CA2419575A1 - Breathing circuits to facilitate the measurement of non invasive cardiac output and other physiological parameters during controlled and spontaneous ventilation - Google Patents

Abstract

A method of quantifying VA utilizing any of the circuits described and illustrated herein in Figures 1 to 9 inclusive.

Description

TITLE OF THE INVENTION
BREATHING CIRCUITS TO FACILITATE THE MEASUREMENT OF NON
INVASIVE CARDIAC OUTPUT AND OTHER PHYSIOLOGICAL PARAMETERS
S DURING CONTROLLED AND SPONTANEOUS VENTILATION
BACKGROUND OF THE INVENTTON
Our previously patented partial rebreathing circuit has the following crucial design feature: when minute ventilation ( VE ) exceeds fresh gas flow, during inhalation, fresh gas will be presented (to the patient) first followed by previously exhaled gas.
However, this circuit has some limitations.
1) It can be used only with spontaneous ventilation.

2) The manifold of 3 valves must be close to the patient's airway in order to minimize the effect of equipment dead-space and retain the characteristics of sequential delivery of fresh gas followed by previously exhaled gas on each breath. Positioning the manifold close to the patient airway is problematic when the patient's head is in a confined space (such as MRI cage, or during ophthalmologic examination) or when extensive access to the head and neck is required such as during surgery.

3) The valve in the bypass limb is designed to open during inspiration after the fresh gas reservoir collapses. The resistance in this valve has to be low in order to minimize the resistance to inspiration. With vigorous exhalation, as occurs during exercise or after a sigh, the pressure in the expiratory limb may rise sufficiently to open the bypass valve and blow some expired gas into the inspiratory limb. The expired gas in the inspiratory limb displaces the same volume of fresh gas so on the next breath both fresh gas and previously exhaled gas enter the lungs together rather than in sequence.

4) The configuration of the circuit does not lend itself to the addition of a COZ
absorber on the bypass limb in order to deliver anesthetics efficiently at low fresh gas flows as this would make the manifold even more bulky and further restrict access to the head.
Previous Art In the past many attempts have been made to measure VOz during anesthesia. The methods can be classified as 1) Empirical formula based on body weight: e.g., a) The Brody equation (1) VOz = 10*BW3~4 is a 'static' equation that cannot take into account changes in metabolic state.
2) Determination of oxygen loss (or replacement) in a closed system Severinghaus (2) measured the rate of N20 and Oz absorption during anesthesia. Patients breathed spontaneously via a closed breathing circuit (gas enters the circuit but none leaves). The flow of N20 and Oz into the circuit was continuously adjusted manually such that the total circuit volume and concentrations of Oz and N20 remain unchanged over time. If this is achieved, the flow of N20 and Oz will equal the rate of N20 and Oz absorption.
Limitations: Unsuitable for clinical use.
1. Method only works with closed circuit, which is seldom used clinically.
2. Requires constant attention and adjustment of flows. This is incompatible with looking after other aspects of patient care during surgery.
3. The circuit contains a device, a spirometer, that is not generally available in the operating room.

4. Because the spirometer makes it impossible to mechanically ventilate patients, the method can be used only with spontaneously breathing patients.

5. Method too cumbersome and imprecise to incorporate assessment of flux of other gases that are absorbed at smaller rates, such as anesthetfc vapors.
In our method i) Patients are maintained with low fresh gas flows in a semi-closed circuit, the commonest method of providing anesthesia. No further manipulations by the anesthesialogist are required.
ii) Method uses information normally available in the operating room without additional equipment or monitors.
iii) The calculations can be made with any flow, or combination of flows, of Oz and N20.
iv) Patients can be ventilated or be breathing spontaneously.
v) Our method can be used to calculate low rates of absorption such as those of anesthetic vapors 3) Gas collection and measurement of 02 concentrations:
a) Breath-by-breath: measurement of Oz concentration and expiratory flows at the mouth For this method, one of the commercially available metabolic carts can be attached to the patient's airway. Flow and gas concentrations are measured breath-by-breath. The device keeps a running tally of inspired and expired gas volumes.
Limitations:
1. Metabolic carts are expensive, costing US$30,000-$50,000.
2. The methods they use to measure Oz flux are fraught with potential errors. They must synchronize both a flow and gas concentration curves. This requires the precise quantification of the time delay for the gas concentration curve and corrections for the effect of gas mixing in the sample line and time constant fox the gas sensor. The error is greatest during inspiration when there are large and rapid variations in gas concentrations. We have not found any reports of metabolic carts used to measure VOz during anesthesia with semi-closed cixcuit.
3. Metabolic carts do not measure fluxes in N20 and anesthetic vapor.
Our method measures flux of Oz, NzO, and anesthetic vapor with a semi-closed anesthesia circuit using the gas analyzer that is part of the available clinical set-up.
b) Collecting gas from the AI'L valve and analyzing it for volume and gas concentration. This will provide the volumes of gases leaving the circuit.
This can be subtracted from the volumes of these gases entering the circuit.
This requires timed gas collection in containers and analysis for volume and concentration.
Limitations i) The gas containers, volume measuring devices, and gas analyzers are not routinely available in the operating room.
ii) The measurements are labor-intensive, distracting the anesthetist's attention from the patient.
With our method, there is no need to collect gas or make any additional measurements.

4) Tracer gases Henegahan(3) describes a method where argon (whose rate of absorption by, and elimination from, the patient is negligible) is added to the inspired gas of an anesthetic circuit at a constant rate. Gas exhausted from the ventilator during anesthesia is collected and directed to a mixing chamber. A constant flow of Nz enters the mixing chamber. Gas concentrations sampled at the mouth and from the mixing chamber are analyzed by a mass spectrometer. Since the flow of inert gases is precisely known, the concentrations of the inert gases measured at the mouth and from the mixing chamber can be used to calculate total gas flow.
This, together with concentrations of Oz and NzO, can be used to calculate the fluxes of these gases.
This method uses the principles of the indicator dilution method. It requires gases, flowmeters, and sensors not routinely available in the operating room, such as argon, N2, precise flowmeters, a mass spectrometer, and a gas-mixing chamber.
With our method, we use only routinely available information such as the settings of the Oz and N20 flowmeters and the concentrations of gases in expired gas as measured by the standard operating room gas monitor.
5) VOz from variations of the Foldes (1952) method:
Foldes formula: FIOa = Oa flow-VOZ
FGflaw -VO
Where F~02 is the inspired fraction of 02; OZflow is the flow setting in ml/min; V02 is the Oz uptake as calculated from body weight and expressed in ml/min ; and FG flow is the fresh gas flow setting in ml/min.
a) Biro(4) reasoned that since modern sensors can measure fractional airway concentrations, the Folder equation can be used to solve for V02.

