Abstract:
Disclosed is an apparatus and method for the delivery of supplemental oxygen gas to a person combined with the monitoring of the ventilation of the person with both being accomplished without the use of a sealed face mask. Preferred embodiments of the present invention combine an oxygen delivery device, a nasal airway pressure sampling device, an oral airway pressure sampling device, and a pressure analyzer connected to the sampling devices to determine the phase of the person&#39;s respiration cycle and the person&#39;s primary airway. The oxygen delivery device is connected to a controller such that it delivers a higher flow of oxygen to the person during the inhalation phase of the person&#39;s respiratory cycle. The invention thus increases end tidal oxygen concentrations with improved efficiency comparative to known open airway devices. Embodiments of the invention can include carbon dioxide sampling tubes that continuously sample air from the nose and mouth to determine carbon dioxide concentration during exhalation.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to an apparatus and method for the delivery of supplemental oxygen gas to a person combined with the monitoring of the ventilation of the person, and more particularly to an apparatus and method where such delivery of oxygen and monitoring of ventilation is accomplished without the use of a sealed face mask. 
   2. Description of Related Art 
   In various medical procedures and treatments performed on patients, there is a need to deliver supplemental oxygen (O 2 ) gas to the patient. In procedures involving the delivery of anesthesia or where a patient is otherwise unconscious and ventilated, the delivery of oxygen (and other gaseous drugs) is typically accomplished via a mask that fits over the patient&#39;s nose and mouth and is sealed thereto or by a tracheal tube. In other procedures, however, for example, where a patient may be sedated, but conscious and breathing on their own, the delivery of supplemental oxygen gas may be accomplished via a mask or by nasal cannulae (tubes placed up each nares of a patient&#39;s nose), connected to a supply of oxygen. 
   The primary goal of oxygen supplementation (whether mask-free or otherwise) is to enrich the oxygen concentration of the alveoli gas, namely, the mixture of gas in the alveoli (microscopically tiny clusters of air-filled sacs) in the lungs. In a person with normal lung function, the level of oxygen in the deepest portion of the alveolar sacs is essentially reflected at the end of each “tidal volume” of exhaled gas (the volume of gas in one complete exhalation). The gas sample measured at the end of a person&#39;s exhalation is called the “end-tidal” oxygen sample. 
   So, for example, if a person breathes room air, room air contains 21% oxygen. When the person exhales, the end tidal gas will have about 15% oxygen; the capillary blood has thus removed 6% of the oxygen from the inhaled gas in the alveoli, to be burned by the body in the process of metabolism. Again, a simple goal of any form of oxygen supplementation is to increase the concentration of oxygen in the alveolar sacs. A convenient method of directly measuring or sampling the gas in alveolar sacs is by continuously sampling the exhaled gas at the mouth or nose and identifying the concentration of oxygen at the end-tidal point, a value that is reasonably reflective of the oxygen concentration in the alveolar sacs. Thus, one can compare the effectiveness of oxygen delivery systems by the amount that they increase the end tidal oxygen concentration. 
   If a person breathes through a sealing face mask attached to one-way valves and inhales a supply of 100% oxygen, the end tidal concentration of oxygen goes up to 90%. More specifically, once inert nitrogen gas has been eliminated from the lungs (after pure oxygen has been breathed for several minutes), alveolar gas will contain about 4% water vapor and 5% carbon dioxide. The remainder (about 90%) will be oxygen. Thus, the best oxygen delivery systems typically increase end tidal oxygen from a baseline of 15%, when breathing non-supplemented room air, to 90% when breathing pure oxygen. Although sealed face-masks are relatively effective oxygen delivery systems, conscious patients, even when sedated, often find masks significantly uncomfortable; masks inhibit the ability of a patient to speak and cause anxiety in patients. 
   Nasal cannulae, on the other hand, do not typically cause the level of discomfort or anxiety in conscious patients that masks do, and thus, from a patient comfort standpoint, are preferable over masks for the delivery of oxygen to conscious patients. Nasal cannulae are, however, significantly less effective oxygen delivery systems than sealed face masks. Nasal cannulae generally increase the end tidal oxygen concentration to about 40% (as compared to 90% for a sealed mask). Nasal cannulae are less effective for at least two reasons. 
