medium, this
problem is compensated for.
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The potential for blockage of the filter by wetting out of
the hydrophilic medium - that is to say by the hydrophilic
medium becoming saturated with water - is avoided by
selecting a cellulose material of appropriate pore size
and thickness so that it has a bubble point pressure low
enough to allow clearance of the excess water during gas
flow. For example, a cellulose material is available from
Pall Corporation having an alcohol bubble point of 64mm to
114mm (2.5in to 4.5in) H20. This is measured in
accordance with the method of the American Institute of
Testing Materials.
The connecting together of the layers, where provided,
makes it easier to pleat the layers without forming gaps
between the layers for the collection of water. In
addition the bonding is beneficial in mitigating or
preventing wetting-out of the hydrophilic layer 11.
The fact that the media 10,11 fill the housing 12
minimizes the dead space in the housing 12. This is
advantageous because it minimizes the volume of
re-breathed gas.
The following is a description of tests of a device of the
kind described above with reference to the drawings in
comparison with two commercially available filters of the
second category described in the introduction to this
specification (designated 2A and 2B respectively), two
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filters of the third category referred to in the
introduction of the specification (designated 3A and 3B
respectively) and two filters of the third category with
the addition of a hygroscopic material to retain moisture
(referred to as M3A and M3B).
The exemplary filter according to the invention was formed
as described above with an area of about 640 to 650 cm2.
The hydrophobic medium of the exemplary filter according
to the invention was of pre-blended ceramic fibres bound
with a suitable de-stabilised resin and having an alcohol
wetted bubble point greater than 710 mm (28 in) H O
2
measured as described above. The amount of resin was 10%
relative to the fibres (weight/weight). The bound fibres
were then rendered hydrophobic by any one of the methods
that are known in the art.
The hydrophilic filter medium of the exemplary filter of
the invention was of cellulose fibres bound with a binder
and having the following composition and properties:
Fibres: 100% hemp
Binder Viscose
PreSSUre Drop 74
(mm water column) (2.9 inches water column)
Tensile Strength 92-115 (kg/mm) (8-10 lbs/in)
Thickness 0.081 mm (3.2 in x 10'3)
Tensile Strength 103 kg/:nm (8.9 lb/in)
wetted with oil
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Tensile Strength 44 kg/mrfr' ~3'.~ "1bs/in)
wetted with water
Burst Strength) 3520-4400 kg/mm2 {12-15 lb/in2)
(Muller Test)
All the filters were tested for water loss, removal
efficiency of bacteria and removal efficiency of viruses,
using the tests now to be described.
- I~SS TEST
This test will be described with reference to Figures 4 to
6.
The artificial patient shown in Figure 4 comprises a
humidifier 20 capable of supplying expired air at a
predetermined moisture content and temperature. A first
outlet 21 of the humidifier is connected to a rubber lung
22 of 2 litres capacity and to a connector tube 23 via a
check valve that prevents flow from the outlet 21 to the
connector tube 23 and permits flow in the opposite
direction.
The second outlet 25 of the humidifier 20 is connected to
a T-connector 26 via a check valve 27 and permits flow
from the second outlet 25 to the T-connector 26 and
prevents flow in the opposite direction. The T-connector
26 is connected to the other end of the connector tube 23
by a third check valve 28 that allows flow from the
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T-connector 26 to the connector tube 12 but prevents flow
in the opposite direction.
The humidifier .20 includes a temperature control 29 that
controls the temperature of air outputed by the humidifer
20.
In use, the humidifier 20 is filled with distilled water.
Then, as seen in Figure S, the outlet 30 to the
T-connector 26 is connected to the stem 31 of a
Y-connector 32. One branch 33 of the Y-connector is
connected by a tube 34 to an inlet to a ventilator 35 and
the second branch 36 of the Y-connector 32 is connected by
tube 37 to an outlet of the ventilator 35.
The ventilator supplies by breathable gases in pulses
whose volume and frequency can be controlled. The volume
sup.lied in any pulse is referred to as the "tidal volume",
and the frequency is measured in "breaths/minute".
In testing the heat and moisture exchanger efficiency, the
tidal volume of the ventilator 35 is set to a known value,
For example, 660 ml and the frequency is set to a known
value, for example, 15 breaths per minute. Each volume of
air outputed by the ventilator 35 passes along the tube 37
to the T-connector 26. As a result of the check valves
24,27,28 this gas passes around the connector tube 23 and
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into the rubber lung 22 which expands to receive the air.
