NL194750C - Filters for exchanging heat and moisture. - Google Patents

Description

1 194750

Filters for exchanging heat and moisture

The invention relates to a filter for exchanging heat and moisture, which comprises a housing with a first part for connecting to a source of ventilation gas and an exhalation line and a second part for connecting to a person who enters the gas. and exhales, wherein the housing is provided with a plate of hydrophilic medium and a plate of hydrophobic filter medium arranged in series in a flow path between the first and second parts, the hydrophilic media being located closer to the first part in the flow path.

Such a filter is known from the European patent application EP-A2 0.265.163. This describes a layer of hydrophobic filter material and a layer of hydrophilic foam in a housing. The layers are formed by flat sheets that do not adhere to each other and touch each other. The hydrophobic layer is made of polypropylene fibers that are electrostatically charged to perform viral and bacterial filtration and acts as an electret. The foam layer is treated to absorb moisture.

In humans, inhaled air is filtered through the nasal cavities and upper respiratory tract.

In addition, in most climates the inhaled air contains a portion of water vapor and during its passage to the lungs the inhaled air is completely saturated with vapor taken up from the mucus secreted by the Goblet cells from the mucous membranes in the airways lie. With certain medical procedures and, for example, also with the supply of air in confined spaces such as aircraft cabins, the humidity levels in the inhaled air can be lower than optimal for satisfactory breathing.

For example, in methods such as pipe insertion or cutting of the trachea, a bypass is placed around these upper airways and therefore no filtration or saturation of gases inhaled from the ventilator used in these methods takes place. The clinical consequences of inhaling unfiltered and unsaturated gases are known, see for example the article "Filtration and Humidification" by Lloyd and Roe in part 4, number 4 of the October / December 1991 edition of the publication "Problems in Respiratory Care Reference is also made to the article entitled "Humidification for Ventilated Patients" by Ballard, Cheeseman, Ripiner and Wells on biz. 2-9 of part 8 (1992) of the publication "Intensive and Critical Care Nursing".

To overcome this problem, it is customary in practice to include a device in a ventilator that filters both the exhaled air and heat and humidifies the inhaled 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 Exchanges" by Hedley and Allt-Graham in Anesthesia 1992, part 47, biz. 414 to 420, and in the article "An Alternative Strategy for Infection Control of Anesthesia Breathing Circuits: A Laboratory Assessment of the Pall HME 35 Filter" by Berry and Nolte, biz. 651-655 of the publication "Anesth.Analg", 1991; 72.

The Lloyd and Roe publication distinguishes between three categories of filters for exchanging heat and moisture. The first category is called "hygroscopic (first generation)" filters for exchanging heat and moisture. These include wool, foam or paper-like materials that are usually impregnated with hygroscopic chemicals such as lithium chloride or calcium chloride to chemically absorb water vapor molecules that are in the exhaled air is present. The second category is called "hygroscopic (second generation)" filters for exchanging heat and moisture. These are the same as those of the first generation but are now supplemented with electret filter material made of felt. Electetics are materials that maintain a constant electrical polarity and form an electric field without an external electric field. Such materials remove 45 microorganisms through electrostatic action. An example of this is shown in the European patent application EP-A1 0.011.847.

The third category includes the use of a hydrophobic membrane that removes microorganisms through pure filtration and retains moisture on the surface of the membrane due to its hydrophobility

All of these three categories of filters generally work in the same way. Upon exhalation, 50 exhaled water vapor is condensed on the filter and, upon inhalation, the inhaled gases collect water vapors (and heat) from the device through evaporation. Microorganisms such as bacteria and viruses are removed by their filters from the exhaled and inhaled air.

The first category of filters is rarely used today. They are not very effective in removing bacteria in the air, even when they are impregnated with bactericidal drugs. Moreover, because of their method, they do not simultaneously achieve the maximum efficiency in the exchange of moisture and heat and have a relatively long acclimatization period before 194750 2 stable equilibrium levels have been reached. In addition, these have relatively large pores and a relatively large thickness that allow liquids to penetrate into the pores and pass through the material, leading to a water-saturated state and therefore to an increase in resistance.