VOz = OZ~ow -( Fr02 * FGf low ) 1- FrOz where FGflow and Ozf iow are obtained from the settings of the flowmeters.
Drawbacks of the approach:
1. This approach requires knowing the F~Oz. F~02 varies throughout the breath and must be expressed as a flow-averaged value. This requires both flow sensors and rapid Oz sensors at the mouth; it therefore has the same drawbacks as the metabolic cart type of measurements.
2. Even if FIOz can be measured and timed volumes of 02 calculated, its use in the equation given in the article is incorrect for calculating VOz. Biro calculated ~Oz of 21 patients during elective middle ear surgery using his modification of the Foldes equation. His calculations were within an expected range of VOz as calculated from body weight but he did not compare his calculated VOz values to those obtained with a proven method. Recently Leonard et al (5) compared the VOz as measured by the Biro method with a standard Fick method in 29 patients undergoing cardiac surgery. His conclusion was the Biro method is an "unreliable measure of systemic oxygen uptake" under anesthesia. We alsa compared the VOz as calculated by the Biro equation with our data from subjects in whom ~Oz was measured independently and found a poor correlation.
Our approach:
VOz = Ozin - Ozout Ozout = TFout * FE'rOz TFout = TFin - ~Oz ~Oz = Ozin - (TFin - X02) '~ FE'rOz Solving for VOz VOz = (Ozin - TFin * FE'rOz) / 1-FETOz where VOZ is oxygen consumption TFin is total flow of gas entering the circuit TFout is total flow of gas leaving the circuit Ozout is total flow of Oz leaving the circuit Ozin is total flow of Oz entering the circuit FErOz is the fractional concentration of Oz in a gas Our equation takes the same form as that presented by Biro except that Biro's has F~Oz instead of FETOz in analogous places in the numerator and denominator of the term on the right side of the equation. This will clearly result in different values for VOz compared to our method. In addition, the difference is that FETOz is a steady number during the alveolar phase of exhalation and therefore can be measured and its value is representative of alveolar gas whereas FrOz is not a discreet number; FiOz varies during inspiration and no value at any particular time during inspiration is representative of inspired gas.
b) Viale et a1(6) calculated VOz from the formula VOz =~E* (FIOz * FENz/FINz-FEOz) Where FIOz and FEOz are inspired and expired fractional concentrations of Oz, respectively; FINz and FENz are inspired and expired N2 fractional concentrations, respectively.
The method requires equipment not generally available in the operating room-- a flow sensor at the mouth to calculate VE and a mass spectrometer to measure FENz and F~Nz. Furthermore, it is then like the breath-by-breath analyzers in that means must be provided to integrate flows and gas concentrations in order to calculate flow-weighted inspired concentrations of Oz and N2.
With our method, we do not require F~02, FEN2, FiN2 or the patient's gas flows.
c) Bengston's method (7) uses a semi-closed circle circuit with constant fixed fresh gas flow consisting of 30% 02 balance NzO. X02 is calculated as VOZ = Vfg02 - 0.45(VfgN20) - (kg : 70.1000.t-0~s )) where Vfg02 is oxygen fresh gas flow; VfgN20 is the Nz0 fresh gas flow and kg is the patient weight in kilograms. The method was validated by collecting the gas that exited the circuit and measuring the volumes and concentrations of component gases.
Limitations of the method:
i) The N20 absorption is not measured but calculated from patient's weight and duration of anesthesia.
ii) The equation is valid only for a fixed gas concentration of 30% 02, balance N2.
iii) The validation method requires collection of gas and measurement of its volume and gas composition.
Our method does not require knowledge of the patient's weight or duration of anesthesia. Our method can be performed with any ratio of Oz/N20 flow into the circuit. Our method does not require expired gas collection or measurements of gas volume.

6) Anesthetic absorption predicted from pharmacokinetic principles and characteristics of anesthetic agent.
a) The equation described by Lowe HJ. The quanitative practice of anesthesia.Williams and Wilkins. Baltimore (1981), p16 VAA = f*MAC*~,B~c* Q * t-l~z Where VAA is the uptake of the anesthetic agent, f*MAC represents the fractional concentration of the anesthetic as a fraction of the minimal alveolar concentration required to prevent movement on incision, ~,B~c is the blood-gas partition coefficient, Q is the cardiac output and t is the time.
Limitations:
i) In routine anesthesia, cardiac output (Q) is unknown.
ii) The formula is based on empirical averaged values and does not necessarily reflect the conditions in a particular patient. For example, it does not take into account the saturation of the tissues, a factor that affects VAA.
b) Lin CY. (8) proposes the equation for uptake of anesthetic agent (VAA ) VAA = VA * Fi *(1-FA/ FI) Where VAa is the uptake of the anesthetic agent; VA is the alveolar ventilation, Ft is the inspired concentration of anesthetic and FA is the alveolar concentration of anesthetic.
Limitations:
i) This formula cannot be used as VA is unknown with low flow anesthesia;
ii) FI is complex and may vary through out the breath so a volume-averaged value is required.
iii) FI is not available with standard operating room analyzers.

7) Calculations directly from invasively-measured values a. Pestana (9) and Walsh (10) placed catheters into a peripheral artery and into the pulmonary artery. They used the oxygen content of blood sampled from these catheters and the cardiac output as measured by thermodilution from the pulmonary artery to calculate V02. They compared the results to those obtained by indirect calorimetry.
Limitations i) The method uses monitors not routinely available in the operating room.
ii) The placement of catheters in the vessels has associated morbidity and cost.
Our method uses only routinely available information such as the flowmeter settings and end tidal Oz concentrations. It does not require any invasive procedures.
SUMMARY TABLE
StandardAdditionalRequires Uses Mea-curesUsesWrong Based Can additionalexpiredgas on -twt -AtiestheticManipulatnnmeasure- gas availabk"Rl~,assutttptionspredictionmeasure meats colkctkmon Crcuit clinicaltnonitar" oreyutuionfrom absorptio pooled data n of other anesthetic Bay -. y~ No ~dy formula weight needed SeveringhausNo. Yes. Yes. Circuit Yes No Uses Constantvolunx -closedadjusdnent circuitof flow Metabolic Yes. Flow Yes Yes No carts at the tthuth Ti rimed No. Yes. Volume.Yes Yes. - Y~
gas volurnes collection Tracer Vaile No. Yes. Yes Yes. Yes Yes- No gases Inserted - -nonrebreat4in -Nt asswnes RQ

g valve V];
u, separate gages Hene Yes. Yes Yes. Yes Possibl - han Foldes Biro YesYes No BengsonNn. Yes. Yes Yes -Far No.

