U.S. patent application number 14/925725 was filed with the patent office on 2016-06-16 for method of measuring cardiac related parameters non-invasively via the lung during spontaneous and controlled ventilation.
This patent application is currently assigned to THORNHILL SCIENTIFIC INC.. The applicant listed for this patent is THORNHILL SCIENTIFIC INC.. Invention is credited to Takafumi Azami, Joseph Fisher, Steve Iscoe, David Preiss, Eitan Prisman, Ron Somogyi, Alex Vesely.
Application Number | 20160166154 14/925725 |
Document ID | / |
Family ID | 32873343 |
Filed Date | 2016-06-16 |
United States Patent
Application |
20160166154 |
Kind Code |
A1 |
Fisher; Joseph ; et
al. |
June 16, 2016 |
METHOD OF MEASURING CARDIAC RELATED PARAMETERS NON-INVASIVELY VIA
THE LUNG DURING SPONTANEOUS AND CONTROLLED VENTILATION
Abstract
An apparatus to measure pulmonary blood flow and cardiac output
(Q) comprising: a) a breathing circuit which, at exhalation, keeps
exhaled gas separate from inhaled gas and at inhalation, when
V.sub.E is greater than first gas set (FGS) flow, results in a
subject inhaling FGS first and then a second gas set (SGS), for the
balance of inhalation; b) a gas sensor for monitoring gas
concentrations at the patient-circuit interface; c) a gas flow
control means for controlling the rate of FGS flow into the
breathing circuit; d) machine intelligence consisting of a computer
or logic circuit capable of controlling the gas flow control means
which receives the output of the gas sensor means and outputs
pulmonary blood flow.
Inventors: |
Fisher; Joseph; (Thornhill,
CA) ; Preiss; David; (Thornhill, CA) ; Azami;
Takafumi; (Nagoya, JP) ; Vesely; Alex;
(Victoria, CA) ; Prisman; Eitan; (Toronto, CA)
; Somogyi; Ron; (Toronto, CA) ; Iscoe; Steve;
(Kingston, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
THORNHILL SCIENTIFIC INC. |
Toronto |
|
CA |
|
|
Assignee: |
THORNHILL SCIENTIFIC INC.
Toronto
CA
|
Family ID: |
32873343 |
Appl. No.: |
14/925725 |
Filed: |
October 28, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13914292 |
Jun 10, 2013 |
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14925725 |
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10545562 |
May 30, 2006 |
8460202 |
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PCT/CA2004/000234 |
Feb 18, 2004 |
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13914292 |
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10509068 |
Mar 17, 2005 |
7913690 |
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10545562 |
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Current U.S.
Class: |
600/532 |
Current CPC
Class: |
A61M 16/205 20140204;
A61M 16/085 20140204; A61B 5/7278 20130101; A61M 16/0833 20140204;
A61M 16/202 20140204; A61M 16/22 20130101; A61B 5/083 20130101;
A61M 16/08 20130101; A61M 16/208 20130101; A61M 16/0858 20140204;
A61B 5/0205 20130101; A61B 5/7225 20130101; A61M 16/206 20140204;
A61B 5/14551 20130101 |
International
Class: |
A61B 5/0205 20060101
A61B005/0205; A61B 5/00 20060101 A61B005/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 19, 2003 |
CA |
2419622 |
Claims
1-61. (canceled)
62. A method of identifying alveolar ventilation ({dot over
(V)}.sub.A) in a subject, the method comprising: (1) using a
breathing circuit configured to: i. on exhalation by the subject,
keep exhaled gas substantially separate from inhaled gas, and ii.
on inhalation by the subject, when minute ventilation ({dot over
(V)}.sub.E) of the subject is less than a first gas set (FGS) flow,
first provide FGS flow to the subject, and then provide a balance
of the minute ventilation that is substantially a second gas set
(SGS); (2) setting the FGS flow into the breathing circuit at a
rate greater than the subject's minute ventilation ({dot over
(V)}.sub.E); (3) measuring an end tidal CO.sub.2 concentration
(P.sub.ETCO.sub.2) in a steady state; (4) progressively lowering
the FGS flow into the circuit, either breath by breath or
continuously, until after a time equal to a recirculation time of
CO.sub.2 within the subject after a rise in P.sub.ETCO.sub.2 values
above a threshold value is observed; (5) deriving {dot over
(V)}.sub.A as the rate of FGS flow at a point of the intersection
between two lines comprising: (a) an average P.sub.ETCO.sub.2 in
steady state; and (b) a line fit to the P.sub.ETCO.sub.2 values
after the rise in P.sub.ETCO.sub.2 values begins until the
recirculation time.
Description
FIELD OF THE INVENTION
[0001] This invention discloses a method that calculates
non-invasively, via the lung, the total cardiac output, pulmonary
blood flow, shunt flow, anatomical and alveolar deadspace, true
mixed venous O.sub.2 saturation, true mixed venous PCO.sub.2, and
PaCO.sub.2. Furthermore the method can be performed in ventilated
subjects, subjects breathing spontaneously, even in the presence of
variations in their tidal volume and breathing frequency. Subjects
need not perform any respiratory manoeuvre such as hyperventilation
or breath holding to perform the test.
BACKGROUND OF THE INVENTION
[0002] 1. Importance of Cardiac Output
[0003] A physician's ability to determine a patient's cardiac
output ({dot over (Q)}, the volume of blood pumped by the heart
each minute) is important in the assessment of critically ill
patients. There are various devices and methods that provide a
direct or indirect measure of {dot over (Q)} (see table 1). The
most Common method used in clinical practice is thermo-dilution,
established by Ganz et al (1). Commercially manufactured catheters
(referred to as Swan-Ganz catheters, named after the inventors)
contain multiple lumina, an embedded thermister, and a balloon at
the tip. The method requires the insertion of the catheter through
the skin to access a large central vein such as the internal
jugular, subclavian, cephalic or femoral. When the balloon at the
end of the catheter is inflated, the catheter tip is carried along
with the flow of blood to the right ventricle of the heart and then
into the pulmonary artery. The part of the catheter that remains
outside the body has connections that can be attached to electrical
sensors that determine the pressure and temperature in the
pulmonary artery where the tip of the catheter is positioned.
Calculation of {dot over (Q)} requires the injection of a fixed
volume of cool liquid of known temperature into a lumen of the
catheter that has its opening part way along its length (usually in
a part of the catheter in the right atrium). The thermister at the
tip of the catheter will register changes in temperature as the
cool liquid, carried by the blood, passes. The extent of dilution
of the cold bolus of liquid by warm blood will determine the
temporal profile of the temperature change at the tip of the
catheter. This is referred to as the thermodilution method of
measuring cardiac output (TD{dot over (Q)}).
[0004] The popularity of TD{dot over (Q)} stems from ease of use
once the catheter is in place. However, the placing and maintenance
of the catheter entails considerable risk and expense. Insertion of
the Swan-Ganz catheter is associated with complications that are
frequently fatal such as puncture of the carotid or subclavian
artery with associated internal haemorrhage or stroke, tension
pneumothorax, rupture of the right ventricle, malignant arrhythmias
(including fatal ventricular fibrillation), and rupture of the
pulmonary artery. As a foreign body violating the skin barrier, a
pulmonary artery catheter is a constant threat as a source of
blood-born infection that is the greatest risk to heart valves,
artificial joints, and other implants. Such infections are medical
disasters leading to severe morbidity and death. Furthermore, the
use of pulmonary artery catheters to measure TD{dot over (Q)} is
very expensive as it requires admission to an intensive care
facility where there is continuous presence of critical care
nursing and medical staff. Despite these risks, it is still not the
ideal method to measure {dot over (Q)} as it tends to overestimate
{dot over (Q)} by as much as 10% compared to the Fick method (see
below) and, for greatest accuracy, requires repeated measurements
as its precision is poor. The variability of repeated single
measurements is about 22% and can be reduced to 10% by repeated
averages of 3 measurements (2). A single thermodilution measurement
is considered to be plus or minus 33% the true value. (3)
[0005] Because of the expense and risks of keeping the catheters in
place, they are removed as soon as practical, often within 24-48
hours of major heart surgery. Often they are removed while the
information they provide can still be clinically useful and well
before the patient is no longer at significant risk for relapse. If
the patient's health deteriorates, a decision must be made about
re-inserting the catheter.
[0006] An automated non-invasive method of {dot over (Q)}
monitoring would be very useful in the following clinical
scenarios:
[0007] a) Selected low risk patients now routinely undergoing
pulmonary artery catheterization for intra- and postoperative
monitoring. [0008] b) Patients whose {dot over (Q)} would be
clinically important to know but in whom the risks and costs of
insertion of a pulmonary catheter cannot be justified; this
includes ward patients, outpatients or patients in the emergency
department or doctor's office. [0009] c) Patients who are too sick
to warrant the added risk of pulmonary artery catheter insertion
[0010] d) High and moderate cardiac risk patients undergoing minor
and moderate non-cardiac surgical procedures [0011] e) Severely ill
patients with non-cardiac disease. [0012] f) Relatively healthy
patients undergoing major stressful surgery. [0013] g) Situations
in which {dot over (Q)} is clinically indicated but there is no
access to the expertise and critical care facilities required for
the use of the pulmonary artery catheters. [0014] h) Means of
monitoring response to cardiovascular therapy such as for
hypertension and heart failure. [0015] i) As a non-invasive
diagnostic test of cardio-pulmonary status. [0016] j) As a means of
assessing cardiovascular fitness.
[0017] Despite these many applications, non-invasive methods of
{dot over (Q)} measurements have not obtained widespread clinical
acceptance. The most commonly researched methods include ECG
bio-impedance (Imhoff, 2000 (4)), and pulsed-wave Doppler
esophageal sonography. These methods have good repeatability (5-12)
and good limits of agreement with either thermodilution or
Fick-based methods but only in some populations of subjects. Each
method fails in certain patients groups with such pathologies as
very high or low {dot over (Q)} states as occur in surgical
patients, septic shock, exercise or cardiogenic shock.
[0018] 2. Background Physiology and Definition of Terms
[0019] Venous blood returns to the right side of the heart from the
muscles and organs with reduced oxygen (O.sub.2) and increased
carbon dioxide (CO.sub.2) levels. Blood from various parts of the
body is mixed in the right side of the heart and pumped to the
lungs via the pulmonary artery. The blood in the pulmonary artery
is known as the mixed venous blood. In the lungs the blood vessels
break up into a network of small vessels that surround tiny lung
sacs known as alveoli. This network of vessels surrounding the
alveoli provides a large surface area for the exchange of gases by
diffusion along their partial pressure gradients. After a breath of
air is inhaled into the lungs, it dilutes the CO.sub.2 left in the
alveoli at the end of the previous expiration, thereby establishing
a pressure gradient between the partial pressure of CO.sub.2
(PCO.sub.2) in the mixed venous blood (P.sub.vCO.sub.2) arriving at
the alveoli and the alveolar PCO.sub.2 (PACO.sub.2). The CO.sub.2
diffuses into the alveoli from the mixed venous blood diminishing
the PCO.sub.2 in the blood, and increasing the PCO.sub.2 in the
alveoli until equilibrium is established between the PCO.sub.2 in
alveolar capillary blood and the PCO.sub.2 in the alveoli. The
blood then returns to the left side of the heart via the pulmonary
vein and is pumped into the arterial system by the left ventricle.
The PCO.sub.2 in the arterial blood (PACO.sub.2) is now the same as
that in the alveoli. When the subject exhales, the gas at the very
end of exhalation is considered to have come from the alveoli and
thus simultaneously reflects the PCO.sub.2 in the pulmonary
capillaries and the alveoli; the PCO.sub.2 in this gas is called
the end-tidal PCO.sub.2 (PETCO.sub.2).
[0020] The volume of gas breathed per minute, or minute ventilation
({dot over (V)}E), is measured at the airway opening (nose
and/mouth) and is expressed in L/min. The volume of breathed gas
distributed to the alveoli (and thus contributing to gas exchange)
is termed the alveolar ventilation ({dot over (V)}A) and is also
expressed in L/min. The part of {dot over (V)}E that does not
contribute to gas exchange is termed dead space ventilation. This
is divided into the anatomical dead space that consists of the
trachea and other gas-conducting tubes leading from the nose and
mouth to the alveoli, and the alveolar dead space that is
collectively the alveoli that are ventilated but not perfused with
blood.
