HK1145541B - Sensor and apparatus for analysing gases present in blood - Google Patents

Description

Sensor and instrument for analyzing gases present in blood

The present invention relates to sensors and instruments for analyzing gases present in blood, and in particular for determining gases present in blood, such as ammonia, hydrogen sulfide and nitric oxide, in minimum quantities on the order of parts per million or less.

It is known that a variety of pathological conditions can be identified by analyzing the gases present in blood. The techniques commonly used for these analyses require the extraction of blood samples by various methods and the subsequent storage of these samples in an environment under conditions of isolation, thermostating, etc., until the actual analysis is carried out. This has various drawbacks known to the person skilled in the art and the impossibility of continuous monitoring of the various gas pressures present in the blood. To overcome these drawbacks, it has been proposed to dispense with taking a blood sample and to determine the presence of gases in the blood by another means of the sample, such as by transcutaneous means in real time or by analysing a saliva sample. In addition to being non-invasive, these techniques also allow for the continuous monitoring of blood gases and techniques for sampling gases, in particular by transcutaneous means, for determining the presence of oxygen and CO in blood₂The percutaneous approach is used just after the prenatal diagnosis of (a).

Instruments for analyzing blood gases are known, generally consisting of a gas sampling probe connected via a duct to an instrument provided with a sensor for measuring the gas. Various sensors for analyzing blood gases are known, such as sensors based on galvanic cells for measurement, which allow to measure the concentration of one or more gases.

Patent US5007424, for example, describes a polarographic/amperometric sensor for measuring the partial pressure of oxygen in blood by means of a Clark (Clark) -type electrode arrangement. The sensor is provided with means for simultaneous determination of CO in blood₂A pH electrode for partial pressure.

Patent US4840179 discloses a device for simultaneous and continuous measurement of oxygen and CO present in blood based on the principle of pH measurement in electrolytes₂The thermostat of (1). Gas sampling is performed percutaneously. However, to ensure oxygen and CO₂Satisfactory measurement of skin temperature, it is necessary to heat the skin to about 42 ℃ to enhance its permeability and thus gas flow.

A problem with amperometric sensors (galvanic sensors) known in the art is that they do not allow the detection of the presence of minute amounts of blood gases, such as ammonia, hydrogen sulfide and nitric oxide, which can be involved in a variety of pathological situations. In particular, the presence of gaseous ammonia in the blood can show disturbances of liver and kidney function, in which case its concentration can increase to physiological values exceeding 0.1-0.6 ppm.

Measurement and monitoring of gaseous ammonia allows rapid and assured diagnosis of diseases such as hyperammonemia and hypo-ammoniaemia, diabetes and hypertension, as well as diagnosis of Helicobacter Pylori (Helicobacter Pylori) infection. Transdermal determination of gaseous ammonia can also be used in hemodialysis treatments and periodic tests.

Nose et al, in the article "Identification of ammonia gas in gases emanating from human skin and its correlation with hydrogen gas in blood (Identification of ammonia in gas emitted from human skin and its correlation with hydrogen gas in blood)" published in Analytical Sciences (Analytical Sciences) at 2005, 12.21, 1471 and beyond, describe an experimental study by which the detection of the presence of gaseous ammonia originating from the skin and the measurement of its amount are possible. This article highlights the necessity of collecting gaseous ammonia transcutaneously by using a painless and real-time method for the patient, thus allowing continuous monitoring of the evolution of gaseous ammonia in the blood, and making the measuring instrument also for home use.

It is therefore an object of the present invention to provide a sensor and a device for determining blood gases, in particular trace gases such as ammonia, hydrogen sulphide and nitric oxide, in real time and in a non-invasive, non-manual and non-destructive manner. The object is achieved by a sensor and an instrument, the main features of which are described in claims 1 and 11, respectively, while other features are described in other claims.

The sensor according to the invention is a measuring galvanic cell which is tailored to detect and measure gases like ammonia, hydrogen sulfide and nitric oxide, which are present in blood in quantities of the order of parts per million or less.

