Abstrict A number of different parameters related to carbon dioxide output
of a patient are routinely determined in ventilator/anaesthetic
systems. For this purpose, a ventilator/anaesthetic system for determining
carbon dioxide parameters includes a ventilator unit in which a
first flow meter is arranged to measure an expired flow of gas,
and a carbon dioxide meter is arranged to measure the concentration
of carbon dioxide in expired gas. Arranging the carbon dioxide meter
in the ventilator/anaesthetic unit minimizes the equipment which
must be located in the immediate vicinity of patient, and a faster,
more sensitive carbon dioxide meter thus can be used.
Claims We claim as our invention:
1. A ventilator/anaesthetic system comprising:
a ventilator/anaesthetic unit;
an inspiratory tube connected to said ventilator/anaesthetic unit
and adapted for communication with a patient for carrying a breathing
gas from said ventilator/anaesthetic unit to a patient;
an expiratory tube connected to said ventilator/anaesthetic system
and adapted for communication with a patient for carrying expired
breathing gas from a patient to said ventilator/anaesthetic unit;
a patient tube connected to said inspiratory tube and to said expiratory
tube and adapted for connection to airways of a patient;
a flow meter disposed downstream of said patient tube for measuring
a flow of expired breathing gas and for generating a flow signal
corresponding to said flow of expired breathing gas;
carbon dioxide meter means, juxtaposed to said flow meter downstream
from said patient tube, for measuring carbon dioxide in expired
breathing gas and for generating a measurement signal and a reference
signal identifying carbon dioxide in expired breathing gas at respectively
different times; and
calculating means, supplied with said measurement signal, said
reference signal and said flow signal, for calculating a concentration
of carbon dioxide in expired breathing gas from a ratio of said
measurement signal and said reference signal and for calculating
a parameter related to production of carbon dioxide by a patient
from said concentration and said flow signal over at least two respiratory
cycles of a patient.
2. A ventilator/anaesthetic system as claimed in claim 1 wherein
said carbon dioxide meter means are disposed inside said ventilator/anaesthetic
unit.
3. A ventilator/anaesthetic system as claimed in claim 1 wherein
said expiratory tube has a first end connected to said patient tube
and an opposite, second end, and wherein said carbon dioxide meter
means is disposed downstream of said second end of said expiratory
tube.
4. A ventilator/anaesthetic system as claimed in claim 1 wherein
said calculating means comprises integrator means for calculating
a minute volume of carbon dioxide in expired breathing gas by integrating
a product of said production of carbon dioxide and said flow signal,
as said parameter related to carbon dioxide concentration.
5. A ventilator/anaesthetic system as claimed in claim 1 wherein
said calculating means comprises means for determining a peak value
for said concentration of carbon dioxide in expired breathing gas
for each respiratory cycle of a patient and for using the peak value
in a respiratory cycle as an end tidal concentration of carbon dioxide
in a next successive respiratory cycle.
6. A ventilator/anaesthetic system as claimed in claim 1 wherein
said expiratory tube and said patient tube have a combined length
extending between said carbon dioxide meter means and a patient,
and wherein said combined length contains a known volume of expired
breathing gas, and wherein said calculating means comprises integrator
means for integrating said flow signal and an end tidal concentration
of carbon dioxide in one breath while said carbon dioxide meter
means measures a peak concentration of carbon dioxide in expired
breathing gas in a next breath, after a volume of expired breathing
gas equal to said known volume has passed said flow meter.
7. A ventilator/anaesthetic system as claimed in claim 1 wherein
said ventilator/anaesthetic unit comprises means for supplying a
continuous flow of breathing gas to said inspiratory tube, and said
ventilator/anaesthetic system further comprising a further flow
meter disposed for measuring a flow of breathing gas supplied to
said inspiratory tube, said further flow meter generating a further
flow signal corresponding to said flow of breathing gas supplied
to said inspiratory tube, and wherein said calculating means comprises
means for correcting, dependent on said further flow signal, said
parameter related to carbon dioxide concentration in expired breathing
gas.
Description BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a ventilator/anaesthetic system
wherein the patient's carbon dioxide output is monitored.
