Abstract:
A pressure compensating IV rate flow regulator having a housing with a metering port and an outlet. The flow rate is set by adjusting a flexible disk to vary the clearance between metering port and flow passageway. The controlled fluid passes to a land surrounding the outlet. A flexible control membrane having specific design and material characteristics extends across the land and will move or oscillate away or towards the outlet port in response to changes in head pressure, venous back pressure or patient movement. The resulting oscillation of the membrane serves to maintain the flow rate originally set by the clinician. The regulator is provided with safety and operational convenience features such as tactile indicators for “off” and KVO settings and has provision for use with accessory items such as a pole mount holder or anchoring device secured at the forearm or chest.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of patent application Ser. No. 10/342,797, filed Jan. 16, 2003, entitled “Pressure Compensating IV Flow Control Regulator”, which claims priority from provisional patent application Ser. No. 60/349,834, filed Jan. 16, 2002, entitled “Pressure Compensated Flow Regulator.” 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a device for regulating and controlling the flow of intravenous solutions to a patient in a gravity infusion system. More particularly, the present invention relates to a disposable, single-use, gravity-flow regulator which is adjustable to closely maintain a pre-set flow rate independent of pressure change due to changes in elevation of the fluid container, venous backpressure or patient movement. 
     Gravity administration of fluids by IV infusion, also known as IV therapy, is a widely practiced medical procedure. Drugs and fluids such as blood, plasma, dextrose and isotonic saline solutions are administered to patients in this manner. 
     In the typical administration set, a container containing the IV solution is provided to the attending medical personnel. The container has a seal which is broken by insertion of a piercing spike on the drip chamber. A flexible tubing line delivers the IV fluid to the patient. The purpose of the drop chamber is to facilitate the determination of flow or drip rate through the tubing. Infusion rates may be regulated by the use of an external pinch valve or roller clamp associated with the tubing for less critical gravity-type infusions. 
     The procedure involves initially purging the tubing and needle of air by initiating a flow of fluid through the tubing. Once this is done, the needle is then inserted into the venipuncture site, such as a location in the forearm or wrist of the patient, and fluid flow is initiated. Medical personnel will normally adjust a pinch valve or roller clamp to restrict the IV tubing. The number of drops passing through the drip chamber is visually counted or timed. The appropriate flow rate is established by trial and error by progressively restricting or opening the lumen in the tubing using the pinch valve or roller clamp. Roller clamps and pinch valves compress the delivery tubing and are not very accurate as they deform the tubing due to the physical properties of the tubing, the inner diameter may change during use. 
     The administration procedure described above requires the attention of medical personnel for a substantial period of time. However, once a drop rate is set, the rate can be subject to substantial deviation as a result of a number of factors. As the administration of the IV progresses, the fluid level within the solution container will lower, reducing the effective head pressure, causing the drop rate to reduce. The drop rate may also be significantly affected by a change in the elevation of the container or by movement of the patient. Accordingly, the traditional clamping procedures are subject to error and deviation and may deliver fluid at too low or too high a rate which may be adverse to the patient, particularly critical care patients. 
     Accordingly, various approaches can be found in the prior art to maintaining constant IV flow rates in gravity systems. Flow can be regulated by controlling pressure or resistance either mechanically or electro-mechanically. For example, U.S. Pat. No. 4,343,505, discloses an adjustable rate, constant output infusion set having a connector piece connectable to a container and a headpiece rotatably attached to the connector to adjust the flow rate. An elastically stretchable diaphragm is interposed between the connector piece and the head piece. The connector piece forms a first chamber with a diaphragm which is in direct communication with the container by an inlet port. The headpiece forms a second chamber with a diaphragm, which is in communication with the patient by a control board. The headpiece is rotatably attached to the connector to adjust the flow rate. A passageway connects the first chamber and the second chamber and the diaphragm and by virtue of elasticity maintains a constant pressure drop between the chambers so that the fluid passes the control port at a constant rate of flow. 
     U.S. Pat. No. 4,515,588 shows a flow regulator for use in an IV administration arrangement which establishes and maintains the rate of flow regardless of changes of pressure. The regulator utilizes a diaphragm control orifice and bypass with a valve to establish the flow rate. The diaphragm adjusts the effective orifice opening to maintain the constant flow selected by the valve setting. 
     U.S. Pat. No. 4,769,012 discloses a flow regulating device for gravity infusion and transfusion of fluids which has an upper and lower housing and having inlet and outlet channels respectively. A continuously adjustable valve is connected between the inflow and outflow channels. The outflow channel has an outlet opening which defines a valve seat and a membrane extends across the outlet opening and is movable toward and away from the outlet opening, depending on pressure occurring on opposite sides of the membrane. In this way, extraneous factors such as patient venous pressure can be compensated by the membrane to maintain substantially constant flow of fluid once the valve is set. 
