Patent Publication Number: US-2013239965-A1

Title: Humidification system with signal transmission optimization

Description:
This patent application claims the priority benefit under 35 U.S.C. §119(e) of U.S. Provisional Application No. 61/295,299 filed on Jan. 15, 2010, the contents of which are herein incorporated by reference. 
    
    
     The invention relates to a pressure support device configured to provide pressure support therapy to a subject, wherein the pressure support device comprises a humidifier configured to control the humidity of gas provided to the subject by the pressure support device. 
     Pressure support devices that provide pressure support therapy to the airway of a subject are known. Some conventional pressure support devices include humidifiers configured to control the level of humidity of gas provided to the subject during pressure support therapy. In conventional pressure support devices, increasing a capacity to hold liquid for use in the humidifier may enhance the convenience of the pressure support device to users. However, typically, increasing the capacity to hold liquid in a pressure support device humidifier may increase the volume of the flow path formed by the humidifier through which gas must travel on the way to the subject. 
     One aspect of the invention relates to a pressure support device configured to generate a pressurized flow of breathable gas for delivery to an airway of a subject, the pressure support device comprising a humidifier configured to humidify the pressurized flow of breathable gas. In one embodiment, the humidifier comprises a chamber, a heating element, and a partition. The chamber includes a gas inlet configured to receive a flow of gas into the chamber and a gas outlet configured to release the flow of gas from the chamber. The chamber is configured to hold liquid. The heating element is configured to controllably elevate the temperature of fluid within the chamber to vaporize liquid within the chamber such that the gas flowing through the chamber from the gas inlet to the gas outlet is humidified by the vaporized liquid. The partition is configured to separate the chamber into a first section and a second section. The first section forms a flow path from the gas inlet to the gas outlet. The partition permits fluid communication between liquid held in the first section and liquid held in the second section while restricting contact between the gas within the second section and the pressurized flow of breathable gas as the pressurized flow of breathable gas flows through the first section such that the flow of gas is maintained within the first section. 
     Another aspect of the invention relates to a method of humidifying a pressurized flow of breathable gas generated by a pressure support device for delivery to an airway of a subject. In one embodiment, the method comprises holding liquid in a chamber that includes a gas inlet configured to receive a flow of gas into the chamber and a gas outlet configured to release the pressurized flow of breathable gas from the chamber; separating a first section of the chamber from a second section of the chamber such that the first section forms a flow path from the gas inlet to the gas outlet, and such that liquid held in the first section and liquid held in the second section are in fluid communication while restricting contact between the gas within the second section and the pressurized flow of breathable gas as the flow of gas flows through the first section such that the flow of gas is maintained within the first section; and controllably elevating the temperature of fluid within a first section of the chamber that forms a flow path between the gas inlet and the gas outlet such that liquid within the first section on of the chamber is vaporized and humidifies gas flowing through the first section of the chamber from the gas inlet to the gas outlet. 
     Yet another aspect of the invention relates to a pressure support device configured to generate a pressurized flow of breathable gas for delivery to an airway of a subject, the pressure support device comprising a system configured to humidify the pressurized flow of breathable gas. In one embodiment, the system comprises means for holding liquid that includes a gas inlet configured to receive a flow of gas and a gas outlet configured to release the flow of gas; means for separating the means for holding liquid into a first section and a second section such that the first section forms a flow path from the gas inlet to the gas outlet, and such that liquid held in the first section and liquid held in the second section are in fluid communication while restricting contact between the gas within the second section and the flow of gas as the pressurized flow of breathable gas flows through the first section such that the flow of gas is maintained within the first section; and means for controllably elevating the temperature of the liquid such that liquid in the first section is vaporized and humidifies gas flowing through the flow path. 
    
    
     
       These and other objects, features, and characteristics of the present invention, as well as the methods of operation and functions of the related elements of structure and the combination of parts and economies of manufacture, will become more apparent upon consideration of the following description and the appended claims with reference to the accompanying drawings, all of which form a part of this specification, wherein like reference numerals designate corresponding parts in the various figures. In one embodiment of the invention, the structural components illustrated herein are drawn to scale. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not a limitation of the invention. In addition, it should be appreciated that structural features shown or described in any one embodiment herein can be used in other embodiments as well. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention. 
