GB2267661A

GB2267661A - Heat and moisture exchanging filters

Info

Publication number: GB2267661A
Application number: GB9311889A
Authority: GB
Inventors: Graham Desmond Lowe; Richard Guy Gutman; Keith Stuart Morris; Roger Edward Page
Original assignee: Pall Corp
Current assignee: Pall Corp
Priority date: 1992-06-11
Filing date: 1993-06-09
Publication date: 1993-12-15
Anticipated expiration: 2013-06-09
Also published as: FI932661A0; ITTO930424A0; GB9212399D0; NL194750B; FI932661A; ITTO930424A1; NO932126L; DE4319458B4; ES2074002B1; SE9301970D0; GR930100236A; NO932126D0; FI110236B; JPH0663141A; SE510976C3; NL9301022A; CH687297A5; SE510976C2; DK174117B1; DK70193A

Abstract

A pleated filter in a housing comprises hydrophobic (10) and hydrophilic (11) sheets, preferably laminated together, and is connected between a patient and a ventilator in an open breathing system, with the hydrophobic sheet nearest to the patient. The hydrophobic sheet comprises resin-bonded ceramic fibres and has an alcohol bubble point of more than 710 mm of H2O to capture micro-organisms. The hydrophilic sheet comprises cellulose, may be continuous or slotted and captures moisture from exhaled breath, allowing it to be picked up again by inhaled air. <IMAGE>

Description

1 2267661 HEAT AND MOISTURE EXCHANGING-FILTERS 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 2 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 article "A Comparison of the Filtration Properties of Heat and Moisture Exchangers" by Hedley and AlltGraham in Anaesthesia 1992, Volume 47, pages 414420, 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 651655 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 3 present in exhaled breath. The second category are called "hy.groscopic (second generation)" heat and moisture exchanging filters. These are the same as the first generation but with the addition of electret felt filter material. Electrets are materials that maintain a permanent electric polarity and form an electric field around them without an external electric field. Such materials remove micro-organisms by electrostatic interaction. An example of this is shown in EP-Al-0011847.

EP-A2-0265163 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-Al- 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.

4 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 microorganism 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 sizes. In addition, and as discussed in the Lloyd and Roe reference, the filter efficiencies may not achieve the 99.9977% 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) H 2 0. 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 99.9977% 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 form 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, as discussed in the technical report 6 entitled "Use of The Pall Heat and Moisture Exchange Filter (HMEF) with Cold Humidification,' by Belkowski and Brandwein of Pall Corporation, published in 1992 as Pall Technical Publication PCC19210M, a small number of long term ventilator patients may require greater humidification than a heat and moisture exchange filter of the third category can provide. The technical report suggests 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 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 7 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) H 2 0 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) H 2 0 and having two opposed surfaces, taking a sheet of hydrophilic medium having two opposed surfaces, 8 and connecting one surf ace of the hydrophobic medium to one surface of the hydrophilic medium.

The following is a more detailed descripti on 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 4 is a schematic view of an artificial patient for use in testing the heat and moisture exchanger efficiency of a filter, Figure 5 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 4 connected to a ventilator in a second configuration for testing heat and moisture exchanger efficiency.

9 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) H 2 0. 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 H 2 0 at an air flow of 60 1/min and an aerosol bacterial removal efficiency 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.

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 throug h 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.

12 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) H 2 0. 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

13 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 cm 2.

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 2 0 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 14 Tensile Strength 44 kg/mm (3.8 lbs/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.

WATER LOSS 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 Tconnector 26 via a check valve 27 and permits flow from the second outlet 25 to the Tconnector 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 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 5, 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 suplied 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 16 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 30 0 C or 34 0 C. 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 Yconnector 32.

17 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 0 C and should not exceed 23 0 C. If the temperature rises above 23 0 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.f.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.

18 In addition, the percentage area of the circuit is also calculated in the formula:- E = 100.(wi wf) Wl where E = the ef iciency, 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 rerun.

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 CHALLENGE 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 19 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 2 0 C to ensure optimal growth.

Next, two 5 ml aliquots of the overnight culture are centrifuged (at approximately 2300g for 10 minutes). The supernatant is discarded and the cell pellets are resuspended in 3m1 sterile water. The washed cells are then collected by recentrifuging at approximately 2300g for 10 minutes. The washed cell pellets are then resuspended in sufficient sterile water to give a cell suspension of approximately 1x10 8 bacteria/ml.

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 Pseudomonas diminuta should be gram-negative, small rod shaped organisms about 0.3 - 0.4 gm by 0.6 - 1.0 gm 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 5 ml 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 56 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 55 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 21 run for 20 minutes to ensure that the airflow 28 1/min is maintained.

The equipment is then validated with regards to recovery of the Pseudomonas diminuta. 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 10 8 /ml pseudomonas diminuta suspension is diluted to 1 x 10 4 /ml using sterile water. The nebulizer 50 is filled with 5 ml of this suspension.

With a control valve 56 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.

22 Next 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 makeup air and maintains the airflow at 28 1/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 2/C. The colony forming units (cfu) are counted after 24 and 48 hours.

The equipment is validated if pseudomonas diminuta 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 x 10 8 ml Pseudomonas diminuta suspension.

