Method for flushing and calibrating a sensor in a body fluid analysis system

Abstract: The present invention relates to a method for flushing and calibrating at least one sensor in a system for body fluid analysis, said system comprising a first container, connected to the sensor, holding a first flushing liquid, and a second container, connected to the sensor, holding a second flushing liquid, said flushing liquids having different analytic contents. The method is characterised by performance of a calibration measurement with one of the flushing liquids before every body fluid analysis determination, whereupon the different flushing liquids are used alternately according to a predetermined pattern, a pattern governed by certain criteria or a combination thereof.

Description: The present invention relates to a method for flushing and calibrating at least one sensor in a system for body fluid analysis. When a body fluid, such as blood, undergoes extracorporeal analysis by a known system for continuous or semi-continuous analyses with the aid of a venous or arterial catheter, the heart or an external pump pumps blood to a sensor outside the body. After the blood analysis performed by the sensor, blood is forced back to the body by pumping flushing liquid in the opposite direction. When most of the blood has been forced back to the patient, additional flushing liquid flushes the sensor to remove all remaining blood residue. This flushing liquid can either be infused into the patient or collected in a special bag. When this flushing is performed, the flushing liquid, which is then in contact with the sensor, is sometimes used for calibration. The sensor's characteristics nevertheless change over time. Periodic two-point calibration is commonly performed in order to attain acceptable accuracy and a reasonable operating life for the sensor. An article entitled "Integrated pO 2 , PCO 2 , pH sensor system for on-line blood monitoring", Sensors and Actuators B, 18-19 (1994), pp. 704-708, by Gumbrecht et al. described a blood analysis system prepared for two-point calibration. The blood analysis system comprises two calibration pumps, a sampling pump, two liquid containers, a collection container and a sensor means. In traditional and known two-point calibration, as in the use of the aforementioned blood analysis system, two-point calibration is always performed in two calibration determinations using different flushing liquids between two blood analyses. In the aforementioned blood analysis system, two-point calibration is performed at certain intervals. To ensure that the flushing liquids really have the right analytic content when intervals between calibrations are long, the tubing must be flushed, a process which is time-consuming and requires a fast pumping rate. Instability and a shortened operating life for the system in continuous or semi-continuous blood analyses are other disadvantages of this calibration method, since frequent two-point calibration greatly increases intervals between blood analysis determinations. The objective of the present invention is solve the aforementioned problems. This is achieved with a method for flushing and calibrating at least one sensor in a system for body fluid analysis according to claim 1. The known system comprises a first container, connected to the sensor, holding a first flushing liquid and a second container, connected to the sensor, holding a second flushing liquid. These flushing liquids have different analytic contents. The method according to the present invention entails steps in which every body fluid analysis is preceded by no more than one calibration measurement using one of the flushing liquids. The different flushing liquids are then used alternately according to a predetermined pattern, a pattern governed by certain criteria or a combination thereof. One advantage of the method according to the present invention is that the body fluid analysis system's accuracy and reliability can be increased with no need to increase the interval between body fluid analyses. This becomes possible when both flushing liquids are used for calibration and these liquids are used alternately, e.g. every other time. The sensor can then undergo two-point calibration in conjunction with every measuring point when the preceding flushing is used as one point and the following flushing is used as the second point. Calibration will then be much better, without any risk of sudden, unexpected changes in performance causing systematic error over a long period of time. Another advantage of the method according to the invention is that neither of the flushing liquids needs to remain in tubing very long, so they do not lose their specific gas content (analytic content). Turnover of the two flushing liquids can therefore be relatively fast. According to a first aspect of the invention, the second liquid is used for every n:th calibration measurement, n being a whole number and n≥2. The first calibration liquid can then be used for other calibrations. Here, performing every other calibration with the first flushing liquid and every other calibration with the second flushing liquid would be particularly advantageous. This would result in maximal turnover of the flushing liquids with minimal time between each body fluid analysis. According to a second aspect of the invention, the second flushing liquid is automatically used for calibration measurements when certain criteria are met. According to a third aspect, the second flushing liquid is used for calibration measurements after manual operation. In conjunction herewith, it would be advantageous, in a body fluid analysis system which additionally comprises a collection means connected to the sensor, a first pumping means connected between the first container and the sensor, a second pumping means connected between the second container and the sensor and a third pumping means connected between the collection means and the