U.S. patent application number 10/361437 was filed with the patent office on 2003-09-04 for airway-based cardiac output monitor and methods for using same.
Invention is credited to Mault, James R..
Application Number | 20030167016 10/361437 |
Document ID | / |
Family ID | 22459833 |
Filed Date | 2003-09-04 |
United States Patent
Application |
20030167016 |
Kind Code |
A1 |
Mault, James R. |
September 4, 2003 |
Airway-based cardiac output monitor and methods for using same
Abstract
A respiratory gas analyzer for measuring the cardiac output of a
subject includes a flow meter and an oxygen sensor interconnected
with one another between a mouthpiece and a source of respiratory
gases which may be a controlled source or the atmosphere. An
oximeter provides measurements of the oxygen saturation of the
subject. A computer connected to receive the signals from the flow
meter, oxygen sensor, and oximeter can then calculate the subject's
cardiac output.
Inventors: |
Mault, James R.; (Evergreen,
CO) |
Correspondence
Address: |
GIFFORD, KRASS, GROH, SPRINKLE
ANDERSON & CITKOWSKI, PC
280 N OLD WOODARD AVE
SUITE 400
BIRMINGHAM
MI
48009
US
|
Family ID: |
22459833 |
Appl. No.: |
10/361437 |
Filed: |
February 10, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10361437 |
Feb 10, 2003 |
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09674899 |
Nov 7, 2000 |
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6517496 |
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09674899 |
Nov 7, 2000 |
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PCT/US00/12745 |
May 10, 2000 |
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60133685 |
May 10, 1999 |
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Current U.S.
Class: |
600/529 |
Current CPC
Class: |
A61B 5/029 20130101;
A61B 5/083 20130101; A61B 5/087 20130101; A61B 5/1455 20130101 |
Class at
Publication: |
600/529 |
International
Class: |
A61B 005/08 |
Claims
1. An apparatus for determining a cardiac output of a subject, the
apparatus comprising: a respiratory analyzer, having a flow path
through which respiratory gases pass, a flow rate sensor, and an
oxygen sensor, the respiratory analyzer providing a flow signal
correlating with a flow rate of respiratory gases through the flow
path, and a respiratory oxygen concentration signal correlating
with an oxygen concentration of respiratory gases; and a
computation unit, wherein said computation unit receives the
respiratory oxygen concentration signal and the flow signal, the
computation unit being operable to determine an oxygen consumption
of the subject, to determine an end-tidal partial pressure of
oxygen of at least one breath of the subject, and to determine the
cardiac output of the subject using the oxygen consumption, the
end-tidal partial pressure of oxygen, and an arterial oxygen
saturation.
2. The apparatus of claim 1, further comprising a pulse oximeter
operable to provide the arterial oxygen saturation.
3. The apparatus of claim 1, wherein the arterial oxygen saturation
is a predetermined value for the subject.
4. The apparatus of claim 1, wherein the computation unit
determines the cardiac output (C.O.) of the subject using a formula
8 C . O . = VO 2 CaO 2 - CvO 2 ,wherein VO.sub.2 represents the
oxygen consumption of the subject, CaO.sub.2 represents an oxygen
content of arterial blood, and CvO.sub.2 represents an oxygen
content of venous blood.
5. The apparatus of claim 4, wherein the computation unit is
operable to determine the oxygen content of arterial blood
(CaO.sub.2) using an equation of the
formCaO.sub.2=A(SaO.sub.2)(Hgb)+B(PaO.sub.2),wherein A and B
represent numerical values, SaO.sub.2 represents the arterial
oxygen saturation, Hgb represents a hemoglobin concentration, and
PaO.sub.2 represents a dissolved arterial oxygen concentration.
6. The apparatus of claim 5, wherein the computation unit uses a
predetermined value for SaO.sub.2 when calculating the oxygen
content of arterial blood.