     U.S. Pat. No. 5,240,035 discloses a pressure compensator for maintaining essentially constant flow rates in an IV system having a source of IV fluid and a controller. The compensator is connectable to the controller and has a housing with a flexible membrane extending transversely therein defining opposite control chambers. One control chamber communicates with the source of IV fluid and also connects to the inlet of the flow controller. The second control chamber receives the regulated flow from the control valve. The second control chamber is generally conical or convex, sloping to an outlet port which is connected to the tubing line leading to the patient. The outlet from the second chamber is smaller than the inlet to the second chamber and the diaphragm serves to control flow to the patient by compensating for pressure changes, such as change in fluid head. The configuration of the second chamber and the configuration of the outlet in the second chamber minimize air entrapment and provide greater flow capacity. 
     While devices of the type described above improve accuracy in IV delivery systems, there nevertheless exists a need for a cost-effective, reliable, accurate and effective IV regulator which will maintain a substantially constant flow of IV fluid once a flow rate is set. There further exists a need for a device which requires minimal manipulation so that when the regulator is placed in use in an IV administration system, the requirement for drop counting and readjustment is either minimized or eliminated. 
     BRIEF SUMMARY OF THE INVENTION 
     Briefly, the present invention provides a gravity flow rate regulator for administering IV solutions. The regulator is connectable to a line leading to the IV fluid container at a tubing connector. The solution container is supported in a position above the venipuncture site so that fluid will flow by means of gravity through the tubing to the venipuncture site. The regulator is adapted to be interposed in the tubing line between the IV solution container and patient. 
     The regulator has a housing which supports a flexible membrane interposed between an inlet and outlet, establishing inlet and outlet chambers. Fluid from the IV container enters the housing through an inlet port which, in one embodiment, is disposed at an angle with respect to the regulator body. As the fluid flows through the regulator to the outlet it passes across a metering port and into an outlet chamber surrounding the outlet. The outlet chamber has a raised land or annular rim surrounding an orifice opening into the outlet. A flexible membrane is disposed across the land so that flexing or oscillating of the membrane toward or away from the orifice will control the flow of liquid through the outlet orifice. The membrane is subject to outlet pressure against its lower surface and to inlet or head pressure against its opposite upper surface. The pressure differential across the membrane operates to maintain the preset flow through the outlet by oscillating to adjust the effective opening of the orifice to compensate for changes in fluid pressure. 
     A valving device such as a screw attached to a large control dial provides precise control so the medical attendant may easily and accurately establish the rate of flow through the metering port by progressively advancing or retracting an elastomeric disk with respect to a flow passage in the fluid flow path in the regulator. 
     The control dial for manually setting the flow rate is provided with a detent that provides a tactile indication of settings such as KVO (keep vein open), an “off” or locking position as well as providing a stop at the full flow position. The dial is further provided with a safety lock to prevent inadvertent or unauthorized changing of the dial setting once it is set. The dial has spaced grooves extending around the dial engaging a fixed projection so that the user is provided tactile feedback as the dial is manually rotated between settings. 
     The regulator membrane and metering disk are selected having specific characteristics to ensure accuracy. Other features of the regulator include internal sealing to minimize leakage and a large radius outlet port for debubbling. In an alternate embodiment, the inlet port extends through an arcuate slot in the dial so a large, manually adjustable dial may be utilized. The regulator may be used with various types of IV fluids, even high viscosity fluids and lipids. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects and advantages of the present invention will become more apparent from the following descriptions, claims and drawings in which: 
         FIG. 1  is a schematic view of a representative IV administration system incorporating the flow control regulator of the present invention; 
         FIG. 2  is a bottom exploded view of the flow regulator of the present invention; 
         FIG. 3  is a top exploded view of the flow regulator of the present invention; 
         FIG. 4  is a perspective view of the screw housing; 
         FIG. 5  is an exploded vertical cross-sectional view of the regulator; 
         FIG. 6  is an enlarged cross-sectional view of the regulator; 
         FIG. 7  is a perspective view of the dial showing detent which provides a tactile indication and retention of the “off” position; 
         FIG. 8  shows the dial safety lock position preventing inadvertent positioning of the dial; 
         FIG. 9  is a schematic diagram illustrating the flow path through the regulator; 
         FIG. 10  is a perspective view illustrating another configuration of the regulator body; 
         FIG. 11  shows the regulator attached to an accessory pole mount holder by an anchoring or securement device; 
         FIG. 11A  shows the regulator of the invention attached to a securement device, the securement device being securable to the chest or forearm of the patient; and 
         FIG. 12  shows the top surface of the dial. 
     
