Patent Publication Number: US-2012029312-A1

Title: System and method for location tracking of patients in a vital-signs monitor system

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     The following applications disclose certain common subject matter with the present application: A Vital-Signs Monitor with Encapsulation Arrangement, docket number 080624-0612; A Vital-Signs Monitor with Spaced Electrodes, docket number 080624-0623; A Vital-Signs Patch Having a Strain Relief, docket number 080624-0624; A Temperature Probe Suitable for Axillary Reading, docket number 080624-0625; System and Method for Monitoring Body Temperature of a Person, docket number 080624-0626; A System and Method for Storing and Forwarding Data from a Vital-Signs Monitor, docket number 080624-0627; System and Method for Saving Battery Power in a Vital Signs Monitor, docket number 080624-0628; A System and Method for Conserving Battery Power in a Patient Monitoring System, docket number 080624-0629; A System and Method for Saving Battery Power in a Patient Monitoring System, docket number 080624-0630; A System And Method for Tracking Vital-Signs Monitor Patches, Docket Number 080624-0631; A System And Method for Reducing False Alarms Associated with Vital-Signs Monitoring, docket number 080624-0632; A System And Method for Reducing False Alarms Based on Motion and Location Sensing, docket number 080624-0634; all of the listed applications filed on ______. 
    
    
     FIELD 
     The present disclosure generally relates to systems and methods of physiological monitoring, and, in particular, relates to a system and method for location tracking of patients in a vita-signs monitor system. 
     DESCRIPTION OF THE RELATED ART 
     Some of the most basic indicators of a person&#39;s health are those physiological measurements that reflect basic body functions and are commonly referred to as a person&#39;s “vital signs.” The four measurements commonly considered to be vital signs are body temperature, pulse rate, blood pressure, and respiratory rate. Some clinicians consider oxygen saturation (S 02 ) to be a “fifth vital sign” particularly for pediatric or geriatric cases. Some or all of these measurements may be performed routinely upon a patient when they arrive at a healthcare facility, whether it is a routine visit to their doctor or arrival at an Emergency Room (ER). 
     Vital signs are frequently taken by a nurse using basic tools including a thermometer to measure body temperature, a sphygmomanometer to measure blood pressure, and a watch to count the number of breaths or the number of heart beats in a defined period of time which is then converted to a “per minute” rate. If a patient&#39;s pulse is weak, it may not be possible to detect a pulse by hand and the nurse may use a stethoscope to amplify the sound of the patient&#39;s heart beat so that she can count the beats. Oxygen saturation of the blood is most easily measured with a pulse oximeter. 
     When a patient is admitted to a hospital, it is common for vital signs to be measured and recorded at regular intervals during the patient&#39;s stay to monitor their condition. A typical interval is 4 hours, which leads to the undesirable requirement for a nurse to awaken a patient in the middle of the night to take vital sign measurements. 
     When a patient is admitted to an ER, it is common for a nurse to do a “triage” assessment of the patient&#39;s condition that will determine how quickly the patient receives treatment. During busy times in an ER, a patient who does not appear to have a life-threatening injury may wait for hours until more-serious cases have been treated. While the patient may be reassessed at intervals while awaiting treatment, the patient may not be under observation between these reassessments. 
     Measuring certain vital signs is normally intrusive at best and difficult to do on a continuous basis. Measurement of body temperature, for example, is commonly done by placing an oral thermometer under the tongue or placing an infrared thermometer in the ear canal such that the tympanic membrane, which shared blood circulation with the brain, is in the sensor&#39;s field of view. Another method of taking a body temperature is by placing a thermometer under the arm, referred to as an “axillary” measurement as axilla is the Latin word for armpit. Skin temperature can be measured using a stick-on strip that may contain panels that change color to indicate the temperature of the skin below the strip. 
     Measurement of respiration is easy for a nurse to do, but relatively complicated for equipment to achieve. A method of automatically measuring respiration is to encircle the upper torso with a flexible band that can detect the physical expansion of the rib cage when a patient inhales. An alternate technique is to measure a high-frequency electrical impedance between two electrodes placed on the torso and detect the change in impedance created when the lungs fill with air. The electrodes are typically placed on opposite sides of one or both lungs, resulting in placement on the front and back or on the left and right sides of the torso, commonly done with adhesive electrodes connected by wires or by using a torso band with multiple electrodes in the strap. 
     Measurement of pulse is also relatively easy for a nurse to do and intrusive for equipment to achieve. A common automatic method of measuring a pulse is to use an electrocardiograph (ECG or EKG) to detect the electrical activity of the heart. An EKG machine may use 12 electrodes placed at defined points on the body to detect various signals associated with the heart function. Another common piece of equipment is simply called a “heart rate monitor.” Widely sold for use in exercise and training, heart rate monitors commonly consist of a torso band, in which are embedded two electrodes held against the skin and a small electronics package. Such heart rate monitors can communicate wirelessly to other equipment such as a small device that is worn like a wristwatch and that can transfer data wirelessly to a PC. 
     Nurses are expected to provide complete care to an assigned number of patients. The workload of a typical nurse is increasing, driven by a combination of a continuing shortage of nurses, an increase in the number of formal procedures that must be followed, and an expectation of increased documentation. Replacing the manual measurement and logging of vital signs with a system that measures and records vital signs would enable a nurse to spend more time on other activities and avoid the potential for error that is inherent in any manual procedure. 
     SUMMARY 
     For some or all of the reasons listed above, there is a need to be able to continuously monitor patients in different settings. In addition, it is desirable for this monitoring to be done with limited interference with a patient&#39;s mobility or interfering with their other activities. 
     Embodiments of the patient monitoring system disclosed herein measure certain vital signs of a patient, which include respiratory rate, pulse rate, blood pressure, body temperature, and, in some cases, oxygen saturation (S O2 ), on a regular basis and compare these measurements to defined limits. 
     In one aspect of the present disclosure, a method of tracking a patient within a facility via a vital-signs patch attached to the patient is provided. The method can comprise receiving a first signal from a bridge, the first signal comprising information indicative of a vital-signs patch attached to a patient. The method can comprise identifying the patient based at least in part on the information. The method can comprise determining location of the patient based on a known location of the bridge within the facility. 
     In one aspect of the present disclosure, a vital-sign monitoring system is provided. The system can comprise a plurality of vital-sign monitor patches configured to monitor one or more vital-signs of patients to whom the vital-sign monitor patches are attached. The system can further comprise a surveillance server configured to collect data relating to the one or more vital-signs of the patients from the plurality of vital-sign monitor patches. The system can further comprise a plurality of bridges at respective locations within a facility and configured to provide data connections between the plurality of vital-sign monitor patches and the surveillance server. The surveillance server can be configured to: receive a first signal from a bridge in the monitoring network, the first signal comprising information indicative of a vital-sign monitor patch attached to a patient; identify a patient based at least in part on the first signal; and determine location of the patient based on a known location of the bridge within the facility. 
     It is understood that other configurations of the subject technology will become readily apparent to those skilled in the art from the following detailed description, wherein various configurations of the subject technology are shown and described by way of illustration. As will be realized, the subject technology is capable of other and different configurations and its several details are capable of modification in various other respects, all without departing from the scope of the subject technology. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature and not as restrictive. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are included to provide further understanding and are incorporated in and constitute a part of this specification, illustrate disclosed embodiments and together with the description serve to explain the principles of the disclosed embodiments. In the drawings: 
         FIG. 1  is a diagram illustrating an exemplary embodiment of a patient monitoring system according to certain aspects of the present disclosure. 
