Abstract:
The gases temperature supplied to a patient when the patient is undergoing treatment such as oxygen therapy or positive pressure treatment for conditions such as Obstructive Sleep Apnea (OSA) or Chronic Obstructive Pulmonary Disease (COPD) is often measured for safety and to enable controlling of the humidity delivered to the patient. The apparatus is related to measurement of properties, particularly temperature (thermister  23 ), of gases flowing through a heated tube ( 3 ), supplying gases to a patient, which utilizes the heating wire ( 21, 28 ) within the tube.

Description:
INCORPORATION BY REFERENCE TO ANY PRIORITY APPLICATIONS 
     Any and all applications for which a foreign or domestic priority claim is identified in the Application Data Sheet as filed with the present application are hereby incorporated by reference under 37 CFR 1.57. 
     This application is a continuation of U.S. patent application Ser. No. 11/572,822, filed Oct. 18, 2007, now U.S. Pat. No. 8,453,641, issued Jun. 4, 2013, which is a National Phase filing of PCT/NZ2005/000219, having an International filing date of Aug. 19, 2005, which claims priority of NZ534853 having a filing date of Aug. 20, 2004. 
    
    
     BACKGROUND 
     1. Field 
     This invention relates to an apparatus for measuring properties, such as temperature and humidity, of gases being supplied to a patient. Humidifiers are commonly controlled by measuring the temperature of gas at two points, adjacent to the output of the humidifier and proximal to the patient. This invention predominantly relates to the measurement of temperature of gas supplied to a patient at a point proximal to the patient. 
     2. Description of the Related Art 
     The gases temperature supplied to a patient when the patient is undergoing treatment such as oxygen therapy or positive pressure treatment for conditions such as Obstructive Sleep Apnea (OSA) or Chronic Obstructive Pulmonary Disease (COPD) is often measured for safety and to enable controlling of the humidity delivered to the patient. Measurement of temperature near the patient is commonly performed using a probe inserted into the breathing tube, such as that of Fisher &amp; Paykel Healthcare Limited, U.S. Pat. Nos. 6,272,933 and 6,584,972. Such a temperature probe is connected to the humidifier through a cable that runs external to the breathing circuit This approach has some drawbacks. In particular, the user must correctly install the temperature probe. If the probe is not correctly installed then the humidification system may malfunction which may increase risk to the patient. Existing end of breathing tube sensors require sensor wires to be run down the outside of the breathing tube. This lowers reliability of the sensors due to the vulnerability of these wires. Alternatively, if these wires are run down the inside of the breathing tube there would be an increase of the resistance to airflow and the hygiene of the breathing circuit would be lowered. 
     SUMMARY 
     It is an object of the present invention to provide a method of measuring properties of gases supplied to a patient that goes some way to overcoming the abovementioned disadvantages in the prior art or which will at least provide the industry with a useful choice. 
     Accordingly in a first aspect the present invention consists in an apparatus for measuring properties of gases being supplied to a patient comprising: 
     a gases supply, 
     at least one delivery conduit including a heater wire for heating said conduit, 
     wherein said heater wire is utilised in an electrical circuit to determine said properties of said gases. 
     Preferably said electrical circuit is connected in series with said heater wire and provides a measurement or enables a calculation of an indication of at least one of temperature, humidity, pressure and composition of said gases. 
     Preferably said electrical circuit is mounted and sealed on a printed circuit board that at least partially extends into the gases supplied to said patient through said at least one delivery conduit. 
     Preferably said electrical circuit is at least partially moulded into the wall of said delivery conduit. 
     Preferably said electrical circuit includes a sensing means with known properties at ambient temperature such that said sensing means can be matched with said at least one delivery conduit. 
     Preferably said sensing means is a temperature sensor. 
     Preferably said electrical circuit includes at least one measuring means in series with said heater wire. 
     Preferably said at least measuring means is a temperature measuring means. 
     Preferably said temperature measuring means includes a thermistor and diode in parallel and a reference resistor. 
     Preferably said thermistor and said diode are located at the end of said delivery conduit near to said patient and said reference resistor is included in said gases supply means. 
     Preferably said gases supply means includes a device to supply gas flow, such as a blower, and a humidifier to humidify said gases from said blower. 
     Preferably said gases supply means is a humidifier. 
     Preferably said electrical circuit includes a gases property measuring means. 
     Preferably said gases property measuring means includes at least one of a sensor, band pass filter or thermistor and at least one reference resistor. 
     Preferably said at least one of a sensor, band pass filter or thermistor are located at the end of said delivery conduit near to said patient and said at least one reference resistor and at least one band pass filter is included in said gases supply means. 
     The invention consists in the foregoing and also envisages constructions of which the following gives examples. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred forms of the present invention will now be described with reference to the accompanying drawings. 
         FIG. 1  is an illustration of a respiratory humidifier system that may be used with the method of the present invention of measuring temperature of gases supplied to a patient. 
