NL9301022A - FILTERS FOR EXCHANGE OF HEAT AND MOISTURE. - Google Patents

Description

Filters for exchanging heat and moisture.

The invention relates to filters for retaining heat and moisture.

In humans, inhaled air is filtered through the nasal passages and the 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 contained in the mucus secreted by the Goblet cells from the mucous membranes located in the airways . For certain medical procedures and, for example, for the supply of air in enclosed spaces such as aircraft cabins, the humidity levels in the inhaled air may be less than optimal for satisfactory breathing.

For example, in methods such as introducing tubing or cutting 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 occurs. The clinical consequences of inhaling unfiltered and unsaturated gases have been well described. See, for example, the article "Filtration and Humidification" by Lloyd and Roe in Volume 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 volume 8 (1992) of the publication "Intensive and Critical Care Nursing".

In order to overcome this problem, it is common practice to include in a ventilator a device that filters both the exhaled air and heat and humidifies the inhaled gases. Such devices are discussed in the two publications referenced above and in the article "A Comparison of the Filtration Properties of Heat and Moisture Exchanges" by Hedley and Allt-Graham in Anesthesia 1992, vol. 47, biz. 4l4 to 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, biz. 651-655 of the publication "Anesth.Analg", 1991; 72.

Lloyd and Roe's 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 present in the exhaled air The second category is called "hygroscopic (second generation)" filters for exchanging heat and moisture, which are the same as those of the first generation but are now supplemented with felt filter material. Electretes are materials that maintain a constant electrical polarity and form an electrical field without an external electrical field, such materials remove microorganisms by electrostatic action, an example of which is shown in EP-A1-0011847.

EP-A2-0265163 discloses the use of a layer of hydrophobic filter material and a layer of hydrophilic foam in a house. The layers are formed by non-adhering flat plates that touch each other. The hydrophobic layer is made of polypropylene fibers that are electrostatically charged to perform viral and bacterial filtration and also act as an electret of the second category of filters as described above. The foam layer is treated to absorb moisture. This works in a similar way as a material of the device of EP-A1-0011847 with the associated drawbacks.

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

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

The first category of filters is rarely used today. These are not very effective in removing bacteria in the air, even when impregnated with bactericidal agents. Moreover, because of their method, they do not simultaneously attain maximum efficiency in exchanging moisture and heat and have a relatively long acclimation period before stable equilibrium levels are reached. In addition, they 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 consequently to an increase in resistance.

The second category of filters provides improved levels of filtration of microorganisms compared to the first generation of hygroscopic filters. However, the problem of contaminated liquids passing through the layers still persists due to the relatively large pore size. In addition, as also discussed in the Lloyd and Roe reference, the filters cannot achieve an efficiency of 99.9977 #, which has been suggested as the minimum degree of removal before a filter is suitable for clinical use.

The third category filters, using hydrophobic membranes, have extremely small pores, typically alcohol bubble wetted bubble point greater than 710 mm (28 inches) H 2 O. The bubble point is measured according to the method of the American Society of Testing Materials. These prevent the passage of contaminated liquids at normal ventilation pressures. These filters also act as a barrier to micro-organisms present in water φη allowing an efficiency greater than 99.9977 # to be achieved. In many cases, hydrophobic membrane filters have been shown to provide an exchange of heat and moisture comparable to normal breathing through the nose. An optimally efficient humidification occurs almost immediately.

Exhaled air contains not only water in the form of water droplets and spheres, but also water in the form of water vapor. It is possible that such water vapor passes through the filter and is lost in the system. This means that when inhaling, not all exhaled water is available for humidification. In general, this is not a problem. However, as discussed in the Pall Corporation technical report "Use of The Pall Heat and Moisture Exchange Filter (HMEF) with Cold Humidification" * published by Belkowski and Brandwein, published in 1992 as Pall Technical Publication PCC19210M, respiratory 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. this problem is overcome to combine the hydrophobic material with a hygroscope material of the category used in the first and second categories to absorb the water.

GB-A-2167307 discloses a filter for exchanging heat and moisture, which alternately comprises hydrophobic and hydrophilic washers mounted in a house, the hydrophilic washers being impregnated with a hygroscopic material. These are likely to result from water saturation.

According to a first aspect according to the invention, there is provided a filter for exchanging heat and moisture comprising a housing with a first part for connecting to a supply of gas that can be inhaled and an exhalation line and a second part being connected to a person who inhales and exhales the gas, the housing being 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 medium closer to the first part is in this flow path and the hydrophobic medium has a bubble point for alcohol greater than 710 mm (28 inches) H2 O to remove microorganisms.

