Monitoring a parameter during cardiac perfusion

Abstract

An apparatus for monitoring a parameter during cardiac perfusion receives a plurality of samples 304, each sample representing the value of the parameter at a respective moment in time. The apparatus

Description

The present disclosure relates to a system and method for monitoring a parameter during cardiac perfusion. 5 Cardiac perfusion is a medical procedure involving extracorporeal oxygenation of a patient’s blood. Cardiac perfusion is performed, for example, when a patient is unable to oxygenate their own blood by breathing, such as during heart and / or lung surgery. Extracorporeal oxygenation usually uses a pump that acts in place of the patient’s own 10 heart and an oxygenator that acts in place of the patient’s own lungs. The oxygenator removes carbon dioxide from, and adds oxygen to, the patient’s blood. Cardiac perfusion is sometimes known in the art as extracorporeal perfusion or extracorporeal circulation. 15 It is necessary to monitor certain parameters during cardiac perfusion to ensure a patient’s safety. These parameters may be physiological parameters of the patient or parameters of equipment used to perform cardiac perfusion. As an example of a physiological parameter, it is known to measure instantaneous values of the saturation of oxygen in a patient’s blood during a cardiac perfusion procedure. As an example of 20 a parameter of the cardiac perfusion equipment, it is known to measure instantaneous values of the partial pressure of oxygen in an oxygenation gas supplied to an oxygenator. In these examples, a clinician observes the instantaneous values of the parameters and adapts the cardiac perfusion procedure (e.g., by adjusting one or more settings of the cardiac perfusion equipment) to ensure that the values are optimal for a 25 patient’s safety. Summary The present disclosure relates to improvements in monitoring parameters during 30 cardiac perfusion to provide further assurance of a patent’s safety and / or to ensure clinical efficacy of an ongoing cardiac perfusion procedure. In accordance with a first aspect of the disclosure, there is provided an apparatus for monitoring a parameter during cardiac perfusion. The apparatus is configured to 35 receive a plurality of samples, each sample representing the value of the parameter at 24 07 24 a respective moment in time. The apparatus is further configured to compare each sample with a first threshold value. When a sample does not meet the first threshold value, the apparatus is configured to add the value of the sample to a running total. The apparatus is further configured to generate an alert when the running total exceeds 5 a second threshold value. The apparatus is further configured to reset the running total to zero when a sample meets the first threshold value. As used herein, ‘cardiac perfusion’ refers to a medical procedure in which blood is removed from a patient, oxygenated, and returned to the patient. Cardiac perfusion 10 encompasses extracorporeal membrane oxygenation (ECMO) and cardiopulmonary bypass (CPB). The parameter may be any measurable physical property relevant to an ongoing cardiac perfusion procedure. In particular, the parameter may be a physiological 15 parameter of the patient undergoing cardiac perfusion, or may be a parameter of the equipment used to perform cardiac perfusion (referred to herein as a ‘cardiac perfusion system’). The first threshold value may be a target value for the parameter (e.g., a value that the parameter should not fall below or exceed). The second threshold value may be a cumulative value of the parameter (in the time since the first threshold value 20 was not met) that, when exceeded, poses a risk to the patient’s safety or could result in a clinically sub-optimal outcome for the patient. Depending on a clinician’s preferences, the second threshold value may be set below the cumulative value that might pose a risk to the patient’s safety or result in a clinically sub-optimal outcome for the patient. 25 The running total represents the cumulative value of the parameter in the time since the first threshold value was not met. By generating an alert when the running total exceeds the second threshold value, the monitoring apparatus disclosed herein warns a clinician that remedial action needs to be taken. The clinician can then take remedial 30 action to correct the value of the parameter before the patient is harmed, and while there is still time to ensure a clinically efficacious outcome from the cardiac perfusion procedure. The present disclosure is thus a significant improvement upon existing cardiac 35 perfusion systems that monitor instantaneous values of parameters, but which do not 24 07 24 consider (let alone provide a way to prevent) the risk to a patient’s safety when the value of a parameter is sub-optimal over a prolonged period of time. The present disclosure also differs from known “area under the curve” (AUC) analyses, which retrospectively calculate oxygen delivery. Although known AUC analyses provide 5 valuable information on how well a clinician performed a particular cardiac perfusion procedure, they do not provide any information while the cardiac perfusion procedure is ongoing. As such, known AUC analyses do not allow a clinician to take remedial action to ensure a patient’s safety or clinical efficacy during a cardiac perfusion procedure. 10 The apparatus may monitor multiple parameters at once, where each parameter has a respective first threshold value and second threshold value. The apparatus can generate an alert when the running total of any of the monitored parameters exceeds the respective threshold value. 15 A parameter of interest may not be directly measurable by a single sensor. The apparatus may be configured to process one or measurements made by one or more sensors to generate a sample, where the processing may include deriving the value of the sample from measurements made a plurality of sensors and / or performing one or more arithmetic or logic operations on a measurement made by a sensor. 20 As mentioned above, the apparatus is configured to reset the running total to zero when a sample meets the first threshold value. In many clinical scenarios, the risk to the patient’s safety is reduced as soon as the parameter being monitored returns to the first threshold value. Therefore, resetting the running total to zero allows each period 25 of time in which the parameter fails to meet the first threshold value to be considered independently of other such periods. In this manner, the alert is only generated when the parameter fails to meet the first threshold for a continuous and prolonged period of time. 30 The apparatus may optionally be configured to receive the first threshold value and / or the second threshold value from a user. A clinician can thus select values for the first threshold value and / or the second threshold value to ensure that the alert is generated before there is a risk to the patient’s safety and before a clinically sub-optimal outcome for the patient occurs. The clinician may select values that are tailored to the clinical 35 circumstances of a particular patient. 24 07 24 The first threshold value may be non-zero. The first threshold value can thus be a nonzero target value that is appropriate to clinical circumstances. 