AU2001268349A1

AU2001268349A1 - Apparatus and method for mask free delivery of an inspired gas mixture and gas sampling

Info

Publication number: AU2001268349A1
Application number: AU2001268349A
Authority: AU
Inventors: Randall S Hickle; Samsun Lampotang
Original assignee: Scott Laboratories Inc
Current assignee: Scott Laboratories Inc
Priority date: 2000-06-13
Filing date: 2001-06-13
Publication date: 2002-03-14
Anticipated expiration: 2021-06-13

Description

APPARATUS AND METHOD FOR MASK FREE DELIVERY OF AN INSPIRED GAS MLXTURE

AND

GAS SAMPLING

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. patent

application Serial No. 09/592,943 filed June 13, 2000, the contents

of which are incorporated herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an apparatus and method for the

delivery of an inspired gas (e.g., supplemental oxygen (O₂) gas) to a

person combined with sampling of the gas exhaled by the person, such sampling for use, for example, in monitoring the ventilation of

the person or for inferring the concentration of a drug or gas in the

person's blood stream. More particularly, the invention relates to

an apparatus and method where such delivery of the inspired gas

and gas sampling are 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 a desired inspired gas composition, e.g., supplemental oxygen, to the patient. In procedures involving the delivery of anesthesia or where a patient is

otherwise unconscious and ventilated, the delivery of oxygen and gaseous or vaporized or nebulized drugs is typically accomplished

via a mask that fits over the patient'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 or

inspired gas may be accomplished via a mask or by nasal cannulae

(tubes placed up each nare of a patient's nose), connected to a

supply of oxygen or the desired gas composition.

Taking oxygen as one example of an inspired gas to be delivered to a person, the primary goal of oxygen supplementation

(whether mask-free or otherwise) is to enrich the oxygen

concentration of the alveolar 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's exhalation is called the "end-tidal" gas 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 some

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 weighed 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 2-5 liters/min.) so the person still dilutes the

inhaled oxygen with a supply of 21% O2 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 30 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

generally find increased flow very uncomfortable. Second, increased

inspired gas flow dilutes (washes away) exhaled gases like carbon

dioxide and/or exhaled vapors of intravenous anesthetics or other

drugs. When this happens carbon dioxide cannot be accurately sampled as a measure of respiratory sufficiency. Also, a drug such

as an inhalational or intravenous anesthetic, cannot be accurately

sampled as a measure of the arterial concentration of the drug from

which, for example, the level of sedation might be inferred. There is

a need in various medical procedures and treatments to monitor patient physiological conditions such as patient ventilation (the

movement of gas into and out of the lungs, typically measured as a

volume of gas 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 diagnosed early and

corrected) whereas a pulse oximeter often only diagnoses 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'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). Certain methods of monitoring

ventilation measure the "effect" of ventilation (pressure oscillations,

gas flow, breath sound and exhaled humidity, heat or CO2 at the

airway). Other ventilation methods measure the "effort" of

ventilation (e.g., transthoracic impedance plethysmography, chest

belts, respiratory rate extraction from optoplethysmograms). Effort-based ventilation monitors may be less desirable because

they may fail to detect a blocked airway where the patient

generates the effort (chest expansion, shifts in blood volume, etc.)

but does not achieve the desired effects that accompany gas exchange. There are a variety of ventilation monitors such as 1) airway

flowmeters and 2) capnometers (carbon dioxide analyzers). These monitors are used routinely for patients undergoing general

anesthesia. These types of monitors work well when the patient'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, humidity, heat,

sound and respiratory rate derived from the pulse oximeter's

plethysmogram. Some of the limitations 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 gas stream 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. Some 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 CO₂ up inside the nose (BCI); providing oxygen in the nose while sampling CO₂ from the mouth (BCI); and supplying

oxygen up one nostril and sampling for CO2 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 or account for the possibility that one nostril may be completely or partially obstructed compared

to the other one. 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 CO2 sample as apnea (stoppage in breathing), resulting once

again, in frequent false positive alarms. Dilution of CO2 may also

mask hypoventilation (detected by high CO2) by making a high CO2

value appear artifactually normal and thus lull the clinician into a

false sense of security, that all is well with the patient.

Impedance plethysmography and plethysmogram respiratory

rate counting also suffer drawbacks as primary respiratory

monitors. Both devices measure the "effort" of the patient (chest expansion, shifts in blood volume). 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 affect reliability. Respiratory

rate derived from the pulse oximeter'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).

