CONTROL SYSTEM

Abstract

An extracorporeal life support device control system and method arranged to provide suitable gas and blood flow rates through an extracorporeal life support device. The control system comprises: a sensor arranged to detect and output a measurand, wherein the measurand is characteristic of a single autonomic nervous system output defining a metabolic demand; and a controller arranged to receive the measurand, and further arranged to control, according to the measurand: gas and/or liquid flow rates through an extracorporeal life support device; wherein the flow rates are arranged to provide blood gas concentrations similar to those arising from healthy lungs at the metabolic demand. In the case of patients with healthy lungs, the control system can control the blood flow rate without controlling gas flow rates through an oxygenator.

Claims

1. An extracorporeal life support device control system arranged to provide a gas flow rate through an extracorporeal life support device; the control system comprising: a sensor arranged to detect and output a measurand, wherein the measurand is characteristic of a single autonomic nervous system output defining a metabolic demand; and a controller arranged to receive the measurand, and further arranged to control, according to the measurand: a first gas flow rate of a first gas through an extracorporeal life support device, the first gas having a first oxygen concentration; wherein the first gas flow rate is arranged to provide blood gas concentrations similar to those arising from healthy lungs at the metabolic demand.

2. The extracorporeal life support device control system as claimed in claim 1, wherein the controller is further arranged to control, according to the measurand: a blood flow rate through the extracorporeal life support device.

3. The extracorporeal life support device control system as claimed in claim 2, wherein the controller is arranged to control a blood pump of the extracorporeal life support device to control the blood flow rate.

4. The extracorporeal life support device control system as claimed in claim 3, wherein the blood pump is one selected from the group: a peristaltic pump; a continuous flow pump; a centrifugal pump; a pulsatile pump; any future blood pump design.

5. The extracorporeal life support device control system as claimed in claim 1, wherein the first oxygen concentration is selected from the range: 20% to 100%.

6. The extracorporeal life support device control system as claimed in claim 1, wherein the controller is further arranged to control, according to the measurand: a second gas flow rate of a second gas through the extracorporeal life support device, the second gas having a second oxygen concentration.

7. The extracorporeal life support device control system as claimed in claim 6, wherein the second oxygen concentration is selected from the range: 0% to 25%.

8. The extracorporeal life support device control system as claimed in claim 6, wherein the second oxygen concentration is lower than the first oxygen concentration.

9. The extracorporeal life support device control system as claimed in claim 6, wherein the second gas comprises a second carbon dioxide concentration selected from the range: 0% to 4%.

10. The extracorporeal life support device control system as claimed in claim 1, wherein the extracorporeal life support device is an extracorporeal membrane oxygenator.

11. An extracorporeal life support device control system arranged to provide a gas flow rate through an extracorporeal life support device; the control system comprising: a sensor arranged to detect and output a measurand, wherein the measurand is characteristic of a single autonomic nervous system output defining a metabolic demand; and a controller arranged to receive the measurand, and further arranged to control, according to the measurand: a blood flow rate through an extracorporeal life support device; wherein the blood flow rate is arranged to be similar to that arising from a healthy heart at the metabolic demand.

12. The extracorporeal life support device control system as claimed in claim 11, wherein the extracorporeal life support device comprises a ventricular assist device.

13. The extracorporeal life support device control system as claimed in claim 11, wherein the measurand is characteristic of one selected from the group: a pulse rate; a blood flow rate from a heart; a ventilation rate.

14. The extracorporeal life support device control system as claimed in claim 11, wherein the control system does not measure any blood gas concentration or any blood gas partial pressure.

15. An extracorporeal life support device, the device comprising: a blood oxygenator comprising a blood conduit and a gas conduit; the blood conduit having a blood conduit inlet arranged to receive unprocessed blood, and a blood conduit outlet arranged to output processed blood; a gas conduit having a gas conduit inlet arranged to receive a first gas having a first oxygen concentration, and a gas conduit outlet arranged to output processed gas; a first gas supply arranged to provide the first gas from the gas conduit inlet to the gas conduit outlet at a first gas flow rate; wherein the blood conduit comprises a blood conduit lumen and the gas conduit comprises a gas conduit lumen, the blood conduit lumen and the gas conduit lumen being separated by a semi-permeable membrane disposed therebetween; and wherein the device further comprises a control system according to any one of the preceding claims arranged to control, according to the measurand: the first gas flow rate.

