WO1999045988A1

WO1999045988A1 - Metabolic gas exchange and noninvasive cardiac output monitor

Info

Publication number: WO1999045988A1
Application number: PCT/US1998/005184
Authority: WO
Inventors: James R. Mault
Original assignee: Mault James R
Priority date: 1997-03-11
Filing date: 1998-03-13
Publication date: 1999-09-16
Also published as: US5836300A

Abstract

This invention is a respiratory gas analyzer (10) for measuring metabolic activity. The cardiac output of a subject includes a bi-directional flow meter (40), a capnometer sensor (16) interconnected by conduits to a mouthpiece (12), a pass through carbon dioxide scrubber (34) for absorption of the carbon dioxide, and a computer (18) for calculating the patient's cardiac output.

Description

1

METABOLIC GAS EXCHANGE AND NONINVASIVE CARDIAC OUTPUT MONTTOR

Field of the Invention

This invention relates to a respiratory gas analyzer employing a flow

sensor and a capnometer which may be interconnected in a first configuration to measure metabolic activity of a patient or in a second configuration to measure

the cardiac output of the patient.

Background of the Invention

My U.S. Patent No. 5,179,958 and related patents including 5,038,792

and 4,917,708 disclose respiratory calorimeters connected to a mouthpiece

which measure the volume of gas inhaled by a patient over a period of time and pass the exhaled gasses through a carbon dioxide scrubber and then a flow

meter. Broadly, the integrated flow differences between the inhalations and the carbon dioxide scrubbed exhalations are a measure of the patient's oxygen

consumption and thus the patient's metabolic activity. These devices may incorporate a capnometer to measure the carbon dioxide concentration of the

exhaled air. A computer receiving signals from the flow meter and the

capnometer may calculate, in addition to the oxygen consumption of the patient,

the Respiratory Quotient and the Resting Energy Expenditure of the patient as

calculated from the Weir equation.

The cardiac output of a patient, that is the volume of blood ejected from

the heart per unit time, is another important measured parameter in hospitalized

patients. Currently, cardiac output is routinely measured by invasive techniques 2 including thermal dilution using an indwelling pulmonary artery catheter. This

technique has several disadvantages including the morbidity and mortality 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, continuous cardiac output measurement

device would substantially improve patient care and reduce hospital costs.

The partial rebreathing technique is a known method for cardiac output

measurement. As described in Capek and Roy, "The Noninvasive Measurement

of Cardiac Output Using Partial CO₂ 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. In its original form, the Fick method of

measuring cardiac output requires blood gas values for arterial and mixed venous

blood as follows:

VC c o. =-

Ca 0₂ -Cv0₂

where CO. is cardiac output, VO₂ is oxygen consumption, CaO₂ is the arterial

oxygen content and CvO₂ is the venous oxygen content. By substituting carbon

dioxide for oxygen in the Fick equation, the partial rebreathing method allows 3 computation of cardiac output without invasive blood gas measurements as follows:

VCO_^

C O .

Ca C0₂ ~CvC0₂

The partial rebreathing technique uses the change in CO₂ production (VCO_j) and

end-tidal CO₂ in response to a brief change in ventilation. The change in CO₂ production divided by the change in CO₂ content of arterial blood (CaCO₂), as

estimated from end-tidal CO₂, equals pulmonary capillary blood flow as follows:

ΔVCO. C . O . = -

ΔetCO„

Clinical studies have verified the accuracy of this partial rebreathing method

relative to more conventional invasive techniques. Despite the advantages of the

partial rebreathing technique it has not achieved extensive usage.

I have discovered that minor modifications of my respiratory calorimeter

will enable it to practice cardiac output measurement using the partial carbon

dioxide rebreathing technique as well as making the metabolic related

measurements described in my patent.

Summary of the Invention

The present invention is accordingly directed toward a respiratory gas

analyzer capable of measuring either the metabolic activity or the cardiac output

of a subject. The configuration of the preferred embodiment of the analyzer

substantially resembles the indirect calorimeter disclosed in my previous patents 4 in that it incorporates a bi-directional flow meter, a capnometer and a carbon

dioxide scrubber. Conduits connect the flow meter between a source of

respiratory gasses, which is typically atmospheric air, and a mouthpiece, so that

the flow meter measures the gas volume during inhalation. During exhalation

the gas is passed through a capnometer to the carbon dioxide scrubber and the output of the scrubber is fed back through the flow meter to the atmosphere. In

this configuration the computer connected to receive the electrical outputs of the flow meter and capnometer calculates the patient's oxygen consumption either

alone or along with one or more of the derivative measurements of Respiratory

Quotient and Respiratory Energy Expenditure.

