Patent Publication Number: US-11648368-B2

Title: Hyperthermic humidification system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 17/099,007 filed on Oct. 16, 2020, now allowed, which is a continuation of U.S. patent application Ser. No. 17/001,257 filed on Aug. 24, 2020, now U.S. patent Ser. No. 10/894,141, which is a continuation of U.S. patent application Ser. No. 16/120,923 filed on Sep. 4, 2018, now U.S. Pat. No. 10,933,212, which is a continuation of U.S. patent application Ser. No. 14/547,012 filed on Nov. 18, 2014, now U.S. Pat. No. 10,092,722, which is a continuation of U.S. patent application Ser. No. 11/973,061, filed on Oct. 5, 2007, now U.S. Pat. No. 8,905,023. The specifications of each of the foregoing applications are hereby incorporated by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to apparatus and methods for respiratory tract therapy. More particularly, this invention relates to an apparatus configured to deliver heated and humidified breathing gas to a patient. 
     BACKGROUND OF THE INVENTION 
     Respiratory airway therapies are recognized medical treatments that enhance breathing by delivering breathing gas to the respiratory tract of patients. Respiratory devices such as humidifier/ventilator systems, however, include parts that may be at risk of contamination due to contact with water or water vapor. While disinfection protocols have been developed to minimize and control bacterial growth, there remains a need for an improved apparatus for respiratory tract therapy that can be used in various settings including clinical and hospital settings that reduces the risk of bacterial contamination. There also remains a need for improved methods of respiratory airway therapy. 
     SUMMARY OF THE INVENTION 
     In one aspect, the invention provides a humidification system configured to deliver humidified breathing gas to a patient. The humidification system includes a vapor transfer unit and a base unit. The vapor transfer unit includes a liquid passage, a breathing gas passage, and a vapor transfer device positioned to transfer vapor to the breathing gas passage from the liquid passage. The system includes a base unit that releasably engages the vapor transfer unit to enable reuse of the base unit and selective disposal of the vapor transfer unit. The liquid passage is not coupled to the base unit for liquid flow therebetween when the vapor transfer unit is received by the base unit. 
     In another aspect, the humidification system configured to deliver humidified breathing gas to a patient includes a vapor transfer unit and a base unit. The vapor transfer unit includes a liquid passage, a breathing gas passage, and a vapor transfer device positioned to transfer vapor to the breathing gas passage from the liquid passage. The base unit releasably engages the vapor transfer unit. The base unit has at least one sensor positioned to sense a parameter in the liquid passage of the vapor transfer device. 
     In yet another aspect, the humidification system configured to deliver humidified breathing gas to a patient includes a vapor transfer unit and a base unit. The vapor transfer unit has a liquid passage and a first pump portion positioned to advance liquid through the liquid passage. The base unit releasably engages with the vapor transfer unit. The base unit has a second pump portion adapted to operationally mate with the first pump portion to advance liquid through the liquid passage of the vapor transfer unit when the base unit engages the vapor transfer unit. 
     In still another aspect, the humidification system is configured to deliver heated and humidified breathing gas to a patient and includes a vapor transfer unit and a base unit. The vapor transfer unit has a liquid passage and a first heater portion positioned to heat liquid in the liquid passage. The base unit releasably engages the vapor transfer unit to enable reuse of the base unit and selective disposal of the vapor transfer unit. The liquid passage is not coupled to the base unit for liquid flow therebetween when the vapor transfer unit is received by the base unit. The base unit has a second heater portion adapted to conduct heat to the first heater portion to heat liquid in the liquid passage of vapor transfer unit. 
     In still yet another aspect, the invention provides a vapor transfer unit for use with a base unit of a humidification system for delivering heated and humidified breathing gas to a patient. The vapor transfer unit is configured to be releasably mounted to base unit to accommodate reuse of base unit and selective disposal of vapor transfer unit. The vapor transfer unit includes liquid and breathing gas passages and a vapor transfer device is positioned to transfer vapor to the breathing gas passage from the liquid passage. An impeller is positioned to advance liquid through the liquid passage and a sensor is positioned to sense a level of liquid in the liquid passage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is best understood from the following detailed description when read in connection with the accompanying drawings, with like elements having the same reference numerals. This emphasizes that according to common practice, the various features of the drawings are not drawn to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Included in the drawings are the following figures; 
         FIG.  1    is a schematic representation of a humidification system according to an exemplary aspect of this invention; 
         FIG.  2    is a front perspective view of an exemplary embodiment of the humidification system in accordance with the present invention; 
         FIG.  3 A  is a side perspective view of the humidification system shown in  FIG.  2   , with a vapor transfer cartridge partially inserted into a base unit; 
         FIG.  38    is a side perspective view of the humidification system shown in  FIG.  2   , with the vapor transfer cartridge fully inserted into the base unit; 
         FIG.  4    is rear perspective view of the humidification system shown in  FIG.  2   ; 
         FIG.  5    is a schematic representation of the humidification system shown in  FIGS.  2 ,  3 , and  4    according to aspects of the invention; 
         FIG.  6 A  is a perspective view of a first portion of a gas blending device according to an aspect of the invention; 
         FIG.  6 B  is an interior view of the first portion of the as blending device shown in  FIG.  6 A ; 
         FIG.  6 C  is an exterior view of the first portion of the gas blending device shown in  FIG.  6 B ; 
         FIG.  7 A  is a perspective view of a second portion of the gas blending device configured to mate with the first portion shown in  FIG.  6 A ; 
         FIG.  7 B  is an interior view of the second portion of the, gas blending device shown in  FIG.  7 A ; 
         FIG.  7 C  is an enlarged view of laminar fins shown in  FIG.  7 B ; 
         FIG.  7 D  is a cross-sectional view of the laminar fins, taken along lines  7 D- 7 D of  FIG.  7 C ; 
         FIG.  7 E  is an exterior view of the second portion of the gas blending device shown in  FIG.  7 B ; 
         FIG.  8 A  is a front perspective view of a chassis of the humidification system; 
         FIG.  8 B  is a rear elevation view of the chassis shown in  FIG.  8 A ; 
         FIG.  8 C  is an exterior side elevation view of the chassis shown in  FIG.  8 A ; 
         FIG.  8 D  is an interior side elevation view of the chassis shown in  FIG.  8 C ; 
         FIG.  8 E  is a cross-sectional view of the chassis, taken along lines  8 E- 8 E of  FIG.  8 D ; 
         FIG.  8 F  is a top view of the chassis shown in  FIG.  8 A ; 
         FIG.  8 G  is a top perspective view of an exemplary base unit that may be used with an embodiment of a humidification system according to the present invention; 
         FIG.  9    is a perspective view of a pump portion the humidification system according to an aspect of the invention; 
         FIG.  10 A  is front perspective view of a fluid pathway module of the humidification system shown in  FIG.  3   ; 
         FIG.  10 B  is rear perspective view of the fluid pathway module shown in  FIG.  10 A ; 
         FIG.  11 A  is an exploded view of the fluid pathway module shown in  FIGS.  10 A and  10 B ; 
         FIG.  11 B  is another exploded view of the fluid pathway module shown in  FIGS.  10 A and  10 B ; 
         FIG.  12    is an exploded view of the fluid pathway module shown in  FIG.  11 B ; 
         FIG.  13    is a front elevation view of the humidification system according to aspects of the invention; and 
         FIG.  14    is a flowchart showing operation of an exemplary embodiment of a humidification system according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Aspects of the invention will now be described with reference to the figures. Such figures are intended to be illustrative rather than limiting and are included herewith to facilitate the explanation of the present invention. 
