Patent Publication Number: US-2011068932-A1

Title: Bed exit alarm of hospital bed mattress

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 11/559,529, filed Nov. 14, 2006, which issued as U.S. Pat. No. 7,849,545 on Dec. 14, 2010 and which is hereby incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     The present disclosure relates to hospital bed mattresses and particularly, to control systems for hospital bed mattresses in which portions of the control systems may control inflation of one or more air bladders included in the mattresses and portions of the control systems may control bed exit alarms. The present disclosure also relates to hose assemblies that interconnect control units with associated mattresses. 
     Hospitals use a variety of different mattress types to support patients having different medical needs. Some patients benefit from being supported on alternating pressure mattresses or rotation therapy mattress. Other patients may not need to be supported on these types of mattresses having dynamic therapies, but rather, so-called static air mattresses that simply operate to provide a relatively low interface pressure may be suitable for such patients. To address these various mattress requirements, manufacturers may market a number of mattress products, each with its own particularized therapeutic function, or may market mattresses that have control units allowing users to program or select different modes of operation. It can be expensive for hospitals or other healthcare institutions to purchase a large number of mattresses, each having its own dedicated therapeutic functionality. However, mattresses allowing user selectable functionality introduce the possibility that users may inadvertently configure the mattress with the wrong type of therapy or operation among the plurality of the available therapies. 
     It is sometimes desirable for patients to remain in bed and, in such situations, hospitals are interested in having some type of bed exit alarm for these patients to provide an alarm or alarm signal indicating that the patient has exited, or is about to exit, the bed. Some bed frames have built-in bed exit alarm systems, oftentimes using the load cells of a weigh scale system, to determine the presence or absence of a patient on the bed and triggering a bed exit alarm when a threshold amount of weight is determined to have been removed from the bed. The addition of a weigh scale system to a bed frame adds cost and therefore, hospitals purchase many bed frames without the weigh scale system option and consequently, therefore, without any bed exit alarm capability. Separate bed exit systems having strips or mats that are placed on top of a mattress and underneath a patient are known in the art, but such strips or mats may result in increased interface pressure on the patient and thus, may compromise the ability of the mattress to support the patient with relatively low interface pressure. 
     SUMMARY OF THE INVENTION 
     The present invention comprises an apparatus or system having one or more of the features recited in the appended claims and/or one or more of the following features, which alone or in any combination may comprise patentable subject matter: 
     A mattress system may comprise a mattress having at least one inflatable bladder and a control unit. The control unit may comprise a main housing, a source of pneumatic pressure carried by the main housing and operable to inflate the at least one inflatable bladder, and a plurality of user interface modules that are coupleable to the main housing. Each user interface module may be programmed to control inflation of the at least one air bladder differently in at least one operational mode of each of the plurality of user interface modules. 
     The user interface modules may include an alternating pressure module and a continuous low pressure module. Additionally or alternatively, the user interface modules may include a rotation therapy module. One or more the user interface modules may be programmed to provide two different types of therapies. Each of user interface modules may have user inputs that are used to change a mode of operation of the associated user interface module. One or more of the user interface modules may be programmed to provide a maximum inflation mode in which the at least one inflatable bladder is inflated to a maximum threshold pressure which renders the at least one air bladder suitably hard for patient transfer. One or more of the user interface modules may include at least one visual indicator that is operable to provide visual indication of the operational status of the mattress system. 
     The control unit may have a latch coupled to the main housing and movable between a locked position in which a selected one of the plurality of user interface modules is locked to the main housing and an unlocked position in which the selected one of the plurality of user interface modules is detachable from the main housing. The latch may have a hook and each of the user interface modules may have a post that is captured by the hook when the associated user interface module is locked to the main housing by the latch. The main housing may have at least one slot and each of the plurality of interface modules may have at least one tab that is received in the slot provided in the main housing when the associated user interface module is coupled to the housing. 
     Each of the user interface modules may comprise a main body. The post and the at least one tab of each of the plurality of user interface modules may be located on opposite ends of the associated user interface modules. The post and the at least one tab of each of the plurality of user interface modules may extend away from the associated main body in opposite directions. 
     The main housing of the control unit may have a recess into which each of the user interface modules is insertable. The control unit may include a first electrical connector coupled to the main housing. Each of the user interface modules may include a second electrical connector that automatically mates with the first electrical connector upon insertion of the user interface module into the recess of the main housing. 
     At least one of the plurality of user interface modules may have a lockout switch that is usable to lockout at least one operational mode of the associated user interface module. The lockout switch may be located adjacent a surface, such as a back surface, of the associated user interface module that is inaccessible to a user when the associated user interface module is coupled to the main housing. The lockout switch may be used to lockout the maximum inflation mode of the associated user interface module. 
     One or more of the plurality of user interface modules may include inputs that are engageable to enable a bed exit alarm system of the control unit. The bed exit alarm system may receive an input signal from the mattress indicative of a pressure sensed by a pressure sensor situated in an interior region of the surface. 
     The source of pneumatic pressure of the control unit may comprise an air compressor within a noise-dampening housing that is situated within an interior region of the main housing. The noise-dampening housing may have an air chamber that serves as a pressure reservoir which stabilizes air flow to the at least one air bladder of the mattress. 
     The mattress may at least one valve located within an interior region of the mattress. The valve may be opened and closed to control pressure in the at least one inflatable bladder in a manner dictated by the programming of the selected user interface module that is coupled to the main housing. The mattress may have at least one pressure sensor situated within an interior region of the mattress and an output signal from the at least one pressure sensor may be communicated to the selected user interface module that is coupled to the main housing. At least one cardiopulmonary resuscitation (CPR) input may be coupled to the mattress and may be movable mechanically to deflate the at least one air bladder of the surface. Movement of the CPR input may result in a CPR signal being communicated to the selected user interface module that is coupled to the main housing. The selected user interface module may deactivate operation of the source of pneumatic pressure in response to receiving the CPR signal. 
     The control unit may have circuitry that receives a signal from the mattress which is indicative of a pressure sensed by a pressure sensor situated within an interior region of the mattress. The circuitry may use the signal to establish at least one target pressure to which the at least one inflatable bladder is controlled. The circuitry may also use the signal to determine whether to activate a bed exit alarm. The bed exit alarm may comprise a flashing light that is carried by the control unit. Alternatively or additionally, the bed exit alarm may comprise a sound-producing device, such as a speaker or buzzer, carried by the control unit. 
     The pressure sensor may comprise a bag filled with silicon oil. The mattress comprises an inflatable underlay having a space that receives the bag filled with silicon oil. The at least one inflatable bladder of the mattress may comprise at least one inflatable layer that overlies the pressure sensor. The at least one inflatable layer may comprise first and second inflatable layers that overlie the pressure sensor. The first inflatable layer may comprise a plurality of laterally extending air bladders, at least two of which are inflatable to different pressures. The second inflatable layer may extend approximately the full length of the mattress and may be inflatable to a single target pressure. 