[0021] The {dot over (V)}E during normal breathing provides the
{dot over (V)}A that is required to eliminate the CO.sub.2 brought
to the lungs. {dot over (V)}E is controlled by a feedback system to
keep PaCO.sub.2 at a set level of approximately 40 mmHg. Under
steady state conditions, the rate at which CO.sub.2 is exhaled from
the lungs ({dot over (V)}CO.sub.2) is equal to the rate that it is
brought to the lungs, which in turn is equal to the metabolic
CO.sub.2 production. We define steady state as the condition in
which the flux of CO.sub.2 at the lungs is equal to the CO.sub.2
production and the ({dot over (V)} CO.sub.2, PvCO.sub.2, and
PaCO.sub.2 remain steady. If the {dot over (V)} CO.sub.2 is
diminished, the CO.sub.2 extraction from the mixed venous blood
passing by the alveoli will be reduced resulting in an increase in
the PaCO.sub.2 when that blood reaches the arterial system. As the
blood traverses the body, it will pick up additional CO.sub.2 and
will return to the pulmonary artery with a higher PCO.sub.2 than on
its previous passage. The time between the change in {dot over
(V)}CO.sub.2 and reappearance of the blood with raised PCO.sub.2 in
the mixed venous circulation is termed the recirculation time which
is generally taken as 20-30 s in resting subjects.
[0022] 3. The Fick Equation
[0023] The approach for respiratory-based methods for measuring
{dot over (Q)} non-invasively is described by the Fick equation, a
mass balance of any substance across the lungs. The Fick method was
originally described for O.sub.2 as a method for determining
pulmonary blood flow. The Fick relation states that the O.sub.2
uptake by the lung is equal to the difference between the pulmonary
artery and systemic arterial O.sub.2 contents times the {dot over
(Q)}. The blood contents originally had to be obtained invasively
from blood samples. The same relation holds with respect to
CO.sub.2. The advantage of using CO.sub.2 as the tracer is that
mixed venous and arterial blood contents of CO.sub.2 may be
determined non-invasively. The Fick mass balance equation for
CO.sub.2 is:
Q . = V . CO 2 C v _ CO 2 - Ca CO 2 ##EQU00001##
where {dot over (Q)} is the cardiac output, {dot over (V)}CO.sub.2
is the rate of elimination of CO.sub.2 at the lungs, CvCO.sub.2 and
CaCO.sub.2 are the mixed venous and systemic arterial contents of
CO.sub.2, respectively. {dot over (V)} CO.sub.2 can be measured by
a timed collection of expired gas and measuring its volume and
CO.sub.2 concentration. The term CaCO.sub.2 can be calculated using
an estimate of arterial PCO2 (PaCO.sub.2) as derived from the
PCO.sub.2 of end tidal gas (PETCO.sub.2). The hemoglobin
concentration (easily obtained from a venous blood sample or a drop
of blood from a finger prick) and the relation between blood
PCO.sub.2 and CO.sub.2 content (available from standard physiology
texts) are then used to calculate CaCO.sub.2.
[0024] However, CvCO.sub.2 is difficult to estimate. The PCO.sub.2
of mixed venous blood (PvCO.sub.2) is difficult to determine as
true mixed venous blood is present only in the pulmonary artery,
which is inaccessible from the surface. The air in the lungs is in
intimate contact with mixed venous blood, but CO.sub.2 diffuses
rapidly from the mixed venous blood into the alveoli before an
equilibrium is established. The PCO.sub.2 of the expired gas
therefore reflects this equilibrium PCO.sub.2 and not the PCO.sub.2
of mixed venous blood. The PvCO.sub.2 can be determined from
expired gas only when there has been full equilibration with
continuously replenished mixed venous blood or partial
equilibration under controlled conditions that allow for back
calculation of PvCO.sub.2 from the PCO.sub.2 in expired gas. Hence
during rebreathing, the alveolar gas is not refreshed and the mixed
venous blood continuously passes the alveoli such that an
equilibrium is established whereby the PETCO.sub.2 reflects the
PCO.sub.2 in mixed venous blood.
[0025] However, even in this scenario, the PCO.sub.2 is not that
which exists in the pulmonary artery. Blood in the pulmonary artery
has a relatively low PO.sub.2. Because of the Haldane effect, the
low PO.sub.2 allows the CO.sub.2 to be carried by the hemoglobin at
a relatively low PCO.sub.2. When the mixed venous blood is exposed
to gas in the alveoli, O.sub.2 diffuses into the blood, binds to
the hemoglobin and increases the PCO.sub.2 needed for a given
CO.sub.2 content on the hemoglobin (the complimentary aspect of the
Haldane effect). All methods based on full or partial equilibration
of alveolar gas with PvCO.sub.2 take into account that the
equilibration is to a virtual PCO.sub.2 that would exist if the
CO.sub.2 content of the hemoglobin were the same as in mixed venous
blood but the hemoglobin were saturated with O.sub.2. We refer to
this as the oxygenated mixed venous PCO.sub.2 (PvCO2-oxy). Because
the relationship between PCO.sub.2 and content of CO.sub.2 in blood
is known, CvCO.sub.2 can be calculated from both the true
PvCO.sub.2 (as obtained, for example, from a pulmonary arterial
blood sample) and PvCO.sub.2-oxy (as obtained by some of the
non-invasive methods described below).sup.1. .sup.1 The
PvCO.sub.2-oxy does not really exist but is a virtual number
created by instantaneously oxygenating mixed venous blood before
and diffusion of CO.sub.2 into the alveoli. The CvCO.sub.2 is the
same in each.
[0026] 4. Rebreathing-Equilibration Method
[0027] One method of measuring PvCO.sub.2-oxy was introduced by
Collier in 1956, and is known as the equilibration method. A bag is
pre-filled with a high concentration of CO.sub.2 (.about.10-13%)
and the subject exhales and inhales rapidly to and from the bag and
PCO.sub.2 is monitored continuously at the mouth. The object of the
test is to find the combination of bag volume and bag concentration
of CO.sub.2 such that once the gas in the bag mixes with that in
the lungs (the concentration of CO.sub.2 in the residual gas in the
lung at the end of a breath in a healthy person is .about.5.5%),
the partial pressure of CO.sub.2 in the lung is equal to that in
mixed venous blood. A flat segment of the PCO.sub.2 tracing segment
indicates that inspired and expired PCO.sub.2 are equal. To
identify the true PvCO.sub.2-oxy, the flat segment must occur
within the first 3-4 breaths, before recirculation raises the
PvCO.sub.2-oxy (see FIG. 8).
[0028] 4.1.1 Advantages of the Equilibration Method
[0029] The capnograph reading is of gas equilibrated with
PvCO.sub.2-oxy and can be considered a directly measured value as
opposed to a value obtained from calculation or extrapolation.
[0030] 4.1.2 Limitations of the Equilibration Method [0031] 4.1.2.1
The CO.sub.2 concentration in the bag depends on bag size, the
patient's lung volume, and the PvCO.sub.2-oxy--the last being the
unknown value. Therefore, the concentration of CO.sub.2 in the bag
must be individualized to the patient and thus found by trial and
error. The method is therefore difficult to automate fully. [0032]
4.1.2.2 In practice, since the characteristic of a suitable
endpoint (the plateau of PCO.sub.2) is subjective, identification
of a suitable plateau is difficult to automate. [0033] 4.1.2.3 The
manoeuvre of rebreathing from a bag is difficult to perform in
mechanically ventilated patients and is therefore not suitable for
such patients. [0034] 4.1.2.4 Inhaling 10-13% CO.sub.2 is very
uncomfortable and most people cannot tolerate it. It is
particularly uncomfortable to someone who is short of breath or
exercising. [0035] 4.1.2.5 The method requires an external source
of CO.sub.2. This makes testing equipment bulky and awkward. [0036]
4.1.2.6 The method requires that the subject hyperventilate in
order to mix thoroughly the gas in the bag and the lungs before
recirculation of blood takes place. This requirement limits the
test to those subjects who can perform this manoeuvre and who can
provide this degree of cooperation. This excludes patients who have
severe lung disease, those who are too young, too confused or too
ill to cooperate. [0037] 4.1.2.7 The test loads a considerable
volume of CO.sub.2 into the subject's lungs and at the same time
prevents CO.sub.2 from leaving the blood for the duration of the
test. This has negative consequences for the subject: [0038]
4.1.2.7.1 Following the test, the subject must hyperventilate to
eliminate the applied CO.sub.2 load as well as the volume of
metabolically-produced CO.sub.2 not eliminated during the test.
This may pose a considerable burden for some subjects with lung
disease or exercising subjects who are already expending
considerable effort to cope with their existing metabolic CO.sub.2
load. [0039] 4.1.2.7.2 A period of hyperventilation following the
test is required to eliminate the CO.sub.2. This may be difficult
for some subjects to perform and, consequently, they may experience
respiratory distress for some time until their PCO2 is decreased.
[0040] 4.1.2.7.3 Repeated tests must be delayed until the extra
CO.sub.2 is eliminated and the baseline state re-established.
[0041] 4.1.2.7.4 The test itself may distress the subject and alter
the {dot over (Q)}.
[0042] 5. Rebreathing-Exponential Method
[0043] In this technique, a small amount of CO.sub.2 is placed in a
bag and the subject asked to rebreathe from the bag. The
PETCO.sub.2s of successive breaths will rise exponentially towards
PvCO.sub.2-oxy. A rising exponential curve is then fit to the
PETCO.sub.2s of these breaths to predict an asymptotic value that
is assumed to be the PvCO.sub.2-oxy (See FIG. 9).
[0044] 5.1 Advantages of the Exponential Method [0045] 5.1.1 There
is no requirement for respiratory manoeuvres by the patient. [0046]
5.1.2 A smaller CO.sub.2 load is placed on the subject in order to
perform the test.
[0047] 5.2 Limitations of the Exponential Method [0048] 5.2.1 This
is an indirect test in which the PvCO.sub.2-oxy is not measured
directly but calculated from data generated by a test. [0049] 5.2.2
As the metabolic production of CO.sub.2 is small compared to the
size of the lung and bag, the rise of PCO.sub.2 occurs over a
prolonged period. This severely limits the number of useful data
points for accurate extrapolation from an exponential curve, before
recirculation. [0050] 5.2.3 The most important limitation of this
and other methods that use partial equilibration during rebreathing
to extrapolate to an asymptote using a single exponential is that
the assumptions underlying the method are incorrect. In fact, the
method produces two different mathematical profiles: the one
describing the washout of CO.sub.2 from the lung into the bag is a
decreasing exponential whereas the second describing the build-up
of CO.sub.2 released from the blood into the lung-bag mixture is an
increasing exponential (13). Only after the gases in the lung-bag
system have become well mixed do the two exponentials resolve to a
single exponential. By then, very few breaths (if any) that can
provide suitable data for extrapolation from a single exponential
can be taken before recirculation. [0051] 5.2.4 A continually
rising level of CO2 makes this test unpleasant in conscious
patients, especially in those exercising or very ill. [0052] 5.2.5
The manoeuvre of rebreathing from a bag is difficult to perform in
mechanically ventilated patients and is therefore not suitable for
such patients. [0053] 5.2.6 The method requires an external source
of CO.sub.2. This makes testing equipment bulky and awkward. [0054]
5.2.7 The test loads a volume of CO.sub.2 into the subject's lungs
and at the same time prevents CO.sub.2 from leaving the blood for
the duration of the test. Although the extent of the CO.sub.2 load
on the subject is less than with the equilibration method, the
negative consequences for the subject, outlined in the section on
the equilibration method discussed above, must be considered.
[0055] 5.2.8 Priming the rebreathing bag with some CO.sub.2
improves the predictive qualities of the asymptote since every data
point lies closer to the asymptote, but the increased CO.sub.2
concentrations increase the discomfort and the limitations approach
those outlined above for the equilibration method.
[0056] 6.0 Calculating {dot over (Q)} without First Calculating
PvCO.sub.2-Oxy
[0057] Gedeon in 1980 described a method of calculating {dot over
(Q)} in ventilated patients via a differential Fick method that
circumvents the need to calculate P v CO.sub.2-oxy. The underlying
assumptions of the method are that {dot over (Q)} and PvCO.sub.2
will remain unchanged during a step change in lung CO.sub.2
elimination and alveolar PCO.sub.2 (PACO.sub.2) lasting less than a
recirculation time (about 30 seconds). Gedeon proposed reducing
lung CO.sub.2 elimination by reducing either the tidal volume or
respiratory frequency setting of the ventilator. As a modification
of this method, Orr et al. proposed leaving the ventilator settings
unchanged and reducing lung CO.sub.2 elimination by temporarily
interposing a dead space between the ventilator and the patient's
airway resulting in a transient period of rebreathing previously
exhaled gas.
