Dehumidifying device

Abstract: A dehumidifying device (8) is described for reducing the relative humidity of the expired air of a living creature. The dehumidifying device (8) comprises a plurality of tubes (9) , permeable to moisture, arrayed in a bundle and encircled by a loosely wound spiral harness between a first connection section (10) and a second connection section (11) in the dehumidifying device (8) . When gas expired by the living creature reaches the dehumidifying device (8), the water vapor diffuses into the tubes (9) , and the relative humidity of expired gas declines.

Description: The invention relates to a dehumidifying device in an expiratory line of a ventilator, connected upstream from an expiratory flow meter in the ventilator. The task of a ventilator, or a respirator, is to supply a patient with a breathing gas or anesthetic gas. This may occur in intensive care, in surgery when anesthesia is administered to the patient, to support spontaneous breathing or to impose a respiratory rate on a patient incapable of spontaneous respiration. The flow of expiratory gas is measured in the ventilator's expiratory section as a part of the ventilator's control function. Sometimes a bacteria filter is also connected to the expiratory section to prevent the patient's bacteria and virus from escaping into ambient air. Since gas from the patient is expired gas, its temperature is relatively high, about 30 to 35° C, and the gas is saturated with moisture, i.e. its relative humidity is 100%. This high humidity can cause major problems due to water condensation in the flow meter and bacteria filter. Condensation in the bacteria filter makes it harder for the patient to breathe, since condensate increases resistance in the expiratory section. One prior art flow meter is described in the Operating Manual for the Servo Ventilator 900 C, Siemens-Elema AB, publication no. AG 0888 12, August 1988, pp. 10:2-10:6. This flow meter contains a mesh through which expired air flows from the patient. If the relative humidity of the patient's expired air is too high when passing the mesh, condensation could form on the mesh, thereby disrupting flow meter function sufficiently to produce erroneous measurement of air flow and affect the whole ventilator's function. Another type of flow meter is described in Bulletin PP101, Gould Godart Fleish Flow Transducers, Gould Inc. This flow meter has a system of channels which the gas runs through. Determination of the pressure on both sides of the channel system provides a measure of the flow from the pressure gradient. At a high relative humidity, water could condense in the channel and obstruct same with an ensuing increase in the pressure gradient across the channel system, leading to an erroneous value for flow. When such a fault occurs, the flow meter must be removed from the ventilator, cleaned, dried, reinstalled and calibrated. Since calibration is particularly important and cannot be performed with a patient connected to the ventilator, one flow meter cannot merely be replaced with another. The entire ventilator must be replaced. This naturally creates needless, irritating extra work for staff. So avoiding this type of error is particularly desirable. A drop of approximately 20% is sufficient to guarantee flow meter operation. This may appear to be simple, and many different methods and designs have been tried over the years in attempts to solve the problem. One such design is described in the aforementioned Operating Manual for the Servo Ventilator 900C, page 1:3, and is based on heating the flow meter to about 60° C in order to reduce the relative humidity. As is well-known, the ability of air to hold water varies directly with its temperature. Another solution to the problem is described in the brochure Star Exhalation Isolation System, Operating Instructions, Siemens version, Infrasonic Inc., form no. 9910053, April 1988. A device is connected to the ventilator's expiratory inlet to heat expired air passing through the device; the device is also equipped with a water trap to collect condensed water. Yet another solution employs a cooling device for condensing water vapor in expired gas before the gas is fed into the ventilator's flow meter. The device is described in the article "Expired Gas Cooling Device" by J. Attwood and L. Bartel, of the U.S. However, the different methods and designs have the disadvantage of either being inadequate in reducing relative humidity sufficiently or being bulky, clumsy units which have to be connected to the ventilator or which are dependent on a reliable power supply. This means that greater reserve power capacity or batteries would be needed in the event of a power failure or when a portable ventilator is used. Additional equipment also increase the staff's work load when connecting the patient, and equipment must be monitored the whole time the patient is connected. The equipment's bulky design is also a problem, especially when patients have to be transported somewhere. So a simple, compact, efficient and energy-sparing dehumidifying device would be highly desirable. This is achieved according to the invention with a dehumidifying device, according to the initial remarks above, comprising a plurality of tubes permeable to moisture, preferably made of fluorsulfonyl polymer, and having a predetermined total contact surface exposed to ambient air, where by the humidity in expired gas passing through the tubes is reduced to a predetermined maximum