The arterial concentration of an inhalational or intravenous

drug or gas is clinically useful and may be inferred from the end-

tidal concentration of the drug or gas measured in the gases exhaled by the patient. The end-tidal concentration of a desired component

of the exhaled gas mixture can be monitored and used to infer the

arterial concentration. Examples of drugs and gases that can be

monitored include, among other things: propofol, xenon,

intravenous anesthetics and sedatives, and water vapor.

Various inspired gas compositions may be administered to

patients for different purposes. Oxygen diluted with air may be

used instead of pure O₂ to reduce the risk of an oxygen-enriched

micro-environment that may support or promote ignition of a fire,

especially for those procedures using lasers (such as laser resurfacing of the face). An oxygen-helium mixture may be used to

reduce the resistance to flow. An oxygen/air/bronchodilator mixture may be used to treat bronchoconstriction, bronchospasm or chronic

obstructive pulmonary disease (COPD). A mixture of O2 and water

vapor may be used to humidify and loosen pulmonary secretions.

In view of the above drawbacks to current systems for

delivering inspired gas and gas sampling, including 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 concentration of an inspired gas, such as oxygen, without

the requirement for a patient to wear a face mask. This is done by, among other things: a) determining the patient's breath phase,

namely whether the person is in the inhalation or exhalation phase of their respiratory cycle; and b) delivering a higher flow of inspired

gas during the inhalation part of the respiratory cycle thereby

making this higher flow of inspired gas acceptable to patients. In one aspect of the invention the inspired gas flow may be provided to

all three respiratory orifices (i.e., both nostrils and the mouth) or

directly in front of the mouth, during the inhalation cycle. Thus,

dilution of inhaled gas by room air at an inhalation portal is

reduced.

A second purpose of the invention is to more effectively sample exhaled gases, such sampling could be used, for example, to monitor patient ventilation, in combination with mask-free delivery of inspired gas to the patient. In this aspect, the invention includes

placing pressure lumens and gas-sampling lumens inside, or near,

at least one of a patient's nostrils and, in some embodiments, the mouth. The pressure lumens are connected to pressure transducers

that in turn are connected to a controller or processor running custom software algorithms for determining breath phase

(inhalation or exhalation) and rate. The pressure samples from the

respective lumens are compared with one another to determine the

primary ventilatory path. The gas sampling tubes may be

connected to gas analyzers or monitors, e.g., CO₂ analyzers, that

measure the level of a gas or drug in the exhaled gas.

Other aspects of the invention will be apparent from the

description below.

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 one embodiment of the invention.

FIG. 2 shows a perspective exterior view of the disposable

portion of the apparatus in accordance with one embodiment of 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 one embodiment of the invention.

FIG. 4 shows an embodiment of the disposable portion of the

apparatus with an oral collection chamber in accord with one

embodiment of the invention.

FIG. 5A is a schematic diagram of a gas delivery and gas sampling system in accordance with one embodiment of the

invention.

FIG. 5B is a schematic diagram of a gas delivery and gas

sampling 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 a pressure waveform during a

respiration cycle used in one method of the invention.

FIG. 8 is a flow chart of a preferred embodiment of one method of the invention.

FIG. 9 is a schematic diagram of a gas delivery and gas

sampling system in accordance with an alternative embodiment of

the invention.

FIG. 10 is a perspective diagram of an alternative embodiment of an oronasal gas diffuser and gas sampling device in accord with the invention. FIG. 11 is a side-elevation frontal view of the device shown in

FIG. 10.

FIG. 12 is a plan view of the bottom of the device shown in

FIG. 10. FIG. 13 is a side-elevation back view of the device shown in

FIG. 10.

FIG. 14 is a cross-sectional view of the tubing that connects

the device in FIG. 10 to the circuitry in FIG. 9.

FIG. 15 is a view of a connector that interfaces the machine

end of the extruded tubing of FIG. 14 to a medical device.

DESCRIPTION OF PREFERRED EMBODIMENTS

Single Capnometer Embodiment

The concept of the invention will now be described using, merely by way of example, supplemental oxygen as the inspired gas

mixture and gas sampling of carbon dioxide in the patient's

exhalations. It should be understood that the concept of the

invention is not limited to supplemental O2 administration and CO2 sampling.

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 O2 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's nose and mouth. Two thin lumens (tubes) are

mounted adjacent one another to portion 4 and placed in one of the

patient's nostrils (nasal lumens 18). Another two thin lumens are also mounted adjacent to one another to portion 4 and placed in

front of the patient's mouth (oral lumens 20).

