Patent Publication Number: US-2005121033-A1

Title: Respiratory monitoring during gas delivery

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application claims priority under 35 U.S.C. § 119(e) from provisional U.S. patent application No. 60/536,132 filed Jan. 13, 2004, the contents of which are incorporated herein by reference. This application also claims priority under 35 U.S.C. § 120 as a Continuation-In-Part (CIP) from U.S. patent application Ser. No. 10/372,381 filed Feb. 21, 2003, which is a Continuation of Ser. No. 09/961,618 filed Sep. 24, 2001, now U.S. Pat. No. 6,544,192, which is a continuation-In-Part of Ser. No. 09/438,081 filed Nov. 10, 1999, now U.S. Pat. No. 6,342,040, which is a Continuation of Ser. No. 09/030,221 filed Feb. 25, 1998, now U.S. Pat. No. 6,017,315, the contents of all of these patents and application are hereby incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention pertains to a method and apparatus for monitoring a patient&#39;s respiratory status during the continuous delivery of a gas, such as supplemental oxygen.  
      2. Description of the Related Art  
      Supplemental oxygen is widely used for the long-term treatment of chronically ill patients suffering from various respiratory diseases, such as COPD and emphysema. In emergency situations, supplemental oxygen is administered on a short-term basis to relieve acute symptoms, such as shortness of breath and lowered oxygen saturation. Supplemental oxygen is also commonly administered throughout the hospital setting, such as in the operating room during surgery, after surgery in post-op, and in the intensive care units to critically ill patients. In addition, supplemental oxygen is provided to patients on a short-term basis outside the hospital, for example during surgical procedures performed outside the hospital in a doctor&#39;s office, dentist&#39;s office, surgicenter, or any other location where such procedures are performed.  
      Conventional practices for administering supplemental oxygen to a patient include fitting a nasal cannula or an oxygen mask on the patient. A conventional nasal cannula typically consists of single lumen tubing with a pair of stubs provided along the length of the tube. The stubs are sized and configured to be situated within the nostrils of the patient, and each stub includes a port through which oxygen flows. The nasal cannula provides more freedom of movement for the patient than other methods of interfacing a flow of supplemental oxygen to a patient, but drawbacks of using the nasal cannula are well known and include unknown delivered FiO 2 , irritation of the nose, and potential dislodgment of the cannula from the patient&#39;s nostrils. Oxygen masks are simple, inexpensive to use, not subject to easy dislodgment, and reliably administer oxygen levels of 40-60% O 2  to the patient. Oxygen masks designs vary based upon the intended use of the particular mask, but typically include a body that is sized to seat over the nose and mouth of the patient. Oxygen is introduced to an interior of the mask via a single lumen through an oxygen inlet defined in the mask. Expiratory gases are typically vented from the mask through apertures defined in the sides of the mask.  
      During the administration of oxygen, whether via nasal cannula or mask, it is often desirable to monitor the patient&#39;s respiratory status. For example, it is desirable to monitor timing related parameters, such as the patient&#39;s breath rate, inspiratory and expiratory times, pressure related parameters, such as the end expiratory pressure, and volume related parameters, such as inspiratory and expiratory tidal volume. Additionally, it is desirable to recognize abnormal breathing patterns, such as Cheyne-Stokes breathing, cessation of breathing (apnea), a reduction in the flow during breathing (hypopnea) or disconnection of the patient from the supplemental oxygen, while the patient is receiving supplemental oxygen. It is particularly desirable to monitor a patient&#39;s respiration during the administration of supplemental oxygen while the patent is being given anesthesia, sedative, and/or painkiller, which can occur across the spectrum of care, including at the physician&#39;s office, at a surgicenter, in a dentist/orthodontist office, and at a hospital ward.  
      Existing approaches to measuring the aforementioned parameters include placing sensing elements, such as thermistors, directly in the airflow path of the subject so that the gas flowing into or out of the patient flows across the sensing element. See, e.g., U.S. Pat. Nos. 5,190,048 and 5,413,111 both to Wilkinson. It is also known to place a single lumen at the patient&#39;s airway to sense the pressure variations related to the subject&#39;s breathing. See, e.g., U.S. Pat. Nos. 5,535,739 and 6,165,133 both to Rapoport. However, these conventional single lumen pressure sensing systems cannot be used when supplemental oxygen is to be provided to the patient because they are unable to provide a clear indication of the pressure variations produced by the patient.  
      Dual lumen cannula have been developed. A conventional dual lumen cannula includes a first lumen for delivering the supplemental gas to the patient and a second lumen for sampling the exhaled carbon dioxide (CO 2 ). The second lumen is connected to a vacuum pump that draws a continuous sample of the exhaled breath to the CO 2  monitor. Both lumens are contained in a common conduit housing. See, e.g. U.S. Pat. No. 5,335,656 to Bowe et al. and U.S. Pat. No. 4,989,599 to Carter. Given that these conventional approaches require additional sensing devices at the patient, such as a thermister, and/or a more complicated measurement technique, such as drawing a sample with a vacuum pump, it is desirable to measure these parameters without the associated expense and complexity of the aforementioned approaches.  
      In addition, conventional dual lumen monitoring systems are used either to monitor the patient&#39;s exhaled CO 2  levels, or are used in oxygen conserving devices (OCDs) that pulse or dose the oxygen delivered to the patient. There are instances, however, where it is desirable to provide a continuous flow of oxygen to a patient over multiple breaths, i.e., without pulsing or dosing the oxygen, while monitoring the patient. It is also preferable to provide a system that is less costly and more robust than CO 2  monitoring systems.  
     SUMMARY OF THE INVENTION  
      Accordingly, it is an object of the present invention to provide a simple system to monitor a subject&#39;s breathing while the subject is receiving a supplemental gas, such as supplemental oxygen or a gas mixture, that overcomes the shortcomings of conventional monitoring techniques. This object is achieved according to the present invention by providing a respiratory therapy and monitoring apparatus that includes a first conduit having a proximal end, a distal end, and a first lumen defined, therethrough from the proximal end to the distal end. A continuous flow of gas is delivered from a gas supply to an airway of a patient over a plurality of respiratory cycles. A first sensor is operatively coupled to the airway of the patient while the continuous flow of gas is passing through the conduit. The first sensor monitors a gas flow characteristic indicative of pressure or flow in the first conduit. In addition, a processor is provided that is adapted to determine at least one respiratory variable of such a patient based on the gas flow characteristic.  
      In a further embodiment of the present invention, the offset or bias present in the gas flow characteristic due to continuous introduction of the supplemental gas is removed. This is accomplished by programming executed by the processor, dedicated hardware, or both. In this manner, the present invention provides a system that both delivers a continuous flow of supplemental gas to a patient and that allows the patient to be monitored by a simple and reliable pressure or flow sensor, so that the condition of the patient can be evaluated in real-time during the delivery of supplemental gas.  
      It is yet another object of the present invention to provide a respiratory therapy and monitoring method that does not suffer from the disadvantages associated with conventional measurement techniques. This object is achieved by providing a method that includes (1) providing a first conduit, (2) delivering, via the first conduit, a continuous flow of gas from a gas supply to an airway of a patient over a plurality of respiratory cycles, (3) monitoring a gas flow characteristic using a pressure sensor, a flow sensor, or both operatively coupled to the airway of the patient while the continuous flow of gas is passing through the conduit, and (4) determining at least one respiratory variable of such a patient based on the gas flow characteristic. In a further embodiment of the present invention, the offset or bias present in the gas flow characteristic due to continuous introduction of the supplemental gas is removed, by the processor, dedicated hardware, or a combination of the two.  
      It is a further object of the present invention to provide a method of displaying a respiratory characteristic of a patient. This object is achieved by displaying a time varying respiratory characteristic over at least a portion of a patient&#39;s respiratory cycle during a current (n) respiratory cycle and a time varying respiratory characteristic over at least a portion of such a patient&#39;s respiratory cycle during a prior respiratory cycle. These time varying respiratory characteristics during at least two respiratory cycles are displayed in a superimposed fashion.  
      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. 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. As used in the specification and in the claims, the singular form of “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic diagram of a first embodiment of a monitoring and therapy delivery system according to the principles of the present invention;  
       FIG. 2  is a schematic diagram of the system of  FIG. 1  shown in use on a patient;  
       FIGS. 3-5  are schematic diagrams illustrating three alternative techniques for accounting for an offset in a measured gas flow characteristic caused by introducing a continuous flow of gas to the airway of the patient while monitoring the patient;  