validation -only -weight valid for fixed inspired gas ratio PhanncokineticL.owe Yes. Yes -Yes Yes Yes.
-prixiples Q

-time Lm YesYes No VA

Yea OBTECT OF THE INVENTION
It is therefore a primary object of this invention to obviate the deficiencies in the prior art by providing circuits which will allow for calculations to be made which are simple and based on equipment which is commonly available to the practitioner.
It is yet a further object of this invention to provide methods of measurement of the flux of 02, N20, and an anesthetic vapour with a semi-closed or closed anesthetic circuit using a gas analyzer that is part of the available clinical set-up.
It is yet a further object of this invention to provide methods of calculations and uses utilizing routinely available information such as flowmeter settings and end tidal Oz concentrations.
It is a further object of this invention to utilize techniques that involve non-invasive procedures.
Further and other objects of the invention will become apparent to those skilled in the art when considering the following summary of the invention and the more detailed description of the preferred embodiments illustrated herein.
Reference List (1) Brody S. Bioenergetics and Growth. New York: Reinhold, 21945.
(2) Severinghaus JW. The rate of uptake of nitrous oxide in man. J Clin Invest 1954; 33:1183-1189.
(3) I~eneghan CP, Gillbe CE, Branthwaite MA. Measurement of metabolic gas exchange during anaesthesia. A method using mass spectrometry. Br J
Anaesth 1981; 53(1):73-76.

(4) Biro P. A formula to calculate oxygen uptake during low flow anesthesia based on FI02 measurement. J Clin Monit Comput 1998;14(2):141-144.
(5) Leonard IE, Weitkamp B, Jones K, Aittomaki J, Myles PS. Measurement of systemic oxygen uptake during low-flow anaesthesia with a standard technique vs. a novel method. Anaesthesia 2002; 57(7):654-658.
(6) Viale JP, Annat GJ, Tissot SM, Hoen JP, Butin EM, Bertrand OJ et al. Mass spectrometric measurements of oxygen uptake during epidural analgesia combined with general anesthesia. Anesth Analg 1990; 70(6):589-593.
(7) Bengtson JP, Bengtsson A, Stenqvist O. Predictable nitrous oxide uptake enables simple oxygen uptake monitoring during low flow anaesthesia.
Anaesthesia 1994; 49(1):29-31.

(8) Lin CY. [Simple, practical closed-circuit anesthesia]. Masui 1997;
46(4):498-505.

(9) Pestana D, Garcia-de-Lorenzo A. Calculated versus measured oxygen consumption during aortic surgery: reliability of the Fick method. Anesth Analg 1994; 78(2):253-256.

(10) Walsh TS, Hopton P, Lee A. A comparison between the Fick method and indirect calorimetry for determining oxygen consumption in patients with fulminant hepatic failure. Crit Care Med 1998; 26(7):1200-1207.
SUMMARY OF THE INVENTION
In this patent application we:
1) Describe a significant modification of the previously patented circuit that allows it to be used with controlled ventilation.

Page 1.3 2) Describe a new partial rebreathing circuit that retains the characteristic of the previous circuit with respect to the sequential delivery of fresh gas and previously expired gas during inhalation whenever VE exceeds fresh gas flow, and has the further advantages over the previous circuit:
~ The valves and gas reservoir bags can be moved away from the interface with the patient without affecting its ability to sequentially deliver fresh gas then previously expired gas during inhalation whenever VE exceeds fresh gas flow.
~ The configuration of the valves precludes any expired gas entering the inspiratory limb of the circuit after a vigorous exhalation.
~ The circuit can be used with spontaneous ventilation or controlled ventilation.
3) Describe a new circuit that retains all of the advantages in efficiency of delivery of anesthesia of a circle circuit and has the additional advantages that ~ by changing the contents of the C02 absorber to zeolyte or other absorber or adsorber of anesthetic, the circuit can be used to accelerate the elimination of volatile anesthetics without the use of a self-inflating bag, demand regulator, or outside source of Oz-C02 mixture.
~ By removing the CO~ absorber from the circuit, the circuit reverts to a partial rebreathing circuit that presents fresh gas first and then previously exhaled gas during inhalation whenever VE exceeds fresh gas flow Importance:
With a partial rebreathing circuit that presents fresh gas first and then previously exhaled gas during inhalation whenever VE exceeds fresh gas flow:
1. For a given VE , a fresh gas flow can be found that it is equal to alveolar ventilation ( VA ), where VA is defined as VE minus anatomical deadspace ventilation. We refer to this fresh gas flow as the "critical fresh gas flow"
or FGFc.
2. When fresh gas flow is equal to FGFc, increases in VE will not affect VA or end tidal concentrations of C02 or 02.
3. When fresh gas flow is equal to FGFc, reductions in fresh gas flow will result in identical reductions in VA such that 4'A remains equal to fresh gas flow and independent of increases in VE (as per statement 2).
Having a breathing circuit with these characteristics is important because it allows us to know the VA and to alter the Va . VA has previously been difficult to measure and very difficult to control to fine tolerance even in paralyzed mechanically ventilated animals/subjects as changes in breathing frequency and tidal volume changes the VA in ways that cannot be precisely quantified.
The first control of VA independent of VE was described in our first patent "accelerated elimination of anesthetics". In that patent, the important concept was the sequential delivery of fresh gas and a "reserve gas" when VE exceeded fresh gas flow. The reserve gas had a PC02 equal to that in mixed venous blood. All component gases of reserve gas were from an external source of supplied gas (otherwise clear of any endogenous gases) and thus the approach was used to eliminate gases other than C02 -- for example, anesthetics and carbon monoxide.
We subsequently realized that the concentration of gases we wanted to conserve during hyperventilation should be equal to that, not in mixed venous blood, but in arterial blood. As such, we could use the expired gas (which has equilibrated with arterial blood) as the reserve gas. The subsequent patent corrected the equation and described a partial rebreathing circuit that provides gas with a PCOZ equal to arterial PCOZ (or end tidal PCOZ) in the reserve gas.