2. Description of the Prior Art
Ventilator/anaesthetic systems are known which in general include
a ventilator/anaesthetic unit, an inspiratory tube to carry a breathing
gas from the ventilator/anaesthetic unit to a patient, an expiratory
tube to carry expired breathing gas from the patient to the ventilator/anaesthetic
unit, a patient tube, connected to the inspiratory tube and the
expiratory tube and connectable to the patient's airways, a flow
meter arranged to measure the flow of expired gas, a carbon dioxide
meter to measure the concentration of carbon dioxide in expired
breathing gas, and a calculation unit connected to the carbon dioxide
meter and the flow meter to determine at least one parameter related
to the patient's carbon dioxide output.
Ventilator systems are normally used for supporting or controlling
the respiration of patients with respiratory problems or who are
incapable of breathing without assistance. The problems can be caused
by lung disease or damage to the lungs. Anaesthetic systems are
used for inducing anaesthesia in patients about to undergo surgery.
With both systems, it is important to obtain some measure on the
efficacy of the patient's ventilation. Information as to whether
the patient's blood is being oxygenated to a sufficient degree is
especially important. One highly useful procedure in this context
is to study some parameter related to the patient's carbon dioxide
output. One such parameter is end tidal concentration, i.e. the
carbon dioxide concentration in the last gas expired by the patient
in an expiration phase. The end tidal concentration of carbon dioxide
is indicative of arterial blood gas pressure and, accordingly, shows
whether or not the patient is being correctly ventilated.
Another parameter related to carbon dioxide output is the minute
production of carbon dioxide, normally expressed as an expired volume
of carbon dioxide per minute. This parameter is indicative of the
patient's general metabolism. Other parameters indicative of the
efficacy of patient ventilation are also well-known.
One known carbon dioxide analyzer is described in the Operating
Manual for the CO.sub.2 Analyzer 930 AG 0291 2.5 July 1981 Siemens-Elema
AB. This known carbon dioxide analyzer is connected to a cuvette
on the Y-piece of the intubation system connected to the patient.
Carbon dioxide is measured using conventional IR spectrophotometry.
The analyzer includes a light source, a filter and a detector. The
filter allows passage of a light wavelength at which carbon dioxide
absorbs the light. Since the analyzer is located in the Y-piece,
gas passes the gas cuvette in two directions, i.e. during inspiration,
when fresh gas is carried from the ventilator unit to the patient,
and during expiration, when gas is carried from the patient in an
expiratory tube back to the ventilator unit. At the end of the inspiratory
phase, the carbon dioxide analyzer is zeroed to obtain a reference
level for 0% carbon dioxide. Zeroing is necessary with this kind
of analyzer, since the detector signal would otherwise generate
erroneous values for the carbon dioxide concentration. The placement
of the analyzer on the Y-piece makes it necessary to position it
near the patient, but since it is heated during operation (to avoid
condensation on surfaces through which light must pass) it must
not come into contact with the patient's skin. A heat shield is
also often used to further protect the patient from the hot analyzer.
In addition to heat generation, other problems are associated with
this arrangement for a carbon dioxide analyzer. As noted above,
the analyzer is zeroed in the final phase of inspiration. Gas supplied
to the patient is normally dry, and the zero value for carbon dioxide
is therefore for dry air. Before the gas is delivered to the patient's
lungs, it can pass a humidifier which humidifies the gas. Regardless
of whether a humidifier is used, gas expired by the patient is saturated
with water. Gas expired by the patient can also contain secretion
etc. Since the carbon dioxide concentration is therefore measured
from gas saturated with moisture, for which correction must be made
in determinations of the carbon dioxide concentration, the deposition
of condensation or secretion on the cuvette's windows, or something
else preventing the light beam from passing the cuvette unimpeded,
is a risk. Increasing the length of the common tube for inspiration
and expiration also increases dead space.
Another problem, which could develop when the carbon dioxide meter
is used in anaesthetic systems, is that certain anaesthetic devices
operate with closed systems which re-use expired gas. Carbon dioxide
is removed from the gas before it is returned to the patient, but
inspired gas could still contain small amounts of carbon dioxide,
and the carbon dioxide analyzer might therefore be zeroed when the
concentration is actually greater than 0%. This would obviously
occur also with ventilator systems when ordinary air is supplied
to the patient via the ventilator system.
In practice, every manufacturer places the carbon dioxide meter
next to the patient, as a matter of principle, so the best possible
measurement value is obtained for end tidal concentration. Technical
developments have accordingly attempted to minimize and simplify
carbon dioxide analyzers, without affecting accuracy, in order to
minimize the size of the equipment which needs to be placed near
the patient.