[0058] 6.1 Theoretical Basis of Gedeon/Orr Method:
[0059] The method applies to a subject being ventilated under
control conditions in which CO.sub.2 elimination and PETCO.sub.2
are measured. A test manoeuvre consisting of a transient alteration
in the CO.sub.2 elimination for a time less than a recirculation
time is effected and the resulting "equilibrium" PETCO.sub.2 is
noted. It is assumed that the {dot over (Q)} and PvCO.sub.2-oxy
during the test are unchanged from control conditions. The Fick
equation for these two conditions can be written as
Q . = V . CO 2 C v _ CO 2 - CaCO 2 ##EQU00002## Q . = V . CO 2 ' C
v _ CO 2 - CaCO 2 ' ##EQU00002.2##
where {dot over (V)}CO.sub.2' is the CO.sub.2 flux at the lungs
during the test and CaCO.sub.2' is the corresponding `new` arterial
content of CO.sub.2. These two equations can be combined to yield
the differential form of Fick's equation:
Q . = .DELTA. V CO 2 .DELTA. CaCO 2 ##EQU00003##
where .DELTA. denotes a "difference in". Since the PaCO.sub.2 and
PvCO.sub.2-oxy lie on the same CO.sub.2 dissociation curve, partial
pressures of CO.sub.2 can be substituted for CO.sub.2 content to
yield the following relation:
Q . = .DELTA. V . CO 2 S * .DELTA. P a CO 2 ##EQU00004##
where S is the slope of the CO.sub.2 dissociation curve. Like the
conventional non-invasive CO.sub.2-based Fick method, the
differential Fick method relies on predicting PaCO.sub.2 through
measurements of PETCO.sub.2. However, instead of requiring a
calculation of PvCO.sub.2-oxy, the differential Fick equation
assumes no change in PvCO.sub.2-oxy over the duration of the test,
and uses the measured quantities {dot over (V)}CO.sub.2 and {dot
over (V)}CO.sub.2' and well as PaCO.sub.2 and PaCO.sub.2' (from
PETCO.sub.2) to calculate the remaining unknown value in the
equation: {dot over (Q)}.
[0060] 6.2 Advantages of Gedeon/Orr Method [0061] 6.2.1 The main
advantage is that PvCO.sub.2 does not need to be calculated. [0062]
6.2.2 If the deadspace method is used to alter the {dot over
(V)}CO.sub.2, then no change in breathing pattern is required.
[0063] 6.2.3 The method can, theoretically, be fully automated. (In
its present commercial form, the size of the interposed deadspace
must still be altered manually).
[0064] 6.3 Limitations of Gedeon/Orr Method
[0065] There are a number of limitations in applying Orr's method
to spontaneously ventilating subjects. [0066] 6.3.1 In
spontaneously breathing subjects, there is considerable
breath-to-breath variation in breath size and breathing frequency
resulting in a variation in PETCO.sub.2. This poses problems with
respect to: [0067] 6.3.1.1 Identification of PETCO.sub.2 and
PETCO.sub.2'. Long periods of baseline measurements are needed in
order to average the end tidal values and identify the PETCO.sub.2
to be used as the baseline PETCO.sub.2 in the differential Fick
equation. The test phase cannot last for more than about 30 seconds
(due to recirculation), typically 5 breaths. This leaves little
time to determine an accurate average PETCO.sub.2'. During
prolonged baseline periods of observation, the condition of the
patient may change. [0068] 6.3.1.2 Calculation of {dot over
(V)}CO.sub.2. The variations in PETCO.sub.2 are related to
variations in CO.sub.2 elimination but the relationship is not
consistently reflected by the PETCO.sub.2. For example, assuming a
subject breathing at rest with an average resting breath size, an
interposed smaller breath may result in a lower PETCO.sub.2 (due to
a smaller contribution of alveolar gas to the end tidal sample) but
the CO.sub.2 elimination from that breath will be diminished.
Conversely, a larger breath may result in the same PETCO.sub.2 as
the resting breath but a greater volume of CO.sub.2 is eliminated.
The commercial automated Gedeon method (NICO2, Novametrics Medical
Systems, Wallingford, Conn., U.S.A.) measures the CO.sub.2
eliminated breath-by-breath and therefore must continuously average
the values to measure {dot over (V)}CO.sub.2. The NICO2 method of
calculating {dot over (V)}CO.sub.2 by real-time integration of
continuous measurements of flow (with a pneumotachymeter) and
CO.sub.2 concentration (with a capnograph) is fraught with
potential for errors: a small error in the integration of these two
signals with different time delays and time constants results in a
much larger error in the calculation of {dot over (V)}CO.sub.2. In
addition, the greater the variability of the breath size and
CO.sub.2 concentrations, the longer the measurement time required
for an accurate estimate of {dot over (V)}CO.sub.2. [0069] 6.3.2
Calculation of {dot over (V)}CO.sub.2'. Stable transient changes in
{dot over (V)}CO.sub.2 cannot be achieved in conscious
spontaneously ventilating patients: [0070] 6.3.2.1 Interposing a
deadspace and raising their PCO.sub.2 will stimulate spontaneously
breathing conscious subjects to increase their {dot over (V)}E and
{dot over (V)}CO.sub.2 until the PETCO.sub.2 is restored. [0071]
6.3.2.2 Any change in breath size or frequency during a period of
breathing, (a normal occurrence in spontaneously breathing people)
changes the {dot over (V)}CO.sub.2 during that period. During
inspiration, the deadspace gas is inhaled first followed by fresh
gas. A decrease in a breath size or frequency diminishes the volume
of fresh gas inhaled (and thus the {dot over (V)}CO.sub.2 for that
breath). An increase in breath size or frequency will result in an
increased volume of fresh gas delivered to the alveoli. [0072]
6.3.2.3 Each breath is an independent event and there is no
inherent method to compensate in a subsequent breath for changes in
{dot over (V)}CO.sub.2 in the preceding breath. For the method to
be implemented, therefore, measures must be taken to ensure that
breath size and frequency stay absolutely constant during the test.
The NICO2 method has no such built-in aspects. The method can
therefore be used only in patients who have precisely uniform
breathing pattern such as those that are paralysed and mechanically
ventilated. [0073] 6.3.3 Identification of PETCO.sub.2--PaCO.sub.2
gradient. The Gedeon and Orr methods assume, or require the
establishment of, a constant gradient between the PETCO.sub.2 and
the PaCO.sub.2. The variation in PETCO.sub.2 is due to variations
of distribution of fresh gas to various parts of the lung and any
one breath does not reflect the overall state of CO.sub.2 exchange.
On the other hand, such variations are not reflected in the
PaCO.sub.2 which does reflect the overall exchange of CO.sub.2 and
remains relatively constant. Therefore, variations in PETCO.sub.2
also confound the quantification of the PETCO.sub.2--PaCO.sub.2
gradient under control conditions. Although Orr provides a number
of equations to correct for these limitations, these equations are
empirical and do not necessarily apply to a particular patient. For
example, they are applied whether or not there is irregular
breathing.
[0074] The PETCO.sub.2--PaCO.sub.2 gradient during the test phase
when rebreathing occurs is unknown. In the presence of large
alveolar deadspace (as commonly occurs in many ill patients) the
PETCO.sub.2--PaCO.sub.2 gradient will change during the rebreathing
phase. Orr provides some equations to correct for this but since
the volume of the alveolar deadspace is unknown, the applicability
of the formula to any particular patient is unknown. This further
diminishes the accuracy of calculating PaCO.sub.2'.
[0075] The manoeuvres required to determine each of the terms
required to calculate {dot over (Q)} ({dot over (V)}CO.sub.2, {dot
over (V)}CO.sub.2', PETCO.sub.2, PETCO.sub.2' and PaCO.sub.2') by
the Orr/Gedeon/NICO2 method is awkward to implement and prone to
errors in measurement in the presence of any variation in breath
amplitude or breathing frequency as occurs in spontaneously
breathing humans or animals. [0076] 6.3.4 The parameter calculated
by the differential Fick method as practiced by
Gedeon/Orr/Respironics is pulmonary blood flow ({dot over (Q)}p).
Pulmonary blood flow may be less than the total cardiac output
({dot over (Q)}t) when, for example, some of the {dot over (Q)} is
shunted from the right side of the circulation (superior vena cava,
right atrium, right ventricle, pulmonary artery) into the left side
of the circulation without passing through the lungs. This is
referred to as "shunt" ({dot over (Q)}s). About 5% of venous blood
bypasses the lungs (termed shunted blood) in healthy adults. Much
larger shunts occur in many medical conditions such as congenital
heart disease, surgical repair of some congenital heart diseases,
pneumonia, pulmonary edema, asthma, pulmonary atelectasis, adult
respiratory distress syndrome, obesity, pregnancy, liver disease
and others. The differential Fick method does not include shunted
blood in the calculation of {dot over (Q)} and other empiric
corrections must be made to account for it.
[0077] 7.0 Kim-Rahn Farhi Method
[0078] 7.1 Theory:
[0079] A unique maneuver was proposed by Rim, Rahn and Farhi, (J.
Appl. Physiol. 21:1388-44. 1966.) as a way to calculate the
oxygenated mixed venous PCO.sub.2 (PvCO.sub.2-oxy) as well as the
true PvCO.sub.2 and PaCO.sub.2. It is based on a paradigm of taking
a breath of O.sub.2, holding the breath, and exhaling slowly over a
period equal to the recirculation time. Over this time of
exhalation, the CO.sub.2 from the mixed venous blood will diffuse
into the alveoli and O.sub.2 will be absorbed. The low PO.sub.2 in
the red blood cells in the mixed venous blood maximizes the volume
of CO.sub.2 that can be carried by hemoglobin. Oxygen from the
alveoli diffuses into the red blood cells, raising the PO.sub.2 and
decreasing the affinity of hemoglobin for CO.sub.2 (Haldane
effect). This releases CO.sub.2 from the binding sites on the
hemoglobin, making it available for diffusion into the alveoli.
With breath holding, CO.sub.2 will accumulate in the alveoli and
the alveolar PCO.sub.2 (PACO.sub.2) will rise until it no longer
provides a gradient for diffusion from the blood. (This PCO.sub.2
is known as the oxygenated mixed venous PCO.sub.2
(PvCO.sub.2-oxy).) However, O.sub.2 will continue to diffuse as
long as the PAO.sub.2 is greater than PvO.sub.2. Relatively little
CO.sub.2 need diffuse into the alveoli to reach PvCO.sub.2-oxy
compared to the volume of O.sub.2 that is available for uptake
before the PO.sub.2 in the pulmonary capillary blood is in
equilibrium with the PAO.sub.2. In other words, the equilibration
of CO.sub.2 in the alveoli with the mixed venous blood will occur
well before that of O.sub.2.
[0080] Since both O.sub.2 and CO.sub.2 are contained in the same
physical volume, the changes in concentrations of each gas over a
short period will reflect the rates of flux of that gas over the
same period. Therefore, over a short period, the ratio of PCO.sub.2
to PO.sub.2 will reflect the respiratory quotient, RQ (defined as
the rate of CO.sub.2 diffusion from the blood into the alveoli
divided by the rate of O.sub.2 absorption into the blood from the
alveoli). The RQ will initially be highest at the beginning of the
breath when the rate of CO.sub.2 diffusion into the alveoli is
maximal, and will approach 0 when the alveolar PCO.sub.2 equals
PvCO.sub.2-oxy. In vitro studies have shown that PACO.sub.2 equals
the true PvCO.sub.2 when the RQ=0.32 and equals PaCO.sub.2 when RQ
is equal to the patient's steady state RQ (typically
.about.0.8).
[0081] 7.2 Test Method
[0082] The method suggested for performing this test would require
a subject to take a maximum breath of 100% O.sub.2 and exhale very
slowly and maximally. Over the course of this exhalation, expired
gas is sampled and analyzed continuously for both PO.sub.2 and
PCO.sub.2. PO.sub.2 is graphed vs. PCO.sub.2 and the RQ is
calculated from the instantaneous slope of tangents to the curves
at various PCO.sub.2 values as follows:
RQ = slope - ( FeO 2 * slope ) - FeCO 2 1 - ( FeO 2 * slope ) -
FeCO 2 ##EQU00005##
[0083] These RQ values are then plotted against their respective
PCO.sub.2 data points resulting in a linear relation as illustrated
in FIGS. 4 and 5 of T. S. Kin, H. Rahn, and L. E. Farhi cited
above.
[0084] 7.3 Advantages of the Method [0085] 7.3.1 This is the only
known non-invasive method by which true PvCO.sub.2 can be
calculated. [0086] 7.3.2 The method provides an estimate of
PaCO.sub.2 not based on assuming a gradient between PETCO.sub.2 and
PaCO.sub.2. [0087] 7.3.3 Data generated by the method can be used
to calculate the O.sub.2 saturation of mixed venous blood.