       FIG. 6A  is a graph of a representative pressure signal measured by the pressure sensor of the monitoring and therapy delivery system of  FIG. 1 , and  FIG. 6B  is a graph showing the patient&#39;s breathing patterns and offset bias flow signal separated from the measured pressure signal;  
       FIG. 7  is a schematic diagram of a second embodiment of a monitoring and therapy delivery system according to the principles of the present invention;  
       FIG. 8  is a graph illustrating a patient&#39;s breathing frequency recorded over a period of time using the monitoring and therapy delivery system of the present invention;  
       FIG. 9  is a schematic diagram of a third embodiment of a monitoring and therapy delivery system according to the principles of the present invention;  
       FIG. 10  is a detailed view of the distal end of the conduit in the monitoring and therapy delivery system of  FIG. 9 ;  
       FIG. 11  is a schematic diagram of a fourth embodiment of a monitoring and therapy delivery system according to the principles of the present invention;  
       FIG. 12  is a schematic diagram of a fifth embodiment of a monitoring and therapy delivery system according to the principles of the present invention;  
       FIG. 13  is a schematic diagram of a sixth embodiment of a monitoring and therapy delivery system according to the principles of the present invention;  
       FIG. 14  is a schematic diagram of a seventh embodiment of a monitoring and therapy delivery system according to the principles of the present invention;  
       FIGS. 15 and 16  are perspective and front views, respectively, of a cannula with filter and a housing suitable for use in the monitoring and therapy delivery system of the present invention;  
       FIG. 17  illustrates an exemplary waveform display suitable for use in displaying a monitored waveform according to the principles of the present invention;  
       FIG. 18  is a graph of a hypothetical flow measured by a flow in the monitor of the present invention  
       FIG. 19  illustrates an eight embodiment of a monitoring and therapy delivery system according to the principles of the present invention;  
       FIG. 20  illustrates a ninth embodiment of a monitoring and therapy delivery system according to the principles of the present invention; and  
       FIG. 21A  is a rear perspective view,  FIG. 21B  is a top view, and  FIG. 21C  is a front perspective view of a tenth embodiment of a monitoring and therapy delivery system according to the principles of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS OF THE INVENTION  
       FIG. 1  schematically illustrates an exemplary embodiment of a respiratory monitoring and therapy delivery system  100  according to the principles of the present invention. Monitoring and therapy delivery system  100  includes a conduit or cannula  102  having a proximal end  104  and a distal end  106 . The terms “conduit” and “cannula” are used interchangeably. A connector  108  is provided at the proximal end of the conduit to connect the conduit to a supply of gas  110 , such as a tank of oxygen with a pressure regulator or an oxygen concentrator. Of course, the present invention contemplates that any type of gas suitable for delivery to a patient can be used as gas supply  110 . Conduit  102  includes a lumen  103 , i.e., a bore, defined therein to carry a flow of gas between the proximal end and the distal end. In the embodiment illustrated in  FIG. 1 , conduit  102  includes a single lumen, thereby simplifying the manufacturability and use of the system.  
      A patient interface device, generally indicated by reference numeral  112 , is provided at distal end  106  of conduit  102 . In the embodiment illustrated in  FIG. 1 , the patient interface device is a pair of nasal prongs  114 , each of which includes a port for delivering a flow of gas to both nares of a patient. The prongs at the distal end of the conduit do not seal the patient&#39;s nostril, so that some gas is permitted to flow from the patient&#39;s nose to the ambient atmosphere around the prongs. In this embodiment, the distal end of conduit  102  includes a loop  116  to allow the distal end of the conduit to wrap around a patient&#39;s head, as is the case with a standard oxygen tubing.  
      Patient interface device  112  can be any device suitable for coupling the flow of gas from conduit  102  to an airway of a patient. Examples of suitable patient interface devices include a nasal mask, oral mask or mouthpiece, nasal/oral mask, nasal cannula, trachea tube, intubation tube, and hood or full face mask. It is to be understood that this list of suitable interface devices is not intended to be exclusive or exhaustive. For example, in the embodiment illustrated in  FIG. 2  and discussed in detail below, the patient interface device is a nasal/oral mask.  
      Monitoring system  100  includes a respiratory monitoring system, generally indicated by reference numeral  130 , that is preferably provided in a housing  132 . Respiratory monitoring system  130  includes a sensor  134  operatively coupled to conduit  102 . In the present embodiment, sensor  134  is a pressure sensor that monitors a pressure within the conduit and outputs a first pressure signal indicative thereof. More specifically, pressure sensor  134  measures the pressure within the lumen relative to a constant or relatively constant reference pressure, such as absolute or atmospheric pressure. A single pressure sensor port or tap into conduit  102  is provided from communicating the pressure sensor with the lumen in the conduit. This sensing port can have any configuration known in the art, such as a single tap flush to the lumen wall or a single tap protruding into the interior of the lumen.  
      A processor  136  receives the pressure signal and determines from that signal at least one respiratory variable of the patient, as discussed in greater detail below. Processor  136  is preferably a microprocessor capable of implementing a stored algorithm, which determines from the monitored pressure from pressure sensor  134  the respiratory variable of interest. Of course, processor  136  includes the necessary memory and processing capability to implement the features of the present invention.  
      The present invention further contemplates that respiratory monitoring system  130  includes an input/output interface  138  for communicating, information, data, and/or instructions and any other communicatable items, collectively referred to as “data”, between a user and processor  136 . Examples of common input/output interfaces suitable for this purpose include a keypad and display that visually indicates at least one respiratory variable in a human perceivable format.  
      The present invention also contemplates providing other communication techniques, either hard-wired or wireless, for communicating with processor  136  from a remote location. For example, a terminal  140  can be provided that enables data to be loaded from a data storage device, such as a disk, CD-ROM, memory card, smart card, etc., into processor  136  or loaded onto the storage device from the processor. Other exemplary, interface devices and techniques adapted for use with the respiratory measurement system include, but are not limited to, an RS-232 port, CD reader/writer, DVD reader/writer, RF link, modem (telephone, cable, or other). In short, any conventional technique for providing, receiving, or exchanging data with processor  136  is contemplated by the present invention as terminal  140 .  
      In the embodiment illustrated in  FIG. 1 , conduit  102  includes a first portion  142  coupled to housing  132  and patient interface device  112 . It is to be understood that the distance from the tap where pressure sensor  134  measures the pressure to the ports at the patient interface should not be too large. Otherwise, the pressure drop across this portion of the conduit may be too great. For example, in one embodiment of the present invention, the length of this portion of the conduit is approximately seven (7) feet. However, conduit lengths up to fifty (50) feet are also contemplated by the present invention.  
      The embodiment illustrated in  FIG. 1  also shows connector  108  provided at proximal end  104  of conduit  102  as being a tube. It is to be understood that the length of this connector can vary. For example, one embodiment of the present invention contemplates connecting the housing directly to gas supply  110 , thereby minimizing the distance between the gas source and the location where pressure sensor  134  taps into conduit  102 . Still other embodiments of the present invention contemplate making this connector quite long, so long as the pressure drop through connector  108  does not become too large.  
      Referring now to  FIGS. 1-4 ,  6 A and  6 B, a description will be provided as to how processor  136  determines, from the pressure measured by the pressure sensor, at least one respiratory variable of the patient. The pressure (P Total ) measured by pressure sensor  134  is the summation of a pressure drop ΔP 1  and a pressure drop ΔP 2 . Pressure drop ΔP 1  is the pressure drop that occurs along the length of lumen  103  between the location of the pressure sensor and the open end  168  of the lumen inserted into a nare  170  of a nose  172  of a patient  174 . Pressure drop ΔP 2  is a pressure drop from open end  168  of conduit  102  to the ambient atmosphere, which is at the opening of the nostril. Thus, P Total =ΔP 1 +ΔP 2 .  
      Pressure drop ΔP 1  is determined based on the flow Q O2  of the supplement gas through lumen  103  and the resistance to flow R Tube  that exists along the length of conduit  102  between the location of the pressure sensor and open end  168 . Thus, ΔP 1 =Q O2 *R Tube . Pressure drop ΔP 2  is based on the pressure drop at the nose due to the bias flow of the supplemental gas (ΔP O2 ) and the pressure drop at the nose due to the patient&#39;s respiration (ΔP Patient ). Pressure drop ΔP O2  is determined based on the flow Q O2  of the supplement gas through lumen  103  and the resistance to flow at the nose R Nose . Thus, ΔP O2 =Q O2 * R Nose . Pressure drop ΔP Patient  is determined based on the patients respiratory flow Q patient  and the resistance to flow at the nose R Nose . Thus, ΔP Patient =Q Patient *R Nose . Pressure drop ΔP 2  can be expressed as follows: 
 