         FIG. 2  is a circuit diagram of the electronics enabling the measurement of the temperature of gases to a patient, where the circuit is utilised when the system of the present invention is utilising DC heating and measuring voltages. 
         FIG. 3  is a circuit diagram of the electronics enabling the measurement of the temperature of gases to a patient, where the circuit is utilised when the system of the present invention is utilising DC or AC voltages for the heating and signal voltages. 
         FIG. 4  is a cut away of a conduit including a circuit of the present invention on a printed circuit board and residing with the conduit in the area of gases flow. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention seeks to measure various properties, for example temperature or humidity, at the end of a gas delivery tube or conduit using sensors mounted on a wire, such as a wire used for heating the gases flow through the tube or conduit, where the wire resides within the delivery tube or conduit. A heated tube with a heating wire such as that described in Fisher &amp; Paykel Healthcare Limited U.S. Pat. No. 6,078,730 or any other similar tube and heating wire could be utilised with the present invention. 
     Referring to  FIG. 1  a ventilation and humidifying system as might be used with the present invention is shown. A patient  13  is receiving humidified and pressurised gases through a nasal cannula  12  connected to a humidified gases transportation pathway or inspiratory conduit  3  that in turn is connected to a humidifier  8  (including humidification chamber  5 ) supplied with gases from a blower  15  or other appropriate gases supply means. 
     The inspiratory conduit  3  is connected to the outlet  4  of the humidification chamber  5  that contains a volume of water  6 . The humidification chamber  5  is preferably formed from a plastics material and may have a highly heat conductive base (for example an aluminium base) that is in direct contact with a heater plate  7  of humidifier  8 . The humidifier  8  is provided with control means or an electronic controller  9  that may comprise a microprocessor based controller executing computer software commands stored in associated memory. Gases flowing through the inspiratory conduit  3  are passed to the patient by way of the nasal cannula  12 , but may also be passed to the patient by way of other patient interfaces such as a nasal or full face mask. 
     The controller  9  receives input from sources such as user input means or dial  10  through which a user of the device may, for example, set a predetermined required value (preset value) of humidity or temperature of the gases supplied to patient  13 . In response to the user set humidity or temperature value input via dial  10  and other possible inputs such as internal sensors that sense gases flow or temperature, or by parameters calculated in the controller, controller  9  determines when (or to what level) to energise heater plate  7  to heat the water  6  within humidification chamber  5 . As the volume of water  6  within humidification chamber  5  is heated, water vapour begins to fill the volume of the chamber above the surface of the water and is passed out of the humidification chamber  5  outlet  4  with the flow of gases (for example air) provided from a gases supply means or blower  15  which enters the humidification chamber  5  through inlet  16 . 
     The blower  15  may be provided with a variable speed pump or fan  2  which draws air or other gases through the blower inlet  17 . The speed of the variable speed pump or fan  2  may be controlled by a further control means or electronic controller  18  which responds either to inputs from controller  9  or to user-set predetermined required values (preset values) of pressure or fan speed, via dial  19 . Alternatively, the function of this controller  18  can be combined with the other controller  9 . 
     A heating element or wire  11  is preferably provided within, around and throughout the conduit or tubing  3  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 heater element effectively replaces the energy lost from the gases through conduction and convection during transit through the conduit. Thus the conduit heater element ensures the gases delivered are at an optimal temperature and humidity. 
     Such a heater wire is commonly driven either with direct current (DC) or alternating current (AC) and in both cases the heating voltage is usually switched on and off to control the power applied to the heating element. In the present invention the heating element  11 , which is most preferably a wire, is used along with an electronic circuit to determine properties of the gases supplied to the patient. The circuit ( 20  or  40  in  FIGS. 2 and 3 ) is preferably connected in series with the heater wire  11 . The circuit may be on a printed circuit board, or wired within a housing that may be a plastic moulding in the gases flow, or a circuit board that is at least partially moulded within the wall of the conduit or tubing  3 . The properties that may be measured include temperature, pressure, gas composition and humidity. Two embodiments of the present invention are described below, one that operates using only a DC heating voltage and the other that can operate with a DC or AC heating voltage. 
     DC Heating Voltage 
       FIG. 2  shows a circuit  20  that may be utilised for carrying out the method of measuring temperature of the present invention. When a DC heating voltage  25  is applied to the heater wire the diode  22  conducts and current flows through the heater wire  21 ,  28  and the heater wire functions as normal and provides heating to the delivery tube  3 . When the heating voltage  25  is switched off using switch  29 , a measurement voltage  26 , which has opposite polarity to the heating voltage  25  is applied to the heater wire. In this case, the current in the heater wire  21 ,  28  does not flow through the diode  22  but flows through the thermistor  23  and through a reference resistor  24 . The voltage across the reference resistor  24  can then be measured at the output  27  and the temperature of the gases determined. The voltage measurement  27  across the reference resistor,  24 , is converted to a temperature using a look up table or an equation to calculate a value for temperature. This is similar to a commonly used technique where the thermistor  23  forms a potential divider with the reference resistor  24 . 