   First, when a person inhales, they frequently breathe through both nasal passages and the mouth (three orifices). Thus, the weighted average concentration of inhaled oxygen is substantially diluted to the extent of mouth breathing because 21% times the volume of air breathed through the mouth “weights down the weighted average”. 
   Second, even if a person breathes only through their nose, the rate of inhalation significantly exceeds the supply rate of the nasal cannula (typically 25 liters/min.) so the person still dilutes the inhaled oxygen with a supply of 21% room air. If the nasal cannula is flowing at 2 liters per minute and a person is inhaling a liter of air over 2 seconds, the inhalation rate is 60 liters per minute, and thus, most of the inhaled volume is not coming from the nasal cannula, but rather from the room. Increasing the oxygen flow rate does not effectively solve this problem. First, patients find increased flow very uncomfortable. Second, increased oxygen flow dilutes (washes away) the exhaled carbon dioxide, then carbon dioxde cannot be sampled as a measure of respiratory sufficiency. 
   There is also a need in various medical procedures and treatments to monitor patient physiological conditions such as patient ventilation (the movement of air into and out of the lungs, typically measured as a volume of air per minute). If the patient does not move air into and out of the lungs then the patient will develop oxygen deficiency (hypoxia), which if severe and progressive is a lethal condition. Noninvasive monitoring of hypoxia is now available via pulse oximetry. However, pulse oximetry may be late to diagnose an impending problem because once the condition of low blood oxygen is detected, the problem already exists. Hypoventilation is frequently the cause of hypoxemia. When this is the case, hypoventilation can precede hypoxemia by several minutes. A good monitor of ventilation should be able to keep a patient “out of trouble” (if the condition of hypoventilation is diagnoses early and corrected) whereas a pulse oximeter often only diagnosed that a patient is now “in” trouble. This pulse oximetry delay compared to ventilatory monitoring is especially important in acute settings where respiratory depressant drugs are administered to the patient, as is usually the case during painful procedures performed under conscious sedation. 
   Ventilatory monitoring is typically measured in terms of the total volumetric flow into and out of a patient&#39;s lungs. One method of effective ventilatory monitoring is to count respiratory rate and then to measure one of the primary effects of ventilation (removing carbon dioxide from the body). 
   There are a variety of ventilation monitors such as 1) airway flowmeters and 2) capnometers (carbon dioxide detectors). These monitors are used routinely for patients undergoing general anesthesia. These types of monitors work well when the patient&#39;s airway is “closed” in an airway system such as when the patient has a sealing face mask or has the airway sealed with a tracheal tube placed into the lungs. However, these systems work less well with an “open” airway such as when nasal cannulae are applied for oxygen supplementation. Thus, when a patient has a non-sealed airway, the options for tidal volume monitoring are limited. With an open airway, there have been attempts to monitor ventilation using capnometry, impedance plethysmography, and respiratory rate derived from the pulse oximeter&#39;s plethysmogram. The limitations of each are discussed below. 
   Nasal capnometry is the technique of placing a sampling tube into one of the nostrils and continuously analyzing the carbon dioxide content present in the airstream thereof. Nasal capnometry is relatively effective provided that 1) the patient always breathes through his/her nose, and 2) nasal oxygen is not applied. More specifically, if the patient is talking, most of the exhalation is via the mouth, and frequent false positive alarms sound because the capnometer interprets the absence of carbon dioxide in the nose as apnea, when in fact, it is merely evidence of talking. A couple of devices in the prior art have tried to overcome this problem by: manual control of sampling from the nose or mouth (Nazorcap); supplementing oxygen outside of the nose while sampling for CO 2  up inside the nose (BCI); providing oxygen in the nose while sampling CO 2  from the mouth (BCI); and supplying oxygen up one nostril and sampling for CO 2  Up inside the other nostril (Salter Labs). None of these already-existing systems provide oxygen to both the nose and mouth or allow automatic control of sampling from either site. Further, if nasal oxygen is applied to the patient, the carbon dioxide in each exhalation can be diluted significantly by the oxygen supply. In this case, the capnometer may interpret the diluted CO 2  sample as apnea (stoppage in breathing), resulting once again, in frequent false positive alarms. 
