The present disclosure relates to the transfer of fluid to or from a body via a cannula, and in particular concerns an apparatus and method for detecting displacement of the cannula during the transfer of said fluid. The term "cannula" may be taken to encompass all hollow conduits capable of penetrating the human or animal body to deliver or extract fluid, and includes needles, catheters and trocars.

Background Renal dialysis is used to purify the blood of a patient 101 whose kidneys are not functioning as they should be. During this procedure, blood is withdrawn from a vein (or artery) by a first needle 102, purified, and then returned to the vein by a second needle 103. This is illustrated schematically in Figure 1. Displacement (or dislodgement) of the second needle 103 from the vein prevents the purified blood from being delivered to the patient 101. If such an event goes undetected, blood may continue to be extracted from the patient 101 by the dialysis machine resulting in significant blood loss and possibly death of the patient 101.

Modern dialysis machines typically monitor the pressure of the fluid conduit 104 (blood line) between the machine and the second needle 103 and generate an alarm if the pressure drops below a predetermined level. Whilst this method is capable of detecting disconnection of the fluid conduit 104 from the second needle 103, it is not suitable for detecting displacement of the second needle 103 from the patient 101. This is because the drop in blood pressure caused by displacement of the second needle 103 from the patient 101 is often insufficient to trigger the alarm of the dialysis machine.

A number of solutions have been proposed for detecting venous needle displacement. One of these (US 2010/0056976 A1 ) involves mechanically clamping the fluid conduit proximal to the second needle when the needle is displaced. Once the fluid conduit is clamped, the increased blood pressure is detected by the dialysis machine which then triggers an alarm. However, this solution requires the force of displacement to overcome friction provided by the clamping force of a resilient member, which cannot be guaranteed. The apparatus and method disclosed herein may address this issue.

The listing or discussion of a prior-published document or any background in this specification should not necessarily be taken as an acknowledgement that the document or background is part of the state of the art or is common general knowledge. One or more aspects/embodiments of the present disclosure may or may not address one or more of the background issues.

Summary According to a first aspect, there is provided a sensing device for detecting displacement of a cannula relative to a body in which it is inserted, the sensing device comprising: a sensor element having an attachment portion for engaging with the cannula and an insertion portion; and

a sensor unit having a receptacle configured to receive the insertion portion of the sensor element and electronic sensing means configured to detect displacement of the insertion portion relative to the receptacle.

The attachment portion of the sensor element may comprise at least one hook configured to engage with a wing portion of the cannula. The attachment portion of the sensor element may comprise a hole or slot through which the cannula can be inserted to provide a friction fit. The attachment portion of the sensor element may form an integral part of the cannula.

The receptacle may comprise a hole or slot configured to receive the insertion portion. The insertion portion may be shaped to aid insertion of the insertion portion into a cavity of the receptacle. The insertion portion may have a generally planar structure comprising an end plane connected to a central plane. The end plane may be tilted with respect to the central plane to aid said insertion. The sensor element may comprise an abutment portion configured to limit insertion of the insertion portion into the receptacle. The abutment portion of the sensor element may comprise at least one shoulder located at an end of the insertion portion. The insertion portion of the sensor element may have a shorter length than an insertable part of the cannula. This feature helps to ensure that displacement of the insertion portion relative to the receptacle is detected by the sensing means before the cannula is withdrawn from the body. The electronic sensing means may be configured to measure the magnitude of relative displacement of the insertion portion and receptacle. The electronic sensing means may be configured to detect relative displacement of the insertion portion and receptacle beyond a predetermined position. The electronic sensing means may comprise a proximity sensor. Unlike sensors which detect the presence of an object (the insertion portion in this case) via physical contact with that object, proximity sensors do not require physical contact for detection. The electronic sensing means may comprise one or more of the following to interact with the insertion portion: an illumination source, a photodetector, a reed switch, a capacitive sensor, an inductive sensor, a Hall effect sensor and a pair of electrical contacts. At least part of the insertion portion may be optically reflective, optically opaque, magnetic, dielectric and/or electrically conductive.

The sensing device may comprise a controller configured to receive a detection signal from the electronic sensing means and send a control signal in response to the received detection signal. The control signal may be configured to activate an audio alarm, activate a visual alarm, activate a valve on a fluid conduit and/or cease operation of a pump on a fluid conduit. The sensing device may comprise a valve sensor configured to detect the presence of a fluid conduit in the valve.