7. The apparatus of claim 5, wherein the computation unit uses the
oxygen saturation signal and a predetermined value of Hgb when
calculating the oxygen content of arterial blood.
8. The apparatus of claim 4, wherein the computation unit is
operable to determine the oxygen content of venous blood
(CvO.sub.2) using an equation of the
formCvO.sub.2=C(SvO.sub.2)(Hgb)+D(PvO.sub.2),wherein C and D
represent numerical constants, SvO.sub.2 represents a venous oxygen
saturation, Hgb represents a hemoglobin concentration, and
PvO.sub.2 represents a dissolved venous oxygen concentration.
9. The apparatus of claim 8, wherein the computation unit
determines PvO.sub.2 using the end-tidal partial pressure of
oxygen.
10. The apparatus of claim 8, wherein the computation unit
determines SvO.sub.2 from PvO.sub.2 using a predetermined
relationship between SvO.sub.2 and PvO.sub.2.
Description
REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 09/674,899, filed Nov. 7, 2000, which is the
U.S. National Phase of PCT/US00/12745, filed May 10, 2000, which
claims priority from U.S. Provisional Patent Application Serial No.
60/133,685, filed May 10, 1999, the entire content of each being
incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention relates to measurement of cardiac
output of a patient. More specifically, the present invention
relates to an apparatus and method for non-invasive cardiac output
measurement of a subject utilizing a respiratory gas analyzer
employing a flow sensor, an oxygen sensor, and a pulse oximeter
which are interconnected to measure the cardiac output of the
subject.
BACKGROUND OF THE INVENTION
[0003] U.S. Pat. No. 5,836,300 to Applicant discloses a respiratory
gas analyzer for measuring the metabolic activity and the cardiac
output of a subject including a bi-directional flow meter and a
capnometer sensor interconnected by conduits and valving between a
mouthpiece and a source of respiratory gases which can be a
controlled source or the atmosphere. A computer receiving signals
from the flow meter and the capnometer can then calculate the
subject's metabolic activity. When valving is shifted, a portion of
the exhaled gases are stored in the conduit so that upon
inhalation, the subject inhales a substantial portion of rebreathed
gases. The computer can then calculate the patient's cardiac output
as a function of the changes in total carbon dioxide content of the
exhaled gas before and after the valve is shifted from a direct
input to a rebreathed position and the difference in end-tidal
carbon dioxide between the two positions.
[0004] The cardiac output of a patient, that is the volume of blood
ejected from the heart per unit time, is an important measured
parameter in hospitalized patients. Currently, cardiac output is
routinely measured by invasive techniques including thermal
dilution using an indwelling pulmonary artery catheter. This
technique has several disadvantages including the morbidity and
mortality risks of placing an invasive intracardiac catheter, the
infectious disease risks, significant expense and the fact that it
provides an intermittent rather than a continuous measurement. A
noninvasive, reusable cardiac output measurement device would
substantially improve patient care and reduce hospital costs.
[0005] The partial rebreathing technique mentioned above is a known
method for cardiac output measurement. As described in Kapec and
Roy, "The Noninvasive Measurement of Cardiac Output Using Partial
CO.sub.2 Rebreathing," IEEE Transactions on Biomedical Engineering,
Vol. 35, No. 9, September 1988, pp. 653-659, the method utilizes
well known Fick procedures, substituting carbon dioxide for oxygen,
and employing a sufficiently short measurement period such that
venous carbon dioxide levels and cardiac output can be assumed to
remain substantially constant during the measurement.
[0006] U.S. Pat. No. 4,949,724 to Mahutte et al. discloses a method
and apparatus for continuously monitoring cardiac output by
utilizing a modified Fick equation. The Mahutte et al. patent
replaces VO.sub.2 in the Fick equation by VCO.sub.2 divided by a
constant representative of the gas exchange ratio of a patient in
order to eliminate inaccuracies associated with monitoring the rate
of uptake of oxygen.