   Impedance plethysmography and plethysmogram respiratory rate counting also suffer drawbacks as primary respiratory monitors. Impedance plethysmography is done via the application of a small voltage across two ECG electrode pads placed on each side of the thoracic cage. In theory, each respiration could be detected as the phasic change of thoracic impedance. Unfortunately, the resulting signal often has too much noise/artifact which can adversely effect reliability. Respiratory rate derived from the pulse oximeter&#39;s plethysmogram may not diagnose apnea and distinguish it from complete airway obstruction, thus misdiagnosing apnea as a normal condition (a false negative alarm state). 
   In view of the above drawbacks to current systems for delivering supplemental oxygen gas and monitoring ventilation, there is a need for an improved combined system to accomplish these functions. 
   SUMMARY OF THE INVENTION 
   One of the purposes of the current invention is to increase the alveolar oxygen concentration without the requirement for a patient to wear a mask. 
   This is done by:
         1) Delivering a higher flow of oxygen (e.g. 10-15 liters per minute)   2) Making this higher flow of oxygen acceptable to patients by providing it only during the inhalation part of the respiratory cycle (so the patient does not get a continuous sensation of high flow oxygen)   3) Making the higher flow of oxygen acceptable to the patient by opening a variable orifice for oxygen supply slowly rather than going to immediate full-open or otherwise diffusing the oxygen supply, which minimizes the sensation of a rapid burst of oxygen beginning with each inhalation.   4) Providing oxygen flow to all three respiratory orifices (i.e., provide flow over both nostrils and the mouth) during the inhalation cycle. Thus, inhaled gas is not diluted at any inhalation portal by pure room air.       

   5) The supply source for oxygen is a multiplicity of holes rather than single lumen cannula. This decreases the Bernoulli-effect of air entrainment that occurs when a high velocity of gas is delivered through a single cannula. 
   The invention thus increases end tidal oxygen concentrations from the baseline 15% (breathing room air) up to 50-55%. Whereas this is not as effective as face mask oxygen supplementation, it is significantly better than the prior art for open airway oxygen supplementation devices. 
   A second purpose of the invention is to more effectively monitor patient ventilation in combination with mask-free delivery of oxygen gas to the patient. 
   In this aspect, the invention includes placing pressure lumens inside one of a patient&#39;s nostrils and in front of the patient&#39;s mouth. The pressure lumens are connected to pressure transducers which in turn are connected to a processor running software. A carbon dioxide sampling tube accompanies each pressure lumen. The nasal and mouth pressure samples from the respective lumens are continually compared with one another to determine the primary ventilatory path i.e., whether the nose or mouth is the primary respiratory site. That is, whichever orifice is experiencing greater pressure swings is selected as the location of the primary ventilatory path. The carbon dioxide sampling tubes continuously sample gas from the nose and mouth and are connected to a solenoid valve which is in turn connected to a capnometer. Once the comparators (pressure transducers) determine the primary ventilatory path, the solenoid valve is opened so that only the sample from the primary path is run to the capnometer. 
   The software also analyzes the pressures sampled from each orifice to determine whether the patient is inhaling or exhaling. The software opens a solenoid valve connected to an oxygen source so that oxygen flow is high only during the inhalation phase of the patient&#39;s breathing. 
   In addition to being connected to pressure transducers, each pressure lumen is also connected to a microphone that amplifies the patient&#39;s respiratory sounds so they may be heard by a care giver in the room. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a side, cut out view of the disposable portion of the apparatus placed on a patient in accordance with the invention. 
       FIG. 2  shows a perspective exterior view of the disposable portion of the apparatus in accordance with the invention. 
       FIG. 3  is a blow-up view showing the lower, middle and cover portions of the disposable portion of the apparatus in accordance with the invention. 