According to a further aspect, there is provided a sensing device as substantially described herein with reference to, and as illustrated by, the accompanying drawings.

According to a further aspect, there is provided a sensor element for a sensing device, the sensing device configured for detecting displacement of a cannula relative to a body in which it is inserted, the sensor element having an attachment portion for engaging with the cannula and an insertion portion,

the sensing device comprising a sensor unit having a receptacle configured to receive the insertion portion of the sensor element and electronic sensing means configured to detect displacement of the insertion portion relative to the receptacle.

According to a further aspect, there is provided a sensor unit for a sensing device, the sensing device configured for detecting displacement of a cannula relative to a body in which it is inserted,

the sensing device comprising a sensor element having an attachment portion for engaging with the cannula and an insertion portion,

the sensor unit having a receptacle configured to receive the insertion portion of the sensor element and electronic sensing means configured to detect displacement of the insertion portion relative to the receptacle.

According to a further aspect, there is provided a kit of parts comprising one or more of: a sensor element as described herein; a sensor unit as described herein; a cannula which is engageable with the sensor element; and a protective sheath which is configurable to prevent direct physical contact between the sensor unit and a body or body fluid.

According to a further aspect, there is provided a method of detecting displacement of a cannula relative to a body in which it is inserted using a sensing device, the sensing device comprising:

a sensor element having an attachment portion for engaging with the cannula and an insertion portion; and

a sensor unit having a receptacle configured to receive the insertion portion of the sensor element and electronic sensing means configured to detect displacement of the insertion portion relative to the receptacle, the method comprising:

detecting displacement of the insertion portion relative to the receptacle by the electronic sensing means.

The present disclosure includes one or more corresponding aspects, example embodiments or features in isolation or in various combinations whether or not specifically stated (including claimed) in that combination or in isolation. Corresponding means for performing one or more of the discussed functions are also within the present disclosure.

The above summary is intended to be merely exemplary and non-limiting.

Brief Description of the Figures

A description is now given, by way of example only, with reference to the accompanying schematic drawings, in which:-

Figure 1 illustrates schematically the delivery and extraction of blood to and from a renal dialysis patient;

Figure 2 illustrates schematically a sensing device as described herein;

Figure 3a illustrates schematically in plan view a sensor element for the sensing device of Figure 2 according to a first embodiment;

Figure 3b illustrates schematically in side view the sensor element of Figure 3a;

Figure 4a illustrates schematically in plan view a sensor element for the sensing device of Figure 2 according to a second embodiment;

Figure 4b illustrates schematically in side view the sensor element of Figure 4a; Figure 5a illustrates schematically in front view a sensor unit for the sensing device of Figure 2 according to a first embodiment;

Figure 5b illustrates schematically in front view a sensor unit for the sensing device of Figure 2 according to a second embodiment;

Figure 5c illustrates schematically in side view a sensor unit for the sensing device of Figure 2 according to a third embodiment;

Figure 5d illustrates schematically in side view a sensor unit for the sensing device of Figure 2 according to a fourth embodiment;

Figure 6a illustrates schematically in side view a sensing device comprising the sensor element of Figure 3 engaged with a cannula;

Figure 6b illustrates schematically in side view a sensing device comprising the sensor element of Figure 4 engaged with a cannula;

Figure 6c illustrates schematically in side view a sensing device comprising a sensor element which is formed integrally with a cannula; Figure 7a illustrates schematically a first sensing means comprising a pair of electrical contacts detecting no relative displacement between the insertion portion and the receptacle;

Figure 7b illustrates schematically the sensing means of Figure 7a detecting relative displacement between the insertion portion and the receptacle;

Figure 8a illustrates schematically the first sensing means in an alternative configuration detecting no relative displacement between the insertion portion and the receptacle;

Figure 8b illustrates schematically the sensing means of Figure 8a detecting relative displacement between the insertion portion and the receptacle;

Figure 9a illustrates schematically the first sensing means in yet another configuration detecting no relative displacement between the insertion portion and the receptacle;

Figure 9b illustrates schematically the sensing means of Figure 9a detecting relative displacement between the insertion portion and the receptacle;

Figure 10a illustrates schematically a second sensing means comprising a photodetector detecting no relative displacement between the insertion portion and the receptacle;

Figure 10b illustrates schematically the sensing means of Figure 10a detecting relative displacement between the insertion portion and the receptacle;