     The mattress may have one or more foam layers, blocks, pads, and/or one or more other non-inflatable support elements in lieu of, or in addition to, the at least one inflatable bladder. Accordingly, the mattress may have no inflatable bladders at all. A pressure sensor comprising an enclosure containing a liquid, such as a bag filled with silicon oil, may be used as part of a bed exit alarm system in such a foam mattress. With regard to mattresses lacking any inflatable bladders, the control unit may be omitted and the circuitry and sound-producing device of the bed exit alarm system may be included within, or coupled directly to, the mattress itself. In other embodiments, some or all of the circuitry and sound producing device of the bed exit alarm system may be situated in a housing or module that is spaced from the mattress and that is coupled to the pressure sensor via one or more electrical lines, or even wirelessly for that matter. 
     The mattress may have at least one electrical component situated within an interior region of the mattress, a first pneumatic port in communication with the at least one inflatable bladder of the mattress, and a first electrical connector in communication with the at least one electrical component. The circuitry of the control unit may be programmed to control the manner in which the at least one inflatable bladder is inflated. The circuitry may also include a second electrical connector and the control unit may include a second pneumatic port in communication with the source of pneumatic pressure. 
     A connector assembly may be provided to pneumatically connect the first pneumatic port with the second pneumatic port and to electrically connect the first electrical connector with the second electrical connector. The connector assembly may include a pneumatic line, at least one electrical line, and a dual mode plug configured to permit connection of the pneumatic line with the second pneumatic connector substantially simultaneously with connection of the at least one electrical line with the second electrical connector. 
     The connector assembly may include a dual lumen hose having side-by-side first and second lumens. The first lumen serves as the pneumatic line for communication of pneumatic pressure from the second pneumatic port of the control unit to the at first pneumatic port of the mattress and the second lumen serves as an electrical conduit through which the at least one electrical line is routed. A first end of the dual lumen hose may be coupled to the dual mode plug. The connector assembly may have a pneumatic coupler mounted to the first lumen at a second end of the dual lumen hose. The least one electrical line may extend beyond the second lumen at the second end of the dual lumen hose. 
     The dual mode plug may include a plug housing and at least one latch member coupled to the plug housing. The at least one latch member may be movable between a first position locking the dual mode plug to the main housing of the control unit and a second position unlocking the dual mode plug for detachment from the main housing of the control unit. The at least one latch member may be spring-biased toward the first position. The at least one latch member may comprise a pair of latch members, each situated on an opposite side of the plug housing. The plug housing may include a pair of recesses and at least a portion of each latch member may be received within a respective one of the pair of recesses. The pair of latch members may be movable toward the plug housing to move the pair of latch members between the respective first and second positions. 
     The dual mode plug may include a front wall. A pneumatic coupler may extend away from the front wall and an electrical coupler may extend away from the front wall. The pneumatic coupler may be configured to mate with the second pneumatic port of the control unit and the electrical coupler may be configured to mate with the second electrical connector of the control unit. The pneumatic coupler may be formed integrally with the front wall of the dual mode plug and the front wall of the dual mode plug may have an aperture through which the electrical coupler extends. The dual mode plug may include a check valve in communication with the pneumatic coupler. 
     Additional features, which alone or in combination with any other feature(s), such as those listed above and those listed in the claims, may comprise patentable subject matter and will become apparent to those skilled in the art upon consideration of the following detailed description of various embodiments exemplifying the best mode of carrying out the embodiments as presently perceived. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The detailed description particularly refers to the accompanying figures in which: 
         FIG. 1  is an exploded perspective view of a mattress system showing inflatable layers of a mattress of the mattress system situated between top and bottom coverlets of the mattress and showing a control unit situated near a foot end of the bottom of the coverlet coupled electrically and pneumatically by a connector assembly a technical box situated within an interior region of the mattress; 
         FIG. 2  is a perspective view of the control unit of  FIG. 1  showing a first user interface module being removed from a recess formed in main housing of the control unit; 
         FIG. 3  is a front perspective of the first user interface module of  FIG. 2  showing user inputs on a front surface of the first user interface module; 
         FIG. 4  is a front perspective of a second user interface module that is coupleable to the main housing of the control unit in lieu of the first user interface module; 
         FIG. 5  is a rear perspective of the second user interface module showing a pair of tabs extending from one end of main body of the module and a post extending from an opposite end of the main body of the module and also showing a lock out switch that is accessible on a rear surface of the main body when the module is detached from the control unit; 
         FIG. 6  is rear perspective view of the control unit showing a latch being movable in the direction of the arrow to unlock the user interface module for detachment from the main housing of the control unit; 
         FIG. 7   a  is a side elevation view of the control unit of  FIG. 6 , with portions broken away, showing the latch in an unlocked position showing the post situated at an open end of a slot formed in a hook or cam of the latch; 
         FIG. 7   b  is a side elevation view, similar to  FIG. 7   a , showing the latch in a locked position such that the post of the user interface module is captured within the slot of the hook; 
         FIG. 8  is a rear perspective view of the control unit similar to  FIG. 6  showing a dual mode plug of the connector assembly coupled to a socket provided in a rear wall of the main housing of the control unit and showing a coupling hook having a cut out beneath the dual mode plug; 
         FIG. 9  is a perspective view of the connector assembly showing a dual lumen hose having one end coupled to the dual mode plug and having pneumatic and electrical couplers at an opposite end thereof; 
         FIG. 10  is an exploded view of the dual mode plug at the end of the connector assembly that couples to the control unit; 
         FIG. 11  is an enlarged perspective view of the end of the connector assembly that couples to the mattress of the mattress system; 
         FIG. 12  is an enlarged perspective view showing the pneumatic and electrical couplers of the connector assembly coupled to mating pneumatic and electrical connectors, respectively, included in the mattress; 
         FIG. 13  is a diagrammatic view of the mattress system showing a pressure sensor in a middle region of the mattress and having associated sensor circuitry, a technical box at a foot end of the mattress having valves and associated circuitry, and the control unit with its associated circuitry; 
         FIG. 14  is a block diagram showing the logic implemented by the circuitry of the mattress system to control a bed exit alarm; 
         FIG. 15  is a block diagram showing the logic implemented by the circuitry of the mattress system to control a lock out function provided in each of the user interface modules; 
         FIG. 16  is a perspective view of the control unit of  FIG. 1  showing a front shell of the main housing of the control unit pulled away from a back shell of the main housing, a compressor sub-housing situated in the lower region of the back shell, and circuitry of the control unit situated in the back shell above the compressor sub-housing. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     A mattress system  10  includes a mattress  12  and a control unit  14  that is spaced-apart from the mattress  12  and that is coupled pneumatically and electrically with the mattress  12  by a connector assembly  16  as shown in  FIG. 1 . As is discussed in greater detail below, the control unit  14  includes a plurality of user interface modules  50 ,  52  that are able to be selectively coupled to a main housing  18  of the control unit  14  to configure the mattress system  10  with various functionalities depending upon which of the user interface modules  50 ,  52  is coupled to the main housing. For example,  FIG. 3  shows a user interface module  50  having a primary mode of operation that is referred to as a continuous low pressure mode and  FIG. 4  shows a user interface module  52  having a primary mode of operation that is referred to as an alternating pressure therapy mode. The illustrative mattress system  10  is available commercially and is marketed as the ClinActiv™ Therapy Mattress System by Hill-Rom Company, Inc. which is an affiliate corporation of the assignee of the present application. 