One advantage of the sensor according to the invention is that it has a response and recovery time in the order of seconds, and can thus be advantageously used for real-time and continuous measurements.

Moreover, the sensor according to the invention does not require any heating of the patient's skin in order to enhance its permeability to blood gases. In fact, with the benefit of the miniaturization of the measuring electrodes, a minimum amount of gas is sufficient for making a correct and accurate measurement. The risk of skin burning is thus completely eliminated.

Another advantage is that the sensor is very compact, thereby allowing to manufacture at low cost a measuring instrument having a reduced size, being portable and also suitable for home use.

A further advantage of the sensor according to the invention is that it can be used with different types of sampling probes, suitable for transcutaneous sampling and in vivo analysis of blood or saliva samples, thereby allowing maximum flexibility in the use of the measuring instrument with the probe inserted therein.

Patent US3886058 discloses an electrochemical gas detection device comprising a galvanic cell provided with a galvanic reference element and a galvanic measuring element immersed in an electrolyte. The amperometric measurement element comprises a measurement electrode on which a hydrophilic wick is transversely arranged. Both free ends of the wick are immersed in the electrolyte. The wick provides a path for the ohmic contact for electrolysis through the sensing surface of the measurement electrode, thereby achieving an electrode/electrolysis interface for detecting gases.

The device described in the above-mentioned patent provides the same advantages as the present invention with respect to response and recovery time, but does not suggest how to configure the sensor to solve the technical problem of measuring small amounts of blood gases and in particular trace amounts of gases such as ammonia, hydrogen sulfide and nitric oxide.

Further advantages and features offered by the sensor and instrument according to the present invention will become clear to a person skilled in the art from the following detailed and non-limiting description of some embodiments, with reference to the attached drawings, in which:

FIG. 1 shows a cross-sectional view of a sensor according to the present invention;

FIG. 2 shows a schematic diagram of a measurement instrument including the sensor of FIG. 1;

FIG. 3 shows a cross-sectional view of a first embodiment of a sampling probe that may be used with the instrument of FIG. 2;

FIG. 4 shows a cross-sectional view of a second embodiment of a sampling probe that may be used with the instrument of FIG. 2;

FIG. 5 is a graph showing the trend of the concentration of gaseous ammonia measured over time during transdermal sampling by the apparatus of FIG. 2;

FIG. 6 is a graph showing the trend of the concentration of gaseous ammonia over time on blood samples measured by the instrument of FIG. 2, which blood samples were periodically taken during a hemodialysis cycle; and

FIG. 7 is a graph showing the trend over time of the concentration of gaseous ammonia on a spent dialysate sample taken periodically during a hemodialysis cycle as measured by the instrument of FIG. 2.

With reference to fig. 1, it is seen that an amperometric sensor according to the invention comprises a duct 1, said duct 1 being adapted to be crossed by an air flow and being provided with an inlet 2 and an outlet 3. The delivery tube 1 may be made of any suitable material. The delivery tube 1 may be, for example, a glass tube, which in the preferred embodiment has a T-shape. The outlet 3 is arranged at the crossbar 1a of the pipe.

The sensor according to the invention also comprises a reference current element consisting of a container 4 containing an electrolyte 5 and a reference electrode 6 inserted in the container 4. The container 4 is fixed to the delivery pipe 1 by means of, for example, friction or a threaded connection. The current element for measurement of the sensor comprises a measuring electrode 7 arranged substantially transversely to the axis of the delivery tube 1 and a wire element 8 with high capillarity, said wire element 8 being for example a knitted cotton yarn anchored on the container 4 and having a first end 8a contacting the measuring electrode 7 and a second end 8b contacting the electrolyte 5. In the embodiment shown in the figures, the filiform element 8 is placed in a position substantially coinciding with the axis of the delivery tube 1.

The working fluid rises through the filamentary element 8 by capillary action to wet the measuring electrode 7, i.e. the element 8 acts as a wick. Thus, between the measuring electrode 7 and the reference electrode 6, a potential difference exists based on the redox potentials of the two current elements and is measured.

