Abstract:
A patient monitoring system includes a replaceable laminar sensor to be placed on a bed, the sensor including distributed force sensing elements providing output signals to processing apparatus including a near-bed processor and a central processor coupled to the near-bed processor by a wireless communication link. The processing apparatus applies spatial weighting to the sensor output signals to derive the force distribution across the sensor, and processes the force distribution over time to generate patient status information such as patient presence, position, agitation, seizure activity, respiration, and security. This information can be displayed at a central monitoring station, provided to a paging system to alert attending medical personnel, and used to update medical databases. The sensor may be manufactured from layers of olefin film and conductive ink to form capacitive sensing elements.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 09/791,114 filed Feb. 22, 2001, now U.S. Pat. No. 6,546,813; which is a continuation in part of U.S. patent application Ser. No. 09/169,759 filed Oct. 9, 1998, which issued May 1, 2001 as U.S. Pat. No. 6,223,606; which is a divisional of U.S. patent application Ser. No. 08/780,435 filed Jan. 8, 1997, which issued on Oct. 13, 1998 as U.S. Pat. No. 5,821,633. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     The present invention is related to the field of patient monitoring equipment. 
     Patient monitoring systems are used in many settings to assist medical personnel in providing care. In many settings, such as hospital wards and nursing homes, there can be problems associated with patients&#39; getting out of bed without supervision or assistance. A patient may suffer a fall whose effects can range from minor to major. Older patients are at risk of breaking their hips in a fall, requiring extended bed rest and attendant problems. Systems have been known that monitor whether a patient is present in a bed or wheelchair. Essentially, these systems employ a flat sensor laid on the mattress or cushion, and electronic apparatus that responds to signals from the sensor. For example, the strength of a sensor output signal may be proportional to the weight applied to the sensor. The electronic apparatus therefore compares the sensor output signal with one or more predetermined values corresponding to significant thresholds of interest. For example, if the sensor output signal falls below a predetermined low value, the apparatus generates an indication that the patient has gotten out of bed. 
     Prior patient monitoring systems have used sensors having certain drawbacks that limit performance. One such drawback is size. Sensors to be used on a bed are as wide as the bed, but extend only about a foot in the longitudinal direction. These sensors are intended for placement in the middle of the bed, on the assumption that a patient&#39;s weight is concentrated there. However, a patient may move into a position away from the sensor, resulting in a false alarm. Existing sensors have also employed switches as sensing elements, which can provide only a binary indication. Due to the lack of resolution, only limited information can be obtained from the sensor. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with the present invention, a sensor-based patient monitoring system is disclosed incorporating features that overcome limitations of the prior art. In addition to having superior performance for traditional uses, such as reducing the incidence of patient falls, the system can be used for a variety of other clinical purposes to assist medical personnel and enhance the quality of care. 
     The system includes a replaceable laminar sensor placed on a bed or similar surface, the sensor including distributed force sensing elements providing output signals to processing apparatus for processing the force distribution information. The processing apparatus includes a near-bed processor and a central processor coupled to the near-bed processor by a wireless communication link. The processing apparatus applies spatial weighting to the sensor output signals to derive the force distribution across the sensor, and processes the force distribution information over time to generate pertinent patient status information. The information can vary depending on the operational purpose for the monitoring. For example, the information can include patient presence, position, agitation, seizure activity, or respiration. The information can be used to generate a display at a central monitoring station, and to update medical databases coupled to the central processor. The information can also be provided to a paging system to alert attending medical personnel. 