The second category of filters provides improved levels of filtration of microorganisms in comparison to the first generation of hygroscopic filters. However, the problem of contaminated liquids passing through the layers still exists due to the relatively large dimensions of the pores. Moreover, as discussed in the Lloyd and Roe reference, the filters cannot achieve 99.9977% efficiency, which is presented as the minimum removal rate before a filter is suitable for clinical use.

The filters of the third category, using hydrophobic membranes, have extremely small pores typically with an alcohol-wetted bubble point greater than 710 mm H 2 O. The bubble point is measured according to the American Society of Testing Materials method. These prevent the passage of contaminated liquids at normal respiratory pressures. These filters also act as a barrier to microorganisms present in water and allow an efficiency of greater than 99.9977% to be achieved. In many cases, hydrophobic membrane filters have been shown to provide an exchange of heat and moisture that is comparable to normal breathing through the nose. Optimally effective humidification occurs almost immediately.

Exhaled air not only contains water in the form of water droplets and globules, but also water in the form of water vapor. It is possible that such water vapor passes through the filter and gets lost in the system. This means that not all expired water is available for humidification when inhaled. In general this is not a problem. However, as discussed in the technical report "Use of The Pall Heat and Moisture Exchange Filter (HMEF) with Cold Humidification," by Belkowski and Brandin from Pall Corporation, published in 1992 as Pall Technical Publication PCC19210M, a small part of long-term ventilation patients may require greater humidification than a third-category heat and moisture exchange filter can provide.The technical report suggests that this problem can be overcome by including a humidifier in the ventilation circuit. to overcome this problem, it is to combine the hydrophobic material with a hygroscope material of the category used in the first and second categories, to absorb the water.

From British patent application GB-A 2,167,307 a filter is known for exchanging heat and moisture, which alternately comprises hydrophobic and hydrophilic washing devices mounted in a housing, the hydrophilic washing devices being impregnated with a hygroscopic material. These are probably due to water saturation.

The present invention now provides a filter according to the first paragraph on page 1, characterized in that the hydrophobic medium has an alcohol bubble point higher than 710 mm H2 O for removing microorganisms, that the plate is provided with hydrophilic medium of a large number of spaced apart parallel slots extending therethrough, whereby the maximum pressure drop across the filter is controlled by the hydrophobic medium when the hydrophilic medium is saturated with water, that the plates of the filter media are pleated and that the filter has a pressure drop no greater than 3.0 cm 40 H2 O at an air flow of 60 l / min.

In the three categories of filters, it is necessary that the filter medium does not create such a large pressure drop that it is difficult to breathe in and breathe out air. This is normally not a problem with filters of the first and second categories, because the pore size of the filter media is large enough not to produce a pressure drop that is large enough to cause a problem. However, the filters of the third category 45 may suffer from this problem. The plates of the filter medium are therefore pleated. This gives a larger surface area and thus reduces the pressure drop.

The following is by way of example a more detailed description of some exemplary embodiments with reference to the accompanying drawings, in which: Figure 1 is a schematic end view of the filter for exchanging heat and moisture formed from folded filter media, Figure 2 is a schematic view of a second embodiment of a hydrophilic medium of the filter of FIG. 1, FIG. 3 is a schematic overview of an artificial patient for use in testing the efficiency of exchanging heat and moisture from a filter Figure 4 shows the artificial patient from Figure 4 connected to a respirator for the tests, 194750 Figure 5 shows the artificial patient from Figure 4 connected to a respirator in a second embodiment for testing the efficiency of a heat and moisture exchanger, figure 7 gives a schematic overview of the equipment that is used for determining the effectiveness of a filter by aerosol test tests.

5

With reference to Figure 1, the filter comprises a plate of hydrophobic medium 10 pleated with a plate of hydrophilic material 11. The two layers can be separate, or connected to each other, or attached to each other by any conventional method. The media is enclosed by a house with gates that lead to respective opposite sides of the media. As can be seen in Figures 1 and 6, the pleated media fill the housing such that the pleats on one side of the media are closely adjacent to one port and the pleats on the other side of the media are close to the other port .