Of nasal lumens 18, one lumen is a pressure lumen for

sampling the pressure resulting from a patient's nose breathing and

the other lumen continuously samples the respiratory gases so they

may be analyzed in a 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, e.g., after every patient use), and preferably constructed of pliable

plastic material such as extruded polyvinyl 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 gas sampling lumen 26) are

preferably formed from a double-holed, single-barrel piece. Oral

lumens 20 (which include pressure lumen 32 and gas sampling 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 the diffuser portion, whereupon the oxygen exits cover 130 through diffuser grid

136 into the immediate vicinity of the patient'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, dilution of inhaled gas at an inhalation portal by pure room air is reduced. 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 O2 jet stream and includes a foam or filler paper section for

diffusing the jet stream of O2. As is also shown in FIG. 3, feedback tubing 22 enters lower

portion 110 at openings 122. At 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 opening 122 begin grooves 144 and 142, formed in lower portion 110 each for receiving

the feedback CO2 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 perspectively 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 gas the patient has breathed orally. That volume of gas is then sampled by lumens 220 and fed back for

analysis through the respective pressure and CO2 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 gas molecules from volume 214

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's gas stream. 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 gas stream.

The present invention generally provides that in the event

that positive pressure ventilation has to be applied via face mask, it

should be possible to leave the apparatus of the invention in place on the person to minimize user actions during an emergency. Thus,

the apparatus of the invention allows a face mask to be placed over

it without creating a significant leak in the pillow seal of the face

mask. The material of the apparatus in contact with the face is preferably soft (e.g., plasticized PVC, etc.) and deformable. This

prevents nerve injury, one of the most common complications of

anesthesia, which is often caused by mechanical compression or

hyperextension that restricts or shuts off the blood supply to nerves.

FIG. 5A shows a schematic circuit diagram of a preferred

embodiment of the oxygen delivery and gas sampling system of the invention. As described above, disposable portion 304 includes

nasal lumens which sample a nasal (nares) volume 318 of gas breathed through the patient's nostril; an oral sample collector

which creates an oral volume of gas 320 effecting sampling of gas breathed through a patient's mouth; and an oxygen diffuser 336

which enriches the immediate breathing area of a patient with

oxygen, increasing the patient's fraction of inspired oxygen and

thereby increasing the patient'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

(O2 tank or in-house oxygen). If the supply of O2 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 a

Compressed Gas Association standard oxygen female nut and

nipple fitting. A source pressure transducer 342 monitors the

oxygen source pressure and allows custom software running on a

processor (not shown) 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 psig.

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). It is noted that flowrates of 2.0 and 15.0 L/min could also be accomplished by 2

less expensive on/off valves coupled with calibrated flow orifices instead of one expensive proportional flow control valve.

Downstream pressure transducer 350 monitors the functionality of

proportional valve 346. Associated software running on a processor

(not shown) 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 valve 354. The capnometer 352 receives the patient

airway gas sample and monitors the CO₂ content within the gas sample. Software associated with capnometer 352 displays

pertinent parameters (such as a continuous carbon dioxide graphic

display known as a capnogram and digital values for end-tidal CO2

and respiration rate) to the user. A suitable capnometer may be that manufactured by Nihon Kohden (Sj5i2) or Ca dioPulmonary

Technologies (CO₂WFA OEM). Three-way 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 a processor (not shown) 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 gas 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 (such as a Honeywell DCXL01DN, for example)

monitors the pressure in the nares volume 318 through the small

bore tubing described above. Associated software running on a

processor (not shown) 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's breath sounds detected at the nasal sample site.

Associated software allows the user to project sound to the room

and control audio volume. Output from nasal 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's physiological condition.

Oral pressure transducer 362 (such as a Honeywell

DCXL01DN, for example) monitors pressure at the oral collection

volume 320 through the small bore tubing described above. Associated software running on a processor (not shown) determines

via pressure transducer 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 that amplifies and projects

breath sounds to the room. Alternatively, a white noise generator

reproduces a respiratory sound proportional to the amplitude of the respiratory pressure and encoded with a sound (WAV file) of a

different character for inhalation versus exhalation so that they

may be heard and distinguished by a care giver in the room.

A dual chamber water trap 364 guards against corruption of

the CO2 sensors by removing water from the sampled gases. 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. The disposable element 304 is interfaced to the non- disposable elements via a single, multi-lumen connector 344 that

establishes five flow channels in a single action, when it is snapped

to the medical device containing the non-disposable equipment.

