Abstract:
An improved medical conduit including a heating element for controlling the temperature and humidity of a gas to be delivered to a recipient patient is disclosed. A flexible ribbon spanning the width of a flexible tubing and extending generally the length of the tubing is provided, where the flexible ribbon carries a heating element therein. The heating element, preferably an electrically conductive wire or plurality of wires, is connected to a power supply in order to heat the flow of gas traveling within the tube. The flow is heated as it passes over and around the heating element. The flexible ribbon supporting the heating element can be integral with the tubing or comprise an insertable unit which fits into the tubing.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is related generally to temperature and/or humidity control in a conduit for communicating a gas such as air or oxygen to a medical patient, and more particularly to a tubing with an internal heating element to control the temperature and humidity of the gas therein prior to delivery to a patient. 
     2. Description of Related Art 
     Systems are currently in use to aid in the delivery of air or other gases to patients who cannot breathe on their own for various reasons. The human upper airway is a location where inspired air is heated and humidified before passing through the trachea to the lungs. Patients who cannot breathe on their own require the assistance of a ventilator. Air delivered from a ventilator is typically dry, coming from a compressor or liquid oxygen source. When this air is delivered to a patient whose upper airway is bypassed, the mucous layer and sensitive tissues of the lower airway is dried out and an additional metabolic demand is required of the patient to warm the air to body temperature. Patients on long term ventilation without humidification develop mucous plugs in the lower airway, requiring interventional suctioning and may develop damage to the lower airway tissues which may exacerbate drying of the airway. Increased metabolic demand places additional strain on all physiologic systems and threatens patient outcome. Consequently, ventilatory air is often heated and humidified prior to being delivered to patients on ventilatory support where the upper airway is bypassed with an endotracheal or tracheostomy tube. 
     Conventional ventilator air humidification consists of warming water to an elevated temperature and passing an inspired air flow over or through the warm water to pick up the available humidity and equilibrate the resultant water vapor/air mixture to the humidifier temperature. The inspired air is conducted to the patient through a ventilator circuit hose or tube, typically a corrugated polyethylene hose, which is in communication with the endotracheal or tracheostomy tube. The interconnecting hose is designed to provide a low resistance, lightweight, highly flexible flow channel such that inspiration does not require excessive effort. The hose is adapted to mate with a mask or endotracheal or tracheostomy tube where the air can be delivered to the patient. 
     Since the ventilator circuit hose typically has a relatively thin wall and a large surface area due to the corrugations, significant heat transfer occurs between the heated flow and the relatively cooler ambient air surrounding the hose. As the heated air travels along the hose, some heat is lost to the air outside of the hose resulting in a temperature drop in the air flow. The drop in temperature in the humidified air flowing through the hose reduces the capacity of the air to suspend the moisture, causing some of the moisture to precipitate out along the length of the tube. This reduction in the humidity of the airstream as the air cools leads to a collection of water in the corrugations of the tube. The condensed water poses a particular hazard to the patient if it accumulates in sufficient quantity to trickle into the endotracheal or tracheostomy tube, where it may be introduced into the lungs. In addition, the water collecting in the warm hose forms a fertile breeding ground for bacteria which can cause airway infections. Lastly, the drop in temperature creates an uncertainty in the temperature of the air being delivered to the patient in addition to the temperature of the air leaving the ventilator. 
     To reduce the precipitation of water and maintain a suitable air temperature, electrically heated hoses have been used to add additional heat to the flowing air, counteracting the heat lost along the length of the hose. Conventional electrically heated hoses or tubing employ a heating element, in the form of a solid or stranded resistance wire, that is either embedded in the wall or wound around the circumference of the hose. In some cases, the resistance wire is spirally wrapped around a supporting thread before it is wrapped around the hose. These hoses apply heat at the walls, which is communicated to the fluid passing within the hose by convection. Alternatively, the heating element may be loosely strung within the lumen of the hose. In this case, heat is conducted to the fluid passing within the lumen from the heating element through insulation placed over the heating element. 
     A problem particular to spirally wound heater wire elements is the formation of localized hot spots from variations in power density. The variation in power density is caused by inconsistency of the spiral pitch over a short section of the element. The winding pitch seems to be particularly difficult for manufacturers of this element to maintain and necessitates specialized testing and equipment to detect in a high speed extrusion operation. This localized hot spot can melt through the hose wall and pose a fire threat. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is a first object of the present invention to provide a heater for a flexible tubing which overcomes the drawbacks of the prior art and improves the delivery of gases at a prescribed temperature and humidity. 