         FIG. 2A  is a perspective view of the vital-signs monitor patch of  FIG. 1  according to certain aspects of the present disclosure. 
         FIG. 2B  is a cross-section of the vital-signs monitor patch of  FIG. 1  according to certain aspects of the present disclosure. 
         FIG. 2C  is a functional block diagram illustrating exemplary electronic and sensor components of the vital-signs monitor patch of  FIG. 1  according to certain aspects of the present disclosure. 
         FIG. 3A  is a functional schematic diagram of the bridge according to certain aspects of the subject disclosure. 
         FIG. 3B  is a functional schematic diagram of an embodiment of the surveillance server according to certain aspects of the present disclosure. 
         FIG. 4A  is a map depicting a healthcare facility (e.g., a hospital)  400  in which a patient monitoring system such as the one shown in  FIG. 1  is implemented according to certain aspects of the present disclosure. 
         FIG. 4B  is a portion of an exemplary database comprising monitor patches, their linkable bridges, signal levels associated with the communication links between the monitor patches and the linkable bridges, and selected bridges according to certain aspects of the present disclosure. 
         FIG. 5A  is a map of the healthcare facility depicted in  FIG. 4A  after a passage of time. 
         FIG. 5B  is a portion of an updated version of the database shown in  FIG. 4B  according to certain embodiments of the present disclosure. 
         FIGS. 6A-C  show a first set of lists stored in various bridge at the time of  FIG. 4A , and a second set of lists which corresponds to updated lists stored in the bridges at the time of  FIG. 5A . 
         FIG. 7  is a flowchart illustrating a process for tracking locations of monitor patches by keeping and updating a database comprising information indicative of the monitor patches and their linkable and selected bridges according to certain aspects of the present disclosure. 
         FIG. 8A  is a diagram illustrating an exemplary data structure for a message indicating a new communication link and/or loss of an existing communication link according to certain aspects of the present disclosure. 
         FIG. 8B  is a diagram illustrating an exemplary data structure for an alternative message indicating a new communication link and/or loss of an existing communication link according to alternative aspects of the present disclosure. 
         FIG. 9  is a flowchart illustrating an exemplary process for a bridge selection process according to certain aspects of the present disclosure. 
         FIG. 10  is a flowchart illustrating a process for detecting an inoperable bridge and selecting an alternative bridge to replace the inoperable bridge for the monitor patches that were previously associated with the inoperable bridge according to certain aspects of the present disclosure. 
         FIG. 11  is a flowchart illustrating a process for determining locations of patients in a healthcare facility according to certain aspects of the present disclosure. 
         FIG. 12A  is an exemplary database comprising monitor patches, IDs of patients assigned to the monitor patches, and names of assigned patients according to certain aspects of the present disclosure. 
         FIG. 12B  is an exemplary database comprising bridges and their respective locations within the healthcare facility according to certain aspects of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Periodic monitoring of patients in a hospital is desirable at least to ensure that patients do not suffer an un-noticed sudden deterioration in their condition or a secondary injury during their stay in the hospital. It is impractical to provide continuous monitoring by a clinician and cumbersome to connect sensors to a patient, which are then connected to a fixed monitoring instrument by wires. Furthermore, systems that sound an alarm when the measured value exceeds a threshold value may sound alarms so often and in situations that are not truly serious that such alarms are ignored by clinicians. 
     Measuring vital signs is difficult to do on a continuous basis. Accurate measurement of cardiac pulse, for example, can be done using an electrocardiograph (ECG or EKG) to detect the electrical activity of the heart. An EKG machine may use up to 12 electrodes placed at various points on the body to detect various signals associated with the cardiac function. Another common piece of equipment is termed a “heart rate monitor.” Widely sold for use in exercise and physical training, heart rate monitors may comprise a torso band in which are embedded two electrodes held against the skin and a small electronics package. Such heart rate monitors can communicate wire lessly to other equipment such as a small device that is worn like a wristwatch and that can transfer data wirelessly to a personal computer (PC). 
     Monitoring of patients that is referred to as “continuous” is frequently periodic, in that measurements are taken at intervals. In many cases, the process to make a single measurement takes a certain amount of time, such that even back-to-back measurements produce values at an interval equal to the time that it takes to make the measurement. For the purpose of vital sign measurement, a sequence of repeated measurements can be considered to be “continuous” when the vital sign is not likely to change an amount that is of clinical significance within the interval between measurements. For example, a measurement of blood pressure every 10 minutes may be considered “continuous” if it is considered unlikely that a patient&#39;s blood pressure can change by a clinically significant amount within 10 minutes. The interval appropriate for measurements to be considered continuous may depend on a variety of factors including the type of injury or treatment and the patient&#39;s medical history. Compared to intervals of 4-8 hours for manual vital sign measurement in a hospital, measurement intervals of 30 minutes to several hours may still be considered “continuous.” 
     Certain exemplary embodiments of the present disclosure include a system that comprises a vital-signs monitor patch that is attached to the patient, and a bridge that communicates with monitor patches and links them to a central server that processes the data, where the server can send data and alarms to a hospital system according to algorithms and protocols defined by the hospital. 
     The construction of the vital-signs monitor patch is described according to certain aspects of the present disclosure. As the patch may be worn continuously for a period of time that may be several days, as is described in the following disclosure, it is desirable to encapsulate the components of the patch such that the patient can bathe or shower and engage in their normal activities without degradation of the patch function. An exemplary configuration of the construction of the patch to provide a hermetically sealed enclosure about the electronics is disclosed. 
     In the following detailed description, numerous specific details are set forth to provide a full understanding of the present disclosure. It will be apparent, however, to one ordinarily skilled in the art that embodiments of the present disclosure may be practiced without some of the specific details. In other instances, well-known structures and techniques have not been shown in detail so as not to obscure the disclosure. 
       FIG. 1  discloses a vital sign monitoring system according to certain embodiments of the present disclosure. The vital sign monitoring system  12  includes vital-signs monitor patch  20 , bridge  40 , and surveillance server  60  that can send messages or interact with peripheral devices exemplified by mobile device  90  and workstation  100 . 
     Monitor patch  20  resembles a large adhesive bandage and is applied to a patient  10  when in use. It is preferable to apply the monitor patch  20  to the upper chest of the patient  10  although other locations may be appropriate in some circumstances. Monitor patch  20  incorporates one or more electrodes (not shown) that are in contact with the skin of patient  10  to measure vital signs such as cardiac pulse rate and respiration rate. Monitor patch  20  also may include other sensors such as an accelerometer, temperature sensor, or oxygen saturation sensor to measure other characteristics associated with the patient. These other sensors may be internal to the monitor patch  20  or external sensors that are operably connected to the monitor patch  20  via a cable or wireless connection. Monitor patch  20  also includes a wireless transmitter that can both transmit and receive signals. This transmitter is preferably a short-range, low-power radio frequency (RF) device operating in one of the unlicensed radio bands. One band in the United States (US) is, for example, centered at 915 MHz and designated for industrial, scientific and medical (ISM) purposes. An example of an equivalent band in the European Union (EU) is centered at 868 MHz. Other frequencies of operation may be possible dependent upon the International Telecommunication Union (ITU), local regulations and interference from other wireless devices. 