     Illustratively, the mattress  12  includes an upper inflatable bladder layer  20  having a plurality of laterally-extending cells or bladders  22  which cooperate to define various zones of layer  20 . For example, a head section zone includes the first three cells  22  of layer  20 . These three cells  22  are fluidly interconnected and are illustratively regulated to the same pressure. The next ten cells  22  form a torso section zone of layer  20 . Finally, a heel section zone includes the last seven cells  22  of layer  20  which are fluidly interconnected with each other and are generally regulated to a low target pressure. This target pressure is independent of the rest of the zones of the layer  20  and is regulated by electronics within the control unit  14 . 
     When system  10  is operating in the continuous low pressure mode, as dictated by module  50 , the ten cells  22  of the torso section zone are controlled to the same target pressure. However, when system  10  is operating in the alternating pressure mode, as dictated by module  52 , a first group of five of the ten cells  22  of the torso section zone are deflated for a period of time while a second group of five of the ten cells  22  of the torso section zone are inflated and then, after the period of time and after a dwell time in which all ten cells  22  of the torso zone section are inflated, the first group of five of the ten cells  22  of the torso zone section are inflated for a period of time while the second group of five cells  22  of the torso section zone are deflated for a period of time. This sequence then repeats after another dwell time in which all ten cells  22  of the torso section zone are inflated. The cells  22  of the two groups of torso section zone cells  22  are arranged such that each cell of the first group is situated between adjacent cells of the second group, and vice versa, with the exception of the cells that are the head end and foot end of the torso section zone. In alternative embodiments, other cells of layer  20  also inflate and deflate alternately in addition to the alternating of the inflation and deflation of the cells  22  of the torso zone section. 
     In the illustrative example, the head, torso, and heel section zones of the layer  20  are controlled and operate independently from one another. Further, while the various head, torso, and heel section zones are described herein as including a particular number of laterally-extending cells, it is within the scope of this disclosure to include any number of independently operable zones having any number of interconnected cells, and having cells of any suitable size, shape, or orientation, including cells that extend longitudinally. 
     The mattress  12  further includes a first air mattress underlay  24  positioned below the layer  20  as well as a second air mattress underlay  26  positioned below the first air mattress underlay  24 . Thus, underlay  24  serves as an intermediate layer between layer  20  and underlay  26 . Underlay  24  is a single inflatable bladder but has internal walls or connections, such as radio frequency welds between top and bottom sheets of the underlay  24 , running in the longitudinal direction of mattress  12  to prevent ballooning of underlay  24 . Similarly, underlay  26  is also a single inflatable bladder but has internal walls or connections running in the lateral direction of mattress  12  to prevent ballooning of underlay  26 . In the illustrative embodiment, the first air mattress underlay  24  and the second air mattress underlay  26  are interconnected via conduits with each other and with the head section zone of layer  20  and are inflated to the same pressure as the head section zone of the layer  20 . The mattress  12  further includes a top coverlet  28  and a bottom coverlet  30  which are coupled together by suitable couplers, such as one or more zippers, to maintain the layer  20  and underlays  24 ,  26  within an interior region of the mattress  12 . 
     Layer  20  and underlays  24 ,  26  are inflatable patient support elements of illustrative mattress  12 . It should be appreciated that the constructional details of elements  20 ,  24 ,  26  are merely illustrative and mattress  12  may have inflatable elements of other shapes, sizes, or orientations. Furthermore, mattress  12  may have non-inflatable patient support elements, such as foam layers, blocks, pads, and the like, as well as mesh materials, gel layers, quilting, and the like, in lieu of or in addition to elements  20 ,  24 ,  26 . Accordingly, mattresses having no inflatable elements at all are within the scope of this disclosure. 
     The illustrative mattress  12  further includes a pressure sensor  32  and a technical box  34  containing an air distribution system  35  which includes a set of valves  37 , associated circuitry  39 , and a manifold assembly  33  in fluid communication with the various cells  22  of layer  20  as well as with the first and second air mattress underlays  24 ,  26  as shown diagrammatically in  FIG. 13 . The manifold assembly  33  includes a pneumatic input and a plurality of pneumatic outputs, each of which is associated its own valve of the set of valves  37 . In one embodiment, a first valve is in fluid communication with the group of five bladders  22  of the torso section zone while a second valve is in fluid communication with the second group of five bladders  22  of the torso section zone. The first and second sets of bladders  22  in the torso section zone are arranged alternately and the first and second valves  37  are controlled to inflate and deflate the first and second groups of bladders  22  of the torso section zone alternately when mattress system  10  is operating in the alternating pressure mode as dictated by the programming of user interface module  52  as mentioned above. If, on the other hand, user interface module  50  is coupled to the main housing  18  of control unit  14 , then the first and second valves  37  are controlled so that all ten of the bladders  22  of the torso zone section remain inflated simultaneously to the same target pressure. 
     The set of valves  37  includes a third valve in fluid communication with the heel zone of the layer  20  and a fourth valve that is in communication with the head section zone of the layer  20 , the first mattress underlay  24 , and the second mattress underlay  26 . The valves  37  operate to open and close various passageways of the manifold assembly  33  as is appropriate to allow inflation or deflation of the associated groups of bladders of mattress  12 . In some embodiments, each of the valves  37  is a three-way valve having an opened position allowing pressurized air to be pumped to the associated bladders  22 ; a closed position blocking air from being pumped to, and blocking air from escaping from, the associated bladders  22  and underlays  24 ,  26 ; and a vent position allowing air to vent from bladders  22  and underlays  24 ,  26  to atmosphere. In other embodiments, one more of the valves  37  are two position valves and a separate vent valve is provided to vent air from each of the various groups of bladders as needed. In some embodiments, the valves  37  are solenoid valves. However, other suitable valves, such as proportional control valves, may be used if desired. The technical box  34  further houses circuitry  39 , referred to herein sometimes as the manifold printed circuit board (PCB) for controlling the operation of the valves  37  within the technical box  34 . As is discussed in greater detail below, the manifold PCB is in electrical communication with the control unit  14 . 