5 The apparatus may optionally be configured to output a current value of the running total to a user. The running total can be output in a visible and / or audible form. By outputting the current value of the running total during a cardiac perfusion procedure, a clinician can estimate whether and / or when the running total is likely to exceed the second threshold value. This can allow the clinician to make an informed decision as 10 to whether there is a need to adapt the ongoing cardiac perfusion procedure to prevent the second threshold value being exceeded. The apparatus may optionally be configured to output a maximum value of the running total reached during an ongoing cardiac perfusion procedure. The maximum value of 15 the running total can be output in a visible and / or audible form. By outputting the maximum value of the running total during the cardiac perfusion procedure, a clinician is provided with real-time feedback on the success of the ongoing procedure. The parameter may be any of the following parameters: delivery of oxygen to a patient; 20 consumption of oxygen by the patient; production of carbon dioxide by the patient; a ratio of oxygen consumed by the patient to oxygen delivered to the patient; a ratio of oxygen delivered to the patient to carbon dioxide produced by the patient; a pressure; a blood gas saturation; a cardiac index; or a flow rate. This is not an exhaustive list of parameters. Any other measurable physiological parameter of a patient or measurable 25 parameter of a cardiac perfusion system may be used. Any of these may parameters may be indexed (normalised) to a particular patient, where it clinically makes sense to do so. In particular, the delivery of oxygen, consumption of oxygen and / or the production of carbon dioxide may be indexed to a 30 particular patient. A parameter may be indexed to a patient by dividing the value of the parameter by the body surface area of the patient or the mass of the patient. The skilled person may also conceive other ways of normalising a parameter, other than by body surface area or mass. The pressure may be a mean arterial pressure, a central venous pressure or a pulmonary artery pressure. The blood gas saturation may be a 35 cerebral mean regional oxygen saturation. 24 07 24 The first threshold value and / or the second threshold value may optionally be defined as a logical or arithmetic function of one or more parameters. This can allow fine control over the clinical situations in which an alert is generated. 5 The apparatus may be a cardiac perfusion system or a component thereof. The apparatus may optionally be configured to send a control signal to a component of the cardiac perfusion system when the running total exceeds the second threshold 10 value. The control signal may be configured to cause the component of the cardiac perfusion system to correct the value of the parameter. The apparatus can automatically correct the value of the parameter proactively (i.e., before the patient is harmed) to ensure the patient’s safety and to ensure a clinically 15 efficacious outcome of an ongoing cardiac perfusion procedure. The component of the cardiac perfusion system to which the control signal is sent may be, for example, a pump, an oxygenator, a valve or any other suitable actuator in the cardiac perfusion system. 20 In accordance with another aspect of the disclosure, there is provided a method of monitoring a parameter during cardiac perfusion. The method comprises receiving a plurality of samples, each sample representing the value of the parameter at a respective moment in time. The method further comprises comparing each sample with a first threshold value. When a sample does not meet the first threshold value, the 25 method further comprises adding the value of the sample to a running total, and generating an alert when the running total exceeds a second threshold value. The method further comprises resetting the running total to zero when a sample meets the first predetermined value. The method may be implemented by a processor. 30 The method optionally further comprises receiving the first threshold value and / or the second threshold value from a user. The method optionally further comprises outputting a current value of the running total to a user. 24 07 24 The method optionally further comprises outputting a maximum value of the running total reached during a cardiac perfusion procedure. The method optionally further comprises sending a control signal to a component of the 5 cardiac perfusion system when the running total exceeds the second threshold value. The control signal can be configured to cause the component of the cardiac perfusion system to correct the value of the parameter. In accordance with a further aspect of the disclosure, a processor-readable medium is 10 provided. The processor-readable medium comprises processor-executable instructions which, when executed by a processor, cause a monitoring apparatus comprising the processor to perform a method as disclosed herein. The processor-readable medium may be non-transitory (such as a disc or a memory device) or may be transitory (such as a signal). 15 In accordance with a further aspect of the disclosure, a computer program is provided. The computer program comprises processor-executable instructions which, when executed by a processor, cause a monitoring apparatus comprising the processor to perform a method as disclosed herein. 20 Brief Description of the Drawings Embodiments will now be described, purely by way of example, with reference to the accompanying drawings, in which: 25 Figure 1 is a schematic diagram of a cardiac perfusion system that includes an apparatus for monitoring a parameter during cardiac perfusion in accordance with the present disclosure; Figure 2 is a schematic diagram of the apparatus for monitoring a parameter 30 shown in Figure 1; Figure 3 is a flow chart of a method of monitoring a parameter during cardiac perfusion; and Figures 4 to 6 are graphs of parameters against time. 24 07 24 Detailed Description Figure 1 is a schematic diagram of an example of a cardiac perfusion system 100 incorporating a monitoring apparatus 160 in accordance with the present disclosure. The perfusion system 100 comprises one or more sensors 110, a venous reservoir 120, a pump 130, an oxygenator 140 and a controller 150. In use, the cardiac perfusion system 100 receives deoxygenated blood from a patient via a venous line 122, and stores the deoxygenated blood in the venous reservoir 120. The pump 130 pumps deoxygenated blood from the venous reservoir 120, through the oxygenator 140, and returns oxygenated blood to the patient via an arterial line 132 (as shown by arrows A). The one or more sensors 110 are configured to measure parameters associated with the cardiac perfusion system 100 and / or parameters associated with the patient. Although Figure 1 shows a single sensor 110 positioned on the arterial line 132, it should be appreciated that this only illustrative. A sensor 110 could be positioned in, or on, any or all of: the venous reservoir 120; the pump 130; the oxygenator 140; a blood line (e.g., venous line 122, arterial line 132 and / or any of the intermediate blood lines between the venous reservoir 120 and the oxygenator 140); a gas line (e.g., any of the tubes supplying gas to, or removing gas from, the oxygenator 140). Alternatively or in addition, a sensor 110 could be positioned in, or on, the patient. The location of the sensor 110 is chosen in accordance with the parameter that is to be measured. A sensor 110 may be configured to measure, for example: a gas pressure; a gas flow rate; a gas composition (e.g., a partial pressure of oxygen, carbon dioxide or any other gas); a blood pressure; a blood flow rate; a blood composition (e.g., a saturation of oxygen or carbon dioxide in the blood); or a blood volume (e.g., a volume of blood in the venous reservoir 120). The venous reservoir 120 receives deoxygenated blood from the patient via the venous line 122. The venous reservoir 120 may additionally be configured to receive blood from the patient via one or more salvage lines, one or more purge lines, and / or one or more lines configured to carry surgical fluids (e.g. priming solutions, volume expanders, blood, and / or drugs), as represented by line 124 in Figure 1. The venous reservoir 120 is positioned upstream of the oxygenator 140, between the patient and the oxygenator 140. It will be appreciated that, in other implementations, the exact arrangement of the 24 07 24 venous reservoir 120 and the pump 130 may vary. In fact, the system 100 need not necessarily comprise the venous reservoir 120 and the pump 130. In such a case, the oxygenator 140 may be configured to receive the blood directly from the patient. 