Method and apparatus for measuring the concentration of a given component in a gas inhaled and/or exhaled by a patient

For the purpose of measuring the concentration of one or more given components of the breathing gas of a patient there is removed a small part of the breathing gas at a location immediately in front of the mouth of the patient. The withdrawn gas is passed to a measuring instrument sensitive to the component or components of interest through a line which comprises, at least in part, a thin tube of a fluorosulfonyl polymer, and the outer surface of which is in free contact with the ambient air. In this way, the temperature and relative humidity of the breathing gas supplied to the measuring instrument are brought into agreement with the temperature and relative humidity of the ambient air. By calibrating the measuring instrument with respect to the temperature and relative humidity of the surrounding air prior to carrying out the measuring operation, the measuring procedure can be carried out without disturbances caused by variations in the relative humidity and temperature of the breathing gas removed for measuring purposes.

BACKGROUND OF THE INVENTION 
The present invention relates to a method for measuring the concentration 
of a given component in a gas inhaled and/or exhaled by a patient, and to 
apparatus with which the method can be carried out. 
The possibility of determining the composition of the breathing gas in the 
pulmonary alveoli of a patient is of great interest to the medical field. 
In intensive care and under anaesthetic, the prime reason for this is 
because patients could then be more readily supervised and their treatment 
more favorably adapted. In the field of physiological research, the 
composition of the breathing gas is determined in order, among other 
things, to provide improved diagnostic methods. Those components whose 
concentration in the gas is of primary interest are CO.sub.2, O.sub.2 and 
gaseous anaesthetics such as N.sub.2 O, halothane, etc. It must be 
possible to measure the concentration of these components continuously, 
and preferably with a short response time so as also to enable rapid 
variations in concentration of the gaseous components of interest to be 
determined effectively. For example, the amount of oxygen (O.sub.2) 
consumed by a patient can be determined by measuring the O.sub.2 -content 
of the breathing gas taken in and expelled by the patient during the 
inspiration and expiration periods. 
The only location where it is practically possible to determine the 
concentration of a given component of the gaseous mixture inhaled and 
exhaled by the patient is immediately in front of the mouth of the 
patient, in the gas line which is connected to the patient's air passages 
and through which both the inhaled and exhaled gas passes. It should be 
possible to carry out such a determination with the aid of a transducer 
disposed in the path of the gas flowing through the aforesaid gas line, at 
least in the case of certain components of the gas. However, this would 
place high demands on the measuring transducer, which must be very small 
and light in weight, besides being capable of withstanding all manner of 
handling treatment, including cleaning and sterilization. For these 
reasons, it is preferred to withdraw part of the breathing gas flowing 
through the line connected to the air passages of the patient, at a 
location immediately in front of the patient's mouth, and to pass this 
part flow to a suitable instrument for determining the concentration of 
the gas component or components of interest in the gaseous mixture. When 
using this measuring method, however, it is necessary that only a 
relatively small gas flow is withdrawn and supplied to the measuring 
instrument. Moreover, the total volume of withdrawn gas between the 
tapping location and to the measuring instrument must be small, and the 
time used by the withdrawn gas to reach the measuring instrument must also 
be short, in order to obtain a rapid measuring response with no risk of 
different parts of the withdrawn breathing gas mixing together before the 
gas flow reaches the measuring instrument. Otherwise, it will not be 
possible to measure rapid variations in the concentration of the gas 
component of interest in a satisfactory manner. 
One difficult problem encountered when carrying out this procedure, is that 
the relative humidity of the gaseous mixture withdrawn for measurement can 
vary from nearly 0% to about 97%, while the temperature of the mixture may 
vary from room temperature to about 35.degree. C. This means that the 
amount of water carried by the withdrawn gaseous mixture can vary greatly, 
which leads to all manner of difficulties. 
With the total pressure of the gaseous mixture constant, variations in the 
relative humidity of the gaseous mixture withdrawn for measurement will 
naturally lead to corresponding variations in the partial pressures of all 
the other gas components of the mixture. On the other hand, the 
temperature and the relative humidity in the patient's lungs are both 
constant. Consequently, the measuring values obtained with respect to the 
gas component or components of interest must be converted to the 
conditions prevailing in the lungs of the patient. This requires 
complicated measurements to be made of the momentary humidity and 
temperature of the gas mixture supplied to the measuring instrument. It 
will be understood that this problem exists even though the measuring 
instrument used is, in itself, insensitive to water vapor, since the 
variations in the content of water vapor contained in the gas mixture give 
rise to variations in the contents of all other gas components in the gas 
mixture, when said mixture is at constant pressure. 
