reference now will be made in detail to the present preferred embodiments of the present invention , examples of which are illustrated in the accompanying drawings . as used herein , air tightly is a relative phrase that refers to essentially no air leakage at the operating air pressures of the present invention . the preferred embodiment of the modular low air loss patient support system is shown in fig1 and is generally designated by the numeral 20 . the patient support system of the present invention preferably includes a frame , indicated generally in fig1 by the numeral 30 , having at least one articulatable section 32 . the frame carries the components of the patient support system and typically has more than one articulatable section and preferably is mounted on castors for ease of movement in the hospital environment . the hydraulic lifting mechanisms for raising and lowering portions of the frame , including the articulatable sections of the frame , are conventional , and suitable ones are available from hillenbrand industries of batesville , ind ., sold under the hill - rom brand . in accordance with the present invention , a plurality , preferably seventeen in the illustrated embodiment ( fig1 and 13 ), of elongated inflatable sacks are provided . as shown in fig2 for example , each of the sacks 34 of the present invention preferably has a multi - chamber internal configuration , and preferably four chambers are provided . in one embodiment shown in the drawings , the shape of each inflated sack is generally rectangular and preferably has exterior dimensions thirty - two inches long , ten and one - half inches high , and four and one - half inches thick . the patient support surface of each sack is provided by a top 36 which measures four and one - half inches by thirty - two inches , and a bottom 38 ( fig3 ) is similarly dimensioned . depending upon their location on the patient support , the sack may include a plurality of pin holes ( not shown ) to allow a small amount of air to bleed from the sack . the diameters of the holes preferably are about fifty thousandths of an inch , but can be in the range of between eighteen to ninety thousandths of an inch . each exterior end 40 of each sack measures ten and one - half inches by four and one - half inches , and each exterior side 42 measures ten and one - half inches by thirty - two inches . each sack is preferably integrally formed of the same material , which should be gas - tight and capable of being heat sealed . the sacks preferably are formed of twill woven nylon which is coated with urethane on the surfaces forming the interior of the sack . the thickness of the urethane coating is in the range of three ten thousandths of an inch to two thousandths of an inch . vinyl or nylon coated with vinyl also would be a suitable material for the sack . unless the sacks are designed to be disposable , the material should be capable of being laundered . internally , the sack preferably is configured with four separately defined chambers . as shown in fig2 for example , the internal webs 44 of each sack preferably are integral with the outside walls of each sack , and are at least joined in airtight engagement therewith . an end chamber 46 is disposed at an opposite end of each sack . each end chamber is generally rectangular in shape with one of the narrow ends 48 formed by a portion of the top of the sack , and the opposite narrow end 50 formed by a portion of the bottom of the sack . as shown in fig5 for example , the narrow end of each end chamber forming a section of the sack bottom is provided with a sack air entrance opening 52 through the bottom of the sack . as shown in fig2 for example , each multi - chamber sack includes a pair of intermediate chambers 54 disposed between the end chambers . each intermediate chamber preferably is shaped as a right - angle pentahedron . each intermediate chamber 54 has a base wall 56 , an altitude wall 58 , a diagonal wall 60 , and two opposite triangular - shaped side walls 62 . each base wall , altitude wall , and diagonal wall has a generally rectangular shaped perimeter . each base wall 56 is connected at a right angle to each altitude wall 58 . each diagonal wall 60 is connected at one edge to each base wall and at an opposite edge to the altitude wall . the edges of each triangular side wall are connected to oppositely disposed edges of the base , altitude , and diagonal walls . as shown in fig2 for example , each intermediate chamber is disposed within each sack so that its diagonal wall faces toward the center of the sack and toward the other intermediate chamber . one of the intermediate chambers is disposed above the other intermediate chamber so that it becomes conveniently referred to as the upper intermediate chamber , while the other intermediate chamber becomes the lower intermediate chamber . the altitude wall of the upper intermediate chamber preferably is formed by a middle section of the top 36 of the sack 34 . the altitude wall of the lower intermediate chamber preferably is formed by the middle section of the bottom 38 of the sack 34 . as shown in fig1 for example , each sack preferably is disposed to extend transversely across the longitudinal centerline of the patient support , and the intermediate chambers are disposed in the center of each sack . thus , the intermediate chambers also are disposed to extend transversely across the longitudinal centerline of the patient support . as shown in fig2 for example , one of the intermediate chambers is disposed at least partly above the other intermediate chamber and preferably is disposed completely above the other intermediate chamber . because of the symmetrical position of each sack relative to the longitudinal centerline of the patient support system , one of the intermediate chambers is disposed predominantly to the left side of the centerline and has a minority portion disposed to the right side of the centerline . similarly , the other of the intermediate chambers is disposed predominantly to the right side of the longitudinal centerline of the patient support and has a minority portion disposed to the left of the centerline . each sack has a pair of restrictive flow passages , one connecting each of the end chambers to the adjacent intermediate chamber . as shown in fig2 for example , preferably a single web serves as a common wall of an end chamber and the base wall of the adjacent intermediate chamber . as shown in fig2 for example , each restrictive flow passage can be defined by a hole 64 through the web that is common to the intermediate chamber and the adjacent end chamber . hole 64 preferably is defined by a grommet having an opening therethrough and mounted in a web that forms both the base wall of an intermediate chamber and the vertically disposed internal side wall of the end chamber adjacent the intermediate chamber . the grommet is sized to ensure that the end chambers have filling priority over the intermediate chambers and thus are the first to fill with air and the last to collapse for want of air . for sacks dimensioned as described above for example , a grommet having a 1 / 4 inch diameter opening has been suitable for achieving the desired filling and emptying priority . in further accordance with the present invention , means are provided for supplying gas , preferably air , to each sack of the patient support system of the present invention . as embodied herein and shown schematically in fig1 for example , the means for supplying air to each sack preferably includes a blower 66 powered electrically by a motor which runs on a low direct current voltage such as 24 volts . the blower must be capable of supplying pressurized air to the sacks at pressures as high as 30 inches of standard water but should be capable of supplying pressures in a preferred range of 0 to 18 inches of standard water while operating in the blower &# 39 ; s optimum performance range . as shown in fig1 for example , a pressure transducer 246 measures the pressure at the blower outlet . the measured pressure signal is transmitted to a microprocessor ( described hereafter ) via a blower control circuit 67 and a circuit board 150 ( described hereafter ). blower 66 preferably is controlled by voltages supplied by a blower control circuit 67 which receives a control voltage signal from the microprocessor via a circuit board 150 . the microprocessor is preprogrammed to compare the pressure signal received from pressure transducer 246 to a desired pressure signal calculated by the microprocessor . depending upon the result of the comparison , the microprocessor regulates the power supply to the blower control circuit . however , the methodology used by the microprocessor to compare the calculated pressure to the measured pressure contains a built - in delay ( preferably about three seconds ) so that the response to changes in the measured blower pressure is not instantaneous . the deliberate time delay in the response to the measured blower pressure assures control loop stability and prevents unwarranted pressure fluctuations in the sacks . otherwise , instantaneous real time pressure corrections in response to the blower output pressure and control valve output pressure could cause pressure oscillations in the system . as embodied herein and shown in fig4 , and 14 , and schematically in fig1 and 13 , the means for supplying air to each sack preferably further includes a support member carried by the frame . the support member preferably is rigid to provide a rigid carrier on which to dispose sacks 34 and may comprise a plurality