[0088] 7.4 Limitations of the Kim-Rahn-Farhi Breath-Hold Method
[0089] The main limitation of this method is that it requires the
subject to have a large lung capacity, hold his breath, and exhale
over a prolonged duration. Patients with conditions such as
pulmonary fibrosis, pneumonia, adult respiratory distress syndrome,
chronic obstructive lung disease, asthma, obesity, trauma,
abdominal and chest surgery, mental obtundation, confusion,
pregnancy and many others have marked limitations in their ability
to take a large breath. Patients are required to cooperate with
their duration of breath holding and rate of exhalation. Many
patients who are ill, exercising subjects, children and others are
unable to perform this satisfactorily. This method is very awkward
to automate or perform on ventilated patients.
[0090] 8.0 Fisher Method
[0091] 8.1 Theory
[0092] In a steady state, if a subject breathes in a PCO.sub.2
equal PvCO.sub.2-oxy, there will be no gradient for gas exchange
and the difference in PCO.sub.2 between the inspired PCO.sub.2
(PICO.sub.2) and the expired PCO.sub.2 (PECO.sub.2) will be 0. The
volume of CO.sub.2 diffusing into the alveoli will be maximal when
the difference between PICO.sub.2 and PECO.sub.2 is greatest, i.e.,
when the PICO.sub.2 is 0. Since the change in alveolar PCO.sub.2
(PACO.sub.2) varies directly as the volume of CO.sub.2 diffusing
into the alveoli and the volume diffusing into the alveoli varies
directly as the gradient, then the difference between the
PICO.sub.2 and PECO.sub.2 will vary inversely as PICO.sub.2. In
other words, graphing the difference between the PECO.sub.2 and
PICO.sub.2 (PECO.sub.2--PICO.sub.2) vs. FICO.sub.2 will result in a
straight line. Since subjects normally breathe room air (PICO.sub.2
equals 0 or O.sub.2, the control PETCO.sub.2 provides the first
point on the graph. When subjects inhale gas with any constant
value of PCO.sub.2, the PETCO.sub.2 at the end of an equilibration
period not exceeding the time for recirculation will provide a
second data point which can be used to define the straight line
which crosses the X axis where PICO.sub.2 equals
PvCO.sub.2-oxy.
[0093] 8.2 Test Method:
[0094] The subject breathes via a non-rebreathing valve. The
inspiratory limb is provided with either fresh gas or test gas with
any PCO.sub.2. To perform a test, the inspired gas is switched from
control gas to test gas for about one recirculation time. The PICO2
of the test gas, the PETCO.sub.2 just before the test (when
PICO.sub.2 was 0), and the PETCO.sub.2 of the last breath before
recirculation are used to calculate the PvCO.sub.2-oxy.
[0095] 8.3 Advantages of the Prior Disclosed Previous Fisher
Method: [0096] 8.3.1 Any low inspired concentration of CO.sub.2
such as 1% is adequate to generate a data point; therefore the
subject need not get a large CO.sub.2 load. [0097] 8.3.2 This
Fisher method extrapolates to the PvCO.sub.2-oxy from a linear
function and is therefore easier to calculate and more accurate
than with the partial rebreathing test in which data points are fit
to an exponential curve for extrapolation to an asymptote. [0098]
8.3.3 The PICO.sub.2 can be any value, so accurate mixtures of
gases are not required. [0099] 8.3.4 Assuming arterial PCO.sub.2
values (PaCO.sub.2) can be obtained from arterial blood sample, for
example, the method measures total {dot over (Q)}, not just
pulmonary blood flow. [0100] 8.3.5 The subject need not carry out
any respiratory manoeuvre such as breath holding or
hyperventilation. [0101] 8.3.6 The method does not entail any
rebreathing. Therefore, O.sub.2 levels remain stable throughout the
test and supplemental O.sub.2 is not needed.
[0102] 8.4 Limitations of the Fisher Method [0103] 8.4.1 Uniform
breath size cannot be guaranteed in spontaneously breathing
subjects. A change of breath size or breathing frequency during the
latter parts of the test phase will affect the PETCO.sub.2 and thus
the calculation of PvCO.sub.2-oxy. Furthermore, as the subjects are
inhaling gas that contains CO.sub.2, they may be stimulated to take
larger or more frequent breaths. [0104] 8.4.2 The test requires an
external source of CO.sub.2. This must be supplied via a tank of
CO.sub.2 and a gas blender or via a tank of pre-mixed gas. If more
than one test gas is required, then arrangements to blend
additional gases must be made or more than one additional gas tank
is required. This is inconvenient, costly, and adds complexity to
the test method and additional bulk and weight to the test
apparatus. [0105] 8.4.3 It is very complex to configure an
automated system that works for both spontaneously breathing and
mechanically ventilated patients. [0106] 8.4.4 There is no simple
method to adapt currently available ventilators, anaesthetic
machines or breathing circuits to provide a known and constant
PICO.sub.2 for a fixed number of breaths. [0107] 8.4.5 The
technique is difficult to adapt to anaesthetized patients breathing
via a circle circuit in which both the test gas and the anaesthetic
gases enter the circuit, especially in the presence of a CO.sub.2
absorber removing CO.sub.2 from the circuit.
OBJECT OF THE INVENTION
[0108] It is therefore a primary object of this invention to
provide an improved method and apparatus for the purpose of
non-invasively determining cardiac output (Q) which may be utilized
in ventilated subjects, subjects who breath spontaneously or
subjects who are under controlled ventilation such as those
undergoing surgical procedures under general anesthesia.
[0109] It is yet a further object of this invention to provide an
improved method and the apparatus related thereto for the purposes
of non-invasively determining alveolar ventilation ({dot over
(V)}A) and calculating minute CO.sub.2 production ({dot over
(V)}CO.sub.2), oxygenated mixed venous PCO.sub.2 (PvCO.sub.2-oxy),
true mixed venous PCO.sub.2 (true PvCO.sub.2), pulmonary shunt,
anatomical dead space, arterial PCO.sub.2, at a greater accuracy
than prior known non-invasive methods and apparatuses would
provide.
[0110] It is yet another object of the invention to provide a
method of non-invasively calculating the oxygen saturation of mixed
venous blood (SvO.sub.2) which may be utilized to reveal heart
failure of septic shock in a patient or the like.
[0111] It is yet a further object of this invention to provide an
improved method and the apparatus related thereto for the purposes
of determining {dot over (Q)}, ({dot over (V)}A and calculating
{dot over (V)}CO.sub.2, PvCO.sub.2-oxy, true PvCO.sub.2, pulmonary
shunt, anatomical dead space in a non-invasive and fully automated
manner.
[0112] 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.
SUMMARY OF THE INVENTION
[0113] This invention discloses a method and apparatus for
calculating all of the {dot over (Q)} regardless of shunt,
calculating the shunt, anatomical and alveolar deadspace, true
mixed venous O.sub.2 saturation, true mixed venous PCO.sub.2, and
PaCO.sub.2. Furthermore the method and apparatus can be used with
ventilated subjects, subjects breathing spontaneously, even with
marked variations in their tidal volume and breathing frequency, or
subjects undergoing surgery under anaesthesia. Subjects need not
perform any respiratory manoeuvre such as hyperventilation or
breath holding.
[0114] According to one aspect of the invention there is provided
an improved method and apparatus for the purposes of determining
{dot over (Q)} and {dot over (V)} A and calculating {dot over
(V)}CO.sub.2, PvCO.sub.2-oxy, true PvCO.sub.2, PaCO.sub.2,
pulmonary shunt, and anatomical dead space which increases the
accuracy of these determinations in relation to known methods and
apparatus and allows the full automation of the various methods
disclosed herein for these determinations and calculations.
[0115] The new method: [0116] 1. is insensitive to changes in
minute ventilation ({dot over (V)}E), tidal volume and/or
respiratory frequency so that the method can be carried out in
spontaneously breathing subjects; [0117] 2. is simplified and less
expensive to construct compared to other non-invasive automated
methods of performing the differential Fick technique in that
[0118] a. it does not necessarily require any mechanically
activated valves to be actively engaged in the patient circuit
[0119] b. does not require a pneumotachygraph to measure flows
[0120] c. does not require manual adjustment of an interposed dead
space (and thus can be totally automated); [0121] d. The device
will be the same for all sizes of adults (one size fits all) [0122]
3. is compatible with a number of sequential gas delivery breathing
(SGDB) circuits. A SGDB circuit provides for the sequential
delivery of two gas sets to the lungs during inhalation. A gas set
is composed of one or more gases and vapors. The first gas set
(FGS) is provided from the beginning of inhalation and can
terminate at some time during inhalation depending on the FGS flow
and the {dot over (V)} E, at which time inhalation continues with
the delivery of the second gas set (SGS). For the purposes of
measuring {dot over (Q)} and the other physiologic parameters
described herein, it is preferred that there is a distinct
transition from FGS to SGS and there is no mixing of the gas sets.
A small degree of mixing of FGS with SGS during the latter part of
inhalation will reduce accuracy of the measured and calculated
results. Mathematical corrections can be made to minimize effect of
the mixing of FGS with SGS, but cannot completely negate the
effects in all circumstances. Therefore, breathing circuits which
separate the FGS from the SGS are preferred. [0123] 4. the
generation and presentation of data will be substantially the same
for controlled (mechanical) ventilation and rebreathing so that the
algorithms to perform the tests and analyze the data can be
substantially the same; [0124] 5. can institute an equilibrium
steady state within one recirculation time so that the value for
PETCO.sub.2 will be a true measured value rather than one requiring
multiple corrections based on unsubstantiated assumptions; [0125]
6. will allow the measurement of a new steady state PETCO.sub.2
within one recirculation time and thus actualize the assumption
underlying the Differential Fick approach that PvCO.sub.2 is
unchanged; [0126] 7. Will minimize the effect of changes in tidal
volume on the alveolar ventilation. [0127] 8. maintain the alveolar
PO.sub.2 while making pulmonary blood flow measurements; [0128] 9.
make all calculations without a requirement to measure
breath-by-breath volumes of inspired and expired CO.sub.2 or any
flows of tidal gases.
[0129] According to one aspect of the invention there is provided
an improved apparatus and method of identifying the alveolar
ventilation ({dot over (V)}A), substantially as illustrated and
described herein, preferably the {dot over (V)}A so determined is
utilized to calculate the {dot over (V)}CO.sub.2 as {dot over
(V)}A.times.FETCO.sub.2, where FETCO.sub.2 is the fractional
pressure of CO.sub.2 in end tidal gas.
[0130] In one embodiment of the improved apparatus and method:
[0131] a) the Fisher approach is used to determine PvCO.sub.2-oxy
(or) [0132] b) the Kim Rahn Farhi approach is used to determine
[0133] i) PvCO.sub.2-oxy [0134] ii) true PvCO.sub.2 [0135] iii)
PaCO.sub.2 [0136] iv) true PvCO.sub.2 plus the information from a
pulse oximeter to determine mixed venous hemoglobin O.sub.2
saturation (or) [0137] c) the differential CO.sub.2 Fick technique
of Gedeon and Orr is utilized to determine any combination of
[0138] i) PvCO.sub.2-oxy [0139] ii) Q [0140] iii) {dot over
(V)}CO.sub.2 [0141] iv) {dot over (V)}CO.sub.2' [0142] v)
PETCO.sub.2--PaCO.sub.2 gradient determined using the PaCO.sub.2 as
determined by the Kim Rahn Farhi method from data collected while
reducing the VCO.sub.2 in order to perform the Differential Fick
method. (or) [0143] d) {dot over (Q)} is determined via the Kim
Rahn Farhi method performed during partial rebreathing using a
CO.sub.2 Fick method where the [0144] i) {dot over (V)}CO.sub.2 is
calculated with or without the new method as disclosed [0145] ii)
CaCO.sub.2 and CvCO.sub.2 are determined from the PaCO.sub.2 and
PvCO.sub.2 respectively derived by the Kim Rahn Farhi method; (or)
[0146] e) calculation of the respiratory quotient (RQ) is
determined as PETCO.sub.2/(PIO.sub.2--PEO.sub.2); (or) [0147] f)
PaCO.sub.2 is determined directly via analysis of arterial blood
sample, arterialized venous sample, transcutaneous PCO.sub.2
electrode, or other methods known to those skilled in the art.
[0148] wherein said apparatus or method may be utilized for very
accurate non-invasive determination of {dot over (Q)} and the other
indicated parameters.
