Note: Descriptions are shown in the official language in which they were submitted.
CA 02419575 2003-02-18
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
CA 02419575 2003-02-18
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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.
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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
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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.
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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.
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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
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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
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Page 8
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.
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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.
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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
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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.
CA 02419575 2003-02-18
Page 12
(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.
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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
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Page 14
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.
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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
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Page 16
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
CA 02419575 2003-02-18
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.
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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
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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
CA 02419575 2003-02-18
Page 20
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
CA 02419575 2003-02-18
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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.
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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.
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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.
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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
CA 02419575 2003-02-18
Page 25
(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).
CA 02419575 2003-02-18
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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.
CA 02419575 2003-02-18
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).
CA 02419575 2003-02-18
Page 28
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.
CA 02419575 2003-02-18
Page 29
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
CA 02419575 2003-02-18
Page 30
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
CA 02419575 2003-02-18
Page 31
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
CA 02419575 2003-02-18
Page 32
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
CA 02419575 2003-02-18
Page 33
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
CA 02419575 2003-02-18
Page 34
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
CA 02419575 2003-02-18
Page 35
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.