of separate non - integral sections so that a one - to - one correspondence exists between each support member section and each articulatable section of the frame . as shown in fig1 for example , each section of the rigid support member preferably comprises a modular support member 68 and defines a multi - layered plate 70 . each plate 70 preferably is thin and has a flat top surface 72 and an opposite bottom surface , which also preferably is flat . as shown in fig1 for example , each plate has an upper layer 74 , a lower layer 76 , and a middle layer 78 disposed between the upper and lower layers . as shown partially in fig4 for example , the three layers are sealed around the edges to form two opposed ends 80 and two opposed side edges 82 joining between the ends . as shown in fig4 and 13 for example , a plurality of inlet openings 84 are defined through at least one of the side edges 82 . as shown in fig1 for example , depending upon the relative position of the modular support member , some of the modular support members have a plurality of outlet openings 86 defined in an opposite side edge 82 . the modular support manifold of zone iv for example also has a plurality of outlet openings 86 defined through the other of the side edges , while the modular support manifold of zone v only has inlet openings 84 defined through one of the side edges 82 , and lacks outlet openings on the opposite side edge . as partially shown in fig4 for example , the inlet openings 84 of one plate 70 are engaged by fittings 88 and flexible hoses 90 to become connected to the outlet openings 86 of an adjacent modular support member . as shown in fig5 and 14 , and schematically in fig1 , for example , the upper layer defines a plurality of air sack supply openings 92 which extend through the top surface of each plate 70 , and preferably through all three layers of plate 70 . as shown in fig5 for example , these air sack supply openings 92 are used to hold a special connection fitting ( described hereafter ) that connects the air sacks to a supply of controlled pressurized air . as shown schematically in fig1 for example , at least one of the modular support members defines a seat sack support member 94 ( zone iii ) and includes a plurality of pressure control valve openings 96 defined through the lower layer 76 and extending through the bottom surface of the plate 70 . each pressure control valve opening 96 is configured to be connected to a pressure control valve ( described hereinafter ). each of the ten pressure control valve openings 96 shown in fig1 is schematically represented by a circle inscribed within a box . to avoid unnecessarily cluttering fig1 , only three of the pressure control openings are provided with designating numerals 96 . preferably , one end of a rigid elbow 98 ( fig7 and 8 ) has a flexible bellows ( not shown ) which is connected to each pressure control valve opening 96 , and the other end of the elbow is connected to the output end of the pressure control valve . the seat sack support member preferably includes at least one pressure control valve opening for each pressure control valve required by the particular configuration of the patient support system . each pressure control valve opening intersects with a channel ( described hereafter ) for supplying air to the air sacks . as shown in fig5 and 14 , and schematically in fig1 - 13 , for example , the layers of each plate 70 preferably combine to define a plurality of separated enclosed channels therethrough . in an alternative embodiment , the channels can be formed by discrete flexible tubes . the channels are airtight and perform the function of conduits for the transport of pressurized air from the source of pressurized air to the air sacks . the multi - layer construction of plate 70 allows some channels to cross one another without intersecting , if the air flow configuration requires same . as shown schematically in fig1 for example , some channels 100 connect one of the inlet openings 84 of plate 70 to one of the outlet openings 86 defined through the opposite side edge 82 of the plate 70 . some of the channels 102 connect one of the inlet openings 84 defined through one of the side edges 82 to one or more of the sack supply openings 92 defined through the top surface of the plate 70 of the modular support member . each air sack supply opening 92 communicates with at least one of the channels . other channels 104 include one of the pressure control valve openings 96 . as embodied herein and shown in fig2 and 5 for example , the means for supplying gas to the sacks preferably includes a hand - detachable airtight connection , an embodiment of same being designated generally in fig5 by the numeral 106 . the connection comprises two components , one secured to the air sack 34 , and the other secured to the modular support member 70 . the force required to insert one of the components into the other component and to disconnect the components from one another is low enough to permit these operations to be accomplished manually by hospital staff without difficulty . accordingly , both components of the hand - detachable connection 106 preferably are formed of a semi - rigid plastic material with an elastic o - ring 114 secured within the interior of a female connection fitting 108 . as shown in fig5 for example , the component secured to the modular support member comprises an elongated female connection fitting 108 having an exterior configured to engage airtightly with the air sack supply opening 92 defined through the plate 70 . a plenum 93 is defined between the exterior of fitting 108 and air sack supply opening 92 . a lower end of the connection fitting extends through the air sack supply opening 92 , and a locking nut 95 screws onto this end of the fitting to secure same within the air sack supply opening of the modular support member . the female connection fitting 108 has an interior configured with a hollow axially disposed coupling opening 110 , preferably a cylinder , to receive a coupling in airtight engagement therewith . a cylindrical poppet 97 is disposed in the cylindrical coupling opening and is configured to slide within the cylindrical coupling opening . poppet 97 is closed at one end , and a spring rests between the bottom 113 of the interior of fitting 108 and the interior of the closed end of poppet 97 . the spring - loaded poppet is thereby biased to seal off the entrance 111 of coupling opening 110 . the connection fitting further defines a fitting groove 112 completely around the interior of the fitting and preferably near the entrance 111 of coupling opening 110 . the connection fitting also includes a resiliently deformable flexible o - ring 114 held in the fitting groove 112 . as shown in fig5 for example , the coupling cylinder 110 defined in the interior of the connection fitting further includes a channel opening 116 defined therethrough and in a direction normal to the axis of the coupling cylinder 110 . because of plenum 93 , the connection fitting is always disposed in the air sack supply opening 92 so that the channel opening 116 communicates with the channel 102 that connects to the air sack supply opening 92 . as shown in fig2 , 5 , and 6 for example , the other component of the hand - detachable connection includes an elongated coupling 118 that is secured at one end to the air entrance opening 52 of the sack and extends outwardly from the sack . the coupling has an axial opening 120 defined therethrough to permit air to pass through same and between the interior and exterior of the sack . the exterior of coupling 118 is configured to be received within the interior of the connection fitting . the exterior of the coupling has a groove 122 therearound that is configured to seat around and seal against the deformable o - ring 114 of the connection fitting 108 therein when the coupling is inserted into the connection fitting in airtight engagement with the fitting . groove 122 provides a locking detent to mechanically lock and seal o - ring 114 therein . as shown in fig6 for example , the coupling is secured to extend from the air entrance opening 52 of the air sack with the aid of a grommet 126 and a retaining ring 125 . the grommet 126 is heat sealed to the fabric of the air sack on the interior surface of the air sack around the air entrance opening . the coupling extends through the grommet 126 and the air entrance opening . a pull tab 124 is fitted over the coupling and rests against the exterior surface of the air sack . alternative embodiments of pull 124 are shown in fig3 and 6 for example . a retaining ring 125 is passed over the coupling and mechanically locks against the coupling in air - tight engagement with the air sack . the pull tab 124 , which is sandwiched between retaining ring 125 and the sack , can be grasped by the hand of a person who desires to disconnect the coupling from the fitting . in this way , the material of the air sack need not be pulled during disconnection of the coupling from the fitting . this prevents tearing of the air sack near the air entrance opening during the disconnection of the coupling from the fitting . as shown in fig5 for example , connection fitting 108 preferably includes a poppet 97 that is a spring loaded cylindrical member disposed concentrically within coupling cylinder 110 so that one end of the spring 99 rests against the closed end of the poppet , and the other end of the spring rests against the bottom 113 of the interior of connection fitting 108 . thus , when coupling 118 is inserted into coupling cylinder 110 , coupling 118 