Figure 1 1a illustrates schematically the second sensing means in an alternative configuration detecting no relative displacement between the insertion portion and the receptacle;

Figure 1 1 b illustrates schematically the sensing means of Figure 11a detecting relative displacement between the insertion portion and the receptacle;

Figure 12a illustrates schematically a third sensing means comprising a reed switch detecting no relative displacement between the insertion portion and the receptacle;

Figure 12b illustrates schematically the sensing means of Figure 12a detecting relative displacement between the insertion portion and the receptacle;

Figure 13a illustrates schematically the third sensing means in an alternative configuration detecting no relative displacement between the insertion portion and the receptacle;

Figure 13b illustrates schematically the sensing means of Figure 13a detecting relative displacement between the insertion portion and the receptacle; Figure 14a illustrates schematically a fourth sensing means comprising a Hall effect sensor detecting no relative displacement between the insertion portion and the receptacle;

Figure 14b illustrates schematically the sensing means of Figure 14a detecting relative displacement between the insertion portion and the receptacle; and

Figure 15 illustrates schematically a renal dialysis system comprising the sensing device described herein.

Description of Specific Aspects/Embodiments

As mentioned in the background section, venous needle displacement can result in significant blood loss, and if undetected for several minutes, may even cause death of the patient. There is therefore a need for fast and reliable detection of venous needle displacement. This need has increased in recent times due to an increase in home dialysis where the level of nursing care may be significantly reduced relative to a hospital or clinic. The apparatus and method described herein may provide a solution to this problem.

The present apparatus is a sensing device 205 for detecting displacement of a cannula 206 relative to a body in which it is inserted, as shown in Figure 2. The cannula 206 comprises an insertable part 207 which is intended to be inserted into the body of a patient, and a non-insertable part 208 which is not intended to be inserted into the body of a patient. The sensing device 205 comprises a sensor element 209 and a sensor unit 210. The sensor element 209 has an attachment portion 211 for engaging with the cannula 206 and an insertion portion 212, whilst the sensor unit 210 has a receptacle 213 configured to receive the insertion portion 212 of the sensor element 209 and electronic sensing means 214 configured to detect displacement of the insertion portion 212 relative to the receptacle 213.

In use, the cannula 206 is inserted into the body of a patient and the sensor element 209 is attached to the cannula 206. Cannulae often comprise wing portions 215 which extend laterally outward from the cannula shaft on either side. Surgical tape can then be applied to secure the cannula 206 (to which the sensor element 209 is attached) in place on the patient via these wing portions 215. Once the cannula 206 (with sensor element 209) is securely attached to the patient, the sensor unit 210 is positioned relative to the sensor element 209 such that the insertion portion 212 is received by the receptacle 213. The sensor unit 210 is then secured to the patient (e.g. using surgical tape) independently of the cannula 206 and sensor element 209. It will be appreciated that the various components of the sensing device 205 could be attached or assembled in a different order to that described above.

To prevent direct physical contact between the sensor unit 210 and the patient's body (which could infect the insertion site), the sensor unit 210 may be wrapped in a protective sheath 216 (which may be sterile and disposable) before attachment to the patient. The protective sheath 216 also prevents the sensor unit 210 from being contaminated by bodily fluids and adhesive from the surgical tape. The protective sheath 216 may be configured to cover the internal surfaces of the sensor unit 210 as well as the external surfaces. This feature prevents direct physical contact between the sensor unit 210 and the sensor element 209. In this scenario, the protective sheath should be made of a material which does not hinder or prevent displacement detection. For example, if the sensing means are configured to utilise electromagnetic radiation (as described later) for displacement detection, the protective sheath should be made of a material which is substantially transparent to this electromagnetic radiation. Similarly, if the sensing means utilise electric or magnetic fields (also described later) for displacement detection, the protective sheath should be made of a material which does not interfere substantially with these electric or magnetic fields. During the delivery or extraction of fluid via the cannula 206, the electronic sensing means 214 monitor the position of the insertion portion 212 within the receptacle 213 and detect displacement of the insertion portion 212 relative to the receptacle 213. As will be discussed in detail later, this may be achieved by measuring the magnitude of relative displacement of the insertion portion 212 and receptacle 213, or by detecting relative displacement of the insertion portion 212 and receptacle 213 beyond a predetermined position. Unlike the automatic clamping mechanism described in the background section, this approach does not require the displacement force to overcome friction provided by the clamping force of a resilient member. Rather, any force which is capable of displacing the cannula 206 relative to the patient's body is also sufficient to displace the insertion portion 212 relative to the receptacle 213. The present technique may therefore be more reliable than the clamping solution.