5 The pump 130 drives blood through the cardiac perfusion system 100. As shown in Figure 1, the pump 130 is located downstream of the venous reservoir 120 and upstream of the oxygenator 140. In the depicted embodiment, pump 130 is a roller (or peristaltic) pump. However, it will be appreciated that, depending on the circumstance, other types of pump may be used, such as a centrifugal pump. 10 The oxygenator 140 receives deoxygenated blood via a blood inlet 144. The oxygenator also receives gases at one or more gas inlets 142a, 142b. The gas inlets 142a, 142b may receive air, oxygen and / or a mixture of air and oxygen. For example, a first gas inlet 142a may receive air, and a second gas inlet 142b may receive pure 15 oxygen. The air and / or oxygen received by the gas inlets 142a, 142b may optionally be supplemented by carbon dioxide and / or an anaesthetic gas. A gas blender (not shown in Figure 1) may mix various gases in clinically appropriate proportions before supplying gases to the gas inlets 142a, 142b. Blood passes through the oxygenator140, whereupon gases dissolved in the blood are exchanged with gases 20 received via the gas inlets 142a, 142b. The gas exchange within the oxygenator 140 generally involves removal of carbon dioxide from the blood, and addition of oxygen to the blood. In this manner, the oxygenator 140 converts deoxygenated blood to oxygenated blood. Oxygenated blood leaves the oxygenator 140 via a blood outlet 148, whereupon it is returned to the patient via the arterial line 132. Waste gas leaves 25 the oxygenator 140 via a gas outlet 146. The controller 150 controls the operation of the cardiac perfusion system 100. In particular, the controller 150 controls the pumping of blood by the pump 130, and controls the oxygenation of blood by the oxygenator 140. The controller 150 may also 30 control the operation of various valves and / or actuators not illustrated in Figure 1. The controller 150 comprises a monitoring apparatus 160 in accordance with the present disclosure. The structure and functionality of the monitoring apparatus 160 are described below with reference to Figures 2 and 3. In addition to the monitoring functionality described in connection with the monitoring apparatus 160, the controller 24 07 24 150 may also monitor other physiological parameters of the patient or parameters of the cardiac perfusion system 100, as is known in the art. The controller 150 is communicatively connected to the one or more sensors 110, to 5 the pump 130 and to the oxygenator 140, as depicted by the dash-dot lines in Figure 1. That is, the controller 150 may be configured to communicate with the one or more sensors 110, pump 130 and oxygenator 140. This communication may occur via hardware connections (e.g. wired connections) or via wireless communication. In this regard, it will be understood that the dash-dot lines depicted in Figure 1 are merely 10 illustrative and do not necessarily represent physical connections between components. Furthermore, it will be understood that the controller 150 may be communicatively connected to more or fewer components in system 100. For example, the controller 150 may not necessarily communicate directly with the oxygenator 140, but may instead be configured to communicate with valves, actuators 15 and / or other gas flow control mechanisms that control the gas flow into and / or out of the oxygenator 140. Similarly, the controller 150 may not necessarily communicate directly with the one or more sensors 110, but may instead be configured to communicate with an intermediate transceiver that relays measurements from the sensor 110 to the controller 150. As used herein, ‘communication’ with the controller 20 150 refers to both the reception of data (e.g. measurements) by the controller 150 and the transmission of data (e.g. instructions or commands) by the controller 150. The cardiac perfusion system 100 may optionally further comprise other components not shown in Figure 1. For example, the cardiac perfusion system 100 may comprise 25 any or all of: a cardioplegia device to stop the patient’s heart; a heater-cooler device to regulate the temperature of blood; one or more additional pumps to cause blood to flow from the patient, through the cardiac perfusion system 100 and / or to the patient; one or more valves to control the flow of blood and / or gas; and a gas blender to mix gases supplied to the oxygenator 140. 30 Figure 2 is a schematic diagram of the monitoring apparatus 160 shown in Figure 1. The monitoring apparatus 160 comprises a processor 202, a memory 204, an analogue-to-digital converter 208, a user interface 210 and, optionally, a digital output 218. 24 07 24 The processor 202 can be any suitable type of data processing device, such as a microprocessor, microcontroller or application specific integrated circuit (ASIC). The processor 202 is communicatively coupled to the memory 204. The memory 204 can include a volatile memory, a non-volatile memory, or both volatile and non-volatile 5 memories. The memory 204 stores a monitoring program 206. The monitoring program 206 includes processor-executable instructions that, when executed by the processor 202, cause the monitoring apparatus 160 to perform the method of monitoring a parameter described below with reference to Figure 3. 10 The analogue-to-digital converter 208 receives analogue data from the one or more sensors 110 (shown in Figure 1), and produces a digital representation of the analogue data. More specifically, the analogue-to-digital converter 208 outputs a series of digital values, which are referred to herein as samples. Each sample represents the instantaneous value of a parameter measured by a sensor 110 at a particular moment 15 in time. The user interface 210 comprises a display 212, a keyboard 214 and a speaker 216. The display 212 is configured to output a visual indication of information relevant to a cardiac perfusion procedure, such as a visible alert signal and / or a visible indication of 20 particular data collected by the monitoring program (e.g., a parameter and / or a running total of a parameter, as described in more detail below). The display 212 can be any suitable type of output device. For example, the display 212 may include a liquid crystal display (LCD) screen or an organic light-emitting diode (OLED) screen. The keyboard 214 comprises a plurality of buttons, which a user can use to input 25 information (e.g., one or more threshold values) for use by the monitoring program. The display 212 and keyboard 216 may be integrated with one another in the form of a touchscreen. The speaker 216 is capable of outputting audio information, such as an audible alert signal and / or a spoken indication of particular data collected by the monitoring program (e.g., a parameter and / or a running total of a parameter). 