     Humidification System 
     Referring generally to the figures ( FIGS.  1 - 14   ), in accordance with an exemplary embodiment, the invention provides a humidification system  100  to deliver heated and humidified breathing gas  80  to a patient. Humidification system  100  includes a base unit  110  and a vapor transfer unit, or fluid pathway module,  120 . Base unit  110  may include controls for operating humidification system  100  and is configured to operate without liquid flowing through base unit  110  or being exchanged with fluid pathway module  120 . Fluid pathway module  120  is releasably mounted to base unit  110  and is configured to operate with liquid  70 , such as water, flowing through fluid pathway module  120  (but not base unit  110 ) to allow reuse of base unit  110  and selective disposal of fluid pathway module  120 . Thus, cost savings and lowered contamination risk for humidification system  100  can be realized through reuse of base unit  110  and by periodically changing fluid pathway module (e.g., for each patient and/or at a determined time interval), which is the component that contacts the water and water vapor, and therefore more prone to bacterial contamination. 
     Referring now to the individual figures in detail,  FIG.  1    illustrates a schematic representation of humidification system  100 . Humidification system  100  manages the delivery of heated and humidified breathing gas  80  to the patient and includes base unit  110  and fluid pathway module  120 . The illustrated base unit  110  includes the controls for operating humidification system  100  and is configured to receive breathing gas  50   a ,  50   b , such as medical air and oxygen, respectively. Alternatively, the controls may be remote to base unit  110 . In addition, other gases, such as, for example, helium, nitric oxide (INO), carbon dioxide, and/or other gases, may be used. For gases other than air and oxygen, base unit  110  may need to be recalibrated for the specific gases being used. When different types of gas are received through base unit  110 , gases  50   a ,  50   b  may be blended by gas blending device  84 , to form blended gas  60 , which is delivered to fluid pathway module  120 . While two different gases may be used with system  100 , those skilled in the art will recognize that system  100  may be used with only one gas, such as, for example pure oxygen or air, in which case gas blending device  84  may be omitted. 
     Fluid pathway module  120  is releasably mounted to base unit  110  and is configured to receive gas  60  from base unit  110  and liquid  70  from an external water source. In an exemplary embodiment, liquid  70  received by fluid pathway module  120  is contained in a reservoir  32  to minimize potential contamination of base unit  110  and to prime a pump used to circulate liquid  70 . Liquid  70  contained in reservoir  32  may be heated by heat conduction  62  from base unit  110 . Vapor transfer device  99  releasably mounted to fluid pathway module  120  combines liquid  70  from reservoir  32  and blended gas  60  to supply heated and humidified breathing gas  80  to a patient. 
     Base Unit 
     Referring now to  FIGS.  2 ,  3 A,  3 B, and  4   , an exemplary embodiment of humidification system  100  according to the present invention is illustrated. Humidification system  100  includes base unit  110 , which contains the controls that operate humidification system  100  and is configured to operate without liquid flowing internally through base unit  110  or being exchanged with fluid pathway module  120 . In the exemplary embodiment, base unit  110  is completely dry so that potential damage to electronics that control humidification system  100  and bacterial contamination of base unit  110  is minimized. 
     Base unit  110  is mountable to a stand  90 , such as an IV pole, via mounting mechanism  95 , shown in  FIG.  4   . In an exemplary embodiment, rear panel  102  of base unit  110  includes a bracket  95   b  and a knob  95   a  that manipulates bracket  95   b  to releasably secure base unit  110  to stand  90 . When knob  95   a  is rotated, for example, bracket  95   b  may be tightened or loosened on stand  90 , thereby securing or loosening humidification system  100  with respect to stand  90 . 
     The rear of base unit  110 , best illustrated in  FIG.  4   , further includes gas inlet ports with filters  101   a ,  101   b  that are configured to connect to gas supply lines (not shown). The gas supply lines supply gas (such as medical air and oxygen) from a portable tank, compressor, or wall outlet into base unit  110 . In an exemplary embodiment, gas supplied to base unit  110  may be filtered and blended to provide a contaminant-free gas mixture. A gas blending device (not shown in  FIGS.  2 - 4   ), for example, may be installed within base unit  110  to blend the gas being supplied into base unit  110 . Additional aspects of the gas blending device and gas blending operation will be described in further detail below. 
     The side of base unit  110 , best illustrated in  FIGS.  2 - 4   , includes a door  103  that may be slid open or closed to expose or cover a component receiving portion  119  of base unit  110 . As shown in  FIGS.  2  and  4   , door  103  may be slid completely closed to cover the component receiving portion  119  from view. As illustrated in  FIGS.  3 A and  3 B , door  103  is slid open to expose component receiving portion  119  of base unit  110 . When door  103  is open, fluid pathway module  120  can be releasably mounted or removed from component receiving portion  119 , e.g., using a handle  121 . A guide  144  extends from the side of component receiving portion  119  to align and secure fluid pathway module  120  to base unit  110 .  FIG.  3 A  shows fluid pathway module  120  partially installed on base unit  110  and  FIG.  3 B  shows fluid pathway module  120  fully installed on base unit  110 . 
     In an exemplary embodiment, when fluid pathway module  120  is mounted to base unit  110 , fluid pathway module  120  is positioned to receive gas from base unit  110 . A gas outlet (not shown in  FIGS.  2 - 4   ) of base unit  110  engages a gas inlet (not shown in  FIGS.  2 - 4   ) of fluid pathway module  120  to form an airtight channel through which gas, received through inlet port  101   a , may be transferred to fluid pathway module  120 . As shown in  FIGS.  2 - 4   , fluid pathway module  120  is also configured to receive liquid from a liquid supply line  75  via liquid inlet  124 . Liquid may be supplied to fluid pathway module  120 , for example, via a sterile water bag (not shown) that is suspended above humidification system  100 . The sterile water bag may be punctured by a tube spike (not shown), with water being gravity fed from the water bag into fluid pathway module  120  via liquid supply line  75 . An exemplary tube spike is disclosed in U.S. patent application Ser. No. 10/918,515 owned by the Assignee of the present invention, which is incorporated herein in its entirety by reference. In an exemplary embodiment, liquid is stored within reservoir  32  (shown schematically in  FIG.  1   ) in fluid pathway module  120  that is provided to receive humidification fluid from the water bag as well as recirculated humidification fluid. The circulated humidification fluid/liquid in fluid pathway module  120  liquid does not flow through base unit  110 . Liquid contained in fluid pathway module  120  is vaporized in vapor transfer device  99  and combined with gas from base unit  110  to generate humidified breathing gas. As shown in  FIG.  3 A , a delivery tube  85  is releasably coupled to a breathing gas outlet  125  of fluid pathway module  120  to deliver humidified breathing gas to the patient. 
     As illustrated in  FIG.  4   , rear panel  102  of base unit  110  includes a pressure relief valve  91  that vents excess gas from base unit  110  if gas pressure supplied to base unit  110  from gas inlet ports  101   a ,  101   b  is too high. Base unit  110  also includes a service access cover  92  which is coupled to the rear panel  102  of base unit  110 . Service access cover  92  may be removed from base unit  110  to provide access to internal components within base unit  110 . 
     As shown in  FIG.  4   , an electrical cord  65  is coupled to base unit  110  to power humidification system  100 . When electrical cord  65  is removed or AC power is temporarily unavailable, an internal battery (not shown) within base unit  110  may provide DC power to humidification system  100 . Humidification system  100  may operate on DC power, for instance, when a patient is being transported from one location to another or during power interruptions, thus providing humidification system  100  portability and continued operations. In order to conserve battery power, the heater (not shown) that heats the fluid in fluid pathway module  120  does not operate in battery mode. 
     As further illustrated in  FIG.  2   , humidification system  100  has a front panel  104  that includes a display panel  105 , such as a liquid crystal display (LCD) or light emitting diode (LED) display that provides visual indication of user settings and status conditions of humidification system  100 . In an exemplary embodiment, the user settings may include user adjustable settings such as temperature  106   a , flow rate  106   b , and oxygen saturation level  106   c  of the breathing gas to be delivered to the patient. User settings may be adjusted, for example, via user interface  107 . User interface  107  includes buttons  108   a ,  108   b , LEDs  109   a ,  109   b , and knob  111  to adjust and monitor operating conditions of humidification system  100 . Additional aspects of the display panel  105  and user interface  107  will be described in further detail below according to aspects of humidification system  100  operation. 