FIG. 5B shows an additional embodiment of the system circuit of the present invention, including a gas sample bypass circuit which keeps the gas sample at the oral and nasal 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 gas sample site between the

capnometer 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 site is connected to the bypass line while the other

sample site is connected to the capnometer. This allows two states:

1) the oral gas sample site fed back to the capnometer, with the

nasal gas sample site connected to the bypass; and 2) the nasal gas

sample site fed back to the capnometer with the oral gas sample site on bypass. As described above, the control software switches the

gas sample site based on logic that determines if the patient is

breathing through the nose or mouth. Oral diverter valve 557

switches the oral gas sample site between the capnometer 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 cc/min.).

The pump 559 also ensures that the gas sample sites are synchronized with one another so that the CO₂ waveform and

respiration rate calculations are not corrupted when gas 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 gas sample line and the bypass line (e.g., maintaining a flow

in the bypass equivalent or near equivalent to the flow within the

CO₂ sampling line, e.g., 200 cc/min) is desired. This prevents corruption of the CO2 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 compensation and zeroing circuit 406 and a low pass

filter 408. The gain and temperature zeroing circuit ensure signal quality for the pressure transducer output. Depending on the

signal to noise ratio of the pressure transducer 402, the low pass

filter 408 may be optional.

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 706 begins, the pressure becomes positive, eventually reaching a peak then dropping back to zero

(atmospheric pressure) as the exhalation completes. The beginning of inhalation 708 is indicated by the pressure becoming negative

(sub-atmospheric). 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^, first A, then B followed by C, and finally D. An O2

source valve is turned up (e.g., to 10-15 liters/min of flow) when

point A, 710, is reached and turned down (e.g., to 2-3 liters/min of flow) when C, 712, is reached, thus providing the higher 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 milliseconds (see FIG. 8,

step 820). During each examination, the software combines the

oral and nasal pressures and then compares the combined pressure

to one of the two thresholds as follows. As shown by the flowchart of FIG. 8, when the software

begins execution, it reads the nasal and oral pressures, step 802, and awaits a combined pressure value less tharJthe upper threshold

(point A), step 804. When this condition is met, the software turns

up the O2 valve, step 806, to a higher desired flow (e.g., 10-15

liters/min) then begins looking for a combined pressure value less

than the lower threshold (point B), step 808. 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 O₂ is

turned down to the lower desired flow rate (e.g., 2-3 liters/min), step

810, 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. In the case of oxygen, the invention may

thus increase end tidal oxygen concentrations from the baseline

15% (breathing room air) up to 50-55%. Whereas this may not be as

effective as face mask oxygen supplementation, it is significantly

better than the prior art for open airway oxygen supplementation

devices.

Also, instead of completely shutting off inspired gas flow during exhalation, the invention selects a baseline lower flow of

inspired gas, e.g., 2 L/min, so that the flow interferes minimally with the accuracy of exhaled gas sampling. The non-zero inspired

gas flow during exhalation enriches the ambient air around the nose and mouth that is drawn into the lungs in the subsequent

inhalation. Further, in the event that O2 is the inspired gas and that the software malfunctions such that the algorithm stays stuck

in the exhalation mode, a non-zero baseline flow of O2 will ensure

that the patient breathes partially O₂-enriched room air rather than

only room air.

As described above, a capnometer may be used to provide

information such as end-tidal CO2 and respiration rate by

continually sampling the level of CO₂ at a single site. Since breathing can occur through the nose, mouth, or both, the software

must activate valve 354 (FIG. 5A) or valves 555 and 557 (FIG. 5B), 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 determines the best

sampling site 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. It is further noted that the gas sampling lumens may be

connected together at a switching valve to minimize the number of

gas analyzers required. Via the switching valve, the gas sampling

lumen connected to the primary ventilatory path is routed to the

gas analyzer. Additionally, in some aspects of the invention, the

user sees a display from one gas analyzer. For example, for a capnometry application, the CO2 tracing that has the highest

averaged value (area under the curve over the last n seconds, e.g.,

15 seconds) is displayed. Because the present invention measures

the "effect," La., the CO2 and airway pressure variations with each

breath, it would not fail to detect a complete airway obstruction. Multiple Capnometer Embodiment

An alternative embodiment of the invention uses two

capnometers as shown in FIG. 9, 912 and 914. Pressure transducer

906 monitors the pressure at nose tap 938. Pressure transducer 908 monitors the pressure at nose tap 940. Each nose tap 938 and 940

samples the pressure in one of the patient's nares. Pressure

transducers 906 and 908 can be momentarily connected to

atmosphere for zeroing purposes via valves 904 and 902

respectively. Pressure is not monitored at the mouth. The primary nasal ventilatory path is determined from analysis of the pressure

trace at each nares. The nare whose pressure trace exhibits the larger amplitude of pressure oscillation is considered to be the

primary nasal ventilatory path.