     Illustratively, the pressure sensor  32  is positioned within a cut-out section  36  formed in the second air mattress underlay  26  as shown in  FIG. 1  while the technical box  34  is positioned at a foot end of the mattress  12  such that foot ends of the air mattress underlays  24 ,  26  generally abut the technical box but do not extend over the top of the technical box  34 . However, the layer  20  (and particularly the heel zone bladders  22  of the layer  20 ) extends over the technical box  34 . When the mattress  12  is assembled, the pressure sensor  32  of the mattress  12  is positioned generally below the torso section zone of the layer  20  in order to sense the pressure exerted by a patient on the torso section zone and provide an output signal to the control unit  14  indicating the sensed pressure. 
     Mattress  12  includes a cardiopulmonary resuscitation (CPR) assembly that is coupled to the technical box  34  and that includes a rotatable knob  40  that is accessible to the caregiver through an aperture  42  formed in the foot end portion of the bottom coverlet  30 . In normal operation, the CPR knob  40  is in a closed position. When the CPR knob  40  is moved manually by a caregiver to an opened position, air rapidly vents through the CPR assembly to atmosphere. In addition, an electrical signal is provided by a switch or other sensor to circuitry  39 , the circuitry of control unit  14 , and the circuitry of sensor  32  to indicate that the CPR knob  40  has been moved to the opened position. In response to this signal, control unit  14  deactivates operation of the source of a pneumatic pressure  43 , such as a compressor, of the control unit  14 . A CPR indicator light flashes on the control unit  14  and a CPR alarm sound beeps regularly every 30 seconds to alert the caregiver that the CPR knob  40  has been activated. Once activated, the bladders of layer  20  and underlays  24 ,  26  quickly deflate to provide a firm surface for performing CPR on the patient lying atop the mattress  12 . 
     Illustratively, the pressure sensor  32  of the mattress  12  comprises a liquid-containing flexible enclosure  45  and associated circuitry  47  as shown in  FIGS. 1 and 13 . Enclosure  45  and circuitry  47  are both situated in the cut-out section  36  of underlay  26 . In some embodiments, the liquid-containing enclosure  45  comprises a bag or bladder filled with a silicon oil, such as polydimethylsiloxane. Circuitry  47  includes a pressure transducer which is exposed to the fluid pressure in the bag via a conduit. The transducer of circuitry  47  detects the pressure exerted upon it by the liquid and relays a pressure signal to the control unit  14  via circuitry  47  and circuitry  39 . The pressure sensed by the transducer correlates to the amount of weight and pressure exerted on enclosure  45 , which correlates to the weight of the patient on the mattress. As is discussed in greater detail below, circuitry within the control unit  14  operates to establish the target pressures of the zones of layer  20  and underlays  24 ,  26  and also determines whether one or more zones of the layer  20  are to be inflated or deflated based upon information received from the pressure signal. The principles of operation of the pressure sensor  32  is described in greater detail in U.S. Pat. No. 6,094,762 which is owned by the same assignee as the present application and which is hereby incorporated by reference herein. 
     As mentioned above, the pressure sensor  32  is positioned below the torso section zone of layer  20 . Accordingly, as the patient&#39;s torso applies pressure on the mattress  12 , this pressure is transferred as a force to the enclosure  45  of pressure sensor  32 . The pressure in the enclosure  45  is detected by the transducer of circuitry  47  and a pressure signal is sent to the control unit  14 . Circuitry within the control unit  14  then determines whether to vent air from the layer  20  and/or underlays  24 ,  26  of the mattress  12  or to turn on the pneumatic source  43  within the control unit  14  to inflate the layer  20  and/or underlays  24 ,  26  in order to reach respective target pressures which are established by control unit  14  based on the signal received from sensor  32 . Accordingly, the target pressures of cells  22  of layer  20  and underlays  24 ,  26  of the mattress  12  are adjusted automatically to accommodate each individual patient&#39;s weight and position. In some embodiments, a pressure comparator is provided in circuitry  47  to make a comparison between the pressure in enclosure  45  and the pressure in the head section zone bladders  22  of layer  20  and the underlays  24 ,  26  and then control unit  14  and valves  37  are operated to equalize the pressure therebetween. In such an arrangement, any additional pressure sensor to sense the pressure in head section zone bladders  22  of layer  22  and underlays  24 ,  26  may be omitted. 
     In some embodiments, additional pressure sensors  41  are provided in technical box  34  and are associated with respective pneumatic lines used to inflate layer  20  and underlays  24 ,  26 . The additional pressure sensors detect and communicate the pressure within each zone of the layer  20  and within the underlays  24 ,  26  to the control unit  14 . Such measured pressures are compared with the target pressure values established by the circuitry of control unit  14  based on the pressure signal originating from pressure sensor  32 . In other embodiments, technical box  34  has only a single pressure sensor  41  which is coupled sequentially, by appropriate opening and closing of the various valves  37 , to each of the zones of layer  22  and to underlays  24 ,  26  to measure sequentially the pressures in each of the zones. 
     The data from pressure sensor  32  is used in three distinct operation modes of system  10  including stand-by regulation, patient egress surveillance, and main zone pressure regulation. As is discussed in greater detail below, the stand-by regulation mode means that system  10  operates to achieve a relatively low pressure in the mattress  12  so as to provide a ready-to-use, and yet comfortable, support when the patient is initially placed on the mattress  12 . The patient egress surveillance mode means that system  10  operates to monitor the presence and absence of the patient on the mattress  12 . The main zone pressure regulation mode means that system  10  operates to control the internal pressures in the bladders  22  of layer  20  and underlays  24 ,  26 . Illustratively, the circuitry  47  of pressure sensor  32  is in electrical communication with the circuitry  39  situated within the technical box  34 . As noted above, the circuitry  39  within the technical box  34  is in electrical communication with the control unit  14  via the connector assembly  16 . 
     Referring now to  FIGS. 2-6 , the control unit  14  includes the main housing  18 , a source of pneumatic pressure  43  such as a pump, blower, or compressor (shown diagrammatically in  FIG. 13 ) contained within the main housing  18 , and first and second user interface modules  50 ,  52  that are selectively coupleable to the main housing  18 . The compressor  43  of the control unit  14  is enclosed in a sub-housing  49 , shown in  FIG. 16 , within the main housing  18  of the control unit  14  to dampen noise while the compressor  43  is running. Power is provided to unit  14  via a power cord  19 , shown in  FIG. 1 , which plugs into unit  14  and into a standard electrical outlet to receive power therefrom. Illustratively, the sub-housing  49  is made of aluminum; however, is should be understood that the sub-housing  49  may be made of other suitable materials to dampen the noise of the compressor  43  contained therein. Further, an air chamber  51  of the control unit  14  is positioned generally in top region of the compressor sub-housing  49  in order to serve as a pressure reservoir that stabilizes the air flow within the control unit  14  as well as reduce noise output of the control unit  14 . 
