Respiratory calorimeter with bidirectional flow monitors for calculating of oxygen consumption and carbon dioxide production

A calorimeter for generating signals representative of the oxygen consumption and carbon dioxide production of a subject over a test period includes two flow sensors and a carbon dioxide scrubber connected to a patient mouthpiece. Interconnections are such that the air inhaled by the subject passes through one flow meter and the exhaled breath passes first through the other flow meter the carbon dioxide the scrubber and then through the first flow meter. The electrical output signals from the flow meters are provided to a microprocessor based computer which integrates the volume differences between the inhaled gas, and the exhaled gas after the carbon dioxide has been removed from it, over the period of the test. The inhaled volume less that part of the exhaled volume which does not constitute CO.sub.2 measures the oxygen comsumption per breath, the integral difference is a function of the patient's oxygen consumption over the period of the test. The computer also integrates the difference in flow volumes through the first and second sensors during an exhalation to calculate the subjects's CO.sub.2 production during the test.

FIELD OF THE INVENTION 
This invention relates to indirect calorimeters for measuring respiratory 
oxygen consumption and carbon dioxide production and more particularly to 
such a calorimeter employing bidirectional flow meters for measurement of 
the inhaled and exhaled gases and a CO.sub.2 scrubber which removes 
CO.sub.2 from the exhaled gas to allow the computation of the difference 
between the inhaled gas volume and the volume of the scrubbed exhaled gas 
to calculate oxygen consumption, and the difference between the exhaled 
gas volumes before and after scrubbing to calculate CO.sub.2 production. 
BACKGROUND OF THE INVENTION 
My U.S. Pat. No. 4,917,708, issued Apr. 17, 1990, discloses an indirect 
calorimeter, or oxygen consumption meter, which may be used to measure the 
resting energy expenditure of a subject. This measurement is important for 
determination of the proper caloric content for feedings of hospitalized 
patients and also is useful in connection with weight loss diets since the 
basal energy requirement may vary during the period of the diet. 
Similarly, knowledge of caloric expenditure and oxygen consumption during 
exercise are useful for cardiac rehabilitation and athletic training. 
That patent discloses a calorimeter which utilizes a unidirectional flow 
meter operative to generate electrical signals proportional to the 
respiratory gases passing through it, a carbon dioxide scrubber operative 
to remove CO.sub.2 from the exhaled gas and valving and conduits 
connecting the flow meter and the scrubber between a source of respiratory 
gases, which may be either the ambient air or some form of positive 
pressure ventilator, and a patient mouthpiece. The inhaled air has a 
negligible content of carbon dioxide and the exhaled gas contains 
lung-contributed carbon dioxide of essentially the same volume as the 
oxygen consumed by the subject. Accordingly, the difference in volumes 
between the inhaled and scrubbed exhaled gases passed through the flow 
meter provides an indication of patient's oxygen consumption. By 
integrating these differences over a test period, which may last for 
several minutes, an accurate measurement of the subject's oxygen 
consumption during the trial may be obtained. 
My U.S. patent application Ser. No. 726,922, filed Jul. 8, 1991, discloses 
a calorimeter utilizing a bidirectional flow meter which passes the 
inhaled gases before they are scrubbed for CO.sub.2 and the exhaled gases 
after they are scrubbed, resulting in a simplified design and the 
potential for disposability after a single use, eliminating the 
requirement for sterilization. 
One embodiment of that invention employs a capnometer disposed in the flow 
path between the subject's mouthpiece and the CO.sub.2 scrubber so that 
the exhaled gases are passed through the capnometer before being scrubbed. 
The capnometer generates an electrical signal which is a function of the 
CO.sub.2 concentration of the exhaled gases. The electrical output of the 
capnometer, along with the flow meter signal, may be used to generate the 
ratio of carbon dioxide to consumed oxygen, or the Respiratory Quotient 
(RQ) as well as the Resting Energy Expenditure (REE), another important 
measure of a subject's metabolism. 
SUMMARY OF THE INVENTION 
The present invention is directed toward a calorimeter which generates a 
signal proportional to the carbon dioxide production of the subject 
without the need for a capnometer, though use of a second flow meter which 
is disposed in the flow path between the subject's mouthpiece and the 
CO.sub.2 scrubber. The volume of CO.sub.2 production is calculated by 
subtracting the exhaled volume which passes out of the scrubber from the 
exhaled volume which passes into the scrubber. The volume of oxygen 
consumed may be calculated by subtracting the exhaled volume after it has 
passed through the scrubber from the inhaled volume. 