depresses poppet 97 and connects channel opening 116 to axial opening 120 of coupling 118 . when no coupling 118 is inserted into coupling cylinder 110 , the spring forces the poppet to seal against o - ring 114 and thereby seal the coupling cylinder opening 110 at the entrance 111 thereof near the top layer 74 of plate 70 . this permits one sack to be detached while air is being supplied to the others without leakage of air through the coupling cylinder opening 110 . the sealing effect of the poppet also prevents fluids from entering the channels of plate 70 , and this is advantageous during cleaning of the upper surfaces of plate 70 . in keeping with the modular configuration of the patient support system of the present invention , the means for supplying air to each sack further preferably includes a modular manifold for distributing air from the blower to the sacks plugged into the modular sack support member . the modular manifold provides means for mounting at least two pressure control valves and for connecting same to a source of pressurized air and to an electric power source . because its elongated shape resembles a &# 34 ; log ,&# 34 ; such modular manifold is sometimes referred to as the log manifold , and one embodiment is designated by the numeral 128 in fig1 for example . log manifold 128 includes an elongated main body 130 that is hollow and defines a hollow chamber 132 within same . as shown in fig1 for example , main body 130 is shaped as a long rectangular tube which preferably is formed of aluminum or another light weight material such as a hard plastic or resin . as shown in fig1 , an air supply hose 134 , which suitably is one and one quarter inches in diameter , carries pressurized air from blower 66 to chamber 132 of main body 130 . a first end wall 136 is defined at one narrow end of main body 130 , and a second end wall ( not shown ) is defined at the opposite end of main body 130 . a conventional pressure check valve 138 such as shown in fig1 for example , is provided in each end wall to permit technicians to gauge the pressure inside chamber 132 . one section of main body 130 defines a mounting wall 140 on which a plurality of pressure control valves 162 ( such as shown in fig7 and 8 for example and described in detail hereafter ) can be mounted . a plurality of ports 142 are defined through the mounting wall and spaced sufficiently apart from one another to permit side - by - side mounting of pressure control valves 162 . each port 142 has a bushing 144 mounted therein . the bushing is configured to receive and secure a valve stem 146 ( fig8 ) of a pressure control valve 162 . as shown in fig7 for example , valve stem 146 typically has one or more o - rings 148 engage with bushing 144 to form an airtight connection that nonetheless is easily detachable and engageable , respectively , by manual removal and insertion of the pressure control valve . this permits easy removal and replacement of the valve and reduces repair time and inoperative time for the patient support system as a whole . the log manifold further includes a circuit board 150 preferably mounted on the exterior of the main body adjacent the mounting wall 140 . as shown in fig1 for example , an electrical connector 152 is provided for receiving a direct current power line to furnish electric power to operate circuit board 150 . the circuit board includes a plurality of electrical connection fittings defined therein . each electrical connection fitting 154 or plug outlet is preferably disposed in convenient registry with one of the ports 142 defined in the mounting wall . electrical connection fittings 154 receive an electrical connector , e . g ., plug 156 , of a pressure control valve 162 to transmit electrical power and signals thereto to operate the various electrical components of the pressure control valve . in addition , a plurality of fuses 158 are provided on circuit board 150 to protect circuit board 150 and components connected thereto , such as a microprocessor 160 ( described hereinafter ), from electrical damage . as shown in fig1 for example , the fuse receptacles are on the exterior of the log manifold 128 to provide technicians with the unobstructed access that facilitates troubleshooting and fuse replacement . in further accordance with the patient support system of the present invention , means are provided for maintaining a predetermined pressure in the sacks . the predetermined pressure is kept at a constant predetermined value for each of a number of groups of sacks in the standard mode of operation or may be constantly varying over time in a predetermined sequence in yet other modes of operation of the patient support system of the present invention . as embodied herein and shown schematically in fig1 ( in which electrical connections are shown in dashed lines and pneumatic connections are shown in solid lines , in both cases arrows indicate the direction of electrical or pneumatic flow ) for example , the means for maintaining a predetermined pressure preferably includes a programmable microprocessor 160 and at least one and preferably a plurality of pressure control valves 162 , each of the latter preferably monitored by a pressure sensing device ( not shown in fig1 separately from valves 162 ). as embodied herein and shown in fig7 and 8 for example , the means for maintaining a predetermined pressure in the sacks includes a pressure control valve 162 . preferably , a plurality of pressure control valves are provided , and each valve 162 can control the pressure in a plurality of sacks 34 by means of being connected to a gas manifold ( such as modular support member channels 100 , 102 , 104 ) which carries air from the pressure control valve to each of the sacks . each pressure control valve includes a housing 164 , which preferably is formed of aluminum or another light weight material . as shown in fig8 for example , an inlet 166 is defined through one end of the housing for receiving air flow from a source of pressurized air . an outlet 168 is also defined through the housing for permitting the escape of air exiting the pressure control valve . an elongated valve passage 170 is defined within the housing and is preferably disposed in axial alignment with the inlet . the passage has a longitudinal axis that preferably is disposed perpendicularly with respect to the axis of the valve outlet , which is connected to the valve passage . the valve housing further defines a chamber 172 disposed between the inlet and a first end 174 of the valve passage . the pressure control valve includes a piston 176 disposed in the chamber . the piston is displaceable in the chamber to vary the degree of communication through the chamber that is permitted between the valve inlet and the valve passage . the piston preferably is formed of a hard polymeric or resinous material such as polycarbonate for example . the pressure control valve further includes an electric motor 178 that preferably is mounted outside the housing and near the chamber . the pressure control valve preferably includes means for connecting the motor to the piston in a manner such that the operation of the motor causes displacement of the piston within the chamber . as embodied herein and shown in fig8 for example , the connecting means preferably includes a connecting shaft 180 that has one end non - rotatably secured to the rotatable shaft 182 of the motor 178 . connecting shaft 180 has its opposite end non - rotatably connected to one end of the piston . as shown in fig9 b for example , piston 176 has a groove 183 disposed diametrically through one end of the piston to non - rotatably secure the end of connecting shaft 180 therein . chamber 172 preferably is cylindrical and has its longitudinal axis disposed perpendicularly relative to the longitudinal axis of the valve passage . the piston preferably is cylindrical and rotatably displaceable in the chamber with a close clearance between the piston and the chamber so as to minimize any passage of air thereby . one end of the piston has a cam stop 181 which engages a stop ( not shown ) in chamber 172 to restrict piston 176 from rotating 360 ° within chamber 172 . as the motor shaft 182 rotates , the connecting shaft 180 and piston 176 are rotatably displaced relative to the chamber . as shown in fig8 for example , the piston has a flow slot 184 extending radially into the center of the piston so that depending upon the position of this slot 184 relative to the inlet and the passage , more or less flow is allowed to pass from the inlet 166 , through this slot 184 , and into the passage 170 . thus , the position of the piston within the chamber determines the degree of communication that is permitted through the chamber and the degree of communication permitted between the valve passage and the valve inlet . this degree of communication effectively regulates the pressure of the air delivered by the valve . as shown in fig9 a , 9b , 9c , and 9d for example , piston slot 184 preferably is configured to result in a linear relationship between the air flow permitted through the valve and the rotation of the piston . as shown in fig9 d for example , piston slot 184 preferably comprises three distinctly shaped sections . the section designated 185 is closest to the surface of the piston and is formed as a spheroidal section . the intermediate section is designated 187 and is formed as a semicylinder . the section extending deepest into the center of the piston is designated 189 and is formed as an elongated cylinder with a spherical end . as shown in fig7 and 8 for example , the