     Referring now to  FIG.  5   , a detailed schematic diagram of humidification system  100  showing gas and fluid flow paths is illustrated. Schematic representations of base unit  110  fluid pathway module  120 , and vapor transfer device  99  are shown. Fluid pathway module  120  is configured to be releasably mounted to base unit  110 , and vapor transfer device  99  is configured to be releasably mounted to fluid pathway module  120 . 
     Base unit  110  includes controls for operation of humidification system  100  and has inlet ports configured to receive gas  50   a ,  50   b , such as medical air and oxygen. Gas input into base unit  110  is controlled by two proportional solenoids PSOL 1 , PSOL 2  that regulate the flow of gas  50   a ,  50   b , respectively, into base unit  110 . Proportional solenoids PSOL 1 , PSOL 2 , respectively, to regulate gas input flow into base unit  110 . Gas pressure sensors PS 1 , PS 2  monitor gas pressure upstream of solenoids PSOL 1 , PSOL 2 , respectively. Check valves  51   a ,  51   b  direct gas flow into gas blending device  84  and prevent reverse flow of gas  50   a ,  50   b.    
     In an exemplary embodiment, gas flow rate of air  50   a  and oxygen gas  50   b  are monitored by mass flow sensor MFS 1 , MFS 2 , respectively, positioned on gas blending device  84 . Air  50   a  and oxygen gas  50   b  are blended in gas blending device  84  and blended gas pressure is monitored by gas pressure sensor PS 4 . An oxygen sensor OS 1  is coupled to a three-way solenoid valve SOL 1  and monitors the oxygen saturation level of the blended gas. If the oxygen saturation level of the blended gas is below a user setpoint, proportional solenoid valve PSOL 1  feeds additional oxygen gas  50   b  into the blended gas. Likewise, if oxygen saturation level of the blended gas is above a user setpoint, proportional solenoid valve PSOL 1  reduces the amount of oxygen gas  50   b  into the blended gas. Gas pressure sensor PS 4  is coupled to a microcontroller (not shown) and monitors the pressure of the blended gas. If the blended gas pressure exceeds a certain safety threshold, humidification system  100  emits an audible and a visual alarm. Additionally, when gas pressure sensor PS 4  senses a low pressure, gas flow is limited by system  100 . 
     As shown schematically in  FIGS.  1  and  5   , blended gas  60  is delivered to vapor transfer device  99 , which is mounted to fluid pathway module  120 . Sensors CS 1 , CS 2  positioned within a base unit interface, such as component receiving portion  119 , shown in  FIG.  3   , of base unit  110  detect the presence of vapor transfer device  99  coupled to fluid pathway module  120 . For example, sensors CS 1 , CS 2  may read a barcode or optically detect an indicator on vapor transfer device  99  when fluid pathway module  120  is mounted to base unit  110 . 
     Fluid pathway module  120  receives water, e.g., from a water bag  33 , into reservoir  32 . Two water level sensors LS 1 , LS 2  on base unit  110  monitor water level within reservoir  32 . For example, water level may be monitored by optical detection, as will be described in further detail below. When fluid pathway module  120  is mounted to base unit  110 , water from reservoir  32  is pumped by a pump portion PMP 1  of fluid pathway module  120 . Pump portion PMP 1  is operationally coupled to a stator STA 1  of base unit  110  to pump water from reservoir  32  to a heater HTR 1 . Heater HTR 1  receives thermal energy from base knit heater HTR 2  to heat water to a user specified temperature. A temperature switch (OVERTEMP SWITCH) controls heater HTR 2  on base unit  110  to provide a safety backup to prevent water in fluid pathway module  120  from overheating. Heated water is pumped to a closed double lumen of a patient delivery tube  85  that is coupled to fluid pathway module  120 . Heated water is recycled from delivery tube  85  into vapor transfer device  99 . In an exemplary embodiment, heated water is supplied through patient delivery tube  85  to minimize water condensation of breathing gas  80  and to maintain the temperature of breathing gas  80  as it makes its way to the patient. Infrared temperature sensors IR 1 , IR 2  monitor the temperature of the water being delivered to and returned from delivery tube  85  and provide feedback to system controller (not shown) in order to maintain a desired temperature of the breathing gas at the outlet of delivery tube  85 . Additional aspects of exemplary delivery tube  85  and vapor transfer device  99  are described in U.S. Patent Application Publication No. 2003/0209246 and U.S. Patent Application Publication No. 2004/0245658, which are incorporated herein fully by reference. 
     Blended gas from base unit  110  is combined with heated water vapor in vapor transfer device  99  to generate heated and humidified breathing gas  80 . The heated and humidified breathing gas  80  is delivered to a breathing gas lumen of the patient delivery tube  85 . Excess heated water delivered to vapor transfer device  99  may be recycled into water reservoir  32 . Bubble sensor BS 1  monitors air bubbles in reservoir  32  and bubble sensor BS 2  monitors the presence of water droplets in the breathing gas  80  to determine when vapor transfer device  99  and/or fluid pathway module  120  should be replaced. In an exemplary embodiment, fluid pathway module  120  has a continuous duty life of about 720 hours and about a 1000 hour test life. 
     Gas Blending Device 
     Referring now to  FIGS.  6 A,  6 B,  6 C,  7 A and  78   , gas blending device  84  is illustrated. Reference to these figures also includes references to sensors represented in  FIG.  5   . Gas blending device  84  may be constructed from a first, generally planar portion  71  ( FIGS.  6 A,  6 B,  6 C ) and a second, generally planar portion  81  ( FIGS.  7 A,  7 B ). Planar portions  71 ,  81  each include complementary channels such that when planar portions  71 ,  81  are assembled together, flow passages are formed within gas blending device  84 . For example, channels  73   a  and  83   a  combine to form a first passage, channels  73   b  and  83   b  combine to form a second passage, and channels  73   c  and  73   c  combine to form a third passage. First portion  71  is configured to mate with second portion  81  shown in  FIGS.  7 A- 7 B  to provide a fully assembled gas blending device  84 . When first portion  71  is secured to second opposing portion  81 , edges  72   a ,  72   b  of first portion  71  form an air tight seal with edges  82   a ,  82   b  of second portion  81  to prevent gas leakage and provide efficient gas blending operation. Gas blending device  84  is configured to be installed internally within base unit  110 . 
     Pilot holes  77   a - c  and pins  78   a - c , shown in  FIGS.  6 A- 6 C , are positioned on first portion  71  of gas blending device  84  to align and secure the first portion  71  to the second opposing portion  81  shown in  FIGS.  7 A- 7 B . In an exemplary embodiment, pilot holes  77   a - c  and pins  78   a - c  provide additional connection surface area and mechanical strength to the gas blending device  84 . Pilot holes  87   a - c  and pin inserts  88   a - c  are positioned on the body of second portion  81  and are configured to mate with pilot holes  77   a - c  and pins  78   a - c  of the first portion  71 . 
     An interior view of the first portion  71  of gas blending device  84  is illustrated in  FIGS.  6 A and  6 B . First portion  71  of gas blending device  84  includes two channels  73   a ,  73   b  that are each configured to receive gas through inlets  74   a ,  74   b , respectively. Channels  73   a ,  73   b  direct gas to blending channel  73   c . Blending channel  73   c  leads to main terminal channel  73   d  where blended gas may exit through second portion  81  and flow to a gas outlet, shown as gas outlet  649  in  FIG.  8 C , of base unit  110 . Upstream of main terminal channel  73   d  is a pressure relief outlet  74   c  that functions as a secondary gas exit if gas pressure exceeds a predetermined value. In an exemplary embodiment, pressure relief outlet  74   c  is part of a pressure relief valve system, which may open or close pressure relief valve  91  (shown schematically in  FIG.  5   ), depending on gas pressure within gas blending device  84 . Passage  73   e  provides for gas flow to oxygen sensor OS 1  (shown schematically in  FIG.  5   ). 