One embodiment of the sensor element 309 is shown in plan view and side view in Figures 3a and 3b, respectively. In this embodiment, the attachment portion 311 comprises hooks 317 configured to engage with the wing portions of the cannula. In practice, however, the attachment portion 311 could be any feature of the sensor element 309 which is configured to couple to the cannula in such a way that movement of the cannula in at least one direction (i.e. the direction of withdrawal of the cannula from the patient) causes consequential motion of the sensor element 309. Another embodiment is shown in plan view and side view in Figures 4a and 4b, respectively. This time, the attachment portion 411 comprises a hole or slot 418 through which the cannula is inserted to provide a friction fit. Figures 5a-5d show more detail of the receptacle 513 of the sensor unit. Figures 5a and 5b are front views of the receptacle 513, whereas Figures 5c and 5d are side views of the receptacle 513. The receptacle 513 may comprise any open cavity which provides at least partial confinement of the insertion portion. For example, the open cavity may be a hole 519 or a slot 520 configured to receive the insertion portion of the sensor element. The term "slot" as used herein refers to a cavity in which the sidewalls 521 of the receptacle 513 do not completely surround the insertion portion once it has been received (i.e. a slot 520 allows insertion and withdrawal of the insertion portion via the sides 523 of the receptacle 513 as well as via the front 522 of the receptacle 513). A slot 520 is shown in Figure 5a. In contrast, the term "hole" refers to a cavity in which the sidewalls 521 of the receptacle 513 completely surround the insertion portion once it has been received (i.e. a hole 519 prevents insertion and withdrawal of the insertion portion via the sides 523 of the receptacle 513). A hole 519 can be seen in Figure 5b. In practice, a hole 519 may help to reduce the chances of erroneous detection caused by withdrawal of the insertion portion via the sides 523 of the receptacle 513 whilst the cannula remains fully inserted in the body of the patient. This could occur, for example, if the receptacle 513 comprises a slot 520 and the patient moves during the medical procedure.

As illustrated in Figure 5c, the slot 520 or hole 519 may a through-slot or through-hole, respectively, meaning that the cavity extends through the receptacle 513 from one end to the other. Alternatively, the slot 520 or hole 519 may be a blind slot or blind hole, respectively, meaning that the cavity terminates somewhere within the receptacle 513. This latter configuration is shown in Figure 5d. A blind slot or hole may be useful in the sense that it limits insertion of the insertion portion. This feature may be used to facilitate the positioning of the sensor element relative to the electronic sensing means at the start of the medical procedure. Nevertheless, even if the receptacle comprises a through-slot or through-hole, the sensor element may comprise an abutment portion configured to limit insertion of the insertion portion into the receptacle. One example of such an abutment portion is shown in Figure 3a. In this example, the abutment portion comprises shoulders 324 located at an end of the insertion portion 312 configured to contact an external surface of the receptacle once the insertion portion has been inserted to the correct position.

Figures 6a-6c show side views of the sensing device 605 as assembled in use. In Figure 6a, the hooks 617 of the attachment portion 611 are engaged with the wing portions 615 of the cannula 606 (although the hooks 617 would normally fit more snugly to the wing portions 615 than shown in the figure), and in Figure 6b, the cannula 606 is inserted through the cavity 618 of the attachment portion 611 to provide a friction fit. The friction fit has to be sufficiently snug to ensure that the cannula 606 does not become detached from the sensor element 609 during use. In Figure 6c, on the other hand, the attachment portion 611 forms an integral part of the cannula 606. This embodiment facilitates on- site assembly of the apparatus, as there is no need for the medical practitioner to manually attach the sensor element 609 to the cannula 606. In practice, the distance between the receptacle 613 and the cannula 606 may be larger than shown in Figures 6a-6c to accommodate different insertion site locations and also to reduce the chances of infection at the insertion site. Furthermore, the insertion portion may be shaped to aid insertion into the cavity of the receptacle 613. One example of how the insertion portion could be shaped is shown in Figures 3a and 3b. In this example, the insertion portion 312 has a generally planar structure comprising an end plane 325 connected to a central plane 326, the end plane 325 being tilted with respect to the central plane 326. The tilt of the end plane 325 facilitates insertion of the end of the insertion portion 312 into the cavity of the receptacle 613. The electronic sensing means for detecting displacement of the insertion portion relative to the receptacle may utilise one or more of a number of different sensing technologies. A relatively simple option involves the use of a pair of electrical contacts 727 within the slot or hole 719 of the receptacle 713. These contacts 727 can be connected to an electrical circuit 728 which is completed by the insertion portion 712 when it is inserted into the receptacle 7 3. As shown in Figure 7a, the electrical circuit 728 may comprise a light 729 (e.g. a green light emitting diode) which is lit on completion of the circuit 728 to indicate a non-displaced state of the cannula ("0" state). If the insertion portion 712 is then displaced 734 relative to the receptacle 7 3 such that the circuit 728 is broken, the light 729 will go out to indicate a displaced state of the cannula ("1" state).