       FIG. 4  shows an embodiment of the disposable portion of the apparatus with an oral collection chamber in accord with the invention. 
       FIG. 5A  is a schematic diagram of an oxygen delivery and ventilatory monitoring system in accordance with one embodiment of the invention. 
       FIG. 5B  is a schematic diagram of an oxygen delivery and ventilatory monitoring system in accordance with an alternative embodiment of the invention. 
       FIG. 6  is a schematic diagram of pressure transducer circuitry in one embodiment of the invention. 
       FIG. 7  is a diagram of the pressure wave form during a respiration cycle used in the method of the invention. 
       FIG. 8  is a flow chart of a preferred embodiment of the method of the invention. 
   

   DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1  shows a cut-out view of the disposable portion  4  of an apparatus in accordance with the invention placed on a patient  10 . 
   The apparatus provides for the mask-free delivery of supplemental oxygen gas to the patient combined with the monitoring of patient ventilation. Oxygen gas is supplied to the patient from an O 2  supply tube  12  and exits portion  4  from a diffuser grid  14  in housing  16  (shown in more detail in FIG.  2 ). Diffuser grid  14  blows diffused oxygen into the immediate area of the patient&#39;s nose and mouth. Two thin lumens (tubes) are mounted adjacent one another to portion  4  and placed in one of the patient&#39;s nostrils (nasal lumens  18 ). Another two thin lumens are also mounted adjacent to one another to portion  4  placed in front of the patient&#39;s mouth (oral lumen&#39;s  20 ). 
   Of nasal lumens  18 , one lumen is a pressure lumen for sampling the pressure resulting from a patient&#39;s nose breathing and the other lumen continuously samples the respiratory gases so they may be analyzed in the capnometer to determine the concentration of carbon dioxide. This arrangement is essentially the same for oral lumens  20 , namely, one lumen is a pressure lumen (samples pressure in mouth breathing) and the other lumen continuously samples the respiratory gases involved in mouth breathing. Nasal lumens  18  and oral lumens  20  are each connected to their own pneumatic tubes, e.g.,  22 , which feed back the nasal and oral pressure samples to pressure transducers (not shown) and which feed back the nasal and oral gas samples to a capnometer (not shown). All of portion  4 ; lumens  18 ,  20 ; oxygen supply tubing  12  and feedback tubing  22  are disposable (designed to be discarded after every patient use), and preferably constructed of pliable plastic material such as extruded poly-vinyl chloride. 
   As shown in  FIG. 2 , lumens  18 ,  20  and tubings  12  and  22 , although shown as a portion cut-out in  FIG. 1  in a preferred embodiment, are housed in cover  30 . Also, in  FIG. 2 , nasal lumens  18  (including pressure lumen  28  and CO 2  lumen  26 ) are formed from a double-holed, single-barrel piece. Oral lumens  20  (which include pressure lumen  32  and CO 2  lumen  34 ) are preferably formed from a double barrel piece. Diffuser grid  36  is formed in cover  30  and functions as an oxygen diffuser which releases a cloud of oxygen into the immediate oral and nasal area of the patient  10 . 
     FIG. 3  shows disposable portion  4  including cover  30  in more detail in cut-out fashion. Specifically, lower portion  110 , formed from a suitably firm, but not rigid, plastic, has an opening  112  for insertion of oxygen supply tube  12 . Slot  114  in portion  110  receives the oxygen gas from the tube  12 , retains it, and forces it up through opening  148  in middle portion  112 . Middle portion  112  is affixed to lower portion  110  lying flat on portion  110 . From opening  148 , the oxygen gas travels into cover  130  (affixed directly onto middle portion  112 ) and travels lengthwise within cover  130  to diffuser portion  135 , whereupon the oxygen exits cover  130  through diffuser grid  136  into the immediate vicinity of the patient&#39;s nose and mouth in a cloud-like fashion. It is preferable to supply oxygen flow to all three respiratory orifices (both nostrils and mouth) to increase the concentration of oxygen provided to the patient. By providing flow to all three orifices inhaled gas is not diluted at any inhalation portal by pure room air. Also, a diffused stream such as that created by grid  136  is a preferred embodiment for the oxygen stream delivered to the patient. This is because a stream of oxygen delivered through a single lumen cannula is typically uncomfortable at the higher flow rates necessary for sufficient oxygen delivery. Further, at those flow rates, a single lumen can create an undesirable Bernoulli effect. It is noted that an alternative to the diffuser grid  136  is a cup-shaped or other chamber which receives the O 2  jet-stream and includes a foam or filler paper section for diffusing the jet stream of O 2 . 