[0149] According to yet another aspect of the invention there is
provided an improved method of apparatus for determining {dot over
(V)}A, {dot over (V)}CO.sub.2 and calculating {dot over (Q)},
PvCO.sub.2-oxy, true PvCO.sub.2, PaCO.sub.2, pulmonary shunt,
anatomical dead space, and O.sub.2 saturation in mixed venous
blood; which increases the accuracy of these determinations and
calculations in relation to known methods and apparatuses and
allows for full automation thereof if necessary by using automated
means well known to those skilled in the art, to: [0150] i) induce
a step change in {dot over (V)}CO.sub.2 by providing a step change
in FGS flow to a SGDB circuit to create, with the control data at
rest, two sets of data for said determination utilizing the
differential Fick equations; (or) [0151] ii) change the partial
pressure of CO.sub.2 in FGS of a SGDB circuit to create, with the
control data at rest, two sets of data for said determination
utilizing the Fisher or the differential Fick equations; (or)
[0152] iii) change FGS flow or change the partial pressure of
CO.sub.2 in FGS in a SGDB circuit to simulate complete or partial
breath holding and utilizing the Kim-Rahn-Farhi technique, wherein
the PETCO.sub.2 of each breath is equivalent to a sequential
alveolar sample;
[0153] thereby providing more relevant data to calculate desired
parameters.
[0154] In yet another embodiment of the invention a ventilation
circuit and method is provided for using sequential delivery of gas
sets in order to identify the minute volume of gas entering the
anatomical dead space and the minute volume entering the alveoli
and thereby available for gas exchange ({dot over (V)}A).
Subsequently, setting FGS flow to substantially equal to or less
than {dot over (V)}A substantially controls {dot over (V)}A. A step
reduction in {dot over (V)}A can then be induced by a step
reduction in FGS flow, and resultant effects on end tidal gases
such as CO.sub.2 can be used in the to calculate {dot over (Q)} and
other parameters as previously set out herein in the Background,
disclosures and figures.
[0155] In yet another embodiment there is provided a method and
apparatus of determining {dot over (Q)} and the other parameters
disclosed by utilizing any SGDB circuit for example, the circuits
described and illustrated herein by reducing the FGS flow to said
circuit or increasing the PCO.sub.2 of FGS to said circuit,
independent of the breathing rate thereby allowing for calculations
to be made via Differential Fick equations, and/or Fisher method,
and/or the Kim-Rahn-Farhi method.
[0156] Preferably the method or apparatus previously described
wherein the CO.sub.2 content as calculated from PvCO.sub.2-oxy and
true PvCO.sub.2, may be utilized to determine the O.sub.2
saturation of mixed venous blood with known relations between
CO.sub.2 content, O.sub.2 saturation and PCO.sub.2.
[0157] In one embodiment the method or apparatus disclosed may be
utilized wherein the arterial O.sub.2 hemoglobin saturation, as
determined by a non-invasive pulse oximeter, which makes the
measurement by shining infrared light through a finger, is utilized
with the O.sub.2 saturation value in the pulmonary artery as
calculated by the Kim Rahn Farhi method, to calculate the fraction
of shunted blood (assuming fully oxygenated blood in the end
pulmonary capillary) thereof.
[0158] Preferably said method or apparatus is utilized to determine
the fraction of shunted blood {dot over (Q)}s, which in conjunction
with determination of total cardiac output {dot over (Q)}T
(utilizing PaCO.sub.2 as determined by the Kim Rahn Farhi method,
or available from analysis or arterial blood or determined by
transcutaneous PCO.sub.2 determination or otherwise known to those
skilled in the art, as a term in the Fick equation) and pulmonary
blood flow {dot over (Q)}.sub.p (utilizing PETCO.sub.2 in the Fick
equation) may be used to determine {dot over (Q)}.sub.s the
pulmonary output via the relationship.
{dot over (Q)}.sub.s={dot over (Q)}.sub.t-{dot over (Q)}.sub.p
[0159] Preferably the method or apparatus disclosed wherein the
O.sub.2 saturation of haemoglobin in mixed venous blood
(SaO.sub.2), as determined therewith, is utilized to reveal a
condition in a patient such as septic shock, or heart failure.
BRIEF DESCRIPTION OF THE FIGURES
[0160] FIG. 8: PCO.sub.2 vs. time tracing during a rebreathing
equilibrium test for determining oxygenated mixed venous
PCO.sub.2
[0161] FIG. 9: PCO.sub.2 vs. time tracing during exponential method
of finding oxygenated mixed venous PCO.sub.2
[0162] FIG. 2 is a SGDB Circuit as taught by Fisher in U.S. Pat.
No. 6,622,725 referred to herein as the Fisher circuit.
[0163] FIG. 3 is similar to FIG. 2 wherein the reservoir bags are
remote from the patient.
[0164] FIG. 5 is a new circuit for use with spontaneous
ventilation.
[0165] FIG. 3B is similar to FIG. 5 wherein bypass limb, bypass
valve, and passive expiratory valve are replaced by an active
expiratory valve.
[0166] FIG. 3D is similar to FIG. 2 wherein an active valve has
been added to the inspiratory limb to prevent mixing of FGS with
SGS during inhalation.
[0167] FIG. 5A is similar to FIG. 5 wherein an active valve has
been added to the inspiratory limb to prevent mixing of FGS with
SGS during inhalation.
[0168] FIG. 3E is similar to FIG. 2 wherein an active valve has
replaced the passive inspiratory valve.
[0169] FIG. 5B is similar to FIG. 5 wherein an active valve has
replaced the passive inspiratory valve.
[0170] FIG. 3C is similar to FIG. 3B wherein an active valve has
replaced the passive inspiratory valve.
[0171] FIG. 4 shows a modification of any of the circuits shown in
FIGS. 2, 3-3E, 5-5B for use with a mechanically ventilated
patient.
[0172] FIG. 4B shows the preferred embodiment modified for use on
ventilated patients.
[0173] FIG. 6 is a modification of the above circuits to include
co-axially arranged inspiratory and expiratory limbs between the
valves and the patient.
[0174] FIG. 6A shows the preferred embodiment of the cardiac output
circuit where inspiratory and expiratory limbs are co-axially
arranged with the circuit of FIG. 5A.
[0175] FIG. 7 is a new circuit designed to allow measurement of
cardiac output while delivering anesthetics or removing volatile
agents from a patient.
[0176] FIG. 5C shows a detail of a circuit design where the passive
valves are surrounded by the exhaled gas reservoir
[0177] FIG. 10: Apparatus for non-invasive cardiac output apparatus
consisting of a breathing circuit, gas sources, gas flow
controllers, gas concentration sensors, and microprocessor capable
of receiving and storing analog and digital input from sensors and
operators, storing and following a decision tree, and generating
output signals to a computer screen and to flow controllers.
[0178] FIG. 11 Flow diagram describing automated sequence of events
performed by the non-invasive cardiac output apparatus in order to
automatically generate and record data non-invasively and calculate
{dot over (Q)} and other physiologic parameters.
[0179] FIG. 12 is a schematic of a standard anesthetic circle
system herein provided as reference for discussion of disclosed
system. Gas entering the anesthetic circuit consisting of oxygen,
with the possible addition of air and/or nitrous oxide (N.sub.2O),
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 CO.sub.2 absorber (5) up the inspiratory limb (8).
During exhalation, the inspiratory valve (7) closes and the fresh
gas passes through the CO.sub.2 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 pressure 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).
[0180] FIG. 13 A detail of the computer screen output of an
automated analysis of test finding VA by progressive reduction in
SGF flow method in a subject is illustrated in FIG. 13. The figure
illustrates that progressive reduction of SGF (labelled "FGF" in
the figure) results in a distinct inflection point when either
PETCO.sub.2 or PETO.sub.2 is graphed as a function of SGF.
DETAILED DESCRIPTION OF THE INVENTION
[0181] Detailed Description of the Apparatus
[0182] Referring now to Figure ??, an apparatus is shown with the
following components: [0183] 1) a breathing circuit (202), said
breathing circuit preferably has the characteristic that, on
exhalation, exhaled gas is kept separate from inhaled gas and on
inhalation, when {dot over (V)}E is greater than the flow of a
first gas set (FGS) into the circuit, the subject inhales FGS gas
first and then inhales a second gas set (SGS) gas, preferably said
SGS containing CO.sub.2 and where SGS may be mostly previously
exhaled gas. Any SGDB circuit can be used to greater or lesser
benefit, according to its characteristics. We provide below
detailed descriptions of several alternate configurations and
outline their particular advantages and drawbacks with respect to
measuring {dot over (Q)} and related parameters outlined above.
[0184] 2) a gas sample line (204.1) leading to a gas analyzer (204)
that monitors the concentration of gases, for example CO.sub.2,
O.sub.2, at the patient-circuit interface and outputs preferably an
electric signal corresponding to the concentrations (204.2) (for
example if the gases of interest are O.sub.2 and CO.sub.2, the
"#17500 O.sub.2 and CO.sub.2 analyzer set" (Vacumed, Ventura
Calif., USA)) [0185] 3) a precise gas flow controller (200),
preferably one that can control the flow of one or more pressurized
gases (such as oxygen, air, CO.sub.2) singly or in combination, and
that can be set manually or via an automated system such as via
machine intelligence (for example, the Voltek gas flow controller
by Voltek Enterprises, Toronto, Canada); [0186] 4) a source of FGS
(201), preferably containing O.sub.2 and/or air with or without
CO.sub.2; [0187] 5) means (205) to identify phase of breathing, for
example using electronic pressure sensors with tubing to sample
pressures at the patient-circuit interface (205.1) or in other
locations in the circuit and generating electrical signal
corresponding to the sensed pressures. Such means will provide
electrical signal (205.2). Phase of breathing can also be
determined from analysis of gas sensor output by machine
intelligence. [0188] 6) a computer or machine intelligence (207)
which records, stores, analyzes signals from gas analyzer (204) and
pressure transducer (if present), contains a predetermined set of
instructions regarding the analysis of data such as calculation of
{dot over (Q)} and physiologic parameters, determination of phase
of respiration, display of information on a computer screen, and
control of gas flow controller (200) including the timing, sequence
and flow of gas. [0189] 7) wherein said device may be utilized for
non-invasive measurement and determination of {dot over (Q)} and
other parameters such as {dot over (V)}A, {dot over (V)}CO.sub.2,
PvCO.sub.2-oxy, true PvCO.sub.2, PaCO.sub.2, pulmonary shunt, and
anatomical dead space
DETAILED DESCRIPTION OF BREATHING CIRCUITS
[0190] FIG. 5 shows a breathing circuit which provides sequential
delivery of the FGS followed by the SGS when {dot over (V)}E
exceeds FGSF, with the manifold containing the valves and the FGS
reservoir bag and the expiratory gas reservoir bag remote from the
patient. This improvement reduces the bulk of the patient manifold,
and eliminates the possibility of the SGS mixing with the FGS due
to vigorous exhalation.
[0191] Referring to FIG. 5, Patient (38) breathes via a Y connector
(40). Valve (31) is an inspiratory valve and valve (33) is an
expiratory valve. Valve (35) is a bypass valve in the bypass limb
(34) that bypasses the expiratory valve (33) and has an opening
pressure greater than inspiratory valve (31). Valves (35, 33) may
be close to or distal from the patient manifold as desired, as long
as they are on the expiratory limb (39). However, in the preferred
embodiment, they are distal to the patient to reduce the bulk of
the patient manifold. Inspiratory valve (31) may be close to, or
distal from, the patient manifold as desired, as long as it is on
the inspiratory limb (32). In the preferred embodiment, it is
distal to the patient as well. FGS enters the circuit via port
(30).
[0192] Function:
[0193] 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). FGS enters via port (30)
and fills FGS reservoir (37). During inhalation, inhalation valve
(31) opens and FGS from the FGS reservoir (37) and FGS port (30)
enter the inspiratory limb (32) and are delivered to the patient.
If FGSF is less than {dot over (V)}E, the FGS 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 SGS.
[0194] Thus when FGSF is less than {dot over (V)}E, the subject
inhales FGS, then SGS, and no contamination of FGS occurs.
[0195] FIG. 3B shows an alternate embodiment of the circuit
illustrated in FIG. 5 where the passive expiratory valve (33) and
expiratory bypass limb (34), and expiratory limb bypass valve (35)
are replaced with a control valve that is triggered by the collapse
of the inspiratory reservoir. Referring to FIG. 3B, a control valve
(401) is placed in the expiratory limb (16) anywhere along its
length between the patient port (10) and the expiratory reservoir
bag (18). When the patient's VE exceeds the FGSF during inspiration
the reservoir bag (20) collapses. This is detected by pressure
sensing means (405) through port (406) as an acute reduction in
pressure. Pressure sensing means (405) could be an electronic
pressure transducer capable of detecting changes 2 cm H.sub.2O
pressure, for example. Immediately afterwards, valve (401) is then
opened by control means (403), which could be an electronic signal
for activating a solenoid valve, for example, leading to
depressurization and collapse of a balloon valve, as is known to
those skilled in the art, resulting in SGS is being inhaled for the
balance of inhalation. During exhalation, patient exhales through
expiratory tube (16) past valve (401) into the SGS reservoir (18).