     The disclosed laminar sensor is made of layers of olefin film having patterns of conductive ink deposited thereon to form capacitive sensing elements, ground planes, and signal traces. The layers are laminated with a foam core selected to provide desired sensitivity of the capacitive sensing elements for a range of expected patient weights. Both a low-cost process and a high-volume process for manufacturing the sensor are shown. 
     Other aspects, features, and advantages of the present invention are disclosed in the detailed description that follows. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
       The invention is more fully understood by reference to the following Detailed Description in conjunction with the Drawing, of which: 
         FIG. 1  is a block diagram of a patient monitoring system in accordance with the present invention; 
         FIG. 2  is a diagram showing the arrangement of a multi-layer sensing sheet used in the system of  FIG. 1 ; 
         FIG. 3  is a layout diagram of a top layer of the sensing sheet of  FIG. 2 ; 
         FIG. 4  is a layout diagram of a bottom layer of the sensing sheet of  FIG. 2 ; 
         FIG. 5  is a first detailed layout view of a connection edge of the bottom layer of  FIG. 4 ; 
         FIG. 6  is a second detailed layout view of the connection edge of the bottom layer of  FIG. 4 ; 
         FIG. 7  is a detailed view of the connection between a cable lead and the top layer of  FIG. 3  or the bottom layer of  FIG. 4 ; 
         FIG. 8  is a detailed layout view of an area of the bottom layer of  FIG. 4  in which a capacitor plate is formed; 
         FIG. 9  (consisting of  FIGS. 9   a  and  9   b ) is a flow diagram of a single-station process of manufacturing the sensing sheet of  FIG. 2 ; 
         FIG. 10  is a diagram showing the manner in which heat pressing is used to laminate the layers of  FIGS. 3 and 4  and additional layers to form the sensing sheet of  FIG. 2 ; and 
         FIG. 11  (consisting of  FIGS. 11   a  and  11   b ) is a flow diagram of a multiple-station process of manufacturing the sensing sheet of FIG.  2 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The disclosure of U.S. patent application Ser. No. 09/169,759, filed Oct. 9, 1998 and entitled Center Of Weight Sensor, is hereby incorporated by reference herein. 
     In  FIG. 1 , the motion of a patient  10  is transmitted to a sensing sheet  12  by direct physical contact, such as exists when the sensing sheet  12  is placed on a bed and the patient  10  lies on top of the sensing sheet  12 . The sensing sheet  12  includes a number of spaced-apart sensing elements or transducers (not shown in  FIG. 1 ) capable of converting applied forces into an electrical signals representative of the forces. One example of such a sensing sheet  12 , described in detail below, employs sensing elements that function as variable capacitors whose capacitance changes in response to applied forces. Other types of sensing elements may also be employed, such as piezoelectric sensing elements, force-sensing resistors, etc. 
     The signals generated by the sensing sheet  12  are communicated to a nearby sensor signal processing unit (SSPU)  14 . The unit  14  contains analog-to-digital (A/D) converters  16 , a signal processor  18 , and a radio-frequency (RF) modem  20 . The A/D converters  16  continually translate the analog signals from the sensing sheet  12  into corresponding digital values. The signal processor  18  applies spatial weighting to the digital output streams from the A/D converters  16  to reflect the respective locations of the sensing elements on the sensing sheet  12 , and uses the spatially-weighted digital signal streams in performing one or more analysis processes. Spatial weighting is described further below. 
     In general, the processor  18  monitors the outputs of the sensing elements to detect the occurrence of certain predetermined “patient states” that pertain to a particular analysis being performed. Generally, the patient states are defined by one or more thresholds associated with certain analysis variables. For example, an analysis process for determining whether the patient  10  is present may simply integrate the force distribution over the sensing sheet  12 , as reported by the various sensing elements, and compare the integrated value with a predetermined threshold representing the minimum value that would be expected if a patient were present. Appropriate values to use for the threshold can be determined analytically or empirically. There may be a selectable threshold based on certain parameters, such as the patient&#39;s weight. 
