Conduit with heated wick

Abstract: A conduit for a breathing circuit includes a heater (110) associated, at least in part, with a portion of hydrophilic material (108). The purpose of the heater is to evaporate any condensed liquid collecting in the conduit, which is first sucked up by the hydrophilic material (108). The heated wick (100) reduces the risk of collected water being passed to the patient and causing choking fits or discomfit. It is preferred that the heated wick (100) lies freely in the conduit to settle at low points in the conduit where condensation may collect.

Description: BACKGROUND TO THE INVENTION Field of the Invention The present invention relates to conduits and in particular to conduits for use in a breathing circuit. Summary of the Prior Art In assisted breathing, particularly in medical applications, gases having high levels of relative humidity are supplied and returned through conduits of a relatively restricted size. Build up of condensation on the inside wall of the conduit is a frequent result of this high humidity. In the prior art, attempts have been made to reduce the adverse effect of this condensation by either reducing the level of condensation or providing collection points in the conduit for draining condensed liquid from the conduit. Reducing the condensation has generally been by maintaining or elevating the temperature of the gases flow and/or of the conduit wall to reduce the formation of condensation. SUMMARY OF THE INVENTION It is an object of the present invention to provide a conduit, which will at least go some way towards improving on the above or which will at least provide the public and the medical profession with a useful choice. In a first aspect the invention consists in a conduit as defined in claim 1. A conduit according to the preamble of claim 1 is disclosed in any of documents US-A-5 411 474 or US-A-6 090 998. In a second aspect the invention consists in a breathing circuit as defined in claim 5. A breathing circuit according to the preamble of claim 5 is disclosed in document US-A-5 172 686. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a cross sectional elevation of a conduit for the expiratory limb of a breathing circuit. Figure 2 is a cross sectional view of a section of conduit wall according to one possible construction. Figure 3 is a cross sectional view of a co extrusion die head for extruding a conduit including two longitudinal strips of permeable material, similar to the conduit of Figure 1. Figure 4 is a cross sectional elevation of a coaxial breathing circuit incorporating a heated wick in the expiratory gases flow path. Figure 5 is a side elevation in partial cross section of a coaxial breathing circuit including a heated wick in both the inspiratory and expiratory gases flow paths. Figure 6 is a representation of a breathing circuit with an expiratory limb fashioned and including a heated wick. Figure 7 is a cut-away perspective view of a heated wick. Figure 8 is a side elevation partially in cross section of an expiratory limb conduit. DETAILED DESCRIPTION A preferred embodiment involves the provision of a heated wick within one of the lengths of conduit making up a breathing circuit. By heated wick we refer to a heater associated with a portion of hydrophilic material. The heated wick is disposed freely within the conduit so that at least part of it lays in low points of the conduit at which condensation accumulates. Accumulated condensation is absorbed by the hydrophilic material and re-evaporated by heat from the heater. As seen in Figures 1A and 7 the heated wick 100 is comprised of an outer hydrophilic material 108 covering an inner insulating hydrophobic layer 112 which in turn covers a heater element 110. Any water that collects in the conduit 102 is attracted to and drawn into the hydrophilic material 108, and is then re-vaporised as it is heated by the heater element 110. The intermediate hydrophobic insulating layer 112 is provided to electrically insulate the inner heater element 110 from the rest of the system. Such a heated wick 100 as shown in Figure 7 may be constructed by co-extruding the hydrophobic insulating layer 112 and hydrophilic layer 108 onto the heater wire 110. Suitable materials for the hydrophilic layer include polyester or polyurethane foam, or a braid of hydrophilic material e.g cotton. Suitable materials for the hydrophobic layer include polypropylene or silicone coatings. An alternate form for the heated wick is shown in each of Figures 1b, 1c and 1d. In Figure 1b the heated wick includes a looped back heater element 110, coated in a hydrophobic insulating layer 112, and the whole encased within a hydrophilic surrounding layer 108. In a further variation depicted in Figure 1c the heater element is an electrical resistance heater and includes a length 120 of higher resistance and a length 121 of lower resistance, insulated from one another and joined at their remote ends. In a still further variation depicted in Figure 1d the heated wick 100 is disposed in the conduit as a simple loop. Each of these variations provides both ends of the heated wick at the same end of the conduit, allowing a single connection of the heater element to an energising source. The