     Surveillance server  60  may be a standard or virtualized computer server connected to the hospital communication network and preferably located in the hospital data center or computer room, although other locations may be employed. The server  60  stores and processes signals related to the operation of the patient monitoring system  12  disclosed herein including the association of individual monitor patches  20  with patients  10  and measurement signals received from multiple monitor patches  20 . Hence, although only a single patient  10  and monitor patch  20  are depicted in  FIG. 1 , the server  60  is able to monitor the monitor patches  20  for multiple patients  10 . 
     Bridge  40  is a device that connects, or “bridges”, between monitor patch  20  and server  60 . Bridge  40  communicates with monitor patch  20  over communication link  30  operating, in these exemplary embodiments, at approximately 915 MHz and at a power level that enables communication link  30  to function up to a distance of approximately 10 meters. It is preferable to place a bridge  40  in each room and at regular intervals along hallways of the healthcare facility where it is desired to provide the ability to communicate with monitor patches  20 . Bridge  40  also is able to communicate with server  60  over network link  50  using any of a variety of computer communication systems including hardwired and wireless Ethernet using protocols such as 802.11a/b/g or 802.3af. As the communication protocols of communication link  30  and network link  50  may be very different, bridge  40  provides data buffering and protocol conversion to enable bidirectional signal transmission between monitor patch  20  and server  60 . 
     While the embodiments illustrated by  FIG. 1  employ a bridge  20  to provide communication link between the monitor patch  20  and the server  60 , in certain alternative embodiments, the monitor patch  20  may engage in direct wireless communication with the server  60 . In such alternative embodiments, the server  60  itself or a wireless modem connected to the server  60  may include a wireless communication system to receive data from the monitor patch  20 . 
     In use, a monitor patch  20  is applied to a patient  10  by a clinician when it is desirable to continuously monitor basic vital signs of patient  10  while patient  10  is, in this embodiment, in a hospital. Monitor patch  20  is intended to remain attached to patient  10  for an extended period of time, for example, up to 5 days in certain embodiments, limited by the battery life of monitor patch  20 . In some embodiments, monitor patch  20  is disposable when removed from patient  10 . 
     Server  60  executes analytical protocols on the measurement data that it receives from monitor patch  20  and provides this information to clinicians through external workstations  100 , preferably personal computers (PCs), laptops, or smart phones, over the hospital network  70 . Server  60  may also send messages to mobile devices  90 , such as cell phones or pagers, over a mobile device link  80  if a measurement signal exceeds specified parameters. Mobile device link  80  may include the hospital network  70  and internal or external wireless communication systems that are capable of sending messages that can be received by mobile devices  90 . 
       FIG. 2A  is a perspective view of the vital-signs monitor patch  20  shown in  FIG. 1  according to certain aspects of the present disclosure. In the illustrated embodiment, the monitor patch  20  includes component carrier  23  comprising a central segment  21  and side segments  22  on opposing sides of the central segment  21 . In certain embodiments, the central segment  21  is substantially rigid and includes a circuit assembly ( 24 ,  FIG. 2B ) having electronic components and battery mounted to a rigid printed circuit board (PCB). The side segments  22  are flexible and include a flexible conductive circuit ( 26 ,  FIG. 2B ) that connect the circuit assembly  24  to electrodes  28  disposed at each end of the monitor patch  20 , with side segment  22  on the right shown as being bent upwards for purposes of illustration to make one of the electrodes  28  visible in this view. 
       FIG. 2B  is a cross-sectional view of the vital-signs patch  20  shown in  FIGS. 1 and 2A  according to certain aspects of the present disclosure. The circuit assembly  24  and flexible conductive circuit  26  described above can be seen herein. The flexible conductive circuit  26  operably connects the circuit assembly  24  to the electrodes  28 . Top and bottom layers  23  and  27  form a housing  25  that encapsulate circuit assembly  28  to provide a water and particulate barrier as well as mechanical protection. There are sealing areas on layers  23  and  27  that encircles circuit assembly  28  and is visible in the cross-section view of  FIG. 2B  as areas  29 . Layers  23  and  27  are sealed to each other in this area to form a substantially hermetic seal. Within the context of certain aspects of the present disclosure, the term ‘hermetic’ implies that the rate of transmission of moisture through the seal is substantially the same as through the material of the layers that are sealed to each other, and further implies that the size of particulates that can pass through the seal are below the size that can have a significant effect on circuit assembly  24 . Flexible conductive circuit  26  passes through portions of sealing areas  29  and the seal between layers  23  and  27  is maintained by sealing of layers  23  and  27  to flexible circuit assembly  28 . The layers  23  and  27  are thin and flexible, as is the flexible conductive circuit  26 , allowing the side segment  22  of the monitor patch  20  between the electrodes  28  and the circuit assembly  24  to bend as shown in  FIG. 2A . 
       FIG. 2C  is a functional block diagram  200  illustrating exemplary electronic and sensor components of the monitor patch  20  of  FIG. 1  according to certain aspects of the present disclosure. The block diagram  200  shows a processing and sensor interface module  201  and external sensors  232 ,  234  connected to the module  201 . In the illustrated example, the module  201  includes a processor  202 , a wireless transceiver  207  having a receiver  206  and a transmitter  209 , a memory  210 , a first sensor interface  212 , a second sensor interface  214 , a third sensor interface  216 , and an internal sensor  236  connected to the third sensor interface  216 . The first and second sensor interfaces  212  and  214  are connected to the first and second external sensors  232 ,  234  via first and second connection ports  222 ,  224 , respectively. In certain embodiments, some or all of the aforementioned components of the module  201  and other components are mounted on a PCB. 
     Each of the sensor interfaces  212 ,  214 ,  216  can include one or more electronic components that are configured to generate an excitation signal or provide DC power for the sensor that the interface is connected to and/or to condition and digitize a sensor signal from the sensor. For example, the sensor interface can include a signal generator for generating an excitation signal or a voltage regulator for providing power to the sensor. The sensor interface can further include an amplifier for amplifying a sensor signal from the sensor and an analog-to-digital converter for digitizing the amplified sensor signal. The sensor interface can further include a filter (e.g., a low-pass or bandpass filter) for filtering out spurious noises (e.g., a 60 Hz noise pickup). 
     The processor  202  is configured to send and receive data (e.g., digitized signal or control data) to and from the sensor interfaces  212 ,  214 ,  216  via a bus  204 , which can be one or more wire traces on the PCB. Although a bus communication topology is used in this embodiment, some or all communication between discrete components can also be implemented as direct links without departing from the scope of the present disclosure. For example, the processor  202  may send data representative of an excitation signal to the sensor excitation signal generator inside the sensor interface and receive data representative of the sensor signal from the sensor interface, over either a bus or direct data links between processor  202  and each of sensor interface  212 ,  214 , and  216 . 