[0007] In its original form, the Fick method of measuring cardiac
output requires blood gas values for arterial and mixed venous
blood as follows: 1 C . O . = VO 2 CaO 2 - CvO 2
[0008] where C.O. is cardiac output, VO.sub.2 is oxygen
consumption, CaO.sub.2 is the arterial oxygen content, and
C.sub.vO.sub.2 is the venous oxygen content.
[0009] By utilizing a respiratory analyzer with a fast-response
oxygen sensor, the cardiac output can be determined based on the
end-tidal oxygen concentration (EtO.sub.2). End-tidal oxygen
concentration is the lowest value of oxygen concentration in
breath. The end-tidal oxygen concentration approximates the
pulmonary capillary oxygen concentration.
[0010] Alternatively, at different points in time, it is also true
that 2 C . O . = VO 2 ( 1 ) CaO 2 ( 1 ) - CvO 2 ( 1 ) = VO 2 ( 2 )
CaO 2 ( 2 ) - CvO 2 ( 2 ) .
[0011] If the oxygen concentration of the inspired gas is
temporarily increased or decreased, the change in alveolar oxygen
concentration will cause a transient uptake or release of oxygen
across the pulmonary capillaries, thereby resulting in a change in
the measured VO.sub.2 and arterial oxygen content (CaO.sub.2). If
these parameters are measured during an interval of time less than
the circulation time (i.e., less than approximately thirty-fifty
seconds), then the venous oxygen content (C.sub.vO.sub.2) level
remains essentially constant during this period and can be removed
from the equation. Therefore, cardiac output can be determined
based on the equation 3 C . O . = VO 2 CaO 2
[0012] The use of these novel concepts in combination with the
apparatus and method of the present invention therefore allows for
the non-invasive measurement of cardiac output utilizing
measurements of airway gases and arterial oxygen concentrations,
both of which can be done by non-invasive techniques.
SUMMARY OF THE INVENTION
[0013] The present invention is accordingly directed toward an
airway-based respiratory gas analyzer for measuring the cardiac
output of a subject. In a preferred embodiment of the analyzer of
the present invention, the analyzer includes a respiratory
connector operative to be supported in contact with a subject so as
to pass inhaled and exhaled gases as the subject breathes. A flow
meter operatively connected to the respiratory connector generates
electrical signals as a function of the volume of gases which pass
therethrough and, in combination with the signals generated by an
oxygen sensor, allows for the determination of oxygen consumption
(VO.sub.2) by integrating the flow and oxygen concentration signals
over an entire breath. The oxygen sensor can also provide for the
measurement of end-tidal (EtO.sub.2) concentration. An oximeter
provides measurements of the subject's oxygen saturation. A
computation unit receives the output signals from the flow sensor,
oxygen sensor and oximeter and calculates the cardiac output based
on the generated signals.
[0014] An alternative mechanism for performing measurements of the
subject's cardiac output includes the subject placing the
mouthpiece of the analyzer into their mouth and breathing a first
oxygen concentration for a first period of time. Typically, the
source of respiratory gases is atmospheric air. As the subject
breathes, oxygen consumption (VO.sub.2) is determined as the
integral of the flow and oxygen concentration signals over the
entire breath. The oximeter provides a measurement of the subject's
oxygen saturation which is utilized to calculate the subject's
arterial oxygen content. After obtaining the measurement of the
oxygen consumption (VO.sub.2) and arterial oxygen content
(CaO.sub.2) over the first time period, the oxygen blender is
caused to provide an increase or decrease in the airway oxygen
concentration of the subject for a second period of time which is
less than the subject's circulation time. The oxygen consumption
(VO.sub.2) and arterial oxygen content (CaO.sub.2) are measured
over this second time period on a breath-by-breath basis and are
utilized in calculating the subject's cardiac output.