 ΔP   2   =ΔP   O2   +ΔP   Patient , or   (1) 
 
 ΔP   2   =Q   O2   *R   Nose   +Q   Patient   *R   Nose , or   (2) 
 
 ΔP   2 =( Q   O2   +Q   Patient ) R   Nose    (3) 
 
      It can thus be appreciated that the pressure (P Total ) measured by pressure sensor  134  can be rewritten as: 
 
 P   Total   =Q   O2   *R   Tube +( Q   O2   +Q   Patient ) R   Nose    (4) 
 
      Because the signal of primary interest in the present application is the pressure resulting from the patient breathing into the patient interface (ΔP Patient ), the present invention compensates or removes the pressure drops that are due to the administration of the continuous supplemental flow of gas. That is, the present invention contemplates removing the bias flow Q O2  from equation (4).  
      One embodiment by which the bias flow Q O2  is removed from the total pressure P Total  is shown in  FIG. 3 . In this embodiment, the measured (total) pressure P Total  is provided to a high pass filter  200 . The cutoff frequency of the high pass filter is set such that it is less than the lowest breathing rate that the system would be expected to encounter. For example, the human breathing frequency is typically in a range of 0.1 Hz to 3 Hz. Thus, the cutoff frequency is set to be less than 0.1 Hz. Output  201  of high pass filter  200 , which corresponds to ΔP Patient , is provided to an amplifier  202 , so that output  203  of amplifier  202  is a signal or waveform that corresponds to ΔP Patient  times a Gain.  
      It should be noted that the present invention contemplates that the function of high pass filter  200 , amplifier  202 , or both can be implemented in hardware, software, or a combination thereof. It should also be understood that other cutoff frequencies can be used in the high pass filter. For example, the present invention contemplates that the respiratory therapy and monitoring system of the present invention can be used on other species of animals that my have different breathing frequencies. In which case, a breathing frequency appropriate for the species using the device should be selected as the cutoff frequency for the high pass filter.  
      Another embodiment by which the effects of the bias flow Q O2  is removed from the total pressure P Total  is shown in  FIG. 4 . In this embodiment, the measured (total) pressure P Total  is provided to an averaging device  204 , such as a microprocessor or digital signal processor (DSP), that determines the average for the measured pressure signal. The output of averaging device  204  and the measured pressure P Total  are provided as input to a subtracting element  206 , such as a difference amplifier, such that the output of averaging device  204  is subtracted from P Total  yielding a signal or waveform that corresponds to the patient breathing pattern or ΔP Patient . This signal is preferably amplified by an amplifier  208 , as needed, to produce a signal having the desired fidelity, i.e., ΔP Patient ×Gain, where the Gain is a gain provided by amplifier  208 .  
      As with the previous embodiment, the function of averaging device  204 , subtracting element  206 , amplifier  208 , or any combination thereof can be implemented in hardware, software, or in a combination of thereof. While two exemplary techniques have been described above and shown in  FIGS. 3 and 4 , it can be appreciated that a variety of other techniques are contemplated by the present invention. Thus, the present invention is not intended to be limited to the specific techniques described herein.  
      The signal from pressure sensor  134  is not identical to the pressure at the patient interface, i.e., at the airway of the patient, due to the pressure drop ΔP 1  that occurs along the length of the conduit between the patient&#39;s airway and the pressure sensor. If the pressure at the patient&#39;s airway is desired, the present invention contemplates using the signal from pressure sensor  134  to determine pressure level at the patient interface. This is accomplished by measuring the pressure at a location along conduit  102  via pressure sensor  134  and offsetting this measurement by the known pressure drop associated with the conduit from the location where the pressure measurement is taken, to the distal end of the conduit where the gas exists the conduit. It can be appreciated that accounting for the pressure drop in the conduit ΔP 1  requires determining that pressure drop in advance. In one embodiment of the present invention, the known pressure drop is stored in memory and the stored value is used to determine the pressure at the patient based on the measured pressure after being processed to remove the offset due to the supplemental gas flow.  
      Another embodiment contemplates accounting for the known pressure drop in the conduit using additional hardware. For example, the present invention contemplates adding a flow sensor  146  along conduit  102  to measure the continuous flow of oxygen delivered to the patient. The signal from flow sensor  146  is provided to control  136 , which includes a look-up table or other suitable correlation function relating predetermined pressure drops over a range of flows for the conduit being used. That is, by measuring the gas flow, a look-up table can be used to determine the pressure drop in the conduit ΔP 1  associated with that flow rate. Thus, the pressure at the patient interface can be found by subtracting ΔP 1  from the pressure P Total  measured via pressure sensor  134 .  
      A further embodiment of the present invention contemplates storing the known pressure drop for different conduits, i.e., conduits of different length, inside diameter, or both. The specific conduit attached to the monitoring system is then entered or learned by the system using any conventional technique so that the appropriate pressure drop for that conduit can be used in determining the pressure at the patient interface. For example, the present invention contemplates using any conventional connector encoding or identifying technique for automatically indicating to processor  136  that type or size conduit is being used when the conduit is attached to housing  132 . The present invention also contemplates calculating the pressure drop ΔP 1  based on the length and the diameter of the conduit, which can be easily determined at the time the system is assembled.  
      Measurement of the pressure level allows pressure related respiratory variables, such as the inspiratory positive airway pressure (IPAP), expiratory positive airway pressure (EPAP), positive end expiratory pressure (PEEP), and continuous positive airway pressure (CPAP) to be determined. Of course, measuring IPAP, EPAP, and CPAP are only possible when the appropriate pressure support therapy is being provided to the patient in addition the gas flow through conduit  102 . The use of a pressure support therapy in combination with the flow of gas through conduit  102  is discussed below with reference to  FIG. 7 .  