Gas sample lumens are placed at both nares and at the

mouth. The oral gas sample lumen 932 is directly connected to the oral capnometer 914. The nasal capnometer 912 can be connected

to either of the nasal gas sampling lumens 934 or 936 via a

switching valve 910. Once the pressure transducers and the software determine the primary nasal ventilatory path, the

switching valve routes the gas sample from the primary nasal

ventilatory path to the nasal capnometer 912. Thus, exhaled gas is

sampled continuously from either the right or left nasal passage.

The software analyzes the sum of the pressures sampled from

the two nasal orifices to determine whether the patient is inhaling or exhaling. Obviously, different algorithms may be possible like

determining the breath phase from only the pressure trace at the

primary nasal ventilatory path, instead of adding the pressures

from both nares. Software running on a processor (not shown)

opens a valve 922 connected to an oxygen source so that oxygen flow

is high (e.g., 15 L/min) during the inhalation phase of the patient's

breathing. A high pressure relief valve 918 relieves pressure if the

O2 supply pressure exceeds 75 psig. A pressure transducer 920 monitors the O₂ supply pressure such that the software can adjust

the opening of the valve 922 to compensate for O2 supply pressure fluctuations. A pressure relief valve 924 downstream of the valve

922 prevents pressure buildup on the delivery side. Components

918, 920, 922 and 924 are mounted on a gas manifold 916 with

internal flow passages (not shown) to minimize the number of

pneumatic connections that have to be manually performed.

An audio stimulus generated by sub-system 926 is used to

prompt the patient to perform a specific action like pressing a

button as a means of assessing responsiveness to commands as an

indirect measure of patient consciousness. This automated responsiveness test is useful in a conscious sedation system like, for

example, that described in U.S. patent application Serial No.

09/324,759 filed June 3, 1999.

The oronasal piece 1000 in FIG. 10 is intended for use with the circuit in FIG. 9. A pressure sampling lumen 1008 and a gas sampling lumen 1006 are contained within left nostril insert 1004

that fits into the left nare of the patient. A pressure sampling

lumen 1058 and a gas sampling lumen 1056 are contained within

right nostril insert 1054 that fits into the right nare of the patient.

A multiplicity of holes 1012 diffuse O2 near the region of the nares.

A similar multiplicity of holes 1026 (FIG. 12) diffuse O₂ near the

region of the mouth, to account for the possibility of mouth

breathing. The oronasal piece 1000 is held onto the patient's face via an adjustable loop of cord or elastic band 1014 that is designed to be rapidly adjusted to the patient. A single cord or elastic band is

made to form a loop by passing both cut ends via an adjustment

bead 1018. The loop is attached in one motion to bayonet-type

notches 1020 on oronasal piece 1000 that securely hold the cord in

place on the oronasal piece while it is being wrapped around the

back of the patient's head. The adjustment bead 1018 is then slid

along the loop to adjust the tension on the cord. Once adjusted, the loop is then released over the stud 1016 such that the stud tends to

splay the two pieces of cord apart, thus locking the adjustment bead

to prevent inadvertent loosening of the adjustment bead. The gas

sample lumen 1024 (FIG. 11) is contained within protuberance 1022 which is designed to stick out into the stream of gas flowing to and from the mouth.

Referring now to FIG. 13, lumen 1038 on the oronasal piece

1000 is internally connected to the gas sample lumen 1006 (FIG. 10)

for the left nare. Lumen 1036 (FIG. 13) on the oronasal piece 1000 is internally connected to the oral gas sample lumen 1024 (FIG. 11).

Lumen 1034 (FIG. 13) on the oronasal piece 1000 is internally

connected to the pressure sampling lumen 1008 (FIG. 10) for the

left nare. Lumen 1030 (FIG. 13) on the oronasal piece 1000 is internally connected to the gas sample lumen 1056 (FIG. 10) for the right nare. Lumen 1028 (FIG. 13) on the oronasal piece 1000 is

internally connected to the multiplicity of holes 1012 and 1026 (FIGS. 10 and 12) that allow O2 to diffuse into the regions close to

the nose and mouth. Lumen 1032 (FIG. 13) on the oronasal piece

1000 is internally connected to the pressure sampling lumen 1058

(FIG. 10) for the right nare. The details of the internal flow passages in oronasal piece 1000 to accomplish the above connections

will be evident to one skilled in the art.

