AU2005211826B2 - Capacitance change patient monitor - Google Patents

Description

WO 2005/077316 PCT/AU2005/000102 1 CAPACITANCE CHANGE PATIENT MONITOR 5 Field of the Invention This invention relates to a patient monitor device and method of using same. In a particular non-limiting aspect, it relates to a patient monitor which can detect a patient rising from a support such as a bed or chair and can be automatically configured to 10 interface with an existing nurse call system. Background of the Invention In hospitals, nursing homes and other similar institutions, it is important to monitor 15 the presence or movements of patients for various reasons. For example, a patient who is not fully aware of his or her surroundings by virtue of dementia or sedation may be subject to injury risk from falls or wandering if they rise from the bed or chair unsupervised. 20 It is therefore important to monitor the presence and/or movements of the patient to reduce the likelihood of injury in this fashion. As it is clearly impractical to have a person directly watching over the patient 24 hours a day, a remote monitoring system is the only practical approach. To this end, various attempts to remotely monitor patients have been proposed with varying degrees of success. For example, pressure 25 mats which sense the pressure of a patient sitting on a chair or lying on a bed have enjoyed some success. Such pressure mats usually comprise two sheets of carbon inked vinyl separated by a perforated foam insulator. Unfortunately, they are prone to damage from folding and creasing. Furthermore, they are somewhat sensitive to variability of the patient's weight, and because of the mechanical nature of the 30 pressure mat, they tend to have a short lifespan.

2 Other applications use two conductive films separated by a compressible insulator capacitance changes as pressure is applied. These are essentially mechanical in nature and are therefore subject to failures over time. 5 It is therefore an object of the invention to provide a patient sensor which gives an alternative approach to the currently known sensor types and is less likely to suffer from the same disadvantages. Disclosure of the Invention 10 The invention provides in one aspect apparatus for monitoring presence, orientation or movement of a patient in relation to a support surface such as a bed or chair supporting the patient comprising, a primary conductive member interposed between the patient and the support 15 surface, a secondary conductive member fixed in a location removed from the patient and primary conductive member, comparative means for measuring the capacitance between the primary and secondary conductive members and for generating an actuation signal in response to a 20 variation in capacitance beyond a predetermined limit for capacitance readings taken at different times, and warning means responsive to the actuation signal for generating a warning signal, wherein the comparative means comprise a microprocessor programmed to 25 operate continuously so that it generates an actuation signal in response to a rolling average value for capacitance changes over a period of time and the microprocessor includes programming which allows the apparatus to analyze a nurse call pendant system to which it is connected whereby to configure the application to operate with the nurse call pendant system. 30 The secondary conductive member may comprise an antenna. The comparative means may take into account the presence or absence of a person interposed between the conductive members. 35 Suitably, there is an air gap between the patient and the secondary conductive member.

WO 2005/077316 PCT/AU2005/000102 3 The secondary conductive member and comparative means may be located in a common housing. An electrical lead may extend between the common housing and primary conductive member. Suitably, the primary conductive member and housing may be located within 10 metres of each other, more preferably 3 metres. 5 The primary conductive member may be located on a bed mattress or on the squab of a seat. It may be made up of a plurality of conductive members. The plurality of conductive members may be electrically connected. Alternatively they may be insulated from each other, in which case each of the plurality of conductive members 10 acts as a primary conductive member. The primary conductive member may comprise a laminate of conductive and non conductive materials. It may comprise a laminate having at least three layers wherein the two outer layers are non-conductive and the inner layer is conductive. The outer 15 layers may comprise flexible plastic sheets such as vinyl sheets. The inner layer comprises ahuninium foil. The aluminium foil may be reinforced. It may be reinforced with spun fibre and paper to add strength and flexibility. Suitably, the comparative means operates continuously. It may be configured so that 20 it only generates an actuation signal in response to a rolling average value for capacitance changes over a period of time. For example, it may respond to a rolling average capacitance change of greater than 1% more preferably 5%. The period over which the capacitance change occurs may be in excess of 0.5 seconds, more preferably 1 second. 25 The rolling average value for capacitance may be adjusted for drift over time, temperature and normal human movement. The system may be arranged so the patient is only ever capacitively coupled to the 30 system, never directly coupled.