    
    
     DETAILED DESCRIPTION 
     Turning now to the drawings, particularly  FIGS. 1 to 9 , an embodiment of the pressure compensated flow control regulator of the present invention is shown and generally designated by the numeral  10 . The flow control regulator  10  is a gravity-flow regulator and preferably is a disposable, single-use device provided to the user in a sterilized package. Gravity devices rely on fluid head pressure to overcome the back pressure of the patient&#39;s venous system. As seen in  FIG. 1 , in use the regulator  10  is interposed at a location between an IV fluid source  12  and the patient P. The flow regulator is connected to tubing line  16  which leads to an IV drip chamber  15  inserted into an IV fluid source  12  at a piercing spike. It is customary to locate the flow control regulator at an intermediate elevation below the level of the IV fluid. 
     The outlet of the flow control regulator  10  is connected to flexible tubing line  18  which leads to the patient and terminates at an administration needle at the venipuncture site “V.” As discussed above, one problem with non-compensated flow control devices is that changes in head pressure due to changes in the fluid level within the IV container, changes in elevation of the fluid container on the IV stand or movement of the patient&#39;s limb can adversely affect flow control rates. The major determination of pressure in gravity administration systems is head height which refers to the distance between the fluid level in the IV solution container and the IV site “V.” 
     In conventional systems, a roller clamp may be interposed between the IV source and the regulator. In such systems, the roller clamp is used to establish a drop rate by counting drops. However, with the regulator of the present invention, use of a roller clamp is not necessary for adjustment and may be incorporated for use only when initiating or stopping flow. 
     The flow control regulator  10  has a housing or body  24  consisting of components including an upper housing  36 , an intermediate or middle housing  28  and lower housing  31 . These housing components are preferably fabricated by injection molding using a suitable medical grade plastic material such as ABS or polypropylene. Although the housing components are shown as separate components for ease of fabrication and assembly, they may be unitary. 
     Housing  28  has an inlet port  32  which defines an inlet passage  29 . The inlet port  32  is generally cylindrical and is sized to be connectable to conventional medical tubing  16 . The inlet port  32  is preferably disposed at an angle greater than 45° with respect to the vertical axis of the regulator and may extend to 90°. The angular orientation of the inlet port  32  provides significant functional advantages in that the angular orientation facilitates use of an easy-to-adjust dial  140  having a large diameter. The angular orientation of the inlet port also allows the regulator to hang in a suspended, in-line position with the surface of dial  140  in a vertical or near vertical, easy-to-read position, as seen in  FIG. 1 . 
       FIG. 10  shows the regulator  10  with a large circular dial  140 A which operates as has been described. To accommodate the large diameter dial  140 A, the surface  141 A is provided with an arcuate groove  143 . This allows the inlet  32 A to extend axially through the slot  143 . 
     Circular wall surface  34  is provided with a concentric, downwardly depending circular flange  40  which engages the lower housing  31 . An annular flange  46  extends upwardly from the upper surface  34  at a location offset from its center. At the center of the top surface area defined by the upper flange  46  is a vertically extending orifice  50  which has a generously radiused upper edge  51  which transitions to the top surface of the wall. A small annular sealing ridge  52  on the upper surface of wall  34  extends adjacent the flange  46 . 