We also realized that the fresh gas flow should be equal to VA , not just VE .
As such, we provided a method to quantify the anatomical deadspace ventilation and thereby VA . [This provides all of the rationale for settings on the Clearmate and Hi-Ox~, Hi-Ox-SR and high altitude circuit and Efficient Oz delivery circuit.]
Being able to quantify VA is important as it is central to many other concepts in respiratory physiology. Nevertheless, it has not previously been quantified.
Many methods in respiratory physiology are designed to account for the fact that VA
is unknown. One application of VA is to multiply it times the expired concentration of a gas to obtain the elimination (or flux) of that gas. Thus C02 elimination (VCOz ) is simply Va times the end tidal PCO2. We addressed this in the last patent application.
In this application, we provide improvements to the Gedeon method for measuring cardiac output. The Gedeon method requires instituting a step change in VCOz .
The commercial automated method temporarily interposes a deadspace between the patient and the breathing circuit. This means temporary placement of a hose at the mouth for the patient to breathe through. During exhalation, the expired gas fills the anatomical deadspace and the hose. On inhalation, the previously exhaled gas in the hose enters the patient, followed by fresh gas from the breathing circuit.
This decreases the Va as long as the patient doesn't try to compensate for the deadspace by taking a bigger breath and getting more fresh gas. The commercial NIC02 (Resperonics)/Gedeon method requires patients to be intubated, paralyzed and mechanically ventilated in order to prevent them from altering their tidal volume (breath size) or breathing frequency when the deadspace is added.
With our circuits, a transient reduction in fresh gas flow is used to temporarily reduce VA and thereby VCOz . Since, during inhalation, the fresh gas is inspired first followed by previously exhaled gas, taking a larger breath results in the same volume of fresh gas with the balance of the breath made up of previously exhaled gas. As the volume of fresh gas in the lungs is unchanged, the VCOz remains unchanged. Thus using one of the previously described circuits or one of our new circuits described below, we can transiently reduce VCOZ independent of changes in the breathing pattern and thereby calculate cardiac output using the Gedeon method in spontaneously breathing subjects. Similarly, a partial rebreathing circuit can improve the accuracy of other methods of measuring cardiac output such as the Fisher method of measuring mixed venous PCOz, and the Kim-Farhi-Rahn single breath method.
What follows in the detailed description is a description of the preferred circuits the previously described circuit modified .for use with controlled ventilation and new circuits suitable to be used with spontaneous and controlled ventilation.
We also describe a circuit that can be used to deliver anesthetics with the same efficiency as the circle circuit and which can be easily modified to maintain VA constant and accelerate the elimination of anesthetics.
According to one embodiment of the invention there is provided the use of the circuits described herein and illustrated in the Figures as a means to improve the measurement of VCOa, mixed venous PCOZ and cardiac output with the aforementioned approaches. Further the use of the aforementioned circuits as an anesthetic circuit, depending on the contents of the canister on the bypass limb, can be used for delivery of anesthetic ~Tapours, rapid elimination of anesthetic vapours, or maintaining isocapnia independent of VE .
During an anesthetic where, a) the patient is breathing from a circle circuit and fresh gas flow is less than minute ventilation (the circuit is then considered to be semi-closed), b) the gas concentrations are monitored by a gas concentration analyzer sampling from the connector to the patient airway. (Such monitors are Page l7 considered the standard of care and therefore are available in most modern operating rooms), c) the anesthetic is in the maintenance phase such as the fresh gas flow and anesthetic concentration settings have been unchanged for more than 20 min., S we provide a method for the continuous measurement of oxygen consumption ( VOz ) and absorption and elimination of anesthetic agents during anesthesia In our method i) Patients are maintained with low fresh gas flows in a semi-closed circuit, the commonest method of providing anesthesia. No further manipulations by the anesthesiologist are required.
ii) Method uses information normally available in the operating room without additional equipment or monitors.
iii) The calculations can be made with any flow, or combination of flows, of 1 S Oz and N20.
iv) Patients can be ventilated or be breathing spontaneously.
v) Our method can be used to calculate low rates of absorption such as those of anesthetic vapors Our method measures flux of 02, N20, and anesthetic vapor with a semi-closed anesthesia circuit using the gas analyzer that is part of the available clinical set-up.
Our method does not require knowledge of the patient's weight or duration of anesthesia. Our method can be performed with any ratio of Oz/Nz0 flow into the circuit. Our method does not require expired gas collection or measurements of gas volume.
Our method uses only routinely available information such as the flowmeter settings and end tidal Oz concentrations. It does not require any invasive procedures.

Our approach:
~Oz = Ozin - Ozout Ozout = TFout ~' FETO2 TFout = TFin - VOz VOz = Ozin - (TFin - ~TOz) ~' FETOz Solving for VOz VOz = (Ozin - TFin * FE'rOz) / 1-FE'rOz where V02 is oxygen consumption TFin is total flow of gas entering the circuit TFout is total flow of gas leaving the circuit Ozout is total flow of Oz leaving the circuit Ozin is total flow of Oz entering the circuit FE'rOz is the fractional concentration of Oz in a gas Our equation takes the same form as that presented by Biro except that Biro's has FIOz instead of FETOz in analogous places in the numerator and denominator of the term on the right side of the equation. This will clearly result in different values for ~Oz compared to our method. In addition, the difference is that FETOz is a steady number during the alveolar phase of exhalation and therefore can be measured and its value is representative of alveolar gas whereas FIOz is not a discreet number; FIOz varies during inspiration and no value at any particular time during inspiration is representative of inspired gas.
BRIEF DESCRIPTION OF THE FIGURES