     Each of the first and second user interface modules  50 ,  52  are programmed to control inflation of the layer  20  and/or underlays  24 ,  26  of the mattress  12  differently in various operational modes of each of interface module  50 ,  52 . Illustratively, the front side of the main housing  18  includes a recess  54  provided therein. The interface modules  50 ,  52  are each selectively received within the recess  54  and are locked in place by a latch  90  of the control unit  14 , as is discussed in greater detail below with regard to  FIGS. 7   a  and  7   b . The user interface modules  50 ,  52  operate as the main caregiver controls of the mattress system  10  while also providing feedback to the caregiver as to the operational status of the system  10 . Each of the modules  50 ,  52  includes circuitry that is programmed with instructions that dictate the operational modes of system  10 . 
     Referring to  FIG. 3 , the first user interface module  50  includes a user interface panel  55  having a front surface  56  and a main body  57  that is coupled to the user interface panel  55  and that has a rear surface  58 . Illustratively, the first user interface module  50  is a continuous low pressure (CLP) therapy module. The CLP mode of the module  50  is automatically activated when the CLP module  50  is placed in the recess  54  of the control unit  14  and the main switch  53  of control unit  14  (see  FIG. 16 ) is switched on. Illustratively, the CLP module  50  and module  52  each include an electrical connector  59  on the rear surface  58  of the main housing  57  as shown in  FIG. 5  with regard to module  52 . The electrical connector  59  is coupled to an electrical coupler  61 , shown in  FIG. 2 , within the recess  54  of the control unit  14  to provide electrical communication between the module  50  and internal circuitry  63 , shown in  FIG. 16 , of the control unit  14 . A ribbon cable  65 , also shown in  FIG. 16 , interconnects connector  61  and circuitry  63 . 
     During the CLP mode, the ten cells  22  in the torso section of the layer  20  are regulated to the same target pressure (all targets pressures referred to herein have associated tolerance ranges as is well known in the art) as set by the pressure sensor  32 . As mentioned above, the three head section cells  22  of the layer  20 , the first underlay mattress  24 , and the second underlay mattress  26  are fluidly interconnected with each other and are inflated to the same pressure as the cells  22  of the torso section of the layer  20  during the CLP mode. The seven foot section cells  22  of the layer  20  are regulated to a lower target pressure which is determined independently of the rest of the mattress sections. 
     Various user inputs and indicators are provided on the front surface  56  of the user input panel  55  of the module  50  as shown in  FIG. 3 . For example, an on/off button  58  is engaged by a user to turn the module  50  on or off. A light indicator  60  illuminates to denote that the module  50  has been turned on. Upon first connecting the control unit  14  to the main power supply, the mattress system  10  operates to automatically inflate the layer  20  and underlays  24 ,  26  to a maximum inflation pressure value to fully expand the cells  22  and underlays  24 ,  26 . The mattress system  10  then automatically reverts to the therapy mode provided by the selected module  50  or  52  after approximately eight minutes. During start up, the caregiver may press the on/off button  58  twice to override the maximum inflation function, if desired. 
     An internal pressure indicator provides an indication of the pressure variation in the torso zone of the layer  20 . Segments of the light indicator  62  will illuminate as the pressure within the torso zone automatically varies. A maximum inflation (P-max) button  64  is engaged by user to inflate layer  20  and underlays  24 ,  26  to the maximum inflation pressure at the user&#39;s discretion. Thus, the maximum inflation button  64  may then be activated at any time in order to inflate the layer  20  and underlays  24 ,  26  to the maximum inflation pressure. Oftentimes, for example, a caregiver may use the maximum inflate button  64  in order to provide a firmer patient support surface for patient transfers or bedding changes. Illustratively, pressing the maximum inflation button  64  a second time will immediately end the maximum inflation mode and return the mattress  12  to the therapy mode. The maximum inflation pressure is a target pressure that is typically higher than the pressures to which layer  20  and/or underlays  24 ,  26  are inflated during other modes of operation of system  10  and it should be understood that layer  20  and underlays  24 ,  26  are capable of withstanding even higher pressures than the one referred to as the maximum inflation pressure in this disclosure. 
     Pressing a bed exit alarm button  66  of the user interface module  50  arms or enables a patient egress surveillance system (sometimes referred to herein as a “bed exit system” or “bed exit alarm system” or the like). As is discussed in greater detail below, the pressure sensor  32  of the mattress  12  operates to detect the presence and absence of a patient on the mattress  12  by sending a pressure signal to the control unit  14 . Circuitry of the control unit  14  then determines, based upon the value of the pressure signal, whether the patient is present on, or absent from, the mattress  12 . The determination regarding the presence of absence of the patient is made by comparing the pressure sensed by sensor  32  with a threshold value. If it is determined that the patient is absent from the mattress  12 , an alarm to alert a caregiver is activated if the bed exit alarm system is enabled. The alarm may be a visual alarm such as a light indicator on the module  50  and/or may be an audible alarm such as a beeping noise made by a sound-producing device such as a speaker or piezoelectric buzzer, for example. When the bed exit alarm function is enabled, a light indicator  68  on the module  50  is illuminated. 
     An alarm silence button  70  of the module  50  may be engaged by a user to temporarily silence any audible alarm associated with the maximum inflation function, the CPR function, a power failure, and/or a mattress malfunction. For example, when one or more of the above-referenced alarms are sounding, the caregiver may press the alarm silence button  70  to temporarily silence the sounding alarm(s). The alarm(s) will then sound again after approximately 10 minutes if the condition causing the alarm is not rectified. The alarm(s) may also be permanently silenced by pressing the alarm silence button  70  until the light indicator  72  flashes. In some embodiments, the alarm silence button  70  also may be engaged by the user to silence a bed exit alarm. 
     A CPR light indicator  74  of the module  50  flashes when the CPR function, discussed above, is activated. A seat cushion light indicator  76  illuminates when an optional seat cushion (not shown) of the mattress system  10  is being inflated. The indicator  76  turns off after approximately five minutes once the seat cushion is completely inflated. Illustratively, during inflation of the seat cushion, the maximum inflation function is automatically activated without audible indication. The seat cushion inflation may be stopped at any time by pressing the maximum inflation button  64 . Once inflated, the seat cushion is disconnected from control unit  14  and remains inflated for use on a wheel chair or other type of seating surface. A valve coupled to the seat cushion may be open to allow deflation of the seat cushion to atmosphere. 