     It is another object of the present invention to provide a heater which reduces the occurrence of tubing failure due to burn-through. 
     It is another object of the present invention to provide a heater which is adaptable to currently gas delivery systems. 
     It is yet another object of the present invention to provide a heater which heats gases more efficiently than prior heating systems. 
     The objects of the present invention are achieved using a flexible tubing having an elongate flexible plastic ribbon disposed inside the tubing generally along a center region and extending predominantly the length of the tubing, and a heating element carried by the ribbon to heat the air. A preferred embodiment of the present invention uses a temperature gage at the point of delivery which is used to adjust the heater to maintain the temperature of heated and humidified air at the point of delivery to the patient. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The objects and features of the present invention, which are believed to be novel, are set forth with particularity in the appended claims. The present invention, both as to its organization and manner of operation, together with further objects and advantages, may best be understood by reference to the following description, taken in connection with the accompanying drawings. 
     FIG. 1 is a perspective view of a humidifying system including humidified air generation for use with a preferred embodiment of the present invention. 
     FIG. 2 is a side view, partially cut-away, of an endotracheal tube connected to the heated tubing; 
     FIG. 3 is an elevated perspective view, partially cut away of a preferred embodiment of the present invention; 
     FIG. 4 is a perspective cross-sectional view taken along the indicated line in FIG. 3; and 
     FIG. 5 is a perspective, cross-sectional view of a flexible ribbon of the present invention illustrating a heating element therein. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description is provided to enable any person skilled in the art to make and use the invention and sets forth the best modes contemplated by the inventors of carrying out their invention. Various modifications, however, will remain readily apparent to those skilled in the art, since the general principles of the present invention have been defined herein specifically to provide a heater for use with flexible tubing especially suited for delivering heated, humidified air to a patient. 
     An embodiment of a system for delivering heated air using the present invention is illustrated generally in FIG. 1. A flexible corrugated tubing  20  made of polyethylene or other suitable material provides a channel for humidified air, which is delivered to a patient. The flexible tubing  20  is connected at a tubular adapter  23  to an air humidifying machine  102  while a second end  24  is connected at a “Y” adapter  25  to an endotracheal tube (see FIG. 2) or tracheostomy tube, or in some cases, a mask to be placed over the patient&#39;s mouth. The air humidifying machine  102  heats and moisturizes air to conditions which are suitable for the patient to accept. 
     Referring to FIG. 1, a water supply reservoir  104  is connected to a tube  106  which feeds water to a heated platen (not shown) within the air humidifying machine  102 . The heated platen is housed in a chamber  112 , and the platen elevates the water temperature to liberate a sufficient amount of vapor into the chamber  112 . An air supply hose  100  connected to a ventilator (not shown) delivers the air (or other gas to be delivered) into the chamber  112  where it encounters the water vapor. The air coming into the chamber  112  mixes with the water vapor and is heated by the presence of the water vapor to a temperature which is controlled by the temperature of the platen. The air leaves the chamber  112  heated and “humidified” at exit port  114  and connects to the adapter  22  of the tubing  120 . 
     At the entrance of tubing  20  is a connector  26  (see FIG. 3) which mates to an interconnecting connector  122  and electrical cable  116  from the controller/monitor of the humidifying machine  102  to receive an electrical voltage from the controller/monitor. The connector  26  is tethered by a strip  30  that extends from or is mounted to a port  32 . The connector  26  is electrically connected to a heating element  42  supported by or within a ribbon  34 . As described more fully below, the flexible ribbon  34  supports the heating element  42  which is used to control the temperature and humidity inside the tubing  20 . 
     The tubing  20  receives the humidified air from the chamber  112  and the air flows along the length of the tubing. The tubing  20  contains within its interior the plastic ribbon  34  which is flexible enough to bend with the tubing  20 , but rigid enough to maintain its flat, solid profile upon bending. The flexible ribbon  34  has a profile (see FIG. 4) which extends up to the inner wall  38  of the tubing  20  with enough tolerance to allow the elongated flexible ribbon  34  to be slid in and out of the tubing  20 , but maintains the flexible ribbon  34  centered within the tubing  20 . Other profiles are possible for the flexible ribbon  34 , such as triangular profile with three spokes, but the planar profile as shown in FIG. 2 is generally preferred because it provides better flexibility than more complicated profiles. The flexible ribbon  34  is preferably made of a plastic material with a relatively high melting point, the preferred plastic material being polyethylene similar in formulation to the composition of the hose material. 