In all these three categories of filters it is imperative that the filter medium does not make such a large pressure drop that it is difficult to inhale and exhale 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 large enough to cause a problem. However, the third category filters can suffer from this problem.

The plates of the filter medium are preferably pleated. This gives a larger medium area and therefore reduces the pressure drop.

According to a second aspect of the invention, there is provided a method of manufacturing a filter for exchanging heat and moisture which comprises a plate of hydrophobic medium with an alcohol bubble point greater than 710 mm (28 inches) H 2 O and two opposite each other located surfaces, is joined to one face of a hydrophilic medium plate with two opposed faces to one face of the hydrophilic medium.

The following is by way of example a more detailed description of some embodiments of the invention, referring 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 first embodiment of a hydrophilic medium of the filter of Figure 1, and

Figure 3 is a schematic view of a second embodiment 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 efficiency of heat and moisture exchange of a filter,

Figure 5 shows the artificial patient of Figure 4 connected to a ventilator for the tests,

Figure 6 shows the artificial patient of Figure 4 connected to a ventilator in a second embodiment for testing the effectiveness of a heat and moisture exchanger.

Figure 7 gives a schematic overview of the equipment used to determine the effectiveness of a filter by aerosol testing.

Referring to Figure 1, the filter comprises a plate of hydrophobic medium 10 pleated with a plate of hydrophilic material 11. The two layers may be separate, or joined together, or bonded together by any conventional method. The media is enclosed in a housing 12 with ports 13, 14 leading to respective opposite sides of the media. As can be seen in Figures 1 and 6, the pleated media fills the housing 12 such that the pleats on one side of the media are close to one port 13 and the pleats on the other side of the media are close to the other port 14.

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 (28 inches) H 2 O. This is measured according to the American Society of Testing Materials method for such a test. The hydrophilic medium is preferably a cellulose material. The material must not separate particles.

Preferably, the filter has a pressure drop not greater than 3.0 cm H 2 O at an air flow of 60 l / min and an efficiency in removing aerosol bacteria, as measured according to the so-called aerosol challenge test described below, of> 99.999%.

The hydrophilic medium, as shown in Figure 2, can be a continuous sheet of material. As can be seen in Figure 3, it may be interrupted by a series of rows of parallel spaced slots provided 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 were to become soaked, i.e. saturated with water.

The device has been used at the "patient end" of open ventilation systems of the type mainly used in intensive care units. Such systems are used by patients undergoing prolonged ventilation and by patients who, due to the nature of their clinical condition, require additional humidification while ventilating.

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

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

The hydrophilic medium 11 acts in the following way. Water passing 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 the normal absorption of additional moisture from the hydrophobic medium. This therefore has the advantage of increasing the humidification levels, eliminating the need for the use of an additional humidifier.

In addition, it overcomes another problem of the hydrophobic medium. Namely, since water 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 wetting occurs. Since moisture spreads evenly over the hydrophilic medium, this problem is overcome.

The possibility of blocking the filter by soaking the hydrophilic medium - that is, by saturating the hydrophilic medium with water - is avoided by choosing a cellulose material of suitable pore size and thickness so that it has a bubble point pressure that is low is enough to allow the removal of excess water during the gas flow. For example, a cellulose material is available from the Pali Corporation with an alcohol bubble point of 64 mm to 115 mm (2.5 inch to 4.5 inch) H 2 O. This is measured according to the method of the American Institute of Testing Materials.

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

The fact that the media 10, 11 fills the housing 12 reduces dead space in the housing 12. 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 compared to two commercially available filters of the second category described in the preamble of this patent (denoted respectively by 2A and 2B), two filters of the third category referred to in the preamble of the patent (denoted 3A and 3B respectively) and two filters of the third category supplemented with a hygroscope material to retain moisture (denoted as M3A and M3B).

The exemplary filter according to the invention was formed as described above with an area of about 640 to 65 cm 2.

The hydrophobic medium of the filter according to the invention which serves as an example consisted of premixed ceramic fibers impregnated with a suitable destabilized resin and an alcohol-wetted bubble point larger than 710 mm (28 inches) H 2 O, measured in the manner as above has been described. The amount of resin was 10% relative to the fibers (weight / weight). The resin-impregnated fibers were then rendered hydrophobic by any of the methods known in the art.