embodiment of Figure 1c has the additional advantage that the heater element voltage at the remote end will be lower than half the supply voltage, and with appropriate selection can be very close to zero. The heated wick may also be provided in both the inspiratory and expiratory conduits. In this case a single length of heated wick may run down the inspiratory conduit and back up the expiratory conduit, with the ends ofthe conduits being insufficiently close proximity to enable easy electrical connection to both ends. The heated wick is provided with connections at its ends for connecting to an energising source. The ends of the wick maybe directly electrically connected to electrical connectors in the connector of the tube or conduit. These connectors may for example be a socket for receiving a plug from a voltage source. Alternatively the heated wick may be a fixture of an assisted breathing device, such as a ventilator or humidifier, and may extend from within the breathing conduit connection port of the device, or be plugged into a socket within such port. Many other configurations for supplying power to the heated wick will also suggest themselves. The heater element 110 is also effective to supply heat to the gases stream to reduce the overall level of condensation occurring within the conduit. At the same time any condensation that does occur is sucked up by the wick and re-evaporated by heat from the heater element 110. Accordingly where a heated wick is provided in the inspiratory arm of the breathing circuit humidity supplied to the gases stream prior to entry into the breathing circuit is not lost through condensation, instead being re-evaporated by the heated wick. This reduces the total humidification load of the breathing circuit as well as eliminating the requirement for conduit drainage ports. Where the heated wick is provided in the expiratory conduit it eliminates the need for conduit drainage ports. Furthermore it provides additional advantages when used in conjunction with an expiratory conduit in which at least a part of the conduit wall is formed from a breathable material. Such an arrangement is shown in Figure 6. A breathable material, as used herein, is a material that allows the passage of water vapour without allowing the passage of liquid water or respiratory gases. Materials may be breathable due to their composition, physical structure a combination thereof. One such breathable material is an activated perfluorinated polymer material having extreme hydrophilic properties. An example of this polymer material is marketed under the trade mark NAFION by DuPont Fluoro products of Fayetteville USA. This material is useful due to its extreme hydrophilic properties and due to its ability to be extruded, particularly to be co-extruded in combination with other plastic materials. Alternative materials are also envisaged including: (a) Hydrophilic thermoplastics, (b) woven treated fabric products exhibiting breathable characteristics The preferred material is a hydrophilic polyester block copolymer formed into a homogeneous flat film. An example of such a film is sold under the brand SYMPATEX. This material is particularly suited to thin film productions. An example of application of the conduit with heated wick is shown in Figure 6. A heater element 110 coated with a hydrophilic material, runs the length of the semipermeable conduit 102 and the inspiratory conduit 101. During operation humidified gases are drawn through inspiratory conduit 101, then flow through the T connector 103, and are then delivered to the patient (not shown). When the patient expires the gases flow through the T connector 103, and then flow through the breathable expiratory conduit 102. The expiratory gases will be almost saturated with humidity and as the wall of the breathable expiratory conduit 102 will be relatively cool, some portion of the vapour in the gases will condense and therefore water will collect in the conduit and run towards the lowest point 106. As already mentioned such collection of water is undesirable and therefore the heated wick 100 is provided to revaporise the water that collects. This is particularly important where the breathable material is one, such as SYMPATEX, which transmits water vapour but does not transmit liquid water. While such materials are advantageous for their ability to stop harmful bacteria and viruses this advantage is offset by their inability to transmit liquid water. By re-evaporation of any collected water by the heated wick it can be transmitted through the breathable membrane in its vapour state. Referring to Figure 1, in one embodiment, the conduit 4 ofthe expiratory limb of a breathing circuit is formed having one ormore longitudinal strips 2,3 of semi permeable membrane as part of the wall 1 thereof. Referring to Figure 8 an alternative embodiment of the expiratory limb conduit is shown in which the entire flexible wall membrane of the conduit is formed from a breathable plastic membrane, extruded and wound helically with edges of adjacent turns sealed to one another. Referring to Figures 4 and 5, further aspects are shown in which an exp iratory limb conduit according to the present invention is provided as a gases flow path of a