     The processor  202  is also capable of communication with the receiver  206  and the transmitter  209  of the wireless transceiver  207  via the bus  204 . For example, the processor  202  using the transmitter and receiver  209 ,  206  can transmit and receive data to and from the bridge  40 . In certain embodiments, the transmitter  209  includes one or more of a RF signal generator (e.g., an oscillator), a modulator (a mixer), and a transmitting antenna; and the receiver  206  includes a demodulator (a mixer) and a receiving antenna which may or may not be the same as the transmitting antenna. In some embodiments, the transmitter  209  may include a digital-to-analog converter configured to receive data from the processor  202  and to generate a base signal; and/or the receiver  206  may include an analog-to-digital converter configured to digitize a demodulated base signal and output a stream of digitized data to the processor  202 . 
     The processor  202  may include a general-purpose processor or a specific-purpose processor for executing instructions and may further include a memory  219 , such as a volatile or non-volatile memory, for storing data and/or instructions for software programs. The instructions, which may be stored in a memory  219  and/or  210 , may be executed by the processor  202  to control and manage the wireless transceiver  207 , the sensor interfaces  212 ,  214 ,  216 , as well as provide other communication and processing functions. 
     The processor  202  may be a general-purpose microprocessor, a microcontroller, a Digital Signal Processor (DSP), an Application Specific Integrated Circuit (ASIC), a Field Programmable Gate Array (FPGA), a Programmable Logic Device (PLD), a controller, a state machine, gated logic, discrete hardware components, or any other suitable device or a combination of devices that can perform calculations or other manipulations of information. 
     Information, such as program instructions, data representative of sensor readings, preset alarm conditions, threshold limits, may be stored in a computer or processor readable medium such as a memory internal to the processor  202  (e.g., the memory  219 ) or a memory external to the processor  202  (e.g., the memory  210 ), such as a Random Access Memory (RAM), a flash memory, a Read Only Memory (ROM), a Programmable Read-Only Memory (PROM), an Erasable PROM (EPROM), registers, a hard disk, a removable disk, or any other suitable storage device. 
     In certain embodiments, the internal sensor  236  can be one or more sensors configured to measure certain properties of the processing and sensor interface module  201 , such as a board temperature sensor thermally coupled to a PCB. In other embodiments, the internal sensor  236  can be one or more sensors configured to measure certain properties of the patient  10 , such as a motion sensor (e.g., an accelerometer) for measuring the patient&#39;s motion or position with respect to gravity. 
     The external sensors  232 ,  234  can include sensors and sensing arrangements that are configured to produce a signal representative of one or more vital signs of the patient to which the monitor patch  20  is attached. For example, the first external sensor  232  can be a set of sensing electrodes that are affixed to an exterior surface of the monitor patch  20  and configured to be in contact with the patient for measuring the patient&#39;s respiratory rate, and the second external sensor  234  can include a temperature sensing element (e.g., a thermocouple or a thermistor or resistive thermal device (RTD)) affixed, either directly or via an interposing layer, to skin of the patient  10  for measuring the patient&#39;s body temperature. In other embodiments, one or more of the external sensors  232 ,  234  or one or more additional external sensors can measure other vital signs of the patient, such as blood pressure, pulse rate, or oxygen saturation. 
       FIG. 3A  is a functional block diagram illustrating exemplary electronic components of bridge  40  of  FIG. 1  according to one aspect of the subject disclosure. Bridge  40  includes a processor  310 , radio  320  having a receiver  322  and a transmitter  324 , radio  330  having a receiver  332  and a transmitter  334 , memory  340 , display  345 , and network interface  350  having a wireless interface  352  and a wired interface  354 . In some embodiments, some or all of the aforementioned components of module  300  may be integrated into single devices or mounted on PCBs. 
     Processor  310  is configured to send data to and receive data from receiver  322  and transmitter  324  of radio  320 , receiver  332  and transmitter  334  of radio  330  and wireless interface  352  and wired interface  354  of network interface  350  via bus  314 . In certain embodiments, transmitters  324  and  334  may include a radio frequency signal generator (oscillator), a modulator, and a transmitting antenna, and the receivers  322  and  332  may include a demodulator and antenna which may or may not be the same as the transmitting antenna of the radio. In some embodiments, transmitters  324  and  334  may include a digital-to-analog converter configured to convert data received from processor  310  and to generate a base signal, while receivers  322  and  332  may include analog-to-digital converters configured to convert a demodulated base signal and sent a digitized data stream to processor  310 . 
     Processor  310  may include a general-purpose processor or a specific-purpose processor for executing instructions and may further include a memory  312 , such as a volatile or non-volatile memory, for storing data and/or instructions for software programs. The instructions, which may be stored in memories  312  or  340 , may be executed by the processor  310  to control and manage the transceivers  320 ,  330 , and  350  as well as provide other communication and processing functions. 
     Processor  310  may be a general-purpose microprocessor, a microcontroller, a Digital Signal Processor (DSP), an Application Specific Integrated Circuit (ASIC), a Field Programmable Gate Array (FPGA), a Programmable Logic Device (PLD), a controller, a state machine, gated logic, discrete hardware components, or any other suitable device or a combination of devices that can perform calculations- or other manipulations of information. 
     Information such as data representative of sensor readings may be stored in memory  312  internal to processor  310  or in memory  340  external to processor  310  which may be a Random Access Memory (RAM), flash memory, Read Only Memory (ROM), Programmable Read Only Memory (PROM), Erasable Programmable Read Only Memory (EPROM), registers, a hard disk, a removable disk, a Solid State Memory (SSD), or any other suitable storage device. 
     Memory  312  or  340  can also store a list or a database of established communication links and their corresponding characteristics (e.g., signal levels) between the bridge  40  and its related monitor patches  20 . In the illustrated example of  FIG. 3A , the memory  340  external to the processor  310  includes such a database  342 ; alternatively, the memory  312  internal to the processor  310  may include such a database. 
       FIG. 3B  is a functional block diagram illustrating exemplary electronic components of server  60  of  FIG. 1  according to one aspect of the subject disclosure. Server  60  includes a processor  360 , memory  370 , display  380 , and network interface  390  having a wireless interface  392  and a wired interface  394 . Processor  360  may include a general-purpose processor or a specific-purpose processor for executing instructions and may further include a memory  362 , such as a volatile or non-volatile memory, for storing data and/or instructions for software programs. The instructions, which may be stored in memories  362  or  370 , may be executed by the processor  360  to control and manage the wireless and wired network interfaces  392 ,  394  as well as provide other communication and processing functions. 
     Processor  360  may be a general-purpose microprocessor, a microcontroller, a Digital Signal Processor (DSP), an Application Specific Integrated Circuit (ASIC), a Field Programmable Gate Array (FPGA), a Programmable Logic Device (PLD), a controller, a state machine, gated logic, discrete hardware components, or any other suitable device or a combination of devices that can perform calculations or other manipulations of information. 
     Information such as data representative of sensor readings may be stored in memory  362  internal to processor  360  or in memory  370  external to processor  360  which may be a Random Access Memory (RAM), flash memory, Read Only Memory (ROM), Programmable Read Only Memory (PROM), Erasable Programmable Read Only Memory (EPROM), registers, a hard disk, a removable disk, a Solid State Memory (SSD), or any other suitable storage device. 
     Memory  362  or  370  can also store a database of communication links and their corresponding characteristics (e.g., signal levels) between monitor patches  20  and bridges  40 . In the illustrated example of  FIG. 3B , the memory  370  external to the processor  360  includes such a database  372 ; alternatively, the memory  362  internal to the processor  360  may include such a database. 