         FIG. 1  illustrates a pressure support device configured to provide pressure support therapy to a subject, in accordance with one or more embodiments of the invention; 
         FIG. 2  illustrates a humidifier of a pressure support device, according to one or more embodiments of the invention; 
         FIG. 3  illustrates a unitary partition structure of a humidifier of a pressure support device, according to one or more embodiments of the invention; 
         FIG. 4  illustrates a humidifier of a pressure support device, according to one or more embodiments of the invention; and 
         FIG. 5  illustrates a humidifier of a pressure support device, according to one or more embodiments of the invention. 
     
    
    
     As used herein, the singular form of “a”, “an”, and “the” include plural references unless the context clearly dictates otherwise. As used herein, the statement that two or more parts or components are “coupled” shall mean that the parts are joined or operate together either directly or indirectly, i.e., through one or more intermediate parts or components, so long as a link occurs. As used herein, “directly coupled” means that two elements are directly in contact with each other. As used herein, “fixedly coupled” or “fixed” means that two components are coupled so as to move as one while maintaining a constant orientation relative to each other. 
     As used herein, the word “unitary” means a component is created as a single piece or unit. That is, a component that includes pieces that are created separately and then coupled together as a unit is not a “unitary” component or body. As employed herein, the statement that two or more parts or components “engage” one another shall mean that the parts exert a force against one another either directly or through one or more intermediate parts or components. As employed herein, the term “number” shall mean one or an integer greater than one (i.e., a plurality). 
     Directional phrases used herein, such as, for example and without limitation, top, bottom, left, right, upper, lower, front, back, and derivatives thereof, relate to the orientation of the elements shown in the drawings and are not limiting upon the claims unless expressly recited therein. 
       FIG. 1  illustrates a pressure support device  10  configured to provide pressure support therapy to a subject  12 . Pressure support device  10  is configured to provide the pressure support therapy in the form of a pressurized flow of breathable gas that is delivered to the airway of subject  12 . The pressure support therapy may be dynamic in that one or more parameters of the pressurized flow of breathable gas generated by pressure support device  10  may be adjusted based on detection of one or more parameters. For example, the pressure of the pressurized flow of breathable gas may be increased based on changes to one or more parameters that indicate a respiratory event (e.g., an apnea, snoring, etc.). In one embodiment, pressure support device  10  includes one or more of a pressure generator  14 , electronic storage  16 , a user interface  18 , a sensor  20 , a processor  22 , a humidifier  24 , and/or other components. 
     In one embodiment, pressure generator  14  is configured to generate a pressurized flow of breathable gas for delivery to the airway of subject  12 . Pressure generator  14  may control one or more parameters of the pressurized flow of breathable gas (e.g., flow rate, pressure, volume, humidity, temperature, gas composition, etc.) for therapeutic purposes, or for other purposes. By way of non-limiting example, pressure generator  14  may be configured to control the flow rate and/or pressure of the pressurized flow of breathable gas to provide pressure support to the airway of subject  12 . The pressure generator may include a ventilator, a positive airway pressure generator such as, for example, the pressure generator of the device described in U.S. Pat. No. 6,105,575, hereby incorporated by reference in its entirety, and/or other pressure generation devices. 
     The pressurized flow of breathable gas is delivered to the airway of subject  12  from pressure support device  10  via a gas delivery circuit  26 . Gas delivery circuit  26  is configured to communicate the pressurized flow of breathable gas generated by pressure generator  14  to the airway of subject  12 . As such, gas delivery circuit  26  includes a conduit  28  and an interface appliance  30 . Conduit  28  conveys the pressurized flow of breathable gas to interface appliance  30 , and the interface appliance delivers the pressurized flow of breathable gas to the airway of subject  12 . Some examples of interface appliance  30  may include, for example, a nasal cannula, a nasal mask, a nasal/oral mask, a full face mask, a total face mask, and/or other interface appliances that communication a flow of gas with an airway of a subject. The present invention is not limited to these examples, and contemplates delivery of the pressurized flow of breathable gas to subject  12  using any subject interface. 
     Although gas delivery circuit  26  is illustrated in  FIG. 1  as a single-limbed circuit for the delivery of the pressurized flow of breathable gas to the airway of subject  12 , this is not intended to be limiting. The scope of this disclosure includes double-limbed circuits having a first limb configured to both provide the pressurized flow of breathable gas to the airway of subject  12 , and a second limb configured to selectively exhaust gas from gas delivery circuit  26  (e.g., to exhaust exhaled gases). 