For this approach to work, at least part of the insertion portion 712 must comprise an electrically conductive material in order to enable completion of the circuit 728. This may simply be a conductive coating on part of the insertion portion 712. On the other hand, if the sensor element is intended to be reused (rather than being disposed of after each medical procedure), it may be better if part of the insertion portion 712 is made entirely of electrically conductive material to prevent the conductive coating from being removed as a result of friction between the insertion portion 712 and the electrical contacts 727. To reduce the risk of electric shock to the patient or medical practitioner via the insertion portion 712, at least the part of the insertion portion 712 which extends outside the cavity 719 of the receptacle 713 when the circuit 728 is complete should be electrically insulating. For example, the left-hand half of the insertion portion 712 of Figure 7a could be coated with or made from an electrically conductive material, and the right-hand half of the insertion portion 712 of Figure 7a could be left uncoated or be made from an electrically insulating material. The same may be applied to the insertion portion 912 of Figure 9.

Furthermore, the electrical contacts 727 should be positioned within the receptacle 713 such that the electrical circuit 728 is broken before the cannula is withdrawn from the patient to the extent that fluid can no longer be delivered to or extracted from the patient. This feature ensures that the light 729 goes out before any loss of fluid occurs, which may increase the chances of a cannula displacement being detected before it becomes fatal (e.g. in the case of renal dialysis). Another option is to spring-mount the electrical contacts within the receptacle in such a way that insertion of the insertion portion forces the contacts apart to break the electrical circuit. There are (at least) two different ways of achieving this. For example, in Figure 8a, the circuit 828 is broken when an electrically insulating insertion portion 812 is inserted into the slot or hole 819 of the receptacle 813 causing the light 829 to switch off. When the insertion portion 812 is displaced 834 relative to the receptacle 813, the tension in the springs 862 causes the electrical contacts 827 to touch one another and complete the circuit 828 (as shown in Figure 8b). In this case, the light 829 may be a red light emitting diode to indicate displacement of the cannula. In Figure 9a, on the other hand, insertion of the insertion portion 912 creates breaks 961 in the circuit 928 causing the light 929 to switch off. The circuit 928 is completed (by removal of the breaks 961 and touching of the electrical contacts 927) when the insertion portion 912 is displaced 934 relative to the receptacle 913 causing the light to switch on. An advantage of the configuration of Figures 9a and 9b is that the insertion portion 912 can be electrically conductive or electrically insulating.

When the sensing means comprise a pair of electrical contacts 727 arranged as shown in Figure 7, a protective sheath configured to prevent physical contact between the sensor unit and the sensor element, body and body fluid would be provided with strategically placed holes corresponding with the positions of the electrical contacts 727 to enable physical (and electrical) contact between the insertion portion 712 and the electrical contacts 727 when the cannula is in the non-displaced "0" state. Such holes would also be included in a protective sheath configured for use with the sensor unit of Figures 8 and 9 to enable physical (and electrical) contact between the electrical contacts 827, 927 when the cannula is in the displaced "1" state.

In the embodiments shown in Figures 7-9, the electronic sensing means are configured to detect relative displacement 734 of the insertion portion 712 and receptacle 713 beyond a predetermined position (i.e. the position of the electrical contacts 727). In this way, the insertion portion 712 switches the sensing means between two distinct states (representing the non-displaced "0" and displaced T states of the cannula). For example, in Figure 7, a low resistance state of the sensing means represents the non- displaced "0" state of the cannula and a high resistance state of the sensing means represents the displaced "1" state of the cannula. In Figures 8 and 9, on the other hand, the resistance states are reversed. In some cases, however (such as renal dialysis), it may be better if the electronic sensing means were configured to measure the magnitude of relative displacement 734 of the insertion portion 712 and receptacle 713.