       FIG. 4A  is a map depicting an exemplary healthcare facility (e.g., a hospital)  400  in which a patient monitoring system such as shown in  FIG. 1  is implemented. The healthcare facility  400  includes a plurality of patient rooms  410 A-H and hallways  420 A,  420 B. Shown in the map are a plurality of vital-sign monitor patches  20 A-O attached to their respectively assigned patients  10 A-O located in the patient rooms  410 A-H and hallways  420 A,  420 B. For ease of illustration and understanding, each patient with attached patch is represented by s triangle in  FIG. 4A . The facility  400  also includes a plurality of bridges  40 A-O located at specified locations in the facility  400  and configured to engage in wireless communication with the monitor patches  20 A-O. The bridges  40 A-O are represented bas circle icons. Although there are other patient rooms, bridges, and patients/monitor patches shown in the map, for the sake of simplicity, the following description will focus on the patient rooms  410 A-H, bridges  40 A-O and patients/monitor patches  10 A-O/ 20 A-O. In the illustrated example, the bridges  40 A-O are in turn connected to WiFi access points  45 A-D (shown as “stars”) that are configured to route data between the bridges  40 A-O and a surveillance server  60  ( FIG. 1 ). 
     Preferably, a bridge  40  is selected for each monitor patch  20  through which the monitor patch  20  sends and receives signals to and from the server  60  via an access point  45 A, B, C, or D. For example, the monitor patch  20 B worn by the patient  10 B in the room  410 B wirelessly transmits one or more signals comprising information indicative of his vital signs (e.g., heart rate) to the bridge  40 B. The bridge  40 B receives the signals and sends the information extracted from the signals to the access point  45  as data via either a wired or wireless connection. The access point  45 A sends the data to the surveillance server  60  via either a wired or wireless connection. As other examples, the monitor patch  20 A worn by the patient  10 A in the room  410 A sends data to the server  60  via the bridge  60 A and the access point  45 A; the monitor patch  20 P worn by the patient  10 P, walking eastward in the hallway  420 A, sends data to the server  60  via the bridge  40 N and the access point  45 A; and the monitor patch  20 M worn by the patient  10 M, walking southward in the hallway  420 B, sends data to the server  60  via the bridge  40 J and the access point  45 D. 
     Because a monitor patch  20  and a bridge  40  have limited wireless ranges, the monitor patch  20  is located in close proximity from the bridge  40  with which the monitor patch has a communicative association. Therefore, it is possible to track the location of a monitor patch  20  by knowing the location of the selected bridge  40 . In certain embodiments of a monitoring network of the present disclosure, the surveillance server  60  is configured to track locations of the monitor patches  20 A-O by maintaining a database comprising information indicative of monitor patches  20 A-O and their selected bridges  40 A-O. In certain embodiments, the database further comprises a list of unselected but linkable bridges with which each of the monitor patches  20 A-O is capable of engaging in a bidirectional wireless data communication. 
       FIG. 4B  is a portion of an exemplary database comprising the monitor patches  20 A- 20 P (first column), their linkable bridges (second column), signal levels associated with the communication links between the monitor patches and the linkable bridges (third column) in an arbitrary unit (e.g., dbm), and selected bridges (fourth column) according to certain aspects of the present disclosure. Such a database may be stored in the memory  370  ( FIG. 3B ) of the server  60  as database  372 , for example. Alternatively, the database may be stored in a memory located outside the server  60  (e.g., on a network) but accessible by the server  60  via, e.g., a wired or wireless interface  992 ,  994 . For ease of illustration, it is assumed that the database of  FIG. 4B  corresponds to the database  372  of  FIG. 3B . 
     For example, the database  372  shows the monitor patch  20 A having one linkable bridge  40 A which is also the selected bridge. The database  372  also shows the monitor patches  20 B and  20 C having the same linkable bridges  40 B,  40 C of which the bridges  40 B is the selected bridge for the both monitor patches. The database  372  also shows the monitor patch  20 D having the same linkable bridges  40 B,  40 C of which the bridge  40 C is the selected bridge. The above portions of the database  372  relating to the monitor patches  20 B-D reflect the fact that the monitor patches  20 B,  20 C are in the room  410 B having the bridge  40 B located therein, while the monitor patch  20 D is in the room  410 C having the bridge  40 C located therein. Therefore, while the bridge  40 B is capable of communicating with the monitor patch  20 D due to their close proximity, the bridge  40 C is selected for the monitor patch  20 D, e.g., by the server  60 , due to the bridge&#39;s closer proximity to the monitor patch  20 D. The database  372  also shows the monitor patch  20 M worn by the patient  10 M having three linkable bridges  40 I,  40 J,  40 K of which the bridge  40 J is the currently selected bridge; and the monitor patch  20 P worn by the patient  10 P having two linkable bridges  40 C,  40 N of which the bridge  40 N is the currently selected bridge. 
     The signal levels (third column) associated with various bridge-patch communication links in the database  372  represent the strengths of wireless signals (e.g., acknowledgment signals) from the bridges  40 A-O received by the monitor patches  20 A-op. As will be described below with respect to  FIG. 9 , the signal levels can be used for a bridge selection by the surveillance server  60 . 
       FIG. 5A  is a map of the exemplary healthcare facility  400  depicted in  FIG. 4A  after a passage of time from the map of  FIG. 4A . The map of  FIG. 4B  is the same as the map of  FIG. 4A  except for the following changes:
         1) The patient  10 A wearing the monitor patch  20 A has left the facility  400 .   2) The patient  10 M wearing the monitor patch  20 M has now returned to her patient room  410 G.   3) The patient  10 P wearing the monitor patch  20 P and walking along the hallway  420 A has now progressed to the middle of the hallway  420 A.       

     In response to the changes, the surveillance server  60  has updated the database  372  discussed above with respect to  FIG. 4B .  FIG. 5B  is a portion of an updated version of the database  372  shown in  FIG. 4B  according to certain embodiments of the present disclosure. The updated database  372  shows the monitor patch  20 A having neither a linkable bridge nor a selected bridge, reflecting the fact that the monitor patch  20 A is longer in communication range of any of the bridges in the monitoring system. The updated database also shows the monitor patch  20 M now having three linkable bridges  40 H,  40 I,  40 K of which the bridge  401  is the currently selected bridge, reflecting the fact that the monitor patch  20 M is now in the room  4100  having the bridge  401  located therein. The updated database also shows the monitor patch  20 P having two linkable bridges  40 N,  40 O of which the bridge  40 O is the currently selected bridge. In the illustrated example of  FIG. 5A , the monitor patch  20 P is substantially equidistant from both the bridge  40 N and the bridge  40 O and the respective signal strengths are similar (19 versus 17). Hence, the monitor patch  20 P can be served equally well by both bridges  40 N,  40 O. Notwithstanding the fact that the signal strength associated with the bridge  40 N is slightly higher than that associated with the bridge  40 O, control software in the surveillance server  60  has selected the bridge  40 O based on the consideration that the monitor patch  20 P worn by the patient  10 P has been moving towards the bridge  40 O and away from the bridge  40 N and, hence, is likely to be served longer by the former bridge  40 O than by the latter bridge  40 N. 
     Henceforth, specific reference numbers (e.g., bridge  40 A, monitor patch  20 C) will be used when referring to specific devices, while generic references (bridge  40 , monitor patch  20 ) will be used when referring to devices in a general sense. 