      A further embodiment of the present invention contemplates adding a flow sensor  146  in respiratory monitoring system  130 , so that flow related variables, such as rate of oxygen flow (Q O2 ) and volume (V O2 ), can be measured. In one exemplary embodiment, this is achieved by providing a flow restriction  144  in conduit  102  and a flow sensor  146  in the form of a differential pressure sensor that measures the differential pressure across the restriction within the lumen. Restriction  144  may be any restriction or obstruction known in the art for flow measurement, and includes but is not limited to, fixed geometries, such as the venturi, and fixed orifice devices as well as variable geometries, such as variable orifice devices. The differential pressure measured by flow sensor  146  is provided to processor  136 , which uses that measurement to determine the rate of flow of gas through conduit  102 , the volume of gas over any given period of time, or both using conventional techniques.  
      It will be readily apparent to one of skill in the art of patient monitoring and/or flow sensing that other techniques for determining the rate of flow of gas in conduit  102  can be used as the flow sensor. Examples of other conventional flow sensing techniques suitable for use with the present invention include, but are not limited to, ultrasonic flow meters, optical flow meters, diverting flow meters, and thermally based flow meters. In addition, the present invention contemplates using flow meter  146  in conjunction with pressure sensor  134  to obtain multiple measurements of gas flow characteristics.  
       FIG. 5  illustrates an exemplary embodiment of a processing configuration by which the bias due to the flow of supplemental gas delivery pressure is removed from the total flow (Q Total ) measured by flow sensor  146  and the pressure measured by pressure sensor  134 . In this embodiment, the measured flow Q Total  is provided to a baseline pressure determining device  209 , such as a microprocessor or digital signal processor (DSP), which determines a baseline pressure ΔP O2  from the measured flow Q Total . This is accomplished as discussed above, by providing a look-up table or other suitable correlation function that relates the measured flow to a pressure drop. This look-up table or pressure versus flow relation is determined in advance for the conduit being used in the monitoring system.  
      As with the embodiment illustrated in  FIG. 4 , the output of baseline pressure determining device  209  and the measured (total) pressure P Total  are provided as input to a subtracting element  206 , such as difference amplifier, such that the ΔP O2  is subtracted from P Total  yielding a signal or waveform that corresponds to the patient breathing pattern or ΔP Patient . This signal is preferably amplified by an amplifier  208  as needed to produce a signal having the desired fidelity.  
      Because pressure or flow variations from a baseline level are generated as a result of changes in pressure at the patient interface due to patient breathing, time related changes, or fluctuations from the baseline pressure or flow can be analyzed to permit the determination of the transitions between the inspiratory and expiratory phases of breathing. This allows time related respiratory variables, such as breathing frequency, to be determined from the time difference of successive breaths of either the inspiratory or expiratory demarcations. The present invention contemplates using any conventional technique for determining the demarcations or transition between the inspiratory and the expiratory phase of the patient&#39;s breathing cycle.  
      Inspiratory time, expiratory time, and derived indices, such as percentage of inspiratory time of total breath, and the I:E (inspiratory-to-expiratory) ratio can be determined once the inspiratory phase and the expiratory phase are determined. Integration of the time related changes in pressure also permit approximations of the inspiratory and expiratory tidal volumes to be made using signal processing methods known in the art, including but not limited to integral functions.  
      A second embodiment of a respiratory monitoring system  100 ′ is shown in  FIG. 7 . In this embodiment, patient interface device  112 ′ is a face mask  148  coupled to distal end  106  of conduit  102 . Additionally, a patient circuit  150  is connected to patient interface device  112 ′ (face mask  148 ) to carry a flow of gas from a ventilator or pressure support system  152  to the patient&#39;s airway in addition to the gas flow provided by conduit  102 .  
      Facemask  148 , when connected via patient circuit  150  allows the administration of continuous positive airway pressure (CPAP), bi-level positive airway pressure, auto-titrating pressure support, PAPP, PAV, ventilator, or any other conventional pressure support therapy. Patient circuit  150  may consist of a single lumen breathing tube with an exhalation port or valve provided on or near the mask, or two breathing tubes, with one tube used to apply inspiratory flow and the other tube to allow for expiratory flow from the patient. It should be readily apparent to one skilled in the art of pressure and flow sensing that other gases or gas mixtures, such as anesthetic agents or helium/oxygen mixtures, may be used with the present invention requiring only changes to the calibration of the respiratory measurement system.  
      Although the present invention contemplates that the information gathered via monitoring system  100 ,  100 ′ can be output, displayed, or transmitted in any one of a variety of formats, one example of such an output is shown in  FIG. 8 . This figure is a graph of breathing frequency recorded over a night derived from the present invention. Five minute averages of minimum and maximum breathing frequency may be plotted to easily identify periods of relatively stable breathing patterns from variable breathing patterns.  
      Breathing pattern analysis includes the identification of abnormal forms of breathing, such as, but not limited to, Cheyne-Stokes breathing, Kussmaul breathing, apnea, hypopnea, and snoring. Breathing pattern analysis may be performed using the respiratory timing variables derived according to the present invention. Cheynes-Stokes breathing is seen with some central nervous system disorders, uremia, and some sleep patterns and is characterized by repeating cycles of waxing and waning in the depth of breathing including a period of apnea. Kussmaul breathing is seen in coma or diabetic ketoacidosis and is characterized by a deep, rapid respiratory pattern. Any conventional technique for determining these breathing patterns can be used in the present invention.  
      A third embodiment of respiratory therapy and monitoring system  300  according to the principles of the present invention is discussed below with reference to  FIGS. 9 and 10 . This embodiment is similar in many respects to the respiratory therapy and monitoring system of the first embodiment shown in  FIG. 1 . However, in this third embodiment, a dual lumen cannula  302  is used to connect gas supply  10  and respiratory monitoring system  304  to the patient, instead of the single lumen cannula of  FIG. 1 .  
      Dual lumen cannula  302  includes a first lumen  306  that connects gas supply  110  to the patient and a second lumen  308  that connects a sensor to the patient. In the illustrated embodiment, the sensor is a pressure sensor. It is to be understood, however, that the sensor to which second lumen  308  is connected could also be a flow sensor. The first lumen has a pair of prongs  307   a  and  307   b  that insert into the user&#39;s nares  309 , and the second lumen has a pair of prongs  311   a  and  311   b  that also insert into the nares. Gas flows out of ports defined in prongs  307   a  and  307   b  into the user&#39;s nostrils due to the continuous supply of gas being delivered to the first lumen from the gas source. This gas flow is illustrated by arrows A in  FIG. 9 . Pressure is applied cyclically to prongs  311   a  and  311   b  in second lumen  308  during patient respiration. The proximal end of cannula  302  is connected to a pressure sensor  134 ′.  