The hydrophobic medium 10 preferably consists of resin-impregnated ceramic fibers and removes microorganisms by direct mechanical interception and has an alcohol-wetted bubble point greater than 710 mm H2 O. This is measured according to the method of the American Society of Testing Materials for such a test. The hydrophilic medium is preferably a cellulose material. The material must not separate any particles.

The filter has a pressure drop that is no greater than 3.0 cm H 2 O at an air flow of 60 l / min and is effective in removing aerosol bacteria as measured by the so-called aerosol test test below of> 99.999%.

The hydrophilic medium is, as can be seen in Figure 2, interrupted by a series of rows of parallel spaced slots arranged through the medium. Where these slots are provided, they ensure that the maximum pressure drop across the device is controlled by the hydrophobic medium. The slots allow flow through the hydrophilic medium if it should be soaked, i.e., saturated with water.

The device has been used at the "patient end" of open respiratory systems of the type primarily used in intensive care units. Such systems are used by patients undergoing long-term respiration and by patients who, due to the nature of their clinical condition , need extra humidification while they are being ventilated.

Such systems include a ventilator, a tube that connects the ventilator to one port of the device on the hydrophilic media side 11, and a tube that connects the other port on the hydrophobic media side to the patient inhaling and exhaling gas. from the ventilator. A valve system is provided that allows exhaled air to be vented via an exhalation line after it has been passed through the device.

During use, the hydrophobic medium 10 is not saturated with the patient fluids, offers a low air flow resistance and also offers a high efficiency in removing microorganisms by means of mechanical interception.

The hydrophilic medium 11 works in the following manner. Water that passes through the hydrophobic medium, almost entirely in the form of water vapor, is trapped by the hydrophilic medium due to its hydrophilic nature. This moisture then spreads over the entire surface of the hydrophilic medium 11. In this way, inhaled gases first pass through the hydrophilic medium 11, where they absorb this moisture for absorbing additional moisture from the hydrophobic medium in the normal manner. This therefore has the advantage that the humidification levels are increased, which makes the need for the use of an additional humidifier unnecessary.

In addition, it overcomes another problem of the hydrophobic medium. Namely, since water 45 does not spread evenly over a hydrophobic medium, there may be areas of the medium that are not covered by water. This can lead to a preferred passage for inhaled gases through the hydrophobic medium, during which passage little or no humidification takes place. As moisture spreads evenly over the hydrophilic medium, this problem is overcome.

The possibility of blocking the filter by soaking the hydrophilic medium - that is, because the hydrophilic medium is saturated with water - is avoided by selecting a cellulose material with suitable pore dimensions and thickness so that it has a bubble point pressure that is is low enough to allow the removal of the excess water during the gas flow. For example, a cellulose material is available on the market with an alcohol bubble point of 64 mm to 115 mm H 2 O. This is measured in accordance with the method of the American Society of Testing 55 Materials.

Joining the layers where they are applied makes it easier to bend the layers without creating space between the layers where water can collect. In addition, 194750 4, bonding is advantageous in eliminating or preventing saturation of the hydrophilic layer 11.

The fact that the media 10,11 fill the house reduces the dead space in the house. This is advantageous because it reduces the volume of the re-inhaled gas. The following is a description of tests of a device of the type described above with reference to the drawings in comparison with two commercially available filters of the second category described in the introduction of this patent specification (designated 2A and 2B, respectively), two filters of the third category referred to in the introduction of the patent specification (designated 3A and 3B respectively) and two filters of the third category supplemented with a hygroscope material to retain moisture (referred to as M3A and M3B).

The filter according to the invention, which serves as an example, was formed as described above, stands with an area of about 640 to 650 cm 2.

The hydrophobic medium of the improved filter, which serves as an example, consisted of pre-mixed ceramic fibers impregnated with suitably destabilized piles and an alcohol-wetted bubble point greater than 710 mm H 2 O, measured in the manner described above. The amount of resin was 10% relative to the fibers (weight / weight). The fibers impregnated with resin were then made hydrophobic according to one of the methods known in the art.