In the preferred embodiment, the measuring electrode 7 and the reference electrode 6 are small metal rods made of stainless steel, but other materials known for use as electrodes may be used.

In the sensor according to the invention, the current element comprising the measuring electrode is very compact, since the quantity of electrolyte that moistens the measuring electrode 7 is determined by the very small size of the contact area between the first end 8a of the filiform element 8 and the measuring electrode 7. For example, if the electrode has a diameter of 1mm and the filiform element has a diameter of 0.1mm, while the filiform element forms a complete coil around the electrode, the amount of electrolyte that moistens the electrode 7 is of the order of 1 microliter.

Based on extensive tests performed by the inventors with a standard liquid containing a known amount of gas, it was possible to verify that such a very small amount of electrolyte obtained by the wicking effect of the element 8 is suitable for detecting amounts of gas of the order of 0.1ppm or less. Similarly, by suitable choice of the diameter of the filiform element, the diameter of the electrode and the size of the contact area between the filiform element and the measuring electrode, it is possible, by suitable calibration, to obtain the desired sensitivity for correct measurement of the desired amount of blood gas present in the blood.

This particular feature of the invention allows the analysis of the gases present in the blood with a minimum amount of sample gas and makes it suitable for measuring only the gases present in small amounts, such as ammonia, hydrogen sulphide and nitric oxide. Thus, in the case of percutaneous sampling, it is not necessary to heat the patient's skin to enhance its permeability and to collect more blood gases. Moreover, the response times of the sensors are faster, since they depend only on the reaction kinetics between the gas to be analyzed and the electrolyte used in the sensor.

The electrolyte 5 used may be, for example, a dilute aqueous solution of ammonium chloride, for example in the case of ammonia.

In addition, the electrolyte used must be chosen so as to avoid interference from other gases present in the blood. In the case of a dilute aqueous solution of ammonium chloride, which does not react with oxygen, there is no interference from oxygen. To avoid CO₂Reacting with water, the fact that CO is reacted with water can be advantageously utilized₂Is lower than ammonia. Therefore, by properly setting the time for the gas flow to pass through the sensor, it is possible to avoid CO altogether₂The interference of (2).

The choice of electrolyte, the material and geometry of the filiform element and the number of its coils surrounding the measuring electrode, as well as the measuring time, are all important parameters in the construction of the sensor, which serve to define both its sensitivity and its response speed.

Fig. 2 shows an instrument for analyzing blood gases comprising an amperometric sensor 9 according to the invention and a first means 10, such as a potentiometer, connected thereto and adapted to detect the potential difference between the electrodes, said first means 10 being for example a potentiometer. A second device 11, such as a personal computer, is connected to the first device 10 and is adapted to process and store the potential difference data detected by the first device 10. As mentioned above, a potential difference exists between the measurement electrode 7 and the reference electrode 6, which is based on the redox potential of the two electrical elements. Therefore, by measuring this potential difference over time using a potentiometer and by continuously acquiring, storing and processing the measured value, it is possible to monitor ammonia contained in blood gas in real time.

As shown, the apparatus according to the invention further comprises a probe 12 for collecting the gas. The downstream end of probe 12 is connected to amperometric sensor 9 and the upstream end is connected to a source 13 of carrier gas, such as ambient air, suitable for carrying the gas present in the blood towards amperometric sensor 9. The carrier gas is pumped from the carrier gas source 13 by a pump 14 and filtered and purified by a series of filters 15 arranged downstream of the pump 14. A flow bypass 16 is arranged between filter 15 and probe 12 to allow carrier gas to be alternately directed to probe 12 and thus to amperometric sensor 9, or directly towards amperometric sensor 9 without passing beyond probe 12.

The connections between the various above-mentioned components of the instrument, i.e. the amperometric sensor 9, the probe 12, the source 13, the pump 14, the filter 15 and the flow bypass 16, pass through the gas-impermeable tube 17. These tubes 17 may be made of PTFE or stainless steel and preferably have an internal diameter of about 1mm, suitable for ensuring a flow rate of the carrier gas preferably between 1 and 5 ml/s.