      It can be appreciated that the flow or pressure of gas in second lumen  308 , e.g., the output of pressure sensor  134 ′, will include a pressure drop ΔP O2  due to the flow of supplemental gas being delivered to the patient. For this reason, this third embodiment of the present invention, like the previous embodiments, contemplates removing or compensating for the pressure drop is generated due to the administration of the supplemental flow of gas ΔP O2 . The techniques discussed above for accomplishing this function are, therefore, applicable to this embodiment.  
      It should also be noted that the proximal end of second lumen  308  can be connected to a flow sensor. That is, pressure sensor  134 ′ can be replaced with a flow sensor, and the proximal end of the second lumen can be open to atmosphere so that a flow is created in the second lumen due to the bias flow of gas. During patient respiration, gas flows into and out of prongs  311   a  and  311   b  cyclically. This flow of gas is illustrated by arrows B. In which case, all of the discussion given above regarding measuring pressure and processing the pressure signal is equally applicable to measuring flow and processing the flow signal, including removing or compensating for the pressure drop ΔP O2  due to the administration of the supplemental flow of gas Q O2 .  
      Respiratory therapy and monitoring system  300  shown in  FIG. 9  also includes a supplemental gas flow control valve  310  associated with first lumen  306 . Valve  310  controls the delivery of the supplemental gas to the patient, and corresponds to the flow control valve found in a variety of oxygen delivery systems. For example, this valve is used to set the flow rate in liters-per-minute for the flow of oxygen to the patient. In the illustrated embodiment, valve  310  operates under the control of processor  136 . Of course, valve  310  can be a manually actuated or remotely actuated valve. Although not shown, the present invention contemplates providing a similar valve in any of the other embodiments of the present invention.  
      One skilled in the art can appreciate that any movement of the cannula connecting the patient to the monitoring system is likely to introduce noise in the pressure or flow measurement conducted by the monitoring system. While this noise can be removed using the filtering and/or averaging techniques discussed above with respect to the bias flow removal, i.e., removing the pressure drop ΔP O2  component from the P Total  signal, another approach that can be used alone or in conjunction with these techniques is shown in  FIG. 11 .  
      Respiratory therapy and monitoring system  320  in  FIG. 11  is similar to that shown in  FIG. 1 , except that conduit  322 , which is connected to the patient, includes an artifact rejection lumen  324  in addition to single lumen  103 . Artifact rejection lumen  324  is preferably physically connected to patient monitoring and gas flow delivery lumen  103  so that any movement in lumen  103  also occurs or is translated into artifact rejection lumen  324 . A distal end  326  of artifact rejection lumen  324  is open to ambient atmosphere, but is situated relative to the patient such that any pressure or flow resulting from respiration of the patient or delivery of the supplemental gas is not “detected” by the open distal end portion  326  of the artifact rejection lumen. For example, one embodiment of the present invention contemplates terminating artifact rejection lumen  324  such that the distal end is located at or near the patient&#39;s ear, where gas flow due to patient respiration or oxygen delivery is unlikely to be detected. The present invention also contemplates that the distal end of artifact rejection lumen  324  can be closed, i.e., not open to the atmosphere.  
      A proximal end of artifact rejection lumen  324  is connected to a second sensor  328 . Second sensor  328  is preferably the same type of sensor used in the gas monitoring system. Thus, if pressure sensor  138  is used, sensor  328  preferably should also be a pressure sensor. If flow sensor  146  is used, sensor  328  is preferably also a flow sensor. If both pressure sensor  138  and flow sensor  146  are used, sensor  328  should include both a pressure sensor and a flow sensor. The present invention also contemplates mixing the pressure and flow sensors to accomplish this same function. It is not necessary to pair a pressure sensor with a pressure sensor and a flow sensor with a flow sensor.  
      Movements in lumen  103  appear as noise in the output of pressure sensor  134  and/or flow sensor  146 . Because lumen  103  and lumen  324  are physically connected, any movement taking place in lumen  103  also takes place in lumen  324 . As a result, any noise due to movement of conduit  322  is detected by sensor  328 , which outputs a signal corresponding thereto. This “noise” signal output by sensor  328  is subtracted from the signal output by pressure sensor  134  and/or flow sensor  146 , thus minimizing the noise due to tube movement. Subtracting out the noise component of the signal measured by pressure sensor  134  and/or flow sensor  146  based on the signal detected by sensor  328  preferably takes place in processor  136  and is accomplished using any signal processing technique.  
       FIG. 12  illustrates another embodiment for removing the noise or artifact resulting from tube movement. Respiratory therapy and monitoring system  330  in  FIG. 12  is similar to that shown in  FIG. 9 , except that conduit  332 , which is connected to the patient, includes an artifact rejection lumen  334  in addition to first lumen  336  and second lumen  338 . First lumen  336  corresponds to lumens  103  and  302  in that its function is to communicate the supplemental gas flow from the gas source to the airway of the patient. Second lumen  338  and artifact rejection lumen  334  provide the patient monitoring and noise rejection functions.  
      Artifact rejection lumen  334  is physically connected to first and second lumens  336  and  338 , such that any movement of the conduit takes place in all lumens. A distal end  340  of artifact rejection lumen  334  is open to ambient atmosphere and situated relative to the patient such that any pressure or flow resulting from respiration of the patient or delivery of the supplemental gas is not detected by open distal end portion  340 , for example, by terminating artifact rejection lumen  334  near the patient&#39;s ear, where gas flow due to patient respiration or oxygen delivery is unlikely to be detected by lumen  334 .  
      A proximal end of artifact rejection lumen  334  and a proximal end of second lumen  338  are connected to opposite sides of a differential pressure transducer  342 . As a result of this connection across the differential pressure transducer, any noise signal or artifact, i.e., movement of lumens  334  and  338 , creates substantially identical pressure variations on each side of the diaphragm in the differential pressure transducer, thereby canceling each other out. Thus, the only remaining pressure sensed by differential pressure transducer  342  is the change in pressure detected by second lumen  338 . In essence, the differential pressure transducer performs, via hardware, the noise measurement and cancellation techniques discussed above with respect to  FIG. 11 . The output of differential pressure transducer  342  is the total patient pressure P Total  with any noise due to motion of conduit  332  effectively suppressed or eliminated.  