The hydrophilic filter medium of the improved filter that serves as an example consisted of cellulose fibers that are bonded with a binder and have the following composition and properties:

Fiber: 100% hemp Binder: viscose Pressure drop 74 mm water column Tensile strength 92-115 kg / mm 25 Thickness 0.081 mm

Tensile strength 103 kg / mm wetted with oil Tensile strength 44 kg / mm wetted with water Crack strength 3520-4400 kg / mm2 (Muller test).

All filters were examined for water losses, effectiveness in removing bacteria and efficiency in removing viruses, during use in the tests that will now be described.

Water loss test

This test will be described with reference to Figures 3-5. The artificial patient shown in Figure 3 comprises a humidifier 20 capable of supplying exhaled air of a certain moisture content and temperature. A first outlet 21 of the humidifier is connected to a rubber lung 22 with a capacity of 2 liters and connected to a connection tube 23, via a check valve 24 which prevents flow from the outlet 21 to the connection tube 23 and allows flow in the other direction.

The second outlet 25 of the humidifier 20 is connected to a T connection 26 via a check valve 27 40 and allows flow from the second outlet 25 to the T connection 26 and prevents flow in the opposite direction. The T-connection 26 is connected to the other end of the connection tube 23 by a third check valve 28 which allows flow from the T connection 26 to the connection tube 12 but prevents flow in the opposite direction

The humidifier 20 comprises a temperature control 29 which controls the temperature of the air produced from the humidifier 20.

In use, the humidifier 20 is filled with distilled water. As can be seen in Figure 4, the outlet 30 of the T-connection 26 is then connected to the leg 31 of a Y-connection 32. One branch 33 of the Y-connection is connected by a tube 34 with an inlet to a ventilator. 35 and the second branch 36 of the Y-connection 32 is connected through tube 37 to an outlet of the ventilator 50.

The respirator delivers respiratory gases by means of pulses whose volume and frequency can be controlled. The volume supplied in a pulse is called the tidal volume (tidal volume) and the frequency is measured in "number of breaths / minute".

When testing the efficiency of the heat and moisture exchanger, the tidal volume of the 55 ventilator 35 is set to a known value, e.g. 660 ml, and the frequency is set to a certain value, e.g. 15 breaths per minute. Each air volume supplied by the ventilator 25 passes along the tube 37 to the T-connection 26. As a result of the 194750 non-return valves 24, 27, 28, this gas passes around the connecting tube 23 and into the rubber lung 22 which expands around the to receive air. At the end of the air pulse, the rubber lung 22 exhales air which, as a result of the check valve, passes through the humidifier 20 and leaves via the second outlet 25 and the T-connection 26 to return to the inlet of the ventilator 35 via the tube 23 The T-connection 26 is insulated to prevent condensation.

The system runs for 30 minutes to allow the system to heat up with the temperature of the artificial patient set at 30 ° C or 34 ° C. After 30 minutes, the artificial device 35 and the temperature control 39 of the artificial patient are switched off. Subsequently, the tube 34 is removed from the first branch 33 of the Y-connection 32 and replaced by a tube that has a housing 38 containing 100 g of desiccant. The housing 38 is therefore connected to the first branch 33 of the Y-connection 32 and to the inlet to the ventilator 35. In addition, the filter 39 to be tested for exchanging the heat and moisture is introduced into the tube between the outlet 30 and the leg 31 of the Y-connection 32. Next, the artificial patient is weighed and the components between the T-connection 26 and the ventilator are weighed. This includes the housing 38 and its contents and the filter 39. The weights are noted.

The ventilator 35 and the artificial patient are then turned on and running for one hour. If the temperature of the ambient air is not controlled during this period, the temperature of the exhalation line (i.e. the temperature in the tube 37) is checked at regular intervals. It must be approximately 20 ° C and may not exceed 23 ° C. If the temperature rises above 23 ° C, ice or cooled water must be placed around the pipes in order to reduce the temperature. The ventilation device 35 and the temperature control 29 are switched off after one hour. The items weighed above are weighed again and their weights are noted.

The water loss is then calculated using the formula below: G = (TV.ft) / 1000 where G = the gas flow In liters per minute, TV = the tidal volume in millimeters, f = the frequency in breaths per minute, t = the time of the test in minutes.