When the pulse of air ends, the rubber lung 22 expires air
which, as a result of the check valve, passes through the
humidifier 20 and exits via the second outlet 25 and the
T-connector 26 to return to the ventilator 35 inlet via
the tube 34.
The T-connector 26 is insulated to prevent condensation
forming.
The system is run for 30 minutes to allow the system to
warm up with the temperature of the artificial patient set
to 30oC or 34oC. After 30 minutes, the ventilator 35
and the artificial patient temperature control 39 is
switched-off.
Next; the tube 34 from the first branch 33 of the
Y-connector 32 is removed and replaced with a tube having
in line a housing 38 containing 100 gms of dessicant. The
housing 38 is thus connected to the first branch 33 of the
Y-connector 32 and to the inlet to the ventilator 35.
In addition, the heat and moisture exchanger filter 39 to
be tested is inserted in the tubing between the outlet 30
and the stem 31 of the Y-connector 32.
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Next, the artificial patient is weighed and the components
between the T-connector 26 and the ventilator are
weighed. This includes the housing 38 and its contents,
and the filter, 39. The weights are recorded to one
decimal place.
The ventilator 35 and the artificial patient are then
turned on and run for one hour. If, during this period,
the ambient air is not temperature controlled, the
expiratory line temperature (i.e. the temperature in the
tube 37) is noted at regular intervals. It should be
approximately 20°C and should not exceed 23°C. If the
temperature rises above 23°C, ice or chilled water
should be packed around the tubing to reduce the
temperature.
After one hour, the ventilator 35 and the temperature
control 29 are turned off. The items weighed above are
re-weighed and their weights recorded to one decimal place.
The water loss is then calculated using the following
formula:-
G = TV.t.t
1,000
where G ~ gas flow in litres per minute,
Tv = the tidal volume in mililitres,
f - the frequency in breaths per minute.
t ~ the time of the test in minutes.
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In addition, the percentage area of the circuit is also
calculated in the formula:-
E = 100.(w; - wf)
W1
where E = the efficiency,
Wi = test circuit Weight before the test in
grammes
Wf = test circuit weight after the test in
grammes
Wl - the weight loss of the artificial patient
in grammes
This figure should not exceed 10%, if it does exceed 10%
the experiment is invalid and should be re-run.
Finally, the water loss of the patient is calculated from
the following formula:-
PL
G
where PL ~ the water loss from the patient in
milligrammes of water per litre of air,
and
Wl and G have the meanings given above.
AEROSOL CHA~I.FNI'F TEST
The equipment comprises a nebulizer 50 of the kind sold by
Devilbiss as Model 40. The inlet to the nebulizer is
connected by filter 51 and a control valve 52 to the
ambient air. The outlet of the nebulizer 50 is connected
to the inlet of the test filter 53 via a tube 54. The
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tube also receives air from a second inlet via an air flow
meter 55, a control valve 56 and a protective filter 57.
The outlet to the test filter 53 is connected to a vacuum
source (not shown) via a protective filter 58 and a vacuum
guage 59. The outlet is also connected to a liquid
impingement sampler 60, having a valve controlled outlet
61.
In use, the first stage is the preparation of a bacterial
suspension. This is achieved by inoculating 100m1
tryptone Soya broth of a single colony from a tryptone
soya agar slope. This culture is incubated overnight in a
shaking water bath at 30 t 2°C to ensure optimal growth.
Next, two 5 ml aliquots of the overnight culture are
centrifuged (at approximately 23008 for 10 minutes). The
supernatant is discarded and the cell pellets are
resuspended in 3ml sterile water. The washed cells are
then collected by recentrifuging at approximately 23008
for 10 minutes. The washed cell pellets are then
resuspended in sufficient sterile water to give a cell
suspension of approximately 1x108 bacteria/ml.
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A gram stain is then prepared. The preparation is
examined with a compound microscope fitted with a
calibrated occular micrometer, and an oil immersion
objective lens .(x 100). Several microscope fields are
observed for organism size and arrangement of cells. The
vseudomonas diminu a should be gram-negative, small rod
shaped organisms about 0.3 - 0.4 Erm by 0.6 - 1.0 ~.m in
size occuring primarily as single cells.