     A communication link between a bridge  40  and a monitor patch associated  20  associated with a patient can be considered established, for example, when the bridge  40  has received one or more regularly transmitted signals (e.g., those indicative of vital signs of the patient) from the monitor patch  20  or when the bridge  40  has received an acknowledgment signal from the bridge  40  in response to a query signal sent out via the bridge  40 . From the perspective of the monitor patches  20 , the bridge is one of linkable bridges for the monitor patches  20 . Conversely, an established communication link between a bridge  40  and a monitor patch  20  can be considered lost when the bridge  40  can no longer receive regularly transmitted signals from the monitor patch  20  or when the bridge  40  does not receive an acknowledgment signal from the monitor patch  20  in response to a query signal. 
     In certain embodiments, upon occurrence of a new communication link or loss of an existing communication link, the bridge  40  automatically sends the message to the surveillance server  60  ( FIG. 1 ), which, in turn, updates the database  372  stored in memory (e.g.,  370 ) associated with the server  60 , such as a hard disk or an external data storage device accessible by the server. In some embodiments, each of the bridges  40 A-O includes memory (e.g.,  312 ,  340  of  FIG. 3A ) for storing a list or database  342  of monitor patches  20 A-O with which the bridge  40  has established communication links. A processor (e.g.,  310  of  FIG. 3A ) executing control software in the bridge  40  can update the list  342  stored in the memory (e.g.,  312 ,  340 ) of the bridge  40  to keep the list current. The processor  310  then sends the updated list  342  or a portion thereof to the surveillance server  60 . 
       FIG. 6A  shows a first list  610 A stored in the bridge  40 A at the time of  FIG. 4A , and a second list  620 A which corresponds to an updated list stored in the bridge  40 A at the time of  FIG. 5A . The first column of the first list  610 A enumerates linkable patches corresponding to all monitor patches with which the bridge  40 A has established communication links at the time of  FIG. 4A . The second column of the first list  610 A enumerates associated patches corresponding to all monitor patches  20  for which the bridge  40 A has been selected for communicative association at the time of  FIG. 4A . As can be seen from  FIG. 4A  and correspondingly reflected in the first list  610 A, the bridge  40 A has an established communication link only with the monitor patch  20 A. The bridge  40 A is also the selected bridge  40  for the monitor patch  20 A, or, conversely, the monitor patch  20 A is an associated patch for the bridge  40 A. As can be seen from  FIG. 5A  and correspondingly reflected in the second list  620 A, the bridge  40 A has neither a linkable patch nor an associated patch at the time of  FIG. 5A , reflecting the fact that between the time of  FIG. 4A  and the time of  FIG. 5A , the patient  10 A wearing the monitor patch  20 A has left the healthcare facility  400 , or the patch  20 A has been removed and deactivated. 
       FIG. 6B  shows a first list  610 B stored in the bridge  401  at the time of  FIG. 4A , and a second list  620 B that corresponds to an updated list stored in the bridge  401  at the time of  FIG. 5A . As can be seen from  FIG. 4A  and correspondingly reflected in the first list  610 B, the bridge  40 A has established communication links with the monitor patches  20 K and  20 L of which the monitor patch  20 L is the associated patch. As can be seen from  FIG. 5A  and correspondingly reflected in the second list  620 B, the bridge  401  has established communication links with the monitor patches  20 K,  20 L, and  20 M of which the monitor patches  20 L and  20 M are the associated patches at the time of  FIG. 5A , reflecting the fact that between the time of  FIG. 4A  and the time of  FIG. 5A , the patient  10 M wearing the monitor patch  20 M has entered the patient room  410 G. 
       FIG. 6C  shows a first list  610 C stored in the bridge  40 N at the time of  FIG. 4A , and a second list  620 C which corresponds to an updated list stored in the bridge  40 N at the time of  FIG. 5A . As can be seen from  FIG. 4A  and correspondingly reflected in the first list  610 B, the bridge  40 N has established communication links with the monitor patches  20 F and  20 P of which the monitor patch  20 P is the associated patch. As can be seen from  FIG. 5A  and correspondingly reflected in the second list  620 B, the bridge  40 N has established communication links with the monitor patches  20 , but has no associated patch, reflecting the condition that between the time of  FIG. 4A  and the time of  FIG. 5A , the patient  10 P wearing the monitor patch  20 P has progressed to the center of the hallway  420 A towards the bridge  400  and the surveillance server  60  has selected the bridge  40 O for the monitor patch  20 P as discussed above with respect to  FIG. 5B . 
       FIG. 7  is a flowchart illustrating a process  700  for tracking locations of monitor patches  20  by keeping and updating a database comprising information indicative of the monitor patches  20  and their linkable and selected bridges  40  according to certain aspects of the present disclosure. For the purposes of illustration only, without any intent to limit the scope of the present disclosure in any way, the process  700  will be described with reference to  FIGS. 1 ,  4 A-B, and  5 A-B. The process  700  begins at start state  701  and proceeds to operation  710  in which a surveillance server  60  receives a message from a bridge  40  indicating that the bridge  40  has established a new communication link with a monitor patch  20  or lost an existing communication link with a monitor patch  20  or both. For example, if the message were sent from the bridge  40 A of  FIGS. 4A and 5A , the message would indicate loss of an existing communication link with the monitor patch  20 A. On the other hand, if the message were sent from the bridge  401 , the message would indicate a new (previously unavailable) communication link with the monitor patch  20 M. 
       FIG. 8A  is a diagram illustrating an exemplary data structure for a message  800 A indicating a new communication link and/or loss of an existing communication link according to certain aspects of the present disclosure. In the illustrated example, the message  800 A includes a header field  810 A for storing a message header, an ID field  820 A for storing an ID for a bridge sending the message, a data field  830 A for storing information relating to a new communication link and/or loss of an existing communication link, and optionally a field  840  for storing a checksum. The header field  810 A can include subfields for indicating a total number of monitor patches with which the bridge has established new communication links and a total number of monitor patches with which the bridge has lost existing communication links. The data field  830 A includes a first subfield  832 A for storing an ID for a monitor patch that the bridge has established a new communication link, a second subfield  833 A for storing data indicative of a signal level or strength associated with the new communication link, and a third subfield  836 A for storing an ID for a monitor patch with which the bridge has lost an existing communication link. If the message  800 A were sent from the bridge  40 A while transitioning from the configuration of  FIG. 4A  to the configuration of  FIG. 5A , the ID field  820  would include data indicative of the bridge  40 A, and the third subfield  836 A would include data indicative of the monitor patch  20 A. The message embodiment shown in  FIG. 8A  is exemplary only, as other message embodiments may be employed. The surveillance server  60  upon receiving the message  830 A can update the database such as the one shown in  FIG. 4B  as further described below. 