In one embodiment of the invention both the inhaled and exhaled volumes are 
passed through the scrubber. In an alternate embodiment of the invention, 
valve means are provided with conduits to direct the inhaled volume to the 
mouthpiece without passing it through either the scrubber or the second 
flow meter, and the exhaled volume is passed first through the second flow 
meter, then through the scrubber and then through the first flow meter. 
This configuration minimizes the volume of exhaled air that remains in the 
scrubber after exhalation that is necessarily inhaled before air from the 
source is inhaled, thereby removing any limitation on the size of the 
scrubber. 
Other objectives, advantages and applications of the present invention will 
be made apparent by the following detailed descriptions of two preferred 
embodiments of the invention The descriptions make reference to the 
accompanying drawings in which:

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The embodiment of the invention illustrated in FIG. 1, generally indicated 
at 12, employs a mouthpiece 14 adapted to engage the inner surfaces of the 
user's mouth so as to form the sole passage for inhaling and exhaling air 
passing through the mouth. A nose clamp of conventional construction (not 
shown) may be employed in connection with a mouthpiece 14 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 a endotracheal tube. 
The mouthpiece connects directly to a flow meter 15. The flow meter is 
preferably of the pressure differential type such as manufactured by 
Medical Graphics Corporation of St. Paul, Minnesota under the trademark 
"MEDGRAPHICS." Alternatively, other forms of flow transducers might be 
used such as differential temperature types. 
A conduit 16 connects the flow meter 15 to one end of a carbon dioxide 
scrubber 18. The scrubber 18 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 calcium hydroxide admixed with silica in a form known 
as "SODALYME." Another absorbent material which may be used is "BARALYME" 
which comprises a mixture of barium hydroxide and calcium hydroxide. 
The other end of the scrubber is connected by a conduit 20 to an artificial 
nose 22 which constitutes a moisture-absorbing filter such as a filter 
formed of fibrous elements or a sponge. The artificial nose 22 acts to 
absorb water vapor from gases passing through it if the water vapor 
content of the gases is higher than the level of moisture contained in the 
filter or to add water vapor to the gases if the filter vapor level is 
higher than that of the gases. 
The artificial nose 22 is connected via conduit 24 to a bacterial filter 26 
which preferably traps particles of about 5 microns in size or larger. The 
conduit 28 connects the bacterial filter to a bidirectional flow meter 30 
preferably of the same type as flow meter 15. Alternatively, other forms 
of bidirectional flow transducers might be used such as differential 
temperature types. 
The other end of the flow sensor 30 is connected via conduit 32 to a 
resistance heater 34 which raises the temperature of air passing through 
it to approximately 37.degree. C. Alternatively, the flow sensor might 
include means for measuring the temperature of the air exhaled by the 
subject and controlling the incoming air to that precise temperature. 
The other end of the heater 34 is connected to an air intake/outlet 36 
which may receive room air or may be connected to a positive pressure 
ventilator in the manner described in my U.S. Pat. No. 5,038,792. 
The electrical output signals from the flow meters 15 and 30 are provided t 
a microprocessor-based computation and display unit 38. The unit 38 
converts the signal from the flow meters to digital form, if they are 
analog signals as employed in the preferred embodiment of the invention. 
Unit 38 is a computation and display unit of the general type disclosed in 
my U.S. Pat. No. 4,917,718. Like that unit, it acts to integrate the 
difference in the signals from flow meter 30 during inhalations and 
exhalations to generate a signal proportional to the volume of oxygen 
consumed during the test. Additionally, it integrates the difference 
between the signal from flow meters 15 and 30 during exhalations to 
develop a signal proportional to the volume of carbon dioxide generated by 
the subject (VCO.sub.2). Essentially, considering the volume of inhaled 
air entering the calorimeter during a patient inhalation, as measured by 
the flow meter 30 as V.sub.1 ; the volume of the full exhalation passing 
through the flow meter 15 during an exhalation as V.sub.2 ; and the volume 
of exhaled air after the CO.sub.2 has been scrubbed, as measured by flow 
meter 30 during an exhalation as V.sub.3, the system makes two following 
computations: 
EQU VO.sub.2 =V.sub.1 -V.sub.3 
EQU VCO.sub.2 =V.sub.2 -V.sub.3 
The keyboard 40 associated with the unit 38 allows storage and display of 
various factors in the same manner as the system of my previous patent. 
In operation, assuming that room air is being inhaled, an inhalation by the 
subject on the mouthpiece 14 draws room air in through the intake 36 where 
it is first heated to essentially the temperature of the exhaled air by 
the heater 32. It then passes through the flow meter 30, generating a 
signal to the computation unit 38. After passing through the bacterial 
filter 28 and then the artificial nose 22, the inhaled air passes through 
the CO.sub.2 scrubber 18. Since there is a negligible carbon dioxide 
content in room air the scrubber will have little effect upon inhaled air 
initially, but after prolonged use may add some water vapor to the 
incoming air by virtue of chemical reactions which occur when the subject 
exhales through the scrubber. 