In order to perform measurements of patient's cardiac output using

partial CO₂ rebreathing the system is convertible into the configuration in which

the exhaled breath is not passed through the carbon dioxide scrubber but is

rather passed directly to the flow meter or into an interior volume within the

analyzer that connects to the flow meter but allows the accumulation of a

fraction of an exhalation which is then mixed with additional air passing through

the flow meter on the next inhalation to increase the carbon dioxide content of

that subsequent inhalation. The analyzer may be formed so that the carbon

dioxide scrubber is completely removable for purposes of taking cardiac output

measurements, or, alternatively, the scrubber may be maintained in position on

the analyzer with the flow passages altered so that the exhaled air is not passed

through the scrubber. 5

The analyzer further includes valving connected to the circuitry to shift

the circuitry between two alternative configurations. In the first configuration

exhaled gasses are passed through the capnometer and then directly to the flow

meter. Upon the subsequent inhalation fresh respiratory gasses are drawn

through the flow meter. In the second alternative configuration, after the valve

is shifted, the exhaled gasses are passed through the capnometer and then fed

into a conduit connecting to the flow meter. The conduit volume thus acts as a

dead space. When the subject then inhales a substantial portion of the inhaled

gasses constitutes rebreathed gasses from the conduit dead space having a high

carbon dioxide content. Preferably from 20% to 70% of the inhaled air

constitutes rebreathed air, with the balance being made up of air drawn in

through the flow meter with the inhalation.

The metabolic measurements are made with the scrubber connected in

operative configuration so that exhaled air passes through the carbon dioxide

scrubber and then the flow meter. A computer connected to the flow meter

integrates the inhaled and exhaled flow signals. Their difference is a function

of the subject's metabolic rate. To use the device to calculate cardiac output, the

scrubber is either removed or its input is blocked and the computer receives

signals from the flow meter and the capnometer while the subject breathes while

the valve is in the first configuration in which the exhaled gas is passed through

the capnometer and then directly out through the flow meter. The computer

integrates the capnometer measurement over the flow volume to determine the 6 carbon dioxide content of the exhalations and also determines the carbon dioxide

content of an exhalation at the end of the breath; i.e. the end-tidal carbon dioxide measurement. The valve is then shifted to bring the circuitry into the alternate

configuration in which the exhaled breath is introduced into the dead space volume within the circuitry so that only a proportion of each exhaled breath

passes out through the flow sensor. Each inhaled breath includes a proportion

of rebreathed air having an increased carbon dioxide content. Measurement is

made for about thirty seconds during which time the computer again measures

the end-tidal carbon dioxide. This measurement is used with the measurements made while the valve was in its first configuration to calculate cardiac output.

Alternatively, the volume of the flow chamber containing rebreathed air

is made adjustable and/or computer controlled so as to adjust the dead space to

the breath volume of the user.

Other objects, advantages and applications of the present invention will

be made apparent from the following detailed description of the preferred

embodiments.

Description of the Drawings

Figure 1 is a schematic diagram illustrating a preferred embodiment of

my invention in the configuration which measures metabolic activity;

Figure 2 is a schematic diagram of the system of Figure 1 in a

configuration for making the first measurements required to determine a

patient's cardiac output; and 7 Figure 3 is a schematic diagram of the system of Figure 1 in a

configuration for making the second measurement required to determine cardiac

output.

Detailed Description of the Invention

The preferred embodiment of the invention, as illustrated in Figure 1 ,

and generally indicated at 10 is in a configuration in which it may be used to

measure a patient's metabolic activity. The analyzer employs a mouthpiece 12

adapted to engage the inner surfaces of the user's mouth, so as to form the sole

passage for flowing respiratory gasses into and out of the mouth. A nose clamp

of conventional construction (not shown) may be employed in connection with

the mouthpiece 12 to assure that all respiratory gas passes through the

mouthpiece. In alternative configurations, a mask that engages the nose as well

as the mouth might be employed or an endotracheal tube could be used.