23 Next, the control valve 56 is opened and the control valve 52 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 10 2 bacteria ml are then filtered through 0.2 gm analysis membrane using sterifils. The analysis membranes are then placed on to tryptone soya agar plates which are incubated at 30 20C. 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.

24 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 0.2[im 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 0 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 =_Bt- 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/m1 Vn = the volume nebulized The bacterial challenge to the filter is then calculated from the formula: C = Vn-Re-nt 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 C 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 - 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 0 C.

The bacteria removal efficiency was tested by the aerosal challenge test described above, with Pseudomonas diminuta. 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.

26 TABLE 1

FILTER WATER LOSS REMOVAL EFFICIENCY TYPE mg/1 BACTERIA VIRUS 2A 10.1 99.9976 99.999 2B 5.3 99.91 no data 3A 8.9 99.9992 (claim) 99.999 3B 10.4 99.999 99.999 M3A 4.2 99.95 99.1 M3B 7.0 99.981 no data Invention 8.4 0.4(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% 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 27 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 1.OOC. In condition B, the tidal volume was 780 ml at 15 breaths/min and an expiratory temperature of 33.2 0 1.0 TABLE 2

FILTER TYPE WATER LOSS (mg(H 2 0)/1 air) CONDITION A B 3A 7.2 1.1(n=12) 12.1 1.2(n=12) INVENTION 5.3 1.0(n=12) 8.8 1.2(n=12) It will be seen that, in both conditions of operation, the 28 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 (32 0 c).

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.

TABLE 3

1. Tidal Volume (ml) 480 660 800 1000 660 660 Frequency (bpm) is is 15 10 15 15 Temp. 0 c 32 32 32 32 30 34 Min.Vol. L/Min 7.2 9.9 12.0 10.0 9.9 9.9 Water loss(mg(H 2 0)l air) 8.1:P 0.2 8.4 T- 0.4 8.6:F 0.8 9.4 1: 0.5 7.3 ':P 0.5 10.05 (n=5) (n=18) (n=3) (n=4) (n=3) (n=2)

Claims

1. 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 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) H 2 0 for removing microorganisms.

2. A filter according to claim 1 wherein the sheet of hydrophilic medium is in contact with the sheet of hydrophobic medium.

3. A filter according to claim 2 wherein the sheets are bonded together.

4. A filter according to claim 2 wherein the sheets are laminated together.

5. A filter according to any one of claims 1 to 4 wherein the hydrophobic medium is of resin bonded-ceramic fibres.

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6. A filter according to any one of claims 1 to 5 wherein the hydrophilic medium is a cellulose material.

7. A filter according to any one of claims 1 to 6 wherein the sheets of filter media are pleated.

8. A filter according to claim 7 wherein the housing comprises a chamber bounded by a peripheral wall and two closures at respective opposite ends of the housing, one closure providing a part for connection to said supply of breathable gas and said expiratory line and the other closure comprising a part for connection to said person inhaling and exhaling the gas, the pleated sheets filling said chamber such that the pleats to one side of the sheets are adjacent one part and the pleats to the other side of the sheets are adjacent the other part.

9. A filter according to any one of claims 1 to 8 wherein the sheet of hydrophilic medium is continuous.

10. A filter according to any one of claims 1 to 8 wherein the sheet of hydrophilic medium is provided with a plurality of spaced parallel slits extending therethrough.

11. A filter according to any one of claims 1 to 10 wherein the filter has a pressure drop not greater than 3.Ocm H 2 0 at an air flow of 60 1/min.

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12. A filter according to any one of claims 1 to 11 wherein the filter has an aerosol bacterial removal efficiency when measured by the aerosol challenge test, of > 99.999%.

13. A filter according to any one of claims 1 to 12 wherein the filter has a water loss (as herein defined) of between 7 mg/1 and 10 mg/1 over a range of minute volume f rom 7 1/min to 12 1/min at a temperature of 32 0 C.

14. A filter substantially as hereinbefore described with reference to the accompanying drawings.

15. A breathing circuit comprising a ventilator, a tube connecting the ventilator to the first part of a filter according to any one of claims 1 to 14 an expiratory line leading from said first part and a tube leading from the second part of said filter for use by a person inhaling and exhaling gas from the ventilator.

16. A breathing circuit substantially as hereinbefore described with reference to the accompanying drawings.

17. 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) H 2 0 and having two opposed surfaces, taking a sheet of hydrophilic medium having two opposed surfaces, 33 and connecting one surface of the hydrophobic medium to one surface of the hydrophilic medium.

18. A method according to claim 17 wherein the connection comprises bonding.

19. A method according to claim 17 wherein the connection comprises laminating.

20. A method according to any one of claims 17 to 19 wherein the hydrophobic media is of resin bonded ceramic fibres and the hydrophilic media is a cellulose material.

21. A method according to any one of claims 17 to 20 wherein the filter has a pressure drop of not greater than 3.0 cm H 2 0 at an air flow of 60 1/min.

22. A method according to any one of claims 17 to 21 and further comprising pleating the connected sheets.

23. A method according to any one of claims 17 to 22 and further comprising inserting said connected media into 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.

24. A method of manufacturing a heat and moisture exchanging filter substantially as hereinbefore described with reference to the accompanying drawings.