   As is also shown in  FIG. 3 , feedback tubing  22  enters lower portion  110  at openings  122 . At one opening  122  begin grooves  146  and  140  formed in lower portion  110  each for receiving the feedback pressure sample from lumens  128  and  132 . At the other opening  122  begin groves  144  and  142 , formed in lower portion  110  each for receiving the feedback CO 2  sample from lumens  126  and  134 . Grooves  146 ,  144 ,  140  and  142 , all formed in lower portion  110 , connect at one end to their respective sampling lumens ( 128 ,  126 ,  132  and  134 ) and at their other end to feedback tubing  22 ; middle portion  112  lies flat on and affixed to portion  110  such that the grooves  146 ,  144 ,  140  and  142  form passageways for the respective feedback samples. As can be seen, when assembled portions  130 ,  112  and  110  together form whole disposable piece  4 , shown perceptively in FIG.  2 . 
     FIG. 4  shows a preferred embodiment of disposable portion  4  (here portions  110  and  112  are shown affixed to one another) with an oral sample collection chamber  210  fitting over oral lumens  220  (nasal lumens are shown) at  218  and the opening for the oxygen supply tube is shown at  212 ). Oral sample collector  210  is preferably constructed of plastic and creates a space in chamber  214  that collects a small volume of air the patient has breathed orally. That volume of air is then sampled by lumens  220  and fed back for analysis through the respective pressure and CO 2  feedback tubing to pressure transducers and the capnometer described above. Collector  210  thus acts as a storage container for better sampling of the oral site. It also serves as a capacitor for better monitoring of oral site pressure (exhalation contributes to volume and pressure increases, while inhalation removes air molecules from volume and pressure decreases). 
   In one preferred embodiment, collector  210  is provided in a variety of sizes and shapes to collect different volumes of air or to facilitate different medical procedures which may be performed in or near the mouth. In another preferred embodiment collector  210  is adjustable in that it is capable of sliding over lumens  220  to enable positioning directly over the mouth&#39;s airstream. In a further embodiment, lumens  220  are themselves also slidably mounted to portion  222  so as to be extendable and retractable to enable positioning of both the lumens and collector directly in front of the oral airstream. 
     FIG. 5A  shows a schematic circuit diagram of a preferred embodiment of the oxygen delivery and ventilatory monitoring system of the invention. As described above, disposable portion  304  includes nasal lumens which sample a nasal (nares) volume  318  of air breathed through the patient&#39;s nostril; an oral sample collector which creates an oral volume of air  320  effecting sampling of air breathed through a patient&#39;s mouth; and an oxygen diffuser  336  which enriches the immediate breathing area of a patient with oxygen, increasing the patient&#39;s fraction of inspired oxygen and thereby increasing the patient&#39;s alveolar oxygen levels. The diffuser  336  ensures that a high rate of oxygen flow is not uncomfortable for the patient. 