pressure control valve further preferably includes a pressure transducer 186 that communicates with the valve passage to sense the pressure therein . preferably , the pressure transducer is mounted to the valve housing . an opening 188 is defined through the housing opposite where the outlet is defined . the pressure transducer has a probe ( not shown ) adjacent the opening to permit the transducer to sense the pressure in the valve passage . the pressure transducer converts the pressure sensed in the valve passage into an electrical signal such as an analog voltage , and this voltage is transmitted to an electronic circuit ( described hereafter as a circuit card ) of the valve . as shown in fig7 for example , the pressure control valve further includes an electronic circuit 190 which is mounted to the exterior of the housing on a circuit card 192 . the valve circuit contains a voltage comparator network and voltage reference chips for example . the valve circuit controls the power being provided to the valve motor . the circuit card is connected to the valve pressure transducer and receives the electrical signals transmitted from the transducer corresponding to the pressure being sensed by the transducer in the valve passage . the circuit card receives a reference voltage signal from a microprocessor ( described hereinafter ) via circuit board 150 . the microprocessor sends an analog voltage signal to the valve circuit 190 via circuit board 150 . the valve circuit compares this signal to the one from the pressure transducer and computes a difference signal . the valve circuit controls the valve motor 178 to open or close the valve according to the magnitude and sign ( plus or minus ) of the difference voltage signal . as shown in fig7 for example , the pressure control valve further includes an electrical lead 194 that is connected at one end ( not shown ) to the valve circuit card 192 and terminates at the other end in a plug 156 . this plug can be connected into a plug outlet such as the electrical connection fitting 154 on the log manifold 128 and thus is consistent with the modular construction of the present invention . as shown in fig7 for example , the pressure control valve further defines a dump outlet hole 196 through the valve housing in the vicinity of the valve chamber . as shown in fig8 for example , a dump passage 198 is defined through the valve piston and is configured to connect the dump hole to the valve passage upon displacement of the piston such that the dump hole becomes aligned with the dump passage of the piston . as shown in fig1 for example , a microswitch 199 is disposed near the hydraulic controls for changing the elevation of the patient support . when a control handle 201 is placed in the cpr mode of operation , microswitch 199 is activated , and the microprocessor turns off the blower and signals all of the valves to align the dump passage of the piston with the dump hole . this causes the rapid deflation of all of the air sacks and places the support into a condition suitable for performing a cardiopulmonary resuscitation ( cpr ) procedure on the patient . as shown in fig1 for example , the control panel of the present invention has a button for seat deflate . when the operator presses the seat deflate button , the microprocessor activates the two pressure control valves which control the pressure in the sacks supporting the seat zone ( zone iii shown in fig1 and 13 for example ) of the support system . the microprocessor signals the pressure control valves controlling the seat zone to align their pistons &# 39 ; dump passages with the dump holes in the valve housings in order to permit all of the air in the sacks in the seat zone to escape to the atmosphere through the dump holes . as shown in fig8 for example , when the valve pistons are aligned in this manner , the valve inlets are blocked by the pistons and thus prevented from communicating with the valve passages and valve outlets . as shown in fig8 for example , a conventional pressure check valve 138 preferably is mounted in a manual pressure check opening 200 defined through the housing , of each pressure control valve . as shown in fig1 , a conventional pressure check valve 138 also preferably is inserted into the end walls of log manifold 128 . as shown in fig1 for example , check valve 138 has a head 202 with a port 204 defined therethrough for receiving a probe of a pressure measuring instrument ( not shown ). a collapsible bladder flange 206 extends from head 202 to the opposite end of check valve 138 . the bladder flange extends through the pressure check opening 200 in the housing of the pressure control valve . a slit 208 is formed axially through the collapsible bladder flange and connects to port 204 . the bladder flange is resiliently collapsible around slit 208 to prevent passage of air therethrough . the probe of the measuring instrument is hollow and is inserted through port 204 until the probe parts the flange 206 to open the collapsible slit 208 . this allows the probe to access the pressure in the control valve or chamber of the log manifold , as the case may be . check valve 138 preferably is formed of a flexible material such as a soft plastic or neoprene rubber . one supplier of such check valves is vernay labs of yellow springs , ohio 45387 . as embodied herein and shown schematically in fig1 for example , the means for maintaining a predetermined pressure preferably includes a programmable microprocessor 160 . the microprocessor preferably has parallel processing capability and is programmed to operate the pressure control valves in conjunction with the blower to pressurize the sacks according to the height and weight of the patient . the height and weight information is provided to the microprocessor by the operator . this is accomplished by providing the desired information via a control panel 210 such as shown in fig1 for example . the height of the patient is displayed on a digital readout 212 in either inches or centimeters , and the weight of the patient is displayed on a separate digital readout 214 in either pounds or kilograms . as shown in fig1 and 13 for example , five pressure zones or body zones preferably include a head zone ( zone 1 or i ), a chest zone ( zone 2 or ii ), a seat zone ( zone 3 or iii ), a thigh zone ( zone 4 or iv ), and a leg and foot zone ( zone 5 or v ). each body zone is supplied with pressurized air from the blower via two separate pressure control valves . in one configuration of the air flow path from the blower to the sacks , one of the pressure control valves controls air supplied to the chambers of each sack on one side of the patient support system for each body zone , and the other pressure control valve controls the air to the chambers on the side of each sack on the opposite side of the patient support system . in yet another configuration of the air flow path from the blower to the sacks , one of the pressure control valves controls the air supplied to all of the chambers of every alternate sack in a body zone , and the other pressure control valve controls the air supplied to all of the chambers in the remaining alternate sacks in the body zone . the microprocessor is programmed to set the reference pressure of each pressure control valve of each body zone into which the patient support system has been divided for purposes of controlling the pressure supplied to air sacks 34 under particular portions of the patient . based upon the height and weight of the patient , the microprocessor is preprogrammed to calculate an optimum reference pressure for supporting the patient in each body zone . this reference pressure is determined at the valve passage where the pressure transducer of each pressure control valve is sensing the pressure . the circuit card 192 performs a comparison function in which it compares the reference pressure signal transmitted to it from microprocessor 160 via circuit board 150 to the pressure which it has received from the pressure transducer . depending upon the difference between this signal received from the valve &# 39 ; s pressure transducer and the calculated desired signal corresponding to the preset reference pressure , the valve circuit 192 signals the valve motor to open or close the pressure control valve , depending upon whether the pressure is to be increased or decreased . this process continues until the desired reference pressure is sensed by the pressure transducer of the pressure control valve . the microprocessor has parallel processing capability and thus can simultaneously supply each of the pressure control valves with the reference pressure for that particular control valve . moreover , the speed of each of the microprocessor and valve circuits greatly exceeds the time in which the motors of the pressure control valves can respond to the signals received from the valve circuits . thus , in practical effect the motor response times limit the frequency with which the pressure control valves can be corrected . moreover , the reference pressure calculated by the microprocessor also can depend upon other factors such as whether one or more articulatable sections of the frame is elevated at an angle above or below the horizontal . another factor which can affect the microprocessor &# 39 ; s calculation of the reference pressure for the particular zone is whether the patient is being supported in a tilted attitude at an angle below the horizontal and whether this angle is tilted to the left side of the patient support system or the right side . still another factor is whether the patient is