At end of exhalation, as detected by pressure sensing means (405)
as a reduction of pressure, valve (401) is closed by control means
(403), which could be an electronic signal for toggling a solenoid
valve, for example, leading to pressurization and inflation of a
balloon valve, as is known to those skilled in the art.
[0196] While the circuits of FIG. 5 and FIG. 3B present the
advantages over the Fisher circuit of reducing the bulk of the
patient manifold, and eliminating the possibility of the SGS mixing
with the FGS due to vigorous exhalation, they still have the
following drawback: When FGS reservoir (20, 37) is emptied and the
patient is breathing SGS for the balance of an inspiration, the
circuit does not deliver SGS alone but a mixture of SGS and FGS.
The FGS continues to flow into the circuit and is drawn by
inhalation past one-way inspiratory valve (31, 3) and allows FGS
gas to be inhaled from the inspiratory limb (32,14). To optimize
the generation of data required to measure of cardiac output, it is
necessary to redirect the FGS into the FGS reservoir (37, 20) for
the balance of inhalation after the initial collapse of the FGS
reservoir. This would prevent mixing of FGS with SGS during the
period of inhalation where the patient breathes SGS. This
limitation of circuits illustrated in FIGS. 5 and 3B with respect
to measuring cardiac output are shared with the Fisher circuit.
[0197] FIG. 3D shows an improved circuit that prevents
contamination of the SGS by FGS when SGS is being delivered to the
patient. Referring to FIG. 3D, FGS control valve (400) is added to
the inspiratory limb (14) at some point between the FGS port (12)
and the inspiratory valve (11). Pop-off valve (425) is connected to
the inspiratory limb on the side of the FGS control valve (400)
that is proximal to the inspiratory reservoir bag (425). 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. FGS 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, FGS entering from the port
(12) and the FGS reservoir (20) passes through the inspiratory
valve (11) and enters the patient. If the FGSF is less than VE, the
FGS reservoir bag (20) collapses, as detected by pressure sensing
means (405) connected to pressure sensing port (406). FGS control
valve (400) is closed via valve control means (403), and valve (17)
in the bypass limb (13) opens, directing previously exhaled gas to
the patient. When the FGS control valve (400) is closed, any FGSF
entering the circuit during the balance of inspiration is directed
only to the FGS reservoir bag (20) and not to the patient, who is
receiving SGS for the balance of inspiration. FGS control valve
(400) may be re-opened any time from the beginning of expiration to
just before the next inspiration. FGS control valve (400) may be
any type of valve, and is preferably an active valve such as a
balloon valve, known to those skilled in the art, that can be
controlled by automated means. The pop-off valve (425) opens when
the reservoir bag (20) is full to prevent the reservoir bag (20)
from overfilling.
[0198] The circuit illustrated in FIG. 5A is similar to that in
FIG. 5 but has the addition of a FGS control valve (400), together
with pressure sensing means (405) and port (406), and valve control
means (403), added to the inspiratory limb of the circuit (32)
distal to the one-way inspiratory valve (31) and proximal to the
FGS inflow port (30). Similarly, a FGS control valve, together with
pressure sensing means and port, and valve control means, may be
added to the inspiratory limb (14) of the circuit illustrated in
FIG. 3B positioned distal to the one-way inspiratory valve (31) and
proximal to the FGS inflow port (12) to achieve the same result,
namely, prevention of contamination of SGS by FGS when {dot over
(V)}E exceeds FGSF and the FGSF reservoir bag is emptied.
[0199] We present two additional circuits that are configured by
adding FGS control valve (400) together with pressure sensing means
(405) and port (406), and valve control means (403), to the Fisher
circuit and the circuit illustrated in FIG. 5 and removing the
passive one way inspiratory valve (11, 31), as shown in FIGS. 3E
and 5B respectively. These circuits function identically to those
illustrated in FIGS. 3D and 5A with respect to complete separation
of FGS and SGS during inhalation. In such a circuit, during
inspiration, FGS control valve (400) is open until FGSF reservoir
bag (20, 37) is emptied, then it is closed so that any additional
FGSF entering the circuit during the balance of inspiration is
directed only to the reservoir bag (20) and not to the patient. As
the patient continues to inspire, bypass valve (17, 35) opens
allowing the patient to inhale SGS for the balance of
inspiration.
[0200] Another embodiment of each of the circuits whereby the
valves can be remote from the patient without loss of sequential
delivery of FGS and SGS, such as those illustrated in FIGS. 5, 3B,
5A, 5B, 3C, 4B, is the replacement of separate inspiratory limbs
and expiratory limbs with co-axially arranged inspiratory and
expiratory limbs as shown in FIG. 6. FIG. 6A shows the preferred
embodiment of the invention: The circuit valves are configured as
in the circuit illustrated in FIG. 5A with the improvement of
co-axially arranged inspiratory (59) and expiratory (51) limbs. The
limbs (51, 59) are co-axial so that one limb is contained within
the other for some length of tubing, with the limbs separating at
some point along its length, such that the expiratory limb (51)
leads to the exhaled gas reservoir (54) and the inspiratory limb
(59) leads to the FGS reservoir (56). This has two important
advantages over the circuit of FIG. 5: [0201] 1. A single tube is
connected to the patient interface making it easier to manage sick
patients [0202] 2. The heat contained in the expiratory limb (51)
warms the FGS entering through the inspiratory limb (59). [0203] 3.
If the inner tube is of a material that allows moisture to pass
through it but not gas, such as Nation, will promote moisture
exchange as well, so that FGS will become slightly moisturized and
more comfortable for the patient to breathe if the SGS is moist. It
should be understood that co-axial tubing may be used with any of
the SGDB circuits described herein.
DESCRIPTION OF A PREFERRED EMBODIMENT
[0204] Referring to FIG. 6A, Patient port (50) opens directly to
the inspiratory limb (59) and expiratory limb (51) without a Y
connector, where the limbs are arranged co-axially. Valve (31) is
an inspiratory valve and valve (33) is an expiratory valve. Valve
(35) is a bypass valve in the bypass limb (34) that bypasses the
expiratory valve (33) and has an opening pressure greater than
inspiratory valve (31). Valves (35, 33) are preferably distal from
the patient on the expiratory limb (51) to reduce the bulk of the
patient interface. Inspiratory valve (31) is also preferably distal
from, the patient on the inspiratory limb (59). FGS enters the
circuit via port (30). FGS control valve (400) is on the
inspiratory limb (59) between port (30) and inspiratory valve (31).
FGS reservoir bag (37) is connected to inspiratory limb (59) distal
to the patient, past port (37). SGS reservoir bag (36) is distal to
the patient on the expiratory limb (51) past expiratory valve (33)
and bypass valve (35). Excess expiratory gas vents to the
atmosphere via port (41). Pressure sensing means (405) is connected
to pressure sensing port (406) which is connected to the patient
port (50), and valve control means (403). Pressure sensing port
(406) may be connected to the co-axial inspiratory (59) and
expiratory limb arrangement (51) anywhere along its length between
the inspiratory valve (31) and the patient port (50) or between the
expiratory valve (33) and the patient. Pop-off valve (425) is
connected to the inspiratory limb on the side of the FGS control
valve (400) that is proximal to the inspiratory reservoir bag
(425).
[0205] Function:
[0206] During exhalation, increased pressure in the circuit closes
inspiratory valve (31) and bypass valve (35). Gas is directed into
the exhalation limb (51), 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). FGS enters via port (30)
and fills FGS reservoir (37). During inhalation, inhalation valve
(31) opens and FGS from the FGS reservoir (37) and FGS port (30)
enter the inspiratory limb (59) and are delivered to the patient.
If FGSF is less than {dot over (V)}E, the FGS 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 SGS. The emptying of FGS reservoir bag (37) is detected by
pressure sensing means (405) such as an electronic pressure
transducer, known to those skilled in the art, connected to
pressure sensing port (406), and FGS control valve (400) such as a
balloon valve known to those skilled in the art, is closed via
valve control means (403) such as access to gas pressure controlled
by an electronically toggled solenoid valve known to those skilled
in the art. When the FGS control valve (400) is closed, any
additional FGSF entering the circuit during the balance of
inspiration is directed only to the FGS reservoir bag (20) and not
to the patient, who is inhaling only SGS for the balance of
inspiration. FGS control valve (400) may be re-opened any time from
the beginning of expiration, as sensed by the reverse of pressure
by the pressure sensing means (405), to just before the next
inspiration, also sensed by pressure changes in the breathing
circuit. Pop-off valve (425) prevents the FGS reservoir bag (20)
from overfilling when FGS exceeds {dot over (V)}E.
[0207] Thus when FGSF is less than VE, the subject inhales FGS,
then SGS, and no contamination of SGS with FGS occurs.
[0208] Use of Circuits for Ventilated Patients
[0209] Any of the SGDB circuits disclosed herein as well as the
Fisher circuit can be used for a patient under controlled
ventilation by enclosing the FGS reservoir (20) and exhaled gas
reservoir (18) within a rigid container (21) with exit ports for
the inspiratory limb of the circuit (24) and expiratory limb of the
circuit (25) and port for attachment to a patient interface of a
ventilator (22) as illustrated in FIG. 4. In FIG. 4, the
inspiratory limb (500) represents the inspiratory limb of any of
the SGDB circuits herein described, and expiratory limb (501)
corresponds to the expiratory limb of any of the SGDB circuits
herein described. The FGS reservoir bag (20) and expiratory gas
reservoir bag (18) are enclosed in a rigid air-tight container such
that the inspiratory limb (500) enters the container via port (24)
and expiratory limb (501) 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).
Detachment from the ventilator allows the circuit to be used with a
spontaneously breathing patient. During the inspiratory phase of
the ventilator, the pressure inside the container (21) rises
putting the contents of the FGS reservoir bag (20) and the
expiratory gas reservoir bag (18) under the same pressure. Since
the opening pressure of the inspiratory valve is less than that of
the bypass valve for circuits using passive bypass valves (for
example those shown in FIGS. 2, 3, 5, 5B, 5A, 3E, and 3D), the FGS
reservoir (20) will be emptied preferentially. When the FGS
reservoir (20) is empty, the pressure in the container (21) and
inside the expiratory gas reservoir (18) will open the bypass valve
(35, 17, 206) and begin emptying exhaled gas reservoir (18)
delivering SGS to the patient. For circuits using an actively
engaged control valve (400) in the inspiratory limb of the circuit,
a valve opening detection means (407) such as an electronic circuit
that is broken by the opening of the valve when the valve is part
of a closed electronic circuit, not shown, detects opening of the
one way valve (35, 17, 206) in the exhalation bypass limb. The FGS
control valve (400) is then closed, directing FGS into the FGS
reservoir bag until the collapse of the FGS reservoir during the
next inspiratory phase.
[0210] During the exhalation phase of the ventilator, the
ventilator's expiratory valve is opened and contents of the
container (21) are opened to atmospheric pressure, allowing the
patient to exhale into the expiratory gas reservoir (18) and the
FGS to flow into the FGS reservoir bag (20). Thus, the FGS and SGS
are inhaled sequentially during inhalation with controlled
ventilation without mixing of FGS with SGS at any time.
[0211] FIG. 4B shows the ventilator configuration described above
as used with the preferred circuit shown in FIG. 6A. This is the
preferred embodiment for ventilated patients.
[0212] The primary difference between the standard anesthetic
circle circuit of the prior art (FIG. 12) and the circuits
disclosed herein is that with the circuits disclosed herein, both a
SGS reservoir (18) and a FGS reservoir (20) are in the rigid box.
With the valve configurations disclosed herein, there will be
sequential delivery of the FGS, then the SGS, when {dot over (V)}E
exceeds the FGSF. This does not occur with the standard anesthetic
circle circuit, even if the CO.sub.2 absorber is removed from the
circuit.