level, preferably 80%. A number of materials permeable to moisture are prior art and have also been used for drying gases. One prior art device (Bullentin 104, Perma Pure Products Inc.) comprises a conduit containing a number of separate tubes made from hygroscopic material. The tubes are connected at each end of the conduit so a gas can traverse the conduit through the tubes. The conduit also has an opening at either end through which a dessicative gas can flow so water vapor is transferred to the dessicative gas. In this way, a completely dry gas can be obtained and fed to a moisture-sensitive equipment for analysis. These materials have also been used in the ventilator/respirator field. SE-A-428345 describes a procedure for drying part of a patient's inspired or expired gas for measurement of the concentration of different component gases. A small part of the patient's inspired or expired gas volume is diverted through a tube made of fluorsulfonyl polymer or some other material with corresponding properties. The tube is completely exposed to ambient air. If a long enough tube is used, the temperature and moisture content of diverted gas will equilibrate to the temperature and relative humidity of ambient air. This is sufficient for obtaining satisfactory measurement results, since measurement instruments can be calibrated for ambient temperature and relative humidity. However, the disadvantage of the latter device is that the room housing the patient must be air conditioned, when a ventilator/respirator has to be connected for a long period of time, as e.g. in intensive care, to ensure that measurements results are reliable. In addition, this system cannot be regarded as appropriate when patients on a ventilator/respirator must be transported, since ambient temperature and relative humidity can then vary considerably. Despite the fact that these materials have been prior art for a long time, even in the ventilator/respirator field, and despite the fact that the problem of condensation in the flow meter has been known even longer, no solution according to the present invention has ever been presented or proposed. To the contrary, professionals in the field, as the above has shown, have made major efforts to make external devices for heating expired gas or designed different kinds of water traps to collect condensed water after expired gas was cooled. Even from the technical point of view, great insight and creativity is needed to achieve this elegant solution to a problem which has been around for such a long time in the field. Since each individual tube has an internal diameter which is less than the internal diameter of the expiratory line, a more effective contact surface between gas and ambient air is obtained. Drying then takes place more rapidly, and tubes can be shorter without any impairment of results. An advantageous refinement of the dehumidifying device according to the invention is obtained by arraying the tubes in a bundle between the respective tube-to-expiratory line attachment areas. This makes it easier to design a compact dehumidifying device while providing the tubes with greater protection than when they are completely separated. Even though the tubes are arrayed in a bundle in which only the outermost tubes are directly exposed to ambient air, experiments have shown that even the innermost tubes in the bundle make a major contribution to the drying of ambient air. To further reduce the risk of damage to tubes by external force, it would be an advantage to surround the tubes with protection permitting ambient air to reach the tubes. Protection in the form of a loosely applied spiral harness which encircles the tubes between the respective tube-to-expiratory line attachment areas would be an advantage. The tubes would then be well-protected while still leaving the peripheral tubes exposed to ambient air. A mesh wound around the tubes, or rings threaded onto the tubes, are other conceivable forms of protection. In order to attain strong, leak-resistant and problem-free tube-to-expiratory line attachment areas, it would be an advantage if each attachment area included a disc with holes, a locking element with holes and a gasket with holes between the disc and the locking element, the tubes passing through the holes, the locking element fixed in a position compressing the gasket, the gasket then expanding in a radial direction and pressing against the metal sockets of the tubes. The compressed gasket keeps the tubes in a fixed position. Radial gasket expansion also creates a contact seal around each tube, thereby minimizing the risk of gas leakage. It would be an advantage if each tube had a socket, preferably made of non-rusting metal, at the gasket. The socket would prevent tube deformation when the gasket pressed against the tubes. Since the internal diameter of the metal sockets is the same as the internal diameter of the tubes, there is no narrowing which could affect gas flow. If the gasket is designed so the diameter of a crosssection through each hole declines towards the middle of the gasket, the gasket's contact with each tube would be more efficient and force more evenly distributed, since the rounding would provide a little additional space for the gasket's radial expansion. To prevent the dehumidifying device from constituting a stricture in the ventilator system, the device has been designed so the total cross-sectional area of the tubes is at least as large as the smallest cross-sectional area