     A second orifice  43  is provided in wall surface  34  at the lower end of the inlet passage  29 . A shallow groove  56  extending in the bottom of wall  34  communicates orifice  43  with the center orifice  50 . A radial flow passageway  58  along the top of wall  34  extends between the center orifice  50  and orifice  54 . Orifice  54  extends through wall  34  radially spaced from center orifice  50 . These passageways are best seen in  FIGS. 2 and 3 . 
     The lower housing  31  has a generally circular planar bottom surface  60  with an upstanding peripheral flange  62  which, in the assembled position, engages the depending flange  40  of the middle housing. An outlet fitting  64  depends at a central location from the outlet housing. The fitting  64  is sized to accommodate attachment of conventional medical tubing  18  which leads to the patient. The outlet fitting  64  is spaced or offset from the location of orifice  54  located in the middle housing. 
     The upper surface of the lower housing wall defines a radially extending passageway  66  which, when the components are assembled, is aligned with orifice  54 . An outlet orifice  70  extends through the bottom wall and is centered within an annular flange  74 . The orifice is located in a raised, circular land  76  which surrounds the outlet orifice  70 . 
     The peripheral outer edges of the upper housing and the lower housing define a peripheral groove  41  about the regulator body. The groove  41  accommodates temporary attachment of the regulator  10  to an accessory such as a pole mount holder or a patient anchoring device such as safety securement device, as seen in  FIGS. 11 and 11A . The device  10  is securable to a support pole S by securement device  200  and having arms  201 ,  202  engageable in groove  41 . The regulator may be easily “snapped” in place with the arms of the holder engaging the groove  41 . The securement device has a clamp  210  adjustably attachable to the pole S. In  FIG. 11A , the regulator  10  is shown with a patient securement device  250  which has a base  252  adhesively securable to the patient. The base  252  has a plurality of projections  256 ,  257  which releasibly engage the body of the regulator along peripheral groove  41 . 
     An elastomeric membrane  80  is seated within chamber  82  defined by the upper surface of the lower housing and with the lower surface of the middle housing. The elastomeric membrane  80  is of a suitable medical grade material such as silicone rubber. The membrane has characteristics critical to the performance of the regulator and the maintenance of accurate flow rates once set by the clinician. The membrane flexes or oscillates to regulate flow to compensate for pressure changes as described above. The elastomeric membrane has a generally annular outer flange  84 . A thin, flexible center section  86  extends across the inner edges of the annular section. Typically, the thin, flexible center section of the membrane will have a thickness within the range of 0.0125″ to 0.0175.″ The membrane is best seen in  FIG. 3 . 
     It has been found that a thinner membrane will generally provide greater accuracy at lower head height and less accuracy as head height increases. It has been found that an optimal thickness for the flexible section of the membrane is about 0.015″. 
     In the assembled position, the central membrane flexible section  86  extends across top of land  76  and across outlet orifice  70 . Small annular sealing projections or ridges  88 ,  89  extend from the surfaces of the lower housing and upper housings positioned to sealingly engage the opposite surface of membrane flange  84  to prevent fluid leakage around the edges of the membrane. The characteristics of the membrane  80  are critical to proper operation. A membrane fabricated in accordance with the following has been found to work well providing the necessary control and accuracy. 
     The membrane is preferably fabricated by compression molding using a silicone material having the following characteristics listed below: 
     