30 The digital output 218 is an optional component that provides a control signal to one or more components of the cardiac perfusion system 100. For example, the digital output 218 may provide a control signal to the pump 130 to cause the pump 130 to start pumping, stop pumping and / or modulate the rate of blood flow caused by the pumping. 35 As another example, the digital output 218 may provide a control signal to the 24 07 24 oxygenator 140 to modulate the amount of one or more gases in the oxygenated blood by the oxygenator 140. Continuing this example, the digital output 218 may provide a control signal to cause the oxygenator to increase or decrease the saturation of oxygen in oxygenated blood and / or to cause the oxygenator to increase or decrease the 5 saturation of carbon dioxide in the oxygenated blood. As another example, the digital output 218 may provide a control signal to any of the one or more sensors 110 to control how a sensor measures a parameter. Continuing this example, the digital output 218 may produce a control signal that increases or decreases the sensitivity of a particular sensor 110. 10 Although the processor 202, memory 204, analogue-to-digital converter 208, user interface 210 and digital output 218 are shown as components of the monitoring apparatus 160 in Figure 1, any or all of these components may also be used to implement other functionality of the controller 150 that is unrelated to the monitoring 15 apparatus 160. Alternatively, the monitoring apparatus 160 may be separate from the controller 150. The operation of the monitoring apparatus 160 will now be described with reference to Figure 3. Figure 3 is a flow diagram of a method 300 of monitoring a parameter during 20 cardiac perfusion. The method 300 may begin at block 302, when the monitoring apparatus 160 receives a first threshold value and / or a second threshold value. The first threshold value and / or a second threshold value may be received from a user via the user interface 210 (e.g., 25 via the keyboard 214). The first threshold value generally represents a target value for a specific parameter being monitored by the monitoring apparatus 160. For example, the first threshold value may be a value that the parameter should not go below. Alternatively, the first threshold value may be a value that the parameter should not exceed. The second threshold value is a cumulative value of the parameter that, when 30 exceeded, causes the monitoring apparatus 160 to generate an alert signal. Block 302 is optional. Either or both of the first and second threshold values may have default values, or their values may have been previously stored in the memory 204. Block 302 need only be performed if a user wishes to modify the first and / or second 24 07 24 values from their default or stored values. If block 302 is not performed, the method 300 begins at block 304. At block 304, the monitoring apparatus 160 receives a sample. For example, the 5 analogue-to-digital converter 208 can receive an analogue voltage signal from a sensor 110, and the analogue-to-digital converter 208 then converts the analogue voltage signal to a digital value representing the value of the parameter measured by the sensor 110 at a particular moment in time. The monitoring program 206 then receives the sample from the analogue-to-digital converter 208. In another example, the sensor 10 110 may output digital data representing the value of the parameter measured by a sensor 110. In this example, the monitoring program 206 can receive digital data from the sensor 110 (e.g., using a digital input, such as a bus) without using the analogue-to-digital converter 208. Continuing this example, the value of the digital data received from the sensor 110 at a particular moment in time is regarded as a sample. 15 In some cases, a parameter of interest may not be directly measurable by a single sensor 110. Some parameters are derived from measurements made by several sensors 110. An example of such a parameter is a perfusion ratio, which is derived from different sensors’ measurements of oxygen delivery and carbon dioxide 20 production. Some parameters require one or more arithmetic or logic operations to be performed to convert a measurement made by a sensor 110 into a value of the parameter. An example of such a parameter is indexed oxygen delivery, which requires a measured value of oxygen delivery to be indexed to a particular patient. Block 304 may optionally include processing one or measurements made by one or 25 more sensors to generate a sample, where the processing may include deriving the value of the sample from the measurements made a plurality of sensors 110 and / or performing one or more arithmetic or logic operations on a measurement made by a sensor 110. 30 At block 306, the monitoring apparatus 160 compares the value of the sample (as received at block 304) with the first threshold value. When the value of the sample meets the first threshold value, the value of the parameter being monitored is acceptable. To put this another way, the first threshold value is met when the value of the parameter is consistent with patient safety or clinical efficacy. Conversely, when 35 the value of the sample fails to meet the first threshold value, the value of the 24 07 24 parameter being monitored is not acceptable. Depending on nature of the parameter in question, a sample may be deemed to meet the first threshold when it is: greater than the first threshold value; greater than or equal to the first threshold value; equal to the first threshold value; less than the first threshold value; less than or equal to the first 5 threshold value; or within a particular range of the first threshold value. For example, the values of some parameters should be greater than or equal to the first threshold value to ensure a safe and efficacious outcome for the patient, whereas the values of other parameters should be less than the first threshold value to ensure a safe and efficacious outcome for the patient. For any given parameter, a clinician will 10 understand whether the value of a sample should be greater than, less than, equal to, or within a particular range of, the first threshold value in order for the sample to be regarded as meeting the threshold. When the sample does not meet the first threshold value, the method proceeds to 15 block 308. Alternatively, when the sample meets the first threshold value, the method proceeds to block 307. At block 308, the monitoring apparatus 160 adds the value of the sample to a running total. The running total represents the cumulative sum of the parameter’s values since 20 the first threshold was not met. The value of the running total is zero before block 308 is performed for the first time during a cardiac perfusion procedure. The value of the running total is also zero immediately after block 309 is performed, as will be discussed in more detail below. The value of the running total increases over time until the value of a received sample meets the first threshold value. 