     In one embodiment, electronic storage  16  comprises electronic storage media that electronically stores information. The electronic storage media of electronic storage  16  may include one or both of system storage that is provided integrally (i.e., substantially non-removable) with system  10  and/or removable storage that is removably connectable to system  10  via, for example, a port (e.g., a USB port, a firewire port, etc.) or a drive (e.g., a disk drive, etc.). Electronic storage  16  may include one or more of optically readable storage media (e.g., optical disks, etc.), magnetically readable storage media (e.g., magnetic tape, magnetic hard drive, floppy drive, etc.), electrical charge-based storage media (e.g., EEPROM, RAM, etc.), solid-state storage media (e.g., flash drive, etc.), and/or other electronically readable storage media. Electronic storage  16  may store software algorithms, information determined by processor  22 , information received via user interface  18 , and/or other information that enables system  10  to function properly. Electronic storage  16  may be (in whole or in part) a separate component within system  10 , or electronic storage  16  may be provided (in whole or in part) integrally with one or more other components of system  10  (e.g., generator  14 , user interface  18 , processor  22 , etc.). 
     User interface  18  is configured to provide an interface between system  10  and subject  12  through which subject  12  may provide information to and receive information from system  10 . This enables data, cues, results, and/or instructions and any other communicable items, collectively referred to as “information,” to be communicated between the subject  12  and one or more of generator  14 , electronic storage  16 , and/or processor  22 . Examples of interface devices suitable for inclusion in user interface  18  include a keypad, buttons, switches, a keyboard, knobs, levers, a display screen, a touch screen, speakers, a microphone, an indicator light, an audible alarm, a printer, a tactile feedback device, and/or other interface devices. In one embodiment, user interface  18  includes a plurality of separate interfaces. In one embodiment, user interface  18  includes at least one interface that is provided integrally with generator  14 . 
     It is to be understood that other communication techniques, either hard-wired or wireless, are also contemplated by the present invention as user interface  18 . For example, the present invention contemplates that user interface  18  may be integrated with a removable storage interface provided by electronic storage  16 . In this example, information may be loaded into system  10  from removable storage (e.g., a smart card, a flash drive, a removable disk, etc.) that enables the user(s) to customize the implementation of system  10 . Other exemplary input devices and techniques adapted for use with system  10  as user interface  18  include, but are not limited to, an RS-232 port, RF link, an IR link, modem (telephone, cable or other). In short, any technique for communicating information with system  10  is contemplated by the present invention as user interface  18 . 
     Sensor  20  is configured to generate output signals conveying information related to one or more parameters of the pressurized flow of breathable gas and/or the breathing of subject  12 . The one or more parameters of the pressurized flow of breathable gas may include, for example, one or more of a flow rate, a volume, a pressure, humidity, temperature, acceleration, velocity, acoustics, changes in a parameter indicative of respiration, and/or other gas parameters. Sensor  20  may include one or more sensors that measure such parameters directly (e.g., through fluid communication with the pressurized flow of breathable gas at pressure generator  14 ). The sensor  20  may include one or more sensors that generate output signals related to one or more parameters of the pressurized flow of breathable gas indirectly. For example, sensor  20  may include one or more sensors configured to generate an output based on an operating parameter of pressure generator  14  (e.g., a valve driver or motor current, voltage, rotational velocity, and/or other operating parameters), and/or other sensors. The one or more parameters of the breathing of the subject (that are not parameters of the pressurized flow of breathable gas) may include other parameters that provide information about the breathing of subject  12 . For example, sensor  20  may include a transducer configured to detect acoustic waves transmitted to pressure support device  10  through gas delivery circuit  26 . These acoustic waves may convey information related to respiratory effort of subject  12 , and/or the noise generated by subject  12  during respiration (e.g., during snoring). 
     Although sensor  20  is illustrated as a single sensor at a single location in pressure generator  14 , this is not intended to be limiting. The sensor  20  may include a plurality of sensors which may be located proximately or separately with respect to each other. Sensors providing the functionality attributed herein to sensor  20  may be disposed in any of a plurality of locations, such as for example, within pressure generator  14 , within (or in communication with) conduit  28 , within (or in communication with) interface appliance  30 , and/or other locations. 
     Processor  22  is configured to provide information processing capabilities in system  10 . As such, processor  22  may include one or more of a digital processor, an analog processor, a digital circuit designed to process information, an analog circuit designed to process information, a state machine, and/or other mechanisms for electronically processing information. Although processor  22  is shown in  FIG. 1  as a single entity, this is for illustrative purposes only. In some implementations, processor  22  may include a plurality of processing units. These processing units may be physically located within the same device (e.g., pressure generator  14 ), or processor  22  may represent processing functionality of a plurality of devices operating in coordination. 