16. The extracorporeal life support device according to claim 15, wherein the device further comprises a blood pump arranged to pump blood from the blood conduit inlet to the blood conduit outlet at a blood flow rate; and wherein the control system is further arranged to the control, according to the measurand: the blood flow rate.

17. The extracorporeal life support device according to claim 15, wherein the device further comprises a second gas supply arranged to provide a second gas from the gas conduit inlet to the gas conduit outlet at a second gas flow rate; and wherein the control system is further arranged to the control, according to the measurand: the second gas flow rate.

18. The extracorporeal life support device according to claim 15, wherein the device does not measure any blood gas concentration or any blood gas partial pressure.

19. A method of controlling a first gas flow rate of an extracorporeal life support device having a first gas supply arranged to provide the first gas flow rate, the method comprising the steps of: i. connecting to an extracorporeal life support device; ii. using a control system having a sensor, detecting a measurand with said sensor, the measurand being characteristic of an autonomic nervous system output; iii. calculating a first gas flow rate using the measurand; and iv. controlling the first gas supply to provide the first gas flow rate.

20. The method as claimed in claim 19, wherein the method further comprises controlling a blood flow rate of the extracorporeal life support device having a blood pump arranged to provide the blood flow rate; the method further comprising the steps of: v. calculating a blood flow rate using the measurand; and vi. controlling the blood pump to provide the blood flow rate.

21. The method as claimed in claim 19, wherein the method further comprises controlling a second gas flow rate of the extracorporeal life support device having a second gas supply arranged to provide the second gas flow rate; the method further comprising the steps of: vii. calculating an optimum second gas flow rate using the measurand; and viii. controlling the second gas supply to provide the optimum second gas flow rate.

22. The method as claimed in claim 19, wherein the method steps are performed by a control system as claimed in any one of claims 1 to 10.

23. The method as claimed in claim 19, wherein the extracorporeal life support device is as claimed in any one of claims 11 to 18.

24. A method applicable where both the lungs and heart are deficient wherein an extracorporeal life support device control system is provided as claimed in claim 1 comprising a blood pump, wherein the sensor senses a measure of ventilation rate and the pump has sufficient power also to augment, replace or mimic healthy functioning of the heart.

25. A heart support device control system arranged to control a blood flow rate from a heart support device, the control being according to a measurand characteristic of a single autonomic nervous system output defining a metabolic demand, wherein the controlled blood flow rate is similar to that which would arise at that metabolic demand from a healthy heart.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0120] Specific embodiments will now be described by way of example only, and with reference to the accompanying drawings.

[0121] FIG. 1 shows a schematic view of an example embodiment of an extracorporeal life support device according the third aspect of the present disclosure comprising a control system according to the first aspect of the present disclosure.

[0122] FIG. 2 shows a shows a schematic view of an example embodiment of an extracorporeal life support control system according to the first aspect of the present disclosure suitable for an ECLS system including a very low pressure drop oxygenator.

[0123] FIG. 3 shows a schematic view of an example embodiment of an extracorporeal life support control system according to the first or second aspect of the present disclosure suitable for an ECLS system applied to a patient who also has heart deficiency and needs a ventricular assist device or artificial heart.

[0124] FIG. 4 shows a schematic view of an example embodiment of a control system according to the second aspect of the present disclosure suitable for a patient with healthy lungs, but with heart deficiency requiring a ventricular assist device or artificial heart.

[0125] FIG. 5 shows a flow chart depicting an example embodiment of a method according to the fourth aspect of the present disclosure, using a control system according to FIG. 1.

DETAILED DESCRIPTION

[0126] Referring to FIG. 1, a schematic diagram of an example embodiment of a control system in use according to the present disclosure is shown. In the embodiment of FIG. 1, a heart and lung system 1 is shown, from which oxygenated arterial blood 3 is pumped around a human body 2. In the body 2, oxygen is consumed and carbon dioxide is produced, resulting in oxygen-depleted venous blood 4, which is returned to the heart and lungs 1, where carbon dioxide is removed from the blood and the blood is re-oxygenated.

[0127] In the heart and lungs 1 shown, the blood-oxygenation function of the lungs 1 is defective, and therefore requires supplementation using an extracorporeal life support device (ECLS) to ensure adequate blood oxygenation.

[0128] In the ECLS shown in FIG. 1, a blood pump 11 is provided, the blood pump 11 being arranged to divert a portion of the venous blood 4, as stream 10, toward a mass exchanger 12. Diverted venous blood 10 is pumped by the blood pump 11 into a blood conduit (not shown) of a mass exchanger 12.