lying on his / her side or back . yet another factor that can affect the reference pressure calculated by the microprocessor is whether the patient comfort adjustment buttons 216 have been manipulated via the control panel to adjust the pressure desired by the patient in a particular zone to a pressure slightly above or slightly below the reference pressure that the microprocessor is preprogrammed to set for that particular zone under the other conditions noted , including , elevation angle , side lying or back lying , and tilt attitude . as shown in fig1 for example , each body support zone has a triangular button 216 pointing upward and a triangular button 216 pointing downward . depression of the upward button 216 increases the reference pressure that the microprocessor calculates for that particular zone . similarly , the depression of the downward pointing button 216 , decreases the reference pressure that the microprocessor calculates for that particular zone . the range of increase and decrease preferably is about twenty percent of the reference pressure that is calculated for the standard mode of operation in each particular zone . this permits the patient to change the pressure noticeably , yet not so much as to endanger the patient by producing a condition that is either over - inflated or under - inflated for the sacks in a particular zone . moreover , the 20 % limitation also can be overridden by pressing the override button shown in fig1 . the override function can be canceled by pressing the reset button shown in fig1 . one form of sack pressure algorithm which is suitable for use by the microprocessor to calculate the reference pressures for different configurations of the patient support system of the present invention is as follows : table 1 provides parameters suitable for several elevation configurations , patients lying on his / her back , side lying , and all five zones . for example , the constants c1 , c2 and c3 for each zone are the same for elevation angles 0 ° through 29 ° with the patient lying on his / her back . the values of c1 , c2 and c3 for side lying are the same for elevation angles of 0 ° through 29 °. table 1______________________________________elevationangle zone c1 c2 c3______________________________________0 °- 29 ° i 0 . 00473 0 . 04208 - 1 . 27789back lying ii 0 . 02088 - 0 . 01288 1 . 73891 iii 0 . 03688 - 0 . 10931 7 . 33525 iv 0 . 00778 - 0 . 01828 2 . 21268 v 0 . 00316 0 . 00482 0 . 6175130 °- 44 ° i 0 . 00857 0 . 02056 - 0 . 22725back lying ii 0 . 02230 - 0 . 03996 3 . 32860 iii 0 . 01971 0 . 08197 - 0 . 68941 iv 0 . 00554 0 . 03495 0 . 38316 v 0 . 00303 0 . 01883 - 0 . 1224845 °- 59 ° i 0 . 00152 0 . 02889 0 . 11170back lying ii 0 . 01349 - 0 . 02296 3 . 06615 iii 0 . 03714 0 . 01023 3 . 37064 iv 0 . 01014 0 . 09399 - 3 . 39696 v 0 . 00298 - 0 . 00337 1 . 4010260 ° and above i 0 . 00571 - 0 . 00976 1 . 77230back lying ii 0 . 01165 0 . 02598 - 0 . 20917 iii 0 . 01871 0 . 04853 4 . 35063 iv 0 . 02273 0 . 06610 - 2 . 94674 v 0 . 00291 0 . 00292 0 . 99296sl i 0 . 01175 0 . 00548 0 . 43111 ( side lying ) ii 0 . 03276 0 . 03607 - 1 . 788990 °- 29 ° iii 0 . 03715 - 0 . 10824 8 . 22602 iv 0 . 01091 - 0 . 00336 1 . 48258 v 0 . 00146 0 . 02093 - 0 . 15271______________________________________ the weight of the patient is supported by the surface tension of the air sack as well as the air pressure within the sack . thus , values of c1 , c2 , and c3 can vary with air sack geometry or the properties , such as stiffness , of the materials used to form the air sack . different air sack geometries may provide more or less stiffness in the air sack . typically , a ribbon cable 218 electrical connector ( fig1 ) connects circuit board 150 to microprocessor 160 . circuit board 150 receives analog signals from microprocessor 160 and distributes same to the valve circuit card 192 of each particular pressure control valve 162 for which the signal is intended . in addition , in some embodiments , circuit board 150 can return signals from the individual pressure control valve circuitry 190 to the microprocessor . the voltage signals from the microprocessor cause the valve circuit card 192 to operate the motor of the pressure control valve to expand or contract the valve opening to attain a reference pressure , which the microprocessor is preprogrammed to calculate . the valve circuit compares the reference signal received from the microprocessor to the signals received from pressure transducer 186 of the pressure control valve . in effect , this enables the support system of the present invention to monitor the air pressure in the valve passage 170 near the valve outlet 168 , which is the location where the sensing probe of the pressure transducer is disposed to sense the pressure supplied to the air sack through the pressure control valve . in further accordance with the present invention , there is provided means for switching between different modes of pressurizing the sacks . as embodied herein and shown schematically in fig1 , 12 and 13 for example , the mode switching means preferably includes at least one flow diverter valve 220 and preferably includes a plurality of flow diverter valves 220 . the number of flow diverter valves depends upon the number of different pressure zones desired for the patient support system embodiment contemplated . a pressure zone includes one or more sacks or sack chambers which are to be maintained with the same pressure characteristics . in some instances , it is desired to have opposite sides of the sack maintained at different pressures . this becomes desirable for example when the rotation mode of the patient support system is operated . in other instances it becomes desirable to have the pressure in every other sack alternately increasing together for a predetermined time interval and decreasing together for a predetermined time interval . this becomes desirable for example when the patient support system is operated in the pulsation mode of operation . as shown in fig1 for example , each flow diverter valve preferably is mounted within a modular support member 68 , and more than one diverter valve 220 can be mounted in a modular support member such as the seat sack support member 94 . however , other sack support members 68 , such as the head sack support member shown in fig1 for example , may lack a diverter valve . each diverter valve preferably is mounted between the top and bottom surfaces of each plate 70 . as shown schematically in fig1 for example , each diverter valve has a first flow pathway 222 with a first inlet 224 at one end and a first outlet 226 at the opposite end . each diverter valve further includes a second flow pathway 228 with a second inlet 230 at one end and a second outlet 232 at the opposite end . the flow pathways are mounted and fixed on a rotating disk 234 , also referred to as a switching disk 234 , that rotates about a central pivot 236 . the so - called switching disk is rotatable for the purpose of changing the path defined by the inlets and outlets . as shown in solid lines in fig1 for example , first flow pathway 222 connects channel a with channel b , and second flow pathway connects channel c with channel d . thus , a first inlet 224 of first pathway 222 is connected to channel a and a first outlet 226 of first pathway 222 is connected to channel b . similarly , a first inlet 230 of second pathway 228 is connected to channel d and a first outlet 232 of second pathway 228 is connected to channel c . in the solid line configuration shown schematically in fig1 , both sides of every alternate sack are connected together and thus maintained at the same pressure by a pressure control valve connected to the sacks via pressure control valve openings 96 . this is the configuration for the so - called pulsation ( p ) mode of operation . as shown by the dotted line configuration of the flow pathways , when the switching disk is rotated 90 ° counterclockwise to the dotted line position ( r ), the first flow pathway connects channel a to channel c , and the second flow pathway connects channel b to channel d . thus , first inlet 224 of first pathway 222 is connected to channel c , and second inlet 230 of second pathway 228 is connected to channel b . first outlet 226 of first pathway 222 becomes connected to channel a , and second outlet 232 of second pathway 228 becomes connected to channel d . in the dotted line configuration shown in fig1 , one side of all of the sacks are connected together and thus can be maintained at a common pressure , and the other side of all of the sacks are connected together and also can be maintained at a common pressure . this is the configuration for the so - called rotation ( r ) mode of operation . the use of the diverter valves by the present invention enables the support system to be operated in either a pulsation mode of operation or a rotation mode of operation with a minimum number of valves and air flow conduits . the diverter valve allows the air flow paths of the support system to be reconfigured between two distinctly different ways of connecting the pressurized air source through the pressure control valves to individual air sacks of the patient support system . the patient support system of the present invention can be operated to automatically rotate the patient , i . e ., turn the patient to one side or the other , at preset intervals of time . referring to the control panel shown in fig1 , the patient support system of the present invention can be set to operate in a rotational mode by pressing the set up button followed by pressing the mode selection