     Processor  22  is configured to control pressure generator  14  to generate the pressurized flow of breathable gas in accordance with the therapy regime. By way of non-limiting example, processor  22  may control pressure generator  14  such that the pressure support provided to subject  12  via the pressurized flow of breathable gas includes, non-invasive ventilation, positive airway pressure support, bi-level support, continuous positive airway pressure support, BiPAP®, and/or other types of pressure support therapy. 
     In controlling pressure generator  14 , the therapy regime may dictate that processor  22  be responsive to the output signals generated by sensor  20 . For example, if the output signals generated by sensor  20  indicate that subject  12  is experiencing a respiratory event, the therapy regime may dictate that the processor control pressure generator  14  to increase pressure of the pressurized flow of breathable gas to help subject  12  overcome the event. Some non-limiting examples of respiratory events include an apnea (central or obstructive), a respiratory obstruction, snoring, hypopnea, flow limitation, and/or other respiratory events. 
     Humidifier  24  is configured to adjust the humidity of the pressurized flow of breathable gas. In one embodiment, humidifier  24  is configured to generate water vapor by heating liquid held within the humidifier. Humidifier  24  includes a gas inlet  32  and a gas outlet  34 . Humidifier  24  is configured such that the pressurized flow of breathable gas is received from pressure generator  14  by humidifier  24  through gas inlet  32  and is humidified within humidifier  24  by water vapor before being released from humidifier  24  through gas outlet  34 . In one embodiment, gas outlet  34  is connected with gas delivery circuit  26  such that the humidified pressurized flow of breathable gas is delivered to the airway of subject  12  through gas delivery circuit  26 . 
     Humidifier  24  is configured such that the amount by which the humidity of the pressurized flow of breathable gas is adjusted within humidifier  24  is controlled by processor  22 . For example, processor  22  may control a heating element (not shown in  FIG. 1 ) configured to heat liquid within humidifier  24  to adjust the amount of moisture added to the pressurized flow of breathable gas within humidifier  24 . The level of humidity to which the pressurized flow of breathable gas is adjusted may be dictated by a therapy regime and/or selected by a user (e.g., subject  12 , a caregiver, a therapy decision-maker, etc.). 
     In one embodiment, sensor  20  includes one or more sensors generating output signals implemented by processor  22  to detect respiratory events are disposed on a side of humidifier  24  opposite from gas delivery circuit  26 . For example, the one or more sensors may be located within pressure generator  14 . 
     In conventional pressure support devices, the sensitivity of such sensors (e.g., sensors disposed on an opposite side of a humidifier from a subject) to respiratory events may be impacted by the size of the humidifier and/or the airspace in the humidifier. Typically, the more liquid that is held by a humidifier of a pressure support device, the greater the volume of a liquid storage chamber (to accommodate the increased amount of liquid) through which gas and/or acoustic waves must pass before coming into contact with the event detecting sensors. Similarly, to reduce occurrences of water ingress (e.g., at startup, during transport of pressure support device  10 , and/or at other times) the volume of a chamber that holds water may be increased. Unfortunately, increasing the volume of this storage chamber tends to attenuate the parameters detected by the event detecting sensors. For example, periodic pressure modulation and/or acoustic waves associated with some respiratory events (e.g., apneas, obstructions, snoring, etc.) are typically attenuated more by larger storage chambers. 
     As such, in conventional designs, there is a design conflict between the amount of liquid that can be held by the humidifier (i.e., the size and configuration of the humidification system) and the sensitivity of respiratory event detection. While holding a larger amount of liquid may enhance the convenience of the pressure support device, reducing the sensitivity of respiratory event detection may reduce the efficacy and/or comfort of the pressure support therapy delivered by the pressure support device. As will be discussed further below. Humidifier  24  of pressure support device  10  is designed to hold an enhanced amount of liquid while maintaining respiratory event detection sensitivity. 