25 Block 308 may optionally include a data normalisation operation to ensure that the running total is consistent or otherwise meaningful. For example, the value of each sample may be divided by the sampling period (i.e., the time interval between two successive samples), which prevents the running total being influenced by the rate at 30 which samples are received. The current value of the running total may optionally be displayed upon the display 212, whereupon it can be seen by a clinician (e.g., a perfusionist) during the cardiac perfusion procedure. Alternatively or in addition, the current value of the running total 35 can be output in an audible form using the speaker 216, such that it can be heard by 24 07 24 the clinician during the cardiac perfusion procedure. The running total displayed on the screen may be refreshed when each sample is received, or it may be refreshed after a particular number of samples have been received, or it may be refreshed at a particular time interval. Similarly, the running total may be output audibly when each sample is 5 received, after a particular number of samples have been received, or at a particular time interval. At block 310, the monitoring apparatus 160 compares the running total with the second threshold value. When the running total does not exceed the second threshold value, 10 the cumulative value of the parameter (in the time since the first threshold value was not met) is consistent with patient safety or clinical efficacy. Conversely, when the running total exceeds the second threshold value, the cumulative value of the parameter being monitored is deemed to pose a risk to the patient’s safety or could result in a clinically sub-optimal outcome for the patient. 15 When the running total does not exceed (i.e., is not greater than) the second threshold, the method returns to block 304 whereupon the method is iterated for another sample. Alternatively, when the running total exceeds (i.e., is greater than) the second threshold, the method proceeds to block 312. 20 At block 312, the monitoring apparatus 160 generates an alert. The purpose of the alert is to warn a clinician (such as a perfusionist) that the second threshold has been exceeded. The alert warns the clinician to take remedial action to correct (e.g., increase or decrease) the value of the parameter being monitored before the patient’s 25 wellbeing is put at risk and / or before the clinical efficacy of an ongoing cardiac perfusion procedure is adversely affected. The nature of the remedial action needed will depend upon the parameter being monitored and the clinician’s judgement. The alert may comprise either or both of an audible alert signal output from the speaker 216 or a visual alert shown on the display 212. 30 Optionally, the cardiac perfusion system 100 may have the ability to perform certain types of remedial action automatically. In this case, the monitoring apparatus 160 may generate an alert that causes the controller 150 to control one or more components of the cardiac perfusion system 100 to correct (e.g., increase or decrease) the value of 35 the parameter being monitored. For example, upon receiving the alert, the controller 24 07 24 150 may send a control signal to the pump 130 to cause the pump 130 to start pumping, stop pumping, increase the flow rate of blood or decrease the flow rate of blood. As another example, upon receiving the alert, the controller 150 may send a control signal to the oxygenator 140 to cause the oxygenator 140 to increase or 5 decrease the saturation of oxygen in the blood. The nature of the remedial action taken by the controller 150 will depend upon the parameter being monitored and the capabilities of the cardiac perfusion system 100. After block 312 is performed, the method returns to block 304 whereupon the method is 10 iterated for another sample. However, the alert may continue to be generated until it is stopped by a user (such as a perfusionist or other clinician) or until the running total is reset at block 309. At block 307, the monitoring apparatus 160 determines whether the current value of the 15 running total is equal to zero. Block 307 is performed when a received sample is determined to meet the first sample at block 306. There are two scenarios in which block 307 is performed. In a first scenario, the two most-recently received samples have both met the first threshold. In the first scenario, the running total will already be equal to zero and, therefore, the method returns to block 304 whereupon the method is 20 iterated for another sample. In a second scenario, the most-recently received sample met the first threshold but at least one immediately preceding sample did not meet the first threshold. In the second scenario, the method proceeds to block 309. At block 309, the running total is reset. In other words, the value of the running total is 25 set to zero. The method then returns to block 304 whereupon the method is iterated for another sample. Hysteresis may optionally be included in the operation of blocks 307 and 309 so that noise in the value of individual samples does not cause the running total to be reset. 30 For example, block 309 may be performed only if a predetermined number of consecutive samples have met the first threshold. The maximum value of the running total reached during an ongoing cardiac perfusion procedure may optionally be displayed upon the display 212, whereupon it can be seen 35 by a clinician (e.g., a perfusionist). The maximum value of the running total may 24 07 24 continue to be displayed until the end of the cardiac perfusion procedure. Alternatively or in addition, the maximum value of the running total can be output in an audible form using the speaker 216, such that it can be heard by the clinician during the cardiac perfusion procedure. The maximum value of the running total is not reset when the 5 running total is reset at block 309. Figures 4, 5 and 6 illustrate the operation of the monitoring system 160 and the method 300. 10 Figure 4 shows a graph 400 of a parameter against time during a cardiac perfusion procedure. The value 402 of each sample is plotted on the vertical axis, and the time at which the sample was measured is plotted on the horizontal axis. The values 402 of the samples are illustrated as a smooth curve. The first threshold value 404 is indicated by a horizontal line. In the example of Figure 4, the first threshold 404 is met 15 when the value of the parameter is greater than the first threshold. Initially, the values 402 of the samples are greater than the first threshold and, therefore, the first threshold is met. The monitoring system 160 does not generate an alert. At time to, the values 402 of the samples fall below the first threshold 404 and, 20 therefore, the first threshold is not met. The monitoring system 160 does not generate at alert at time to, but instead starts adding the values 402 of samples to a running total from time to onwards. The running total is indicated by the hatched area 406, and can be thought of as the area under the curve between the sample values 402 and the first threshold 404. The values 402 of samples remain below the first threshold until time t2 25 and, therefore, the running total increases over time. At time t1, the running total exceeds the second threshold. Hence, the monitoring system 160 generates an alert at time t1. The alert prompts a clinician to take remedial action to correct the parameter, or causes the controller 150 to generate a control signal that causes one or more components of the cardiac perfusion system 100 to take remedial action. As a 30 result of the remedial action, the values 402 of the samples increase towards the first threshold 404 between time t1 and time t2. At time t2, the first threshold 404 is met and the running total is reset. The values 402 of the samples meet the first threshold 404 for the rest of the cardiac perfusion procedure. 