[0213] Circuit for Calculation of Q and Related Physiologic
Parameters while Modifying Second Gas Set
[0214] FIG. 7 shows the preferred circuit for measuring cardiac
output while maintaining the ability to modify the SGS. The circuit
consists of the following components: [0215] 200 patient port
[0216] 201 three-port connector [0217] 202 expiratory limb [0218]
203 expiratory valve [0219] 204 canister on bypass conduit that may
be switched to be empty, contain CO.sub.2 absorbing crystals,
zeolyte, charcoal or similar substance that filters anesthetic
agents, or hopcalite for filtering carbon monoxide [0220] 205
bypass conduit. [0221] 206 one-way bypass valve with opening
pressure slightly greater than that of the inspiratory valve (219)
[0222] 207 SGS reservoir bag [0223] 208 port in rigid container for
entrance of expiratory limb of circuit in an air-tight manner
[0224] 209 exit port for expired gas from expired gas reservoir
[0225] 210 a 2-way manual valve that can be turned so that the gas
in the rigid box (216) is continuous with either the ventilator Y
piece (211) or the manual ventilation assembly consisting of
ventilating bag (212) and APL valve (213) [0226] 211 the ventilator
Y piece [0227] 212 the ventilation bag [0228] 213 APL valve [0229]
214 ventilation port in rigid box (216) [0230] 215 FGS reservoir
[0231] 216 rigid box [0232] 217 port in rigid container for
entrance of inspiratory limb of circuit (220) in an air-tight
manner [0233] 218 FGS inlet port [0234] 219 inspiratory valve
[0235] 220 inspiratory limb [0236] 221 bypass limb proximal to
canister (204) [0237] 400 active FGS Control valve [0238] 403 valve
control means [0239] 407 bypass valve opening sensing means
[0240] Function of the Circuit as an Anesthetic Circuit:
[0241] For spontaneous ventilation, 3-way valve (210) is open
between rigid container (216) and manual ventilation assembly
consisting of ventilation bag (212) and APL valve (213). When the
patient exhales, 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). FGS enters via port (218)
and fills the FGS reservoir (215). During inhalation, inhalation
valve (219) opens and FGS from the FGS reservoir (215) and FGS port
(218) enter the inspiratory limb (220) and are delivered to
patient. If FGSF is less than VE, the FGS 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 exceeded, it opens and gas
from the expiratory gas reservoir (207) passes through the canister
(204) into the rebreathing limb (221) and makes up the balance of
the breath with SGS. The opening of bypass valve (206) is detected
by valve opening sensing means (407) signals are sent to close FGS
control valve (400) by activating valve control means (403). When
the FGS control valve (400) is closed, any additional FGSF entering
the circuit during the balance of inspiration is directed only to
the FGS reservoir bag (215) and not to the patient. When valve
(400) is closed patient receives only SGS for the balance of
inspiration. FGS control valve (400) may be re-opened any time from
the beginning of expiration to just before the next inspiration.
Phase of ventilation is sensed by sensor (407).
[0242] For the purposes of functioning as an anesthetic delivery
circuit, part of the FGS entering the circuit would be the
anesthetic vapor, for example Desflurane, and the canister (204)
would contain CO.sub.2 absorbent material. The SGS passes through
the canister (204) but still contains expired O.sub.2 and
anesthetic, which can both be safely rebreathed by the patient. In
this respect, the circuit in FIG. 7 functions like a circle
anesthetic circuit in which the FGSF containing O.sub.2 and
anesthetic can be reduced to match the consumption or absorption by
the patient. However, by bypassing the canister (204), the circuit
can be used for measuring cardiac output.
[0243] If the canister (204) is filled with hopcalite it can be
used to remove carbon monoxide from the patient, since the SGS
still contains expired O.sub.2 and CO.sub.2. If the canister (204)
is filled with zeolite it can be used to remove volatile agents
such as anesthetics from the patient.
[0244] Advantages of circuit over previous art: [0245] 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. [0246] 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 pneumotach (or a
spirometer if the patient is breathing spontaneously) can be placed
at port (214) and thus remote from the patient. [0247] 3) Sasano
(Anesth Analg 2001; 93:1188-1191) taught a circuit that can be used
to accelerate the elimination of anesthesia. However that circuit
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, Sasano 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 the
apparatus and method disclosed herein, there is no requirement for
an additional external source of gas or demand regulator; [0248] 4)
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. [0249] 5)
Circle circuits cannot deliver FGS and then SGS sequentially. Such
control is required to make physiological measurements such as
cardiac output during anesthesia.
[0250] With the circuit of FIG. 7, if the canister (204) is
bypassed, the circuit becomes the equivalent of the one described
in FIG. 5 with the addition of the ventilator apparatus shown in
FIG. 4. With the circuit of FIG. 7, box (216) could be opened to
atmosphere instead of connected to a ventilator, and the circuit
could be used with spontaneously breathing patients for measuring
cardiac output while modifying SGS.
[0251] It should be recognized to those skilled in the art that
various embodiments of the invention disclosed in this patent
application are possible without departing from the scope
including, but not limited to: [0252] a) using multiple inspiratory
and expiratory limbs in combination provided that: [0253] i) the
inspiratory and expiratory limbs are kept separate except at a
single point prior to reaching the patient where they are joined
[0254] ii) each limb has the corresponding valves as in the
arrangement above, and [0255] iii) the valves have the same
relative pressures so as to keep the inspired gas delivery
sequential as discussed above. [0256] b) using active valves, for
example electronic, solenoid, or balloon valves, instead of passive
valves, provided said valves are capable of occluding the limbs,
and means is provided for triggering and controlling said active
valves. The advantage of active valves is more precise control. The
disadvantage is that they are more costly. [0257] c) replacing
reservoir bags with extended tubes or other means for holding gases
[0258] d) surrounding valves in exhalation limb and/or in the
inspiratory limb of circuit with the exhaled gas reservoir causing
them to be surrounded by warm exhaled air and prevent freezing and
sticking of valves in cold environments. [0259] e) Changing the
composition of FGS and SGS to change alveolar concentrations of
gases other than CO.sub.2, for example O.sub.2. By analogy to CO2,
with respect to O.sub.2: alveolar PO.sub.2 is determined by FGS
flow and the PO.sub.2 of FGS. When PO.sub.2 of SGS is the same as
the PO.sub.2 in the alveoli, inhaling SGS does not change flux of
O.sub.2 in the alveoli. Therefore, those skilled in the art can
arrange the partial pressure of component gases in FGS and SGS and
the flows of FGS such that they can achieve any alveolar
concentration of component gases independent of {dot over (V)}E, as
long as {dot over (V)}E exceeds sufficiently flow of FGS.
[0260] 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.
[0261] To clarify the function of the automated cardiac output
device, we will contrast it to a standard anaesthetic machine which
has the same configuration of listed components.
[0262] 1) The preferred SGDB circuits we describe differ from any
anaesthetic circuit. The SGDB circuit first provides the FGS, then
the SGS. This allows the circuit to compensate for changes in
CO.sub.2 elimination on any particular breath. For example, during
a small breath, the unused FGS remains in the FGS reservoir and is
available to provide the exact additional VA for each gas in the
set when a larger breath is taken or frequency of breathing
increases subsequently. As a result, changes in VCO.sub.2 can be
instituted independent of breathing pattern.
[0263] 2) Anesthetic machines do not automatically alter the fresh
gas flows. Fresh gas flows are manually controlled by the
anesthesiologist.
[0264] 3) Anesthetic machines do not calculate VA and cannot
calculate VCO.sub.2, and Q.
[0265] 4) Anesthetic machines cannot generate the data required to
make the calculations for Q and its associated parameters because
the circuit is inappropriate and the gas flows are not configured
to be controlled by a computer.
[0266] 5) The flowmeters on commonly used anesthetic machines are
too imprecise and inaccurate to perform these tests and
calculations. There is no need for such precision and accuracy of
flow for routine clinical anesthetic care.
[0267] 9.0 Method of generating data required to make calculations
of {dot over (Q)} and related physiologic parameters (see FIG.
11):
[0268] Cardiac Output can be measured in several ways according to
the methods and apparatus disclosed herein. These include:
[0269] 9.1 Set-Up Phase
[0270] 9.1.1 Set Flow of FGS>{dot over (V)}E
[0271] 9.1.2 Access default values
[0272] 9.1.3 Check pressure sensor or PCO.sub.2 sensor during
inhalation. If fresh gas reservoir collapsed or CO.sub.2 is
detected during inhalation, increase FGS flow until the reservoir
until reservoir does not collapse fully and no CO.sub.2 is detected
during inhalation
[0273] 9.1.4 Identify PETCO.sub.2 from the CO.sub.2 gas analyzer
[00266]9.2 Find {dot over (V)}A via one of two methods:
[0274] 9.2.1 Calculate {dot over (V)}A by inducing two reductions
in FGS flow below {dot over (V)}A without first identifying {dot
over (V)}A by following the following steps:
[0275] 9.2.1.1 Calculate a preliminary minimum {dot over (V)}A for
the subject based on body weight, temperature, sex and other
parameters known to those skilled in the art.
[0276] 9.2.1.2 Provide luxuriant FGS flow greater than the
patient's resting {dot over (V)}E until steady state PETCO.sub.2.
is reached
[0277] 9.2.1.3 Impose a VA by setting FGS Flow below assumed {dot
over (V)}A, to {dot over (V)}A.sup.x preferably just below the
calculated preliminary {dot over (V)}A, for a time less than or
equal to a recirculation time, and measure PETCO.sub.2.sup.x, the
end tidal CO.sub.2 concentration during equilibrium if an
equilibrium end tidal value is reached within a recirculation time,
otherwise it is the equilibrium value of end tidal CO.sub.2 as
extrapolated from the exponential rise in end tidal CO.sub.2 values
within the recirculation time.
[0278] 9.2.1.4 Set FGS flow above V.sub.E until steady state
PETCO.sub.2 is reached as identified by a less than a threshold
change in PETCO.sub.2 over a designated time period. The actual
thresholds and time periods are user defined according to the
circumstances of the test and can be determined by those skilled in
the art. [0279] 9.2.1.5 Impose a {dot over (V)}A by setting FGS
Flow below assumed {dot over (V)}A, to VA.sup.y where {dot over
(V)}A.sup.y is less than calculated preliminary minimum {dot over
(V)}A and not equal to {dot over (V)}A.sup.x, for a time
approximately equal to a recirculation time, about 30s at rest.
Measure PETCO.sub.2.sup.x, the end tidal CO.sub.2 concentration
during equilibrium if an equilibrium end tidal value is reached
within a recirculation time, otherwise it is the equilibrium value
of end tidal CO.sub.2 as extrapolated from the exponential rise in
end tidal CO.sub.2 values within the recirculation time. [0280]
9.2.1.6 On a graph of PETCO.sub.2 vs FGS flow, plot the points
(PETCO.sub.2.sup.y, {dot over (V)}A.sup.y) and (PETCO.sub.2.sup.x,
{dot over (V)}A.sup.x). Extrapolate the line formed by connecting
these two point to intersect a horizontal line at
PETCO.sub.2=resting PETCO.sub.2. The FGS flow at the intersection
point is determined to be {dot over (V)}A. [0281] 9.2.2 Progressive
Reduction of FGS flow method of finding {dot over (V)}A: [0282]
9.2.2.1 Use FGS that preferably has no CO.sub.2 [0283] 9.2.2.2 Wait
for steady state as indicated by less than a threshold change in
PETCO.sub.2 over a designated time period. The actual thresholds
and time periods are user defined according to the circumstances of
the test and can be determined by those skilled in the art. [0284]
9.2.2.3 When in steady state, reduce FGS flow by a small fixed
flow, for example 0.1 L/min, preferably at regular intervals of
time or after each breath. Alternate flow reduction rates could be
used, and the reduction need not be linear in time. [0285] 9.2.2.4
When PETCO.sub.2 begins to rise above a threshold value which is
approximately the mean steady state PETCO.sub.2, continue the
reduction in the FGS flow for a time approximately equal to one
recirculation time. [0286] 9.2.2.5 After approximately one
recirculation time, usually about 30 s, raise FGS flow above
resting {dot over (V)}E. A relation of PETCO.sub.2 vs FGS flow is
calculated and two lines of best fit are calculated, one for the
set of steady state PETCO.sub.2 values, and one for the set of
raised PETCO.sub.2 values above the mean of the steady state
values. The FGS flow corresponding to the intersection of said
lines corresponds to {dot over (V)}A. FIG. 13 illustrates that
progressive reduction of SGF (labelled "FGF" in the figure) results
in a distinct inflection point when either PETCO.sub.2 or
PETO.sub.2 is graphed as a function of SGF. We define the SGF
corresponding to this inflection point as equal to {dot over (V)}A.
[0287] 9.2.2.6 These two methods of finding {dot over (V)}A are
physiologically equivalent and one may have some advantages over
the other in particular clinical or research circumstances. The
Progressive Reduction method should be contrasted with the method
for calculating {dot over (V)}A taught by Preiss et al. (Canadian
Patent Application 2346517). In that method, while fresh gas flow
into a sequential gas delivery circuit was reduced stepwise, after
each reduction, the subject was observed for several breaths
looking for an exponential rise in PETCO.sub.2. The Preiss method
requires continued breathing at each fresh gas flow looking for
development of a new steady when fresh gas flow falls below {dot
over (V)}A. This process is very time consuming and is unlikely to
be tolerated by most patients. If, in the attempt to shorten the
time for finding the fresh gas flow below {dot over (V)}A the fresh
gas flow reduction are large, resolution of critical fresh gas flow
is lost. If the steps are small, when the fresh gas flow is just
barely less than {dot over (V)}A, it will be difficult to discern
the small rise in PETCO.sub.2 from the normal variation in
PETCO.sub.2. The progressive breath-by-breath reduction in FGS flow
disclosed herein results in a rapid linear rise in PETCO.sub.2 and
fall in PETO.sub.2, both of which can be used to identify the FGS
flow corresponding to {dot over (V)}A as illustrated in FIG.