[0129] The mass exchanger 12 comprises a blood conduit and a gas conduit. The blood conduit comprises a blood conduit inlet arranged to receive the diverted venous blood 10 from the blood pump 11, and a blood conduit outlet arranged to output processed blood 13 back into the venous blood stream 4 which then returns to the heart and lung system 1.

[0130] The mass exchanger 12 also comprises a gas conduit having a gas conduit inlet arranged to receive a gas mixture 18. The gas conduit also comprises a gas conduit outlet arranged to output processed gas 19. The mass exchanger 12 comprises first gas pump 15 arranged to pump the first gas 14 into the gas conduit through the gas conduit inlet and to the gas conduit outlet at a first gas flow rate. The mass exchanger 12 also comprises a second gas pump 17 arranged to pump the second gas 16 into the gas conduit through the gas conduit inlet and to the gas conduit outlet at a second gas flow rate. The blood conduit comprises a blood conduit lumen and the gas conduit comprises a gas conduit lumen. The blood conduit lumen and the gas conduit lumen are separated by a semi-permeable membrane disposed therebetween. The semipermeable membrane forms a series of channels of the gas conduit such that a flowing gas (comprising the first gas and the second gas) may flow through the series of channels between the gas conduit inlet and the gas conduit outlet. The semi permeable membrane is arranged to permit mass transfer of oxygen and carbon dioxide between the blood conduit lumen and the gas conduit lumen such that the output processed blood 13 comprises a different gas composition to the diverted venous blood 10. ECLS is employed for patients having deficient lungs within the heart/lung system 1 who are unable to sufficiently oxygenate the blood 3. The exchanger 12 then removes carbon dioxide from the blood 10 into the gas mixture 18 within the mass exchanger 12, and oxygen is added from the gas mixture 18 to the blood 10, to produce oxygenated output blood 13 to be returned to the venous blood stream 4. The resulting oxygen-depleted gas stream 19 is discharged from the exchanger 12.

[0131] The limited capability within the heart and lung system 1 then completes the oxygenation of the blood such that the combined action of the mass exchanger 12 and the otherwise inadequate heart and lungs together produce an arterial blood stream 3 of similar composition to that of a healthy person.

[0132] The control system augments the ECLS system by controlling the rate of blood flow 10 and the flow rate and composition of the gas stream 19 in response to surrogates of the output from the autonomic nervous system. In this way, control is delegated to the autonomic nervous system.

[0133] Alternative embodiments will be appreciated in which gas stream 18 is the output from an oxygen concentrator. The oxygen concentration and flow rate from the concentrator is set by the signals 22 and 23 from the controller.

[0134] The control system of the present disclosure comprises a sensor 21, in digital communication with a control unit 20, which in-turn is in digital communication with the blood pump 11, the first gas pump 15, and the second gas pump 17. The blood pump 11 is linked to the controller by a first digital connection 24. The first gas pump 15 is linked to the controller by a second digital connection 23, and the second gas pump 17 is linked to the controller by a third digital connection 22.

[0135] The first, second and third digital connections 22, 23 and 24 in the embodiment shown are wireless connections. Embodiments will be appreciated wherein the connections are wired or wireless, or any combination of wired and wireless. Embodiments will also be appreciated wherein the connections are wired or wireless analogue signals, and may be pneumatic or hydraulic, or optical.

[0136] The sensor 21 is arranged to detect a pulse rate of the heart 2. Embodiments will be appreciated wherein the sensor 21 is arranged to sense any surrogate of the output from the autonomic nervous system which, in a healthy person, controls the rate at which blood is pumped around the body and the breathing rate and depth. In a healthy person, there is a relationship between the rate at which the blood is pumped around the body and the ventilation rate (the quantity of air that enters and leaves the lungs in any defined period of time), provided that the person is in sinus rhythm. The ventilation rate and blood circulation rate together determine the rate of oxygen and carbon dioxide transfer in the lungs.

[0137] The present control system, in the embodiment shown in FIG. 1, takes a measure of the blood flow rate using the sensor 21, and the measurement is transmitted to the control unit 20. The control unit subsequently provides a signal 24 to the blood pump 11, wherein following receipt of the signal, the blood flow rate from the blood pump 11 through the blood conduit of the mass exchanger 12 is altered by the blood pump 11 in proportion to the blood flow rate measured from the heart and lungs 1. The rationale for this control is that the blood flow rate through a vein is roughly proportional to the flow rate from the heart and lungs. The control using the present system ensures that an approximately constant proportion of the blood flow through a vein is taken through the mass exchanger 12.