24 07 24 Figure 5 shows another graph 500 of a parameter against time during a cardiac perfusion procedure. The value 502 of each sample is plotted on the vertical axis, and the time at which the sample was measured is plotted on the horizontal axis. The values 502 of the samples are illustrated as a smooth curve. The first threshold value 5 504 is indicated by a horizontal line. As with the example of Figure 4, the first threshold 504 is met when the value of the parameter is greater than the first threshold. Initially, the values 502 of the samples are greater than the first threshold and, therefore, the first threshold is met. The monitoring system 160 does not generate an 10 alert. At time t4, the values 502 of the samples fall below the first threshold 504 and, therefore, the first threshold is not met. The monitoring system 160 does not generate at alert at time t4, but instead starts adding the values 502 of samples to a running total from time t4 onwards. The running total is indicated by the hatched area 506. The values 502 of the samples increase over time, and meet the first threshold at time t5. 15 Time t5 occurs before the second threshold is exceeded. Therefore, an alert is not generated between t4 and t5, even though the first threshold was not met during this time interval. The running total is reset at time t5. At time t6, the values 502 of the samples again fall below the first threshold 504 and, therefore, the first threshold is not met. The monitoring system 160 does not generate at alert at time t6, but instead 20 starts adding the values 502 of samples to a new running total from time t6 onwards. The new running total is indicated by the hatched area 508. The values 502 of samples remain below the first threshold until time t8 and, therefore, the running total increases over time. At time t7, the running total exceeds the second threshold. Hence, the monitoring system 160 generates an alert at time t7. The alert prompts the 25 clinician to take remedial action to correct the parameter, or causes the controller 150 to generate a control signal that causes one or more components of the cardiac perfusion system 100 to take remedial action. As a result of the remedial action, the values 502 of the samples increase towards the first threshold 504 between time t7 and time t8. At time t8, the first threshold 504 is met and the running total is reset. The 30 values 502 of the samples meet the first threshold 504 for the rest of the cardiac perfusion procedure. Figure 6 shows a graph 600 of a different parameter against time during a cardiac perfusion procedure. The value 602 of each sample is plotted on the vertical axis, and 35 the time at which the sample was measured is plotted on the horizontal axis. The 24 07 24 values 602 of the samples are illustrated as a smooth curve. The first threshold value 604 is indicated by a horizontal line. In contrast to the example of Figures 4 and 5, the first threshold 604 is met when the value of the parameter is less than the first threshold. 5 Initially, the values 602 of the samples are less than the first threshold and, therefore, the first threshold is met. The monitoring system 160 does not generate an alert. At time t9, the values 602 of the samples increase above the first threshold 604 and, therefore, the first threshold is not met. The monitoring system 160 does not generate 10 at alert at time t9, but instead starts adding the values 602 of samples to a running total from time t9 onwards. The running total is indicated by the hatched area 606. The values 602 of the samples remain above the first threshold until time t11 and, therefore, the running total increases over time. At time t10, the running total exceeds the second threshold. Hence, the monitoring system 160 generates an alert at time t10. The alert 15 prompts the clinician to take remedial action to correct the parameter, or causes the controller 150 to generate a control signal that causes one or more components of the cardiac perfusion system 100 to take remedial action. As a result of the remedial action, the values 602 of the samples decrease towards the first threshold 604 between time t10 and time 111. At time t11, the first threshold 604 is met and the running total is 20 reset. The values 602 of the samples meet the first threshold 604 for the rest of the cardiac perfusion procedure. Non-limiting examples of parameters that can be monitored in accordance with the present disclosure include: 25 • Oxygen delivery (DO2) - this parameter is the rate at which oxygen is delivered to tissues throughout the whole body. Oxygen delivered is typically measured in millilitres per minute (mUmin). Inadequate oxygen delivery can cause acute injury to various tissues and / or organ systems of a patient. The renal system is 30 one example of an organ system that can be injured by inadequate oxygen delivery. By monitoring oxygen delivery in accordance with the present disclosure, a clinician is alerted before the oxygen delivery becomes sufficiently inadequate to cause tissue and / or organ injury. The clinician or the controller 150 can thus take remedial action before injury arises, thereby ensuring the 35 safety and wellbeing of the patient. 24 07 24 • Indexed oxygen delivery (DOa) - this parameter is similar to oxygen delivery (DO2), but is indexed to a particular patient. For example, DO2i can be calculated by dividing a DO2 value by the body surface area (BSA) of the patient in m2, or by dividing a DO2 value by the mass of the patient in kg. 5 Indexed oxygen delivery is typically measured in mUmin / m2 when indexed to the patient’s BSA, or in mL / min / kg when indexed to the patient’s mass. The clinical implications of inadequate DO2i are the same as those explained above in connection with DO2. • Oxygen consumption (VO2) - this parameter is the rate at which oxygen is taken 10 up (consumed) by a patient. Oxygen consumption is typically measured in mL / min. Inadequate oxygen consumption can cause acute injury to various tissues and / or organ systems of a patient. • Indexed oxygen consumption (VO2i) - this parameter is similar to oxygen consumption, but is indexed to a particular patient. For example, VOa can be 15 calculated by dividing a VO2 value by the body surface area of the patient in m2, or by dividing a VO2 value by the mass of the patient in kg. Indexed oxygen consumption is typically measured in mL / min / m2 when indexed to the patient’s BSA, or in mL / min / kg when indexed to the patient’s mass. The clinical implications of inadequate VO2i are the same as those explained above in 20 connection with VO2. • Carbon dioxide production (VCO2) - this parameter is the rate at which carbon dioxide is excreted (produced) by a patient. Carbon dioxide production is typically measured in mL / min. Inadequate carbon dioxide production can indicate shunting or abnormal cellular metabolism. 25 • Indexed carbon dioxide production (VCO2i) - this parameter is similar to carbon dioxide production, but is indexed to a particular patient. For example, VCO2i can be calculated by dividing a VCO2 value by the body surface area of the patient in m2, or by dividing a VCO2 value by the mass of the patient in kg. Indexed carbon dioxide production is typically measured in mL / min / m2 when 30 indexed to the patient’s BSA, or in mL / min / kg when indexed to the patient’s mass. The clinical implications of inadequate VCO2i are the same as those explained above in connection with VCO2. 24 07 24 • Extraction ratio (ER) - this parameter is the ratio of oxygen consumption to oxygen delivery, i.e. VO2 divided by DO2. Extraction ratio is a dimensionless parameter, which is typically expressed as a percentage. The “normal” value of extraction ratio is 25% at normal body temperature, but is often in the range of 5 15-20% without adverse consequences in a patient whose body is being cooled during a cardiac perfusion procedure. A lower than normal value of extraction ratio indicates that less oxygen is being taken up by the patient’s tissues, which could indicate that shunting is taking place. If allowed to continue over a prolonged period, shunting can result in lactic acidosis which, in turn, can cause 10 harm to a patient. A greater than normal value of extraction is indicative of either inadequate oxygen delivery (the implications of which are discussed above) or increased tissue metabolic rate. Extraction ratio may be deemed to meet the first threshold only when it is within a few percentage points above or below the normal value for a particular body temperature. 