coaxial conduit configuration, such that expiratory gases and inspiratory gases each flow in one of the inner conduit or the space between the inner conduit and the outer conduit and in use water vapourbut not liquid water is transmitted from the expiratory gases passageway to the inspiratory gases passageway. Referring to Figures 2 & 8, spiral or helical internal (or external) reinforcing members 30, or a series of annular hoop reinforcing members, may be provided outside (or inside) the tubular membrane 6 to provide support to it. The helical, spiral or hoop supporting members may for example be formed from polymer plastic materials, such as the material used in the wall of the conduit (not being the semi permeable regions), or alternatively may for example be a metal wire support, such as drawn steel wire. The conduit shown in Figure 2 may be formed in any one of a number of methods. For example the tubular membrane 6 may be supplied in a continuous tube. Alternatively it might be supplied in tape form, which may result in the conduit of Figure 8. Supplied as extruded tape 81, the membrane may be wound helically onto a former. The helical supporting rib 30, provided in a semi molten state is then laid on the overlap between adjacent turns. The heat from the helical supporting rib 30 bonds the two adjacent strips with itself forming a flexible resilient conduit once cooled. Referring to Figure 8 an additional longitudinal reinforcement may be provided to alleviate the shortcomings of some of the breathable materials. This reinforcement may be in the form of a plurality of reinforcing threads 83. The threads 83 run parallel to the length of the conduit and are supported on the helical reinforcing ribs, spanning between them. As many threads may be provided. For example eight threads may be spaced around the circumference of the tube. The reinforcing threads 83stop accidental stretching of the conduit, and providing they have some stiffness and the rib spacing is not to large, also reduce any longitudinal compression of the conduit under negative relative internal pressures. Referring to Figure 3 the conduit, such as that shown in Figure 1, may alternatively be formed by co extrusion of the semi permeable material (where the material is a suitable extrudable material) with a plastic material forming the remainder of the conduit wall. A suitable co extrusion die 9 is depicted in Figure 3 in which a pair of circumferential sections 7 of the die opening have the semi permeable plastic material extruded therethrough, and the remainder sections 8 of the annular extrusion opening have the non permeable plastic wall material extruded therethrough. The purpose of the breathable region or regions of the conduit wall is to allow diffusion of water vapour (and for some materials liquid water) from the expiratory limb of the breathing circuit along the path thereof independent of specific drain locations. This eliminates the build up of condensation within the expiratory limb by drying the humidified gases during their flow through the expiratory limb. This furthermore reduces the humidity of the gases arriving at ancillary equipment, such as filters, ventilators and the like reducing the risk of condensation accumulation, thereby improving their operation. In accordance with a further aspect of the invention, and as exemplified in Figures 4 and 5 the conduit incorporating one or more longitudinal strips of semi permeable membrane may further be incorporated in a coaxial breathing circuit as a passive humidification device. In particular referring to the cross section in Figure 4 the coaxial breathing circuit may include an outer conduit 11 and an inner conduit 10. Preferably, for heat transfer reasons, the inner conduit 10 carries the inspiratory flow in the space 12 there within. The expiratory flow is carried in the space 13 between the inner conduit 10 and the outer conduit 11, and a doubled back heated wick 100 is provided in the expiratory flow space. The airflow configuration is indicated by arrows 20, 19 respectively in Figure 5. The inner conduit 10 is formed having one or more longitudinal strips 2,3 of semi permeable membrane in the wall 1 thereof, as has previously been described with reference to Figures 1, 2 and 3. Thus humidity in the expiratory flow space 13 may pass through the sections 2, 3 of semi permeable membrane to humidify the inspiratory flow in inspiratory flow space 12. The semi permeable membrane works on relative partial pressures of water vapour so, with the flows in a counter flow arrangement substantial passive humidification of the inspiratory flow can be achieved. Referring to Figure 5 a circuit configuration including the coaxial conduit depicted in Figure 4 is represented, but with a heated wick 100 disposed in both of the inspiratory and expiratory flow paths (for example doubling back at the patient end connector 15). In this circuit the conduit has a patient end connector 15 and a ventilator end connector 16 having inspiratory port 17 and an expiratory port 18. The inspiratory 20 and expiratory 19 counter flows are indicated. So in its broadest form the invention is a conduit for a breathing circuit which includes a heater associated, at least in part with