[0015] According to one aspect of the present invention, a
respiratory gas analyzer for measuring cardiac output of a subject,
is provided. The analyzer includes an apparatus for determining a
cardiac output of a subject having a flow path through which
respiratory gases pass, a flow rate sensor, and an oxygen sensor.
The respiratory analyzer provides a flow signal correlating with a
flow rate of respiratory gases through the flow path, and a
respiratory oxygen concentration signal correlating with an oxygen
concentration of respiratory gases. The analyzer also includes a
computation unit that receives the respiratory oxygen concentration
signal and the flow signal, and is operable to determine an oxygen
consumption of the subject, to determine an end-tidal partial
pressure of oxygen of at least one breath of the subject, and to
determine the cardiac output of the subject using the oxygen
consumption, the end-tidal partial pressure of oxygen, and an
arterial oxygen saturation.
[0016] According to one preferred embodiment of the invention
described below, the analyzer is used in a two-measurement
procedure, wherein the computer calculates the cardiac output
(C.O.) of the subject according to the following equation: 4 C . O
. = VO 2 CaO 2
[0017] wherein: .DELTA.VO.sub.2 is the difference in said consumed
oxygen in the two-measurement procedure, and .DELTA.CaO.sub.2 is
the difference in said arterial oxygen in the two-measurement
procedure;
[0018] and wherein: the two-measurement procedure involves:
[0019] (a) a first measurement of said consumed oxygen and said
arterial oxygen during a first time interval, and
[0020] (b) a second measurement, following a change in the oxygen
content of the inhaled air, during a second time interval having a
duration less than the blood circulation time of the subject.
[0021] According to a second described preferred embodiment, the
computer calculates the cardiac output (C.O.) of the subject
computer calculates the cardiac output (C.O.) of the subject
according to the following equation: 5 C . O . = VO 2 CaO 2 - CvO
2
[0022] wherein: VO.sub.2 is the oxygen consumed during a breath;
CaO.sub.2 is the concentration of oxygen in the subject's arterial
blood; and CvO.sub.2 is the concentration of oxygen in the
subject's venous blood, which is assumed to be the same as the
end-tidal oxygen concentration in the exhaled air.
[0023] Other features, advantages and applications of the present
invention will be made apparent from the following detailed
description of the preferred embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] Other advantages and applications of the present invention
will be made apparent by the following detailed description of
preferred embodiments of the invention. The description makes
reference to the accompanying drawings in which:
[0025] FIG. 1 is a schematic representation of a first embodiment
of the present invention;
[0026] FIG. 2 is a schematic representation of a second embodiment
of the present invention; and
[0027] FIG. 3 is a graph of oxygen concentration over time measured
in milliseconds to illustrate oxygen concentration as recorded with
a fast-response oxygen sensor and is also illustrative end-tidal
oxygen concentration.
DETAILED DESCRIPTION OF THE INVENTION
[0028] Referring to FIG. 1, a preferred embodiment of the present
invention includes an airway-based cardiac output analyzer,
generally indicated at 10, having a mouthpiece 12, a flow sensor
14, and a gas sensor 16. The flow sensor 14 and the gas sensor 16
are disposed in fluid communication with one another.
[0029] The mouthpiece 12 is adapted to engage the inner surfaces of
a user's mouth, so as to form the sole passage for flowing
respiratory gases into and out of the mouth. A nose clamp of
conventional construction (not shown) can be employed in connection
with the mouthpiece 12 to assure that all respiratory gas passes
through the mouthpiece 12. In alternative configurations, a mask
that engages the nose as well as the mouth of the user can be
employed or an endotracheal tube could also be utilized.
[0030] The mouthpiece 12 is located adjacent to a bi-directional
volume flow sensor 14. The flow sensor is preferably an ultrasonic
flow meter such as an ultrasonic transit time flow meter such as
that manufactured by NDD Medizintechnik AG, Zurich, Switzerland,
and disclosed in U.S. Pat. Nos. 3,738,169; 4,425,805; 5,419,326;
and 5,645,071. Preferably, the ultrasonic flow meter transmits and
receives ultrasonic pulses along a path which is either parallel to
or has a substantial component in the direction of the flow. The
gas flow acts to advance or retard the flow of pulses so that the
full transit time of the pulses is a function of the flow rate.