       
         
               
             
               
               
               
               
             
           
               
                   
               
               
                 MEMBRANE 
               
               
                 MATERIAL CHARACTERISTICS 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Durometer, Shore A 
                 50 and above 
                 ASTMD2240 
               
               
                   
                 Tensile Strength 
                 1100 to 1200 
                 ASTM D-412 
               
               
                   
                 Elongation % 
                 200 to 300 
                 ASTM D-412 
               
               
                   
                 Modulus @ 100% 
                 60 to 70 
                 ASTMD-624 
               
               
                   
                 Tear Strength 
                 50 to 60 
                 ASTMD-624 
               
               
                   
                 Bayshore 
                 60 and above 
                 ASTMD-624 
               
               
                   
               
             
          
         
       
     
     A vertical port  90  is provided in the annular portion  84  of the membrane and, when the regulator is assembled, aligns with the orifice  54  in the inlet housing and communicates this passageway with the radially extending groove  66  in the upper surface of the bottom housing. Thus, as seen in  FIG. 9 , a flow path is established from the inlet passageway  29  through the orifice  43  at the bottom of the inlet passage and extending radially along channel  56  to central orifice  50  in the middle housing. The flow path continues upwardly through orifice  50  and then radially along channel  58  to a location registering with the vertical port  90  in the annular portion of the membrane  80 . Fluid will then flow downwardly through the port  90  to the radial passage  66  in the lower housing. Fluid will then pass between the membrane and the bottom surface to an annular outlet chamber  95 . This chamber  95  is defined by annular ring  74 . Raised land  76  is located in the center of ring  74  and defines an outlet orifice  70  communicating with the lumen in the outlet fitting  64 . 
     The flow control regulator further includes an upper housing  36 . The upper housing  36  has a generally cylindrical body  100  which, when assembled, is received within the annular flange  46  of the middle housing  28 . The upper housing  36  defines a vertical bore  104  which is aligned with an orifice in the middle housing. Bore  104  defines the plurality of screw threads  106  which are preferably left hand threads. An elastomeric flow control disk  120  is seated on the upper surface of the middle housing extending across the upper end of the orifice  50 . An adjusting screw  110  is threadably received in the threaded bore  104 . The adjusting screw has a body with external male threads  130  which are engageable with the threads  106  in the upper housing. 
     The pitch of the mating screw threads  106  and  130  are selected to provide ease of adjustment and to spread the range of adjustment over about 360° of dial rotation. A thread pitch of about 48 has been found to work well providing medical technicians “feel” for the precise adjustment. The lower end of the screw body tapers to a rounded point  132  which engages the upper surface of the elastomeric disk  120 . 
     The disk  120  is preferably molded silicon having a thickness approximately 0.070″. The preferred material characteristics for the control disk are set forth in the above Material Characteristic chart below. 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                   
               
               
                 CONTROL DISK MATERIAL CHARACTERISTICS 
               
             
          
           
               
                 Test Method Description 
                 Unit 
                 Lower Unit 
                 Upper Unit 
               
               
                   
               
             
          
           
               
                 COMPRESSION SET 22/350F 
                 % 
                 0 
                 35 
               
               
                 A/B BLEND PB 
                   
                   
                   
               
               
                 DUROMETER A/B BLEND PB 
                 No Unit 
                 57 
                 65 
               
               
                 ELONGATION A/B BLEND PB 
                 % 
                 320 
                 550 
               
               
                 MODULUS 100% PSI A/B 
                 psi 
                 0 
                 900 
               
               
                 BLEND PB 
                   
                   
                   
               
               
