Patent Abstract:
This invention relates to respiratory humidifiers and heated breathing conduits used to couple a patient to the humidifier. A conduit overheating detection system for a conduit having heating wire or element is disclosed. The overheating detection system may be utilized with a in a single limb conduit system or dual limb system. In each of these systems the conduit overheating detection system monitors the current in the heating element(s) and alters the power to the heating element(s) to prevent the occurrence the heating element(s) and/or conduit from overheating.

Full Description:
FIELD OF INVENTION 
   This invention relates to respiratory humidifiers and heated breathing conduits used to couple a patient to the humidifier. A conduit overheating detection system for the conduit heating wire or element is disclosed. 
   SUMMARY OF THE PRIOR ART 
   In order to supply gases to a patient or a person needing such gases, it may sometimes be necessary to first humidify those gases, for example using a respiratory humidifier/ventilator system. In such a case where the gases have been humidified, and therefore laden with water, it is likely that during transport through a conduit to the patient, condensation of that water vapour will occur. In order to overcome this disadvantage it is known to associate a heating wire or element with respiratory humidifier breathing conduits to avoid condensation. Examples of such a heated breathing conduit are disclosed in U.S. Pat. No. 5,537,996 (McPhee) and U.S. Pat. No. 5,392,770 (Clawson et al.). 
   In parts of conduit that contains a heating wire or element, where a temperature probe is incorporated, it is possible to monitor the conduit temperature directly and detect any over-heating. This over-heating may occur under no-flow circumstances, or if excessive insulation such as a blanket is applied to the conduit. In parts of conduit where (to reduce bulkiness, complexity and cost) no temperature probe is incorporated, safety of the equipment or patient may be compromised. This is due to the increased possibility of the conduit material over-heating and melting if no alternative method of monitoring the conduit temperature is implemented. Furthermore, with no sensor in the conduit, the possibility that the patient will receive high temperature gases is increased. 
   In respiratory apparatus where a dual limb breathing circuit is used, often only one of the limbs is controlled, while the other simply follows or acts as a “slave” to the controlled limb. Therefore, with no monitoring or control of the “slave” limb, if this limb was disconnected from flow, blocked or covered it could overheat or melt without a user being aware. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a conduit overheating detection system for a respiratory conduit heating element, which goes some way towards overcoming the abovementioned disadvantages. 
   Accordingly, in a first aspect, the present invention consists in a conduit overheating detection system for a respiratory conduit including a heating element comprising: 
   detecting means which includes means to detect the current in said heating element, and 
   control means, including power supply means, which implements an algorithm that causes the control means to: 
   i) receive input of said current in said heating element from said detecting means, and 
   ii) if said current is outside a safe current region, then reduce the power supplied by said power supply means to said heating element so as to alter the current in said heating element to within said safe current region and prevent occurrence of said conduit and said heating element overheating, then 
   iii) after a predetermined time increase said power supplied by said power supply means to said heating element. 
   In a second aspect the invention consists in a humidification apparatus for humidifying a gases flow to be supplied to a patient or other person in need of such gases comprising: 
   humidification chamber means adapted to hold a quantity of water and having an inlet and an outlet to allow said gases flow to pass through said humidification chamber means, 
   heating means provided adjacent said humidification chamber means and adapted to provide heat to said quantity of water in said humidification chamber means in order to provide water vapour to said gases flow passing through said humidification chamber means, said heating means utilising a measurable quantity of power, 
   gases transportation pathway means connected to said outlet of said humidification chamber means to convey said gases flow to said patient or other person in need of such gases, 
   gases transportation pathway heating means that is energisable to supply heat to said gases flow along at least a part of the length of said gases transportation pathway means, 
   detecting means which includes means to detect the current in said gases transportation pathway heating means, and 
   control means, including power supply means, which implements an algorithm that causes the control means to: 
   i) receive input of said current in said gases transportation pathway heating means from said detecting means, and 
   ii) if said current is outside a safe current region, then reduce the power supplied by said power supply means to said gases transportation pathway heating means so as to alter the current in said gases transportation pathway heating means to within said safe current region and prevent occurrence of said conduit and said heating element overheating, then 
   iii) after a predetermined time increase said power supplied by said power supply means to said gases transportation pathway heating means. 