       FIG. 8B  is a diagram illustrating an exemplary data structure for an alternative message  800 B indicating a new communication link and/or loss of an existing communication link according to alternative aspects of the present disclosure. In the illustrated example, the message  820 B includes a header field  810 B for storing a message header, an ID field  820 B for storing an ID for a bridge sending the message, a data field  830 B for storing information relating to all monitor patches with which the bridge has established communication links, and a field  840 B for storing a checksum. The header field  810 A can include subfields for indicating a total number of linkable monitor patches  20  with which the bridge  40  has established communication links. The data field  830 B includes a first subfield  832 B for storing an ID of a first likable monitor patch  20 , a second subfield  833 B for storing data indicative of a signal strength or level associated with the communication link between the bridge  40  and the first likable monitor patch  20 . The data field  830 B includes other subfields  834 B,  835 B,  836 B,  837 B for storing ID&#39;s and data indicative signal strengths for additional likable monitor patches  20 . If the message were sent from the bridge  401  in the configuration of  FIG. 5A  (corresponding to the list  620 B of  FIG. 6B ), the data fields  832 B,  834 B,  836 B would include data indicative of the monitor patches  20 K,  20 L,  20 M, respectively, for example. In certain embodiments, each of the subfields  832 B,  834 B,  836 B includes a single bit for indicating whether the monitor patch  20  indicated by the subfield is associated with the bridge  40  or not (i.e., whether the bridge  40  is the selected bridge  40  for the monitor patch  20 ). In those embodiments, such a bit would be clear for the subfield  832 B (for the monitor patch  20 K), but set for the subfields  834 B,  836 B (for the monitor patches  20 L,  20 M). The message embodiment of  FIG. 8A  is exemplary only, as other message embodiments may be employed. The surveillance server  60 , upon receiving the message  830 B, can update the database such as the one shown in  FIG. 4B  as further described below. 
     Returning to  FIG. 7 , the process  700  proceeds to decision state  720  in which it is determined whether the received message indicates that the bridge  40  sending the message has established at least one new communication link with a monitor patch  20 , e.g., by starting to receive regularly transmitted signals from the monitor patch. In case of the message  800 A of  FIG. 8A , this determination can involve control software running in a processor of the surveillance server  60  searching for nonzero data in the data field  832 A (“NEW PATCH ID”). In case of the message  800 B of  FIG. 8B , this determination can involve the control software comparing the linkable monitor patches  20  indicated in the data field  830 B of the message  800 B to previously stored linkable monitor patches  20  for the bridge  40  in order to discover one or more monitor patches  20  that are newly present in the message. If it is determined at the state  720  that no new communication link has been established for the bridge  40  (NO), the process  700  proceeds to decision state  730  to be described below. 
     On the other hand, if it is determined at the decision state  720  that at least one new communication link has been established for the bridge  40  (YES), the process  700  proceeds to operation  725  in which database  372  comprising information indicative of communication links between monitor patches  20 A-O and bridges  40 A-O is accessed and the new communication link is added to the database. Examples of such additions include the communication links between the monitor patch  20 M and the bridge  401  and the communication link between the monitor patch  20 P and the bridge  40 O, both of which are not present in the database shown in  FIG. 4A  but present in the updated database shown in  FIG. 5B . The process  700  then proceeds to decision state  730  described below. 
     In the decision state  730  it is determined whether the received message indicates that the bridge  40  sending the message has lost at least one existing communication link with a monitor patch  20 , e.g., by failing to receive regularly transmitted signals from the monitor patch  20 . In case of the message  800 A of  FIG. 8A , this determination can involve control software running in a processor of the surveillance server  60  looking for nonzero data in the subfield  836 A. In case of the message  800 B of  FIG. 8B , this determination can involve the control software comparing the linkable monitor patches  20  indicated in the data field  830 B of the message  800 B to previously stored linkable monitor patches  20  for the bridge  40  in order to discover one or more monitor patches  20  that are not longer present in the message. 
     If it is determined at the decision state  730  that no existing communication link has been lost for the bridge  40  (NO), the process  700  ends at state  703 . On the other hand, if it is determined at the decision state  730  that at least one existing communication link has been lost for the bridge (YES), the process  700  proceeds to operation  725  in which a database comprising information indicative of communication links between monitor patches  20 A-O and bridges  40 A-O is accessed and the communication link is deleted from the database. Examples of such deletion include the communication link between the monitor patch  20 M and the bridge  403  and the communication link between the monitor patch  20 P and the bridge  40 C, which is present in the database shown in  FIG. 5A  but not present in the updated database shown in  FIG. 5B . The process  700  ends at state  703 . 
     Therefore, the surveillance server  60 , by maintaining and updating a database of bridge-patch communication links using a process such as the process  700  based on messages received from bridges  40 , can track locations of monitor patches  20 A-O in the healthcare facility  400 . In certain embodiments, the surveillance server  60 , after receiving one or more of such messages or at scheduled intervals, can select a particular bridge  40  among a set of linkable bridges  40  for a particular monitor patch  20 . After such a bridge selection, in certain embodiments, the server  60  prevents other linkable but unselected bridges  40  from communicating with the particular monitor patch  20 . 
       FIG. 9  is a flowchart illustrating an exemplary process  900  for a bridge selection process according to certain aspects of the present disclosure. The process  900  begins at start state  901  and proceeds to decision state  910  in which it is determined whether there are multiple bridge-patch communication links (e.g., multiple linkable bridges) available for a particular monitor patch  20 . If it is determined at the decision state  910  that there is only one communication link (e.g., one linkable bridge) available for the monitor patch, the process  900  proceeds to another decision state  930  to be described below. On the other hand, if it is determined at the decision state  910  that there are multiple communication links (e.g., multiple linkable bridges  40 ) for the monitor patch  20 , the process  900  proceeds to operation  920  in which signal strengths of the multiple communication links are compared, and a bridge  40  associated with the highest signal strength is identified. For example, in case of the monitor patch  20 M in the configuration of  FIG. 4B , the communication link between the monitor patch  20 M and the bridge  40 J has the highest signal strength (19) among all available communication links. 
     The process  900  proceeds to decision state  930  in which it is determined whether the bridge  40  being considered for selection (e.g., the only bridge in case of one communication link or the identified bridge in case of multiple communication links) is available for communication with the monitor patch  20 . This determination can involve determining by the surveillance server  60  or by the bridge  40  the number of monitor patches  20  with which the bridge  40  is currently associated (e.g., the number of monitor patches to which the bridge is currently the selected bridge) in order to determine whether the bridge  40  is currently overloaded. If it is determined at the decision state  930  that the bridge  40  being considered for selection is not available (NO), the process  900  proceeds to decision state  940  in which it is determined whether there are one or more other linkable bridges  40  with which the monitor patch  20  can be associated. If it is determined at the decision state  940  that there is no other linkable bridge  40  (NO), the process  900  ends at state  903 . On the other hand, if it is determined at the decision state  940  that there are one or more other linkable bridges  40  (YES), the process  900  proceeds to operation  945  in which another linkable bridge  40  associated with the next highest signal strength is identified, and then back to the decision state  930  for determining availability of the other bridge  40 . 
     On the other hand, if it is determined at the decision state  930  that the bridge  40  being considered for selection is available (YES), the process  900  proceeds to decision state  950  in which it is determined whether the selection of the bridge  40  being considered is consistent with other considerations. For example, as indicated above with respect to  FIG. 5B , control software running in the surveillance server  60  selected the bridge  40 O over the bridge  40 N in spite of the condition that the signal strength associated with the bridge  40 N is currently stronger than the signal strength associated with the bridge  40 O. The selection is based on the additional consideration that the patch  20 P has been moving away from the bridge  40 N and towards the bridge  40 O. 