One example of a sensing means which can provide this functionality is shown in Figures 10a and 10b. In this example, the sensing means comprises a photodetector 1030 (e.g. a photodiode with a p-n junction 1031) located within the receptacle which is configured to allow an electrical current to flow when exposed to incident electromagnetic radiation 1032 (e.g. visible light, ultraviolet, infrared, etc). In the embodiment shown in Figures 10a and 10b, the insertion portion 1012 is used to prevent the electromagnetic radiation 1032 from reaching the photodetector 1030. Displacement 1034 of the insertion portion 1012 relative to the receptacle can therefore be detected by measuring the current (e.g. with an ammeter 1033). This embodiment requires at least part of the insertion portion 1012 to be optically opaque in order to block/absorb the electromagnetic radiation 1032. Since the magnitude of current is proportional to the intensity of the incident electromagnetic radiation 1032, it is possible to measure the magnitude of relative displacement 1034. As a result, the sensing device could be configured to raise a first alarm (as discussed later) when the current reaches a first predetermined level and a second alarm when the current reaches a second predetermined level. For instance, the first predetermined level might correspond with a position at which the cannula is displaced relative to the patient's body but is still inserted and able to transfer fluid, whilst the second predetermined level might correspond with a position at which the cannula is withdrawn from the patient's body and is unable to transfer fluid. In this way, the first alarm could be used to pre-warn a medical practitioner (or patient) that venous needle displacement is likely to occur, thereby providing them with sufficient time to reposition the cannula before there is any substantial loss of blood. Alternatively, the first predetermined level may be omitted.

The sensing means may comprise a source 1061 of electromagnetic radiation 1032 (e.g. a light emitting diode), or the photodetector 1030 may be configured to detect light from the surrounding environment instead. In the latter scenario, the sensor unit would be configured to allow light from the surrounding environment to enter the slot or hole of the receptacle. This may be achieved by making at least part of the receptacle from an optically transparent material, or by forming an additional slot or hole in the receptacle (not shown). An alternative embodiment is shown in Figures 11a and 11b in which the insertion portion 1112 is used to reflect the incident electromagnetic radiation 1132 onto the photodetector 1130 when it is inserted within the receptacle. This time, relative displacement 1134 between the insertion portion 1112 and the receptacle prevents reflection of the electromagnetic radiation 1132 onto the photodetector 1130 resulting in an associated decrease in current. This configuration requires at least part of the insertion portion 1112 to be optically reflective. Rather than measuring the current to determine the magnitude of relative displacement, the sensing means of Figures 10 and 11 could be configured to switch between two distinct states based on the current in order to detect displacement beyond a predetermined position. This may be achieved using a phototransistor coupled to a Schmitt trigger as the photodetector (also known as a transmissive optoschmitt sensor). A Schmitt trigger retains its value until the input changes sufficiently to trigger a change. For example, in the non-inverting configuration, the output is high when the input is above an upper predetermined threshold; the output is low when the input is below a lower predetermined threshold; and the output remains the same when the input is somewhere between the two thresholds. In this way, the Schmitt trigger causes the photodetector to act like a switch.

A different type of sensing means is shown in Figures 12a and 12b, and comprises a reed switch 1235 within the receptacle of the sensor unit. A reed switch 1235 is an electrical switch which is operated by a magnetic field 1237. It comprises a pair of electrical contacts 1236 which can be connected or disconnected (depending on the specific configuration) based on the presence of the magnetic field 1237. In the example shown in Figures 12a and 12b, the reed switch 1235 is configured such that the presence of the magnetic field 1237 causes disconnection of the electrical contacts 1236. If the electrical contacts 1236 are connected to an electrical circuit 1228, connection of the electrical contacts 1236 can be used to complete the circuit 1228. As also shown in these Figures, the circuit 1228 may comprise a light 1229 (e.g. a red light emitting diode) to indicate displacement 1234 of the insertion portion 1212 relative to the receptacle. As with the sensing means of Figures 7-9, the reed switch 1235 is used to detect relative displacement 1234 of the insertion portion 1212 and receptacle beyond a predetermined position rather than measure the magnitude of relative displacement 1234.