   In a third aspect, the present invention consists in a conduit overheating detection system for a respiratory conduit heating element comprising: 
   a conduit, comprising two limbs, one limb being an inspiratory limb of said respiratory conduit and the other being an expiratory limb of said respiratory conduit, said conduit having a heating element disposed within it, where, in use, the current flowing in the first part of said heating element in said first limb differs to that of the current flowing in the second part of the heating element in said second limb, 
   detecting means which includes means to detect a first current and a second current in said first part of said heating element and said second part of said heating element respectively, and 
   control means which implements an algorithm that causes the control means to: 
   i) receive input of said first current and said second current from said detecting means, 
   ii) determine the difference between said first current and said second current, and 
   iii) if said current approaches a predetermined limit, then reduce the power supplied by said power supply means to said heating element so as to alter the current in said heating element to retreat from said predetermined limit and prevent occurrence of said conduit and said heating element overheating, then 
   iv) after a predetermined time increase said power supplied by said power supply means to said heating element. 
   To those skilled in the art to which the invention relates, many changes in construction and widely differing embodiments and applications of the invention will suggest themselves without departing from the scope of the invention as defined in the appended claims. The disclosures and the descriptions herein are purely illustrative and are not intended to be in any sense limiting. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     One preferred form of the present invention will now be described with reference to the accompanying drawings in which; 
       FIG. 1  is a schematic diagram of a respiratory humidification system that may incorporate the detection of conduit overheating system of the present invention, 
       FIG. 2  is an illustration of a respiratory humidifier system that may utilise the overheating detection system of the present invention, 
       FIG. 3  is an illustration of the humidifier base of the respiratory humidifier system of  FIG. 2 , 
       FIG. 4  is a graph of the current in a heating element over time during the testing of a conduit under conditions where there is no current detection, 
       FIG. 5  is a graph of the current in a heating element over time where current detection is used to ensure that the conduit does not melt and gases provided to the patient are not of high temperature, and 
       FIG. 6  is a schematic diagram of a respiratory humidification system having inspiratory and expiratory conduits, which may incorporate the detection of conduit overheating system of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   With reference to the accompanying drawings and in particular to  FIG. 1 , an example of humidification apparatus or a respiratory humidification system incorporating preferred embodiments of the present invention is illustrated. Included in the respiratory humidification system is a gases supply means  1  (such as a ventilator, insufflator or blower) having an outlet  2 , which supplies gases (for example oxygen, anaesthetic gases or air) to the inlet  3  of a humidification chamber means  4  via a conduit  6 . Humidification chamber means  4  may, for example comprise a plastics formed chamber having a metal base  7  sealed thereto. Humidification chamber  4  is adapted to hold a volume of water  8 , which is heated by a heater plate means  9  under the control of controller or control means  11  of a humidification device or humidifier  10 . 
   As the water within chamber  4  is heated it will slowly evaporate, mixing water vapour with the gases flow through the humidification chamber from ventilator  1 . Accordingly, humidified gases leave humidification chamber  4  via outlet  12  and are passed to a patient or other person in need of such gases  13  through a gases transportation pathway or inspiratory conduit  14 . In order to reduce condensation within the inspiratory conduit  14  and to raise the temperature of the gases provided to the patient  13  a heating element means  15  is provided which is energised under the control of control means  11 . 
   In  FIG. 1  a gases mask  16  is shown over the patient&#39;s nose and mouth (referred to as “Intact Airways” gases delivery) however it should be understood that many gases delivery configurations exist such as intubation in which a delivery tube is positioned in the patient&#39;s trachea to by-pass the patient&#39;s airways (known as “Intubated Airways” gases delivery). It is also possible to provide a return path for the patient&#39;s exhaled gases back to ventilator  1 . In this case a suitable fitting such as a “Y-piece”  36  (see  FIG. 6 ) may be attached between a patient  40  inspiratory conduit  31  and an expiratory conduit  32 , which is connected to an inlet  42  of the ventilator  33 . 
   Control means  11  may for example comprise a microprocessor or logic circuit with associated memory or storage means which holds a software program which, when executed by control means  11 , controls the operation of the humidification system in accordance with instructions set in the software and also in response to external inputs. For example, control means  11  may be provided with input from heater plate  9  so that control means  11  is provided with information on the temperature and/or power usage of the heater plate  9 . Furthermore, a flow sensing means or flow probe  17  may be provided anywhere in the breathing circuit (“the breathing circuit” comprises the parts of the humidification apparatus through which the gases flow passes). The flow probe  17  is shown in  FIG. 1  may be provided at or near the humidifier outlet  12  to indicate to control means  11  the outlet gases flow. Also provided in such apparatus may be a temperature probe at the outlet to the humidifier and an ambient temperature probe at the inlet to the humidifier. Each of the outputs from these probes may be an input to control means  11 . 
   A still further input to control means  11  may be a user input means or switch  18  which could be used to allow a user (such as a health care professional or the patient themselves) to set a desired gases temperature of gases to be delivered or a desired gases humidity level to be delivered or alternatively other functions could be controlled by switch  18  such as control of the heating delivered by heating element  15  or selecting from a number of automatic gases delivery configurations. 