Fig. 3 shows a first embodiment of a probe 12, particularly suitable for transcutaneous sampling of gases. The probe consists of a bell-shaped member having a base with an opening 18 to allow percutaneous recovery of the gas. The bell-shaped element is also provided with an inlet 19 and an outlet 20 suitable for allowing the carrier gas to pass through the bell. In particular, the inlet 19 is connected to the pipe 17a coming from the bypass 16 and the outlet 20 is connected to the pipe 17b leading to the amperometric sensor 9. The base opening 18 of the bell-shaped member defines no more than 1cm²Is necessary to ensure that sufficient blood gas flows into the clock.

Fig. 4 shows a second embodiment of a probe 12 which can be used for collecting gas transcutaneously or for collecting gas from a blood or saliva sample collected in an analysis unit.

The probe 12 consists of a small tube of porous material, such as PTFE, with a small pore size on the order of microns. Similar to the bell probe, a small tube of porous PTFE is inserted between tubes 17a and 17b and a carrier gas can pass through the small tube. In order to allow the recovery of a sufficient amount of gas, the small tube portion comprised between the ends of the tubes 17a and 17b has a length preferably between 1 and 2 cm.

In the case of percutaneous sampling, the tubule is bent into a "U" shape and placed across the patient's finger, which is grasped by the patient to hold the probe 12.

When collecting gas from a blood or saliva sample contained in the analysis unit, the tubes 17a and 17b are sealingly inserted into the lid of the closure unit, so that the small tubes are suspended above the sample to be analyzed.

During operation of the apparatus, the carrier gas flow is at a preset measurement time t_MIs pumped out, for example 10s, through the tube 12, in the course of which blood gas collected by the probe 12 is taken and conveyed to the amperometric sensor 9, thereby colliding with the measuring electrode 7. When measuring ammonia, a part of the ammonia molecules enters the solution containing ammonia chloride inside the end of the filiform element 8 contacting the measuring electrode 7, thereby forming NH₄ ⁺And OH^-Ions. The negative OH "ions bind to the iron ions already in solution, so the redox potential of the measuring element is changed according to Nernst's law. Thus, the potentiometer 10 connected to the electrodes 6 and 7 detects a potential difference which is different from the initial potential difference and which is related to the concentration of ammonia present in the blood by suitable calibration of the amperometric sensor 9. Next, the carrier gas is allowed to flow for a recovery time t by acting on the flow bypass 16_RInternal direct flow to the sensor, e.g. 50s, during which the initial state of the amperometric sensor is stored.

A standard reference cell 21, containing a solution of a gas to be analyzed of known concentration, such as an aqueous solution of ammonia, can optionally be arranged between the filter 15 and the bypass 16. In this way it is possible to set different starting states of the amperometric sensor 9, thereby obtaining a more or less fast recovery time depending on the operating mode established by the instrument.

By repeated measurements of the sensor over time and the recovery of circulation, a continuous analysis of the gases present in the blood is possible, thereby allowing the diagnosis of different pathologies related to blood gases and the monitoring of the patient.

The following examples illustrate some uses of the instrument and sensor according to the invention.

Example 1

An instrument is prepared for gas analysis comprising an amperometric sensor according to the invention, a potentiometer and a computer adapted to acquire, store and process the measured values of the potential difference obtained by the potentiometer. The instrument is also provided with a probe for the transcutaneous sampling of blood gases of the type shown in fig. 4, and with a source of carrier gas, in particular ambient air, connected to a pump and a series of filters, and a flow bypass, by a tubing made of PTFE and having a diameter of 1.2 mm.

The amperometric sensor is provided with a reference element comprising a dilute aqueous solution of ammonium chloride. The filiform element used was cotton yarn with a diameter of 0.1mm and wound to form a coil around a measuring electrode made of stainless steel and having a diameter of 1 mm.