[0138] The measure of the blood flow rate taken by the sensor 21 is further used by the controller 20 to determine and provide a signal 23 to the first gas pump 15 and a signal 22 to the second gas pump 17. Signals 23 and 22, upon receipt by their respective gas pumps 15, 17 adjust the gas flow rates of the respective gases 14, 16 through the gas pumps 15, 17. The altered gas flow rates through the gas pumps 15, 17 determines the composition and flow rate of the mixed gas stream 18 entering the gas conduit of the mass exchanger 12.

[0139] A specific embodiment of the control algorithm employs equations (1), (2) and (3) described herein, where the measurand is:

[0140] f is the blood flow rate from the heart as estimated by sensor 21;

and the controlled variables are:

[0141] f.sub.D is the blood flow rate of streams 10 and 13 as set by pump 11;

[0142] g.sub.1 is the flow rate of gas stream 14 as set by control valve or pump 15;

[0143] g.sub.2 is the flow rate of gas stream 16 as set by control valve or pump 17;

[0144] Other algorithms are possible in which there are non-linear relationships between the measured variable “f” and the controlled variables set by pumps or control valves.

[0145] In the specific embodiment shown, gas stream 14 is oxygen and gas stream 16 is air. Other embodiments are possible in which gas stream 14 is relatively oxygen-rich and gas stream 14 has a comparatively lower oxygen concentration. Gas stream 14 may also contain a small concentration of carbon dioxide, such as that discussed herein.

[0146] In a further specific embodiment, the control algorithm employs equations (4), (5) and (6), where “v” is the ventilation rate as estimated by the sensor 21. For a person with healthy lungs, it may be the healthy ventilation rate. For a person with deficient lung function, it may be a fraction of the healthy ventilation rate. As for sensed variable “f”, alternative algorithms are possible, and embodiments with gas streams of alternative composition are possible.

[0147] The embodiment shown represents a simple feed-forward control as in the above equations. There is no feedback in the embodiment shown—the feedback is left to the autonomic nervous system.

[0148] In conventional control systems such as those found in the prior art, blood oxygen partial pressure or concentration, x, is typically measured. A perfusionist sets a target “healthy” concentration, say x.sub.T. The control is then along the following lines:

dg.sub.1/dt=K(x.sub.T−x)  (7)

[0149] The equation has the effect of increasing the oxygen flow rate if x is less than the target concentration and decreasing if it is higher. The system settles when the oxygen flow rate matches the target rate. Similar controls are applied to achieve target carbon dioxide concentrations.

[0150] The first gas flow rate and the second gas flow rate are adjusted by the controller to alter the oxygen and carbon dioxide concentrations in the mixed gas stream 18 flowing into the mass exchanger. This adjustment is in accordance with the measurand (the venous blood flow rate or ventilation rate detected by the sensor 21). The adjustments are tailored to produce blood gas concentrations in arterial blood stream 3 from the heart and lungs 1 which mimic those arising in a person with healthy lungs.

[0151] In the embodiment shown, the first gas 14 is oxygen, the second gas 16 is air, and the flow of blood through the blood conduit and the flow of gas through the gas conduit occurs in a co-current fashion. The benefit of a co-current mass exchanger is that outlet blood gas partial pressures (or concentrations) are nearly in equilibrium with the outlet gas stream 19. When the gas flow rate through the mass exchanger 12 is very high, the concentration of carbon dioxide in the outlet stream 19 is very low because the carbon dioxide concentration transferred from the blood is a small proportion of the gas flow rate through the device 12. Conversely, when the gas flow rate is very low, the carbon dioxide concentration in output stream 19 is very high because the carbon dioxide transferred in the exchanger is a significant proportion of the gas flow rate through the device. In this way, the carbon dioxide concentration in gas stream 19 (and hence the partial pressure and concentration in blood stream 13) can be controlled by adjusting the total gas flow rate through the exchanger 12. The oxygen transfer rate (and hence the oxygen partial pressure and concentration in blood stream 13) is controlled by controlling the oxygen concentration in stream 18. The complete control system then operates as follows. At highest metabolic demand (as signalled by maximum blood flow rate detected by sensor 21) oxygen stream 14 is at a maximum flow rate and air stream 16 is at zero flow rate. At minimum metabolic demand the oxygen 14 flow rate is zero and the air 16 flow rate is slightly higher, thus the total gas flow rate 18 is low and the major portion is made up of air 16 (which has a lower oxygen concentration). The resulting system closely mimics healthy lungs by implementing a linear relationship between oxygen flow rate 14 and blood circulation rate (detected by sensor 21), and by implementing a linear relationship between air flow rate 16 and blood circulation rate.