   Referring to  FIGS. 2 and 3  that show a humidifier apparatus  20  in more detail, the humidifier  20  has a humidifying chamber  21  having edges that engage with the collar  22  on the humidifier  20 . The gases to be humidified may be a mixture of air, oxygen and anaesthetic for example, which are supplied to the chamber through gas inlet  23 . This might be connected to a ventilator, source of pressurised oxygen, flow generator, or air compressor. A gases outlet  24  is also provided and the gases outlet  24  is connected to the conduit  25 , which conveys humidified gases to the patient at the end  26  of the conduit. The end  26  of the conduit may have a cannula connected to the patient&#39;s nose, nasal mask or face mask connected to the patient&#39;s face, so as to supply humidified gases to the patient. The humidifier heater plate  27  has a temperature transducer  28  that is in electrical connection with the electronic control circuitry in body  29  of the apparatus so that the control means monitors the temperature of the heating plate. 
   A heating element means  15  is provided within the conduit  25  to help prevent condensation of the humidified gases within the conduit. Such condensation is due to the temperature of the walls of the conduit being close to the ambient temperature, (being the temperature of the surrounding atmosphere) which is usually lower than the temperature of the humidified gases within the conduit. The heating element  15  effectively replaces the energy lost from the gases through conduction and convection during transit through the conduit. Thus the conduit heating element  15  ensures the gases delivered are at an optimal temperature and humidity. 
   The heating element  15 , which is usually a copper filament, has a material property that causes a change in electrical resistance, which is usually significant, when there is a change in temperature of the copper filament. Therefore, the electrical resistance, and indirectly the temperature of the heating element  15  can be monitored by monitoring the current drawn by the heating element  15  when power is applied to the heating element  15 . This monitoring of the heating element  15  may be done by directly using the control means  11 , which is connected to the heating element  15 , or by external detection means, such as a sensor  30  (see  FIG. 1 ) connected to the control means  11 . If the current through the heating element  15  is low then the resistance of the heating element  15  is high, and the heating element temperature is high and the conduit hot. In which case, if the current drawn by the heating element  15  exceeds a predetermined limit or is outside a safe current region, the respiratory humidifier  10  and conduit  14  can be switched to a safe mode by the control means  11 , and then back into operating mode once the temperature of the heating element  15  has reduced to safe levels. 
   Whether the predetermined conduit heating element current limit is an upper or a lower limit depends on the specific resistance-temperature characteristic of the heating element material.  FIG. 4  shows a graph of current (in amperes) versus time for a conduit with heating element where the element is a typical copper filament. In order to simulate an increase in the temperature of the conduit, tests were conducted where a blanket was placed over the conduit at time t=55 minutes and no detection of conduit overheating was used. 
   As can be seen from  FIG. 4 , between 0 to 4 minutes the conduit heating element is in its start-up period and is not significantly powered to cause heating of the humidified gases. Between 4 and 55 minutes the conduit heating element power has been set to a constant duty cycle (in this instance the duty cycle was 95%, but any appropriate level is sufficient) and the heating element current settled at a stable operating level, in this example the operating level is approximately 1.65 amperes, other operating levels appropriate to the heating element may be used. The current operating level ultimately depends upon the flow rate, ambient temperature and conduit dynamics (that is, the dimensions, materials, resistance and wire length of the heating element). However, testing has shown that for a particular conduit design, a current safety limit can be determined, below which the conduit heating element current will not fall (at any flow rate or ambient temperature) unless the conduit is heating to a degree that approaches a safety hazard. 
   In  FIG. 4  at time t=55 minutes, during testing, a blanket was placed over the conduit, this additional insulation caused the current within the heating element to decrease as the temperature within the conduit increased. As can be seen the current in the heating element between t=55 minutes and t=100 minutes continues to decrease below the predetermined current safety limit. Eventually, at time t=100 minutes the conduit temperature is such that the conduit, being made from a plastics material, begins to melt. Also over the period of time where the heating element current is below that of the current safety limit if such a respiratory system was used under these conditions then the patient is likely to be supplied with high temperature gases, causing discomfort and possibly harm to the patient. 
   The method of detecting over-heating of the heating element  15  in the conduit  14  is to monitor the current in the heating element  15  as described above. To prevent unsafe conduit temperatures and eventual conduit melt a heating element current safety limit can be determined, by manual testing or the like, and programmed into the control means  11 . When the current in the heating element  15  exceeds the current safety limit, the humidifier  10  is switched to a safe mode by the control means  11 , decreasing the heating element power to a predetermined safe level for a predetermined time period, then increasing the heating element power to normal operating mode or level. 