4 The apparatus may be coupled to standard monitoring systems already present in places such as hospitals or nursing homes. Thus for example, it may be added to current wiring so as to be coupled to devices such as nursing home call pendants, pagers or alarms. 5 In another aspect the invention provides a method of sensing the ongoing presence of a person on a support surface comprising, interposing a conductive sheet between the person and the support surface, locating an antenna in a fixed location remote from the person to provide an 10 air gap between the person and the antenna, sensing the capacitance between the sheet and the antenna, and generating an actuation signal in response to a variation in the capacitance brought about by removal of the person, wherein the actuation signal is generated in response to a change in rolling 15 average of the capacitance of greater than 5% for a period greater than I second. Preferred aspects of the invention will now be described with reference to the accompanying drawings. 20 Brief Description of the Drawings Figure 1 shows a block diagram of a paging/alarm system incorporating the invention; and Figure 2 shows a partially cutaway plan view of a sensor mat which comprises a 25 primary conductive member for application to the invention. Detailed Description of the Preferred Embodiments The various elements identified by numerals in the drawings are listed in the 30 following integer list. Integer List 2 Reference capacitor WO 2005/077316 PCT/AU2005/000102 5 3 Capacitive comparator 4 Nurse call pendant 5 User interface 6 Micro-controller 5 7 Output interface 8 Nurse call 9 Pager 10 Power supply 11 Battery monitor 10 20 Sensor mat 21 Reinforced aluminium foil 22 Vinyl sheets 23 Electrical contact 24 Flexible hook up wire 15 26 Antenna 31 Calibration button 32 Calibration button Referring to Figure 1, the sensor input for the device comprises a detection capacitor. 20 The detection capacitor comprises three main components, namely: (i) A sensor mat which comprises the primary conductive member of the capacitor. It will be described in more detail hereinafter in relation to Figure 2. The sensor mat is typically placed on the top of a mattress or 25 chair and underneath bedclothes. (ii) The intermediate elements. In this instance, the intermediate elements comprise insulation on the sensor mat, bed or seat coverings over the sensor mat, the patient and air interposed between the sensor mat, and patient. 30 (iii) An antenna 26 placed remotely from the sensor mat and patient comprises the secondary conductive member forming the second plate of the detection capacitor. Typically, the antenna 26 will be located in a WO 2005/077316 PCT/AU2005/000102 6 common housing with a reference capacitor 2 and a capacitive comparator 3. The capacitive comparator 3 is arranged to compare inputs from the reference 5 capacitor and detection capacitor so as to detect changes in capacitance of the detection capacitor. Such changes of course can occur with the patient leaving or falling out of the bed or chair or even in some instances with the patient moving in a manner which changes the capacitance of the detection capacitor. The capacitive comparator suitably should be able to measure changes of capacitance greater than 10 5%, more preferably greater than 0.5%. It also has calibration buttons 31 and 32 to respectively calibrate for a person being present and a person being absent. A micro-controller 6 is arranged to receive input from the capacitive comparator. Micro-controllers suitable for the task are currently available on the retail market. 15 The micro-controller can calculate a rolling average value for the capacitance in the detection capacitor and may adjust for drift over time, temperature and normal human movement. The micro-controller may be directly connected to a pre-existing nurse call pendant 20 system or other form of user interface 5. The user interface may be used to set sensitivity and response speed. It may also have the capacity to analyze the call pendant so that it may configure itself to operate with the call pendant system. Additionally or alternatively, it may be connected to an output interface 7 which in 25 turn may direct signals to a nurse call device 8 or pager 9. The connection between the output interface and the nurse call or pager may be by methods known in the art such as radio or infrared. The device includes a power supply 10 which may comprise battery, mains or other 30 source of power. A battery monitor 11 may be included in the case where the system relies upon the battery power.