The mouthpiece 12 connects through a short passage 14 to a capnometer

sensor 16. The capnometer 16 generates an electrical signal which is a function

of the instantaneous carbon dioxide concentration of gas passing through the

mouthpiece 12. The capnometer may be of a conventional type such as those

described in U.S. Patent Nos. 4,859,858; 4,859,859; 4,914,720 or 4,958,075.

The capnometer provides an electrical output signal to a computation unit 18

incorporating a suitably programmed microprocessor (not shown), a display 20,

and a keypad 22. 8 The capnometer is connected by a short passage 24 to a two position,

three-way valve, generally indicated at 26. The valve has a single input flow

channel from a one-way valve 28 which connects to a gas flow conduit 30. The

valve has a first position, illustrated in Figure 1 , in which output is provided to

a second one-way valve 32 connecting to the input of a carbon dioxide scrubber

34. In its second position, schematically illustrated in Figure 3, the valve is

shifted so as to block gas flow to the valve 32 and thus the scrubber and to direct

flow to an air passage 34 which connects with the gas conduit volume 30.

The carbon dioxide scrubber 34 is a container having a central gas

passageway filled with a carbon dioxide absorbent material such as sodium

hydroxide or calcium hydroxide. Such absorbent materials may include sodium

hydroxide and/or calcium hydroxide mixed with silica in a form known as "Soda

Lime™. " Another absorbent material which may be used is "Baralyme™" which

comprises a mixture of barium hydroxide and calcium hydroxide. The carbon

dioxide scrubber has internal baffles 36 which provide a labyrinth flow of

gasses.

The output 38 of the scrubber is located adjacent to a bi-directional

volume flow sensor 40 which is positioned at the end of the volume 30 opposite

to the valve 26. The flow sensor is preferably of the pressure differential type

such as manufactured by Medical Graphics Corporation of St. Paul, Minnesota

under the trademark "Medgraphics" of the general type illustrated in U.S. Patent

No. 5,038,773. Alternatively other types of flow transducers such as 9 pneumatics or spirameters might be employed. The other end of the flow sensor

is connected to a source and sink for respiratory gasses through a line 42. The

source and sink is typically the atmosphere but may alternatively be a suitable

form of positive pressure ventilator. The electrical output of the bi-directional volume flow sensor is connected to the computation unit 18.

With the valve 26 in the first position schematically illustrated in Figure

1 , the system operates in the same manner as the unit described in my patent

5, 179,958 to calculate various respiratory parameters of the patient such as

oxygen consumption per unit time, the Respiratory Quotient (RQ) which equals VCO₂ divided by VO₂, and the Resting Energy Expenditure (REE) preferably

calculated from the Weir equation.

In this mode of operation, assuming that room air is being inhaled, an

inhalation by the subject on the mouthpiece 12 draws room air in through the

intake 42 through the flow meter 40, generating an electrical signal to the

computation unit 18. The inhaled air then passes through the volume 30 and

through the one-way valve 28, to the passage 24 leading to the capnometer

sensor 16. The sensor 16 generates an electrical signal which is provided to the

computation unit 18. The inhaled air then passes through the passage 14 to the

patient via the mouthpiece 12. When the patient exhales the expired gasses pass

through the capnometer 16 in the reverse direction and then through the one-way

valve 32 to the input of the carbon dioxide scrubber 34. The scrubber absorbs

the carbon dioxide in the exhaled breath and provides its output into the volume 10 30 immediately adjacent the bi-directional volume flow sensor 40 in a direction

opposite to the inhaled gas.

The volume of exhaled air passing through the flow sensor 40 will be

lower than the volume of inhaled air because of the absorption of the carbon

dioxide by the scrubber 34. This difference in volume is a function of the

oxygen absorbed from the inhaled air by the patient's lungs. The computation unit 18 converts the signals from the capnometer 16 and the flow sensor 40 into

digital form if the signals are in analog form, as employed in the preferred

embodiment of the invention. The computation unit 18 otherwise operates in the manner disclosed in my U.S. Patent 4,917,718 to integrate signals representing the difference between the inhaled and exhaled volume for the period of the test

and multiply them by a constant to arrive at a display of kilocalories per unit

time. The Resting Energy Expenditure (REE) and the Respiratory Quotient

(RQ) are similarly calculated. The keyboard 22 associated with the computation

unit 18 allows storage and display of various factors in the same manner as the

systems of my previous patent.