The acquisition probe spans the middle finger of a healthy patient at the metacarpal joint and can be easily held in place by gripping the hand.

Each of the three capsules containing a 0.5g dose of ammonia chloride was initially administered to the patient. Next, the instrument was opened and the carrier gas was flowed at a flow rate of 3 ml/s. By acting on the bypass, a flow of carrier gas is alternately pumped through the probe for a measurement time of 10s, whereby the blood gas recovered by the probe is conveyed to the sensor, and directly to the sensor for a recovery time of 20 s.

The instrument was run continuously for 30 minutes, detecting each interval of the measurement time and recovery time values of the potential difference proportional to the ammonia concentration in the blood gas. These values are listed in table 1 below and in the chart of fig. 5.

It can be seen that, approximately 5 minutes after the ammonia chloride has been delivered, the concentration value of the gaseous ammonia increases gradually to a maximum value and then decreases to a value equal to the initial value.

TABLE 1

Time [ minute ]]	ΔE[mV]	C NH3[ppm atm]
			0	2.7	56
5	2.7	56
			8	3.2	67
9	3.4	71
			10	3.8	79
11	4.8	100
			13	4.3	90
15	3.8	79
			16	3.5	73
17	3.2	67
			18	3.1	65
19	2.7	56
			20	2.7	56
23	2.7	56
			30	2.7	56

Example 2

A gas analysis instrument similar to the one described in example 1 was prepared by tightly inserting the probe into the lid of an analysis cell adapted to receive a blood sample.

The instrument was used in the same manner as described in example 1 during the hemodialysis cycle. During a hemodialysis cycle, the patient typically eats a snack after about 30 minutes from the start of treatment and takes lunch and coffee about 60 minutes after the snack.

Samples of blood of the order of 1g were taken at regular 30 minute intervals, by inserting a syringe into the tube that conveys the patient's blood towards the inlet of the hemodialysis machine, over a period of 4 hours. These blood samples are treated with a buffer to produce a known level of PH, such as 9.1.

The data detected by the sensors are listed in table 2 below and in the graph of fig. 6, and show how the variation of the ammonia concentration contained in the blood gases is correlated with the food consumed by the patient and the subsequent digestion step. In particular, the content of ammonia is initially reduced due to the filtering effect of the operation of the hemodialysis machine and increases after eating the food during the digestion step.

TABLE 2

Time [ minute ]]	ΔE[mV]	C NH3[ppm atm]
			0	-26.5	60.8
30	-26.0	59.6
			60	-23.0	52.8
90	-22.7	52.4
			120	-23.1	53.0
150	-24.6	56.4
			180	-23.9	54.8
210	-21.5	49.3
			240	-21.8	50.0

For comparison purposes, example 2 was repeated on samples of the drained dialysate obtained during the same hemodialysis treatment, thereby providing a correlation between the change in the concentration of gaseous ammonia in the blood and the change in the concentration of ammonia in the drained dialysate. The data detected by the sensors are listed in table 3 below and in the graph of fig. 7.

TABLE 3

Time [ minute ]]	ΔE[mV]	C NH3[ppm atm]
			5	-37.8	20.7
30	-6.5	3.6

60	-6.8	3.7
			90	-9.8	5.4
120	-13.0	7.1
			150	-21.0	11.5
180	-20.9	11.4
			210	-20.8	11.3
240	-21.0	11.5

The above-described and illustrated embodiments of the sensor and instrument according to the invention are only examples of the influence of a large number of variables. In particular, it is possible to make other sampling probes depending on the body part chosen for analyzing the presence of gases in the blood, such as, for example, a small tubular probe made of silicone rubber that can be inserted in the mouth between the patient's palate and tongue.