button until the rotation indicator is lit . then the rotation section of the control panel becomes illuminated and can be operated . the operator selects the amount of time that the patient is to be maintained in a right - tilted position , or a horizontal position , or a left - tilted position . to accomplish this for the horizontal position for example , the operator activates the horizontal button 238 followed by activating the time button . this manipulation enters the time interval during which the patient support is to maintain the patient supported in the horizontal position . this interval of time is displayed on a digital readout 239 . to set the time that the patient is to spend in the right - tilted position , the operator presses the right button 240 followed by the time button . again , the time interval which the patient is to be maintained tilted to the right is displayed digitally on readout 239 . a similar procedure is followed to set the time spent in the left - tilted position . in addition , right button 240 allows the operator to select the attitude of the patient in the right - tilted position . there are a number of illumination bars disposed above the right button . each illumination bar corresponds to a different attitude to which the patient can be tilted to the right . the operator selects the desired attitude by continuously pressing the triangular buttons above and below right button 240 until the bar adjacent the desired attitude is illuminated . for example , the maximum attitude of tilt requires the operator to continue pressing the downward pointing triangular button beneath right button 240 until the lowermost bar above the right button is lit . the same procedure is followed to set the attitude for the left - tilted position . moreover , as shown schematically in fig1 for example , the angle of elevation of the head and chest section of the patient support is monitored by an elevation sensing device 242 , which sends signals to the circuit board 150 of the modular valve mounting manifold 128 . fig1 illustrates electrical signaling pathways by dashed lines and pneumatic pathways by solid lines . the arrows at the ends of the dotted lines indicate the direction of the electrical signals along the electrical pathways . the elevation sensing device detects the angle at which the head and chest section has been positioned , and supplies a corresponding signal to the microprocessor via circuit board 150 . examples of suitable elevation sensing devices are disclosed in u . s . pat . nos . 4 , 745 , 647 and 4 , 768 , 249 , which patents are hereby incorporated in their entireties herein by reference . if this elevation information from the sensing device 242 indicates that the angle of articulation exceeds 30 °, the microprocessor configures the pressure profile to a standard mode of operation and thus cancels any rotation or pulsation that may have been selected by the operator . the rotation mode is canceled to avoid torquing the patient &# 39 ; s body . the pulsation mode is canceled because the elevation of the patient above 30 ° reduces the ability to float the patient in the sacks in the seat zone during pulsation of the three sacks therein . thus , the &# 34 ; bottoming &# 34 ; of the patient during pulsation at elevation angles above 30 ° is avoided . upon reduction of the articulated angle below 30 °, the microprocessor does not automatically resume either pulsation or rotation but requires any mode other than the standard mode to be reset . in accordance with the present invention , the control over blower 66 preferably includes a blower control circuit which controls the power supplied to blower 66 . microprocessor 160 provides a blower control voltage to blower control circuit 67 which controls the power supply to blower 66 according to this blower control voltage signal received from microprocessor 160 . a pressure transducer 246 measures the pressure preferably at the blower and communicates a signal corresponding to the measured blower pressure to the microprocessor 160 via blower control circuit 67 and circuit board 150 . microprocessor 160 has a blower control algorithm which enables microprocessor 160 to calculate a desired reference pressure for the blower . the blower control algorithm preferably calculates this blower reference pressure to be 3 to 4 inches of standard water higher than the highest pressure in the air sacks . typically , the seat zone ( zone iii ) has this highest pressure for a given height and weight setting ( provided by the operator to the microprocessor ) regardless of the elevation of the head and chest sections and whether the patient is lying on his / her side or back . however , a patient with abnormal body mass distribution ( which could be caused by a cast for example ) may require the highest sack pressure in one of the other zones . if zone iii has the highest sack pressure , as the elevation angle increases , the sack pressure in zone iii increases , and the reference pressure for the blower also increases to equal 3 to 4 inches of standard water above the pressure of the sacks in zone iii . microprocessor 160 stores the signal from transducer 246 corresponding to the measured blower pressure in the microprocessor memory , which is updated preferably only once every three seconds . microprocessor 160 calculates the reference blower pressure about four times each second and compares it to the stored measured pressure about once each second . if the measured pressure is more than about one inch of standard water higher than the reference pressure calculated by microprocessor 160 , microprocessor 160 decreases the control voltage by an increment of 1 / 256 of the maximum control voltage signal that microprocessor 160 is programmed to provide to blower control circuit 67 . this maximum voltage corresponds to the maximum output of blower 66 . if the measured blower pressure is more than about one inch of standard water lower than the reference pressure , then microprocessor 160 increases the control voltage signal by an increment of 4 / 256 times the maximum control voltage . the increase or decrease , if any , occurs about once each second . pressure deficits are of a greater concern , and thus correction of such deficits occurs four times faster than correction of excess pressures . the pressure changes resulting from the blower control sequence occur no more frequently than once each second and are no greater than 1 / 256 of the maximum pressure for decreases and 4 / 256 times the maximum pressure for increases . moreover , the microprocessor &# 39 ; s three second delay in updating the measured pressure used in the calculations assures that changes in the measured pressure that have very short durations will not lead to pressure instability because of control loop exacerbation of short - lived pressure fluctuations . this three second time interval can change depending upon the pressure dynamics and control dynamics of the system . the selection of the rotation mode of operation on control panel 210 causes the microprocessor to signal the diverter valves to align their pathways for rotational operation of the support system . once the parameters of operation in the rotation mode have been inputted , the microprocessor recalculates an optimum reference pressure for each pressure control valve . the microprocessor determines the appropriate tilt reference pressure based upon the height and weight of the patient and the angle of tilt selected by the operator . this is accomplished such that the pressure in the low pressure side of the sack and the pressure in the high pressure side of the sack average out to the pressure that would be set for the same sacks in the normal mode of operation , i . e ., without any rotation . thus , the average pressure over the entire sack during the rotational mode of operation is the same as it would be in the nonrotational modes of operation . the operator initiates the rotation by pressing the run button on panel 210 in fig1 for example . when the operator presses the run button , the microprocessor adjusts the pressure control valves 162 to set the new tilt reference pressure in the end and intermediate chambers on the side of the support system to be tilted . this results in a reduction in the pressure in the end and intermediate chambers of the tilted sides of the sacks in each body zone . the microprocessor operates the control valve to prevent this low sack pressure from falling below 1 to 2 inches of standard water , because this is the minimum pressure needed to keep the end chamber inflated while the weight of the patient is squeezing out air from the intermediate chamber . the microprocessor also raises the pressure in the end and intermediate chambers on the opposite side , i . e ., non - tilted side of the sacks of the support system . the increase in pressure in the chambers of the untilted side of the support system is needed to compensate for the loss in pressure in the chambers on the tilted side of the support system . the additional pressure allows the patient to be supported in the tilted position as comfortably as in the non - tilted position . the pressure increase in the chambers of the non - tilted side of the sacks is preferably sufficient so that the average pressure between the two sides of each sack equals the pressure in this sack when the patient is supported thereon in a non - tilted position . in other words , one - half of the sum of the pressure in the high side of the sack and the low side of the sack is equal to the normal base line pressure