The difficulties will, of course, be still greater when the measuring 
instrument used is sensitive to water vapor, so that measurement of the 
gas component or gas components of interest is disturbed by the presence 
of water vapor in the gas mixture. This is the case, for example, with gas 
concentration detectors incorporating a crystal oscillator whose crystal 
has a coating which absorbs the gas component or components to be 
measured, for example a gaseous anaesthetic, and which is also able to 
absorb water vapor, such that the water-vapor content of the gas mixture 
will influence the measuring result. Another example is those instruments 
based on IR-absorption and used for determining, inter alia, CO.sub.2 
-contents. These instruments at present use a wavelength of about 4.3 
.mu.m, which means that the measuring process will be disturbed by the 
presence of N.sub.2 O and O.sub.2 in the gas mixture. For this reason, it 
would be to better advantage if there could be used a wavelength of about 
2.6 .mu.m, for which wavelength many good IR-radiation detectors are 
available. At this wavelength, however, the measuring process is greatly 
disturbed by variations in the amount of water vapor contained in the gas 
mixture. 
Another difficulty which can occur when the relative humidity and the 
temperature of the withdrawn gas mixture to be measured are high, is that 
the water vapor contained in the gas mixture condenses in the pipe or 
similar line leading to the measuring instrument, and/or in the measuring 
instrument itself, resulting in clogging of the pipe and damages to the 
instrument, respectively. 
These difficulties could be minimized by drying the gas mixture withdrawn 
for measurement prior to supplying it to the measuring instrument, either 
by causing the water vapor in the gas mixture to condense and collecting 
the condensation in a water trap, or by passing the gas mixture through a 
suitable drying agent capable of absorbing the water vapor in the gas 
mixture, so that in both cases a substantially dry gas mixture is supplied 
to the measuring instrument. Both of these solutions to the problem, 
however, have been found in practice to be either unusable or highly 
unsuitable. For example, the water trap or the drying device must be 
regularly superintended, a task which is considered troublesome by those 
using the measuring equipment. A more serious disadvantage with these 
solutions, however, is that the presence of a water trap or a drying 
device results in an increase in the volume of gas between the tapping 
location and the measuring instrument and also in the time taken for the 
gas mixture to pass from the tapping location to said measuring 
instrument, which, as mentioned in the aforegoing, results in a 
lengthening of the measurement response time, so that rapid variations in 
the concentration of the gas component of interest cannot readily be 
detected. This problem can only be counteracted by increasing the flow of 
gas withdrawn for measurement. On the other hand, such an increase in the 
withdrawn gas flow is not desirable, since only a small part of the total 
volume of gas inhaled and/or exhaled by the patient should be withdrawn 
for measuring purposes; and said total volume may, in itself, be small, 
such as when treating children for example. 
SUMMARY OF THE INVENTION 
Consequently, an object of the present invention is to provide an improved 
method for measuring the concentration of one or more given components in 
a gas mixture inhaled and/or exhaled by a patient, in which a small part 
of the flow of breathing gas through a breathing-gas line connected to the 
air passages of the patient is continuously withdrawn and supplied to a 
measuring instrument sensitive to the gas component or components of 
interest, said method significantly reducing all of the aforediscussed 
problems. 
This object is realised in accordance with the invention by a method of the 
aforementioned kind in which the temperature and the relative humidity of 
the gas withdrawn for measurement are adjusted to substantially coincide 
with the temperature and relative humidity of the ambient air prior to the 
gas being supplied to the measuring instrument. 
The invention is based on the realisation that it is not at all necessary 
to totally free the gas withdrawn for measurement from water vapor prior 
to passing the gas to the measuring instrument, but that a fully 
satisfactory result can be achieved by bringing the humidity and 
temperature of the gas withdrawn for measurement substantially into 
agreement with the temperature and relative humidity of the ambient air. 
As will be understood, the measuring instrument can be readily calibrated 
prior to the measuring operation with respect to the humidity and 
temperature of the ambient air, which for the duration of the measuring 
operation will not change to any degree likely to greatly influence the 
accuracy of the measurements taken. 