Figure 1 is an anesthesia via a circle circuit. The circuit is designed to efficiently deliver anesthetic gases to a patent. It does so by allowing the patient to rebreathe exhaled anesthetic gases but not COz.
Figure 2 is the Fisher Isocapnia Circuit. The circuit is designed to control the PCOZ
in expired gas (PETC02).
Figure 3 is similar to Figure 1 wherein the manifold remote from the patient.
Figure 4 is similar to Figure 3 used for a mechanically ventilated patient.
Figure 4B is prior art.
Figure 5 is the circuit for spontaneous ventilation.
Figure 6 is a co-axial version of new isocapnia circuit (CAIC).
Figure 7 is the new circuit and controlled ventilation.
Figure 8 is a new circuit with a co-axial extension and controlled ventilation.
Figure 9 is a circuit designed to deliver anesthetics.
DESCRIPTION OF THE INVENTION
Anesthesia via a circle circuit (see figure 1) Fresh gas consisting of oxygen, with the possible addition of air and/or nitrous oxide (N20), and possibly an anesthetic vapor such as isoflurane, desflurane or sevoflurane enters the fresh gas port (6) at a constant and known flow. The gas concentrations entering the circuit are set by the anesthesiologist. The patient inspires through the patient port (1) and draws fresh gas plus gas drawn from the gas reservoir bag (4) through the COZ absorber (5) up the inspiratory limb (8).
During exhalation, the inspiratory valve (7) closes and the fresh gas passes through the C02 absorber (5) towards the gas reservoir bag. Expired gas flows down the expiratory limb (2) displacing gas into the gas reservoir bag (4). When the reservoir bag is full, the pressure in the circuit rises, opening the APL (airway presslure relief) valve (9), and the rest of the expired gas exits the circuit through the APL
valve. Gas is sampled continuously at the patient port and is analyzed for concentrations of constituent gases. The inspiratory (2) and expiratory (8) limbs consist of tubing (T).
This circuit is designed to efficiently deliver anesthetic gases to a patient.
It does so by allowing the patient to rebreathe exhaled anesthetics gases but not C02.
Important characteristics to note:
1) The circuit is designed to be used as a partial rebreathing circuit. It has this characteristic when the fresh gas flow is less than minute ventilation ( VE ) where VE is defined as the volume of gas breathed per minute. Partial rebreathing i) results in inspired gas that is composed of mixtures of fresh gas and previously exhaled gas. Although fresh gas or previously exhaled gas may predominate during part of inhalation, the gases mix and cannot be separated.
ii) increases the efficiency of delivery of expensive anesthetic vapors as the vapor that had been exhaled on a previous breath can be re-supplied to the patient instead of being vented out of the circuit (through the APL
valve (9));
iii) requires the presence of a device that will filter out the carbon dioxide (COz) from the previously exhaled gas. An important function of breathing is to eliminate COZ from the body. If a CO2 filter, known as a COZ absorber, is present in the circuit, then all of the fresh gas inhaled is free of COz and the rate of elimination of COz is a function of VE , as is the case when breathing normally without a circuit. In that portion of previously exhaled gas that is inhaled, the COz is filtered, but its anesthetic vapor is rebreathed as stated in (ii);
2) There are only two one-way valves, (3) and (7).
3) Note that if the COz absorber (5) is bypassed then the fresh gas and previously exhaled gases mix and COz elimination becomes a function of the fresh gas flow until the fresh gas flow is increased to greater than VE , at which time COz elimination becomes a function of VE .
4) The circuit is designed such that the valves and COz absorber can be remote from the patient in order to allow for a maximal surgical field. If the surgery is on the chest or head and neck, there needs to be a minimum of tubing near the surgical field and there needs to be about 1-2 m of clearance between the anesthetic machine and the patient in order to give the nurses and surgeons a sufficiently large sterile field in which to work.
5) The length of the tubing (T) between the patient and the valve and COz absorber manifold does not affect the function of the circuit.
The Fisher Isocapnia Circuit 1 (FICx), figure 2 During exhalation, gas passes from the patient port (10), through the expiratory one-way check valve (15) down the expiratory limb (16) into the expiratory reservoir bag (18). Excess gas exits the expiratory reservoir bag (18) at the opening (19) remote from the entrance. Fresh gas (gas containing no COz) enters the circuit at a constant flow via a fresh gas port (12). As the inspiratory one-way check valve (11) is closed during exhalation, the fresh gas accumulates in the fresh gas reservoir bag (20).
During inhalation, fresh gas from the fresh gas flow and the fresh gas reservoir (20) passes through the inspiratory valve (11) and enters the patient. If the fresh gas flow is less than VE , the fresh gas reservoir bag (20) collapses and valve (17) in the bypass limb (13) opens, directing previously exhaled gas to the patient.

FICi is designed to control the PCOz in expired gas (PETC02).
Important characteristics of the circuit:
1) there are 3 valves.
2) during exhalation, it prevents mixing of exhaled gas with fresh gas 3) when minute ventilation ( VE ) exceeds fresh gas flow, both fresh gas and previously expired gas are inhaled in sequence-fresh gas first followed by mostly previously exhaled gas.
FICA compared to the anesthetic circuit (AC):
1. Control of COZ
FIC1: The precise and predictable control of the elimination of COZ arises from the ability to sequence the delivery of gases. The gas that enters the lungs last (previously exhaled gas) is distributed to the anatomical deadspace.
The anatomical deadspace can contain some fresh gas and some previously exhaled gas depending on how much VE exceeds the fresh gas flow. If the VE exceeds the fresh gas flow sufficiently so that previously exhaled gas fills the anatomical deadspace, all the fresh gas (which was inhaled before the previously exhaled gas) will have passed the anatomical deadspace and been distributed to the alveoli. If at this point the fresh gas flow stays constant and the VE increases, the flow of fresh gas to the alveoli stays constant and only the flow of previously exhaled gas will vary with the VE . As only the flow of fresh gas to the alveoli will determine the elimination of COz, varying VE
will not affect the rate of C02 elimination.
Stated another way, the alveolar ventilation , VA , that eliminates COZ is equal to the fresh gas flow whenever the fresh gas flow is less than the VE minus the ventilation distributed to the anatomical dead space.

AC: When the C02 absorber is in place, P~'rC02 is controlled by VE . If no COz absorber is in place, the PETCOz is controlled by the fresh gas flow and VE . Since the fresh gas and the previously exhaled gas mix, fresh gas is distributed to both the alveoli and the deadspace. The actual VA that eliminates C02 will be less than or equal to the fresh gas flow. Since the flow of fresh gas to the alveoli will depend on the relative amounts of fresh gas flow and VE , VA and C02 elimination cannot be precisely predicted from the fresh gas flow.
2. Mixing of gases FICi: fresh gas and previously exhaled gases are kept separate in separate limbs and separate reservoir bags during exhalation. During inhalation, fresh gas enters the lungs first; if FGF is less than VE , this is followed by previously exhaled gas.
AC: fresh gas and previously exhaled gases mix in the C02 absorber and gas reservoir bag.
3. Joining of previously expired gas and fresh gas FICi: Occurs when the inspired volume exceeds the fresh gas reservoir volume, regardless of inspired flows.
AC: Occurs when the inspired flow exceeds fresh gas flow, regardless of the inspired volume.
4. Composition of inhaled gas FICi: Inhaled gas consists of fresh gas until the fresh gas reservoir collapses; it then consists of mostly expired gas.