a portion of hydrophilic material. The purpose of the heater is to evaporate any condensed liquid collecting in the conduit. The heated wick is not a humidifier and so no liquid is supplied directly to the hydrophilic material from outside said conduit. The heated wick reduces the risk of collected water being passed to the patient and causing choking fits or discomfit. It also improves the predictability of the humidity levels in the gases passed to the patient. It is preferred that the heated wick lies freely in the conduit to settle at low points in the conduit where condensation may collect. Where the conduit in question is an expiratory conduit, or at least where the heated wick is located in an expiratory flow path of a breathing circuit, then the heated wick will have additional benefits where the conduit has at least of potion of its wall formed from breathable material for passive dehumidification of the expired gases. Because the breathable material will pass only vapour, evaporation of any condensed liquid within the conduit will allow that liquid to subsequently be passed. Another aspect to the invention is the construction of the heated wick, which is preferably an elongate heating element covered with an inner hydrophobic insulating layer co-extruded with an outer hydrophilic layer. It will be appreciated that the concepts encapsulated by the described and illustrated embodiments are not restricted to being combined only as described. For example the heated wick described with reference to Figures 6 and 7 may be used in the coaxial conduit of Figures 4 and 5 or the separate limbed conduit as in Figure 6. Similarly the conduit incorporating the breathable membrane, whether it be the inner conduit of the coaxial configuration shown in Figures 4 and 5 or the stand alone expiratory limb of figure 6, may be formed as a co-extrusion as in Figures 1 and 3 or as an extruded tape as in Figure 8 and with the breathable membrane being of a number of alternate materials. While some embodiments have been described as preferred and convey particular advantages over other embodiments many other combinations may prove commercially useful.

Claims: A conduit (102) for a breathing circuit including heating means (100) located within said conduit (102), said heating means (100) comprising an elongate heating element (110) characterised in that said heating element (110) of said heating means (100) is covered with an inner hydrophobic insulating layer (112) and said inner hydrophobic insulating layer (112) is at least partially covered with an outer hydrophilic layer (108), there being no means for direct supply of water or fluid to said hydrophilic material (108) from outside said conduit (102). A conduit as claimed in claim 1, further characterised in that said heating means (100) lies, freely in said conduit (102) to settle over at least some of its length at low points (106) in said conduit (102) where condensed water vapour may collect. A conduit as claimed in claim 1 or 2, further characterised in that said conduit (102) is an expiratory conduit and said heating means (100) is located in an expiratory flow path of said conduit (102) and at least a length of said conduit (102) has a conduit wall (1) wherein at least a region of said conduit wall (1) is of a breathable material. A conduit as claimed in claim 3, further characterised in that said at least a region is or are distributed over said length of said conduit (102). A breathing circuit including an expiratory gases flow path and a heating means (100) located within said expiratory gases flow path, said heating means (100) comprising an elongate heating element (110) characterised in that said heating element (110) of said heating means (100) is covered with an inner hydrophobic insulating layer (112) and said inner hydrophobic insulating layer (112) is at least partially covered with an outer hydrophilic layer (108). A breathing circuit as claimed in claim 5, further characterised in that said heating means (100) lies freely in said expiratory flow path to settle over at least some of its length over low points (106) in said expiratory flow path where condensed water vapour A breathing circuit as claimed in any one of claims 5 or 6, further characterised in that said expiratory gases flow path is defined by at least one conduit wall (1) and at least a region of said conduit wall (1) is of a breathable material. A breathing circuit as claimed in any one of clams 5 to 7, further characterised in that said breathing circuit is a coaxial breathing circuit including an inner conduit (10) and an outer conduit (11), the inner conduit (10) located within the outer conduit (18), such that one of an inspiratory or expiratory flow path are provided within the inner conduit (10) and the other of the inspiratory or expiratory flow paths is provided between the inner conduit (10) and the outer conduit (11) and at least a region of the wall of said inner conduit is of a breathable material.

Inventors: BALDWIN DAVID PETER, SMITH DANIEL JOHN, BALDWIN, DAVID PETER, SMITH, DANIEL JOHN
Applicants: FISHER & PAYKEL HEALTHCARE LTD, FISHER & PAYKEL HEALTHCARE LIMITED
Application year: 2001
Main classification: A61M16
IPC codes: A61M16, A61M25