The relative humidity and temperature of the gas mixture withdrawn for 
measurement can be readily brought to the humidity and temperature of the 
ambient air by passing the withdrawn gas mixture to the measuring 
instrument through a line which over at least a part of its length 
consists of a material exhibiting high, selective and reversible 
water-absorption properties and the outer surfaces of which are in free 
contact with the ambient air. This section of the line may, to advantage, 
comprise a thin tube of a material comprising a fluorosulfonyl polymer or 
a copolymer of fluorosulfonyl and other monomers, such as 
tetrafluorethylene. If a gas is passed through a line, for example a thin 
tube, of a such material, whose outer surfaces are in free contact with 
the ambient air, the relative humidity of the gas flowing through the line 
is rapidly and effectively equalized into agreement with the humidity of 
the ambient air, owing to the fact that water vapor diffuses through the 
wall of the line towards the side of said wall on which the lower relative 
humidity lies. As will be understood, the temperature of the gas flowing 
through the line is, at the same time, brought into agreement with the 
temperature of the ambient air by thermal exchange therewith. All that 
need be ensured is that the outer surfaces of the line are in free contact 
with the ambient air and that the ambient air is able to flow across said 
surfaces.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The drawing shows a breathing gas line 1, one end of which is adapted to be 
connected to the air passages of a patient. The opposite end of the 
breathing line 1 is connected to an inhalation line 2 and an exhalation 
line 3, which are connected to a respirator or lung ventilator (not 
shown). For measuring the concentration of given components of the gaseous 
mixture inhaled and exhaled by the patient, a small part of the gas flow 
through the breathing gas line 1 is withdrawn from the line 1 through a 
thin tube 4 and passed to a suitable measuring instrument 5 of any 
conventional type, which is capable of measuring the concentration of said 
given gas components in the gas received through the tube 4. According to 
the invention, the tube 4 consists, at least over part of its length, of a 
material having high selective and reversible water-absorption properties. 
Further, at least that part of the tube 4 has its outer surfaces in free 
contact with the ambient air. In this way, what is achieved is that the 
relative humidity and the temperature of the gas withdrawn from the 
breathing gas line 1 through the tube 4 and passed to the measuring 
instrument 5 will be brought into substantial agreement with the relative 
humidity and the temperature of the ambient air, before the gas reaches 
the measuring instrument 5. The material in the tube 4 may preferably be a 
fluorosulphonylpolymer. 
When applying the method according to the invention, only a very small flow 
of gas need be withdrawn and supplied to the measuring instrument, and 
since the length of tube needed is relatively short the time taken for the 
gas to pass from the tapping location to said measuring instrument is 
short and the risk of internal mixing of the withdrawn gas is minimal, 
thereby enabling even rapid changes in the concentration of the gas 
component of interest to be measured. When applying the method according 
to the invention, the amount of gas withdrawn for measurement depends upon 
the requirements of the measuring instrument used. 
As will be understood, the requisite length of the tube of the water-vapor 
reversibly absorbing material will depend upon the size of the gas flow 
and the diameter of the tube, and on the magnitude of the difference in 
the relative humidities to be equalized. These values, however, can be 
readily established by tests. 
In order to determine the efficiency of the method according to the 
invention, a number of tests were carried out in which air of a given 
relative humidity was passed into one end of a commercially available tube 
made of a fluorosulfonyl copolymer, the outer surfaces of which tube were 
in free contact with the ambient air, and the relative humidity of the air 
exiting from the other end of the tube was measured. These tests were 
carried out with tubes of mutually different lengths and with two 
different rates of flow through the tube, namely 100 ml/min and 400 ml/min 
respectively. The tubes had an outer diameter of about 1.25 mm and an 
inner diameter of about 1.0 mm in all tests. 
Test 1 
In this test the air introduced into one end of the tube had a relative 
humidity of 96% and a temperature of 23.degree. C., while the air 
surrounding the tube had a relative humidity of 25% and a temperature of 
23.degree. C. Test 2 
In this test, the air introduced to one end of the tube had a relative 
humidity of 2% and a temperature of 23.degree. C., while the air 
surrounding the tube had a relative humidity of 24% and a temperature of 
23.degree. C. 
The results of these tests are shown in the Table below, which sets forth 
the relative humidity of the air exiting from the other end of respective 
tubes of mutually different lengths. 
TABLE 
______________________________________ 
Test 1 Test 2 
Tube length 
Flow rate Flow rate Flow rate 
Flow rate 
cm 400 ml/min 
100 ml/min 
400 ml/min 
100 ml/min 
______________________________________ 
100 24 24 23 23 
90 25 24 23 23 
80 26 24 23 23 
70 28 24 23 23 
60 33 24 23 23 
50 36 24 23 23 
40 39 24 21 23 
30 48 24 18 23 
20 61 32 12 23 
10 79 62 6 20 
______________________________________ 
It will be clear from these results that when practicing the method 
according to the invention using a relatively short tube, the relative 
humidity of the gas flowing through the tube can be brought into agreement 
with the humidity of the surrounding air irrespective of whether the 
original relative humidity of the gas flowing through said tube was much 
higher or much lower than that of the ambient air. 
It will also be understood that the invention can be applied irrespective 
of the form taken by the measuring instrument used to measure the 
concentration of the gas component or components of interest.