       FIG. 2  illustrates an exploded view of one or more implementations of humidifier  24 . In the view shown in  FIG. 2 , a unitary base structure  36  is exploded from a unitary partition structure  38 . The orientation of unitary base structure  36  and unitary partition structure  38  shown is the orientation in which humidifier  24  would be deployed during use (e.g., with unitary base structure  36  positioned underneath unitary partition structure  38 ). The unitary base structure  36  and/or unitary partition structure  38  may be formed from polycarbonate, plastic, and/or other materials. Unitary base structure  36  forms gas inlet  32 , and unitary partition structure  38  forms gas outlet  34 . As is discussed below, a flow path is formed between gas inlet  32  and gas outlet  34  by unitary base structure  36  and unitary partition structure  38 .  FIG. 3  illustrates reverse elevation of one or more implementations of unitary partition structure  38 . It will be appreciated that the description of humidifier  24  as being formed by unitary base structure  36  and unitary partition structure  38  is not intended to be limiting. The scope of this disclosure includes apparatuses having more or fewer pieces, and/or with pieces having different specific shapes. 
       FIGS. 4 and 5  illustrate sectional views of humidifier  24  with unitary base structure  36  and unitary partition structure  38  assembled. Specifically,  FIG. 4  shows a sectional view taken along section line  4 - 4 , and  FIG. 5  shows a sectional view taken along section line  5 - 5 . When assembled, humidifier  24  includes a chamber  39 , a heating clement  42 , and a partition  46 . 
     Chamber  39  is configured to hold liquid, and forms a flow path from gas inlet  32  to gas outlet  34 . As such, the pressurized flow of breathable gas flowing through humidifier  24  flows through the flow path defined by chamber  39  between gas inlet  32  and gas outlet  34 . During operation, the temperature of at least a portion of the liquid held in chamber  39  is elevated to vaporize the liquid. The pressurized flow of breathable gas is humidified by the vaporized liquid as it flows through the flow path. The base of chamber  39  is formed by unitary base structure  36 . The chamber is divided into a first section  40  and a second section  44  by partition  46 . 
     The flow path formed by chamber  39  is formed entirely within first section  40 . A ceiling of first section  40  is formed by unitary partition structure  38 . First section  40  holds a first reservoir of liquid  48 . During use, the portion of the liquid held by chamber  39  that is vaporized is the first reservoir of liquid  48 . The first section is sealed (or substantially sealed) from ambient atmosphere). 
     Heating element  42  is configured to controllably elevate the temperature of fluid within first section  40 . In the embodiment shown in  FIGS. 4 and 5 , heating element  42  is positioned at the bottom of first section  40  to be in proximity to first reservoir of liquid  48  such that heat emitted by heating element  42  is dispensed directly into first reservoir of liquid  48 . This emission of heat by heating element  42  into first reservoir of liquid  48  vaporizes first reservoir of liquid  48 . As was discussed above with respect to  FIG. 1 , the amount of heat emitted by heating element  42  is controllable by a processor (e.g., processor  22  shown in  FIG. 1  and described above) to bring the humidity of the pressurized flow of breathable gas to a selected level. As can be seen in  FIGS. 4 and 5 , in one embodiment, heating element  42  does not directly heat fluid within second section  44 . 
     Second section  44  is positioned adjacent to first section  40 . In one embodiment, second section  44  at least partially surrounds first section  40 . For example, in  FIGS. 4 and 5 , second section  44  surrounds first section  40  on three sides. An outer side wall of first section  40  is formed by unitary base structure  36 . In one embodiment, second section  44  is held in isolation from ambient atmosphere. In one embodiment, a secondary opening  50  in humidifier  24  provides for restrictive communication between second section  44  and ambient atmosphere. Second section  44  is configured to hold a second reservoir of liquid  52 . 
     As was mentioned above, partition  46  is configured to divide first section  40  from second section  44 . In dividing first section  40  from second section  44 , partition  46  defines an opening  54  between first section  40  and second section  44 . Opening  54  is located such that first reservoir of liquid  48  is placed in fluid communication with second reservoir of liquid  52 . For example, in the implementations shown in  FIGS. 4 and 5 , opening  54  is formed toward the bottom of first section  40  and second section  44 . More specifically, partition  46  is formed to extend not all the way to the base of first section  40  and second section  44  such that opening  54  is formed underneath partition  46 . The gap between the bottom of partition  46  and unitary base structure  36  forming opening  54  may be about 1.5 mm. In one embodiment, partition  46  and opening  54  are formed such that fluid communication between first reservoir of liquid  48  and second reservoir of liquid  52  is unrestricted by any valve or nozzle, but instead enables liquid to pass back and forth between first reservoir of liquid  48  and second reservoir of liquid  52 . 
     The size and/or shape of opening  54  enables gas within first section  40  to be held in isolation from gas within second section  44  even as first reservoir of liquid  48  and second reservoir of liquid  52  are in fluid communication. For example, as is shown in  FIGS. 4 and 5 , opening  54  may be formed toward the bottom of chamber  39  such that until the amount of liquid held in chamber  39  falls to about 5% of capacity, or some other level near empty, the gas held in first section  40  is isolated from gas in second section  44 . 