The unit may incorporate an artificial nose and/or a bacterial filter as

described in my previous patents or may incorporate a temperature sensor which

provides a signal to the computation unit 18 to adjust the measurements as a

function of the breath and external air temperature.

In order to use the analyzer to noninvasively measure the patient's

cardiac output, the connections between scrubber 34 and the main body of the 11 unit are blocked. The scrubber may be physically removed from the main unit

or may continue to be supported on the main unit with appropriate valving (not

shown) shifted to block off the scrubber so it is inoperative during the measurement.

Figure 2 illustrates the unit with the scrubber 34 physically detached and

with wall sections 50 and 52 blocking off the ports in the main body to which

the input and output connections of the scrubber 34 connect. This creates a

relatively narrow, low volume passage 54 connecting the output of the one way

valve 32 to the area adjacent the flow meter 40.

In this position, when the patient inhales air or respiratory gasses are drawn in through the inlet 42, passed through the bi-directional sensor 40,

passed through the volume 30 and the one way valve 28, through the capnometer

16 to the mouthpiece 12. When the patient exhales, gasses are passed from the

mouthpiece 12, through the passage 14, through the capnometer 16, through the

one way valve 32 and the passage 54 and out the bi-directional sensor 40.

The computation unit 18 may control the two position valve 26 and move

it to a second position, illustrated schematically in Figure 3, in which the flow

passage to the one-way valve 32 is blocked and the passage 34 is open to the

flow volume 24 adjacent the capnometer 16. The shifted valve prevents exhaled

gasses from entering the passage 56 and instead returns the exhaled gasses back

in the direction of the flow sensor 40 through the conduit volume 30. This

creates a temporary increase in dead space that causes rebreathing of carbon 12 dioxide enriched air from the volume 30 when the patient inhales to create

changes in the carbon dioxide content of the exhalation (VCO₂) and in the end-

tidal carbon dioxide (etCO₂) so that the computation unit 18 may generate a signal which is a function of the cardiac output.

The measurement sequence is as follows:

1. With valve 26 in the position illustrated in Figure 2, VCO₂ and

etCO₂ are recorded over three minutes. The volume of VCO₂ is

calculated by integrating the instantaneous measurements of the capnometer sensor over the flow volume as indicated by sensor

40. The etCO₂ is calculated on a breath-by-breath basis using a

peak detection algorithm which stores the maximum value of the transient CO₂ signal from the capnometer 16 for each breath.

The inhaled air is not admixed to any appreciable degree with previously exhaled air.

2. The computation unit 18 then switches the valve 26 to the

position illustrated in Figure 3. The volume of the conduit 30 is

then filled with exhaled breath, with the overflow being passed

out through the bi-directional flow sensor 40. The volume of the

passage 30 is preferably about 15-25% of the tidal volume of the

subject. Typical tidal volumes range between 600 ml and 1000

ml and the volume of the chamber 30 is preferably about 150 ml.

The subject therefore rebreathes carbon dioxide from the 13 temporary dead space chamber for approximately thirty seconds.

During this thirty second period breath-to-breath end-tidal carbon

dioxide and total integrated volume of carbon dioxide are

recorded. 3. The collected data are than processed by the computation unit 18

and the results are displayed or printed.

The unit can thus calculate and display the following parameters: oxygen

consumption (VO₂), measured energy expenditure (MEE), carbon dioxide

production (VCO₂), cardiac output (CO), respiratory exchange ratio (RER), minute ventilation (V), and end-tidal carbon dioxide (etCO₂).

The computation unit 18, in the cardiac output mode may employ a

computation algorithm of the type described in the Capek and Roy paper.