of the rest of the expiratory section. A further refinement of the dehumidifying device according to the invention is obtained by having the dehumidifying device removably attached to the expiratory line, the respective attachment area consisting of a first connector section and a second connector section. This makes the dehumidifying device compact and easy to replace, thereby facilitating device cleaning. In this context, it would be an advantage if all the components in the dehumidifying device were made of materials capable of withstanding autoclaving. An embodiment of a dehumidifyer device according to the invention is described in conjunction with the four figures, Fig. 1 showing a ventilator with a dehumidifying device connected to a patient; Fig. 2 showing the dehumidifying device; Fig. 3 showing an enlarged part of the dehumidifying device in cross-section, and Fig. 4 showing a cross-section of a gasket in the dehumidifying device. A patient 1 requiring a special mixture of breathing gas, or requiring mechanically assisted artificial ventilation, is usually connected to a ventilator 2 , only shown in part in Fig. 1. Breathing gas is fed from the ventilator 2 to the patient 1 through an inspiratory line 3 and then to the lungs through a breathing rube 4. An expiratory line 5 conducts expired gas from the patient 1 to the ventilator 2 . The flow of e.g. expired gas is measured in the ventilator 2 . The inspiratory line 3 has a first water trap 6 whose task is to collected any condensed water forming in the inspiratory line 3 . A second water trap 7 located in the expiratory line 5 has the same task. The risk of water condensation is much greater in the expiratory line 5 , since expired gas is saturated, i.e. it has a relative humidity of 100%, and is relatively warm, about 33° C. When expired gas is cooled in the expiratory line 5 by the lower ambient temperature, water condenses and runs down into the second water trap 7 . However, the relative humidity of expired gas remains unchanged, and if this expired gas is allowed to pass the flow meter in the ventilator 2 , the risk of disruption to flow meter operation is very considerable. So part of the expiratory line 5 consists of a dehumidifying device 8 which reduces the relative humidity of expired gas to a level harmless to the flow meter, i.e. a maximum relative humidity of about 75-80%. Figs. 2 and 3 show how the dehumidifying device 8 is designed with a plurality of tubes 9 , e.g. 37, made of a material permeable to moisture, such as fluorsulfonyl polymer. The tubes 9 are attached between a first connector section 10 and a second connector section 11 . The tubes 9 are about 300 mm long, but the length can be varied, depending on how large or small a reduction in the relative humidity of expired gas is desired. A 300 mm length is sufficient for expired gas to attain the same temperature and relative humidity as ambient air. The tubes 9 are arrayed in a bundle encircled by a loosely wrapped spiral harness 12 between the first connector section 10 and the second connector section 11 . The harness 12 has only been drawn at the respective connector sections 10, 11 in Fig. 2 to illustrate how the tubes 9 are bundled. The spiral harness 12 protects the tubes 9 from external force. However, it is still sufficiently loose to permit ambient air to come into contact with the peripheral tubes 9 . As shown in Fig. 3, the connector section 10 has a disk 13 against a boss 14 in the first connector section 10 . The disk 13 has holes 15 which the tubes 9 can pass through. A gasket 16 , also equipped with holes 17 for the tubes 9 , is adjacent to the disk 13 , and there is a locking element 19 , also equipped with holes 19 , next to the gasket 16 . After the tubes 9 are inserted into in the holes 15, 17, 19 , the locking element 18 is pressed against the disk 13 and then fixed in this position when an elevation 21 in the diameter of the locking element 18 is pressed against an elevation 20 on the inside of the first connector section 10 and then snapped in place. Since the locking element 18 has a spiral track 22 , the spiral harness 12 can be fixed in position at the first connector section 10 without any additional fittings. The second connector section 11 is devised in an identical manner. Compression of the gasket 16 causes it to expand radially, leading to a reduction in the size of the holes 17 in the gasket 16 , and the gasket 16 is pressed against the outer sides of the tubes 9, thereby fixing them radially in position while simultaneously sealing the first connector section 10 at the entry point for the tubes 9. To prevent deformation of the tubes 9 by pressure from the gasket 18 , a non-rusting metal socket 23 has been incorporated as a part of each tube 9 at the gasket 16 position. The metal socket 23 supports the tube 9 and improves sealing. Since the internal diameter of the metal socket 23 is identical to the inner diameter of the tube 9, there are no problems with strictures impeding air flow. As shown in Fig. 4 which shows the gasket 16 in an uncompressed state, the holes 17 in the gasket 16 are rounded in a cross-section through the gasket 16 so the diameter of the holes 17 declines from the upper and lower side respectively 24, 25, of the gasket towards the middle of the gasket 16 . This design facilitates compression of the gasket at the same time as the force against the tubes 9 is more even and more controlled than if the holes 17 had a straight cross-section.