Moreover, the percentage range of this circuit is also calculated in the formula: 100. (w, - Wf) W, where E = the efficiency, 35 W, = the weight of the test circuit for the test in grams,

Wf = the weight of the test circuit after the test in grams, W1 = the weight loss of the artificial patient in grams.

This number cannot exceed 10%; if this is above 10%, the experiment is invalid and must be repeated.

40 Finally, the water loss of the patient is calculated from the following formula:

PL = W, / G

where PL = the water loss of the patient in milligrams of water per liter of air and W1 and G have the meaning given above.

45 Aerosol test test

The device comprises a nebulizer 50. 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 test filter 53 via a tube 54. The tube also receives air from a second inlet via an air flow meter 50 55, a control valve 56 and a protective filter 57. The outlet to test filter 53 is connected to a vacuum source (not shown) via a protective filter 58 and a vacuum pressure meter 59. The outlet is also connected to a collision sampling device 60 for liquids (a liquid Impingement sampler) with an outlet 61 controlled by a valve

In use, the first step is to prepare a bacterial suspension. This is obtained by inoculating 55 ml of trypton-soya liquid medium with a single colony of a trypton-soya oblique agar culture. This culture is incubated overnight in a shaking water bath at a temperature of 30 ± 2 ° C to ensure optimum growth.

* 194750 6

Subsequently, two equal parts of 5 mm of the culture that developed overnight are centrifuged (at approximately 2300 g for 10 minutes). The supernatant is discarded and the cell spheres are resuspended in 3 ml of sterile water. The washed cells are then collected by re-centrifugation (at approximately 2300 g for 10 minutes). The washed cell beads are then resuspended in sufficient sterile water to give a cell suspension of approximately 1 x 10® bacteria / ml.

A gram strain is then prepared. The preparation is examined with a compound microscope equipped with a calibrated ocular micrometer, and an oil-immersed objective lens (x 100). Several microscope fields are viewed for organism size and arrangement of the cells. The pseudomonas diminuta must be gram-negative, with small rod-like organisms occurring approximately 0.3-0.4 µm times 0.6-1.0 µm in size primarily as individual cells.

The device is then made valid for volume flow. With this validation, the test filter 53 is removed and replaced with a flow meter (not shown). The atomizer 50 is filled with 5 ml of sterile water and the collision sampling device with 20 ml of sterile water. The control valve 52 to the atomizer 50 is closed and the control valve 56 to the air flow meter 55 is opened, a vacuum is applied and air is drawn into the device for 30 seconds. At 0.5 bar vacuum or higher, the air flow must be 28 l / min, which is controlled by the critical opening of the collision device 60 (impinger).

The atomizing device 50 is then activated by fully opening the associated control valve 52. At the same time, the control valve 56 of the air flow meter is partially closed to maintain an air flow of 28 l / min through the device. The volume flow rate on the air flow meter 55 by the associated control of valve 56 is noted. The device is operated for 20 minutes to ensure that the air flow of 28 l / mln is maintained

The device is then validated with regard to the recovery of the pseudonomas diminuta.

To be able to do this, the test filter 53 from Figure 6 is removed and replaced by a six-step Anderson sampling device (a six-stage Anderson sampler). The glass petri dish that belongs to the sampling device is filled with tryptone soya agar in each step. The air to be sampled enters the inlet to the sampling device (sampler) and then proceeds stepwise through the successive opening steps with successively higher opening speeds from step 1 to step 6. Successively smaller particles are initially impacted on the surfaces of agar sets of every step.

Subsequently, one ml of the approximately 1 x 108 / ml pseudomonas diminuta solution is diluted to 1 x 104 / ml using sterile water. The atomizer 50 is filled with 5 ml of this suspension.

While a non-return valve 56 is open and the non-return valve 52 is closed, vacuum is applied and air is drawn into the device for 30 seconds. At 0.5 bar vacuum or higher, the air flow will be 28 l / min, which will be controlled by the collision device of the critical opening.