     Referring now to  FIGS.  7 A and  7 B  an interior view of second portion  81  of gas blending device  84  is shown. Second portion  81  of gas blending device  84  is configured to mate with the first portion  71  shown in  FIGS.  6 A- 6 C . Second portion  81  includes channels  83   a ,  83   b  having fins  66   a ,  66   b  within each channel  83   a ,  83   b . Fins  66   a ,  66   b  promote laminar flow across mass flow sensors MFS 1 , MFS 2  (shown schematically in  FIG.  5   ) installed through openings  76   a ,  76   b  of the first portion  71  ( FIG.  7 B ) of gas blending device  84 . Walls  69   a ,  69   b  define channel  83   a  such that when gas blending portion  71  ( FIG.  6 B ) mates with the second opposing portion  81  ( FIG.  7 B ), air tight channels are formed. 
     Channels  83   a ,  83   b  direct gas to a blending channel  83   c  that includes a tortuous path  67  to efficiently mix gas together prior to reaching terminal channel  83   d . Channel  83   e  mates with channel  73   e  to form a passage to oxygen sensor OS 1  (shown schematically in  FIG.  5   ). 
     In an exemplary embodiment, gas metering operation of humidification system  100  determines the precise flow rate of gas within each channel  73   a ,  83   a  to obtain a blended gas mixture having an oxygen saturation level between 21% and 100% O 2 . Oxygen saturation level of blended gas mixture  60  shown in  FIG.  1    may be monitored by oxygen sensor OS 1 , which is shown in  FIG.  5   . 
     As shown in  FIGS.  6 A- 6 C , first portion  71  of gas blending device  84  includes sensor openings  76   a ,  76   b  which open into either end of passage  76   c . Passage  76   c  is in fluid communication with mass flow sensor MFS 1  (not shown in  FIGS.  6 A- 6 C ), which monitors gas flow rate in channel  73   a  to control the gas metering operation of humidification system  100 .  FIG.  6 C  illustrates an exterior view of first portion  71  of gas blending device  84 . First portion  71  includes threaded inserts  79   a ,  79   b  to mount mass flow sensor MFS 1 . 
     As shown in  FIG.  7 B , channel  83   b  of second portion  81  includes sensor openings  86   a ,  86   b  which open into either end of passage  86   c . Passage  86   c  is in fluid communication with mass flow sensor MFS 2  (not shown in  FIGS.  7 A- 7 B ), which monitors gas flow rate in channel  83   a  to control the gas metering operation of humidification system  100 .  FIG.  7 E  illustrates an exterior view of second portion  81  of gas blending device  84 . Second portion  81  includes threaded inserts  89   a ,  89   b  to mount mass flow sensor MFS 2  through openings  86   a ,  86   b . Main gas outlet  68  provides an exit for blended gas to flow to gas outlet  649 , shown in  FIG.  8 C , of base unit  110 . 
     The gas flow rate detected by mass flow sensor MFS 1 , MFS 2  in channel  73   a  may be sent to a microcontroller that controls proportional solenoid valves PSOL 1 , PSOL 2 . Proportional solenoid valve PSOL 1  or PSOL 2  may vary gas input flow in channel  73   a  by increasing or decreasing gas flow through the inlet  74   a . Thus, an adequate ratio of gas flow may be supplied to channel  73   a  to obtain a desired blended oxygen saturation level. In an exemplary embodiment, oxygen sensor OS 1 , which may be positioned in oxygen sensor opening  74   d , is calibrated to 100% O 2  during a system power up sequence. Once calibrated, the oxygen sensor OS 1  measures oxygen content of blended gas to ensure that blended gas is within 98% to 102% of a selected oxygen percentage setpoint. If detected oxygen content falls below 98% of the selected oxygen level, the microcontroller may adjust proportional solenoid valves PSOL 1 , PSOL 2  to increase the flow of oxygen gas  50   b  and/or decrease the flow of air  50   a . Alternatively, if detected oxygen content is above 102% of the selected oxygen level, the microcontroller may adjust proportional solenoid valves PSOL 1 , PSOL 2  to decrease the flow of oxygen gas  50   b  and/or increase the flow of air  50   a  through inlet  74   b  or  74   a.    
     Base Unit Chassis 
     Referring now to  FIGS.  8 A- 8 G , an exemplary embodiment of a base unit chassis  640  is illustrated. Base unit chassis  640  contains the interfaces with fluid pathway module  120 . References to these figures also include references to sensors BS 1 , BS 2 , electronic readers CS 1 , CS 2 , water level sensors LS 1 , LS 2 , temperature sensors IR 1 , IR 2 , pump stator STA 1 , and heater HTR 2 , which are shown schematically in  FIG.  5    and with reference to base unit  110  in  FIG.  8 G . It will be appreciated that base unit chassis  640  houses the electronic components of humidification system  100  and is configured such that liquid does not flow internally through base unit  110 . 
     As shown in  FIGS.  2  and  8 A , base unit chassis  640  includes a front portion  640   a  into which display panel  105  may be installed. Threaded inserts  641   a - d  are provided on front portion  640   a  of chassis  640  to secure display panel  105  to base unit chassis  640 . When display panel  105  is secured, front panel  104  may be fitted over a front edge  642  of base unit chassis  640  to prevent access to the electrical connections of display panel  105 . After installation of front panel  104 , slidable door  103  as shown in  FIG.  3   , may be secured to base unit chassis  640  via threaded inserts  643   a ,  643   b  located on the top and side of base unit chassis  640 . 
     As shown in  FIGS.  3 A,  3 B, and  8 A , base unit chassis  640  includes a component receiving portion  619  configured to receive fluid pathway module  120 . A guide  644   a  extends along the side of the component receiving portion  619  to align and secure fluid pathway module  120  to base unit  100 . Base unit chassis  640  has a seat  645  to support fluid pathway module  120  and a seat opening  646  in which a pump portion such as stator STA 1  may be installed via threaded inserts  647   a - c . When fluid pathway module  120  is seated in component receiving portion  619 , stator STA 1  (shown schematically in  FIG.  5   ) of base unit  110  operationally mates with pump portion PMP 1  (shown schematically in  FIG.  5   ) of fluid pathway module  120 . In an exemplary embodiment, stator STA 1  may drive pump portion PMP 1  which may be an impeller that is magnetically driven by stator STA 1  to advance liquid through a liquid passage of fluid pathway module  120 . An exemplary embodiment of a pump having a separate pump portion PMP 1  and a separate stator portion STA 1  is manufactured by Laing Thermotech, Inc., located in Chula Vista, Calif. In yet another embodiment (not shown), the entire pump portion may be provided only on base unit  110  to provide a low cost, disposable fluid pathway module  120 . Alternatively, the entire pump portion may be provided only on fluid pathway module  120   
     Base unit chassis  640  includes a recessed portion  648  that has a gas outlet  649 . When fluid pathway module  120  is inserted on base unit  110 , recessed portion  648  aligns with and supports a gas receiving portion  130 , shown in  FIG.  10 B , of fluid pathway module  120 . The gas receiving portion of fluid pathway module  120  includes a gas inlet which is configured to couple to the gas outlet  649  of base unit  110  to provide an air tight seal through which gas may be transferred from base unit  110  to the gas passage of fluid pathway module  120 . 