Alternatively, the flow sensor 14 can be of the pressure
differential type such as manufactured by Medical Graphics
Corporation, St. Paul, Minn. under the trademark MEDGRAPHICS and of
the general type illustrated in U.S. Pat. No. 5,038,773.
Alternatively, other types of flow transducers such as pneumatics
or spirometers could also be employed. The electrical output of the
bi-directional flow sensor 14 is connected to a computation unit 20
through a conductive line 22.
[0031] The other end of the flow sensor 14 is connected to the gas
sensor 16. The gas sensor 16 is preferably a fast-response (i.e.
50-80 millisecond response time), flow-through type oxygen sensor
and is preferably of the fluorescent quench type as disclosed in
U.S. Pat. Nos. 3,725,658; 5,517,313; and 5,632,958. The preferred
embodiment can employ a sensor manufactured by Sensors for Medicine
and Science, Inc., Germantown, Md. The electrical output of the gas
sensor 16 is connected to the computation unit 20 through a
conductive line 24. The computation unit 20 can include a source
(not shown) for directing exciting radiation to a fluorescent
coating disposed on the oxygen sensor 16 and sensing the resulting
fluorescence intensity which is diminished as a function of the
concentration of oxygen in the gas flowing over its surface to
produce a direct measurement of oxygen concentration. The exciting
radiation and fluorescent signal can be carried to the sensor 16 by
an optical fiber (not shown).
[0032] A pulse oximeter 26 can be utilized to monitor oxygen
saturation by pulse oximetry. The pulse oximeter 26 provides an
output signal which is received by the computation unit 20 which is
indicative of saturation percentage. The output signal of the pulse
oximeter 26 is connected to the computational unit 20 through a
conductive line 28. In a preferred embodiment, the pulse oximeter
is preferably of the type manufactured by Datax-Ohmeda, Louisville,
Colo. Alternatively, for most healthy individuals, the pulse
oximeter 26 can be omitted and the oxygen saturation can be assumed
to be approximately 95-96%.
[0033] Utilizing the Fick equation, in combination with the
airway-based respiratory gas analyzer 10 having the flow sensor 14
and the fast-response oxygen sensor 16, allows for the
determination of a subject's cardiac output by utilizing
measurements of end-tidal oxygen concentration and VO.sub.2. The
airway-based gas analyzer 10 allows for the determination of
end-tidal oxygen concentration (EtO.sub.2) as illustrated in FIG.
3. If one assumes that EtO.sub.2 .quadrature. PvO.sub.2 (dissolved
venous oxygen concentration in the plasma), then using the
PvO.sub.2 and the hemoglobin concentration, the SvO.sub.2 can be
determined based on the oxygen dissociation curve. The pulse
oximeter 26 can be used to obtain oxygen saturation movement so
that based on the Fick equation of 6 C . O . = VO2 CaO 2 - CvO
2
[0034] wherein VO.sub.2 is measured by the airway based respiratory
analyzer, CaO.sub.2 and CvO.sub.2 are determined according to the
equations
CaO.sub.2=[(SaO.sub.2)(Hgb)(1.36)+(PaO.sub.2)(0.0031)] and
CvO.sub.2=[(SvO2)(Hgb)(1.36)+(PvO.sub.2)(0.0031)],
respectively,
[0035] wherein SaO.sub.2 is the oxygen saturation measurement
obtained by pulse oximetry, Hgb is the hemoglobin concentration
(which is entered as a known value or by direct measurement), and
PvO.sub.2 is obtained from the measurement of EtO.sub.2. It is
assumed that EtO.sub.2 approximates PvO.sub.2 and, if the PvO.sub.2
and the hemoglobin concentrations are known, using the oxygen
dissociation curve, SvO.sub.2 can be determined. The pulse oximeter
26 measures SaO.sub.2 (alternatively, SaO.sub.2 and PaO.sub.2 can
be reasonably assumed).