Figures 13a and 13b show another embodiment in which the presence of the magnetic field 1337 is configured to cause connection of the electrical contacts 1336. In this embodiment, the circuit 1328 may comprise a green light 1329 (e.g. a green light emitting diode) to indicate that the cannula is still within the body of the patient. Displacement 1334 of the insertion portion 1312 relative to the receptacle would then be detected by a medical practitioner when the light 1329 goes out.

In order for a reed switch 1335 to be used to detect relative displacement 1334 between the insertion portion 1312 and the receptacle, the insertion portion 1312 must be capable of generating a magnetic field 1337. This could be achieved, for example, by coating the insertion portion 1312 in a magnetic material, attaching a piece of magnetic material to the insertion portion 1312, forming the insertion portion 1312 from a magnetic material, winding a current carrying conductor around the insertion portion 1312, or forming the insertion portion 1312 from a current carrying conductor.

Alternative sensing means could be used in combination with a magnetic insertion portion. For example, Figures 14a and 14b show a Hall effect sensor 1438 (comprising a Hall probe 1439). A Hall probe 1439 produces a change in output voltage in response to a magnetic field 1437. If the output voltage is measured (e.g. using a voltmeter 1440) and the system is calibrated, it is possible to determine the relative displacement 1434 of the insertion portion and receptacle based on this output voltage. Furthermore, since the output voltage is proportional to the magnetic field strength at the Hall probe 1439 (which varies with distance from the insertion portion 1412), this embodiment would enable a measurement of the relative displacement 1434 to be obtained as with the photodetector 1030, 1130 of Figures 10 and 1 . Inductive and capacitive proximity sensors (not shown) could also be used to detect relative displacement between the insertion portion and receptacle. Inductive proximity sensors comprise a coil which generates an alternating magnetic field when an AC current is passed through it. When a metallic object is within the range of detection, the alternating field induces Eddy currents in the metal which generate opposing magnetic fields. These opposing fields vary the impedance of the coil which can be measured to determine the position of the metallic object. An inductive proximity sensor could therefore be mounted in the receptacle and used to detect relative displacement between the insertion portion and receptacle provided that at least part of the insertion portion is metallic.

Capacitive proximity sensors provide the additional benefit that the insertion portion need not be metallic. Capacitive sensors comprise an electrode to which an electrostatic potential is applied. When another object is brought into proximity of the electrode, the capacitance associated with the electrode varies. The variation in capacitance depends on the surface area of the object and its distance from the electrode. In this way, the capacitance can be measured and used to determine the position (and displacement) of the object.

The inductive and capacitive proximity sensors could be configured to measure the magnitude of relative displacement of the insertion portion and receptacle based on the impedance of the coil or the capacitance of the electrode, respectively. Alternatively, they may be configured to activate an alarm when the measured impedance/capacitance reaches a predetermined threshold or varies by a predetermined amount in order to detect relative displacement beyond a predetermined position.

Regardless of the electronic sensing means used for displacement detection, the sensor element and sensor unit should be configured such that the electronic sensing means switch from one state to the other before the cannula is withdrawn from the patient to the extent that fluid can no longer be delivered to or extracted from the patient. In practice, this may be achieved by appropriate positioning of the sensing means within the receptacle. Additionally or alternatively, the sensor element may be formed such that the insertion portion is shorter in length than the portion of the cannula which is inserted (or insertable) into the patient's body. The latter arrangement helps to ensure that the insertion portion is withdrawn from the slot or hole of the receptacle before the cannula is withdrawn from the patient's body.

Figure 15 shows a renal dialysis system 1550 comprising the sensing device 1505 described herein. The system 1550 further comprises a dialysis machine 1551 for purifying the blood of a patient 1552; a first blood line 1553 (fluid conduit) connected between a first cannula (not shown) and the dialysis machine 1551 for transporting blood from the patient 1552 to the dialysis machine 1551 ; a second blood line 1554 connected between the dialysis machine 1551 and a second cannula 1506 for transporting blood from the dialysis machine 1551 to the patient 1552; and a pump 1555 on the first blood line 1553 for extracting blood from the patient 1552. In practice, however, the pump 1555 would typically be located within the dialysis machine 1551 itself.