                 TENSILE PSI A/B BLEND PB 
                 psi 
                 1159 
                 2000 
               
               
                 TEAR B PPI A/B BLEND PH 
                 ppi 
                 143 
                 400 
               
               
                 SPECIFIC GRAVITY 
                 No Unit 
                 1.11 
                 1.15 
               
               
                 DENSITRON ASTM792-A 
                   
                   
                   
               
               
                 GOETTFERT T90 MINUTES 
                 min 
                 4.5 
                 6.5 
               
               
                 GOETTFERT RHEOMETER 
                 ° C. 
                 113.0 
                 120.0 
               
               
                 C.I.T. LINEAR PRG. 
               
               
                   
               
             
          
         
       
     
     The upper end of the screw  110  has a plurality of axial ribs  134  which are received within the cooperating opening  138  in the control dial  140 . The control dial  140  has an upper generally planar surface  141  which has a substantial diameter for ease of use and which may be provided with flow rate indicia graduations  180  from 0 to full flow which are positioned relative to the screw when the regulator is calibrated, as seen in  FIG. 12 . The graduations are indexed with reference to indicator  146  located on the inlet  32  as best seen in  FIG. 12 . The outer edge of the control dial forms a downwardly depending surface  142  which may be ribbed or knurled on its exterior for ease of use. An inner, annular flange  144  closely cooperates with the smooth, outer surface  105  of the housing  36  having cooperating grooves  148  so the dial may be “snapped” in place over the upper end of the screw and secured to the screw in a desired calibration position at assembly with respect to the upper housing and disk  120 . 
     It will be apparent that by rotating the control dial, rotation will also be imparted to the adjusting screw  110 . As the adjusting screw is advanced by rotating the dial in a counterclockwise direction, the elastomeric disk  120  will be forced downwardly toward the upper radiused edge  51  of the control orifice  50  reducing the clearance between the orifice  50  and passageway  59  to reduce the flow rate. Rotation of the control dial in the opposite direction will open the clearance at the control orifice permitting greater flow rate. The upper housing  36  carries flow rate indicia  180  which cooperates with indicator  146  along the center portion of the inlet port  32 . The graduations  180  on the upper surface  141  of the control dial indicate flow regulating positions, full off, full flow and purge positions and with intermediate flow rate or graduations indicated between these positions. The flow control regulator is calibrated at the time of assembly as will be explained hereafter. 
     Referring to  FIGS. 4 ,  7 , and  8 , several safety features are shown. The control dial  140  may be rotated approximately 360° from “off” to a full open or purge position. In order to provide a positive indication of the “off” position so that a low rate of fluid flow is not inadvertently delivered to the patient, the dial is provided with a detent feature. As seen in  FIGS. 4 and 7 , a circular flange  155  on the underside of the dial is provided with two adjacent projecting ridges  150 ,  152  connected by a land area  154 . Ridge  150  is shallower than ridge  152 . A stop  160  is provided on the housing  36  at the “off” position and has a detent  161  located to be engageable with the ridges. When the dial is rotated counterclockwise to the “off” position, the first ridge  150  will encounter the detent  161  and the stop indicating a slow or KVO position has been reached. Further rotation of the dial will cause the detent  161  to pass over ridge  150  seating in the land  154  in a positive “off.” The dial material will deflect slightly. This feature also prevents continued rotation of the dial to prevent it from being removed. Rotation in the opposite direction will cause the detent to abut the ridge  152  in the full open position. 
     The user is provided tactile input as the dial is rotated to enhance and refine the adjustment procedure. The inner surface of the peripheral dial flange is provided with spaced grooves  164 . The stop  160  carries a V-shaped projection  166  which engages the grooves when the dial is assembled. The engagement is a light, frictional engagement so that as the dial is rotated, the user will sense the “click” as the projection  166  rides over the dial surface seating in each groove. The engagement will also assist in preventing inadvertent rotation of the dial caused by incidental contact. 