Next, the equipment is validated for flow rate. In this
validation, the test filter 53 is removed and replaced by
a flow meter (not shown). The nebulizer 50 is filled with
mI of sterile water and the impingement sampler 60 with
20 ml of sterile water. The control valve 52 to the
nebulizer 50 is closed and the control valve S6 to the
airflow meter 55 is opened, vacuum is applied and air is
drawn into the equipment for 30 seconds. At 0:5 bar
vacuum or greater, the airflow should be 28 1/min,
regulated by the critical orifice of the impinger 60.
The nebulizer 50 is then activated by fully opening the
associated control valve 52. Simultaneously, the control
valve 56 of the airflow meter SS is closed partially to
maintain an airflow of 28 1/min through the apparatus.
The flow rate on the airflow meter 55 through the
associated control of valve 56 is noted. The apparatus is
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run for 20 minutes to ensure that the airflow 28 1/min is
maintained.
The equipment i.s then validated with regards to recovery
of the nseudomonas dim'n"ta. To do this, the test filter
53 of Figure 7 is removed and replaced with a six stage
Anderson sampler. The glass petri dish supplied with the
sampler is filled with tryptone soya agar at each stage.
The air to be sampled enters the inlet to the sampler and
cascades through the succeeding orifice stages with
successively higher orifice velocities from stage 1 to
stage 6. Successively smaller particles are initially
impacted onto the agar collection surfaces of each stage.
Next, 1 ml of the approximately 1 x 108/ml pseudomonas
diminuta suspension is diluted to 1 x 104/ml using
sterile water. The nebulizer 50 is filled with 5 ml of
this suspension.
with a control valve 55 open and the control valve 52
closed, vacuum is applied to the equipment and air is
drawn into the equipment for 30 seconds. At 0.5 bar
vacuum or greater, the airflow will be 28 1/min regulated
by the critical orifice impinger.
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Neat the nebulizer 50 is activated by opening fully the
associated control valve 52. Simultaneously, the control
valve 56 is partially closed to the predetermined level as
ascertained by. the flow rate validation test. This
provides make-up air and maintains the airflow at 28 I/min
through the apparatus. After a test time of 15 minutes,
the valve 52 is closed and the valve 56 opened fully:
After a further 30 seconds to clear the system of aerosol,
the vacuum source is turned off.
The agar collection plates are then removed from the
Anderson sampler and incubated at 30 t 2/C. The colony
forming units (cfu) are counted after 24 and 48 hours.
The equipment is validated if nseudomonas dimin~t-a are
recovered on the Anderson sampler at stages 6 or 5. This
confirms that monodisperse organisms are being produced by
the equipment.
After these validation tests, the equipment is used to
test the efficiency of filters in the following way.
A test filter 53 is inserted into the equipment as shown
in Figure 7. 20 ml of sterile water are placed in
the liquid sampler 60 and the nebulizer is filled with
ml of the approximately 1 a l0aml pseudomonas diminu~-a
suspension.
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Next, the control valve 56 is opened and the control valve
S2 is closed. Vacuum is applied to the equipment and air
is drawn into the apparatus for 30 seconds. At 0.5 bar
vacuum or greater, the airflow will be 28 1/min regulated
by the critical orifice of the liquid sampler 60. Then,
the nebulizer 50 is activated by fully opening the
associated control valve 52 and partly closing the control
valve 56 to the level determined by the validation test to
maintain an airflow of 28 1/min.
After a test time of 15 minutes, the valve 52 is closed
and the valve 56 opened fully. After a further 30 seconds
to clear the system of aerosol, the vacuum is shut off.
The liquid remaining in the nebulizer 50 is then withdrawn
using a 5 ml syringe and needle. The volume remaining is
measured using a 10 ml glass measuring cylinder and the
volume is serially diluted tenfold with water seven
times. Dilutions containing approximately 102 bacteria
ml are then filtered through 0.2 ~t,m analysis membrane
using sterifils. The analysis membranes are then
placed on to tryptone Soya agar plates which are
incubated at 30 t2oC. The cfu are counted after 24
and 48 hours and the number of colonies are recorded on
membranes showing 20 to 200 colonies. The nebulizer
challenge titre is then calculated.
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The liquid in the impingement sampler is then also
withdrawn. The volume is measured using a 20 ml glass
measuring cylinder and this volume is tenfold serially
diluted in sterile water three times. The remaining neat
solution and the resultant dilutions are filtered through
a O.ZEun analysis membrane using sterifils. The analysis
membranes are placed on tryptone soya agar plates and the
orifice of the impingement sample is checked to ensure
that it is not occluded.