     The flexible ribbon carries a heating element  42  near the center of its profile which is used to heat the flow traveling in the tubing  20 . The heating element  42  is preferably in the form of a solid resistance wire carried on or in the flexible ribbon  34 . The heating element  42  may be located on top of the flexible ribbon  34 , or in a preferred embodiment the heating element  42  is extruded in the flexible ribbon (FIG. 4) as long as the ribbon is thin enough to conduct the heat generated by the heating element to the flow. A single wire  42   a  may be used to traverse the length of the flexible ribbon  34 , or a plurality of wires  42   b ,  42   c ,  42   d  may be used (see FIG.  5 ). Wires  42  or other electrically conductive elements supported by the ribbon  34  can be connected in a series or in parallel, or combination of series and parallel connections in order to make up a desired total heater circuit resistance. The heating element or elements  42  preferably start and terminate at a single end to form a loop or loops along the flexible ribbon  34  such that a power source can be connected at the single end. 
     As described above, the heating element  42  is connected to an electrical power supply within the air humidifying machine  102  via cable  116  (see FIG.  1 ). Voltage is applied creating an electrical current in the heating element, which in turn generates heat due to the resistance of the wire. The heat energy generated by the heating element  42  is transferred to the humidified air traveling along the tubing  20 . If more voltage is applied to the heating element  42 , then more heat energy will be supplied to the load, and the amount of heat can be adjusted to maintain the proper temperature of the air at the exit  24 . Having the heating element  42  extend the length of the tubing permits heating of the air right up to delivery to the patient and reduces the likelihood of condensation of the humidified air. To further control the temperature of the air being delivered to the patient, a temperature probe  118  may be placed at the Y connector  25  at the exit  24  of the tubing  20  to measure the exit temperature of the air delivered to the patient. The temperature is communicated via cable  124  to the controller/monitor within the humidifying machine  102 , which then adjusts the power supplied to the heating element  42  or heated platen in order to maintain the desired exit temperature. 
     The tubing terminates at a “Y” juncture  25  or alternately a “T” joint, which is preferably plastic, and the Y juncture is connected to a delivery apparatus such as an endotracheal tube  130  as shown in FIG.  2 . Other delivery systems are possible, such as breathing masks and tracheostomy tubes, and operation of each of these delivery systems are well known in the art. 
     The air being expelled by the patient is carried away by a tubing  21  which may also include a heating element similar to tubing  20 . This is because the air taken away from patient is humidified, and the same risks of condensation such as bacteria build-up and maintenance are present in the evacuation side of the tubing. 
     The present invention is advantageous over the prior art loose heating element in several ways. Pseudo-laminar air flow exists within the hose where the majority of flow occurs close to the center of the hose cross section. A velocity profile of the flow in the tubing  20  would also show a maximum velocity along the center region of the tubing, decreasing radially to a minimum at the tubing wall  38 . The location of the heating element  42  in the center of the air flow assures that the heat is being conducted more efficiently to the air flowing through the lumen. The efficiency of the heat exchange by the heating element  42  to the air flow is enhanced by promoting heat exchange where most air travels. Without a support for the heating element, such as the flexible ribbon of the present invention, the position of the heating element cannot be guaranteed to lie at the center of the tubing and insufficient heat transfer occurs where the heater lies close to or against the internal hose wall. This results in more heat energy transferred to the tubing wall than the air and localized condensation may occur. Additionally, the location where the heating element touches the wall becomes a site of dangerous melt through and possible ignition. 
     In the present invention, however, the heating element  42  is held away from the ventilator circuit hose wall  38  and near the center of the air flow by the supporting ribbon  34 . The temperature of a high dissipation location at a high air flow location is minimized because heat is spread across the supporting ribbon by conduction and convected away by the surrounding air. The possibility for melting the hose or heater insulation is therefore minimized by supporting the heating element  42  in the center of the air stream where the dissipation is spread and transfer of energy is maximized. 
     In an alternate embodiment, the heating element may be resistance wire that is spiral wound over a supporting element and extruded in a similar plastic support structure. Construction and electrical connections are similar to the previous embodiment. It should also be recognized that the heat transfer and electrical connections can be realized with resistance elements in the form of conductive bands, etched foils in straight or serpentine patterns, or conductive films applied or embedded in the ribbon. The ribbon can be removably disposed in the tubing, fixedly disposed or integrally formed with the tubing. Additionally, the ribbon can be mechanically extruded with the tubing as an integral unit. 
     Those skilled in the art will appreciate that various adaptations and modifications of the just-described preferred embodiment can be configured without departing from the scope and spirit of the invention. Therefore, it is to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described herein.