       FIG. 8  illustrates another feature which provides a safety lock to prevent inadvertent movement of the dial. A deflectable lever  170  extends upwardly from the middle housing  28  having an upper end with a detent  172  engageable in grooves  164  to prevent dial rotation. The frictional engagement, which is greater than the engagement between projection  166  and grooves  164 , will serve to maintain a dial setting even if the dial is attempted to be rotated. The medical attendant must apply an inward force to the lever  170  while holding the regulator body to disengage the detent  172  from the groove in which it is seated. The dial setting can then be changed using the other hand. Release of the pressure applied to the lever  170  will cause it to return to a position engaging the inner grooves  164  on the dial and again locking the dial in the set position. 
     Operation 
     In the operation of the flow regulator, a roller or slide clamp, if used, is placed in a closed position and the flow regulator is placed in the tubing line at a convenient location between the drip chamber and the administration needle, as seen in  FIG. 1 . The flow regulator needs to be positioned below the fluid level of the container as the device is gravity-driven. Conventional connectors and adaptors are conventionally used and are employed to make the connection. 
     Air is purged from the system by opening the flow regulator to the full open or purge position as indicated by the indicia on the control dial  140  and disengaging the roller clamp/slide clamp. Once the air is purged from the system, the control knob is rotated to a position to a fully closed position by turning on the knob to force the disk downwardly. Once a needle is inserted in the venipuncture site “V” the control dial can be rotated to the suitable selected position indicated on the dial to establish a desired flow rate. In some instances it may be desirable to check or confirm the flow rate by timing the drop rate through the drip chamber. The medical clinician will confirm the flow rate by timing the drop rate through the drip chamber. 
     Once the device is set at the desired flow rate, the control membrane  80  will function and maintain an essentially constant flow rate regardless of changes in fluid level within the IV container or patient movement. As indicated, flow will pass from the inlet across the middle housing to the rate control orifice  50 . The position of the screw  110  will determine the position of the elastomeric disk  120  relative to the orifice  50  and the radial passageway  58  of the middle housing  28  controlling the rate of flow passing passing through the aperture and radially to the passageway in the membrane. Fluid will then pass downwardly and radially through the passageway in the upper surface of the lower outlet housing and will enter the orifice at the upper end of the outlet. The opening or space between the orifice and lower surface of the flexible membrane will vary in response to pressure in the chambers or either side of the thin membrane. It will be appreciated that the upper surface of the flexible membrane is subjected essentially to the pressure existing in the inlet. The pressure existing at the underside of the membrane at the outlet orifice is the pressure existing at the venipuncture site. 
     If the pressure within the inlet chamber is reduced due to a decrease in head pressure or patient movement towards the fluid source the resulting reduction in inlet pressure relative to outlet pressure will force the membrane away from surface  76  of the land to effectively increase the orifice opening to the outlet passageway with a compensating increase in flow. Conversely, by increasing the elevation of the IV fluid container, head pressure will increase and similarly have an opposite effect by forcing the elastomeric membrane down on its landing and closer to the outlet port, decreasing the effective orifice opening of the outlet passageway with a compensating decrease in flow. 
     Calibration 
     Calibration is accomplished by connecting the inlet to a source of static fluid pressure (air or liquid). The dial has not been assembled and the screw  110  is used to turn the regulator to an “off” position. The regulator is tested for leakage by observation and by pressure readings on a pressure gauge. Calibration is generally performed with reference to a head height of 32″ which is representative of most IV set-ups. 
     After leak testing, the dial  140  may be affixed to the adjusting screw by aligning the proper dial indicia “off” with the “off” position of the screw. The dial is then rotated to the full open position so that the disk is not subject to pressure from the screw tip  132  that may deform it. The unit may be packaged and subject to sterilization by gas or gamma ray as is conventional. 
     It will be obvious to those skilled in the art to make various changes, alterations and modifications to the invention described herein. To the extent these various changes, alterations and modifications do not depart from the spirit and scope of the appended claims, they are intended to be encompassed therein.