AC: Inhaled gas consists of the gas mixture in the inspiratory limb of the circuit (8) between the inspiratory valve (7) and the patient port (1) followed by the fresh gas accumulated in the COz absorber (5), followed by previously exhaled gas from the gas reservoir bag drawn through the COz absorber.
5. Moving the manifold remote from the patient (as is required in the operating room and for some experiments).
FICi: The fresh gas reservoir bag (20) and expiratory gas reservoir bag (18) can be moved remotely but the inspiratory valve (11), expiratory valve (15), or bypass valve (17) must be kept close to the patient port (10) in order to retain the advantages of the FICi in maintaining isocapnia. Moving the valves and bypass limb distally from the patient will result in previously expired gas mixing with fresh gas in the inspiratory limb (14) before it is delivered to the patient. The precise sequential delivery of gases will be lost.
AC: Inspiratory (7) and expiratory (3) valves are remote from the patient.
Previously expired gas and fresh gas mix. There is no sequential delivery of gas.
6. Use -to deliver anesthesia FICi: cannot be used with low fresh gas flow as the COz will build up. A COz absorber (typically 1-4 L in volume) on the bypass limb would make the manifold very bulky. Moving the bypass limb distally and adding a COz absorber would not have any advantages over the circle circuit with respect to delivery of anesthesia and would lose the advantages of the IC in maintaining isocapnia when the COz absorber is out of the circuit.
7. Controlled ventilation:
FICi: Is described for spontaneous ventilation only. We describe a modification of the circuit so that it can be used for controlled ventilation (Figure 4). The fresh gas reservoir bag (20) and expiratory gas reservoir bag (18) can be enclosed in a rigid air-tight container such that the inspiratory limb (14) enters the container via port (24) and expiratory limb (16) enters the container via port (25) such that the junctions of the outside of the limbs form an air-tight seal with the inside surface of the ports. A further port (22) is provided for attachment of the Y piece of any ventilator (23). During the inspiratory phase of the ventilator, the pressure inside the container (21) rises putting the contents of the fresh gas .reservoir bag (20) and the expiratory gas reservoir bag (28) under the same pressure. As the opening pressure of the inspiratory valve (11) is less than that of the bypass valve (17), the fresh gas reservoir (20) will be emptied preferentially. When the fresh gas reservoir (20) is empty, the pressure in the container (21) and inside the expiratory gas reservoir (18) will open the bypass valve (17) and deliver the previously expired gas to the patient. During the exhalation phase of the ventilator, the contents of the container (21) are opened to atmospheric pressure, allowing the patient to exhale into the expiratory gas reservoir (18) and the fresh gas reservoir bag to fill with fresh gas. Thus, fresh gas and previously exhaled gas are delivered sequentially during inhalation with controlled ventilation.
Previous art During controlled ventilation, the APL valve (309) is closed. In the exhalation phase, the gas from the patient port (301) passes down the expiratory tubing (302) past the one-way expiratory valve (303) and into the common gas reservoir (312). When the common gas reservoir (312) is full, additional gas is vented through the spill valve (313) that contains a one-way valve (310). Fresh gas enters the fresh gas port (306) and flows into the COz absorber (305), displacing gas that was in the COz absorber (305) into the gas reservoir (312) or out of the spill valve (313). As the common gas reservoir (312) fills, it displaces gas from the rigid container (311) out through the expiratory port (317) of the ventilator Y piece (316).

During inhalation, the mushroom valve at the expiratory port of the ventilator Y
piece (317) inflates, blocking off this port, and a volume of gas is delivered from the ventilator (315) into the rigid container (311). This displaces an equal volume of gas from the common gas reservoir (312) into the circuit. Fresh gas and previously exhaled gas from the gas reservoir bag (312) and C02 absorber (305) pass the inspiratory one-way valve (306), travel down the inspiratory limb (308) towards the patient port (301).
The primary difference between the previous art (figure 4B) and our circuit is that with our circuit both an expired gas reservoir and a fresh gas reservoir are in the rigid box. In the presence of the disclosed configuration of 3 valves, such that the opening pressure of the bypass valve is greater than the opening pressure of the inspiratory valve, there will be sequential delivery of fresh gas, then previously exhaled gas, when VE exceeds fresh gas flow. This does not occur with the previous art, even if the C02 absorber is removed from the circuit.
Description of a new circuit to deliver fresh ,gas then previously exhaled has seguentially A: Spontaneous ventilation:
The modification of FICi for controlled ventilation still has the limitation that the manifold must be kept close to the face. It is therefore the purpose of this application to further improve on the ventilated version of the FICa circuit by describing an isocapnia circuit that will maintain isocapW a by the sequential delivery of fresh and previously exhaled gas when VF exceeds fresh gas flow and will allow the placement of the manifold containing the valves and the fresh gas reservoir bag and the expiratory gas reservoir bag remote from the patient. This improvement will further reduce the bulk of tubing near the face by allowing the use co-axial tubing.

Page 2~
1. Description of the circuit for spontaneous ventilation Layout Patient (38) breathes via a Y connector (40). Valve (31) is an inspiratory valve and valve (33) is an expiratory valve. Valve (34) is a bypass limb valve that has an opening pressure greater than valve (31).
Function:
During exhalation, increased pressure in the circuit closes inspiratory valve (31) and bypass valve (35). Gas is directed into the exhalation limb (39), past one-way valve (33) into the expiratory gas reservoir bag (36). Excess gas is vented via port (41) in expiratory gas reservoir bag (36). Fresh gas enters via port (30) and fills fresh gas reservoir (37). During inhalation, inhalation valve (31) opens and fresh gas from the fresh gas reservoir (37) and fresh gas port (30) enter the inspiratory limb (32) and are delivered to the patient. If fresh gas flow is less than VE , the fresh gas reservoir (37) empties before the end of the breath, and continued respiratory effort results in a further reduction in pressure in the circuit. When the opening pressure of the bypass valve (35 ) is reached, it opens and gas from the expiratory gas reservoir (36) passes into the expiratory limb (39) and makes up the balance of the breath with previously exhaled gas.
Thus when fresh gas flow is less than VE , the subject inhales fresh gas, then previously expired gas. As described by Fisher, this will maintain normocapnia independent of minute ventilation.
The new circuit maintains all of the functional advantages of the FICi circuit with respect to maintaining isocapnia independent of VE during spontaneous ventilation.
The circuit has additional advantages not attainable with the FICi such as 1) The circuit can retain all the advantages of sequential gas delivery while moving the gas reservoir bags and valves remote from the patient by extending the inspiratory and expiratory limbs or by the use of co-axial tubing, yet still be used with controlled ventilation (see below).