[0152] Alternative embodiments are possible in which the mass exchanger may be cross-current or counter-current. For a cross-current mass exchanger, a wider range of total gas flow rate 18 into the exchanger is required because outlet blood gas partial pressures have a more complex relationship with outlet gas composition which is a mixture between gas that exits in contact with the inlet blood stream and some exits in contact with the blood outlet stream. For a counter-current mass exchanger, the outlet blood stream is in contact with the inlet gas stream. Hence, in this case, stream 16 will need to contain carbon dioxide to give a sufficiently high partial pressure (and concentration) of carbon dioxide in outlet blood stream 13.

[0153] In the embodiment shown, the flowing gas may flow freely through the series of cylindrical channels of the gas conduit formed by the semipermeable membrane. Embodiments will be appreciated wherein access to, or flow rate through, one or more of the series of channels may be controllable by the control system. As such, the control system may, according to the measurand, increase or reduce access of the flowing gas to the one or more of the series of channels in order to increase or reduce the effective area of the mass exchanger. It will be appreciated that, in substantially the same manner in an embodiment wherein the series of channels are instead comprised within the blood conduit, the control system may, according to the measurand, increase or reduce access of the blood to the one or more of the series of channels in order to increase or reduce the effective area of the mass exchanger.

[0154] Embodiments will be appreciated wherein a flat-sheet mass-exchanger is provided, wherein the gas conduit or the blood conduit may comprise a series of parallel channels.

[0155] In further embodiments, the sensor may sense pulse rate instead of blood flow rate. Blood flow rate is the product of pulse rate and stroke volume and for modest exertion, stroke volume does not change greatly with metabolic demand. Hence, pulse rate can stand in as a surrogate for blood flow rate and for output from the autonomic nervous system. It would be expected that the ability of the autonomic nervous system to acclimatize would adapt to such a control.

[0156] Various non-invasive methods are available for capturing the measurand, which for blood flow rate is made up from the product of pulse rate and heart stroke volume. For example, the pulse rate may be measured with an ECG and the stroke volume estimated from methods based on measuring pulse wave velocity. Alternatively, a fixed value of stroke volume may be employed which may be related to the size of the patient; a typical value would be 70 ml. Ventilation rate is made up from the product of breathing rate and volume of each breath; estimates of both are available without using a face mask.

[0157] The blood pump 11 is a positive displacement pump, so that the flow rate can be directly controlled. Where a pump that is not positive displacement (such as a centrifugal pump) is employed, the blood flow rate can be set as a set-point on a feedback control that measures flow rate.

[0158] Alternative embodiments will be appreciated wherein the first gas is not pure oxygen, but comprises oxygen at a suitable concentration or partial pressure. Embodiments will be appreciated wherein the second gas is not air, but simply comprises oxygen at a lower concentration or partial pressure compared to the first gas. The first gas and the second gas may be used having differing oxygen concentrations and possibly carbon dioxide concentrations to those discussed.

[0159] The first gas supply and the second gas supply in the embodiment shown are provided through metering pumps 15, 17. The first gas supply and the second gas supply in alternative embodiments may comprise flow metering valves from high pressure (for example, bottled) gas supplies. One of the gas supplies may be from a portable oxygen concentrator with controllable flow rate and composition. Alternatively, there may be only one gas supply delivered from a controllable portable concentrator.

[0160] In some cases, the controller can be calibrated by determining two points on a calibration curve. The first point corresponds to the maximum metabolic rate that can be supported with the maximum oxygen concentration available and adjusting the relevant gas flow rate to give a stable carbon dioxide concentration. The second point corresponds to resting conditions, when the lowest oxygen concentration to support rest is established together with a relevant gas flow rate to give a stable carbon dioxide concentration. The controller is then programmed to interpolate smoothly between these two points (for example, with linear interpolation). Note that the resting rate must not be the lowest possible oxygenation rate and highest carbon dioxide concentration capable of being set by the control unit. Conditions can arise at which the patient's metabolic rate falls below the resting rate. Under such conditions, the control must supply a sufficiently low gas flow rate, or high carbon dioxide concentration, to prompt the autonomic nervous system to increase the respiration rate. If this condition is not met, the respiration rate may continue to fall without limit. The calibration may be performed directly on a patient, or may be performed on a mathematical model of the patient's respiratory system. Undertaking the calibration on a mathematical model may minimize the stress on a patient already suffering with breathing difficulties.