Referring to FIG. 14, the oronasal piece 1000 of FIG. 10 is

connected to the circuit of FIG. 9 via the extruded tear-apart tubing

of FIG. 14. The extruded tubing contains seven lumens grouped in

three clusters (1142, 1144 and 1146) that can be separated from

each other by manually tearing along the tear lines 1143 and 1145.

Lumen 1130 in cluster 1142 channels the flow of O2 to the oronasal

piece and is of larger bore to accommodate the high flow of O2 and present minimal flow resistance. Lumen 1128 in cluster 1146

carries the audio stimulus that prompts the patient to squeeze a

button as part of an automated responsiveness test (ART) system.

Lumen 1132 in the middle of cluster 1144 carries the oral gas

sample. Lumens 1138 and 1134 in cluster 1142 carry the pressure and gas samples from one nasal insert. Lumens 1140 and 1136 in

cluster 1144 carry the pressure and gas samples from the other

nasal insert. The cross-section of each cluster is shaped like an aerofoil to adapt to the indentation of the facemask pillow seal and

the cheek of the patient when a facemask is placed over the separated clusters. The lumens are arranged such that the larger bore lumens are in the middle of each cluster, taking advantage of

the aerofoil like cross-section of each cluster.

An additional feature of the invention is that the pneumatic

harness (shown in cross-section in FIG. 14) can be connected to a

standard, male, medical O2 barbed outlet connector commonly referred to as a "Christmas tree," so that the oronasal piece of the invention can also be used post-procedurally to deliver O2-enriched

air to the patient. Another feature of the invention is that the

pneumatic harness of FIG. 14 can be snapped onto a medical device

with a single action. To accomplish both design objectives, the connector of FIG. 15 is used to adapt the pneumatic harness of FIG.

14 for connection to a medical device. The pneumatic harness of

FIG. 14 is mounted onto adapter 1148 using seven male ports like

ports 1150 and 1152. Port 1152 carries the oxygen inflow and port 1150 pipes in the audio stimulus. The adapter 1148 has a tapered

inlet connected to the O2 delivery lumen 1130 (FIG. 14). The

tapered inlet is made of soft material and is designed to mate to a

standard male O2 barbed connector known as a Christmas tree.

The connector snaps into a socket on the medical device to establish

seven airtight pneumatic connections with only one action. Tapered male port 1158 on the medical device delivers oxygen into lumen

1130 via port 1152. Port 1156 brings in the pressure signal from nose pressure tap 2. Pegs 1154 allow the multi-lumen connector

1148 to be held in tightly and securely once snapped into the

medical device to prevent accidental disconnection.

The above-described systems and methods thus provide improved delivery of inspired gas and gas sampling, including CO2

sampling, without use of a face mask. The system and method may

be 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 for patient-

controlled analgesia, among others. It should be understood that

the above describes only preferred embodiments of the invention. It

should also be understood that while the preferred embodiments discuss gas sampling, such as CO2 sampling and analysis, the

concept of the invention includes sampling and analysis of other

medical gases and vapors like propofol, oxygen, xenon and intravenous anesthetics. It should further be understood that

although the preferred embodiments discussed address

supplemental O2 delivery, the concept of the invention is applicable

to delivery of pure gases or mixtures of gases such as 02/helium,

02/air, and others.

Claims

WHAT IS CLAIMED: 1. A method for supplying an inspired gas to a person, the method

comprising the steps of: a) determining whether the person is in

the exhalation or inhalation phase of a respiratory cycle; and b) delivering an increased flow of inspired gas to the person during the

inhalation phase of the respiratory cycle.
2. The method of claim 1, wherein the inspired gas includes pure

gas.
3. The method of claim 2, wherein the pure gas includes oxygen.
4. The method of claim 1, wherein the inspired gas includes a gas

mixture.
5. The method of claim 4, wherein the gas mixture includes a

mixture of oxygen and air.
6. The method of claim 4, wherein the gas mixture includes a mixture of oxygen and nitrogen.
7. The method of claim 4, wherein the gas mixture includes a

mixture of oxygen and water vapor.
8. The method of claim 4, wherein the gas mixture includes a

mixture of oxygen and bronchodilators.
9. The method of claim 4, wherein the gas mixture includes a mixture of oxygen and helium.
10. The method of claim 1, wherein the inspired gas may be released