The Agar plates are then incubated at 30 ~ 2°C. The
cfus are counted after 24 and 48 hours and the number of
colonies on membranes showing 20 to 200 colonies are
recorded and the number of bacteria recovered downstream
of the filter are calculated. The equipment efficiency is
calculated from the formula:-
Re = Bfi x 100
_Tf . Vn
Where Re = the rig efficiency in percent
Bt = the total number of bacteria recovered
Tf = the final nebulized titre in CFU/ml
Vn = the volume nebulized
The bacterial challenge to the filter is then calculated
from the formula:
Vn,R~:nt
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Where C = the total challenge
Vn and Re have the meanings given above, and
Nt is the nebulizer challenge titre in CFU/ml
Next the filtered titre reduction is calculated from the
formulae:
TR =
Bt
where TR = the filter titre reduction, and
C and Bt have the meanings given above
From this the filter efficiency in percent can be
calculated from the formula:
Filter efficiency - 1 - ~ X 100
TR
The water loss was measured as described above. The tidal
volume was 660 ml, the rate 15 breaths per minute, and the
artificial patient expiratory temperature 32°C.
The bacteria removal efficiency was tested by the aerosal
challenge test described above, with pseudomonas
dirninuta. The virus removal efficiency was tested by an
aerosol challenge test of the kind described above with
M52 bacterophage.
The results of the test are given in Table 1.
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TABLE 1~
FILTER WATER LOSS REMOVAL EFFICIENCY
TYPE mg/l. BACTERIA ($) VIRUS (%)
2A 10.1 99.9976 99.999
2B 5.3 99.91 no data
3A 8.9 99.9992 (claim) 99.999
3H 10.4 99.999 99.999
M3A 4.2 99.95 99.1
M3B 7.0 99.981 no data
Invention8.4t0.4(n=18) 99.999 99,999
It will be seen that the second category filters had
either a high water loss with a relatively high removal
efficiency (2A) or a much lower water Loss but with a
corresponding reduction in removal efficiency (2B)
(c.f.the 99.9977°s efficiency suggested by Lloyd and Roe as
the minimum for clinical use). The third category filters
had much higher removal efficiencies but comparatively
high water losses. The modified third category filters
had much lower water losses due to the presence of the
hygroscopic material, but had comparatively lower removal
efficiencies. In contrast, the filter described above
with reference to the drawings had a high removal
07 JUN '93 11:51 MHTHISEN MACRRA & CO F.12
27 2098132
efficiency and comparatively low water loss (less than
8.5mg/1).
The filter described above with reference to Figure 1 and
having the test results given in Table 1, was also
incorporated in a ventilator in a clinical trial and
compared with the commercially available filter 3A of
Table 2. The filters were tested under two conditions (A
and B) and the water loss based on a use of 24 hours is
given in Table 2. In condition A the tidal volume was
480 ml at 15 breaths/min and an expiratory temperature of
32.4 t 1.0°C. In condition B, the tidal volume was 780
ml at 15 breaths/min and an eapiratory temperature of 33.2
t 1.0 .°
TABLE 2
FILTER TYPE WATER LOSS (mg(H20)/lair)
CONDITION A g
3A 7.2t1.1(n=12) l2.1t1.2(n=12)
INVENTION 5.3t1.0(n=12) 8.8t1.2(n=12)
It will be seen that, in both conditions of operation, the
87 JUN '93 11:52 MRTHISEN MACRRR & CO P.13
28 2098132
filter described above with reference to the drawings has
much reduced water loss.
This reduced water loss is maintained over a wide range of
operating conditions. Table 3 below gives the water loss
found at the specified different operating conditions of
the ventilator. It will be seen that over a wide range of
minute volumes (tidal volume x frequency) from 7.2 1/min
to 12 1/min the water loss does not vary significantly
(7 mg/1 to 10 mg/1) at constant temperature (32oC).
The above are examples using specific media. It will be
appreciated, however, that other suitable hydrophobic and
hydrophilic media may be utilized in filters of the kind
described above.
Although the examples given above relate to medical uses,
the combination of hydrophilic and hydrophobic media may
be used in other systems where inspired and expired air is
filtered and a problem arises that requires the use of a
heat and moisture retaining filter. for example, the air
in an aircraft cabin is supplied through a filter and may
not be at a suitable temperature and humidity. The use of
a filter of the kind described above can provide a supply
of air to an aircraft cabin that is of required
temperature and humidity.
87 JUN '93 11:52 MRTHISEN MRCRRR & CO P.14
. 29 2098132
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