2) The lengths of the inspiratory and expiratory limb can be as long as desired without affecting the ability of the circuit to maintain isocapnia. (Note that connecting coaxial tubing to the patient port of FICi disrupts its ability to provide fresh gas and previously rebreathed gas sequentially when fresh gas flow is less than VE and would have the same effect as moving the bypass limb distally (discussed above)).
2. Description of co-axial version of new isocapnia circuit (LAIC) (fig 6) Layout Patient port (50) opens directly to the inspiratory limb (59) and expiratory limb (51) without a Y connector. Valve (58) is an inspiratory valve and valve (52) is an expiratory valve. Valve (60) is a bypass limb valve that has an opening pressure greater than valve (58).
Function:
During exhalation, increased pressure in the circuit closes inspiratory valve (58) and bypass valve (60). Gas is directed into the exhalation limb (51), past one-way valve (52) into the expiratory gas reservoir bag (56). Excess gas is vented via port (55) in expiratory gas reservoir bag (56). Fresh gas enters via port (57) and fills fresh gas reservoir (56). During inhalation, inhalation valve (58) opens and fresh gas from the fresh gas reservoir (56) and fresh gas port (57) enter the inspiratory limb (59) and are delivered to the patient. If fresh gas flow is less than VE , the fresh gas reservoir (56) empties before the end of the breath, and continued respiratory effort results in a further reduction in pressure in the circuit. When the opening pressure of the bypass valve (60) is reached, it opens and gas from the expiratory gas reservoir (54) passes into the expiratory limb (51) and makes up the balance of the breath with previously exhaled gas.