15 • Perfusion ratio (PR) - this parameter is the ratio of oxygen delivery to carbon dioxide production, i.e. DO2 divided by VCO2. Perfusion ratio is a dimensionless parameter, which is typically expressed as a floating point number. The “normal” value of perfusion ratio is in the range of 5 to 5.3 at normal body temperature, but is often above 5.3 without adverse consequences 20 in a patient whose body is being cooled during a cardiac perfusion procedure. A lower than normal value of perfusion ratio is indicative of either inadequate oxygen delivery (the implications of which are discussed above) or increased tissue metabolic rate. A higher than normal value of perfusion ratio could indicate that shunting is taking place. If allowed to continue over a prolonged 25 period, shunting can result in lactic acidosis which, in turn, can cause harm to a patient. Perfusion ratio may be deemed to meet the first threshold only when it is within a particular range above or below the normal range for a particular body temperature. • Mean arterial pressure (MAP) - if mean arterial pressure is too low, the patient 30 is not being provided with enough energy to perfuse their tissue beds adequately, which could lead to injury to various tissues and / or organ systems of a patient. The glomerulus of the kidney is one example of a tissue that can be injured by low mean arterial pressure. On the other hand, if mean arterial pressure is too high, blood vessels anywhere in the arterial system may be 24 07 24 ruptured. This, in turn, may result in haemorrhagic stroke, bleeding, or increased oedema in tissues. Mean arterial pressure may be deemed to meet the first threshold only when it is within a particular range above or below its normal value. 5 • Central venous pressure (CVP) - if central venous pressure is too low, this may indicate inadequate fluid return to the right heart. This, in turn, may be indicative of blood loss, blood vessel dilation or blood vessel obstruction. If central venous pressure is too high, this may indicate excess blood volume or blood vessel constriction. This, in turn, can cause oedema and / or poor tissue 10 perfusion (because the difference between MAP and CVP is what creates the gradient for normal blood flow through tissues). Central venous pressure may be deemed to meet the first threshold only when it is within a particular range above or below its normal value. • Pulmonary artery pressure (PA) 15 • Cerebral mean regional oxygen saturation (CMRO2) - this parameter can be indicative of inadequate global brain flow or one brain hemisphere being under / over-perfused compared with the other hemisphere. This could be due to an occlusion (which can occur when a patient’s head is incorrectly positioned, causing the carotid artery to kink), a blood clot or a dissecting aneurysm. 20 • Blood flow rate - this parameter could be either the flow rate of oxygenated blood returned to a patient (e.g., via the arterial line 132), or the flow rate of deoxygenated blood received from a patient (e.g., via venous line 122). If the blood flow rate is too low, there is a risk of inadequate blood delivery. If cardiac index is too high, there is a risk of increased haemolysis (whereby blood is 25 damaged by the pump 130). Haemolysis can contribute to oedema and / or bleeding. Blood flow rate may be deemed to meet the first threshold only when it is within a particular range above or below a clinically acceptable value. • Cardiac index (Cl) - this parameter is a blood flow rate indexed to a particular patient. Cardiac index is typically measured in mL / min / m2 when indexed to a 30 patient’s BSA, or in mL / min / kg when indexed to the patient’s mass. The clinical implications of inadequate cardiac index are the same as those explained above in connection with blood flow rate. 24 07 24 This list of parameters is not intended to be exhaustive. Any other parameter that is measurable and relevant to an ongoing cardiac perfusion procedure can be monitored in accordance with the present disclosure. In general, the parameter being monitored is a parameter that, if allowed to remain at an abnormal value for a prolonged period of 5 time, could directly affect the wellbeing of a patient (e.g., by causing injury to a tissue and / or organ system of the patient), could be an indirect indicator of a situation in which the patient’s wellbeing may be adversely affected, or could otherwise lead to a clinically sub-optimal outcome for the patient. 10 The first threshold value and / or the second threshold value may be defined as a logical and / or arithmetic function of one or more parameters. By defining either or both of the first and second threshold values as a logical and / or arithmetic function of the monitored parameter(s), the monitoring apparatus 160 can be configured to generate an alert in very specific clinical situations. In this manner, the monitoring apparatus 15 160 can be configured to be more likely to generate an alert in some clinical situations, and less likely to generate an alert in other clinical situations. This, in turn, can reduce the number of “false positive” alerts generated produced by the monitoring apparatus 160, and thus reducing distractions for a clinician during a cardiac perfusion procedure. 20 The logical function may include any Boolean function, such as AND, OR, NOT or XOR. The logical function may also include an inequality operation (e.g., greater than; greater than or equal to; equal to; less than or equal to; or equal to). The arithmetic function may include addition, subtraction, division or multiplication. The logical and / or arithmetic function can be defined when the monitoring apparatus 160 is manufactured, 25 or it can be edited or entered by a clinician (e.g., using the keyboard 214). For example, consider a monitoring apparatus that is configured to monitor three parameters, which are referred to as A, B and C. The first threshold can be defined as a logical expression such as “(A >100) AND (B = 0 OR C <0)”. In this case, the 30 monitoring apparatus will start adding the value of a parameter (say, parameter A) to the running total only when the value of parameter A is greater than 100 and either the value of parameter B is equal to zero or the value of parameter C is less than zero. The second threshold can be defined as another logical expression such as “RUNNING_TOTAL_A >500 AND B = 0”. In this case, the alert will be generated only 35 if the value of parameter B is equal to zero when the running total of parameter A exceeds 500. It will be appreciated that this is just a simple example of how the first and second thresholds may be defined using logical and / or arithmetic functions and, in practice, a clinician can define either or both thresholds in whatever way is most useful to them. 5 It will be understood that the invention has been described above purely by way of example, and that modifications of detail can be made within the scope of the claims. In particular, the sequence of operations shown in Figure 3 is merely exemplary. Any of the operations shown in method 300 may be performed in a different order that 10 achieves substantially the same result. 24 07 24 24 07 24