Having thus described my invention I claim:

Claims

14 CLAIMS

1. A respiratory gas analyzer for measuring the metabolic activity

or cardiac output of a subject, comprising:

a respiratory connector operative to be supported in contact with a

subject so as to pass inhaled and exhaled gasses as the subject breathes;

means for connecting to a source of respiratory gasses;

a bi-directional flow meter adapted to generate electrical signals as a function of the volume of gasses which pass through it in either direction;

a pass-through carbon dioxide scrubber operative to absorb carbon

dioxide from gasses which pass through it;

a capnometer;

a valve shiftable between a first configuration and a second

configuration;

conduits interconnecting said respiratory connector, said means for

connecting to a source of respiratory gasses, said scrubber, said flow meter, said

capnometer and said valve;

computer means for receiving the outputs of the flow meter and the

capnometer;

means for controlling the position of the valve;

said computer being connected to said means for controlling the valving

so as to interconnect the components in either a first configuration in which,

upon inhalation by a subject substantially the entire inhaled volume is passed 15 from the source of respiratory gasses, through the flow meter to the subject and

upon exhalation by a subject substantially all of the exhaled gasses are passed

through the flow meter in a direction opposite to the inhaled gasses, or a second configuration in which upon inhalation by a subject only a fraction of the inhaled

gasses are passed through the flow meter from the source of respiratory gasses,

with the balance constituting previously exhaled gasses stored in said conduits,

and upon exhalation by a subject only a fraction of the exhaled gasses are passed

through the flow meter in a direction opposite to the inhaled gasses, the balance

being stored in said conduits, whereby the computer may calculate the cardiac

output of a subject based on the difference between the carbon dioxide content

of the exhaled gasses between the time the valves are in the first configuration

and the time the valves are in the second configuration and the difference in the

end-tidal carbon dioxide content of the exhaled breath between the times the

valves are in the first configuration and the valves are in the second

configuration; and means operative at such time as said valve is in said first

configuration, whereby upon exhalation by a subject, the exhaled gasses are

passed first through the scrubber, then through the flow meter in a direction

opposite to the inhaled gasses, whereby the computer may generate a signal

proportional to the integral of the difference between the inhaled and exhaled gas

volumes over a period of time to calculate a subject's metabolic rate.

16 2. The metabolic gas analyzer of claim 1 wherein the scrubber may

be connected to the conduits so that upon exhalation by a subject the exhaled

gasses are passed first through the scrubber, then through the flow meter in a

direction opposite to the inhaled gasses, or the scrubber may be removed from

the conduit so that upon exhalation by a subject exhaled gasses passing through

the flow meter in a direction opposite to the inhaled gasses are not first passed

through the scrubber.

3. A respiratory gas analyzer useful for calculating the respiratory oxygen consumption per unit time or the cardiac output of a subject, comprising:

a respiratory connector operative to be supported in contact with the

mouth of a subject so as to pass inhaled and exhaled gasses as the subject

breathes;

means for connecting to a source of respiratory gasses;

a pass-through bi-directional flow meter adapted to generate electrical

signals as a function of the volume of gasses which pass through it in either

direction;

a pass-through carbon dioxide scrubber operative to absorb carbon

dioxide from gasses which pass through it;

a capnometer;

a computer for receiving generated electrical signals from the flow meter

and the capnometer; 17 conduits interconnecting said respiratory connector, said means for

connecting to a source of respiratory gasses, said capnometer and said flow meter;

a connector for removably attaching the scrubber to the conduits; and

valve means connected to the conduits for arranging the respiratory gas

analyzer in either a first configuration in which, upon inhalation by a subject

gasses are passed from the source of respiratory gasses through the flow meter

to a subject so that substantially all of the inhaled gasses are passed through the

flow meter or a second configuration wherein upon inhalation by a subject a

portion of the inhaled gasses are derived from previously exhaled gas stored

within the conduits and the balance is derived from the source of respiratory

gasses through the flow meter, the computer being operative to calculate an

electrical signal which is a function of the cardiac output of a subject based upon

the difference in the carbon dioxide content in the expired gasses between the

time that the valve is in the first configuration and the valve is in the second

configuration, and the difference between the end-tidal carbon dioxide content

of the exhaled breath between the times that the valve is said first configuration

and the time that the valve is in said second configuration; and

means for connecting said scrubber to the conduits when said valve is in

said first configuration, so that exhaled gasses are passed first through the

scrubber, then through the flow meter in a direction opposite to the inhaled

gasses, whereby the flow meter is operative to generate a signal proportional to 18 the integral of the difference between the inhaled and exhaled gas volumes over

a period of time to calculate the metabolic activity of a subject.

4. The respiratory gas analyzer of claim 3 in which the source of

respiratory gasses is the atmosphere.

5. The respiratory gas analyzer of claim 3 in which said portion of

inhaled gasses which are derived from the previously exhaled gasses represents

about 10-60% of the entire inhaled gas.