The hydrophilic filter medium of the filter according to the invention which serves as an example consisted of cellulose fibers bonded with a binder and having the following composition and properties:

Fiber: 100 # hemp

Binder: viscose

Pressure drop 74 (mm water column) (2.9 inches water column)

Tensile strength 92-115 (kg / mm) (8-10 lbs / inches)

Thickness 0.08l mm (3.2 inches x 10-3)

Tensile strength 103 kg / mm (8.9 lbs / inches) wetted with oil

Tensile strength 44 kg / mm (3.8 lbs / inches) wetted with water

Burst strength 3520-4400 kg / mm2 (12-16 lbs / inches2) (Muller test).

All filters were tested for water loss, bacteria removal efficiency and virus removal efficiency, using the assays now described.

WATER LOSS TEST

This test will be described with reference to Figures 4-6.

The artificial patient shown in Figure 4 comprises a humidifier 20 capable of supplying exhaled air of a certain moisture content and temperature. A first humidifier outlet 21 is connected to a 2 liter capacity rubber lung 22 and connected to a connection tube 23, via a check valve 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 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.

Humidifier 20 includes a temperature controller 29 that controls the temperature of the air produced from humidifier 20.

In use, the humidifier 20 is filled with distilled water. As can be seen in Figure 5, the outlet 30 to 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 through a tube 34 with an inlet to a ventilator. 35 and the second branch 36 of the Y-joint 32 is connected through tube 37 to an outlet of the ventilator 35.

The ventilator supplies ventilation gases by means of pulses, the volume and frequency of which can be controlled. The volume supplied in a pulse is called the tidal volume (tidal volume) and the frequency is measured in "breaths / minute".

When testing the efficiency of the heat and moisture exchanger, the tidal volume of the ventilator 35 is set to a known value, for example, 660 ml, and the frequency is set to a certain value, for example, 15 breaths per minute. Any volume of air supplied by the ventilator 25 passes along the tube 37 to the T-connection 26. Due to the check valves 24, 27, 28, this gas passes around the connecting tube 23 and into the rubber lung 22 which expands to expel the air. receive. When the air pulse ends, the rubber lung 22 exhales air which, as a result of the check valve, passes through the humidifier 20 and exits through the second outlet 25 and the T-connection 26 to return to the inlet of the ventilator 35 through . the tube 34.

The T-connection 26 is insulated to prevent condensation from forming.

The system runs for 30 minutes to warm the system to the temperature of the artificial patient set at 30 ° C or 34 ° C. After 30 minutes, the ventilator 35 and the temperature control 39 of the artificial patient are turned off.

Then, the tube 34 from the first branch 33 of the Y-joint 32 is removed and replaced with a tube having 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 exchanged for exchanging heat and moisture is introduced into the tube between the outlet 30 and the leg 31 of the Y-joint 32.

Then, the artificial patient is weighed and the components between the T-connection 26 and the ventilator are weighed. This includes the body 38 and its contents and the filter 39 · The weights are noted with one decimal place.

The ventilator 35 and the artificial patient are then turned on and run for one hour. If the ambient air temperature 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 should be about 20 ° C and should not exceed 23 ° C. If the temperature rises above 23 ° C *, ice or chilled water should be placed around the pipes 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 are recorded with one decimal place.

The water loss is then calculated using the Formula below:

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.

In addition, the percentage area of this circuit is also calculated in the formula:

J

where E = efficiency,

Wi = the weight of the test circuit for the test in grams,

Wf = the weight of the test circuit after the test in grams,

Wx = the weight loss of the artificial patient in grams.

This number should not be above 10 #, if it is above 10 # the experiment is invalid and must be repeated.

Finally, the patient's water loss is calculated from the following formula:

PL = Mi G.

where PL = the patient's water loss in milligrams of water per liter of air, and

Wx and G = have the meaning given above.

AEROSOL TEST TEST

The device includes a nebulizing device 50 of the type sold by Devilbiss as Model 40. The inlet to the nebulizing device is connected by filter 51 and a control valve 52 to the ambient air. The outlet of the misting device 50 is connected to the test filter 53 via a tube 5 ^ D® 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 test filter 53 is connected to a vacuum source (not shown) via a protective filter 58 and a vacuum pressure gauge 59. The outlet is also connected to a liquid impingement sampler collision sampler 60 with an outlet 6l controlled by a valve.

In use, the first step is to prepare a bacterial suspension. This is obtained by inoculating 100 ml of trypton-soya liquid medium with a single colony of a trypton-soya slant agar culture. This culture is incubated overnight in a shaking water bath at 30 ± 2 ° C to ensure optimal growth.