     A safety lock-out light indicator  78  of the module  50  illuminates whenever the user interface module  50  is locked-out to prevent a patient or caregiver from adjusting the settings. Illustratively, as shown in  FIG. 5  with regard to module  52 , the rear surface of the module  50  and the module  52  each includes a lock-out switch  80 . The caregiver may move the lock-out switch  80  to an activated position in order to lock-out the controls of the module  50  or the module  52 , as the case may be, to prevent a patient or other caregiver from tampering with the particular settings of the module  50 ,  52 , thereby to prevent intentional or unintentional changes in the operating mode of system  10 . Illustratively, the lock-out switch  80  is located on the rear surface  58  of the module  50 ,  52  and is, therefore, generally inaccessible by the patient. The safety lock-out indicator  78  is illuminated when the lock-out switch  80  has been activated. Illustratively, the safety lock-out indicator light  78  will then flash in the event of a control button being pressed in order to signal that the user interface is locked-out. 
     To unlock the lock-out function, the caregiver removes the module  50  from the housing  18  of the control unit  14  and moves the lock-out switch  80  to the deactivated or unlocked position. Alternatively, the caregiver may temporarily unlock the lock-out function by removing the module  50  from the housing  18  and pressing a lock-out button  81  adjacent the switch  80 . Illustratively, pressing the lock-out button  81  (while maintaining the switch  80  in the activated position) operates to deactivate the lock-out function for a predetermined period of time, such as 20 seconds, for example. This function allows the user then to resecure the module  50  to the housing  18  and manipulate the controls of the module  50  without having to again remove the module  50  from the housing to reinitiate the safety lock-out function. Once the caregiver has manipulated the controls of the module  50 , the lock-out function will be automatically reactivated once the predetermined period of time has elapsed. 
     A power failure light indicator  82  illuminates when the control unit  14  is disconnected from the main power supply or in the event of a power failure. The power failure light indicator  82  will also illuminate during transport. A mattress malfunction light indicator  84  of module  50  illuminates in the event of a pressure fault and a continuous low pressure (CLP) therapy indicator  86  of module  50  illuminates to indicate that CLP therapy is being used. 
     Illustratively, as noted above, an audible alarm is sounded, as well as a visual indicator being illuminated, when the maximum inflation function has been activated, when the CPR function has been activated, when a patient has exited the bed (assuming the bed exit alarm system is enabled), when the control unit  14  has been disconnected from the main power supply or when a power failure has occurred, and when a mattress defect is detected. 
     Referring now to  FIG. 4 , the second user interface module  52  similarly includes the user interface panel  55  having the front surface  56  and the main body  57  having the rear surface  58 . Illustratively the second user interface module  52  is an alternating pressure (AP) therapy module. The AP therapy mode is automatically activated when the AP module  52  is placed within the recess  54  of the main housing  18  of the control unit  14 . Illustratively, the AP mode affects the ten cells  22  within the torso section zone of the layer  20  to sequentially inflate and deflate every other cell  22  within the torso section zone. Accordingly, none of the three head section cells  22  and none of the seven heel section cells  22  are alternately inflated and deflated in the illustrative embodiment, but may do so in other embodiments. 
     As noted above, the pressure in the torso section cells  22  is controlled by the pressure sensor  32 . Illustratively, the AP mode may be described in three phases. In the first phase, five non-adjacent cells  22  (forming a first AP zone of cells) in the torso section zone of the layer  20  are deflated. This deflation takes approximately four minutes. In the second phase, the pressure in all the cells  22  within the torso section is the same (i.e., at a continuous low pressure). The duration of the second phase is approximately 1 minute. The purpose of the second phase of the AP therapy mode is to enhance patient comfort by providing a soft surface between the deflated phases. In the third phase, the other five non-adjacent cells  22  (forming a second AP zone of cells) in the torso section zone of the layer  20  are deflated. Similar to the first phase, the third phase lasts approximately four minutes. A similar discussion of the AP mode is presented above. 
     Various user inputs and indicators are provided on the front surface  56  of the user input panel  55  of the module  52 . Illustratively, many of these user inputs are the same as those discussed above with respect to the first user interface module  50 . As such, like reference numerals have been used to denote like inputs and/or indicators. However, rather than including the CLP indicator  86  of the module  50 , the module  52  includes an alternating pressure (AP) therapy indicator  88  to indicate that AP therapy is being used. 
     Although the CLP user interface module  50  and the AP user interface module  52  are provided in the illustrative examples, it is within the scope of this disclosure for the mattress system  10  to include other user interface modules that are selectively coupleable with the control unit  14  as well. For example, a rotation therapy user interface module may provide a continuous lateral rotation therapy (CLRT) function and a low air loss user interface module may provide a low air loss function for cooling the patient and/or wicking moisture away from the patient. Of course, it should be understood that any number of user interface modules having any number of therapy functions may be provided for use with the control unit  14  and mattress  12  of the mattress system  10 . Further, it is within the scope of this disclosure to include more than one therapy function on a single user interface module. For example, while each of the CLP and AP modules  50 ,  52  are dedicated to a single therapy, a single user interface module may be provided which includes both CLP and AP therapies. Accordingly, any number and any combination of desired therapies may be provided on a single user interface module. Each user interface module includes circuitry that is programmed to implement its associated functionalities. Thus, the programming of the various types of modules is different. This provides a cost savings to healthcare facilities because, to achieve different types of patient therapies, only the associated user interface modules need to be purchased rather than having to purchase an entirely different mattress system. 
     Each of the user interface modules  50 ,  52  further includes a locking post  92  extending from a first end of  94  the main body  57  of the respective module  50 ,  52 , as shown in  FIGS. 3 and 4 . Referring now to  FIG. 5 , a pair of tabs  96  of each user interface module  50 ,  52  extend outwardly from a second end  98  of the main body  57  of the respective module  50 ,  52 . Thus, post  92  and tabs  96  extend from main body  57  in opposite directions. As is discussed below, the locking post  92  and tabs  96  are used to secure the respective modules  50 ,  52  within the recess  54  formed in the main housing  18  of the control unit  14 . 
     As noted above, the user interface modules  50 ,  52  may be selectively attached to and removed from the main housing  18  of the control unit  14 . In order to attach one of the first and second user interface modules  50 ,  52  to the main housing  18  of the control unit  14 , the caregiver slides the pair of tabs  96  of the selected module  50  or  52  into corresponding slots  67 , shown in  FIG. 2  (in phantom), formed in the main housing  18  of the control unit  14 . In the illustrative embodiment, the slots of the housing  18  are formed in a side wall of housing  18  defining part of the recess  54 . Once the tabs  96  of the selected module  50  or  52  are received within the corresponding slots of the housing  18 , the main body  57  of the selected module  50  or  52  may further moved into the recess  54  such that the locking post  92  is received within a slot  97  defined by hook portion  98  of a pivotable cam  99  coupled to the latch  90  to further secure the selected module  50  or  52  within the housing  18 . Illustratively, as shown in  FIGS. 7   a  and  7   b , movement of the latch  90  in a first direction  101  operates to rotate the cam  99  in a clockwise direction such that the slot  97  of the hook portion  98  captures the post  92  of the selected module  50  or  52  therein (as shown in  FIG. 7   b ). 