       FIG. 13  is a schematic diagram of a sixth embodiment of a monitoring and therapy delivery system  400  according to the principles of the present invention. This embodiment is also similar to that of  FIG. 1  except that an exhaust conduit  402  is provided from conduit  102  to ambient atmosphere. A flow sensor  404  is provided to measure the flow of gas passing through exhaust conduit  402 . The flow of gas Q Total  measured by flow sensor  404  will include (1) a flow of gas Q O2  due to the continuous flow of oxygen being introduced into conduit  102 , and (2) a flow of gas Q Breathing  due to patient breathing. Thus, flow sensor  404  provides a flow measurement comparable to the flow measurement made by flow sensor(s) in the previous embodiments.  
       FIG. 14  is a schematic diagram of a seventh embodiment of a monitoring and therapy delivery system  406  according to the principles of the present invention. This embodiment is similar to that of  FIG. 13 , except that a gas receiving reservoir  408  is provided at the end of exhaust conduit  402 . Reservoir  408  is closed so that gas passing through flow sensor  404  does not exhaust to atmosphere. Reservoir  408  is also formed from a compliant material, so that its volume can change as the pressure in conduit  102  changes. For example, during inhalation, reservoir  408  has a first volume (Volume 1), and during exhalation, in which the pressure in conduit  102  is greater, the reservoir has a second, greater volume (Volume 2). This expansion of reservoir  408  as the pressure in the conduit increases is illustrated by the dashed line in  FIG. 14 .  
      Because reservoir  408  is compliant, it allows gas to flow in exhaust conduit  402  during breathing. However, because the reservoir is closed, there is no continuous flow of gas to atmosphere. Thus, the flow measurement made by flow sensor  404  is directly related, i.e., proportional to, the a flow of gas Q Breathing  due to patient breathing.  
      It is common when using a cannula to provide a bacterial filter or similar filtering element in the gas flow path of the cannula. The bacterial filter protects the sensor from contaminants carried in the gas. The present invention contemplates using the bacterial filter, and, more specifically, the bacteria filter housing, to assist in connecting any one of the cannula (lumens) discussed above to housing  132 . One example of this concept is illustrated in  FIGS. 15 and 16 .  
      Housing  132  includes a receptacle  352  that is sized and configured to receive a housing  354  of a bacteria filter attached to a cannula  356 . Receptacle  352  and housing  354  of the bacteria can have any shape imaginable so long as they are complementary, allowing at least a portion of housing  354  to fit within receptacle  352 . For example, in the illustrated embodiment, bacteria filter  354  includes an optional stem portion  358  and receptacle  352  includes a stem receptacle  360  into which the stem portion inserts.  
      This configuration for receptacle  352  and bacteria filter  354  serves two purposes. First, placing all or at least a portion of the bacteria filter within the receptacle protects the bacteria filter from damage, such as from bumping, and streamlines the cannula running from the housing to the patient so that no bulky items are provided on this length of relatively slim, lightweight tubing. Second, this configuration allows the bacteria filter to provide a solid attachment between the cannula and the housing. The relatively large size of the bacteria housing also makes it easy for a user to plug the bacteria filter into the receptacle.  
      In the embodiment shown in  FIGS. 15 and 16 , cannula  356  corresponds to cannula  103 ,  308 ,  324 ,  334 , or  338  in the above-described embodiments. A second cannula  360  is connected to a supplemental gas port  362  via attachment portion  364 . Cannula  360  corresponds to lumen  306  or  336  from the above-described embodiments.  
       FIG. 17  illustrates an exemplary display  420  suitable for use in displaying a waveform or a plurality of waveforms corresponding to the monitored pressure resulting from the patient breathing into the patient interface. More specifically, the present invention contemplates displaying the ΔP Patient  waveforms for the last n breaths in a superimposed fashion, so that the user can quickly visually compare the current waveform with one or more previous waveforms.  
      For example, waveform  422  shown in display  420  represents the waveform of ΔP Patient  for the current respiratory cycle n. Waveform  424  represents the waveform of ΔP Patient  for the previous respiratory cycle (n- 1 ). Waveform  426  represents the waveform of ΔP Patient  for the respiratory cycle (n- 2 ). Waveform 428 represents the waveform of ΔP Patient  for the previous respiratory cycle (n- 3 ). It can be appreciated that this visual representation allows the user to quickly see that, in this hypothetical example, the patient is experiencing an abrupt reduction in breathing, which is indicative, for example of the patient experiencing an apnea. It can also be appreciated that any monitored parameter, such as patient flow or volume can also be displayed using this technique.  
      Referring back to  FIG. 9 , a further embodiment of the present invention will now be described. This embodiment contemplates adding a flow sensor  430  to conduit  302  to measure the flow of gas Q O2  in conduit  302 . By measuring the flow of gas via sensor  430  and the pressure via pressure sensor  134 ′ the following two important physiological parameters can be determined: (1) the volumetric patient flow in liters per minute (1 pm); and (2) the fractional inspired oxygen concentration (FIO 2 ) can be determined using this configuration and these measurements.  
      The volumetric patient flow rate is a quantitative measure of the amount of gas passing into and out of the patient during the respiratory cycle.  FIG. 18  is a graph illustrating a hypothetical flow waveform  432  which corresponds to the flow measured by flow sensor  430  with the oxygen flow  434  removed, while a constant flow of supplemental oxygen is being delivered, and while a patient is breathing into patient interface device  112 ′. Removing the bias due to the supplemental oxygen flow from the output of flow sensor  430  is accomplished, for example, using a high pass filter, or using any of the other techniques discussed herein.  
      Preferably using software, a point  436  on waveform  432 , when the patient flow is substantially zero, can be determined. At this point, all of the pressure inside the nose is caused by the supplemental oxygen flow. The pressure drop (ΔP) is measured by pressure sensor  134 ′ when the patient flow is zero and while supplemental oxygen is being delivered. In addition, the oxygen flow rate (Q O2 ) is measured at this time. These measured values, when patient flow is zero, are used to determine the resistance (R) of the system according to equation (1) as follows: 
 