Two equal aliquots of 5 µm of the overnight culture are centrifuged (at about 2300 g for 10 min). The supernatant is discarded and the cell spheres are resuspended in 3 ml of sterile water. The washed cells are then collected by centrifugation at about 2300 g for 10 min. The washed cell spheres are then resuspended in enough sterile water to give a cell suspension of about 1 x 108 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 should be gram negative, with small rod-like organisms about 0.3-0.4 µm times 0.6-1.0 µm in size acting mainly as single cells.

The device is then validated for volume flow strength. In this validation, the test filter 53 is removed and replaced with a flow meter (not shown). The atomiser 50 is filled with 5 ml of sterile water and the collision sampler with 20 ml of sterile water. The control valve 52 to the atomizing device 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 should be 28 rpm, which is controlled through the critical opening of impact device 60 (impinger).

The atomizing device 50 is then activated by fully opening the associated control valve 52. Simultaneously, the air flow meter control valve 56 is partially closed to maintain an air flow of 28 1 / min through the device. The volume flow rate on the air flow meter 55 through the associated control of valve 56 is noted. The device is operated for 20 minutes to ensure that the airflow of 28 1 / min is maintained.

The device is then validated with respect to the recovery of the pseudonomas diminuta. To do this, the test filter 53 from Figure 7 is removed and replaced with a six-stage Anderson sampler (a six stage Anderson sampler). The glass Petri dish associated with the sampler is filled with tryptone soya agar in each step. The air to be sampled enters the inlet to the sampler (sampler) and then cascades through the successive opening steps with successively higher opening rates from step 1 to step 6. Successive smaller particles are initially bumped onto the surfaces of agar pools of each step.

Then, one ml of the approximately 1 x 108 / ml pseudomonas diminuta solution is diluted to 1 x 10 ml using sterile water. The nebulizer 50 is filled with 5 ml of this suspension.

While a check valve 56 is open and the check 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 rpm, controlled by the critical orifice impactor.

Then, the atomizing device 50 is activated by fully opening the associated check valve 52. Simultaneously, the control valve 51 is partially closed to the determined level as determined by the volume flow validity test. This provides spent air and maintains the air flow at 28 1 / min through the device. After a 15 minute test period, valve 52 is closed and valve 56 is fully opened. After another 30 sec to clean the aerosol system, the vacuum source is turned off.

The agar pool plates are then removed from the Anderson sampler and incubated at 30 ± 2 ° C. Colony forming units (CFUs) are counted after 24 and 48 hours.

The device is valid if pseudomonas diminuta are found on the Anderson sampler 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 fogging device is filled with 5 ml of the approximately 1 x 108 ml nseudonomas diminuta suspension.

Then the check valve 56 is opened and the check valve 52 closed. A vacuum is applied to the device and air is drawn into the device for 30 seconds. At 0.5 bar vacuum or higher, the air flow will be 28 rpm and controlled by the critical opening of the liquid sampling device 60. Then, the misting 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 1 / min.

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

The liquid remaining in the misting device 50 is then drained using a 5 ml injection needle. The volume remaining is measured using a 10 ml glass graduated cylinder and the volume is serially diluted tenfold with water seven times. Dilutions containing about 102 ml of bacteria are then filtered through 0.2 µm analytical membrane using steriles. The assay membranes are then placed on the tryptone soya agar plates incubated at 30 * 2eC. The cfu's 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 misting device test is then calculated.

The liquid in the impact sampling device (impingement sampler) is thus discharged. The volume is measured using a 20 ml glass graduated cylinder in this volume is serially diluted 3-fold 10-fold in sterile water. The remaining undiluted solution and the resulting dilutions are filtered through a 0.2 μ® analysis membrane using steriles. The analytical membranes are placed on tryptone soya agar plates and the opening of the impact tester is checked to ensure that it is not excluded.

The agar plates are then incubated at 30 ± 2®C. The cfu's are counted after 24 and 48 hours and the number of colonies on membranes showing 20 to 200 colonies are noted and the number of bacteria recovered downstream from the filter calculated. The efficiency of the device is calculated according to the formula;

where Re = the efficiency of the arrangement in percent,

Bt = the total number of bacteria recovered,

Tf = the final nebulized titer in cfu / ml,

Vn = the atomized volume.

The bacterial test on the filter is then calculated according to the formula;

where C = the total test, /

Vn and Re have the meanings given above, and Nt = the test titer of the nebulizer in cfu / ml. Then the decrease of the filtered titer is calculated from the formula;

where TR = the decrease in filter titer, and C and Bt have the meanings given above.