     The component receiving portion  619  of base unit chassis  640  has a rectangular opening  651  into which heater HTR 2 , shown in  FIG.  5   , such as a heat conduction plate may be installed. When fluid pathway module  120  is releasably mounted to base unit  110 , heater HTR 2  of base unit  110  contacts heater HTR 1 ,  FIG.  5   , of fluid pathway module  120 ,  FIG.  3   . Heater HTR 1  of fluid pathway module  120  may also be a heat conduction plate HTR 2  such that when electrical current is supplied to the heat conduction plate of base unit  110 , energy is transferred to heat conduction plate HTR 1  of fluid pathway module  120 . Thermal energy received by heat conduction plate HTR 1  of fluid pathway module  120  is used to heat the liquid contained within fluid pathway module  120  for delivery to the patient at a temperature specified by the user. In an exemplary embodiment, the user may adjust the temperature setting of humidification system in 1° C. steps to a maximum temperature of 43° C. and a minimum temperature of 33° C. A temperature sensor opening  652   a  in base unit chassis  640  is configured to receive temperature sensor IR 1  to monitor the temperature of the liquid that is supplied to delivery tube  85 , 
     Base unit chassis  640  also includes a bubble sensor opening  653   a  adjacent temperature sensor opening  652   a . Bubble sensor opening  653   a  is configured to receive bubble sensor BS 1  that monitors the formation of air bubbles in liquid reservoir  32  of fluid pathway module. Additional aspects of bubble sensor BS 1  will be described in further detail below. 
     Referring to  FIGS.  5  and  8 A , two water level sensor openings  654   a ,  654   b  are positioned on base unit chassis  640 . Water level sensor openings  654   a ,  654   b  are each configured to receive a water level sensor LS 1 , LS 2  to monitor the water level within liquid reservoir  32  of fluid pathway module  120 . In an exemplary embodiment, optical water level sensors LS 1 , LS 2  are aimed at reflectors  128   a ,  128   b , shown in  FIG.  10 A , located at the top and bottom of liquid reservoir  32  to determine when the liquid reservoir  32  is full, low, or empty. Additional aspects of water level sensors LS 1 , LS 2 , shown in  FIG.  5   , will be described in further detail below. 
       FIG.  8 B  illustrates a rear view of base unit chassis  640  in which water level sensors LS 1 , LS 2  may be installed. As described above, water level sensor openings  654   a ,  654   b  are each configured to receive a water level sensor LS 1 , LS 2 , respectively. Water level sensors LS 1 , LS 2  may be mounted to base unit chassis  640  via threaded inserts  655   a - d . Rear panel  102 , shown in  FIG.  4   , may be fitted over rear edge  656  of base unit chassis  640  to restrict access to the electrical connections of water level sensors LS 1 , LS 2 . 
       FIG.  8 C  illustrates a side view of base unit chassis  640  in which fluid pathway module  120 , shown in  FIG.  3   , may be releasably mounted. An electronic reader opening  657  on base unit chassis  640  is configured to receive electronic reader CS 1 , CS 2  that detects the type of vapor transfer device  99  coupled to fluid pathway module  120 . Vapor transfer device  99  may be, for example, a disposable, cartridge that is labeled with an indicator  194  such as a sticker or barcode. In an exemplary embodiment, a high flow vapor transfer device  99  is labeled with an all-reflective sticker while a low flow vapor transfer device  99  is labeled with a portion reflective/portion non-reflective sticker. Electronic reader CS 1 , CS 2  may monitor the optical properties of the sticker to identify the type of vapor transfer device  99  installed in humidification system  100 . When indicator  194  is read by electronic reader CS 1 , CS 2 , a signal is sent to a microprocessor (not shown) to control the gas metering operation of humidification system  100 . For example, when a low flow rate vapor transfer device  99  is installed, the microcontroller may limit the flow rate of the breathing gas being delivered to a setpoint between 0.5 LPM (liters per minute) and 8 LPM. If a high flow rate vapor transfer device  99  is installed, the microcontroller may limit the flow rate between 8 LPM and 40 LPM. In an exemplary embodiment, when a user attempts to adjust the flow rate setpoint beyond the limits defined by the microcontroller, humidification system  100  generates an auditory warning and prevents the setpoint from deviating beyond the maximum and minimum flow rate limits of a high flow or low flow vapor transfer device  99 . 
     Referring to  FIGS.  5  and  8 C , a temperature sensor opening  652   a  of base unit chassis  640  is configured to receive infrared temperature sensor IR 1  that detects the temperature of liquid in fluid pathway module  120 . A second temperature sensor opening  652   b  is configured to receive second infrared temperature sensor IR 2  that detects the temperature of liquid returning back to fluid pathway module  120  from delivery tube  85 . In an exemplary embodiment, monitoring of the two temperatures allows humidification system  100  to efficiently operate heater HTR 2 , which is mounted through heater opening  651  of base unit chassis  640 . For example, activation and deactivation of HTR 2  may be controlled by a PID (proportional-integral-derivative) feedback controller to maintain a consistent temperature of the breathing gas being delivered to a patient. 
     Adjacent the temperature sensor openings  652   a ,  652   b  of base unit chassis  640  are bubble sensor openings  653   a ,  653   b  that are each configured to receive a bubble sensor BS 1 , BS 2 , respectively. During operation of humidification system, air bubbles may be detected in liquid reservoir  32  of fluid pathway module  120  due to air permeating under pressure through the exchange media in vapor transfer device  99  of fluid pathway module  120 . Under normal operating conditions, the water and gas passages of fluid pathway module  120  are connected to vapor transfer device where a portion of the liquid is transferred to the gas. Over time, as gas and liquid flow internally through vapor transfer device, the core of vapor transfer device may begin to degrade such that the mixing interface between the gas and liquid passages erodes. As the interface degrades, gas from the gas passage may pass into the liquid passage such that air bubbles begin to form in liquid reservoir  32  of fluid pathway module  120 . Conversely, liquid from the liquid passage may pass into the gas passage such that liquid droplets are mixed into the gas flow. Bubble sensors BS 1 , BS 2  that are mounted in bubble sensor openings  653   a ,  653   b  of base unit chassis  640  detect these conditions and send appropriate signals to the microcontroller to warn a user of when either or both conditions exist. 
     In an exemplary embodiment, bubble sensor BS 1 , shown in  FIG.  5   , of humidification system  100  detects the rate at which air bubbles are formed in liquid reservoir  32  of fluid pathway module  120 . If the bubble formation rate rises above a predetermined level, an auditory warning may be generated and vapor transfer device fault icon  112   c , shown in  FIG.  13   , may illuminate on display panel  105  to indicate that vapor transfer device  99  should be replaced with a new cartridge. Alternatively, second bubble sensor BS 2  detects the rate at, which liquid droplets form in the gas passage of vapor transfer device  99 . If the droplet formation rate exceeds a predetermined level, the gas metering and warming operations of humidification system  100  may be suspended. An auditory warning may be generated and fluid pathway module  120  icon  115 , shown in  FIG.  13   , may illuminate on display panel  105 , shown in  FIG.  13   , to indicate that fluid pathway module  120  should be replaced. 
       FIG.  8 D  Illustrates a side view of base unit chassis  640  in which electronic reader CS 1 , CS 2 , temperatures sensors IR 1 , IR 2 , bubble sensors BS 1 , BS 2 , and heater HTR 2  may be installed. As described above, electronic reader opening  657  is configured to receive electronic reader CS 1 , CS 2  which may be installed on base unit chassis  640  via threaded inserts  658   a ,  658   b . Temperature sensor openings  652   a ,  652   b  are each configured to receive temperature sensor IR 1 , IR 2 , which may be mounted to base unit chassis  640  via threaded inserts  659   a ,  659   b  and bubble sensors BS 1 , BS 2  may be mounted to bubble sensor openings  653   a ,  653   b  via threaded inserts  661   a - c . Heater HTR 2  such as a heat conduction plate may be mounted to the heat conduction plate opening  651  via threaded inserts  663   a - d . A side panel (not shown) may be fitted over the rear edge  664  of base unit chassis  640  to restrict access to the electrical connections of electronic components, such as readers CS 1 , CS 2 , bubble sensors BS 1 , BS 2  ( FIG.  5   ), temperature sensors IR 1 , IR 2 , and heater HTR 1 , which are all shown schematically in  FIG.  5   . 