In the determination of a plurality of the parameters, such as
the volume of expired carbon dioxide, the minute production of carbon
dioxide etc., respiratory gas flow is also measured. This is normally
performed with a flow meter arranged in the ventilator/anaesthetic
unit. Pressure changes in the expiratory tube, however, can cause
compression of the volume, and the measured flow will then fail
to correspond to the patient's expired flow. Errors also occur in
calculations of the parameter.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a ventilator/anaesthetic
system in which accurate measurement of parameters related to the
patient's carbon dioxide output can be performed while the aforementioned
problems are simultaneously resolved.
Such a ventilator/anaesthetic system is achieved in accordance
with the invention having a carbon dioxide meter which generates
a measurement signal and a reference signal, the carbon dioxide
concentration then being determined from the relationship between
the measurement signal and the reference signal, the carbon dioxide
meter being arranged downstream from the patient tube, near the
flow meter, to measure the concentration of carbon dioxide in expired
breathing gas. A calculation unit determines the parameter from
the values measured for flow and the carbon dioxide concentration
in at least two breaths.
Carbon dioxide meters, which generate a measurement signal and
a reference signal, are well-known. They can, e.g., be constructed
so a filter wheel rotates and alternately interposes different filters
in the beam path between the light source and the light detector.
One filter then passes a light wavelength at which carbon dioxide
absorbs light, and another filter passes a wavelength at which carbon
dioxide does not absorb light. Alternately, the number of light
detectors can be doubled, and a fixed filter placed in front of
the respective detector, each filter passing a specific wavelength.
A carbon dioxide meter according to the latter design is disclosed
in European Application 0 584 519. Both types of carbon dioxide
meters have the advantage of not needing to be zeroed periodically,
since they generate a reference signal at a wavelength at which
carbon dioxide does not absorb light. This is necessary for obtaining
the advantage in the invention of moving the carbon dioxide meter
from the Y-piece to the flow meter itself. This means that all the
additional equipment related to carbon dioxide measurement is moved
away from the patient, thereby greatly facilitating work for staff
around the patient. Dead space will also decrease. The measurement
instruments can be arranged in different ways. Both the flow meter
and the carbon dioxide meter can advantageously be arranged inside
the ventilator/anaesthetic unit.
Another advantage of the new location for the carbon dioxide meter
is a reduction in the impact on measurements caused by the patient's
moisture-laden expired air, and any secretion in it. Secretion is
collected in a special container near the patient, and expired gas
can be dehumidified to a greater or lesser degree in a dehumidifier
before it reaches the carbon dioxide meter and the flow meter. In
addition, prevention of condensation etc. on the windows of the
measurement cuvette is facilitated, since the carbon dioxide meter
can easily be heated to a much higher temperature at its new location
than is acceptable from the safety point of view when located at
the Y-piece.
Moving the carbon dioxide meter from the Y-piece, near the patient,
to a position close to, or inside, the ventilator/anaesthetic unit
may initially appear a relatively simple measure, since a location
near the patient does cause some problems. There are a number of
reasons, however, why this has not been possible before. First,
the end tidal concentration of carbon dioxide has often been cited
as one of the most important carbon dioxide parameters. For correct
measurement of this parameter, measurement has been performed as
close to the lungs as possible. It must also be remembered that
expired gas fills the entire Y-piece and the expiratory tube, like
a column of gas, during expiration. The diffusion of gas between
breaths obliterates the sharp demarcations at the beginning and
end of this gas column. With a system employing continuous flows
of gas, i.e. bypass flow, existing gas in the patient tube and expiratory
tube is indeed expelled, but it then mixes with gas, which does
not occur in the common output of the Y-piece.
This therefore makes it impossible to measure the end tidal concentration
of carbon dioxide in a single breath (respiration cycle) when the
position of the carbon dioxide meter has been changed. It will be
realized that any such change in the position of the carbon dioxide
meter is by no means self-evident. A different method for the calculation
of the parameters by the calculation unit, and the extraction of
same from measurement values, has been needed. This is achieved
by virtue of the calculation unit of the invention determining the
parameters from values measured for flow and carbon dioxide concentration
in at least two respiratory cycles. When the volume of gas in the
expiratory tube and Y-piece and the delay until carbon dioxide measurement
are known, diffusion in the column of gas can be determined, and
the parameters can be corrected. It should be emphasized that continuous
measurement throughout two respiratory cycles is not necessary for
determining the parameters. Measurement during parts of the cycles
is fully sufficient measurement during the measurement periods can
be performed in a known manner, e.g. analog (continuous) measurement
or digital measurement (with a predefined sampling rate).