Δ P=R*Q   2    (5) 
 
      Solving this equation for R yields, 
 
 R=Q   2 /ΔP   (6) 
 
      Thus, the resistance of the system can be determined using Q O2  for Q when ΔP is measured corresponding to time  436 . This calculated resistance can then be used to determine the quantitative flow Q(t) at all other times by solving equation (1) for flow as follows: 
 
 Q ( t )={square root}{square root over (Δ P ( t )· R )},   (7) 
 
 where ΔP(t) is the pressure measured by pressure sensor  134 ′. 
 
      It should be noted that equation (7) may not accurately represent patient flow because the flow at the nose may include turbulent and laminar flow, while equation (7) presumes that the flow will be turbulent. The relationship between pressure and flow for a laminar flow is a first order equation similar to that of equation (5), ΔP=Q*R. In addition, the value of R is determined based on pure, i.e., 100%, oxygen flow. A patient, however, does not inhale and exhale 100% oxygen at the nose using the system of  FIG. 9 , for example. Thus, the present invention contemplates applying a correction factor to equation (7) to maintain the accuracy of the quantitative flow determination.  
      The correction factor to apply to the calculated flow value is determined by comparing the volume of gas inhaled to the volume of gas exhaled. It is assumed that over a period of time, e.g., over the last five respiratory cycles, the volume inhaled should equal the volume exhaled. If it does not, it can be assumed that calculated flow Q(t) should be corrected.  
      The method for compensating for these factors involves using the following expression: 
 
Δ P=R*Q   (x) ,   (8) 
 
 where x has a value between 1 (e.g., purely laminar flow) and 2 (e.g., purely turbulent flow). The present invention contemplates adjusting the value for x based on the inspiratory to expiratory volume match. If the inspiratory volume exceeds the expiratory volume, the value of x is decreased toward 1, and vice versa. The amount by which x is changed, how frequently this change is made, and the threshold levels that give rise to a change, can be determined based on predetermined criteria or can be changed based on the monitored condition of the patient. 
 
      The present invention also contemplates estimating the volumetric flow rate through a given conduit, such as through the nostril Q Patient . However, to do so requires that the pressure drop from this inside of the nostril to ambient atmosphere ΔP 2  and the resistance of the nostril R Nose  be known. See  FIG. 2 . It can be appreciated that each patient has a uniquely sized nostril. For this reason, the resistance of the nostril is different for each patient.  
      The present invention contemplates estimating the resistance of the nostril R Nose  by estimating the size of the patient&#39;s nostril. To this end, the present invention contemplates providing a set of nostril gages, where the resistance associated with each different sized gage in the set of gages is predetermined. The gages are used to determine the gage size that best fits a particular patient. Once the nostril size is gauged, the associated resistance for that gage size is used to estimate the volumetric flow. For example, R Nose  in equation (4) is now known (actually estimated) based on the nostril gage. The other variables in this equation are also known except for Q Patient . Equation (4) can be solved for Q Patient  to provide a volumetric estimation of the patient flow. It should be noted that equation (4) is a first order equation. Thus, it assumes a laminar flow. For a turbulent flow a second order variation is used as follows: 
 
 P   Total   =Q   O2   2   * R   Tube +( Q   O2   +Q   Patient ) 2   R   Nose    (9) 
 
      The present invention also contemplates that a combination of equation (4) and (9) representing the presence of both laminar and turbulent flow can be used for the purposes discussed herein, such as solving for Q Patient .  
      Once the patient flow rates, volumes inhaled (V inhaled ), and volumes exhaled (V exhaled ) are determined, the FIO 2  can be estimated by calculating the volume of oxygen inhaled (V O2inhaled ) and by using equation (10). The volume of oxygen inhaled (V O2inhaled ) is determined based on the flow measurement from flow sensor  430 .  
               FIO   2     =           (       (       V   inhaled     -     V   O2inhaled       )     ⁢     (   0.21   )       )     +     (       (     V   O2inhaled     )     ⁢     (   1.0   )       )         V   inhaled       .             (   10   )             
 