     Much more sophisticated analysis processes can also be performed. Analyses may also include time as a parameter. For example, an analysis process may be used to help reduce the incidence of bedsores, which can develop if a patient remains in a given position too long. The movement of the center of the patient&#39;s mass over time can be monitored, and appropriate action taken when the extent of movement is less than a predetermined threshold for more than a predetermined time. Processes may be employed for detecting and providing information about patient agitation, respiration, reaction to drugs, sleep disorders, or seizures. The system can also be used to enhance patient safety and security. By monitoring weight changes on a patient&#39;s bed, the system can provide an indication that a patient has gotten up, or that an additional person is on the bed. 
     When significant patient states or state transitions are detected by the processor  18 , a corresponding information message is generated by the processor  18  and transmitted on a wireless communications link  22  via the RF modem  20 . In general, the information message contains information identifying the patient, such as the patient&#39;s name, room number, etc., and information about the detected patient state. In addition, the processor  18  may also update a local data collection log (not shown) maintained for administrative or diagnostic purposes. 
     In the illustrated system, it is desirable that the RF modem function as a “slave” with respect to a “master ” modem  24  residing in a central information-processing unit (CIPU)  26 . Because the CIPU  26  communicates with a number of SSPUs  14 , it would be inefficient to continually maintain individual communications links  22  between the CIPU  26  and each SSPU  14 . By employing a master-slave arrangement, a link  22  is in existence only when needed. When the slave modem  20  receives a message from the processor  18 , it requests a connection with the master modem  24  using a separate, low-rate signaling channel (not shown). The master modem  24  informs the slave modem  20  when the link  22  has been established, whereupon the slave modem  20  transmits the information message. Preferably, the master modem  24  transmits a positive acknowledgement message to the slave modem  20  if the information message is received correctly. 
     It may be desirable that the master modem  24  also be capable of initiating the establishment of the link  22 . This capability can be useful, for example, when configuration information, updates, or other information is to be transferred from the CIPU  26  to the SSPU  14 . When directed by the master modem  24 , the slave modem  20  monitors the link  22  for incoming messages containing such information and forwards these messages to the processor  18 . Software executing in the processor  18  responds in a desired predetermined fashion. 
     When a patient state information message is received at the CIPU  26 , the data is used to update a central database archive  30  and is also provided to a user interface platform  32 . The information in the database archive  30  can be used for a variety of generally offline activities, such as administrative record keeping, statistics gathering, etc. The user interface platform  32  provides the information to one or more real-time users, who in general are medical personnel responsible for the care of the patient  10 . For example, the platform  32  may include a graphical display at a nurses&#39; desk to provide the information to a desk nurse  34 . The platform  32  may also include paging equipment programmed to send an alert message to a floor nurse  36  or other personnel. The alert message preferably includes patient identifying information, such as the identity and room number of the patient  10 , and a brief description of the detected patient state. For example, when a “patient not present” state is detected, an alert message such as “Jones, 302, Out Of Bed” may be generated. 
     As shown, the CIPU  26  may also communicate with other entities via a local- or wide-area network. There may be inter-departmental communications with other departments  38  of a medical facility, such communications typically occurring over a local-area network. Examples include communications with medical laboratories and administrative offices such as a patient billing department. There may also be wider-area communications with remote entities  40 , such as a patient&#39;s family, affiliated research facilities, physicians&#39; offices, and insurance companies, for example. 
     As a scaled-back alternative to the system of  FIG. 1 , the SSPU  14  may itself include a pager (not shown) in place of the slave RF modem  20 , and the CIPU  26  and its network connections dispensed with. In such a system, the SSPU  14  itself sends a paging signal to the desk nurse  34 , floor nurse  36 , or other personnel as appropriate. While such a system has overall less functionality than the system of  FIG. 1 , it retains the important core functions of the sensing sheet  12  and SSPU  14 , and can provide greater cost effectiveness and flexibility in deployment. Of course, other system configurations are also possible. 