Claims: A dehumidifying device (8) in an expiratory line (5) of a ventilator (2), connected upstream from an expiratory flow meter in the ventilator (2), characterized in that the dehumidifying device (8) comprises a plurality of tubes (9) made of a material which is permeable to moisture, preferably made of fluorsulfonyl polymer, and having a predetermined total contact surface exposed to ambient air, whereby moisture in a wet expiratory gas, entering the tubes (9) at one end, is transmitted through the contact surface into the ambient air, and the humidity of the gas leaving the tubes (9) at the other end, i.e. the flow meter end, is thereby reduced to a predetermined maximum relative humidity, preferably 80%. A dehumidifying device according to claim 1 , characterized in that every individual tube (9) has an internal diameter which is less than the internal diameter of the expiratory line (5) . A dehumidifying device according to claims 1 or 2 characterized in that the tubes (9) are arrayed in a bundle between the respective attachment areas (10, 11) for the tube (9) and the expiratory line (5) . A dehumidifying device according to any of the above claims characterized in that the tubes (9) are surrounded by protection permitting ambient air to reach the tubes (9) . A dehumidifying device according to claim 4 characterized in that the protection consists of a spiral harness (12) which loosely encircles the tubes (9) between the respective attachment areas (10, 11) for the tubes (9) and the expiratory line (5) . A dehumidifying device according to any of the above claims characterized in that the respective attachment areas includes a disc with holes (13) , a locking element with holes (18) and a gasket with holes (16) between the disc (13) and the locking element (18) , in that the tubes (9) passes through the holes (15, 17, 19), and in that the locking element (18) is fixed in a position compressing the gasket (16) the gasket (16) then expanding in a radial direction and pressing against the metal sockets (23) for the tubes (9). A dehumidifying device according to claim 6 characterized in that each tube (9) is equipped with a socket (23) , primarily of non-rusting metal, at the gasket (16) . A dehumidifying device according to claim 7 characterized in that the internal diameter of the sockets (23) is the same as the internal diameter of the tubes (9) . A dehumidifying device according to one of claims 6, 7 or 8 characterized in that the gasket (16) is designed so that at every hole (17) the diameter of the cross-section of the hole (17) declines towards the middle of the gasket (16). A dehumidifying device according to any of the above claims characterized in that the total cross-sectional area of the tubes (9) is at least as large as the smallest cross-sectional area of the rest of the expiratory section. A dehumidifying device according to any of the above claims characterized in that the dehumidifying device (8) is removably attached to the expiratory line (5) , the respective attachment area (10, 11) consisting of a first connector section (10) and a second connector section (11). A dehumidifying device according to claim 11 characterized in that all the components in the dehumidifying device (8) are made of materials capable of withstanding autoclaving.

Inventors: PSAROS GEORGIOS, PSAROS, GEORGIOS
Applicants: SIEMENS AG, SIEMENS ELEMA AB, SIEMENS AKTIENGESELLSCHAFT
Application year: 1992
Main classification: A61M16
IPC codes: A61M16, A61B5, A61B16, B01D53, B01D71