A more sensitive carbon dioxide meter can be used when the carbon
dioxide meter is placed inside the ventilator/anaesthetic unit.
Preferably the calculation unit is an integrator and the calculation
unit determines a minute volume of carbon dioxide from the integral
of the product of the measured carbon dioxide concentration and
flow. Compared to known carbon dioxide meters and equipment, a direct
product of carbon dioxide concentration and flow can be obtained
with a system according to the invention. Since the carbon dioxide
meter and flow meter are near each other, the product directly designates
the concentration of carbon dioxide in the flow. The integral of
the product yields the volume. As noted above, the measurement does
not require all the values from two respiratory cycles.
The end tidal concentration, considered in the art as the most
interesting parameter, can be determined in at least two ways by
the system according to the invention.
The end tidal concentration of carbon dioxide in one breath can
be determined as the peak carbon dioxide concentration, measured
by the carbon dioxide meter, in the following breath. In principle,
the contents of the expiratory tube and patient tube consist of
a column of gas. In the final phase of an expiration, there is therefore
a column of expired gas in the expiratory tube and the patient tube.
During the next inspiration, fresh breathing gas flushes out part
of the patient tube and is supplied to the patient. When the patient
again exhales, a smaller column of fresh breathing gas, which does
not contain any carbon dioxide, pushes the preceding breath's column
of gas ahead of it through the ventilator unit and carbon dioxide
meter. Measuring the peak value for carbon dioxide concentration
in one expiration yields a good value for the end tidal concentration
in the preceding breath. One small difference, compared to known
measurement systems, may develop because of the diffusion of gas
between different gas columns and because of mixing effects between
gas columns, if any turbulence occurs. This is not a serious adverse
effect, however, since the carbon dioxide curve in expiration rises
relatively quickly, as shown in FIG. 1 to a level which is maintained
throughout the rest of the expiration, so the true end tidal concentration
(ETCO.sub.2) does not differ very much, even if there has been some
mixture of expired gas and fresh gas. As already noted, diffusion
can be determined and calculation of the end tidal concentration
can thereby be corrected for that diffusion.
Alternately, the end tidal concentration can be obtained by utilizing
the fact that a known volume of expired breathing gas fills the
expiratory tube and the patient tube between the carbon dioxide
meter and the patient, the calculation unit comprises an integrator
for integrating the measurement value from the flow meter and an
end tidal concentration of carbon dioxide in one breath is determined
as the peak concentration measured by the carbon dioxide meter in
the next breath, when a volume corresponding to the known volume
has passed the flow meter.
In principle, this method is based on the same reasoning as in
the above-described method for obtaining the end tidal concentration.
In this instance, however, the known volume is regarded as a unit,
and the end tidal concentration is determined at the time the known
volume has passed the flow meter.
In practice, the two described methods supply essentially the same
value for end tidal concentration. The delay between the end tidal
concentration for a specific expiration until measurement of same
has occurred, however, can exceed one breath, depending on the tidal
volume supplied to the patient and the volumes contained in tubes.
In an embodiment of the ventilator/anaesthetic system in accordance
with the invention, the ventilator/anaesthetic system supplies a
continuous flow of breathing gas which flows through the inspiratory
tube, the patient tube and the expiratory tube, and an additional
flow meter is arranged in the ventilator/anaesthetic system, to
measure the flow of gas supplied to the inspiratory tube. The additional
flow meter supplies a signal to the calculation unit and the calculation
unit corrects the parameter's determination from the continuous
flow.
Bypass flows occur in different contexts in conjunction with ventilator/anaesthetic
systems. For example, a patient, who only requires limited respiratory
support in order to breathe, can then breathe, with relative ease,
from the passing flow of breathing gas, however, determination of
the parameters must be corrected for this continuous flow.
In the case of end tidal concentration, for example, this means
that the column of gas which would otherwise have filled the inspiratory
tube and part of the patient tube will be expelled more rapidly
from the ventilator/anaesthetic system. In addition, this column
of gas will mix with fresh gas, thereby affecting measurement of
concentration. Since the supplied flow of breathing gas is known,
thanks to the additional flow meter, however, the impact of this
flow on determination of the parameters can be corrected. The parameter
is still determined with measurement values from two respiratory
cycles, since the final volume expired by the patient is expelled
by the continuous flow following each concluded expiration.