to the ambient environment.
11. The method of claim 1 also comprising the step of determining

the primary respiratory site; and sampling the person's breath gas

stream at least in accordance with the determination of the primary respiratory site.
12. The method of claim 11 whereby the gas stream at the mouth is

continuously sampled, in addition to sampling at the determined

primary respiratory site.
13. The method of claim 11, wherein the step of sampling the breath gas stream includes the step of monitoring the ventilation of the

person at least in accordance with the determination of the person's

primary respiratory site.
14. The method of claim 13 whereby the gas stream at the mouth is continuously sampled, in addition to sampling at the determined

primary ventilatory site.
15. The method of claim 1 wherein the inspired gas is delivered to

the person in the area of the person's nose and mouth.
16. The method of claim 1, wherein the inspired gas is delivered to

the person in the area in front of the person's mouth.
17. The method of claim 1 wherein the determining of whether the

person is in the exhalation or inhalation phase is accomplished by

analyzing the pressure in the person's breath gas stream.
18. The method of claim 17 also comprising the step of monitoring the respiratory rate in accord with the pressure analysis.
19. The method of claim 17 also comprising the step of monitoring

the inspiratory/expiratory time ratio in accord with the pressure analysis.
20. The method of claim 17, wherein the pressure in the person's

breath gas stream is determined by sampling pressure at at least

one respiratory site.
21. The method of claim 17, wherein the determining of whether the

person is in the exhalation or inhalation phase is accomplished by

analyzing the humidity in the person's breath gas stream.
22. The method of claim 21 also comprising the step of monitoring

the respiratory rate in accord with the humidity analysis.
23. The method of claim 21 also comprising the step of monitoring

the inspiratory/expiratory time ratio in accord with the humidity

analysis.
24. The method of claim 17, wherein the determining of whether the

person is in the exhalation or inhalation phase is accomplished by

analyzing the temperature in the person's breath gas stream.
25. The method of claim 24 also comprising the step of monitoring

the respiratory rate in accord with the temperature analysis.
26. The method of claim 24 also comprising the step of monitoring

the inspiratory/expiratory time ratio in accord with the temperature analysis.
27. The method of claim 11, wherein the determining of the primary

respiratory site is accomplished by sampling pressure at the respiratory sites and comparing said pressures.
28. The method of claim 11, wherein the step of sampling the

exhaled gas stream includes sampling the level of CO2 in the

person's breath gas stream.
29. The method of claim 13, wherein the monitoring of the

ventilation is accomplished by measuring the CO2 levels in the

person's breath stream.
30. The method of claim 29, wherein the monitoring of the

ventilation is accomplished by measuring the end-tidal CO2 value.
31. The method of claim 29, wherein the monitoring of the ventilation is accomplished by determining the area under the

expired CO2 time pilot.
32. The method of claim 1 also comprising the step of delivering a

decreased flow of inspired gas to the patient during exhalation.
33. The method of claim 11, wherein the step of sampling the breath

gas stream includes monitoring the level of a drug in the person's

breath gas stream.
34. The method of claim 33, wherein the drug is an intravenous anesthetic.
35. The method of claim 33 wherein the drug is propofol.
36. The method of claim 11, wherein the sampled gas is xenon.
37. An apparatus that delivers inspired gas to a person comprising:

a) an inspired gas delivery device; b) at least one respiratory site

sampling device which samples the pressure at at least one

respiratory site; c) and wherein the respiratory site sampling device

is connected to a pressure analyzer which determines the phase of

the person's respiration cycle; d) and wherein the inspired gas

delivery device is connected to a controller that modulates the flow

of inspired gas in accordance with the phase of the person's

respiratory cycle.
38. The apparatus of claim 37, wherein the respiratory site sampling device comprises at least one nasal sampling device which samples

the pressure in the person's nasal airway and an oral sampling

device which samples the pressure in the person's oral airway.
39. The apparatus of claim 37, wherein the controller delivers a higher flow of inspired gas during the inhalation phase of the

person's respiratory cycle.
40. The apparatus of claim 38, wherein at least two of the nasal and

oral sampling devices are connected to a pressure comparator which

determines the person's primary respiratory site.
41. The apparatus of claim 37 also comprising a gas sampling

device.
42. The apparatus of claim 41, wherein the gas sampling device is a capnometer.
43. The apparatus of claim 41, wherein the gas sampling device

comprises a nasal gas sampling device and an oral gas sampling

device and wherein the controller selects at least the gas stream

from the primary respiratory site for monitoring.
44. The apparatus of claim 43, wherein the oral and nasal gas