   Oxygen gas is supplied to diffuser  336  from an oxygen supply (O 2  tank or in-house oxygen). If the supply of O 2  is from an in-house wall source, DISS fitting  340  is employed. The DISS fitting  340  (male body adaptor) has a diameter indexed to only accept Compressed Gas Association standard oxygen female nut and nipple fitting. A source pressure transducer  342  monitors the oxygen source pressure and allows software running on processor  344  to adjust the analog input signal sent to proportional valve  346  in order to maintain a user-selected flow rate as source pressure fluctuates. Pressure relief valve  348  relieves pressure to the atmosphere if the source pressure exceeds 75 psi. Proportional valve  346  sets the flow rate of oxygen (e.g. 2.0 to 15.0 liters per minute) through an analog signal and associated driver circuitry (such circuitry is essentially a voltage to current converter which takes the analog signal to a dictated current to be applied to the valve  346 , essentially changing the input signal to the valve in proportion to the source pressure, as indicated above). Downstream pressure transducer  350  monitors the functionality of proportional valve  348 . Associated software running on processor  344  indicates an error in the delivery system if source pressure is present, the valve is activated, but no downstream pressure is sensed. 
   As described above, the nares volume  318  and oral collection volume  320  are fed back to the capnometer  352  via a three-way solenoid valve  354 . The capnometer  352  receives the patient airway sample and monitors the CO 2  content within the sample. Software associated with capnometer  352  displays pertinent parameters (such as a continuous carbon dioxide graphic display and digital values for end-tidal CO 2  and respiration rate) to the user. A suitable capnometer may be that manufactured by Nihon Kohden (Sj5i2). Three-way solenoid valve  354  automatically switches the sample site between the oral site and the nasal site depending on which site the patient is primarily breathing through. This method is described in more detail below, but briefly, associated software running on processor  344  switches the sample site based on logic that determines if the patient is breathing through the nose or mouth. It is preferable to have a short distance between the capnometer and valve  354  to minimize dead space involved with switching sample sites. 
   Also as described above, the nares volume  318  collected is fed back to a nasal pressure transducer  356  and nasal microphone  358 . Transducer  356  monitors the pressure in the nares volume  318  through the small bore tubing described above. Associated software running on processor  344  determines through transducer  356  if the patient is breathing primarily through the nose. Associated offset, gain and temperature compensation circuitry (described below) ensures signal quality. Nasal microphone  358  monitors the patient&#39;s breath sounds detected at the nasal sample site. Associated software allows the user to project sound to the room and control volume. Output from microphone  358  may be summed with output of the oral microphone  360  for a total breath sound signal. In an additional embodiment the breath sound signals are displayed to the user and/or further processed and analyzed in monitoring the patient&#39;s physiological condition. 
   Oral pressure transducer  362  monitors pressure at the oral collection volume  320  through the small bore tubing described above. Associated software running on processor  344  determines via monitor  362  if the patient is primarily breathing through the mouth. Offset gain and temperature compensation circuitry ensure signal quality. Oral microphone  360  operates as nasal microphone  358  described above to project breath sounds to the room. 
   Dual chamber water trap  364  guards against corruption of the CO 2  sensors by removing water from the sample volumes. Segregated chambers collect water removed by hydrophobic filters associated with the nasal and oral sites. This segregation ensures that the breathing site selected as the primary site is the only site sampled. 
     FIG. 5B  shows an additional embodiment of the system circuit of the present invention, including a sample bypass circuit which keeps the sample sites flowing at the same rate, regardless of whether the site is being sampled by the capnometer or bypassed. Specifically, nasal diverter valve  555  switches the nasal sample between the capnometer for CO 2  sampling and the bypass line. Activation of the valve  555  is linked to activation of oral diverter valve  557  in order to ensure that one sample is connected to the bypass line while the other sample is connected to the capnometer. This allows two states: 1) the oral site fed back to the capnometer, with the nasal site to the bypass; and 2) the nasal site fed back to the capnometer with the oral site on bypass. As described above, the control software switches the sample site based on logic that determines if the patient is breathing through the nose or mouth. Oral diverter valve  557  switches the oral sample between the capnometer for CO 2  sampling and the bypass line and operates as described with respect to nasal diverter valve  555 . 
   Bypass pump  559  maintains flow in the bypass line  561  that is equivalent to flow dictated by the capnometer (e.g., 200 cm/min.). The pump  559  also ensures that the sample sites are synchronized with one another so that the CO 2  waveform and respiration rate calculations are not corrupted when sample sites are switched. Flow sensor  563  measures the flow rate obtained through the bypass line  561  and provides same to electronic controller  565  necessary for flow control. Controller  565  controls the flow of pump  559 . 