[0036] Referring to FIG. 2, in an alternative embodiment of the
present invention is shown wherein like numerals represent like
elements among the embodiments, a gas blender 18 is disposed
directly adjacent to and in fluid communication with the gas sensor
16. The gas blender 18 is also in fluid communication with the
atmosphere or a source and sink of respiratory gases. The gas
blender 18 can also be connected to a ventilator or source of
calibrated or known gases such as an external oxygen tank 30. The
gas blender 18 is preferably computer controlled and is in
electrical communication with the computation unit 20 through a
conductive line 32. That is, the computation unit 20 can transmit
signals to the gas blender 18 in order to modify, mix, or change
the composition of the inhaled air passing through the cardiac
output analyzer 10 to the subject. In other words, the gas blender
18 can be caused to allow an increase/decrease in the concentration
of airway oxygen for a given or predetermined period of time.
[0037] In a further alternative embodiment, the pulse oximeter 26
can be replaced by a synchronized, side-port sampling oxygen sensor
as is well known in the art. That is, a portion or sample of the
gases flowing through the analyzer is directed via a port to an
oxygen sensor.
[0038] The analyzer 10 can also incorporate an artificial nose
and/or a bacterial filter as described in Applicant's previous
patents or can incorporate a temperature sensor which provides a
signal to the computation unit 20 to adjust the measurements as a
function of breath and external air temperature.
[0039] In operation, in order to non-invasively obtain a
measurement of the cardiac output of a subject, the subject
attaches the pulse oximeter 22 to a suitable portion of their body
such as a finger or earlobe, the subject then places the mouthpiece
12 into their mouth and the oxygen consumption (VO.sub.2) is
determined as the integral of the flow of oxygen concentration
signals over an entire breath. The arterial oxygen concentration is
calculated according to the formula:
CaO.sub.2=(SaO.sub.2)(Hbg)(1.36)+(0.0031)(PaO.sub.2)
[0040] where SaO.sub.2 is the oxygen saturation measurement
obtained by the pulse oximeter 22, Hbg is the hemoglobin
concentration (which is entered as a known value or obtained by
direct measurement), and PaO.sub.2 is the dissolved arterial oxygen
concentration. After obtaining a stable measurement of VO.sub.2 and
CaO.sub.2 over a first time period of approximately two to three
minutes, the gas blender 18 is caused to increase/decrease the
concentration of oxygen (preferably, at least a 10% change in
FIO.sub.2, e.g., 40% increased to 50%) supplied to the subject for
a second time period less than the subject's circulation time of
approximately thirty to fifty seconds. VO.sub.2 and CaO.sub.2 are
monitored on a breath-by-breath basis during this time period and
the cardiac output is then determined. Accordingly, the method and
apparatus of the present invention take advantage of the phenomenon
that if the oxygen concentration of the inspired gas is temporarily
increased or decreased, the change in alveolar oxygen concentration
will cause a transient uptake or release of oxygen across the
pulmonary capillaries thereby resulting in a change in the measured
VO.sub.2 and arterial oxygen content (CaO.sub.2). If these
parameters are measured during an interval less than the
circulation time (i.e., less than approximately thirty to fifty
seconds), then the venous oxygen content (CvO.sub.2) can be ignored
and the cardiac output of the subject can be calculated based on
the equation 7 C . O . = VO 2 CaO 2
[0041] In view of the teaching presented herein, other
modifications and variations of the present invention will readily
be apparent to those of skill in the art. The discussion and
description are illustrative of some embodiments of the present
invention, but are not meant to be limitations on the practice
thereof. It is the following claims, including all equivalents,
which defines the scope of the invention.
* * * * *