sampling devices are capnometers.
45. The apparatus of claim 37 also comprising a flow control valve

and wherein the controller runs software that indicates an error to

a user if while the flow control valve is open, the controller detects

pressure at the source of inspired gas but fails to detect pressure downstream of the flow control valve.
46. The apparatus of claim 37 also comprising an auditory breath

sonification device that amplifies breath sounds.
47. The apparatus of claim 46, wherein the auditory breath

sonification device is a microphone that amplifies actual breath

sounds.
48. The apparatus of claim 46, wherein the auditory breath

sonification device comprises a white noise generator that provides

simulated breath sounds.
49. The apparatus of claim 48, wherein said simulated breath

sounds distinguish between inhalation and exhalation breath sounds.
50. The apparatus of claim 41, wherein the gas sampling device

samples CO2 gas.
51. The apparatus of claim 41, wherein the gas sampling device

samples xenon gas.
52. The apparatus of claim 41, wherein the gas sampled is a drug.
53. The apparatus of claim 52, wherein the drug is an intravenous

anesthetic.
54. The apparatus of claim 52, wherein the drug is propofol.
55. The apparatus of claim 37, wherein the inspired gas delivery

device comprises a diffuser.
56. The apparatus of claim 37, wherein the controller reduces the

flow of inspired gas during the exhalation phase.
57. A method for delivering an inspired gas, the method comprising

the steps of: a) determining the breath phase; b) delivering a higher

flow of inspired gas during the inhalation phase; and c) monitoring

gases in the breath gas stream.
58. The method of claim 57 also comprising the step of determining at least one of the breath rate and inspiratory/expiratory time ratio.
59. The method of claim 57, wherein the step of determining at least

one of the breath phase, breath rate and inspiratory/expiratory time

ratio is accomplished by analyzing the pressure waveform at at least one respiratory site.
60. The method of claim 57, wherein the step of determining at least one of the breath phase, breath rate and inspiratory/expiratory time

ratio is accomplished by monitoring the humidity at at least one

respiratory site.
61. The method of claim 57, wherein the step of determining at least one of the breath phase, breath rate and inspiratory/expiratory time

ratio is accomplished by monitoring the temperature at at least one

respiratory site.
62. The method of claim 57 also comprising the step of reducing the

flow of inspired gas during the exhalation phase.
63. The method of claim 57, wherein the monitoring of exhaled gas

is performed during a period of low gas flow in the exhalation

phase.
64. The apparatus of claim 37 also comprising a plurality of lumens which effect one or more of delivering of inspired gas, respiratory

site sampling and gas sampling and wherein said lumens are

affixed to one another along separable tear lines.
65. The apparatus of claim 64, wherein the lumen that

accommodates the flow of inspired gas is of larger circumference

than the other lumens.
66. An apparatus according to claim 64 wherein one of said lumens is a stimulus channel that carries an auditory prompt to the person.
67. A pneumatic harness for a medical device comprising a plurality

of lumens grouped in one or more clusters, said clusters being manually separable from one another.
68. The pneumatic harness of claim 67, wherein the harness also

comprises tear lines to permit separation of the lumens from one

another.
69. The pneumatic harness of claim 67, wherein at least one of the lumens is larger than the other lumens.
70. The pneumatic harness of claim 67, wherein the cross section of

each cluster is of aerofoil shape.
71. The pneumatic harness of claim 67 also comprising a connector that permits delivery of supplemental oxygen from standard

medical oxygen connectors using an oronasal piece.
72. The pneumatic harness of claim 67 also comprising an adapter

that connects the pneumatic harness to a medical device.
73. A method of determining which of the two nares is less

obstructed, said method comprising the steps of: a) sampling the

pressure in the gas stream of each nare; b) comparing the pressure

variations in the gas stream within each nare; c) comparing the

extent of variation of said pressures as between the nares; and d)

selecting the nare with the larger pressure variation as the nare that is less obstructed.
74. The method of claim 73, wherein the nare that is less obstructed

is selected to receive inspired gas.
75. The method of claim 73, wherein the nare that is less obstructed

is selected for gas sampling.
76. The method of claim 73, wherein the nare that is less obstructed

is selected for pressure sampling.
77. The method of claim 73, wherein the nare that is less obstructed

is selected for determination of respiration phase.
78. The method of claim 73, wherein the nare that is less obstructed is selected for determination of respiration rate.
79. The method of claim 73, wherein the nare that is less obstructed

is selected for determination of inhalatory/expiratory time ratio.