sensor, if the method comprised the following steps which do not need to be performed in the stated order: The first pumping means pumps the first flushing liquid for flushing the sensor, whereupon the sensor performs a calibration measurement of the first flushing liquid; The sensor performs a body fluid determination; and The second pumping means pumps the second flushing liquid for flushing the sensor, whereupon the sensor performs a calibration measurement of the second flushing liquid. In conjunction herewith, it would be advantageous if the method also comprised the following step: The third pumping means pumps the flushing liquids to the collection means during the flushing stages. An additional advantage in conjunction herewith is achieved if the method also comprises the following step: At least one of the pumping means is used to pump body fluid to the sensor during body fluid analysis measurements. In conjunction herewith it would be advantageous, with a body fluid analysis system which also comprises a pressure sensor and bubble detector means between each of the pumping means and the sensor, if the method also comprised the following step: The pressure sensor and bubble detector means monitors the pressure and presence of bubbles during blood analysis and calibration measurement stages. The invention will now be described in greater detail, referring to attached drawings, in the following description of preferred embodiments of same. FIG. 1 shows a blood analysis system as one embodiment of a body fluid analysis system; and FIG. 2 is a flow chart showing a method according to the invention for flushing and calibrating at least one sensor in the blood analysis system shown in FIG. 1. FIG. 1 shows a blood analysis system 10 for extracorporeal blood analysis as one embodiment of a body fluid analysis system. The blood analysis system 10 comprises a first container 12 holding a first flushing liquid 14 and a second container 16 holding a second flushing liquid 18. These flushing liquids 14, 18 have different analytic contents. The blood analysis system 10 also comprises a collection means 20 for collecting used flushing liquid, i.e. "waste" liquid 22. The blood analysis system 10 also comprises a sensor 24 for performing blood analysis measurements. The sensor 24 can e.g. be a chemical sensor. The sensor 24 is connected to the first container 12 by a first tube 26, to the second container 16 by a second tube 28 and to the collection means 20 by a third tube 30. The sensor 24 is intended for connection, via a fourth tube 32, to a patient's blood circulation by means of a venous or arterial catheter (not shown). The blood analysis system 10 also comprises a first pumping means 34 connected between the first container 12 and the sensor 24, a second pumping means 36 connected between the second container 16 and the sensor 24 and a third pumping means 38 connected between the collection means 20 and the sensor 24. All the pumping means 34, 36, 38 are double-acting pumps, i.e. they are capable of pumping in both directions, as schematically designated with the arrows in FIG. 1. The blood analysis system 10 also comprises a pressure sensor means and bubble detector means 40, 42, 44, connected between each pumping means 34, 36, 38 and the sensor 24, which monitors pressure and the presence of bubbles during the blood analysis and calibration stages. The blood analysis system 10 shown in FIG. 1 is only one example of a known system. This type of blood analysis system 10 can be devised in a plurality of different ways, e.g. the sensor can be located in different ways. FIG. 2 is a flow chart depicting a method, according to the invention, for flushing and calibrating at least one sensor in a blood analysis system, e.g. a system such as the one described in conjunction with the description of FIG. 1. Even if FIG. 2 shows and the description concerns a method for flushing and calibrating at least one sensor in a blood analysis system, the method is equally applicable to a sensor in a body fluid system. The method starts at block 50. A calibration measurement using one of the flushing liquids, i.e. referred to here as "flushing liquid 1", e.g. the first flushing liquid 14 in FIG. 1, starts at block 52. A blood analysis determination, performed by the sensor 24, then occurs at block 54. Block 56 queries about whether two-point calibration is to be performed. The answer, "yes" or "no", to this query depends, according to the present invention, on the way the various flushing liquids are to be used alternately. According to the method, use is according to a predetermined pattern, a pattern governed by certain criteria or a combination thereof. Blocks 52, 54, 56 are repeated if the reply at query block 56 is "no". But if the reply is "yes", the method continues from block 58, whereupon a calibration measurement is made with flushing liquid 2, i.e. the second flushing liquid 18 in FIG. 1. The sensor 24 then performs a new blood analysis determination at block 60 (cf. FIG. 1). Block 62 queries about whether two-point calibration is to be performed. Whether the reply is "yes" or "no" is determined the same way as in query block 56. If the reply at block 62 is "no", blocks 58, 60 and 62 are repeated. However, if the reply is "yes", the method continues from block 52 etc. The method accordingly continues to be performed in this loop until someone e.g. turns off the blood analysis system 10 or disconnects it from the patient. A pre-defined pattern could be e.g. use of the second flushing liquid 18 for every n:th calibration, n being a whole number and n≥2. Here, it would be particularly advantageous if every other calibration were performed with the first flushing liquid and every other calibration with the second flushing liquid (i.e. n=2). This would result in maximal turnover of the flushing liquids with minimal time between each body fluid analysis. A pattern governed by certain criteria could entail e.g. automatic use of the second flushing liquid 18 for calibration measurement whenever certain criteria are met. These criteria could be based on e.g. signal drift in the system. Another pattern governed by certain criteria could entail use of the second calibration liquid 18 for manual calibration measurement, i.e. calibration initiated by a physician when needed. This step, performed at block 52, can be described in greater detail, referring to the system according to the FIG., as follows: The first pumping means 34 pumps the first flushing liquid 14 to flush the sensor 24. The sensor 24 then performs a calibration measurement of the first flushing liquid 14. The step performed at block 58 can be described in greater detail as follows: The second pumping means 36 pumps the second flushing liquid 18 to flush the sensor 24. The sensor 24 then performs a calibration measurement of the second flushing liquid 18. The method also comprises the following steps: The third pumping means 38 pumps used flushing liquid 14, 18 to the collection means 20 during flushing operations. The method also comprises the following step: At least one of the pumping means is used for pumping blood to the sensor 24 during blood analysis determinations. The method also comprises the following step: A pressure sensor and bubble detector means 40, 42, 44 monitors pressure and the presence of bubbles during the blood analysis determination and calibration steps. In the above-described method for flushing and calibration, two-point calibration is always executed in such a way that calibration is only performed between two blood analysis determinations. In two-point calibration according to the prior art, two calibration measurements are always performed between every two blood analysis determinations. With two-point calibration, the output signal's offset (A) and sensitivity (B) can be determined according to y = A + Bx. The invention is not restricted to the cited embodiment. A plurality of variations are possible within the scope of the attached claims. A number of variations in the calibration procedure is obvious in use of the above-described method according to the invention. It is not even necessary for every explicit body fluid analysis to be preceded by a calibration. The choice of sensor, type of body fluid etc. dictate this need. Thus, the method can e.g. be performed so calibration is only carried out before two out of three analyses, two different flushing liquids being utilised for these calibrations. The result is running two-point calibration at three-analysis intervals.

Claims: A method for flushing and calibrating at least one sensor (24) in a system (10) for body fluid analysis, said system (10) comprising a first container (12), holding a first flushing liquid (14), connected to the sensor (24) and a second container (16), holding a second flushing liquid (18), connected to the sensor (24), said flushing liquids (14, 18) having different analytic contents, characterised in that each body fluid analysis is preceded by no more than one calibration measurement using one of the flushing liquids, the different flushing liquids being alternately used according to a predetermined pattern, according to a pattern governed by certain criteria or a combination thereof. The method for flushing and calibration according to claim 1, characterised by use of the second flushing liquid (14;18) for every n:th calibration, n being a whole number and n≥2. The method for flushing and calibration according to claims 1-2, characterised in that the second flushing liquid (14;18) is used/also used automatically for calibration measurements whenever certain criteria are met. The method for flushing and calibration according to any of claims 1-3, characterised in that the second flushing liquid (14;18) is used/also used in calibration measurements when calibration is manually controlled. The method for flushing and calibration according to any of claims 1-4, said system also containing a collection means (20) connected to the sensor (24), a first pumping means (34) connected between the first container (12) and the sensor (24), a second pumping means (36) connected between the second container (16) and the sensor (24) and a third pumping means (38) connected between the collection means (20) and the sensor (24), characterised in that the method comprises steps which need not be performed in the following sequence: The first pumping means (34) pumps the first flushing liquid (14) to flush the sensor (24), whereupon the sensor (24) performs a calibration measurement of the first flushing liquid (14); The sensor (24) performs a body fluid analysis; and The second pumping means (36) pumps the second flushing liquid (18) to flush the sensor (24), whereupon the sensor (24) performs a calibration measurement of the second flushing liquid (18). The method for flushing and calibration according to claim 5, characterised in that the method also comprises the following step: The third pumping means (38) pumps the flushing liquids (14, 18) to the collection means during the flushing steps. The method for flushing and calibration according to claim 6, characterised in that the method also comprises the following step: at least one of the pumping means (34, 36, 38) is used to pump body fluid to the sensor (24) during body fluid analyses. The method for flushing and calibration according to claim 7, said system also comprising a pressure sensor and bubble detector means (40, 42, 44), connected between each of the pumping means (34, 36, 38) and the sensor (24), said method also being characterised by the following step; The pressure sensor and bubble detector means (40, 42, 44) monitors pressure and the presence of bubbles during body fluid analysis and calibration measurement steps.

Inventors: VON BAHR PONTUS, VON BAHR, PONTUS
Applicants: SIEMENS ELEMA AB, SIEMENS-ELEMA AB
Application year: 1998
Main classification: G01N33
IPC codes: A61B5, G01N33