       FIG.  8 E  illustrates a cross-sectional view of base unit chassis  640  along the  8 E- 8 E line shown in  FIG.  8 D . As shown in  FIG.  8 E , base unit chassis  640  includes a recessed portion  648  configured to receive a gas receiving portion of fluid pathway module  120 . Gas outlet  649  of recessed portion  648  is configured to receive a gas inlet of fluid pathway module  120  to supply gas into the gas passage of fluid pathway module  120 . Water level sensor openings  654   a ,  654   b  are disposed along the top and bottom of the component receiving portion  619  and are configured to receive water level sensors LS 1 , LS 2  to detect water level in fluid pathway module  120 . Guide  644   a  extends from the side of the component receiving portion  619  to align and secure fluid pathway module  120  to base unit  110 . 
       FIG.  8 F  illustrates a top view of base unit chassis  640 . As described above, base unit chassis  640  includes a component receiving portion  619  having a seat  645  that is configured to seat fluid pathway module  120 . A seat opening  646  is provided to receive pump portion STA 1  of base unit  110  which may be secured to base unit  110  via threaded inserts  647   a - c . Guides  644   a ,  644   b  extend from the side of the component receiving portion  619  to align and secure fluid pathway module  120  to base unit  110 , A recessed portion  648  of base unit chassis  640  includes a gas outlet  649  which receives gas inlet of fluid pathway module  120 . Opening  651  is positioned on base unit chassis  640  to receive heater HTR 2 . 
     Referring now to  FIG.  9   , a pump portion  96  of base unit  110  is illustrated. Pump portion  96  is configured to mount to seat opening  646 , shown in  FIG.  8 A , of base unit chassis  640  via openings  97   a - d  on pump portion  96 . Installation bracket  99  facilitates alignment and installation of pump portion  96  Pump portion  96 , such as stator STA 1  shown in  FIG.  5   , is configured to operationally mate with pump portion PMP 1  of fluid pathway module  120  to advance liquid through the liquid passage of fluid pathway module  120 . In an exemplary embodiment, pump portion PMP 1  of fluid pathway module  120  is a rotatable impeller  98  that magnetically couples to pump portion  96 . When impeller  98  is operationally mated with pup portion  96 , rotation of impeller  98  drives the rotation of pump portion PMP 1 , thereby increasing pressure and liquid flow within fluid pathway module  120 . 
     Fluid Pathway Module (Vapor Transfer Unit) 
     Referring now to  FIGS.  3 A,  3 B,  10 A and  10 B , an exemplary embodiment of fluid pathway module  120  shown in  FIG.  3    is illustrated. Fluid pathway module  120  is configured to be releasably mounted to base unit  110  to accommodate reuse of base unit  110  and selective disposal of fluid pathway module  120 . 
     Referring to  FIGS.  3 A,  5 , and  10 A , features and sensors of fluid pathway module  120  are discussed. Fluid pathway module  120  includes handle  121  that may be used to insert and remove fluid pathway module  120  from base unit  110 . When fluid pathway module  120  is mounted to base unit  110 , heater HTR 1  contacts heater HTR 2 . Pump portion STA 1  of base unit  110  engages pump portion PMP 1  of fluid pathway module  120 . As shown in  FIG.  10 A , fluid pathway module  120  includes liquid inlet  124  and gas outlet  125 . Liquid inlet  124  receives liquid from supply line  75  and gas outlet  125  delivers heated and humidified breathing gas to a patient via delivery tube  85 . 
     When liquid (such as water) is supplied to liquid inlet  124 , liquid is stored within reservoir  32  of fluid pathway module  120 . A sight glass  126  on the side of fluid pathway module  120  provides visual indication of liquid amount in reservoir  32  via a plastic ball  127  floating within reservoir  32 . Two reflectors  128   a ,  128   b  are visible through the sight glass  126  and are positioned to align with water level sensors LS 1 , LS 2 . Water level sensors LS 1 , LS 2  of base unit  110  optically sense water level in fluid pathway module  120  by monitoring light reflection off reflectors  128   a ,  128   b . For example, when reservoir  32  is full, light reflection from reflector  128   a  is blocked by plastic ball  127  and the humidification system microcontroller (not shown) determines that water level in fluid pathway module  120  is full. When reservoir  32  is empty, light reflection from reflector  128   b  is blocked by plastic ball  127  and humidification system  100  may cease operation until water is added. If light is reflected from both reflectors  128   a ,  128   b , plastic ball  127  is floating between reflectors  128   a ,  128   b  and microcontroller (not shown) may illuminate a low water icon  116 , shown in  FIG.  13   , on display panel  105 , shown in  FIG.  3   , of base unit  110  to indicate a low water condition. 
     Referring to  FIGS.  5 , and  10 B , features and sensors of fluid pathway module  120  are discussed.  FIG.  10 B  illustrates a side view of fluid pathway module  120 . Fluid pathway module  120  includes heater HTR 1  in the form of heat conduction plate  122  that heats liquid stored in fluid pathway module  120  by receiving thermal energy from heater HTR 1 . Adjacent heat conduction plate  122  are two temperature reflectors  129   a ,  129   b  that are positioned to align with two infrared temperature sensors IR 1 , IR 2 . In an exemplary embodiment, temperature sensor IR 1 , aligned with temperature reflector  129   a , monitors the temperature of liquid heated by heat conduction plate  122  and temperature sensor IR 2 , aligned with temperature sensor reflector  129   b , monitors the temperature of liquid returning to fluid pathway module  120  from delivery tube  85 . 
     Fluid pathway module  120  includes gas inlet  130  which is configured to receive gas from base unit  110 . When fluid pathway module  120  is mounted to base unit  110 , as shown in  FIG.  3   , an air tight seal is formed between gas inlet  130  and gas outlet  649  of base unit chassis  640 , shown in  FIG.  8 C . As shown in  FIGS.  5  and  10 B , gas received through gas inlet  130  and liquid stored in reservoir  32  flow through passages within fluid pathway module  120  into vapor transfer device  99 . Vapor transfer device  99  is mounted to fluid pathway module  120  to form a vapor transfer assembly. Vapor transfer device  99  combines water vapor and gas received from fluid pathway module  120  to form heated and humidified breathing gas. Heated and humidified breathing gas flows from vapor transfer device  99  through outlet  125 , shown in  FIG.  10 A  of fluid pathway module  120 , and to the patient via delivery tube  85 . Exemplary embodiments of vapor transfer devices that may be used with the present invention are disclosed in U.S. patent application Ser. No. 11/851,713, and U.S. patent application Ser. No. 10/810,768, which are both incorporated by reference herein in their entireties. 
     Indicator  194 , such as a barcode or sticker, is positioned on vapor transfer device  99  such that when vapor transfer device  99  is coupled to fluid pathway module  120  and mounted on base unit  110  ( FIG.  3   ), an electronic reader CS 1 , CS 2  aligns with indicator  194 . Thus, the type of vapor transfer device  99  installed on fluid pathway module  120  such as a low flow or high flow cartridge, described above, may be determined by a microcontroller (not shown) of humidification system  100 . 