      It should be noted that the value “1.0” in equation (10) is used assuming that 100% oxygen is being delivered to the patient. This value can be adjusted to correspond to other delivered oxygen concentrations.  
      The respiratory therapy and monitoring system of the present invention can be a stand-alone system or it can be combined with other medical devices. When combined with other medical devices or systems, it can be integrated into that system or provided as a module that selectively attaches to the other medical device.  
      The respiratory therapy and monitoring system of the present invention is particularly well suited for use in situations where a patient is receiving a surgical procedure outside the hospital. In this configuration, the respiratory therapy and monitoring system delivers oxygen to the patient while monitoring his or her respiratory characteristics, such as whether he or she is breathing normally or experiencing an apnea. This occurs before, during, or after the patient receives a medication used during the procedure, such as an anesthetic, i.e., a general anesthetic, a sedative, a pain reliever, or a combination thereof. Until the present invention, there has been little or no patient monitoring in these situations, and deaths have been known to occur. The present invention provides a cost effective monitoring capability in such an environment, so that that the physician or other caregiver can be warned of any deterioration in the patient&#39;s condition.  
      Heretofore, a quantitative measurement of the flow and/or volume of gas entering or exiting the patient is possible by first determining the resistance (R) or the system or the nose (R nose ). This resistance can then be used to determine the flow/volume by measuring the pressure across the resistance. The present invention, as described above, determines the resistance of the system by solving for resistance when the flow in the system (with bias removed) is zero, i.e., see equation (6) above. The above-described embodiments also teach estimating the resistance of the nose using a sizing gage, where the resistance associated with each different sized gage in the set of gages is predetermined. These resistance determination techniques are used if the resistance of the orifice is not known. The present invention also contemplates providing a patient interface having a flow element with a known resistance. However, in this embodiment, all of the gas passing into or out of the patient must pass through the flow element.  
       FIG. 19  illustrates an eight embodiment of a monitoring and therapy delivery system  500  according to the principles of the present invention. System  500  includes a patient interface  502  that includes a flow element  504 , i.e., an exhaust path to atmosphere, with a known resistance. A gas supply  506  provides a constant supply of breathing gas, such as oxygen or an oxygen mixture, to the interior of the patient interface via a conduit  508  and port  509 . Gas supply  506  is any suitable supply, such as an oxygen tank or oxygen concentrator. In the illustrated exemplary embodiment, patient interface  502 , which is a mask, includes a second port  510  that communicates a sensor  512  to the interior portion of the mask via a conduit  514 . It is to be understood that the breathing gas need not be directly provided to the user interface, as shown in  FIG. 19 . On the contrary, the gas can be provided to the conduit  514 , thereby avoiding the need to provide two ports in the mask.  
      A plurality of holes are provided in the mask to define the flow element. It is to be understood, however, that any venting system for communicating the interior of the mask with ambient atmosphere, while creating a pressure drop across the flow element, is contemplated by the present invention. Because the resistance of this flow element is known and all gas exists the mask through this flow element, the flow or volume of gas delivered to or from patient can be quantitatively measured. Techniques for making this quantitative flow or volume pressure determination are discussed in detail in U.S. Pat. Nos. 6,544,192; 6,342,040; and 6,017,315, the contents of which are hereby incorporated herein by reference.  
      A still further embodiment of a monitoring and therapy delivery system  550  is shown in  FIG. 20 . The patient interface  502  of this embodiment is substantially similar to that shown in  FIG. 19 , except that a nebulizer or a medication delivery device  552  is provided in addition to the supply of breathing gas or oxygen. This embodiment allows a supply of aerosolized medication or other therapeutic or medicated gas to be delivered to the patient via a relatively large bore conduit  554  connected to patient interface device  502  at port  509 ′. The present invention contemplates that the internal cross-sectional area of conduit  554  and port  509 ′ should be larger than that of a standard oxygen cannula to allow the aerosolized medication to be delivered efficiently to the patient.  
       FIGS. 21A-21C  illustrate a nasal prong patient interface  600  according to a further embodiment of the present invention suitable for use in the monitoring and therapy delivery system of the present invention. As in the embodiments of  FIGS. 19 and 20 , patient interface  600  is configured to capture all of the flow of gas to and from the user so that the flow/volume can be quantatively measured. Nasal prong patient interface  600  includes a pair of protruding portions  602  supported by a base member  604 . In an exemplary embodiment of the present invention, protruding portions  602  are integral with base member  604  and all are formed from a relatively flexible and biocompatible material. Protruding portions  602  insert into the patient&#39;s nares and seal the nares so that all gas passing through the each nostril must pass through the protruding portions, as indicated by arrows  606 . Flanges  610  are provided to facilitate this sealing function so that no gas escape around the protruding portions. Flanges  610  can have any size and shape or multiple flanges can be provided to achieve this function.  
      In the illustrated exemplary embodiment, the outside diameter of each protruding portion increases as the distance toward base member  604  decreases. This increase in diameter is identified by numeral  612  in  FIGS. 21A and 21B . The increase in diameter serves to seal the protruding portions against the nostrils to prevent gas from leaking around the patient interface.  
      A pair of flow element portions  614  are provided on the side of base member  604  opposite the side that protruding portions  602  are located. In an exemplary embodiment of the present invention, flow element portions  614  are integral with base member  604 . A pair of rigid rings  616  are provided in flow element portions  614 . Rings  616  provide a mounting structure for pressure/flow tubes  618  that are used to measuring the pressure or flow of gas passing through each channel between the patient and ambient atmosphere. More specifically, a small hole is defined in each ring and tube  618  is connected to each hole so that the flow through ring can be measured.  
      In the illustrated embodiment, a relatively small lip  620  is provided along an edge of each ring  616  to produce the necessary pressure drop along each channel for so that the flow or pressure can be measured. Preferably the hole in each ring for pressure or flow measurement purposes is provided adjacent to each lip because this is the location where the pressure differential in the channel is maximized.  
      An optional supplemental gas delivery conduit  622  is shown attached to each ring. This conduit is used, for example, to deliver oxygen to the nasal passages of the patient in the same manner conduit  508  delivers supplemental oxygen in the patient interface in the embodiment shown in  FIGS. 19 and 20 .  
      In this embodiment, holes  624  are provided on each edge of base member  604 . Holes  624  help contain tubes  618  and  622  and prevent them from interfering with other medical equipment or from being distracting or annoying to the patient. Of course, the present invention contemplates any number of techniques for securing the pressure/flow sensing tube and gas delivery tubes to the patient.  
      Rings  616  are preferably rigid so that the cross sectional area through the rings does not change, thereby ensuring flow/pressure measuring accuracy. Because rings  616  are removable from flow element portions  614  of patient interface  600 , the individual structures can be easily cleaned or components replaced as needed. It is to be understood, however, that rings  616  can be integral with flow element portions  614 . Furthermore, the present invention contemplates that separate sensors can be associated with each airflow channel so that the flow of gas through each nostril can be measured independently.  
      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.