If the volume of the expiratory tube is not known, this volume
can be determined by removing the bypass flow for at least one breath
and recording when the carbon dioxide value changes. The integral
of the value for flow then designates the volume of the tube. Moreover,
the curve for the carbon dioxide concentration at the Y-piece can
be recreated, with the known tube volume, by correcting for the
effect of tube volume.
DESCRIPTION OF THE DRAWINGS
FIG. 1 shows the carbon dioxide concentration during a patient's
inspiration and expiration, respectively.
FIG. 2 shows an embodiment of a ventilator system according to
the invention.
FIG. 3 shows the morphology of a measurement signal from a carbon
dioxide meter by the valve system according to the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
As noted above, FIG. 1 shows curves for the concentration of carbon
dioxide in air expired by a patient. Curves 2A, 2B and 2C show how
the concentration of carbon dioxide rapidly levels off during expiration
(exp). At the end of expiration, the concentration rapidly drops
towards zero. The end tidal concentration of carbon dioxide, ETCO.sub.2
is determined at the end of expiration. During inspiration (insp),
the concentration is normally equal to zero.
FIG. 2 shows an embodiment of the invention in the form of a ventilator
system 4. The ventilator system 4 includes a ventilator unit 6 from
which an inspiratory tube 8 carries a breathing gas, via a patient
tube 10 to a patient 12. The patient tube 10 is also referred to
as a Y-piece or Y-tube. Expired gas is carried from the patient
12 via the patient tube 10 and an expiratory tube 14 back to the
ventilator unit 6. Breathing gas supplied to the patient is admitted
via one or more of three gas connections 16A, 16B and 16C and is
mixed in a mixing chamber 18 before being carried to the inspiratory
tube 8.
It should be noted that the ventilator unit 6 also has a number
of other components than those shown in the FIG. 1. In principle,
the ventilator unit 6 can be, e.g., a modified Servo Ventilator
300 Siemens-Elema AB. Check valves can be arranged in the respiratory
system to control the direction of gas flow in the inspiratory tube
8 patient tube 10 and expiratory tube 14.
Expired gas passes a first flow meter 20 which is arranged in
the ventilator unit 6. The flow of expired gas is measured in this
flow meter. A carbon dioxide meter 22 is arranged next to the first
flow meter 20. The carbon dioxide meter 22 measures the concentration
of carbon dioxide in expired gas. In principle, any carbon dioxide
meter will suffice, provided the meter generates a measurement signal
and a reference signal, the concentration of carbon dioxide being
determined from the ratio between the measurement signal and the
reference signal. The carbon dioxide meter 22 is thus an example
of carbon dioxide meter means, disposed next to the flow meter 20
downstream from the patient tube 10 for measuring carbon dioxide
in expired breathing gas and for generating a measurement signal
and a reference signal identifying carbon dioxide in expired breathing
gas at respectively different times, including measurement of a
peak concentration of carbon dioxide in expired breathing gas. The
first flow meter 20 and the carbon dioxide meter 22 are connected
to a calculation unit 24 which calculates or determines at least
one parameter related to the carbon dioxide output of the patient
12. In the event that a continuous flow of gas is admitted via the
inspiratory tube and flows through the patient tube 10 and the expiratory
tube 14 a second flow meter 26 is arranged in the ventilator unit
calculation unit 24 which can accordingly correct the determination
of the parameter, or parameters, for the continuous flow.
In contrast to known systems with carbon dioxide meters or carbon
dioxide analyzers located in the patient tube 10 near the patient,
the calculation unit 24 must be devised to take into account the
altered location of the carbon dioxide meter 22. In particular,
the fact that there is a given volume of expired gas in the expiratory
tube 14 and the patient tube 10 after concluded expiration must
be taken into account. This volume of gas does not normally reach
the carbon dioxide meter 22 until the next expiration. This is illustrated
more clearly in FIG. 3 which shows the measurement signal from the
carbon dioxide meter 22 for the expiratory curves shown in FIG.
1. The curve 28A shows that the carbon dioxide meter 22 does not
measure gas expired in a breath until some point into the patient's
expiration, as shown in the diagram. When the patient terminates
an expiration and commences inspiration, the measured concentration
of carbon dioxide will remain at a constant level, since gas in
the expiratory tube 14 is motionless.