[0161] The novel control system presented here is equally applicable to alternative extracorporeal life support systems, and indeed to control of ventricular assist devices and artificial hearts.

[0162] FIG. 2 shows an ECLS system comprising features as described and shown in FIG. 1, and equivalent numbering is therefore used. The ECLS system of FIG. 2 includes a very low pressure drop for the blood stream through mass exchanger 12. With such a low pressure drop, a blood pump is not necessary and the whole flow through a vein can be directed through the mass exchanger 12 (in this case a membrane oxygenator). In this case, the control unit 20 needs only control the flow rate and composition of mixed stream 18 into the mass exchanger. It does so by controlling the flow rates of gases 14 and 15 in the identical manner to that described for FIG. 1. The criterion for a “very low” pressure drop across the exchanger depends on the vein from which the blood is taken and the pressure differential available in the vein without having an adverse impact on the blood flow.

[0163] FIG. 3 shows an ECLS system employed for a patient who also has impaired heart function. The system comprises features as described and shown in FIG. 1, and equivalent numbering is therefore used. For patients also having impaired heart function, the ECLS system now takes the whole flow in the blood vessel. The blood pump 11 in this case is a ventricular assist device, but can be any number of suitable blood pumps as described herein. The measurand from sensor 21, the surrogate for the output from the autonomic nervous system, is an estimate of the ventilation rate. The control system 20 controls both the blood flow 10 through blood vessel on input to the exchanger 12, the blood flow 13 on output from the exchanger 12 and the gas transfer rate in the exchanger 12. Apart from the higher pressure required from the pump 11, the operation is identical to that described for FIG. 1. Where the patient needs an artificial heart, its function will be integrated with the heart/lung system and it will be located between blood flow 13 and blood flow 10. In these applications, the life support device 12 (in this case a membrane oxygenator) may be located before pump 11 or after the heart/lung system. In the latter case, the oxygenator completes the limited gas transfer in the deficient lungs. As is the case for FIG. 1 and FIG. 2, the control is delegated to the autonomic nervous system; when the blood is insufficiently oxygenated for the level of activity (metabolic demand), the patient breathes harder. The sensor 21 detects the increased breathing effort and increases blood circulation and gas transfer rate until a sufficient rate is established. Similarly, where the blood is oxygenated at a rate higher than required, the breathing effort reduces and the controller reduces blood circulation and gas transfer.

[0164] FIG. 4 shows the control system adapted for use with a patient with healthy lungs but defective heart. In this case, the mass exchanger 12 of FIG. 3 is omitted and control unit 20 controls only the pump 11, which may be a ventricular assist device or an artificial heart. The ventilation rate estimated by the sensor 21 enables the pumping rate to be adjusted to meet the required metabolic demand. This control integrated with the autonomic nervous system is simpler and more responsive than previous control systems. The case for this integrated control is exactly similar to that for ECLS systems.

[0165] Referring to FIG. 5, a flow chart of an example embodiment 30 of the method of the fourth aspect of the present disclosure is shown, the method 30 comprising the steps of: [0166] i. connecting to an extracorporeal blood oxygenator having a blood conduit, a gas conduit and a semipermeable membrane disposed therebetween 32; [0167] ii. using a control system having a blood flow rate sensor, detecting a blood flow rate of a person using said sensor 34; [0168] iii. calculating an oxygen flow rate using the blood flow rate 36; and [0169] iv. controlling an oxygen supply to provide the oxygen flow rate through the gas conduit 38; [0170] v. calculating a blood flow rate using the measurand 40; and [0171] vi. controlling a blood pump to provide the blood flow rate 42; [0172] vii. calculating a second gas flow rate using the measurand 44; and [0173] viii. controlling the second gas supply to provide the second gas flow rate 46.

[0174] The control system used in the method of FIG. 5 is a control system in accordance with that described for FIG. 1, and according to the first aspect of the disclosure. A similar method will be appreciated using a control system according to that described for FIG. 2, FIG. 3 or FIG. 4.

[0175] It will be appreciated that the above described embodiments are given by way of example only and that various modifications thereto may be made without departing from the scope of the disclosure as defined in the appended claims.