     More generally, the thermistor may be replaced by an impedance (for example, a resistor and a capacitive sensor) for pressure or humidity measurement. Either the impedance can be measured by measuring the voltage across the reference resistor  24  or the rise-time could be determined by looking at the voltage across the reference resistor  24  in time. 
     Part of the circuit  20  would be included in the delivery conduit  3  and in particular the diode  22  and thermistor  23  (in parallel with one another) are preferably placed in series with the heater wire  21 ,  28  at a point in the heater wire at or near the end  30  (nearest the user  13 , see  FIGS. 1 ,  2  and  4 ) of the delivery tube  3 , for example they may be interconnected on a printed circuit board, overmoulded with plastic for sealing and mounted in the gases stream through the delivery conduit as shown in  FIG. 4 . Furthermore, the circuit may be formed by interconnected parts in a housing, for example, a plastic housing, that protrudes from the plastic wall of the delivery tube into the gases flow through the conduit, in order to measure that gases properties. All other parts of the circuit  20  including the reference resistor  24  and the switching circuitry  29  would be included in the control circuitry of the humidifier  8 . 
     The thermistor&#39;s value can be chosen to have different resistance curves with known properties at ambient temperature. The choice of a particular thermistor value for use with the circuit allows identification by the control system of the present invention and matching of that thermistor value with a specific conduit or tubing  3 . Such that different thermistor values can be matched with a particular and appropriate conduit types and upon connection of the conduit to a humidifier or blower device, the control system can identify that thermistor and apply the appropriate control strategy to the heating of the conduit. 
     AC or DC Heating Voltage 
     The circuit shown in  FIG. 2  is intended to be used when a DC heating voltage is used in conjunction with the heater wire, delivery conduit and system as shown in  FIG. 1 . An alternative embodiment of a circuit  40  that would provide measurement of the gases properties, such as temperature and is suitable for AC and DC voltages, is shown in  FIG. 3 . A number of voltage signals  51 ,  52 ,  53 , which are at different frequencies, are added together at an adder  50 . These signals include at least one heating signal  51  and at least one measuring signal  53 . The combination of these signals passes down the heater wire  44 , creating currents (heating and measuring) in the heater wire  44 . A number of parallel paths are established  41 ,  43 ,  45  each containing a filter (for example, as shown in  FIG. 3 , one low pass filter  41  and three band pass filters  43 ,  45 ,  48 ) that each pass a different frequency range. These parallel paths (that is, filters, thermistors and/or sensors) are preferably located at the end  30  of the delivery tube  3 , in a similar manner as described in relation to  FIG. 2 . The parallel paths allow the heating current to be passed through a different path to the measurement currents. It also allows multiple measurement signals to be passed through the heater wire so that different properties of the gases (e.g. temperature, pressure, humidity, composition) may be measured. 
     The heating and measurement currents return through the heater wire  46  and can be filtered through a number of measurement filters  47 ,  49 ,  57  in parallel that pass frequency bands that correspond to the filters,  41 ,  43 ,  45  located at the end  30  of the tube  3 . The heating current takes a different path than the measurement currents. The measurement currents each take a different path depending on their frequency and this allows each measurement current to be measured by passing it through a reference resistor  48 ,  54  or similar. Again a look up table or equation may be used to convert the voltage across the reference resistor  48 ,  54  to, for example, a temperature. In the preferred embodiment of the present invention the measurement filters  47 ,  49 ,  57  would be included in the humidifier  8  control circuitry. 
     In a further embodiment one or more of the sensing elements  55 ,  56  at the end  30  of the delivery tube  3  could be replaced by a fixed impedance to allow identification of the tube so that different control algorithms can be used for different conduits or tubes. 
       FIG. 4  shows a cutaway view of a conduit  3  with a printed circuit board  60  housing the parts to one of the circuits of the present invention described above with reference to  FIG. 2  or  3 . The circuit board  60  is connected to the heating wires  21 ,  28  and as such is positioned within the conduit  3 . In this manner, the thermistor  23  included on the board  60  is exposed to the gases flowing through the conduit  3  and can provide measurements of the properties of the gases. 
     The circuits and method of the present invention can be applied to a number of applications of these technologies for humidification and breathing circuit products. For example, the measurement of the temperature or humidity at the end of the delivery tube (or in a patient interface, for example, nasal cannula or mask) can be used to better control the humidifier, such that a more accurate temperature of gases can be supplied to the patient, providing optimal patient comfort and therapy. Additionally, other gases properties may be measured, such as the gases pressure or gas composition near the patient. 
     The apparatus of the present invention eliminates the need for external wires for sensing gases properties, as is required by the prior art. Furthermore the apparatus of the present invention only uses two pins or contacts (as opposed to four pins as used in current heated tube implementations). This means the system of the present invention is likely to be more reliable as the contacts/pins are likely to be less prone to breakage. The utilisation of the heater wire for measuring gases properties may also reduce the cost of the breathing tube  3  and associated parts, especially if the breathing tube is to be disposable.