     Because the flow path between gas inlet  32  and gas outlet  34  is formed wholly within first section  40 , and because this flow path is isolated from second section  44 , the volume of chamber  39  through which the pressurized flow of breathable gas passes is effectively reduced to just first section  40 . This reduction in the volume of chamber  39  through which the pressurized flow of breathable gas passes reduces the attenuation of parameters that are detected by sensor  20  within a pressure generator (or between humidifier  24  and the pressure generator) to facilitate respiratory event detection. Because second reservoir of liquid  52  is still in fluid communication with first reservoir of liquid  48 , this reduction in the attenuation of respiratory event detection sensitivity by sensor  20  is achieved while still providing the advantages of increased liquid storage capacity within humidifier  24 . 
     In one embodiment, opening  54  is formed such that the cross-sectional shape and/or area of opening  54 , in conjunction with typically submersed location of opening  54 , maintains first section  40  substantially in acoustic isolation from second section  44 . The substantial acoustic isolation of first section  40  from second section  44  while still permitting fluid communication between first reservoir of liquid  48  and second reservoir of liquid  52  maintains respiratory event detection sensitivity by sensor  20 , but provides for the advantages associated with increased liquid storage within humidifier  24  and/or increased volume within chamber  39  at the same time. 
     During operation, the pressure within first section  40  is increased by the pressure of the pressurized flow of breathable gas as it flows through first section  40 . This increase in pressure is substantial, and may increase the pressure in first section  40  to at least about 4 cmH 2 O. Opening  54  is formed such that in response to this increase in pressure in first section  40 , liquid in the first reservoir of liquid  48  can flow from first section  40  through opening  54  and into second section  44  to join second reservoir of liquid  52 . Because opening  54  does not enable gas within second section  44  to communicate directly with gas in first section  40 , the flow of liquid from the first reservoir of liquid  48  to the second reservoir of liquid  52  causes the level of liquid in the second reservoir of liquid  52  to rise and the level of liquid in the first reservoir of liquid  48  to rise. For example,  FIGS. 4 and 5  illustrate the manner in which the liquid levels in first section  40  and second section  44  may be adjusted by pressurization of first section  40  by the pressurized flow of breathable gas. 
     It will be appreciated, however, that the changes in the levels of first reservoir of liquid  48  and second reservoir of liquid  52  caused by increased pressure in first section  40  does not impede replenishment of first reservoir of liquid  48  from second reservoir of liquid  52 . As liquid from first reservoir of liquid  48  is vaporized and carried out of first section  40  by the pressurized flow of breathable gas, liquid from second reservoir of liquid  52  passes through opening  54  to replenish first reservoir of liquid  48 . 
     In one embodiment, rather than permitting the pressurization of first section  40  to cause the disparity in liquid levels between the first reservoir of liquid  48  and the second reservoir of liquid  52 , one or more openings (not shown) are formed in partition  46  to provide for restrictive communication between the gas held in second section  44  and first section  42 . However, the openings are restrictive enough that partition  46  still restricts contact between the gas within the second section  44  and the pressurized flow of breathable gas as the pressurized flow of breathable gas flows through the first section  40  such that the pressurized flow of breathable gas is maintained within the first section. This means that the openings restrict communication between the first section  40  and the second section  44  to the point that the flow path through which the pressurized flow of breathable gas travels does not include the second section  44 . Thus, sudden changes in pressure and/or flow rate experienced by the pressurized flow of breathable gas due to, for example, respiration by subject  12  are not dampened by the volume of the second section  44 . For example, in one embodiment, the opening(s) in partition  46  are formed to restrict contact of the pressurized flow of breathable gas with the gas in the second section to a flow rate of less than about 2 LPM at a pressure of about 10 cmH 2 O, less than about 3 LPM at a pressure of about 10 cmH 2 O, less than about 4 LPM at a pressure of about 10 cmH 2 O. 
     Although the invention has been described in detail for the purpose of illustration based on what is currently considered to be the most practical and preferred embodiments, it is to be understood that such detail is solely for that purpose and that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover modifications and equivalent arrangements that are within the spirit and scope of the appended claims. For example, it is to be understood that the present invention contemplates that, to the extent possible, one or more features of any embodiment can be combined with one or more features of any other embodiment.