Claims (20)

1. A sensor for analyzing a gas, the sensor comprising: a duct (1) suitable for being crossed by a gas flow and provided with an inlet (2) and an outlet (3); an amperometric reference element consisting of a container (4), said container (4) containing an electrolyte (5) into which a reference electrode (6) is inserted; and an amperometric measuring element comprising a measuring electrode (7) and a wick (8), said wick (8) being associated with said measuring electrode (7) and being adapted to define an electrode/electrolyte interface thereon, characterized in that said container (4) is fixed to said delivery pipe (1) and in that said measuring electrode (7) is arranged transversely with respect to the axis of the delivery pipe (1), said wick (8) being in the form of a filiform element anchored to said container (4) and having a first end (8a) contacting said measuring electrode (7) and a second end (8b) contacting said electrolyte (5), said wick (8) forming at least one coil around the measuring electrode (7).

2. A sensor according to claim 1, characterized in that the amount of electrolyte (5) wetting the measuring electrode (7) is of the order of 1 microliter.

3. Sensor according to claim 1 or 2, characterized in that the delivery tube (1) is a T-glass tube, the gas inlet (2) being arranged at one end of the tube and the gas outlet (3) being arranged on the crossbar (1a) of the tube.

4. A sensor according to claim 1 or 2, wherein the electrolyte (5) is a dilute aqueous solution of ammonium chloride.

5. Sensor according to claim 1 or 2, characterized in that the measuring electrode (7) is a small metal rod.

6. Sensor according to claim 5, characterized in that the measuring electrode (7) is made of stainless steel.

7. A sensor according to claim 1 or 2, characterized in that the reference electrode (6) is a small metal rod.

8. Sensor according to claim 7, characterized in that the reference electrode (6) is made of stainless steel.

9. A sensor according to claim 1 or 2, characterized in that the wick (8) is made of cotton.

10. A sensor according to claim 9, characterized in that the wick (8) is a woven cotton yarn.

11. An apparatus for analyzing blood gases, the apparatus comprising: an amperometric sensor (9) according to any one of claims 1 to 10; -first means (10) connected to said amperometric sensor (9) and adapted to measure the potential difference between a measuring electrode (7) and a reference electrode (6) of the sensor (9); and second means (11), said second means (11) being connected to said first means (10) and being adapted to obtain, store and process potential difference measurements obtained by the first means (10).

12. The apparatus according to claim 11, characterized in that it further comprises a probe (12) for sampling blood gases, the amperometric sensor (9) being connected downstream of the probe (12), a source (13) of carrier gas being connected upstream of the probe (12), the carrier gas being pumped from said source (13) by a pump (14) and being filtered and purified by a series of filters (15) arranged downstream of said pump (14), a flow bypass (16) being arranged between said filters (15) and said probe (12) and connecting the components (9, 12, 13, 14, 15, 16) of the apparatus through a system of ducts of gas impermeable tubes (17).

13. The apparatus according to claim 12, characterized in that said probe (12) for gas sampling consists of a bell-shaped element having an opening (18) at the base to allow percutaneous recovery of the gas.

14. The apparatus according to claim 13, characterized in that the probe (12) is further provided with an inlet (19) and an outlet (20), said inlet (19) being connected to the tube (17a) from the bypass (16) and said outlet being connected to the tube (17b) leading to the amperometric sensor (9).

15. The instrument according to claim 14, characterized in that the opening (18) in the base body of the probe (12) defines not more than 1cm²The area of (a).

16. The apparatus according to claim 12, characterized in that said gas sampling probe (12) consists of a small tube made of porous material having ends respectively interposed between a tube (17a) coming from the bypass (16) and a tube (17b) leading to the amperometric sensor (9), and having a free portion adapted to allow the recovery of the gas.

17. The apparatus according to claim 16, characterized in that said porous material is PTFE and the pore diameter is of the order of microns.

18. The apparatus according to claim 16 or 17, characterised in that the free portion of the small tube of porous material comprised between the ends of the tubes (17a, 17b) has a length of between 1 and 2 cm.

19. The apparatus according to any one of claims 12-17, wherein the carrier gas source (13) is adapted to provide ambient air.

20. An apparatus according to any one of claims 12-17, characterized in that the tube (17) connecting its components is made of a material selected from PTFE and stainless steel.