     If it is determined at the decision state  930  that the selection of the bridge  40  is not consistent with other considerations (NO), the process  900  proceeds to the decision state  940  and to the operation  945  and back to the decision state  930  as discussed above. On the other hand, if it is determined at the decision state  930  that the selection of the bridge  40  is consistent with other considerations (YES), the process  900  proceeds to operation  960  in which the bridge  40  is selected for the monitor patch  20  and, the database of bridge-patch communication links is updated to reflect the new selection. The process  900  ends at state  903 . 
     At times, a bridge  40  can lose power or break down or otherwise become inoperable and can no longer carry data between associated monitor patches  20  and the surveillance server  60 . For example, if the bridge  401  becomes inoperable, the monitor patches  201 , and  20 M can no longer send data to the surveillance server  60  via the bridge  401 . In certain embodiments, the surveillance server  60 , upon recognition of such an occurrence, selects alternative bridges  40  for the monitor patches  20  so as to route data between the monitor patches  20  and the surveillance server  60  via the alternative bridges  40 . 
       FIG. 10  is a flowchart illustrating a process  1000  for detecting an inoperable bridge  40  and selecting an alternative bridge  40  to replace the inoperable bridge  40  for the monitor patches  20  that were previously associated with the inoperable bridge  40  according to certain aspects of the present disclosure. The process  1000  begins at start state  1001  and proceeds to decision state  1010  in which it is determined whether an inoperable selected bridge  40  has been detected. The determination can include failing to receive regularly transmitted messages from the selected bridge  40  or failing to receive an acknowledgment message from the bridge  40  in response to a query message sent to the bridge  40  by the surveillance server  60 . If it is determined at the decision state  1010  that an inoperable selected bridge  40  has not been detected (NO), the process  1000  loops back to the decision state  1010  to await such an occurrence. On the other hand, if an inoperable selected bridge  40  has been detected (YES), the process  1000  proceeds to operation  1020  in which a monitor patch  20  associated with the selected bridge  40  is identified from, e.g., database (e.g.,  372 ) such as the ones shown in  FIGS. 4B and 5B . For example, if the selected bridge found to be inoperable is the bridge  40 D, the monitor patch  20 E can be identified. After the identification, the process proceeds to decision state  1030 . 
     In the decision state  1030 , it is determined whether there are one or more alternative linkable bridges  40  for the identified monitor patch from, e.g., a list in a database such as the ones shown in  FIGS. 4B and 5B . If it is determined at the decision state  1030  that there is no alternative bridge  40  (NO), the process  1000  proceeds to decision state  1050  which will be described below. On the other hand, if it is determined at the decision state  1030  that there are one or more alternative linkable bridges  40  for the identified monitor patch (YES) (the bridge  40 B for the monitor patch  20 E in the above example), the process  1000  proceeds to operation  1040  in which a bridge selection process such as the one described above with respect to  FIG. 9  is performed in order to select an alternative bridge  40  for the identified monitor patch  20 , 
     The process  1000  then proceeds to decision state  1050  in which it is determined whether there is another monitor patch  20  associated with the selected bridge  40  determined to be inoperable at the decision state  1010 . If it is determined at the decision state  1050  that there is no other monitor patch  20  associated with the inoperable bridge  40 , the operation  1000  ends at state  1003 . On the other hand, if it is determined at the decision state  1050  that there is another monitor patch  20  associated with the inoperable bridge  40  (YES) (the monitor patch  20 E for the bridge  40 D in the above example), the process  1000  loops back to the decision state  1030  in which it is determined where there are one or more alternative bridges  40  for the other monitor patch  20  and then to the selection operation  1040  and decision state  1050 . The loop is repeated until it is determined at the decision state  1050  that there is no other monitor patch  20  associated with the inoperable bridge  40  in which case the process  1000  ends at state  1003 . 
     In certain aspects, the knowledge of locations of monitor patches (e.g.,  20 A-O of  FIGS. 4A and 4B ) can be used for tracking patients (e.g.,  10 A-O) wearing the monitor patches  20  in a healthcare facility (e.g., hospital). As discussed above with respect to  FIG. 7 , a surveillance server (e.g.,  60  of  FIG. 1 ) can track locations of monitor patches  20  by keeping and updating database  372  comprising information indicative of the monitor patches  20  and their linkable and selected bridges  40  based on messages received from various bridges  40 . Therefore, assuming that the locations of various bridges  40  in the facility and the names of patients  10  to whom the monitor patches  20  are assigned are known, locations of the patients  10  can also be tracked. 
       FIG. 11  is a flowchart illustrating a process  1100  for determining locations of patients  10  in a healthcare facility according to certain aspects of the present disclosure. The process  1100  begins at start state  1101  and proceeds to operation  1110  in which a surveillance server  60  receives a signal comprising information relating to a monitor patch  20  attached to a patient  10  from a selected bridge  40  for the monitor patch  20 . The signal can be, for example, one of the messages  800 A and  800 B discussed above with respect to  FIGS. 8A and 8B . After receiving the signal, surveillance server  60  can update database  372  such as the ones shown in  FIGS. 4A and 4B  as discussed above with respect to  FIG. 7 . In certain embodiments, the signal is generated by the bridge  40  in response to a newly established communication link between the bridge  40  and the monitor patch  20  trigged by the patient  10  being moved into her new patient room. In other embodiments, the signal is generated by the bridge  40  in response to a query signal sent to the bridge  40  by the surveillance server  60 . 
     The process  1100  proceeds to operation  1120  in which a patient to whom the monitor patch  20  is attached is identified. In certain embodiments, the identification operation includes control software running in the surveillance server  60  accessing a database such as the one shown in  FIG. 12A  that lists monitor patches  20  (first column) and their assigned patients  10  (second column). In other embodiments, the received signal includes information indicative of the patient  10  (e.g., the patient ID), and the control software extracts the information from the signal. The process  1100  proceeds to operation  1130  in which location of the patient  10  is determined. In certain embodiments, the operation  1130  includes the control software accessing a database such as the one shown in  FIG. 12B  that lists locations of various bridges  40  in the facility. In other embodiments, the received signal includes information indicative of the location of the bridge  40  that sent the signal, and the control software extracts the information from the signal. In some embodiments, the determined location (e.g., “Room  3 ”) is displayed on a display associated with a hospital system (e.g., the workstation  100  of  FIG. 1 ). Alternatively, the display can graphically indicate the patient location on a hospital map such as the ones shown  FIGS. 4A and 4B . 
     The foregoing description is provided to enable any person skilled in the art to practice the various embodiments described herein. While the foregoing embodiments have been particularly described with reference to the various figures and embodiments, it should be understood that these are for illustration purposes only and should not be taken as limiting the scope of the claims. 
     The word “exemplary” is used herein to mean “serving as an example or illustration.” Any aspect or design described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other aspects or designs. 
     A reference to an element in the singular is not intended to mean “one and only one” unless specifically stated, but rather “one or more.” The term “some” refers to one or more. Underlined and/or italicized headings and subheadings are used for convenience only, do not limit the invention, and are not referred to in connection with the interpretation of the description of the invention. All structural and functional equivalents to the elements of the various embodiments of the invention described throughout this disclosure that are known or later come to be known to those of ordinary skill in the art are expressly incorporated herein by reference and intended to be encompassed by the invention. Moreover, nothing disclosed herein is intended to be dedicated to the public regardless of whether such disclosure is explicitly recited in the above description.