During the next expiration, the volume of gas filling the expiratory
tube 14 and part of the patient tube 10 is pushed forward through
the carbon dioxide meter 22 and the first flow meter 20. The rest
of the gas expired in the preceding breath will then pass the carbon
dioxide meter 22 and a value for e.g. the end tidal concentration,
ETCO.sub.2 can be determined for the preceding breath. This determination
can be performed in such a way that the peak value measured for
carbon dioxide in each breath serves as the end tidal concentration
of carbon dioxide in the preceding breath. Presentation of the end
tidal concentration with a delay of one breath is not a major problem
for the physician. If some drastic event were to occur in respect
to the output of carbon dioxide by the patient 12 it would most
likely be manifest even in the part of the curve measured during
the current breath. Such an event could be, e.g., failure of the
carbon dioxide meter 22 to measure any carbon dioxide content, even
though the patient 12 is exhaling.
Other parameters which could de determined are, e.g., effective
and ineffective tidal volumes, the minute volume of carbon dioxide
in expired gas and the minute production of carbon dioxide by the
patient 12. In the same manner as for end tidal concentration, these
parameters are determined from information derived from at least
two breaths. For example, the minute volume of carbon dioxide in
expired gas, which can be determined from the integral of the product
of concentration and flow. It then does not matter that the concentration
measured during an inspiration is consistently high, since the flow
is zero, and flow would have no impact on the determination of the
minute volume of carbon dioxide, nor in determination of the minute
production of carbon dioxide. Tidal volume is obtained in a corresponding
manner by, e.g., integrating the product of concentration and flow
for, e.g., the concentration curve 28A.
When a continuous flow of breathing gas flows through the tubes
8 10 and 14 the calculation unit 24 must correct the calculated
parameters for this continuous flow. The most important difference
is found in the determination of end tidal concentration, since
measurement of concentration alone is then no longer sufficient.
The continuous flow will cause the column of gas, or volume of gas,
in the expiratory tube 14 at the end of expiration, according to
the reasoning above, to be expelled more rapidly from the system.
Since the continuous flow is known from the second flow meter 26
the patient flow can be determined as the difference between flows
measured in the first flow meter 20 and in the second flow meter
26. Because of the integration, passing volumes are known, and the
concentration figure for the entire passing flow, or volume, can
be converted into a concentration for the volume of expired gas.
The ventilator system 4 could also employ an anaesthetic system
according to some other known design. The salient features of the
invention are that measurement of carbon dioxide is made near measurement
of flow, and measurement has been transferred to a point downstream
from the patient in the direction of expiratory flow.
The calculation unit 24 in the various embodiments is thus an example
of calculating means, supplied with the measurement signal, the
reference signal and the flow signal, for calculating a concentration
of carbon dioxide in expired breathing gas from a ratio of the measurement
signal and the reference signal and for calculating a parameter
related to production of carbon dioxide by the patient from the
concentration and the flow signal over at least two respiratory
cycles of the patient. The calculating unit 24 is also an example
of calculating means including integrator means for calculating
a minute volume of carbon dioxide in expired breathing gas by integrating
a product of the production of carbon dioxide and the flow signal,
as the aforementioned parameter related to carbon dioxide concentration.
The calculation unit 24 is also an example of calculating means
including means for determining a peak value for the concentration
of carbon dioxide in the expired breathing gas for each respiratory
cycle of the patient and for using the peak value in a respiratory
cycle as an end title concentration of carbon dioxide in a next
successive respiratory cycle. The calculating unit 24 is also an
example of calculating means including integrator means for integrating
the flow signal and an end title concentration of carbon dioxide
in one breath while the carbon dioxide meter 22 measures a peak
concentration of carbon dioxide in expired breathing gas in a next
breath, after a volume of expired breathing gas equal to the known
volume has passed the flow meter 20. Lastly, the calculation unit
24 is an example of calculating means including means for correcting,
dependent on the further flow signal received from the flow meter
26 the aforementioned parameter related to carbon dioxide concentration
in expired breathing gas.
Although modifications and changes may be suggested by those skilled
in the art, it is the intention of the inventors to embody within
the patent warranted hereon all changes and modifications as reasonably
and properly come within the scope of their contribution to the
art.
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