     During operation of humidification system  100 , the internal core of vapor transfer device  99  may degrade, resulting in the mixing of gas and water vapor within vapor transfer device  199  becoming less efficient. In this instance, gas pockets may enter the liquid passage of fluid pathway module  120  so that air bubbles form in liquid reservoir  32 . In another instance, droplets of liquid may enter the gas passage. These conditions are monitored by bubble sensors BS 1 , BS 2 . Bubble sensors BS 1 , BS 2  align with bubble reflectors  131   a ,  131   b  on fluid pathway module  120 . In an exemplary embodiment, bubble sensor BS 1  aligned with bubble reflector  131   a  monitors air bubble formation within liquid reservoir  32  and bubble sensor BS 2  aligned with bubble reflector  131   b  monitors liquid droplets in the gas passage. When the rate at which air bubbles or liquid droplets are detected exceed predefined detection rates, signals may be sent to humidification system microcontroller to generate auditory warnings and illuminate fault icons  115 ,  112   c  on display panel  105 , shown in  FIG.  13   , to warn a user of these conditions. Additional aspects of warnings and fault icons will be described in further detail below. 
       FIGS.  11 A and  11 B  illustrate exploded views of a portion of fluid pathway module  120  shown in  FIGS.  10 A and  10 B . As shown in  FIG.  11 A , main body  120   a  of fluid pathway module  120  includes liquid reservoir  32  to store liquid received through liquid inlet  124 . A water level sensor plate  133  having upper reflector  128   a  and lower reflector  128   b  is configured to be positioned within the reservoir  32  and is visible through sight glass  126 . A non-reflective level ball  127  is configured to be positioned within the reservoir  32  such that it floats within reservoir  32  as previously described. Optical water level sensors LS 1 , LS 2  monitor the light reflection from reflectors  128   a ,  128   b , thereby determining the water level within reservoir  32 . 
     Main body  120   a  of fluid pathway module  120  includes heater HTR 1  in the form of a heat conduction plate. Throughout device  100 , heat is transferred from heater HTR 2 , located in base unit  110  to heater HTR 1 , located in fluid pathway module  120 , via conduction. Heat is then transferred by conduction from heater HTR 1  to liquid in reservoir  32  when fluid pathway module  120  is mounted to base unit  110 . A temperature and bubble sensor plate  135  is adjacent heater portion  134  and is configured to couple with temperature reflectors  129   a ,  129   b  and bubble reflectors  131   a ,  131   b.    
     As shown in  FIG.  11 B , temperature and bubble sensor plate  135  is configured to fit over temperature reflector portions  129   a ,  129   b  and bubble reflector portions  131   a ,  131   b . Main body  120   a  includes handle  121  which may be used to insert or remove fluid pathway module  120  from base unit  110 . Tubular support structures  136   a - e  extend from the side of main body  120   a . Tubular support structures  136   a - e  are configured to mate with vapor transfer device adapter  120   b , shown in  FIG.  12   . In an exemplary embodiment, vapor transfer device  99  is configured to couple to support structures  136   a - c  such that passages defined by support structures  136   a - c  connect with passages of vapor transfer device  99 . Liquid and gas may be exchanged through the passages to generate heated and humidified breathing gas in vapor transfer device  99 . Heated and humidified breathing gas, for example, may be supplied to gas outlet  125  of fluid pathway module  120 , shown in  FIG.  10 A , through the passage defined by support structure  136   a . In an exemplary embodiment, heated liquid may be supplied to vapor transfer device  99  through the passage defined by support structure  136   c  and recycled through the passage defined by support structure  136   b.    
     Referring now to  FIGS.  11 A and  12   , vapor transfer device adapter  120   b  is configured to couple to main body  120   a  to fully assemble fluid pathway module  120 . Vapor transfer device adapter  120   b  includes passages  137   a - e  that are configured to mate with support structures  136   a - e  of main body  120   a.    
     Referring now also to  FIGS.  10 A and  10 B , vapor transfer device adapter  120   b  includes gas inlet portion  130   a  configured to mate with reciprocal gas inlet portion  130   b  to form gas inlet  130 . Gas inlet  130  receives gas from base unit  110  and channels gas to gas opening  138 . Gas opening  138  is configured to connect to a gas passage of vapor transfer device  99  and supply gas into vapor transfer device  99 . Gas from gas opening  138  and heated water from passage  137   c  are used in vapor transfer device  99  to generate heated and humidified breathing gas. In an exemplary embodiment, heated and humidified breathing gas exits vapor transfer device  99  through passage  137   a  and is delivered to breathing gas outlet  125  of fluid pathway module  120 . As further shown in  FIGS.  5  and  12   , excess water in vapor transfer device  99  from passage  137   c  is recycled back into liquid reservoir  32  through passage  137   b.    
     In an exemplary embodiment, gas inlet  130  includes an air port ball  139   a  that is configured to be contained within cap  130   b . An O-ring  139   c  is positioned at a distal end of cap  139   b  and provides a circumferential seal around a distal opening of the cap  139   b . Cap cover  139   d  is positioned distally from O-ring  139   c  and is configured to seat O-ring  139   c  when cap cover  139   d  is secured around a portion of cap  139   b . In an embodiment of the present invention, when gas inlet portions  130   a ,  130   b  are mated together and coupled to cap  139   b  and cap cover  139   d , the air port ball  130   a  moves freely between O-ring  139   c  and distal opening  193   a ,  193   b  of gas inlet portions  130   a ,  130   b . In one embodiment, gas flow into gas inlet  130  causes air port ball  139   a  to move in a direction opposing gravity. When gas flow through gas inlet  130  exceeds gravitational pull on air port ball  139   a , air port ball  139   a  will “float”. In another embodiment, when gas is not supplied to gas inlet  130 , air port ball  130   a  will contact and seal O-ring  139   c  to prevent air flow into gas outlet  649  ( FIG.  8 C ) of base unit chassis  640  ( FIG.  8 C ). Under normal operating conditions, air port ball  139   a  will “float” between distal opening  193   a ,  193   b  and O-ring  139   c , thereby providing gas flow through gas opening  138 . Thus, as, shown in  FIGS.  1  and  5   , gas only flows in one direction from base unit  110  into fluid pathway module  120 . 
     Referring now to  FIGS.  10 B  and  FIG.  12   , a flexible lip  139   e  is positioned between gas inlet portions  130   a ,  130   b  and adjacent gas opening  138  in one embodiment, when vapor transfer device  99  is coupled to fluid pathway module  120 , lip  139   e  is deformably opened such that gas may flow into vapor transfer device  99 . When vapor transfer device  99  is removed from fluid pathway module  120 , lip  139   e  is deformably sealed to prevent air from flowing in a direction from gas opening  138  into the gas outlet  130 . 
     Operation and Display 
     Referring now to  FIGS.  13  and  14   , exemplary aspects of operational modes, warning indicators and a flow chart of the operation of humidification system is  100  are illustrated. Vapor transfer device  99  is releasably mounted to fluid pathway module  120  such that liquid and gas communication is provided between vapor transfer device  99  and fluid pathway module  120  (STEP  510 ). Fluid pathway module  120  is releasably mounted to base unit  110  such that breathing gas  50   a ,  50   b  supplied into base unit  110  flows into fluid pathway module  120  and through vapor transfer device  99  (STEP  520 ). Breathing gases  50   a ,  50   b  are blended inside a gas blending device  84  to form a blended gas  60  (STEP  530 ). Thermal energy is transferred from base unit  110  to fluid pathway module  120 , thereby heating liquid  70  in a liquid passage of fluid pathway module  120  (STEP  540 ). Liquid  70  is heated in fluid pathway module  120  and is used to heat and humidify blended gas  60  and maintain a desired temperature of breathing gas  80  (STEP  550 ). Heated liquid  70  and blended gas  60  are passed through vapor transfer device  99 , thereby forming heated and humidified breathing gas  80 , which is then delivered to a patient (STEP  560 ), e.g., via a nasal cannula (not shown). 