Claims

1. An apparatus for monitoring a parameter during cardiac perfusion, the apparatus configured to:5 receive a plurality of samples, each sample representing the value of theparameter at a respective moment in time;compare each sample with a first threshold value;when a sample does not meet the first threshold value: add the value of the sample to a running total, and10 generate an alert when the running total exceeds a second thresholdvalue; andreset the running total to zero when a sample meets the first threshold value.

2. An apparatus in accordance with claim 1, wherein the apparatus is configured 15 to receive the first threshold value and / or the second threshold value from a user.

3. An apparatus in accordance with claim 1 or claim 2, wherein the first threshold value is non-zero.20 4. An apparatus in accordance with any of the preceding claims, wherein theapparatus is configured to output a current value of the running total to a user.

5. An apparatus in accordance with any of the preceding claims, wherein the apparatus is configured to output a maximum value of the running total reached during 25 an ongoing cardiac perfusion procedure.

6. An apparatus in accordance with any of the preceding claims, wherein the parameter is:delivery of oxygen to a patient;30 consumption of oxygen by the patient;production of carbon dioxide by the patient;a ratio of oxygen consumed by the patient to oxygen delivered to the patient;a ratio of oxygen delivered to the patient to carbon dioxide produced by the patient;35 a pressure;24 07 24a blood gas saturation;a cardiac index; or a flow rate.5 7. An apparatus in accordance with any of the preceding claims, wherein the firstthreshold value and / or the second threshold value is defined as a logical or arithmetic function of one or more parameters.

8. An apparatus in accordance with any of the preceding claims, wherein the 10 apparatus is a cardiac perfusion system or a component thereof.

9. An apparatus in accordance with any of the preceding claims, the apparatus is further configured to send a control signal to a component of the cardiac perfusion system when the running total exceeds the second threshold value, the control signal 15 configured to cause the component of the cardiac perfusion system to correct the value of the parameter.

10. A processor-implemented method of monitoring a parameter during cardiac perfusion, the method comprising:20 receiving a plurality of samples, each sample representing the value of theparameter at a respective moment in time;comparing each sample with a first threshold value;when a sample does not meet the first threshold value:adding the value of the sample to a running total, and25 generating an alert when the running total exceeds a second thresholdvalue;resetting the running total to zero when a sample meets the first predetermined value.30 11. A method in accordance with claim 10, further comprising receiving the firstthreshold value and / or the second threshold value from a user.

12. A method in accordance with claim 10 or claim 11, wherein the first threshold value is non-zero.24 07 2413. A method in accordance with any of claims 10 to 12, further comprising outputting a current value of the running total to a user.

14. A method in accordance with any of claims 10 to 13, further comprising 5 outputting a maximum value of the running total reached during a cardiac perfusion procedure.

15. A method in accordance with any of claims 10 to 14, wherein the parameter is: delivery of oxygen to a patient;10 consumption of oxygen by the patient;production of carbon dioxide by the patient;a ratio of oxygen consumed by the patient to oxygen delivered to the patient;a ratio of oxygen delivered to the patient to carbon dioxide produced by the patient;15 a pressure;a blood gas saturation;a cardiac index; ora flow rate.20 16. A method in accordance with any of claims 10 to 15, wherein the first thresholdvalue and / or the second threshold value is defined as a logical or arithmetic function of one or more parameters.

17. A method in accordance with any of claims 10 to 16, further comprising sending 25 a control signal to a component of the cardiac perfusion system when the running total exceeds the second threshold value, the control signal configured to cause the component of the cardiac perfusion system to correct the value of the parameter.

18. A processor-readable medium comprising processor-executable instructions 30 which, when executed by a processor, cause a monitoring apparatus comprising the processor to perform a method in accordance with any of claims 10 to 17.

19. A computer program comprising processor-executable instructions which, when executed by a processor, cause a monitoring apparatus comprising the processor to 35 perform a method in accordance with any of claims 10 to 17.

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