In addition to the sensor element 1509 and sensor unit 1510, the sensing device 1505 comprises a control box 1556 (which may be mounted on a stand and positioned at the patient's bedside) and a valve 1557 (e.g. a solenoid pinch valve). The sensor unit 1510, valve 1557 and pump 1555 are electrically connected to the control box 1556. The control box 1556 comprises a controller (e.g. a processor) configured to receive a detection signal (indicating relative displacement between the insertion portion 1509 and the receptacle 1513) from the electronic sensing means and send a control signal in response to the received detection signal. The control signal is configured to activate one or more safety measures. For example, the control signal may be configured to activate an audio alarm 1559 (e.g. a loudspeaker), activate a visual alarm 1558 (e.g. a light emitting diode), activate the valve 1557 and/or cease operation of the pump 1555. In the example shown, the audio 1559 and visual 1558 alarms are located within the control box 1556, but they could be located remote from the control box 1556.

Activation of the valve 1557 causes the second blood line 1554 to be clamped, thereby preventing the leakage of any purified blood. In addition, ceasing operation of the pump 1555 prevents any further blood from being extracted from the patient 1552. Rather than controlling operation of the pump 1555 directly via the control box 1556, however, the increased fluid pressure created by the clamped blood line 1554 may trigger the dialysis machine 1551 to cease operation of the pump 1555 itself (provided that the dialysis machine 1551 incorporates this safety measure). In this scenario, the electrical connection between the control box 1556 and pump 1555 is not required. The sensing device 1505 may also comprise a valve sensor 1560 configured to detect the presence of the second blood line 1554 in the valve 1557. If the second blood line 1554 is not detected in the valve 1557, the valve sensor 1560 sends a detection signal to the control box 1556, which in turn sends a control signal to activate an audio 1559 and/or visual 1558 alarm. Furthermore, the valve 1557 may be configured such that a malfunction of the valve 1557 or a disruption of power to the valve 557 also triggers the valve 1557 to clamp the second blood line 1554 (i.e. the valve 1557 is configured to be in a normally-closed state). The control box 1556 may also be configured to fail-safe. For example, the control box 1556 may be configured to activate the audio 1559 and/or visual 1558 alarm in the absence of the required inputs from the sensor unit 1510. This might occur, for example, if the sensor unit 1510 became disconnected from the control box 1556.

Whilst Figure 15 shows the sensing device 1505 within a renal dialysis system, it is not limited to this particular application. In practice, the sensing device 1505 may be used to detect displacement of a cannula 1506 during the delivery or extraction of any number of different fluids to or from the body. For example, the sensing device 1505 could be used during the delivery of blood, a blood substitute, a medication, a buffer solution and/or a nutritional formula (e.g. via a drip). In addition, the sensing device 1505 could be used during the extraction of blood or urine, or in chemotherapy.

The various components of apparatus described herein may be supplied individually or as a kit of parts. For example, the kit of parts may comprise one or more of the sensor element 1509, the sensor unit 1510, the cannula 1506, the fluid conduits 1553, 1554, the protective sheath 1516, the control box 1556 and the valve 1557. This is because certain components of the apparatus may be intended for disposal after each medical treatment (e.g. to reduce the spread of infection), whilst other components may be intended for use in multiple treatments. For example, the sensor element 1509 (either as an integral part of the cannula 1506 or as a separate attachment), the cannula 1506, the fluid conduits 1553, 1554 and the protective sheath 1516 could be manufactured cheaply enough to render them suitable for one-time use. On the other hand, the sensor unit 1510 (comprising the electronic sensing means), the control box 1556 and the valve 1557 would typically be more expensive to manufacture and would therefore be reusable. The disposable components may be supplied in sterile packs. Other embodiments depicted in the figures have been provided with reference numerals that correspond to similar features of earlier described embodiments. For example, feature number 1 can also correspond to numbers 101 , 201, 301 etc. These numbered features may appear in the figures but may not have been directly referred to within the description of these particular embodiments. These have still been provided in the figures to aid understanding of the further embodiments, particularly in relation to the features of similar earlier described embodiments.

The applicant hereby discloses in isolation each individual feature described herein and any combination of two or more such features, to the extent that such features or combinations are capable of being carried out based on the present specification as a whole, in the light of the common general knowledge of a person skilled in the art, irrespective of whether such features or combinations of features solve any problems disclosed herein, and without limitation to the scope of the claims. The applicant indicates that the disclosed aspects/embodiments may consist of any such individual feature or combination of features. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the disclosure.