     In order to remove one of the first and second user interface modules  50 ,  52  from the control unit  14 , the caregiver may rotate the latch  90  located at the rear side of the control unit housing  18  in direction  103  shown in  FIG. 7   b  from a first, locked position to a second, unlocked position. Illustratively, movement of the latch  90  in direction  103  operates to rotate the cam  99  in a counterclockwise direction such that the post  92  of the selected module  50  or  52  becomes disengaged from the slot  97  of the hook portion  98  (as shown in  FIG. 7   a ). When the latch  90  is in the second, unlocked position, hook  98  of the cam  99  is disengaged from the post  92  of the selected module  50  or  52  to allow the caregiver to remove the particular user interface module  50  or  52  from the main housing  18  of the control unit  14 . 
     The shape of slot  97  is such that when latch  90  is rotated in direction  101 , with hook portion  98  capturing post  92 , post  92  is pulled generally radially inwardly toward the pivot axis of latch  90  to firmly seat the associated module  50 ,  52  against the main housing  18  and to pull the electrical connectors  59 ,  61  together. On the other hand, when latch  90  is rotated in direction  103 , with hook portion  98  capturing post  92 , post  92  is pushed generally radially outwardly way from the pivot axis of latch  90  to unseat the associated module  50 ,  52  from housing and to push the electrical connectors  59 ,  61  apart. After being unseated in this manner, the module  50 ,  52  is far enough away from main housing  18  to provide space for a user to grasp the module  50 ,  52 , as shown in  FIG. 7   a  with regard to module  50 , and completely detach it from housing  18 . 
     Referring now to  FIG. 8 , the control unit  14  further includes a socket  100  formed in a rear wall  102  of the main housing  18  as well as a bracket assembly or coupling hook  104  coupled to the rear side  102  of the main housing  18 . As is discussed in greater detail below, the socket  100  of control unit  14  receives a dual mode plug  110  of the connector assembly  16  therein. Illustratively, the coupling hook  104  is positioned below the socket  100  of the main housing  18  and is provided to allow a caregiver to hang the control unit  14  on a footboard or side rail of a bed frame, for example. Hook  104  has a cutout  101  to accommodate the dual mode plug  110  and a user&#39;s fingers when attaching the connector assembly  16  to, or detaching the connector assembly  16  from, housing  18 . The control unit  14  also includes rubber pads (not shown) on a bottom surface of the main housing  18  in order to enable the control unit  14  to stand on a hard surface as well. 
     Referring now to  FIGS. 9-12 , the connector assembly  16  of the mattress system  10  provides a pneumatic and electrical connection between the control unit  14  and the mattress  12 . The connector assembly  16  includes a dual lumen hose  118  including a pneumatic line  120  for communication of pneumatic pressure from the control unit  14  to the mattress  12  and an electrical line  122  that serves as an electrical conduit through which electrical conductors are routed between the control unit  14  and the mattress  12 . Illustratively, the pneumatic line  120  and the electrical line  122  are connected to each other and positioned side-by-side along a majority of the length of the lines  120 ,  122 , as shown in  FIG. 9 , for example. 
     The connector assembly  16  further includes the dual mode plug  110  at a first end of the dual lumen hose  118 . The dual mode plug  110  is received within the socket  100  of the main housing  18  in order to provide a substantially simultaneous pneumatic and electrical connection with the control unit  14 . Illustratively, the dual mode plug  110  includes a plug housing  124  and two latch members  126  coupled to respective sides of the plug housing  124 . In particular, the plug housing  124  includes a pair of recesses  127  formed in each side of the plug housing  124  such that a portion of each latch member  126  is received within a respective recess  127 . Each of the latch members  126  are movable between a first position locking the dual mode plug  110  to the main housing  18  of the control unit  14  and a second position unlocking the dual mode plug  110  from the main housing  18  of the control unit  14 . Illustratively, the latch members  126  are spring-biased toward the first position by respective springs  128 , shown in  FIG. 10 . Further, in order to move the latch members  126  between the first and second positions, a caregiver squeezes each latch member  126  in a direction generally toward the plug housing  124  and further within the respective recesses  127 . 
     Referring still to  FIG. 10 , the dual mode plug  110  further includes a front wall  130  coupled to the housing  124 , a pneumatic coupler  132  extending away from the front wall  130 , and an electric coupler  134  that extends beyond front wall  130  through an aperture  133  formed therein. A first end of each of the electrical conductors routed through line  122  is coupled to, and terminates at, electrical connector  134 . The control unit  14  includes an electrical connector  107  (shown diagrammatically in  FIG. 13 ) within the socket  100  which is in communication with the internal circuitry  63  control housing  14  and a pneumatic port  109  (also shown diagrammatically in  FIG. 13 ) within the socket  100  which is in communication with the compressor  43  of the control unit  14 . Accordingly, when the dual mode plug  110  is received within the socket  100  of the control unit  14 , the pneumatic coupler  132  of the plug  110  is coupled to the pneumatic port  109  of the control unit  14  and the electric coupler  134  of the plug  110  is coupled to the electrical connector  107  of the control unit  14 . Further, the dual mode plug  110  is configured so that the electrical and pneumatic connections between the connector assembly  16  and the control unit  14  are made substantially simultaneously. 
     A check valve  131  is provided within housing  124  of plug  110  and is in pneumatic communication with port  132 . When plug  110  is coupled to socket  100  and compressor  43  is operated, the check valve  131  is opened to allow pressurized air to move through pneumatic line  120 . When plug  110  is disconnected from socket  100 , check valve  131  is closed to prevent air from escaping from mattress  14  through connector assembly  16 . 
     Latch members  126  each have a rearwardly projecting tab  111  that is captured within a rear pocket  121  of the respective recess  127  and a forwardly projecting tab  113  that extends through a respective aperture  115  of front wall  130 . Apertures  115  are oversized in the lateral direction to allow movement of tabs  113  therein when latch members  126  move between the first and second positions. Each latch member  126  also has a laterally inwardly projecting tab  117  that is received in an associated slot  119  formed in a respective side of housing  124 . Receipt of tabs  111 ,  113  in the associated pockets  121  and apertures  115  retains latch members  126  in place within recess  127 , whereas receipt of tabs  117  in slots  119  guides the lateral movement of latch members  126 . Each of latch members  126  further has a laterally outwardly projecting tabs  123  that is received in associated tab-receiving recess or pocket  125 , shown in  FIG. 8  (in phantom), formed in the sidewalls of socket  100 . Receipt of tabs  123  in recesses  125  locks plug  110  to the socket  100  of the housing  18  of the control unit  14 . When latch members  126  are squeezed toward housing  124  of plug  110 , tabs  123  are withdrawn from recesses  125 , thereby allowing plug  110  to be disconnected from socket  100 . 