   As can be seen from  FIG. 5B , balancing the flow between the active sample site and the bypass (e.g., maintaining a flow in the bypass equivalent or near equivalent to the flow within the CO 2  sampling site, e.g., 200 cc/min) is desired. This prevents corruption of the CO 2  waveform and respiration rate calculations in the event one site became occluded such that the bypass and capnometer lines flowed at different rates. 
     FIG. 6  shows a schematic of the electronic circuitry associated with pressure transducers  356  and  362 . Such circuitry includes a pressure sensor  402 , a hi-gain amplifier  404 , a temperature compensating zeroing circuit  406  and a low pass filter  408 . This gain and temperature zeroing circuitry ensure signal quality for the pressure transducers. 
     FIG. 7  is a diagram of the pressure reading (oral or nasal) during a typical respiration cycle with thresholds A, B, C and D identified in accordance with the preferred method of the invention. As is shown, as exhalation begins, the pressure becomes positive, eventually reaching a peak then dropping back to zero as the exhalation completes. The beginning of inhalation is indicated by the pressure becoming negative. The pressure will become more negative during the first portion of inhalation then trend back towards zero as inhalation ends. 
   The control software of the present invention defines an upper and a lower threshold value  702 ,  704 , respectively. Both are slightly below zero, with the lower threshold  704  being more negative than the upper threshold  702 . During each respiration cycle the software determines when the thresholds  702 ,  704  are crossed (points A, B, C, and D,  FIG. 7 ) by comparing the pressures to one of the two thresholds. The crossings are expected to occur in sequence, i.e., first A, then B followed by C, and finally D. An O 2  source valve is turned up (e.g., to 10-15 liters/min of flow) when point A is reached and turned down (e.g., to 2-3 liters/min of flow) when C is reached, thus providing the highest oxygen flow during the majority of the inhalation phase. 
   To determine when the threshold crossings occur, the software examines the pressures from the oral and nasal pressure sensors at periodic intervals, e.g., at 50 milli/seconds (e, FIG.  8 ). During each examination the software combines the oral and nasal pressures then compares the combined pressure to one of the two thresholds as allows. 
   As shown by the flowchart of  FIG. 8 , when the software begins execution, it awaits a combined pressure value less than the upper threshold (point A). When this condition is met, the software turns up the O 2  valve to a higher desired flow (e.g., 10-15 liters/min) then begins looking for a pressure value less than the lower threshold (point B). When this occurs the software waits for a combined pressure value that is greater than the lower threshold (point C). When this value is read, the O 2  is turned down to the lower desired flow rate (e.g., 2-3 liters min) and the software awaits a pressure value that exceeds the upper threshold (point D). Once this value is read, the cycle begins again for the next breath. 
   As described above, a capnometer is used to provide information such as EtCO 2  and respiration rate by continually sampling the level of CO 2  at a single site. Since breathing can occur through the nose, mouth, or both, the software must activate valves  354  (FIG.  5 ), that switch the capnometer-sampling site to the source providing the best sample, i.e., mouth or nose. 
   As is also shown in  FIG. 8 , the software accomplishes this by examining the oral and nasal pressure readings at periodic intervals. During each examination, the current and prior three oral pressure values are compared to the corresponding nasal pressure values. If the combined nasal pressures exceed the combined oral pressures by more than a factor of three, the capnometer sample is obtained at the nose. If the combined oral pressures exceed the combined nasal pressures by more than a factor of three, the sampling occurs at the mouth. 
   The above-described system and method thus provides improved delivery of supplemental O 2  gas and ventilatory monitoring without use of a face mask. The system and method are particularly useful in medical environments where patients are conscious (thus comfort is a real factor) yet may be acutely ill, such as in hospital laboratories undergoing painful medical procedures, but also in the ICU, CCU, in ambulances or at home in for patient-controlled analgesia, among others. It should be understood that the above describes only a preferred embodiment of the invention and other equivalent embodiments are contemplated.