13.
[0288] 9.3 Calculations with the Differential Fick Equation
[0289] There are two methods of calculating cardiac output with the
Differential Fick equation. (It is understood that the general
methods are disclosed without the details well known to those
skilled in the art of the multiple standard corrections for
temperature, moisture, barometric pressure and the like): [0290]
9.3.1 Find {dot over (V)}A by the Progressive Reduction of FGS flow
method of finding {dot over (V)}A: [0291] 9.3.1.1 Find {dot over
(V)}A [0292] 9.3.1.2 Set FGS Flow={dot over (V)}A and calculate
{dot over (V)}CO.sub.2 using the equation {dot over
(V)}CO.sub.2={dot over (V)}A.times.FETCO.sub.2. [0293] 9.3.1.3
Impose a transient step change in {dot over (V)}A to {dot over
(V)}A' for a time approximately equal to a recirculation time,
about 30s at rest, by changing FGS flow to a value below {dot over
(V)}A. To fully automate the process, select a {dot over (V)}A'
that will be below the {dot over (V)}A. Calculate {dot over
(V)}CO.sub.2'={dot over (V)}A'.times.FETCO.sub.2'. Where
FETCO.sub.2' is the fractional end tidal CO.sub.2 concentration
during equilibrium if an equilibrium end tidal value is reached
within a recirculation time, otherwise it is the equilibrium value
of end tidal CO.sub.2 as extrapolated from the exponential rise in
end tidal CO.sub.2 values within the recirculation time. [0294]
9.3.1.4 Calculate {dot over (Q)} according to the differential Fick
equation using {dot over (V)}CO.sub.2, {dot over (V)}CO.sub.2', and
CCO.sub.2 and CCO.sub.2' where CCO2 and CCO.sub.2' are the contents
of CO.sub.2 of end capillary blood as calculated from PETCO.sub.2,
and PETCO.sub.2' using known relationships between PETCO.sub.2, and
other characteristics related to the blood such as hemoglobin
concentration, temperature oxygen partial pressure and other
parameters that are accessible or can be used as default values by
those skilled in the art. [0295] 9.3.1.5 Calculate {dot over (Q)}
according to the differential Fick equation using {dot over
(V)}CO.sub.2 and PETCO.sub.2 data from steady state phase and step
change phase and the PaCO.sub.2 from the Kim Rahn Farhi method.
This allows the identification of the PETCO.sub.2--PaC02 gradient
without an arterial blood sample. [0296] 9.3.2 Generate required
data by inducing two reductions in FGS flow below {dot over (V)}A
without first identifying {dot over (V)}A by following the
following steps: [0297] 9.3.2.1 Calculate a preliminary minimum
{dot over (V)}A for the subject based on body weight, temperature,
sex and other parameters known to those skilled in the art. [0298]
9.3.2.2 Provide luxuriant FGS flow greater than the patient's
resting {dot over (V)}E until steady state PETCO.sub.2 is reached
[0299] 9.3.2.3 Impose a {dot over (V)}A and hence a {dot over
(V)}CO.sub.2 by setting FGS Flow below preliminary calculated {dot
over (V)}A, to {dot over (V)}A.sup.x preferably just below the
preliminarily calculated {dot over (V)}A, for a time less than or
equal to a recirculation time, and calculate {dot over
(V)}CO.sub.2.sup.x using the equation {dot over
(V)}CO.sub.2.sup.x={dot over (V)}A.sup.x.times.FETCO.sub.2.sup.x
where FETCO.sub.2 is the fractional end tidal CO.sub.2
concentration during equilibrium if an equilibrium end tidal value
is reached within a recirculation time, otherwise it is the
equilibrium value of end tidal CO.sub.2 as extrapolated from the
exponential rise in end tidal CO.sub.2 values within the
recirculation time. [0300] 9.3.2.4 Set FGS flow above V.sub.E until
steady state PETCO.sub.2 is reached as identified by a less than a
threshold change in PETCO.sub.2 over a designated time period. The
actual thresholds and time periods are user defined according to
the circumstances of the test and can be determined by those
skilled in the art. [0301] 9.3.2.5 Impose a transient step change
in {dot over (V)}A to {dot over (V)}A.sup.y where {dot over
(V)}A.sup.y is less than calculated {dot over (V)}A and not equal
to {dot over (V)}A.sup.x, for a time approximately equal to a
recirculation time, about 30 s at rest. Calculate {dot over
(V)}CO.sub.2.sup.y={dot over (V)}A.sup.y.times.FETCO.sub.2.sup.y.
FETCO.sub.2.sup.y is the end tidal CO.sub.2 concentration during
equilibrium if an equilibrium end tidal value is reached within a
recirculation time, otherwise it is the equilibrium value of end
tidal CO.sub.2 as extrapolated from the exponential rise in end
tidal CO.sub.2 values within the recirculation time. [0302] 9.3.2.6
Calculate {dot over (Q)} according to the differential Fick
equation using {dot over (V)}CO.sub.2.sup.x, {dot over
(V)}CO.sub.2.sup.y, and and CCO.sub.2.sup.x and CCO.sub.2.sup.y
where CCO.sub.2.sup.x and CCO.sub.2.sup.y are the contents of
CO.sub.2 of end capillary blood as calculated from
PETCO.sub.2.sup.x, and PETCO.sub.2.sup.y using known relationships
between PETCO.sub.2, and other characteristics related to the blood
such as hemoglobin concentration, temperature oxygen partial
pressure and other parameters that are accessible or can be used as
default values by those skilled in the art. [0303] 9.3.2.7
Calculate {dot over (Q)} according to the differential Fick
equation using {dot over (V)}CO.sub.2 and PETCO.sub.2 data from
steady state phase and step change phase and the PaCO.sub.2 from
the Kim Rahn Farhi method to identify the PETCO.sub.2--PaCO.sub.2
gradient. This allows the identification of the
PETCO.sub.2--PaCO.sub.2 gradient without an arterial blood
sample.
[0304] Difference between this method and previous methods to
perform the differential Fick: [0305] (a) With the new method, the
decrease in {dot over (V)}CO.sub.2 is performed by reducing the FGF
to a SGDB circuit as opposed to insertion of a deadspace at the
patient-circuit interface. As a result, if the subject increases
his breathing rate or breath size, there is no change in {dot over
(V)}CO.sub.2 and the calculations via the differential Fick
equation are not affected. [0306] (b) The VCO.sub.2 is known using
the {dot over (V)}A (identified by one of the new or the previously
disclosed method) and the PETCO.sub.2, two robust and highly
reliable measures. This is unlike the need for a pneumotachymeter
and the error-prone breath-by-breath analysis of {dot over
(V)}CO.sub.2 required by previous art. [0307] (c) {dot over (V)}A
is not identified with the previous differential Fick methods.
[0308] (d) The PETCO.sub.2 to PaCO.sub.2 gradient is calculated
from two independently derived values in the same subject. In the
previous art, this gradient is calculated from empirical formulae
derived from averaged values and do not necessarily apply to the
subject.
[0309] Therefore our method provides more accurate values for {dot
over (V)}CO.sub.2, {dot over (V)}, CO.sub.2' and PaCO.sub.2 than
the previous art.
[0310] 9.4 Kim-Rahn-Farhi [0311] 9.4.1 A period of reduced FGS flow
simulates complete or partial breath holding. The PETCO.sub.2 of
each breath is equivalent to a sequential alveolar sample in the
KRF prolonged exhalation method. The substitution of sequential
PETCO.sub.2 values for sequential samples from a single exhalation
is used to calculate true PvCO.sub.2, PvCO.sub.2-oxy, PaCO.sub.2
and hemoglobin O.sub.2 saturation in mixed venous blood SvO.sub.2
using the Kim Rahn Farhi method. [0312] 9.4.2 {dot over (Q)} can be
calculated using the Fick approach where the PvCO.sub.2-oxy and
PaCO.sub.2 as calculated by the Kim Rahn Farhi method are used to
calculate the respective CO.sub.2 contents using methods well known
to those skilled in the art, and the {dot over (V)}CO.sub.2 is as
calculated from {dot over (V)}A and FETCO.sub.2 as derived in the
sequence of steps described above. [0313] 9.4.3 Mixed venous
O.sub.2 hemoglobin saturation are calculated as follows. {dot over
(V)}O.sub.2 is calculated from {dot over (V)}O.sub.2={dot over
(V)}A.times.(FIO.sub.2-FETO.sub.2) where FIO.sub.2 and FETO.sub.2
are the fractional concentration of inspired and end tidal O.sub.2
respectively. Using {dot over (V)}O.sub.2, {dot over (Q)} as
calculated by Differential Fick or Kim Rahn Farhi or Fisher Method,
end capillary O.sub.2 oxygen content (assuming end capillary blood
is fully saturated with oxygen), Mixed venous O.sub.2 saturation
can be calculated from the standard Fick equation. [0314] 9.4.4
Information regarding the arterial O.sub.2 hemoglobin saturation
(SaO.sub.2) (as read from a non-invasive commonly available pulse
oximeter that makes the measurement by shining an infrared light
through a finger), and the SvO.sub.2 can be used to calculate the
fraction of shunted blood ({dot over (Q)}s) (assuming fully
oxygenated blood in the end pulmonary capillary) by using the
following equation
[0314] Q . s = ( SP O 2 ) Q . t - ( Sa O 2 ) Q . p S v _ O 2
##EQU00006##
[0315] Our method of performing the Kim Rahn Farhi is an
improvement over the previous art in that [0316] (a) Test is
performed simultaneously with a test for differential Fick in
spontaneously breathing subject. [0317] (b) Data are pooled with
the test as outlined above so calculation of CO.sub.2, is
simultaneous to the other calculations. In the previous art, the
{dot over (V)}CO.sub.2, calculation cannot be done during a breath
hold or simulated breath hold by rebreathing. [0318] (c) {dot over
(V)}CO.sub.2, measurement does not require a pneumotachymeter which
is expensive, cumbersome and error-prone. In the previous art, {dot
over (V)}CO.sub.2, required for the calculation of {dot over (Q)}
required additional apparatus such as pneumatchymeter or gas
collection and volume measuring apparatus.
[0319] 9.5 Fisher E-I Test [0320] 9.5.1 Calculate {dot over (V)}A
from the calibration phase, set FGS flow={dot over (V)}A. [0321]
9.5.2 With FGS Flow at {dot over (V)}A, the PCO.sub.2 in the FGS is
changed to any value and held at that value for a time
approximately equal to a recirculation time, about 30 s at rest.
[0322] 9.5.3 PvCO.sub.2-oxy is calculated using the
PETCO.sub.2--PICO.sub.2 method described by Fisher.
[0323] Our method of the Fisher E-I test is an improvement over the
previous art in that the effect of change in breath size on the
equilibrium value of PETCO.sub.2 is minimized by the SGDB circuit
such that a larger breath delivers physiologically neutral
previously expired gas instead of additional test gas.
[0324] 10.0 Method of Finding VE Using Progressive Reduction of FGS
Flow:
[0325] 10.1 Use FGS that preferably has no CO.sub.2
[0326] 10.2 Wait for steady state as indicated by less than a
threshold change in PETCO.sub.2 over a designated time period. The
actual thresholds and time periods are user defined according to
the circumstances of the test and can be determined by those
skilled in the art.
[0327] 10.3 When in steady state, reduce FGS flow by a small fixed
flow, for example 0.1 L/min, preferably at regular intervals of
time or after each breath. Alternate flow reduction rates could be
used, and the reduction need not be linear in time.
[0328] 10.4 Using a means for measuring pressure within the FGS
reservoir in the breathing circuit, for example a pressure
transducer, monitor when the FGS reservoir bag first collapses.
{dot over (V)}E is the FGS flow rate when the reservoir bag first
collapses.
[0329] 11.0 Method for Measuring Anatomical Dead Space
[0330] 11.1 Measure {dot over (V)}E and {dot over (V)}A using any
of the methods disclosed above
[0331] 11.2 Measure the respiratory rate, preferably using the
apparatus for cardiac output disclosed herein.
[0332] 11.3 Calculate Anatomical Dead Space {dot over (V)}DAN=({dot
over (V)}E-{dot over (V)}A)/respiratory rate
[0333] 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.
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