Subsequently, the atomizing device 50 is activated by fully opening the associated check valve 52. At the same time, the control valve 56 is partially closed to the determined level as determined by the validity test for the volume flow. This supplies used air and maintains the air flow at 28 l / min through the device. After a test period of 15 minutes, the valve 52 is closed and the valve 56 is fully opened. After another 30 seconds to clean the aerosol system, the vacuum source is switched off.

The agar pool plates are then removed from the Anderson sampling device and incubated at 30 ± 2 ° C. The colony forming units (colony forming units; cfu's) are counted after 24 and 48 hours.

The device is valid if pseudomonas diminuta are found on the Anderson sampling device in steps 6 or 5. This confirms that monodisperse organisms are produced by the device.

After these validity tests, the device is used to test the efficiency of filters in the following manner.

A test filter 53 is introduced into the device as shown in Figure 7. 20 ml of sterile water is placed in the liquid sampling device 60 and the atomizer is filled with 5 ml of the approximately 1 x 10® ml pseudonomas diminuta suspension.

The check valve 56 is then opened and the check valve 52 closed. Vacuum is applied to the device 55 and air is sucked into the device for 30 seconds. At 0.5 bar vacuum or higher, the air flow will be 28 l / min and be controlled by the critical opening of the fluid sampling device 60. Then, the atomizing device 50 is activated by fully opening the associated control valve 52 and partially closing the control valve 56 to the level determined by the validity test to maintain an air flow of 28 l / min.

After a test time of 15 minutes, the valve 52 is closed and the valve 56 is fully opened. After another 30 seconds to clean the aerosol system, the vacuum is switched off.

The liquid that remains in the atomizing device 50 is then discharged by using a 5 ml injection needle. The volume that remains is measured by using a glass measuring cylinder of 10 ml and the volume is serially diluted seven times with water ten times. Dilutions containing approximately 102 ml of bacteria are then filtered through 0.2 µl of analysis membrane using sterifiles. The assay membranes are then placed on the tryptone soya agar plates 10 incubated at 30 ± 2 ° C. The CFUs are counted after 24 and 48 hours and the number of colonies is written down from membranes showing 20 to 200 colonies. The titre of the nebulizer test is then calculated.

The liquid in the collision sampling device (impingement sampler) is then discharged. The volume is measured using a 20 ml glass measuring cylinder. This volume is serially diluted three times ten times in sterile water. The remaining undiluted solution and the resulting dilutions are filtered through a 0.2 µm analysis membrane using sterifiles. The assay membranes are placed on tryptone soya agar plates and the opening of the collision tester 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 20 number of colonies on membranes showing 20 to 200 colonies are noted and the number of bacteria recovered downstream from the filter is calculated. The efficiency of the device is calculated according to the formula:

Re = 100% fn 25 where Re = the efficiency of the arrangement in percentages,

Bt = the total number of bacteria recovered,

Tf = the final atomized titer in cfu / ml,

Vn = the atomized volume.

The bacterial test on the filter is then calculated according to the formula: C = V '. R ,. n, where C = the total test,

Vn and Re have the meaning given above, and 35 n = the test titer of the nebulizer in cfu / ml.

Subsequently, the decrease of the filtered titer is calculated from the formula:

Tr-b, 40 where TR = the decrease of the filter titer, and C and B, have the meaning given above.

From this the efficiency of the filter can be calculated in percent according to the formula: Filter efficiency = ^ 1 - x 100% 45

The water loss was calculated as described above. The tidal volume was 660 ml, the rate of 15 breaths per minute, and the outlet temperature of the artificial patient 32 ° C.

The effectiveness of removing bacteria was tested by the aerosol test test, as described above, with pseudonomas diminuta. The effectiveness in virus removal was tested by an aerosol assay test of the kind described above with M52 bacterophage.

The results of the test are shown in Table 1.