The inhaled air is then passed through the flow meter 15 and then the 
mouthpiece 14 to the subject. When the subject exhales, the exhaled air is 
again passed through the flow meter 15 and scrubber 18 in the opposite 
direction. The chemicals in the scrubber react with the carbon dioxide in 
the exhaled breath, producing water vapor and raising the temperature of 
the scrubber. The exhaled air is then passed through the artificial nose 
which tends to equalize the moisture vapor content of the exhaled air with 
that of the inhaled air. The exhaled air then passes to the flow meter 30 
through the bacterial filter 26. The exhaled air will at this point have a 
water vapor content and temperature roughly comparable to that of the 
inhaled air so that the flow meter measurements of the inhaled and exhaled 
gases are on a comparable basis. The exhaled air then passes through the 
heater 34 to the air source. 
The volume of exhaled air passed through the flow meter will be lower than 
the volume of inhaled air because of the absorption of the carbon dioxide 
by the scrubber 18. This difference in volume is a function of the oxygen 
absorbed from the inhaled air by the subject's lungs and the signals 
provided by the flow meter 30 to the unit 38 allow the integration and 
calculation of the resting energy expenditure in the manner described in 
my previous patent. The volume of CO.sub.2 produced by the subject is 
similarly calculated. 
The system 12, unlike the devices disclosed in my previous patent, requires 
no one-way valves and is accordingly lower in cost and more reliable in 
operation than the previous devices. Its costs may be sufficiently low 
that the entire unit may be disposed of after a single test. 
Alternatively, since the bacterial filter 26 prevents bacterial 
contamination of the flow meter, the flow meter might be reused and the 
other components, to the right of the flow meter in FIG. 1, discarded 
after a single use. 
An alternative form of the invention, in which the inhaled gases are not 
passed through the carbon dioxide scrubber, is illustrated in FIG. 2. 
Again, a connection to an air source 44 passes inhaled air through a 
heater 46 and then to a bidirectional flow sensor 48. The electrical 
output of the flow sensor is provided to a microprocessor-based 
computation and display unit 50. 
The inhaled air passes from the flow meter 48 through a bacterial filter 52 
and then through a water vapor-absorbent artificial nose 54. It is then 
carried through conduit 56, through a one-way valve 58, to the subject 
mouthpiece 60. 
The exhaled gases pass from the mouthpiece 60 through another one-way valve 
62, which provides an exit from the mouthpiece, and through a second flow 
sensor 64. The electrical signal from the flow meter 64 is provided to the 
microprocessor-based computer 50. In addition to calculating the oxygen 
consumption of the subject, VO.sub.2, and the resting energy expenditure 
in kilocalories per unit time, the computer 50 generates a display of the 
exhaled CO.sub.2 volume per unit time, the Respiratory Quotient (RQ), 
which equals VCO.sub.2 divided by VO.sub.2, and the resting energy 
expenditure. The Resting Energy Expenditure (REE) is preferably calculated 
from the Weir equation: REE (KC/24 hours)=1440 (VO.sub.2 
.times.3.341)+(VCO.sub.2 .times.1.11) where VO.sub.2 and VCO.sub.2 are 
both measured in milliliters per minute. 
The output of the flow meter 64 is provided to the CO.sub.2 scrubber 66 
which removes the CO.sub.2 from the exhaled gases in the same manner as 
the scrubber 18 of the embodiment of FIG. 1 and provides its output to the 
flow passage 56. Since the flow path through the artificial nose 54, the 
bacterial filter 52, the flow meter 48, the heater 46, and the air 
inlet/outlet 44 has a lower resistance than the passage through the 
one-way valve 58, particularly during an exhalation, the output from the 
scrubber takes this low resistance flow path and the exhaled volume again 
passes through the flow meter 48, in the reverse direction from the 
inhalation, and its output signal is provided to the computer 50. Since 
the passageway from the output of the scrubber 66 has a higher resistance 
to flow than the passage through the unidirectional valve 58, the inhaled 
air passes through the valve 58 to the mouthpiece rather than through the 
CO.sub.2 scrubber in the reverse direction. 
The embodiment of FIG. 2, by avoiding passage of the inhaled air through 
the scrubber, eliminates problems caused by the volume of exhaled air that 
remains in the scrubber after an exhalation and is necessarily inhaled 
before air from the source 44 is inhaled, thereby removing any limitation 
on the size of the scrubber.