   In the present invention the safe mode is one where the duty cycle power to the heating element  15  has been reduced from the operating value. As can be seen in  FIG. 5  when the current in the heating element drops below the current safety limit, this is detected by a detecting means, such as a sensor  30 , the reduction of current causes the control means  11  to limit the duty cycle of the voltage supplied to the heating element, in this case the duty cycle has been reduced to approximately 30%, but other appropriate values may be used. The effect of reducing duty cycle is to increase the current in the heating element. The control means  11  which may be either a software program stored in a micro controller or may be electronically implemented by a comparator and current limiting circuit. 
     FIG. 5  shows the current and duty cycle waveforms where the current drops below the current safety limit four times, and each time the detector and controllers act to alter the duty cycle and thus bring the heating element current to safe levels. Preferably the heating element is run at the 30% duty cycle for approximately 15 minutes (although, other appropriate values may be used) before returning to the normal operating mode. Further, if the current limit is again reached then the present invention will act to ensure that the apparatus moves into safe mode operation, reducing the duty cycle and increasing the current in the heating element. 
   In a second embodiment where the respiratory apparatus, incorporating the overheating detection system of the present invention comprises two conduits (such as that shown in  FIG. 6 ), where one conduit is an inspiratory conduit and the other the expiratory conduit, the present invention has a different embodiment. Referring now to  FIG. 6 , an inspiratory conduit  31  is connected to a ventilator and/or humidifier. In  FIG. 6 , the inspiratory conduit  31  is merely connected at it&#39;s proximal end  37  to a ventilator  33 , but in most preferred embodiments a humidifier (such as that described in relation to  FIGS. 1 to 3  is placed between the ventilator exit port  34  and inlet to the inspiratory conduit  31 . The distal end  35  of the inspiratory conduit  31  is connected to a “Y” shaped connector  36  having three inlet/outlet ports. One port  38  of the “Y” shaped connector  36  directs the inspiratory gases flowing through the inspiratory conduit  31  to a patient interface  39  and patient  40  and also received air or gases exhaled from the patient  40 . The expired air is channelled by the “Y” shaped connector  36  to an expiratory conduit  32  via the third port  41  of the “Y” shaped connector  36  so that the expiratory gases may be returned to the ventilator  33  from the end  42  of the expiratory conduit  31 . In the preferred form each of the inspiratory  31  and expiratory  32  conduits has a heating element ( 31 ,  32  respectively) residing within, throughout or about it. These heating elements are of the type as described above in relation to  FIG. 1 . In common ventilator systems the duty cycle of the voltage to the heating elements  43 ,  44  within the conduits  31 ,  32  is usually controlled using inputs, such as conduit temperature from the inspiratory conduit, while the expiratory conduit acts as a slave. Therefore, in order to detect and control any overheating of the expiratory conduit  32 , the current in each of the inspiratory  31  and expiratory  32  conduits need to be detected. Usually, the electrical resistance in each of the heating elements  43 ,  44  within the conduits is different to allow different heating levels during operation; because of this a different current flows through each conduit. Thus, the detecting means, such as a sensor (not shown) or a control means  45 , must be capable of detecting the current in both conduits  31 ,  32 . In this embodiment it is preferred that the current in the heating elements  43 ,  44  is detected by the control means  45 , which compares each of the currents. If the difference between the detected currents in the heating elements  43 ,  44  starts to approach a predetermined limit the control means  45  causes the heating elements  31 ,  32  to be switched to the safe operation mode in the same manner as described above (in relation to the first embodiment of  FIG. 1 ). In this way, if either of the conduits  31 ,  32  is covered during use, or if gases are not flowing in one conduit causing that conduit to overheat, then overheating will be detected and the duty cycle of the voltage supplied to the heating elements  43 ,  44  will be altered by the control means  45  to cause the currents in the heating elements  43 ,  44  to return to safe levels, preventing damage to the conduits  31 ,  32  or harm to the patient  40 . 
   The predetermined limit of the difference in current between the conduits  31   32  depends on the specific resistance-temperature characteristic of the heating element material, and the relative resistances of the inspiratory  31  and expiratory  32  conduits. For example, if the inspiratory conduit heating element  43  has a resistance of 18 ohms and the expiratory conduit heating element  44  has a resistance of 12 ohms, where the heating element is a typical copper filament, the difference in operating currents between the conduits  31 ,  32  is approximately 0.4 amperes. If the expiratory conduit  32  overheats, the current in the expiratory conduit heating element  44  will reduce while the current in the inspiratory conduit heating element  43  remains unaffected. Therefore, the difference in current between the heating elements  43 ,  44  will reduce. In the example given above, the predetermined limit referred to is a difference in current between the conduits of 0.3 amperes.

Technology Classification (CPC): 0