     As shown in  FIG. 2 , the sensing sheet  12  includes a number of layers laminated together. The sheet  12  includes a multi-layer top sheet  42 , a multi-layer bottom sheet  44 , and a foam core  46  disposed therebetween. Both the top sheet  42  and bottom sheet  44  include a layer of olefin film  48  approximately 0.0065″ thick, such as sold by duPont, Inc. under the trademark TYVEK®. Both sides of each layer of film  48  are coated with conductive material. Each outer layer  50  is a ground plane covering substantially the entire surface of the respective film  48  to provide shielding from electrical noise. Each inner layer  52  has patterned conductive traces that define the sensing elements, as described in more detail below. 
     The conductive layers  50  and  52  are preferably made using conductive inks that are applied to the respective surfaces of the films  48  during manufacture of the sensing sheet  12 . These layers are approximately 0.001″ thick. The inner layers  52  are preferably made using a silver-based conductive ink for its excellent electrical properties. The outer layers  50  may be made using a copper-based conductive ink, which will have suitable electrical properties and lower cost than a silver-based ink. 
     The foam core  46  is approximately 0.5″ thick when uncompressed. The compression properties of the foam core  46  can vary depending on the application, more specifically on the range of weights of the patient  10  being monitored. The compression properties of the foam core  46  largely dictate the sensitivity of the sensors, which refers to the change in sensor capacitance due to changes in applied force. For adults in a normal weight range, it is desirable that the foam core  46  deflect about 25% when a pressure of 25 lbs. per square foot is applied. The useful upper limit of deflection is approximately 50% of uncompressed thickness. If the sensing sheet  12  is to be used with a different class of patients  10 , such as infants for example, it may be desirable to use a foam core  46  having different compression characteristics so as to achieve optimal sensitivity. 
       FIG. 3  shows the top sheet  42 , specifically the surface on which the conductive layer  52  ( FIG. 2 ) is formed. The top sheet  42  measures approximately 6.5 feet long by 3 feet wide. The conductive layer  52  comprises a number of conductive planar elements referred to as “plates”  54  interconnected by a conductive trace  56 . A segment  58  of the trace  56  is formed at the bottom of the sheet  42  for purposes of establishing an electrical interconnection between the trace  56  and a separate connector (not shown), as described in more detail below. The plates  54  measure approximately 5″ on a side. 
       FIG. 4  shows the bottom sheet  44 , specifically the surface on which the conductive layer  52  is formed. The bottom sheet  44  also measures 6.5 feet by 3 feet. Conductive plates  60  (shown as  60 - 1  through  60 - 8 ) are formed at respective positions corresponding to the positions of the plates  54  on the top sheet  42  (FIG.  3 ), so as to form eight plate capacitors when the sensing sheet  12  is assembled. The plates  60  are connected to respective traces in a set  62  that extends to the bottom edge of the bottom sheet  44 . The traces  62  are described in more detail below. 
     In operation, a suitable drive signal such as a 5 volt peak-to-peak sine wave of 50 KHz is applied to the plates  54  of the top sheet  42  via the trace  56  formed thereon. This signal is capacitively coupled to each of the plates  60  of the bottom sheet  44 . The capacitance of each plate capacitor formed by a given plate  54  and its opposite plate  60  changes in response to locally experienced forces that change the plate spacing by compressing the foam core  46  (FIG.  2 ). As a result, the respective strengths of the 50 KHz signals appearing on the plates  60  vary accordingly, and these signals are sampled and processed by the SSPU  14  ( FIG. 1 ) as described above. In particular, different two-dimensional weights are applied to the signals from the plates  60  to reflect their respective spatial characteristics, including location, size, and shape. These spatial weights are chosen from a suitable two-dimensional space, such as a rectangular grid with vertices at ( 0 ,  0 ), ( 0 ,  1 ), ( 1 ,  0 ) and ( 1 ,  1 ). For the sheet  12  as shown herein, the plates  60  are of uniform size and are distributed symmetrically on the surface of the bottom layer  44 . In this case, the spatial weights in the following table might be used, where each spatial weight corresponds to a different plate  60  as shown: 
     