From this, the efficiency of the filter can be calculated in percent according to the formula:

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

The bacteria removal efficiency was tested by the aerosol test assay, as described above, with pseudonomas diminuta. Virus removal efficiency was tested by an aerosol challenge test of the kind described above with M52 bacterophage.

The results of the test are shown in Table 1.

] f]]

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 removal efficiency (2B) (namely 99.9977 efficiency # proposed by Lloyd and Roe as 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 similarly lower removal efficiency. However, the filter described above with reference to the drawings had their high removal efficiency and a relatively low water loss (less than 8.5 mg / l).

The filter described above with reference to Figure 1 and with the test results reported in Table 1 was also included in a ventilator 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 the water loss based on a 24 hour use is shown in Table 2. Under condition A, the tidal volume was 480 ml at 15 breaths per minute and at an exhalation temperature of 32.4 ± 1C. In condition B, the tidal volume was 780 ml at 15 breaths per minute and an exhalation temperature of 33.2 ± 1C.

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

This decrease in water loss is maintained over a wide range 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 1 / min to 12 1 / min, there is no significant variation in water loss (7 mg / 1 to 10 mg / 1 ) occurs at a 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 exhaled air has been filtered and where a problem arises involving the use of a filter to retain heat and moisture requires. For example, the air in an aircraft cabin is supplied through a filter and may have an unsuitable temperature and humidity. The use of a filter of the type described above can provide an air supply to an aircraft cabin that has the required temperature and humidity.

Claims (20)

A filter for exchanging heat and moisture, comprising a housing having a first part for connecting to a source of breathing gas and an exhalation line and a second part for connecting a person who inhales and exhales the gas, the housing comprising 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 closer to the first part in the flow path and the hydrophobic medium having an alcohol bubble point higher than 710 mm (28 inches) H20 for removing microorganisms. Filter according to claim 1, characterized in that the hydrophilic medium plate is in contact with the hydrophobic medium plate. Filter according to claim 2, characterized in that the plates are bonded together. Filter according to claim 2, characterized in that the plates are laminated together. Filter according to any one of claims 1 to 4, characterized in that the hydrophobic medium is made of resin-impregnated ceramic fibers. Filter according to any one of claims 1 to 5, characterized in that the hydrophilic medium is a cellulose material. Filter according to any one of claims 1 to 6, characterized in that the plates of filter media are pleated. 8. A filter according to claim 7, characterized in that the housing comprises a chamber bounded by a circumferential wall and two closures at respective opposite ends of the housing, one closure providing a part for connecting to the source of Respiratory gas and the exhalation line and the other termination include the one portion that is connected to the person who inhales and exhales the gas, the pleated plates filling the chamber such that the pleats on one side of the plates are adjacent to a portion and the folds on the other side of the plates are near the other part. Filter according to any one of claims 1 to 8, characterized in that the hydrophilic medium plate is continuous. Filter according to any one of claims 1 to 8, characterized in that the hydrophilic medium plate has a plurality of spaced parallel slots extending therethrough. Filter according to any one of claims 1 to 10, characterized in that the filter has a pressure drop not exceeding 3.0 cm H 2 O at an air flow of 60 l / min. The filter according to any one of claims 1 to 11, characterized in that the filter has an efficiency in removing aerosol bacteria measured by the aerosol test test of> 99,999 # · Filter according to any one of claims 1 to 12, characterized in that the filter has a water loss (as defined here) of between 7 mg / 1 and 10 mg / 1 over a range of minute volumes of 7 1 / min up to 10 1 / min at a temperature of 32eC. Ventilation circuit comprising a ventilator, a tube for connecting the ventilator to the first part of the filter according to any one of claims 1 to 14, an exhalation line leading away from the first part and a tube leading away from the second part of the filter to be used by a person who inhales and exhales gas from the ventilator. A method of manufacturing a filter for exchanging heat and moisture, connecting one face of a hydrophobic medium plate with an alcohol bubble point greater than 710 mm (28 inches) H 2 O and two opposing faces with one face of a hydrophilic medium plate with two opposed faces with one face of the hydrophilic medium. Method according to claim 15. characterized in that the joining comprises bonding. Method according to claim 15. characterized in that the joining comprises lamination. The method according to any one of claims 15 to 17, characterized in that the hydrophobic media are resin-impregnated ceramic fibers and the hydrophilic media is a cellulose material. Method according to any one of claims 15 to 18, characterized in that the filter has a pressure drop not greater than 3.0 cm H 2 O at an air flow of 60 1 / min. A method according to any one of claims 15 to 19, characterized in that it comprises bending the joined plates.