WO 2005/077316 PCT/AU2005/000102 7 The micro-controller may include circuitry to generate alarm signals if the connecting lead from the sensor mat to the housing for the micro-controller and capacitive comparator is pulled out during use. 5 Nursing homes and hospitals in the developed world have installed a nurse call system of one type or another. This system usually incorporates a nurse call pendant, which is left within reach of the patient concerned. When the patient requires nursing assistance all they need do is press the button on the nurse call pendant. 10 In the case of an individual with a cognitive impairment, such as Alzheimers Disease the pendant often becomes redundant. The device of the invention exploits the existence of the nurse call system by activating the pendant on behalf of the person when they rise from the bed or the 15 chair. Unfortunately, there is no real standard protocol for the various nurse call systems in use. Some are based on a simple 2-pole switch, others use complex 6 or 8 wire proprietary connectors. Some have a mechanical latch built into the pendant others 20 have the facility to control over bed lights etc. The two most common types are the simple 6.5mm 'Mono' jack plug type and the more complex 'Stereo' type jack plug. In the first case, there being only 2 wires there is no difficulty in duplicating the function of the pendant. In the second case however 25 there are a surprising number of alternative wiring strategies available. If the option of latching vs. momentary is included the number rises in excess of 12 combinations. The device of the invention overcomes this problem by offering the user the opportunity to 'clone' itself and become an equivalent of the pendant. This is 30 achieved by plugging the existing pendant into the user interface unit incorporating the microprocessor and pressing a 'prog' button provided on the interface. The microprocessor is in turn connected to the nurse call system. Once in programming WO 2005/077316 PCT/AU2005/000102 8 mode the microprocessor examines the state of the connections between the various parts of the plug. I.e. Tip to Ring, Tip to Barrel, Ring to Barrel. If the system finds the ring connected to the barrel then it assumes the pendant to be a simple 'mono' type and goes no further. However, if there is no such connection the user is asked to 5 press the call pendant in the same way they would if they were calling for assistance. The microprocessor will then analyse the connections made and determine what the appropriate configuration should be. Once this is done the microprocessor will store this information in its non-volatile memory and will continue to use it until reprogrammed. 10 If the pendant type includes a button to control the overbed light then each time the system is activated the light will toggle state. ie. If the light were off then it would be switched on, equally, if the light is on then it would be switched off. 15 Referring to Figure 2, the sensor mat 20 comprising the primary conductive member of the invention has a core of reinforced aluminium foil 21 or conductive aluminized rip stop fabric covered by overlying and underlying non-conductive waterproof vinyl sheets 22. The aluminium foil may include any suitable form of reinforcing to improve robustness. However applicants have found from testing that aluminium foil 20 reinforced with spun fibre and paper gives satisfactory strength and flexibility. The sensor mat 20 can have a range of dimensions. It has been found dimensions of 750mm by 350mm made it suitable for use on a standard single bed in one test. A sensor mat having dimensions 750mm by 100mm gave satisfactory performance in another test. 25 An electrical contact 23 in the form of a disc of thin brass shim with strain relief provides the electrical connection to the foil and a flexible hookup wire 24 is soldered to the shim. The shim may optionally be covered with a brass eyelet having an insulating cover (not shown). The whole assembly is pressed into a laminated 30 sandwich so as to maintain both a close electrical connection and a flexible interface between the sensor mat and the connecting lead.

WO 2005/077316 PCT/AU2005/000102 9 In one embodiment of the invention the sensor mat may comprise a plurality of separate sections of foil to enable movement of the patient in a bed to be monitored. Whilst the above description includes the preferred embodiments of the invention, it 5 is to be understood that many variations, alterations, modifications and/or additions may be introduced into the constructions and arrangements of parts previously described without departing from the essential features or the spirit or ambit of the invention. 10 It will be also understood that where the word "comprise", and variations such as "comprises" and "comprising", are used in this specification, unless the context requires otherwise such use is intended to imply the inclusion of a stated feature or features but is not to be taken as excluding the presence of other feature or features. 15 The reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form of suggestion that such prior art forms part of the common general knowledge in Australia.

Claims (7)

1. Apparatus for monitoring presence, orientation or movement of a patient in relation to a support surface supporting the patient comprising, 5 a primary conductive member interposed between the patient and the support surface, a secondary conductive member fixed in a location removed from the patient and primary conductive member, comparative means for measuring the capacitance between the primary and 10 secondary conductive members and for generating an actuation signal in response to a variation in capacitance beyond a predetermined limit for capacitance readings taken at different times, and warning means responsive to the actuation signal for generating a warning signal, 15 wherein the comparative means comprise a microprocessor programmed to operate continuously so that it generates an actuation signal in response to a rolling average value for capacitance changes over a period of time and the microprocessor includes programming which allows the apparatus to analyze a nurse call pendant system to which it is connected whereby to configure the apparatus to operate with the 20 nurse call pendant system.

2. Apparatus according to claim I wherein the comparative means take into account the presence or absence of a person interposed between the conductive members and there is an air gap between the patient and the secondary conductive 25 member.

3. Apparatus according to claim 1 or claim 2 wherein the comparative means comprise a capacitive comparator which is located in a housing with the secondary conductive member and the housing and primary conductive member are located 30 within 10 metres of each other. 11

4. Apparatus according to claim 3 wherein the primary conductive member comprises two outer layers of non-conductive material sandwiching an inner layer of conductive material.

5 5. Apparatus according to any one of the preceding claims wherein the rolling average capacitance change is greater than 1% and the period of time is in excess of 0.5 seconds.

6. Apparatus according to any one of the preceding claims wherein the 10 comparative means are programmed to adjust a rolling average value for capacitance for at least one of, drift over time, temperature and normal human movement.

7. Apparatus according to claim 1 substantially as hereinbefore described with reference to any one of the accompanying drawings.