194750 8 TABLE 1

Filter type Water loss mg / l Removal target 5 Bacteria (%) Virus (%) 2A 10.1 99.9976 99.999 2B 5.399.91 no data 3A 8.9 99.99992 (conclusion) 99.999 10 3B 10.4 99.999 99.999 M3A 4.2 99.95 99.1 M3B 7.0 99.991 no data improved filter 8.4 ± 0.4 (n = 18) 99.999 99.999 15

As can be seen, the second category of filters has either a high water loss with a relatively high removal efficiency (2A) or a much lower water loss, but a corresponding decrease in the removal efficiency (2B) (namely the efficiency of 99 , 9977% proposed (by Lloyd and Roe) as being the lower limit for clinical use). The third category of filters achieves much higher removal efficiency, but has relatively high water losses. The modified third category of filters has much lower water losses due to the presence of hygroscopic material, but had comparatively lower removal efficiencies. However, the filter described above with reference to the drawings had their high removal efficiency and relatively low water loss (less than 8.5 mg / l).

The filter described above with reference to Figure 1 and with the test results listed in Table 1 was also included in a respirator in a clinical test and compared to the commercial filter 3A of Table 2. The filters were tested under two conditions (A and B) and water loss based on 24-hour use is shown in Table 2. At condition A, the tidal volume was 480 ml at 15 breaths per minute and at an exhalation temperature of 32.4 ± 1 ° C . In condition B, the tidal volume was 780 ml at 15 breaths per minute and an exhalation temperature of 33.2 ± 1 ° C.

TABLE 2 35 Filter type Water loss (mg (H20) / lalr)

Condition A B

3A 7.2 ± 1.1 (n = 12) 12.1 ± 1.2 (n = 12) 40 improved filter 5.3 ± 1.0 (n = 12) 8.8 ± 1.2 (n = 12)

It can be seen that under both operating conditions the filter described above with reference to the drawings has a greatly reduced water loss.

45 This decrease in water loss is maintained over a wide area of working conditions.

Table 3 below shows the water loss found under specific different operating conditions of the ventilator. It can be seen that over a wide range of minute volumes (tidal volume times frequency) from 7.2 l / min to 12 l / min there is no significant variation in water loss (7 mg / l to 10 mg / l) with constant temperature (32 ° C).

The examples given above are examples using specific media. It will be understood, however, that other suitable hydrophobic and hydrophilic media can be used in filters of the type described above.

Although the examples given above relate to medical use, the combination of hydrophilic and hydrophobic media can also be used in other systems where Inhaled and 55 exhaled air is filtered and where a problem arises which involves the use of a filter for retaining heat and moisture required. The air in an aircraft cabin, for example, is supplied through a filter and may have an unsuitable temperature and humidity. The use of a filter from

Claims (3)

9 194750 the type described above can provide a supply of air in an aircraft cabin that has the required temperature and humidity. TABLE 3 5 - Tidal volume 480 660 800 1000 660 660 Frequency 15 15 15 10 15 15 Temperature ° C 32 32 32 32 30 34 Minimum volume l / min 7.2 9.9 12.0 10.0 9.9 9.9 10 - Water loss 8.1 ± 0.2 8.4 ± 0.4 8.6 ± 0, Δ 9.4 ± 0.5 7.3 ± 0.5 10.05 (mg CHsOU (n = 5) (n = 1B) (n = 3) (n = 4) (n = 3) (n = 2) 15 A heat and moisture exchange filter comprising a housing having a first part for connecting to a source of respiratory gas and an exhalation line and a second part for connecting to a person inhaling and exhaling the gas, the housing is provided with a plate of hydrophilic medium and a plate of hydrophobic filter medium arranged in series in a storm path between the first and second parts, the hydrophilic media being located closer to the first target in the flow path, characterized in that the hydrophobic medium has an alcohol bubble point higher than 710 mm H 2 O for removing microorganisms, that the hydrophilic medium plate is provided with a large number of spaced apart parallel slots extending thereby, thereby controlling the maximum pressure drop across the filter by the hydrophobic medium when the hydrophilic medium is saturated with water, that the plates of the filter media are pleated and that the t filter has a pressure drop that is no greater than 3.0 cm H2 O with an air flow of 60 l / min. 2. Filter according to claim 1, characterized in that the hydrophobic medium is resin-impregnated ceramic fibers. A filter according to claim 1 or 2, characterized in that the hydrophilic medium is a cellulose material. Hereby 5 sheets of drawing