of this particular sack in a nontilted mode of operation , i . e ., when both sides of the sack are at this same base line pressure . in accordance with the present invention , a method is provided for turning the patient on a low air loss patient support system as in the present invention . as embodied herein , the turning method includes the step of grouping all of the sacks 34 into at least two body zones that correspond to at least two different zones of the patient &# 39 ; s body . each zone of the patient &# 39 ; s body is preferably supported by one or more sacks in one of the two body zones . preferably five body zones are involved . the next step in the method for turning a patient is to pressurize all of the sacks according to a first pressure profile that provides each sack in each body zone with a respective first air pressure . this first air pressure has been chosen so as to provide a first respective level of support to that portion of the patient &# 39 ; s body supported by the sacks in that body zone . the level of support is predetermined depending upon the height and weight of the patient and calculated accordingly by the microprocessor . the height and weight data also affect the respective first air pressure that is chosen for the sacks in that particular body zone . the terms &# 34 ; pressure profile &# 39 ; are used to refer to the fact that the pressure in each body zone may be different because of the different support requirement of that particular body zone . if the individual pressures in the sacks of all the body zones were to be represented on a bar graph as a function of the linear position of the sacks along the length of the patient support , a line connecting the tops of the bars in the graph would depict a certain profile . hence the use of the term &# 34 ; pressure profile &# 39 ; to describe the pressure conditions in all of the sacks at a given moment in time , either when the pressures are changing or in a steady state condition . the next step in turning the patient involves separately controlling the air pressure that is supplied to each side of each of the sacks . this preferably is accomplished by supplying the chambers on one side of the sacks in each body zone via a first pressure control valve and supplying the chambers on the other side of the sacks via a separate pressure control valve , and connecting each pressure control valve to a four - way diverter valve . the diverter valve can then be configured to ensure that the air pressure being supplied to the chambers on one side of each sack is being controlled by one of the pressure control valves , and the pressure being supplied to the chambers on the other side of the sack of a particular zone is being supplied through a separate pressure control valve . the next step in turning the patient involves lowering the pressure in the chambers on the side of the sacks to which the patient is to be tilted . specifically , the pressure must be lowered in the chambers of one side of the sacks from a first pressure profile , previously established , to a predetermined second pressure profile . the second pressure profile is predetermined according to the height and weight of the patient and also according to the attitude to which the patient is to be tilted . the greater the angle below the horizontal to which the patient is to be tilted , the lower the predetermined second pressure profile . another step in the method of turning the patient requires raising the pressure in the chamber on the side of the sacks that is opposite the side to which the patient is being tilted . this involves raising the pressure in the chamber of the non - tilted side of each of the sacks to a predetermined third pressure profile . the raised pressure profile in the non - tilted sacks compensates for the lower pressure profile in the side of the sacks to which the patient has been tilted . when the overall pressure being supplied to support the patient has been reduced in half of the sack , as occurs during tilting , that portion of the patient &# 39 ; s body in that particular body zone would not be maintained at the desired level of support without increasing the pressure in the non - tilted side of the sack . the operator begins by lowering the pressure in one side of the all of the sacks until the patient has been tilted to the desired attitude of tilt beneath the horizontal . as this is occurring , the microprocessor is increasing the pressure in the non - tilted sacks such that one - half of the sum of the pressure in the tilted sacks plus the pressure in the untilted sacks equals the base line pressure of the sacks before the tilting procedure began . in the case just described , the base line pressure corresponds to the pressure in the sack at the first pressure profile . preferably , the raising and lowering of the pressures in the chambers of opposite sides of the sacks occurs practically simultaneously . since preferably the microprocessor has parallel processing capability and thus can control each of the pressure control valves simultaneously , the speed with which the tilting is effected ( or any other pressure changes in the sacks ) is primarily limited by the flow restrictions in the pneumatic circuit , which is primarily a function of the air sack volume and the pressure level in the sacks . in further accordance with the present invention , the patient is maintained in the selected tilted position for a predetermined length of time . at the end of this predetermined length of time , which is clocked by the microprocessor , the patient is returned to the horizontal position by simultaneously increasing the pressure in the side of the sacks to which the patient previously had been tilted while decreasing the pressure in the non - tilted side of the sacks until the pressure in both sides of the sacks returns to the first predetermined pressure profile . the changes in pressure from low to high or from high to low preferably occurs over a time interval of about three minutes . this is done to reduce the likelihood that the patient will experience any uncomfortable sensation during these pressure changes . in still further accordance with the present invention , the method of turning a patient can maintain the patient in the horizontal position for a predetermined interval of time . at the end of this predetermined interval of time , the patient then can be tilted to the side of the patient support system that is opposite the side to which the patient had been tilted prior to being maintained in the horizontal position . moreover , the amount of time which the patient spends in a particular position , namely , left - tilted , horizontal , and right - tilted , can be preselected so that the patient can be maintained in one of the three positions for however long is deemed therapeutic . it is during the turning , i . e ., rotation or tilting , mode of operation that the grommet which defines the hole 6 connecting each intermediate chamber 54 with each end chamber 46 of each sack 34 plays a particularly important role . as the pressure control valve controlling the side of the sack to which the patient is to be tilted begins to close and reduce the pressure being supplied to this side of these sacks , the weight of the patient above the depressurizing intermediate chamber 54 squeezes the air from the intermediate chamber through the grommet and into the end chamber 46 to compensate for the reduced pressure being supplied to the end chamber via the pressure control valve . thus , the reduction in pressure initially serves to deflate the intermediate chamber while maintaining the end chamber as fully inflated as before the pressure control valve began to reduce the pressure supplied thereto . the pressure in the end chamber of course is being reduced . however , the end chamber remains completely inflated , unlike the connecting intermediate chamber which is being squeezed by the weight of the patient that no longer is being supported by the same level of air pressure as was present when the sacks were being maintained according to the first pressure profile that was first set to maintain the patient in the horizontal position atop the sacks . moreover , since the end chamber remains inflated , it acts as a passive constraint to prevent the patient from rolling past the end chamber and off of the patient support . to operate the support system of the present invention in the pulsation mode , the operator pushes the set up button on the control panel illustrated in fig1 for example . then the operator presses the mode selection button until the pulsation indicator illuminates . when the pulsation indicator is illuminated , the pulsation section of the control panel also becomes illuminated . the microprocessor immediately signals the diverter valves to align their pathways for the pulsation mode of operation . in the pulsation alignment of the diverter valves , the channels of the modular support members connect alternately adjacent air sacks . this results in two sets of sacks which can be operated at two separate and opposite patterns of pressurization . as shown in fig1 for example , the operator selects the time interval for a complete pulsation cycle by pressing the time button . the time interval for each pulsation cycle is displayed in a digital readout 244 above the time button . the operator selects the degree of depressurization in the phase of the pulsation cycle in which the pressures in alternating sacks are lowered while the pressures in the other sacks are increased according to the amount that the