     Front panel  104  of base unit  110  includes display panel  105  that provides visual indication of the operating conditions of humidification system  100 . In an exemplary embodiment, when AC power is supplied to humidification system  100  through electrical cord  65 , battery icon  113  may illuminate on display panel  105  to indicate that an Internal battery (not shown) is, charging. Battery icon  113  may flash to indicate that the battery backup time is reduced in the event that AC power is lost during charging, When the battery is fully charged, battery icon  113  automatically switches off. 
     When humidification system  100  is powered on and electrical cord  65  is disconnected from base unit  110 , battery icon  113  may illuminate to indicate that DC power is being used. When a loss of AC power occurs in RUN mode, system  100  automatically enters BATTERY mode. In BATTERY mode, heater HTR 2  and pump stator STA 1  are turned off to conserve battery power. Gas flow control and delivery continues unabated. When AC power is reestablished, system  100  automatically returns to RUN mode. In BATTERY mode, pressing the Run button causes system  100  to enter POWER_OFF mode. If battery capacity is exhausted, system  100  will enter POWER OFF mode. 
     When humidification system  100  is powered off, pressing the “Standby/Run” button  108   b  activates an initial boot-up stage that performs a series of self-tests to verify the proper function of subsystems, sensors, and actuators contained in base unit  110 . During system boot-up, if any self-test diagnosis fails, a system fault icon  114  is illuminated on display panel  105 , and operation of humidification system  100  is disabled. If all self-tests pass, humidification system  100  transitions to “standby” mode and sensors in base unit  110  are activated to detect the presence of fluid pathway module  120  in base unit  110 . If fluid pathway module  120  is not detected on base unit  110  or if bubble sensors BS 1 , BS 2  detect that fluid pathway module  120  needs to be replaced, fluid pathway module fault icon  115  is illuminated. In an embodiment of the present invention, when fluid pathway module  120  is mounted to base unit  110 , fluid pathway module fault icon  115  is switched off and water level sensors LS 1 , LS 2  in base unit  110  are activated to detect water level in fluid pathway module  120 . If the water level is low, a low water icon  116  flashes on/off and an audible alarm sounds to indicate that reservoir  32  of fluid pathway module  120  should be refilled by providing additional water through water supply line  75 . If the water is empty, low water icon  116  in constantly illuminated an audible alarm sounds. 
     When fluid pathway module  120  is mounted to base unit  110 , an electronic reader CS 1 , CS 2  is activated to detect the presence of vapor transfer device  99 . If vapor transfer device  99  is not detected on fluid pathway module  120  or if bubble sensors BS 1 , BS 2  detect that vapor transfer device  99  is worn, vapor transfer device fault icon  112   c  is illuminated. In an exemplary embodiment, when vapor transfer device  99  is coupled to fluid pathway module  120  and installed in base unit  110 , the type of vapor transfer device  99  installed is determined by electronic reader CS 1 , CS 2 . For example, if a high flow vapor transfer device  99  is detected, a high flow icon  112   a  is illuminated on the display panel  105 . If a low flow vapor transfer device is detected  99 , a low flow icon  112   b  is illuminated. In yet another embodiment, detection of fluid pathway module  120  and vapor transfer device  99  is performed concurrently such that when vapor transfer device  99  is detected, the system automatically determines that fluid pathway module  120  is installed on base unit  110 . 
     Gas flow to the patient is a metered blending of the two input gases, such as medical air and oxygen. A closed feedback control loop exists between the proportional solenoids PSOL 1 , PSOL 2  that control the flow of each gas, and mass flow MFS 1 , MFS 2  sensors that measure the gas flow. 
     A gas blending algorithm controls the gas blending process. A gas blending algorithm suitable to control the gas blending process will be understood by one having ordinary skill in the art from the description herein. Mass flow sensors MFS 1 , MFS 2  measure the flow rates of medical air and oxygen gases. Proportional solenoids PSOL 1 , PSOL 2  control the flow rates of the gases. Each valve PSOL 1 , PSOL 2  is controlled by a digital to analog converter (DAC), not shown. 
     A non-linear relationship exists between the output of gas flow sensors MFS 1 , MFS 2  and the corresponding representation in engineering units, such as Standard Liters Per Minute (SLPM). In order to maximize the accuracy of operation and to compensate for part tolerances, a suitable lookup table is provided in the system microprocessor to implement a non-linear transformation function. In one embodiment, the lookup table includes  201  entries that are defined for each of mass flow sensors MFS 1 , MFS 2 . The lookup table is indexed by engineering units in 0.25 SLPM increments, and returns values corresponding to the output of mass-flow sensors MFS 1 , MFS 2  in raw 12-bit A/D counts. Fractional indices may be resolved through linear interpolation between table entries. 
     When system  100  is configured for single gas operation, oxygen saturation level  106   c  is set for 21% for air and 100% for oxygen. An audible alarm sounds if the user attempts to edit or otherwise adjust the value for oxygen saturation level  106   c . To select single gas operation, the user attaches an air or an oxygen supply to one of gas inlet ports  101   a ,  101   b  while system  100  is in standby mode. 
     To select dual gas operation, the user attaches gas supply lines to each of gas inlet ports  101   a ,  101   b  while system  100  is in standby mode. If either gas supply loses pressure while system  100  is in dual gas operation, an audible alarm sounds. 
     It is further contemplated that when any humidification system  100  fault condition exists, auditory warning alarms may be generated. For example, auditory tones and alarms may be generated concurrently when warning indicators are displayed on display panel  105 . In another embodiment, alarms may be programmed with unique auditory patterns depending of the priority of the warning. For example, a low priority auditory warning may sound briefly to indicate the occurrence of an event that does not require immediate user attention, whereas a higher priority auditory warning may sound continuously to indicate that immediate attention is required. 
     Warning alarms may be muted by pressing the mute button  108   a  of the user interface  107 . In one embodiment, pressing the mute button  108   a  illuminates LED  109   a  to provide visual indication that warning alarms are muted. In another embodiment, pressing alarm button  108   a  mutes low priority auditory warnings, while higher priority auditory warnings may remain auditory. In yet another embodiment, alarm button  108   a  function may be programmed with additional user adjustable settings such as controlling the brightness of display panel  105 . For example, pressing alarm button  108   a  for a period of time may adjust the brightness of display panel  105 . 
     When humidification system  100  is in “standby” mode, user settings such as the temperature  106   a , flow rate  106   b , and oxygen saturation level  106   c  of the breathing gas may be adjusted using encoder knob  111  of user interface  107 . In exemplary embodiment, pressing encoder knob  111  cycles through user settings that can be adjusted. Pressing the encoder knob  111  once, for example, may activate the temperature adjustment setting and pressing encoder knob  111  in succession may cycle through additional user settings that can be adjusted. In an exemplary embodiment, pressing encoder knob  111  causes the user setting that is activated to blink on display panel  105 , thus indicating the specific user setting that may be adjusted. In an exemplary embodiment, rotating encoder knob  111  while in an activated user setting allows the current user setting setpoint to be adjusted. For example, clockwise rotation of encoder knob  111  may increase the setpoint and rotating knob  111  counterclockwise may decrease the setpoint. In another embodiment, encoder knob  111  has an acceleration feature, in which turning knob  111  faster causes the setpoint to increase or decrease in larger steps. 
     According to one embodiment, after the desired user setting has been set, pressing ‘Standby/Run’ button  108   b  transitions humidification system from “standby” mode to “run” mode. When the, system is in “run” mode, status LED  109   b  may be illuminated to indicate that the gas metering and heating operations of the system are activated to deliver heated and humidified gas to the patient. Base unit  110  includes gas pressure sensors to detect if the breathing gas delivery tube  85  is blocked and if gas supply into base unit  110  is too low or too high. A tube fault icon  117  may be lit on display panel  105  when base unit senses a pressure indicating that the breathing gas delivery tube  85  is kinked or blocked. Gas supply fault icon  118  may be displayed when a gas supply problem, such as low or high gas pressure is input to humidification system  100 . 
     Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention.