GB2485558A - Blood analysis apparatus for use with an oxygenator - Google Patents

Abstract

A blood analysis apparatus is provided for use in a system in which blood from a patient is provided to an oxygenator 40 and in which blood from the oxygenator is provided back to the patient. The apparatus comprises a first group of sensors 6, 7 for directly measuring a corresponding first group of properties of the blood provided to or from the oxygenator; and a second group of sensors 1, 2, 3, 4 for directly measuring a corresponding second group of properties of a gas provided to or from the oxygenator. The apparatus also comprises a processor 60 for determining a value for the partial pressure of oxygen in the blood and/or a value for the partial pressure of carbon dioxide in the blood in dependence upon the first group of directly measured properties and the second group of directly measured properties. Preferably, the first group of sensors measures its corresponding property without coming into contact with the blood.

Description

I

Blood Analysis Apparatus and Method The present invention relates to a blood analysis apparatus and method.

Known blood gas analysers typically provide direct measurements of various properties of the blood, such as pH, partial pressure of oxygen (P02), partial pressure of carbon dioxide (P002), and oxygen saturation. When relating to arterial blood flow (as opposed to venous blood flow), P02 and P002 are sometimes known respectively as Pa02 and PaCO2, or simply Pa02 and PaCO2.

These direct measurements of P02 and P002 are typically used to evaluate oxygenation F. of the tissues and pulmonary function. For example, P02 reflects the amount of oxygen gas dissolved in the blood, and is an indicator of the effectiveness of the lungs in transferring oxygen into the blood stream from the air breathed in. Decreased P02 levels may be associated with anaemia, hypoventilation or pulmonary disease. On the other hand, P002 reflects the exchange of 002 through the lungs to the outside air. Increased 1::: levels of P002 may be caused by pulmonary oedema or lung disease, while decreased F levels of P002 may be caused by hyperventilation or hypoxia.

Known techniques for measuring P02 and P002 are invasive in the sense that contact is required between the sensor being used and the blood being analysed Even when the measurements are carried out extra-corporeally, such that some degree of invasiveness is anyway required to extract the blood contact with blood is undesirable for reasons of hygiene and sensor re-use Such measurements are also direct in the sense that a sensor is provided whose specific purpose is to measure a value of P02 or P002, as the case may be the values are not derived from other measured properties, although other measured properties may be taken into account for reasons of accuracy or adjustment The present applicant has appreciated the desirability of providing an apparatus and method for the indirect and/or non-invasive determination of P02 and P002, for example during cardiopulmonary bypass surgery or when membrane oxygenation of patients blood is carned out, for example extra-corporeal membrane oxygenation (E0MO) procedures According to a first aspect of the present invention there is provided a blood analysis apparatus for use in a system in which blood from a patient is provided to an oxygenator and in which blood from the oxygenator is provided back to the patient. A first group of sensors is provided for directly measuring a corresponding first group of properties of the brood provided to or from the oxygenator. A second group of sensors is provided for directly measuring a corresponding second group of properties of a gas provided to or from the oxygenator. A processor is provided for determining a value for the partial pressure of oxygen in the blood and/or a value for the partial pressure of carbon dioxide in the blood in dependence upon the first group of directly measured properties and the second group of directly measured properties.

According to a second aspect of the present invention there is provided a blood analysis method. Blood is provided from a patient to an oxygenator. Blood is provided from the oxygenator back to the patient. A first group of properties of the blood provided to or from the oxygenator is directly measured using a corresponding first group of sensors. A second group of properties of a gas provided to or from the oxygenator is directly measured using a corresponding second group of sensors. A value is determined for the partial pressure of oxygen in the blood and/or a value for the partial pressure of carbon dioxide in the blood in dependence upon the first group of directly measured properties and the second group of directly measured properties.

At least one of the first group of sensors may be adapted to measure its corresponding property without coming into contact with the blood. It may be that each of the first group of sensors is adapted to measure its corresponding property without coming into contact with the blood.

The first group of sensors may comprise a blood temperature sensor for measuring a temperature of the blood provided to or from the oxygenator and a blood flow rate sensor for measuring a flow rate of the blood provided to or from the oxygenator The second group of sensors may comprise a carbon dioxide out sensor for measuring a concentration of carbon dioxide in the gas provided from the oxygenator The partial pressure of carbon dioxide in the blood may be determined in dependence upon the measurements of blood temperature, blood flow rate and concentration of carbon dioxide in the gas provided from the oxygenator The second group of sensors may comprise an oxygen in sensor for measuring a concentration of oxygen in the gas provided to the oxygenator. The partial pressure of oxygen in the blood may be determined in dependence upon the measurements of blood temperature, blood flow rate, concentration of carbon dioxide in the gas provided from the oxygenator, and concentration of oxygen in the gas provided to the oxygenator.

The second group of sensors may comprise a carbon dioxide in sensor for measuring a concentration of carbon dioxide in the gas provided to the oxygenator. The partial pressure may be determined in dependence upon the measurement of concentration of carbon dioxide in the gas provided to the oxygenator.

The first group of sensors may comprise one or both of a haemoglobin sensor for measuring a level of haemoglobin in the blood provided to or from the oxygenator and a blood saturation sensor for measuring a level of red blood cell oxygen saturation provided to or from the oxygenator The partial pressure may be determined in dependence upon the measurement of haemoglobin and/or blood saturation level.

The second group of sensors may comprise one or both of a gas temperature sensor for measuring a temperature of the gas provided to or from the oxygenator and a gas flow rate sensor for measuring a gas flow rate of the gas provided to or from the oxygenator.

The partial pressure may be determined in dependence upon the measurement of gas temperature and/or flow rate The blood analysis apparatus may comprise a pressure sensor for measuring atmospheric pressure The blood analysis method may comprise measuring atmospheric pressure F The partial pressure may be determined in dependence upon the measurement of atmospheric pressure The pressure sensor may form part of the second group of sensors The blood analysis apparatus may comprise a second processor for applying a correction to one or more of the measurements of concentration The blood analysis method may comprise applying a correction to one or more of the measurements of concentration The partial pressure of oxygen in the blood may be determined according to function (11) below The partial pressure of carbon dioxide in the blood may be determined according to function (12) below.

Each of the sensors is adapted to measure its corresponding respective property extra-corporeally. Therefore, a method and apparatus according to an embodiment of the present invention is intended to be used extra-corporeally, and not directly on the human or animal body.

According to a third aspect of the present invention there is provided a program for controlling an apparatus to perform a method according to the second aspect of the present invention or which, when loaded into an apparatus, causes the apparatus to become an apparatus according to the first aspect of the present invention. The program may be carried on a carrier medium. The carrier medium may be a storage medium. The carrier medium may be a transmission medium.

According to an fourth aspect of the present invention there is provided an apparatus programmed by a program according to the third aspect of the present invention.

According to a fifth aspect of the present invention there is provided a storage medium containing a program according to the third aspect of the present invention.

Reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 illustrates a system embodying the present invention; and Figure 2 illustrates a method embodying the present invention As mentioned above, the aim of an embodiment of the present invention is to determine the levels of P02 and PCO2 within the arterial blood stream and in particular to achieve this by indirect and/or non-invasive means The present applicant has appreciated that, in a situation where cardiopulmonary bypass surgery is being undertaken or when membrane oxygenation of patients blood (e g EGMO) is being carried out, there is a possibility of determining P02 and PCO2 levels by monitoring the gas conditions being presented to the oxygenator and the gas conditions leaving the oxygenator.

There are several equations that are used in everyday medicine which provide a certain amount of knowledge about the relationship between the gas breathed in and out and the amount of P02 and PCO2 that occurs in the blood stream.

A first equation of interest is the following for arterial oxygen content (Ca02): CaD2 = Hb(grn / di)1.34(O / gm)Hb * SaC2 + PaO, * O.003(m102 / rnmHg / dl) However, the present applicant has appreciated that the measurements of Sa02 (arterial oxygen saturation) and Hb (haemoglobin) are needed at a very high precision for thrs F equation to be effective. Furthermore, Pa02 is required, which is what an embodiment of the present invention is seeking to determine.

The well-known alveolar gas equation is also of interest (where PAO2 is alveolar P02 rather than artenal Pa2 or Pa 02) PAD2 = F102 (BP -47) -1 2PaCO2 In the above equation, F102 is the fraction of inspired oxygen The properties that one might therefore consider to be relevant to the above are Oxygen%in * Atmospheric pressure * Carbon Dioxide % out However, the present applicant has appreciated that, although the above equations are physiologically correct, they cannot be used to determine P02 or PCO2 when an oxygenator is involved The oxygenator efficiency and performance has been found to override the equations to make the accuracy unacceptable However some of the basic principles can be used in an embodiment of the present invention The present applicant has therefore appreciated the importance of taking account of and characterising the oxygenator system, and having functions that allow a basic equation to be used and to allow each function to modify the result by correcting for changes in various variables.

The variables (or properties) that have been identified as being useful are, but are not limited to: * Oxygen % in * Carbon Dioxide % in * Blood Temperature * Blood flow rate * Carbon Dioxide % out Figure 1 illustrates a system embodying the present invention With a system embodying the present invention, extra-corporeal blood property measurements are made externally of the patient typically during a procedure in which the patients blood is routed through a system such as a heart lung machine during a heart bypass operation.

The system comprises a venous blood flow line 20, an arterial blood flow line 30 and an oxygenator 40, for example as part of a heart-lung machine Thus, in the illustrated example the venous blood flow line 20 receives de-oxygenated blood from the patient, and the arterial blood flow line 30 provides oxygenated blood to the patient Blood in the blood flow lines 20 and 30 pass through a sterile channel such as tubing Gas is passed to the oxygenator 40 via a gas module 50 compnsing sensors 1 to 3 and gas is passed from the oxygenator 40 to a CO2 module comprising a sensor 4 These sensors I to 4 are as follows * Sensor 1 Percentage of oxygen at inlet to the oxygenator (02) * Sensor 2 Percentage of CO2 at inlet to the oxygenator (CO2in) * Sensor 3 Gas line pressure * Sensor 4 Percentage of CO2 at exit from the oxygenator (CO2out) In addition to the above parameters the following are also measured by strategically placed sensors in other locations: Sensor 5: atmospheric pressure * Sensor 6: blood flow rate * Sensor 7: blood temperature Sensor 7 is provided on the blood flow line from the oxygenator 40, sensor 6 is provided on the blood flow line to the oxygenator 40, and sensor 5 is provided in a suitable position for measuring atmospheric pressure. The positions of the sensors 6 and 7 with respect to the oxygenator 40 could be different to that as illustrated.

Although an atmospheric pressure sensor 5 and a line pressure sensor 3 are illustrated, in practice it is possible to employ only one of these. As the gas running through the device I: is vented to atmosphere the line pressure is almost exactly atmospheric pressure, and since the more accurate of the two would generally be line pressure, in practice it would be the line pressure sensor 3 that is used to determine atmospheric pressure Therefore in the description that follows, atmospheric pressure readings are indicated as coming from the line pressure sensor 3, so that a separate atmospheric pressure sensor 5 is not required.

Sensors 6 and 7 can be considered as a first group of sensors for directly measuring a corresponding first group of properties of the blood provided to or from the oxygenator 40 Sensors I to 4 can be considered as a second group of sensors for directly measunng a corresponding second group of properties of a gas provided to or from the oxygenator 40.

A more detailed description of each of the sensors I to 7 is provided below A technique been developed according to an embodiment of the present invention that enables arterial P02 and PCO2 to be determined (or inferred or predicted or calculated) using readings from the above sensors Each of sensors I to 7 is preferably non-invasive, in that they are not required to contact the blood in order to operate For example, a typical non-invasive flow rate sensor 6 measures the flow rate of extra-corporeal blood flow in the tubing by employing ultrasonic energy In order to achieve the aim of being able to determine arterial P02 and PCO2 non-invasively any sensors which are required solely to

B

enable arterial P02 and PCO2 to be determined should be of a non-inva&ve or non-contact type. However, even if an invasive type sensor is used, an embodiment of the present invention still has the advantage of determining P02 and PCO2 levels indirectly; that is, the P02 and PCO2 levels are inferred from other measured properties that might anyway be required in the system for other purposes.

It is preferable that levels of arterial P02 and PCO2 are corrected for the following interfering variables: * Temperature variations * Pressure variations * Blood flow rate variations * Oxygen inlet percentage variations * Carbon dioxide inlet and outlet variations To provide the system with these corrections, various sensors can be used as detailed in the above list.

The range required for the oxygen sensor I is thought to be quite wide, and hence a sensor that could deal with 0 to 100% would be preferable Such a sensor is available from a company called FIGARO, in the form for example of an oxygen battery type sensor This sensor also benefits from being temperature compensated The range required for the CO2 sensors 2 and 4 is thought to be somewhat lower than that F for the oxygen sensor I preferably somewhere in the region of 0 to 5% or 0 to 10% Such a sensor is available from a company called Gas Sensing Solutions, in the form of an IR device that provides both analogue and digital outputs from 0 to 20% However, an alternative (and perhaps preferred) CO2 sensor is available from a company called Dynament, in the form of a device having a full scale range of 0 to 10% CO2 and is temperature compensated this sensor also uses an infrared measurement technique with an analogue output, and has proved to be reliable and accurate, with fewer initial reading errors breakages and zero drift problems compared to other sensors tried For the temperature sensor 7, a thermistor arrangement is suitable for example a 10 K Ohm Negative Temperature Coefficient (NTC) thermistor Such a sensor is supplied by a company called Epcos which has a range of 0 to 70 degrees C. A suitable blood flow rate sensor 6 is found in the Spectrum Medical MS monitor or by Transonic Systems Inc. These provide an accurate measurement of flow over the range of 0 to 10 1/mm with a fast response to flow variations.

For pressure sensors 3 and S these could be individual devices as this would provide more accuracy but, as mentioned above, it is likely that they would be one device that would read line pressure only. This works on the principle that the line pressure is very close to atmospheric pressure. These pressure sensors will likely be a strain gauge type however other types of sensors would be perfectly acceptable. A typical sensor range for this measurement would be between 600 and 1300 mBar absolute. Such a sensor is supplied by a company called Sensortechnics.

In an embodiment of the present invention, values for P02 and PCO2 are derived from measurements from the above-described sensors according to the following functions: Pa2 = Function of {Tblood, Qblood, O2in% , CO2in%, CO2out%, Pa} (1) PCO2 = Function of {Tblood, Qbloocl CO2in%, CO2out% , Pa} (2) where Tblood = Blood Temperature (sensor 7 e g degrees C) Qblood = Blood Flow Rate (sensor 6) O2in% = 02 percentage entering system (various sensors see below) CO2in% = CO2 percentage entering system (various sensors see below) CO2out% CO2 percentage in gas leavFng system removed from blood (various sensors; see below) U Pa Line or Atmospheric pressure (sensor 3 or 5) The values for P02 and PCO2 given respectively by functions (1) and (2) are calculated by the first processor (60) illustrated in Figure 1 in dependence upon readings or a subset of readings from the first group of sensors the second group of sensors and any other sensors provided The above-mentioned values for O2in%, CO2iri% and CO2out% are derived from sensor readings which benefit from correction in order to improve performance and accuracy.

In this respect, the 02 measurement should ideally be accurate over a range of pressure and temperature conditions. As the chosen sensor is temperature compensated internally further compensation is not required. It has been determined that the following is an appropriate relationship to employ for pressure compensation: O2in% = Function of {02 I Pa} (3) where: Pa = Line or Atmospheric pressure (sensor 3 or 5; e.g. mbar) 02 = Raw 02 sensor reading (sensor 1; e.g. % concentration) There are two CO2 measurements of interest, one being the gas entering the system and the other being the difference between the gas entering and the gas leaving. These measurements should also ideally be accurate over a range of a pressure and temperature conditions As the chosen sensor is temperature compensated internally further compensation is not required It has been determined that the following is an appropnate relationship to employ for pressure compensation CO2in% = Function of {CO2in Pa} (4) CO2out% = Function of {CO2out Pa} -CO2in% (5) where CO2in = Raw CO2in sensor reading of gas entenng the system (sensor 2, e g % concentration) CO2out = Raw CO2out sensor reading of gas leaving the system (sensor 4 e g % concentration) Pa = Atmosphenc pressure (sensor 3, e g mbar) As the gas passes through the oxygenator 40 oxygen is absorbed by the blood and carbon dioxide is released As a result of this, the gas exchange mix that is present as it finishes passing over the blood is not the same as the gas concentration that entered the oxygenator chamber. This final condition of the gas can be approximated by subtracting the amount of CO2 exiting from the 02 entering. It has been determined that the following is an appropriate relationship to employ for providing this corrected reading: O2out% Function of {O2in%, CO2out%} (6) The corrected values for O2in%, O2out%, CO2in% and CO2out% given respectively by functions (3) to (6) are calculated by the second processor (70) illustrated in Figure 1.

Each of the basic functions set out in (1) to (6) above can now be described in more detail, so that the specific interactions of each of the above variables can be shown Turning first to function (3), and as mentioned above, in order to ensure that the measured O2in% is correct over a range of known use conditions it should ideally be corrected for pressure In order to do this, the following variables are used to act upon the pressure measurements which are then used to calculate O2in%: O2PacaI = Line or Atmospheric pressure when 02 sensor was calibrated (sensor 3 orb; eg mbar) And based on the above the function (3) for O2in% becomes O2in% 02* (1 -((Pa -O2Pacai)/1 000)) (7) Turning now to functions (4) and (5), and as mentioned above, in order to ensure that the measured CO2in% and CO2out% values are correct over a range of known use conditions they should ideally be corrected for pressure In order to do this, the following variables are used to act upon the pressure measurements which are then used to calculate CO2in% F and CO2out% C02Pa31 = Line or Atmospheric pressure when CO2 sensors were calibrated (sensor 3 orb,eg mbar) F And based on the above the functions (4) and (5) for CO2in% and CO2out% become CO2in% = CO2in * (1 -((Pa -C02Pa21)/1 000)) (8) CO2out% = CO2out * (1 -((Pa -CO2Pacai)/1 000)) -CO2in% (9) Turning now to function (6), as mentioned above, in order to provide a corrected reading for O2in% which can be used for the final condition of the gas mix within the oxygenator the following variable are used to create O2out%: O2out% = (O2in% -CO2out%) (10) The system uses a synchronisation point which is where the user provides to the system a set of reading that have been obtained from a Blood Gas Analyser and the system takes a set of predefined readings from sensors These readings are used by the system to synchronise itself with this point The readings required from the blood gas analyser are P02 and PCO2 measurements, other measurements that are used are taken from the sensors attached to this device Another way of thinking about this is that synchronisation is a snapshot in time of all sensor readings and results Turning now to function (1) the P02 function uses a number of vanables and sensor values to produce a result including the derived value from function (9) above For a more accurate result the following variables are also available for use VarQb02 P02 blood flow correction constant (see Appendix 3) Vartemp4 = Temperature correction constant at O2out% of 4 (see Appendix 3) Vartem p50 = Temperature correction constant at O2out% of 60 (see Appendix 3) Tblood8 = Blood Temperature at time of synchronisation (sensor 7, e g degrees C) Qblood = Blood flow at time of synchronisation (sensor 6, e g I/mm) O2out%SYflC = O2out% at time of synchronisation (function 10) P025 = PD2 value supplied by the user at time of synchronising (e g mmHg) MasterPO2LUl = A table of two columns, first column is O2out% and second column is P02 Values (see Appendix 1) These are used to denve the following Qb02 = (Qblood -Qblood5) * VarQb02 MasterPO24 = P02 value from MasterPO2LU[ at an O2out% of 4 MasterPO260 = P02 value from MasterP021 at an O2out% of 60 Synctempgajn = ((Vartemp50 * (Tblood5 -37) * MasterPO250 + MasterPO260) -(Vartemp4 * (Tblood5-37) + MasterPO24)) / (MasterPO260 -MasterPO24) SynctempP02LU = ((Each P02 value in MasterPO2LUL) -MasterPO24) * Synctempgajn + (Vartemp4* (Tblood-37) + MasterPO24) SyncP02= (Each P02 value in SynctempP02L) * (P025 / (Value from SynctempP021 when 02out% is input)) SyncPO24 = P02 value from SYnGPO2LUt at an O2out% of 4 SyncPO250 = P02 value from SyncP02L at an O2out% of 60 Working981 = ((Vartemp * (Tblood -Tblood) * SyncPO260 + SyncP0260) -(Vartemp4 * (Tblood -Tblood30) + SyncPO24)) I (SyncPO250 -SyncPO24) And from this the following can be derived: WorkingP02L = (((Each P02 value in SYnGPO2LUt) -SyncPO24) * Working + (Vartemp4 * (Iblood -Tb!ood5) + SyncPO24)) -((Qblood -Qblood) * VarQb02) P02 = Value from WorkingP02LU when O2out% is input (11) Turning now to function (2), the PCO2 function uses a number of variables and sensor values to produce a result, including the denved value from function (8) above, and the P02 functions above For a more accurate result, the following variables are also available for use C02out%5, = CO2out% (function 9) at time of synchronisation MasterPC021 = A table of three columns, first column is Qblood second column is X values and third column is C values (see Appendix 2) TempPCO21 = A table of two columns first column is blood temperature difference (between blood temperature at time of synchronisation and run time blood temperature) and second column is PCO2 temperature corrections value (see Appendix 2) Note X and C values referred to above are gain and offsets for the generation of a straight line.(e.g. Y=mX+C) in this case rn = Oblood, so for each Qblood entry in the table a bespoke straight line can be generated.

PCO2SyflC = PCO2 value supplied by the user at time of synchronising (e.g. mmHg) PCO2x6 = X value from MasterPCO2LUl when Qblood is input PCO2c = C value from MasterPCO2LU when Qblood5 is input PCO2x = X value from MasterPCO2LU when Oblood is input PCO2c = C value from MasterPCO2LUl when Oblood is input PCO2temp = Temperature correction value from TempPCO21 when (Tbood -Tblood3) is input MasterPCO2 ((Qblood * CO2out%) * PCO2x) + PCO2c QbCO2 = Qblood * CO2out% And from this the following can be derived PCO2 = (((QbCO2 * PCO2X) + (QbCO2 * PCO2tomp) + PCO2c) * (MasterPCO2 / PCO2) (12) A blood analysis method according to an embodiment of the present invention is illustrated schematically in the flow chart of Figure 2 Blood is provided from a patient to the F oxygenator 40 in step P1, and blood is provided from the oxygenator 40 back to the patient in step P2, in a continuous loop The above-mentioned first group of sensors is provided in step Si, while the above-mentioned second group of sensors is provided in step S2 Step 63 comprises directly measuring a first group of properties of the blood provided to or F from the oxygenator 40 using corresponding respective ones of the first group of sensors Step S4 comprises directly measuring a second group of properties of a gas provided to or from the oxygenator 40 using corresponding respective ones of the second group of sensors Step S5 comprises applying corrections to various ones of the measured properties, for example as set out above with reference to functions (6) to (10). In step 56 a value for the partial pressure of oxygen in the blood is determined in dependence upon the first group of directly measured properties (from step 53) and the second group of directly measured properties (from step 84), or a selection thereof. In step 87 a value for the partial pressure of carbon dioxide in the blood is determined in dependence upon the first group of directly measured properties (from step 33) and the second group of directly measured properties (from step 84), or a selection thereof. The properties used in steps 86 and 37 may be different. Processing then loops back to step 83, at least until sufficient determinations of P02 and/or PCO2 have been made.

It will be appreciated that, for any sensors illustrated in Figure 1 but not included in any function described above, those sensors may be useful for further enhancements to an embodiment of the present invention, for example in order to provide a more accurate determination of P02 and PCO2. It will also be understood that the specific calculations described above with reference to functions (1) to (12) are intended to be illustrative and not limiting it is perfectly feasible to apply different specific calculations in other embodiments of the present invention.

It will be understood that although the oxygen and carbon dioxide sensors 1, 2 and 4 are F descnbed as measuring a percentage of oxygen or carbon dioxide (as the case may be) those sensors need not measure actual percentages but may instead provide their measurements in any other suitable unit of concentration with appropriate adjustments to the functions which use those measurements It will be appreciated that operation of one or both of the first and second processors 60, could be controlled or provided at least in part by a program operating on the device or apparatus A single processor or processing unit may be arranged to perform the function of both the first and second processors 60 70 Such an operating program can be stored on a computer-readable medium, or could for example, be embodied in a signal such as a downloadable data signal provided from an Internet website The appended claims are to be interpreted as covering an operating program by itself or as a record on a carrier, or as a signal, or in any other form It will be understood that not all of the variables included in the above-described functions, and in particular in the basic functions (1) and (2), are essential Generally, the more variables that are used in the function, the more accurate the resulting derivation of P02 and PCO2, but it is possible to sacrifice a little accuracy by dropping those variables that contribute less to the overall accuracy. This would in turn reduce the overall complexity of the calculations, and also the overall number of sensors required. For example, it might also be possible to drop CO2in% from function (1). Depending on the types of sensors employed, and whether correction for pressure is required, it may also be possible to drop Pa from functions (1) and (2). What variables are included will depend on the particular application, and the accuracy required.

Therefore, the following cut-down version of functions (1) and (2) might be considered: P02 = Function of {Tblood, Oblood, O2in%, CO2out%} (1') PCO2 Function of {Tblood, Qblood, CO2out%} (21) Therefore, at least in respect of function (2') above, the second group of sensors might comprise a single sensor, for measuring a concentration of carbon dioxide in the gas from the oxygenator (40).

A similar situation applies to the correction functions (3) to (6), certain variables could be F left out depending on the accuracy required. For example, CO2in% could be dropped k from function (5). Again, what variables are included will depend on the particular application.

Likewise, it is also possible to include more variables in functions (1) and (2), and the associated more detailed functions derived from these basic functions For example one might consider including the current value of PCO2 in function (1) One might also consider adding further sensors to the first and/or second group of sensors to provide further vanables for inclusion in the functions For example, one might include a further sensor in the first group of sensors for measunng hematocrit I haemoglobin concentration and include a corresponding variable Hct / Hb in one or both of functions (1) and (2) One might include a further sensor in the second group of sensors for measunng gas flow rate and include a corresponding vanable Qgas in one or both of functions (1) and (2) One might include a further sensor in the second group of sensors for measuring gas temperature and include a corresponding variable Tgas in one or more of functions (3) to (5). One might include a further sensor in the first group of sensors for measuring blood saturation, and include a corresponding variable SaO2% in one or more of the functions.

It will also be appreciated by the person of skill in the art that various modifications may be made to the above-described embodiments without departing from the scope of the present invention as defined by the appended claims.

Appendix I Example of MasterPO2LU Look Up Table: O2out P02 % mmhg F: 4.000 41.500 6.000 44.400 8.000 48.300 10.000 51.200 12.000 56.000 14.000 60.000 16.000 64.700 18000 70500 000 76 300 22 000 81 780 24 000 88 800 26 000 96 600 28000 105300 30000 115900 F 32000 126300 34000 137200 36000 148800 F 38000 161400 40000 174700 42000 188500 44 000 203 300 46M00 217.400 48.000 234.800 50.000 252.200 52.000 270.500 54.000 287.900 56.000 302.400 58.000 317.900 60.000 334.300 62.000 347.800 64.000 359.400 66 000 371 000 68.000 382.600 000 396 100 72 000 406 800 74000 419300 76000 427100 78 000 435 700 000 443 500 Appendix 2 (A) Examp'e of MasterPCO2L Look up Table Absolute Blood flow Oblood X C 1/miri 204 2 1 18 2 16 2 2 141 2 3 1089 2

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a N) 0)0) (fl o w C C) oc;)&Ppiso -p E NJN)PUM -, C -o a o a-(0