pressures in the first group of alternating sacks have been lowered . the operator accomplishes this selection by pressing one of the two triangular shaped buttons beneath the light bars next to the max - min scale to illuminate the light bar adjacent the desired level of depressurization . once the parameters of operation in the pulsation mode have been inputted , the microprocessor begins calculating a pulsation reference pressure for each pressure control valve . this pulsation reference pressure depends upon the degree of depressurization selected by the operator and the height and weight of the patient . preferably , the microprocessor maintains the pressures in adjacent sacks such that one - half of the sum of the pressures in the adjacent sacks equals the base line pressure for a sack in that zone at the elevation angle , if any , and taking into account whether the patient is side lying or back lying . the operator initiates the pulsation of the sacks by pressing the run button on panel 210 in fig1 for example . in further accordance with the present invention , a method is provided for periodically relieving the pressure of the patient support system against the patient &# 39 ; s body . this method preferably is accomplished by pulsating the pressure in the sacks of the low air loss patient support system having a plurality of sacks disposed transversely across the length of the support system . the pressure in a first group of sacks comprising every alternating sack is depressurized relative to the remaining sacks , which are provided with an increase in pressure . the pressure differential between the two separate sacks is maintained for a predetermined interval of time . at the end of this time interval , the pressure profiles switch so that the other set of alternating sacks becomes depressurized while the first set of alternating sacks receives a slight increase in pressure . this opposite pressurization condition is also maintained for a predetermined interval of time , whereupon the cycle repeats itself until the pulsation mode of operation is discontinued . prior to the initiation of the pulsation mode of operation , all of the sacks in the patient support will be maintained at a first pressure profile according to the height and weight of the patient , the various angles of inclination of any of the articulating sections of the frame , and any tilt angle imposed upon the sacks . however , preferably , the pulsation method will not be operated in conjunction with any tilting of the patient , and thus activation of the pulsation method automatically discontinues operation in the tilting mode . the steps of the method for pulsating the pressure in the sacks of the low air loss patient support system include configuring the ar supply means of the patient support to define two separate groups of alternating sacks . a first group of sacks includes either every odd number sequenced sack in order from one end of the patient support to the opposite end of the patient support or every even number sequenced sack . for purposes of this description , the first of the two groups of sacks will be chosen to be the odd number sequenced sacks . in a preferred embodiment , the sacks are further grouped into body zones to support the patient &# 39 ; s body at a predetermined pressure for all of the sacks in the body zone . thus , all of the sacks in a particular body zone will be pressurized at the same first pressure , and accordingly the individual first pressure will be applied to all of the sacks in each body zone . this step of configuring the sacks is preferably accomplished by configuring a plurality of diverter valves to connect every alternating sack in a body zone . the next step includes reducing the air pressure being supplied to the sacks in the first group . this is accomplished as the microprocessor controls the pressure control valve of this first group to attain a second pressure profile . the second pressure profile corresponds to a decreased pulsation reference pressure calculated by the microprocessor when the degree of depressurization was selected by the operator . the microprocessor controls the pressure control valves supplying air to the sacks in the first group until the decreased pulsation reference pressure has been attained by the sacks in this first group . the next step occurs simultaneously with the first step and includes supplying air pressure to the sacks in the second of the two groups , namely , the group including every even number sequenced sack in order from one end of the patient support to the opposite end of the patient support , at a third pressure profile . this third pressure profile corresponds to an increased pulsation reference pressure which the microprocessor calculated for each pressure control valve controlling the sacks in the second group for each individual body zone . this increased pulsation reference pressure also has been calculated by the microprocessor depending upon the degree of depressurization selected by the operator . this third pressure profile is designed to compensate for the loss of pressurization by the first group of sacks so that the patient support can continue to maintain the patient at the same level of horizontal support during the depressurization of the first group of sacks . in other words , while the pressures in the alternate groups of sacks are changing , the vertical height of the patient above the floor is not changing significantly from what it was prior to the onset of the pulsation mode of operation . thus , the microprocessor maintains the pressures in the two groups of sacks such that one - half the sum of the second and third pressure profiles equals the first pressure profile . the two steps involving the changes in pressurization of the two groups of sacks , occur simultaneously over a first time interval . the method for pulsating the pressure in the sacks further includes the step of maintaining the second and third pressure profiles being supplied to the two groups of sacks during a second interval of time . this is accomplished by the microprocessor controlling the pressure control valves to maintain the increased or decreased pulsation reference pressures calculated by the microprocessor for the respective group of sacks over the time interval selected by the operator . after the predetermined lower pressure has been maintained for the sacks in the one group for the second interval of time , the next step is to increase the pressure being supplied to this one group during a third interval of time until each sack in this one group attains a higher individual pressure corresponding to the third pressure profile . at the same time that the sacks in the first group of sacks are attaining the higher individual pressure , the pressure being supplied to the sacks in the other of the two groups is being decreased to the lower pressure corresponding to the second pressure profile . the pressure in the other of the two groups is decreased until the predetermined lower pressure is being provided to each individual sack in this other group . the pressure decreases over this third interval of time . finally , the third pressure profile in the one group and the second pressure profile in the other group are maintained during a fourth interval of time . preferably , all of the first , second , third , and fourth intervals of time are of equal duration . however , in some embodiments of the method of pulsating the sacks of the present invention , the first interval of time preferably equals the third interval of time , and the second interval of time preferably equals the fourth interval of time . in yet another embodiment of the method of pulsating the sacks of the present invention , not only are the first and third time intervals equal to each other as well as the second and fourth time intervals being equal to each other , but the first and third time intervals are shorter than the second and fourth time intervals . in other words , the time which the sacks spend alternately changing pressures is less than the time during which the sacks remain at the steady state higher or lower pressures . similarly , in yet another embodiment of the method of pulsating the sacks of the present invention , the second and fourth time intervals can be equal to each other and shorter than the first and third time intervals , which also are equal to each other . it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention . thus , it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents .
a method for turning a bed ridden patient uses a low air loss patient support system that includes a plurality of identical multi - chambered inflatable sacks . a restrictive flow hole connects two adjacent chambers disposed predominately to one side of the centerline of the sack , and each side is separately pressurizable under the control of a microprocess and a plurality of pressure control valves with pressure transducers and a plurality of flow diverter valves for switching between different modes of configuring the manner in which the sacks are pressurized . the support system effects a method of rotating or tilting the patient that depressurizes one side of the sacks while increasing the pressurization of the opposite side of the sacks . the end chamber of the depresurizing side of the sacks remains inflated while the intermediate chamber of the depressurizing side of the sacks becomes progressively deflated during depressurization to permit the end chamber to restrain the patient from sliding off the sacks during tilting .