       
         
               
               
               
             
           
               
                   
               
               
                 Plate 
                 X 
                 Y 
               
               
                   
               
             
             
               
                 60-1 
                 0.2 
                 0.3 
               
               
                 60-2 
                 0.2 
                 0.7 
               
               
                 60-3 
                 0.4 
                 0.5 
               
               
                 60-4 
                 0.5 
                 0.1 
               
               
                 60-5 
                 0.5 
                 0.9 
               
               
                 60-6 
                 0.6 
                 0.5 
               
               
                 60-7 
                 0.8 
                 0.3 
               
               
                 60-8 
                 0.8 
                 0.7 
               
               
                   
               
             
          
         
       
     
       FIG. 5  shows the bottom edge of the bottom sheet  44  in more detail. The traces  62  are arranged in two groups, one to the right of the bottom-most plate  60  and the other to the left. The right group includes seven individual traces, consisting of four ground traces interspersed with three signal traces, one for each of the three plates  60  on the right side of the sheet  44  (FIG.  4 ). Similarly, the left group includes nine individual traces, consisting of five ground traces interspersed with four signal traces, one for the top-most plate  60  and one for each of the three plates  60  on the left side of the sheet  44  (FIG.  4 ). 
       FIG. 6  shows the manner in which connections are formed between the traces  62  and a cable  66  at the connection edge of the bottom sheet  44 . The traces  62  are shown as signal traces  62 -S and ground traces  62 -G. Each conductor of the cable  66  is provided with a solderless terminal  68  which is secured to the sheet  44  in contact with a corresponding signal trace  62 -S. A conductive snap  70  is used to electrically couple each ground trace  62 -G to the ground plane on the opposite surface of the sheet. Specifically, a male component (not shown) of the snap  70  extends through a hole in the trace  62 -G and sheet  44 , and the male component is received by a female component (not shown) on the other side. Although it is not shown in the Figures, it is generally desirable to place several such snaps  70  along the length of each ground trace  62 -G, to minimize stray impedance in the ground path that can contribute to noise. Also, it may be desirable that the snaps  70  and/or terminals  68  be epoxied to the sheet  44  for an even more secure attachment. 
       FIG. 7  shows the attachment of a conductive lead of the cable  66  to either sheet  42  or  44  in greater detail. A plastic rivet  72  extends through a copper washer  74 , the sheet  42  or  44 , and the solderless terminal  68  as shown. A rivet head  76  is placed over the rivet  72 , and the rivet  72  and rivet head  76  are then squeezed together in a conventional fashion. By this action, the terminal  68  makes secure connection to the conductive layer  52  of the sheet  42 ,  44 . 
       FIG. 8  shows the area around a typical plate  60 . The plate  60  is connected to a corresponding signal trace  62 -S, which is surrounded on both sides by ground traces  62 -G for shielding purposes. Each pair of ground traces  62 -G extends alongside the entire run of the corresponding signal trace  62 -S from plate  60  to the bottom edge of the sheet  44 . 
       FIG. 9  shows a single-station process for manufacturing the sensing sheet  12 . In step  80 , a silk screening machine is set up with a roll of olefin film. At step  82 , the outer conductive layer  50  ( FIG. 2 ) is silk screened onto a length of film sufficient for  60  top sheets  42  and  60  bottom sheets  44 . Because the outer layer  50  is a ground plane extending across the entire surface of each sheet, this layer can be deposited as one continuous film along 780 feet (120×6.5) of the olefin film. After the layer  50  has been deposited, the individual sheets are cut as each 6.5′ length of film exits the machine. 
     At step  84 , the sheets are placed into an oven to allow the conductive ink to dry. The sheets are then removed from the oven at step  86 . At step  88 , the patterned conductive layer  52  is silk screened onto the  60  top sheets  42 , and these are returned to the oven for curing at step  90 . At the same time, at step  92  a press, assembly machine and testing apparatus can be set up in preparation for the final assembly and testing of the sheets  12 . 
     At step  94 , the cured top sheets  42  are removed from the oven, and at step  96  ground leads are “snapped” to the ground plane of the cured top sheets  42  using snaps as described above with reference to FIG.  6 . At the same time, at step  98  the patterned layer  52  is silk screened onto the bottom sheets  44 . The bottom sheets  44  are then placed in the oven for curing at step  100 , while at step  102  the top sheets  42  are moved to an assembly area and the press is set up for the bottom sheets  44 . At step  104 , ground leads are snapped to the ground plane of the cured bottom sheets  44 , while at the same time at step  106  a cable assembly is riveted to each top sheet  42 . At step  108 , a cable assembly is riveted to each bottom sheet  44 . 
     At step  110 , each sheet  42  and  44  is tested for continuity of connections, such as between each plate  60  and its associated trace  62  for example. Each sheet is also tested for the absence of any short circuits between the outer and inner layers  50  and  52 , which could occur for example if the conductive ink were to bleed through the olefin film. This testing is preferably done prior to the attachment of the cables. Once the cables are attached, additional testing is performed to ensure proper connectivity between the conductors of the cable and the various conductive elements on the sheet. 
     At step  112  the top and bottom sheets  42  and  44  are assembled into the final sheet  12 . The core  46  is pre-treated with a heat-activated adhesive on both surfaces, and then pressed together with the sheets  42  and  44  in a heated press. This process is illustrated in  FIG. 10 , where the elements  114  are heated press elements. 
       FIG. 11  illustrates a process for manufacturing the sensing sheet  12  which follows more of an assembly line model than the process of FIG.  9 . It is assumed that there are separate workers at each station. Also, some of the equipment, such as the silk screening machines and cable assembly stations, are duplicated for improved throughput. The overall process reflected in steps  120 - 142  of  FIG. 11  is generally the same as that shown in FIG.  9 . However, much greater volumes of sheets  12  can be produced due to the assembly line structure. Several batches of material are in process simultaneously, with each batch being in a different stage of completion. The process of  FIG. 11  is capable of yielding approximately 210 finished sensing sheets  12  per day, whereas the single-person process of  FIG. 9  can yield approximately 60 sheets per day. 
     A patient monitoring system employing a laminar sensor sheet has been shown. It will be apparent to those skilled in the art that modifications to and variations of the disclosed methods and apparatus are possible without departing from the inventive concepts disclosed herein, and therefore the invention should not be viewed as limited except to the full scope and spirit of the appended claims.