     Referring now to  FIGS. 9 and 11 , the connector assembly  16  further includes a pneumatic coupler  140  mounted to the second end of the pneumatic line  120  and an electrical connector  142  mounted to an end of a cable  141  of the electrical conductors which exit from the second end of the electrical line  122 . As shown in  FIG. 12 , the pneumatic coupler  140  is coupled to a pneumatic port  144  of the mattress  12 . Specifically, the technical box  34  includes the pneumatic port  140  which is in pneumatic communication with the inflatable layer  20  and underlays  24 ,  26  of the mattress  12  via the manifold assembly  35  within the technical box  34 . Further, the electrical connector  142  of the connector assembly  16  is coupled to an electrical connector  146  of the technical box  34  of the mattress  12 . The electrical connector  146  of the mattress  12  is in electrical communication with the electrical circuitry  39 ,  47  within the mattress  12 . Further illustratively, the technical box  34  includes a first pressure test port  150  for checking the pressure in the torso zone of the layer  20  and a second pressure test port  152  for checking the pressure in the heel zone of the layer  20 . Pressure transducers  41  of technical box  34  are coupled electrically to test ports  150 ,  152  to provide feedback information regarding the pressure within an associated zone of mattress  14 . 
     In operation, air enters the control unit  14  through an air filter (not shown) within the control housing  18  of the control unit  14 . The air then travels into the compressor inlet. Upon exiting the compressor  43 , the air travels through check valve  131  and the pneumatic line  120  of the connector assembly  16  and into the manifold assembly  35  located in the technical box  34  within an interior region of the mattress  12 . The air is then dispatched through valves  37  into the various inflatable mattress layers including the layer  20 , the first air mattress underlay  24 , and the second mattress underlay  26 . In the illustrative embodiment, the check valve  131  located inside plug  110  of the connector assembly  16  provides approximately four hours of inflation of the mattress  12  in the event the control unit  14  is disconnected from the main power supply or if the dual lumen hose  118  is disconnected from the control unit  14 . 
     As mentioned above, each of modules  50 ,  52  is programmed to provide system  10  with a bed exit alarm function.  FIG. 14  is a diagram which illustrates the logic of the bed exit alarm function of system  10 . Sensor  32  provides a signal to circuitry  47  which measures the pressure exerted on sensor  32  as indicated at block  160 . A comparator  162  receives the pressure measurement signal and also receives a threshold setting signal which is represented by block  164  in  FIG. 14 . In one embodiment, the threshold setting is established for a minimum patient weight of fifty pounds but, in other embodiments, other threshold settings may be established at the discretion of the system programmer or designer. 
     If the pressure measurement signal is less than the threshold value signal, which is an indication that the patient has exited the mattress  14 , then the output of comparator  162  is on (e.g., a high logic state), otherwise the output of comparator  162  is off (e.g., a low logic state). If the comparator is turned on, then system  10  automatically operates in the stand by mattress pressure regulation mode in which layer  20  and underlays  24 ,  26  are controlled to reduced pressure settings as indicated at block  166 . If the bed exit or patient egress alarm is enabled, as indicated at block  168  and the comparator is on, then the output of an AND gate  170  is on. The output of AND gate  170  is input to another AND gate  172  which has a second input from a 1 Hertz (Hz) clock  174 . Thus, if the output of AND gate  170  is on, then the output of AND gate  172  will be a 1 Hz signal that is fed to patient egress alarm indicator as indicated at block  176 . 
     As mentioned above the bed exit alarm may include a visual indicator, such as a light emitting diode (LED) and/or a sound producing device, such as a speaker or buzzer. In the illustrative embodiment, the 1 Hz signal being output from AND gate  172  will cause the visual indicator to flash at a rate of 1 Hz and will cause the sound producing device to beep at a rate of 1 Hz. Although, logical AND gates  170 ,  172  are used in  FIG. 14  to explain the operation of the bed exit alarm function of system  10 , it should be appreciated that the logical AND conditions may be implemented by software rather than by use of discrete logic gates, but use of discrete logic gates is within the scope of the disclosure as well. 
     As mentioned above, each of modules  50 ,  52  includes lock-out switch  80  and lock-out button  81  which are used to lock out various functions of the modules  50 ,  52  and to unlock the functions.  FIG. 15  is a diagram which illustrates the logic of the lock out function of system  10 . When switch  80  is in its position enabling various functions of system  10 , an output of an OR gate  178  is turned on (i.e., a high logic state) and is coupled to the input of a set of AND gates  180 , each of which is associated with a respective function as indicated generically at a “function A” block  182  and at a “function Z” block  184  in  FIG. 15 . A “function key A” switch  186  is coupled to the input of the AND gate  180  associated with function A and a “function key Z” switch  188  is coupled to the input of the AND gate  180  associated with function Z. Function A and function Z are intended to generically represent the functions of system  10 . The dotted lines appearing in  FIG. 15  between switches  186 ,  188  and between the output of OR gate  178  and the AND gate associated with function Z are intended to convey the notion that additional functions may be controlled in the same manner as the two that are illustrated. 
     When the output of OR gate  178  is on, which occurs when system  10  is enabled, and button  186  or button  188  is pressed, the associated AND gate has two high logic states at its input resulting in its associated output being turned on (i.e., a high logic state) to signal the operation of the associated function A or function Z. If switch  80  is in its position disabling or locking out the various functions of system  10 , and assuming button  81  is not pressed, then both inputs to OR gate  178  are at low logic states and the output of OR gate  178  is off (i.e., a low logic state) resulting in the output of the associated AND gates  180  being turned off, regardless of whether either of buttons  186 ,  188  is pressed. 
     If switch  80  is in its position locking out the various functions of system  10  and switch  81  is pressed, then a twenty second timer is activated, as indicated at block  190 , resulting in a high logic state being applied at one of the inputs of OR gate  178  for twenty seconds, thereby turning the output of OR gate  178  on, thereby enabling the various functions of system  10  for twenty seconds. During this twenty period, switches  186 ,  188  may be pressed to operate the associated function  182 ,  184 . After the expiration of the twenty second timer, the OR gate  178  once again receives two low logic inputs which turns off the output of the OR gate  178  thereby locking out the functions of system  10 . 
     Switches  186 ,  188  are coupled to the inputs of an OR gate  192  as shown in  FIG. 15 . The output of OR gate  192  is coupled to a lock light  194 . Lock light  194  is turned on by OR gate  192  when switch  80  is in its position locking out the functions of system  10  and lock light  194  flashes if either of switches  186 ,  188  is pressed when system  10  is locked out. Although, logical AND gates  180  and logical OR gates  178 ,  192  are used in  FIG. 15  to explain the operation of the lock out function of system  10 , it should be appreciated that the logical AND and logical OR conditions may be implemented by software rather than by use of discrete logic gates, but use of discrete logic gates is within the scope of the disclosure as well. 
     Although certain illustrative embodiments have been described in detail above, variations and modifications exist within the scope and spirit of this disclosure as described and as defined in the following claims.