Thus when fresh gas flow is less than VE , the subject inhales fresh gas, then previously expired gas. As described by Fisher, this will maintain normocapnia independent of minute ventilation.
S B: The new circuit and controlled ventilation (See figure 7) Our new circuit was described for spontaneous ventilation only. We describe a modification of the new circuit that will allow it to be used for controlled ventilation (Figure 7). The fresh gas reservoir bag (88) and expiratory gas reservoir bag (77) can be enclosed in a rigid air-tight container such that the inspiratory limb (84) enters the container via port (86) and expiratory limb (81) enters the container via port (74) such that the junctions of the outside of the limbs form an air-tight seal with the inside surface of the ports. A further port (89) is provided for attachment of the Y
piece of any ventilator (73). During the inspiratory phase of the ventilator, the pressure inside the container (87) rises, putting the contents of the fresh gas reservoir bag (88) and the expiratory gas reservoir bag (77) under the same pressure. As the opening pressure of the inspiratory valve (85) is less than that of the bypass valve (78), the fresh gas reservoir (88) will be emptied preferentially. When the fresh gas reservoir (88) is empty, the pressure in the container (87) and inside the expiratory gas reservoir (77) will open the bypass valve (78) and deliver the previously expired gas to the patient. During the exhalation phase of the ventilator, the contents of the container (87) are opened to atmospheric pressure, allowing the patient to exhale into the expiratory gas reservoir (77) and the fresh gas reservoir bag to fill with fresh gas. Thus fresh gas and previously exhaled gas are delivered sequentially during inhalation with controlled ventilation.
Previous art: The "bag in the box" method of controlled ventilation is well known:
The primary difference between the previous art (figure 4B) and our circuit is that with our circuit both an expired gas reservoir and a fresh gas reservoir are in the rigid box. In the presence of the disclosed configuration of 3 valves, such that the opening pressure of the bypass valve is greater than the opening pressure of the inspiratory valve, there will be sequential delivery of fresh gas then previously exhaled gas when VE exceeds fresh gas flow. This does not occur with the previous S art, even if the COZ absorber is removed from the circuit. In addition, our circuit differs from the FICi modification for controlled ventilation in that our circuit will maintain isocapnia by the sequential delivery of fresh and previously exhaled gas and will allow the placement of the manifold containing the valves and the fresh gas reservoir bag and the expiratory gas reservoir bag remote from the patient and be connected to the patient via a co-axial tubing.
The new circuit with a co-axial extension and controlled ventilation (See figure 8) Our new circuit with co-axial extension was described for spontaneous ventilation only. We describe a modification of the new circuit with co-axial extension that will allow it to be used for controlled ventilation (Figure 8). The fresh gas reservoir bag (106) and expiratory gas reservoir bag (110) can be enclosed in a rigid air-tight container (105) such that the inspiratory limb (101) enters the container via port (103) and expiratory Limb (114) enters the container via port (110) such that the junctions of the outside of the limbs form an air-tight seal with the inside surface of the ports.
(An alternate configuration is to have the full coaxial circuit entering the container via a single port where the division of the inspiratory and expiratory limbs occurs inside the container. Valves (102), (113) and (115) would also be held inside the container.) A further port (107) is provided for attachment of the Y piece of any ventilator (108). During the inspiratory phase of the ventilator, the pressure inside the container (105) rises, putting the contents of the fresh gas reservoir bag (106) and the expiratory gas reservoir bag (110) under the same pressure. As the opening pressure of the inspiratory valve (102) is less than that of the bypass valve (115), the fresh gas reservoir (106) will be emptied preferentially. When the fresh gas reservoir (106) is empty, the pressure in the container (105) and inside the expiratory gas reservoir (110) will open the bypass valve (115) and deliver the previously expired gas to the patient. During the exhalation phase of the ventilator, the contents of the container (105) are opened to atmospheric pressure, allowing the patient to exhale into the expiratory gas reservoir (110) and the fresh gas reservoir bag to fill with fresh gas. Thus, fresh gas and previously exhaled gas are delivered sequentially during inhalation with controlled ventilation.
The additional advantage of this circuit over the previously described is that only one tube need be at the patient interface.
A circuit designed to deliver anesthetics It is the further purpose of this patent to describe an improved anesthetic circuit that can be used for the efficient delivery of anesthetics with Iow fresh gas flow or closed circuit with spontaneous ventilation and mechanical ventilation, and can also be used for:
1. precise control of COZ elimination independent of VE
2. accelerated elimination of anesthesia while maintaining normocapnia The circuit consists of the following components:
200 Patient port 201 3 port connector 202 expiratory limb 203 expiratory valve 204 cannister on bypass conduit that may be switched to be empty, contain C02 absorbing crystals or zeolyte, charcoal or similar substance that filters anesthetic agents 205 bypass conduit.
206 one-way valve with opening pressure slightly greater than that of the inspiratory valve (219) 207 expiratory gas reservoir bag 208 port in rigid container for entrance of expiratory limb of circuit in an air-tight manner 209 exit port for expired gas from expired gas reservoir 210 a 2-way manual valve that can be turned so that the gas in the box (216) is continuous with either the ventilator Y piece (211) or the ventilation bag (212) and APL valve (213) assembly 211 the ventilator Y piece 212 the ventilation bag 213 APL valve 214 ventilation port of rigid box (216) 215 fresh gas reservoir 216 rigid box 217 port in rigid container for entrance of inspiratory limb of circuit (220) in an air-tight manner 218 fresh gasinlet 219 inspiratory valve 220 inspiratory limb 221 bypass limb proximal to canister (204) Descriptions Function of the circuit as an anesthetic circuit:
During exhalation, increased pressure in the circuit closes inspiratory valve (219) and bypass valve (206). Exhaled gas is directed into the exhalation limb (202), past one-way valve (203) into the expiratory reservoir bag (207). Fresh gas enters via port (2I8) and fills the fresh gas reservoir (215). During inhalation, inhalation valve (219) opens and fresh gas from the fresh gas reservoir (215) and fresh gas port (218) enter the inspiratory limb (220) and are delivered to patient. If fresh gas flow is less than VE , the fresh gas reservoir (215) empties before the end of the breath;
continued respiratory effort results in a further reduction in pressure in the circuit.
When the opening pressure of the bypass valve (206) is reached, it opens and gas from the expiratory gas reservoir (207) passes through a COz absorber (204) into the rebreathing limb (221) and makes up the balance of the breath with partially rebreathed gas.
The rebreathed gas passes through the COz absorber (204) but still contains expired Oz and anesthetic, which can both be safely rebreathed by the patient. In this respect, the circuit in figure 9 functions like a circle anesthetic circuit in which the fresh gas flow containing Oz and anesthetic can be reduced to match the consumption or absorption by the patient.
Function of the circuit as an isocapnic hyperpnea circuit to eliminate anesthetics or other volatile toxins:
During exhalation, increased pressure in the circuit closes inspiratory valve (219) and bypass valve (206). Exhaled gas is directed into the exhalation limb (202), past one-way valve (203) into the expiratory reservoir bag (20~). Fresh gas enters via port (218) and fills the fresh gas reservoir (215). During inhalation, inhalation valve (219) opens and fresh gas from the fresh gas reservoir (215) and fresh gas port (218) enter the inspiratory limb (220) and are delivered to patient. If fresh gas flow is less than VE, the fresh gas reservoir (215) empties before the end of the breath;
continued respiratory effort results in a further reduction in pressure in the circuit.
When the opening pressure of the bypass valve {206) is reached, it opens and gas from the expiratory gas reservoir (207) passes through a gas filter (204) into the rebreathing limb (221) and makes up the balance of the breath with partially rebreathed gas.
The rebreathed gas passes through gas filter (204), which can be used to remove gases such as anesthetics or volatile hydrocarbons (depending on the choice of filter), but still contains expired Oz and COz, which can be used to maintain isocapnia independent of VE if the fresh gas flow is set to ~'~'. In this respect, the circuit in figure 9 functions like a non-rebreathing circuit described by Fisher, where rebreathed gas is cleared of an agent, rather than being delivered from a pressurized 4P~ ~z~ r 9i ee source. '~~~, x . F ,.
Advantages of circuit over previous art:
1) It is comparable to the circle anesthesia circuit with respect to efficiency of delivery of anesthesia, and ability to conduct anesthesia with spontaneous ventilation as well as controlled ventilation.
2) It is often important to measure tidal volume and VE during anesthesia.
With a circle circuit, a pneumotach with attached tubing and cables must be placed at the patient interface, increasing the dead-space, bulk and clutter at the head of the patient. With our circuit, the pneumotachograph (or a spirometer if the patient is breathing spontaneously) can be placed at port (214) and thus remote from the patient.
3) Fisher (Accelerated elimination of anesthetic) taught a circuit that can be used to accelerate the elimination of anesthesia. However that required additional devices such as an external source of gas (reserve gas), a demand regulator, self-inflating bag or other manual ventilating device 3-way stopcock and additional tubing. Furthermore, he did not disclose a method whereby mechanical ventilation can be used. In fact it appears that it cannot be used-patients must be ventilated by hand for that method.
With this circuit, the canister (204) is made to contain an anesthetic gas absorbent such as zeolyte.
a) No other equipment is necessary: specifically, there is no requirement for an external source of gas or demand regulator;
b) the patient can be ventilated with the ventilation bag (212) already on the circuit or the circuit ventilator, or any ventilator; no other tubing or devices are required.
4) Circle circuit cannot deliver fresh gas and then previously exhaled gas sequentially. The ability to do so allows the fresh gas flow to precisely control the '~. Such fine control may be required to make physiological measurements during anesthesia such as cardiac output (see next patent).
With our circuit, if the canister (204) is bypassed, the circuit becomes the equivalent of the one described in fig 7 and fig 8 As many changes can be made to the various embodiments of the invention without departing from the scope thereof; it is intended that all matter contained herein be interpreted as illustrative of the invention but not in a limiting sense.

Claims (6)

1. The various circuits described and illustrated herein in relation to figures 1 to 9 inclusive.

2. A method of quantifying ~A utilizing any of the circuits described and illustrated herein in Figures 1 to 9 inclusive.

3. The method of claim 2 wherein ~A is multiplied by the expired concentration of a gas to determine the flux of that gas.

4. The method of claim 2 wherein ~CO2 = ~A × P ET CO2.

5. The use of the circuits of claim 1 as a means to improve the measurement of ~CO2, PCO2 and cardiac output (Q).

6. A method of calculating oxygen consumption for the various circuits described herein comprising the following relationships;
~O2 = O2in - O2out O2out = TFout * F ET O2 TFout = TFin - ~O2 ~O2 = O2in - (TFin - ~O2) * F ET O2 Solving for ~O2 ~O2 = (O2in - TFin * F ET O2)/1-F ET O2 where ~O2 is oxygen consumption TFin is total flow of gas entering the circuit TFout is total flow of gas leaving the circuit O2out is total flow of O2 leaving the circuit O2in is total flow of O2 entering the circuit FETO2 is the fractional concentration of O2 in a gas wherein the oxygen consumption ~O2 is easily determined.