Patent Publication Number: US-6709417-B1

Title: Valve for intravenous-line flow-control system

Description:
RELATED APPLICATIONS 
     The present application is a divisional of U.S. application Ser. No. 08/917,537 filed Aug. 22, 1997, now U.S. Pat. No. 6,165,154 which, in turn, is a continuation-in-part of U.S. application Ser. No. 08/478,065 filed Jun. 7, 1995, which issued as U.S. Pat. No. 5,755,683 on May 26, 1998, which was concurrently filed with applications Ser. No. 08/472,212, entitled “Intravenous-Line Flow-Control System” for an invention by Heinzmann, Kamen, Lanigan, Larkins, Lund and Manning, which issued on Jun. 30, 1998 as U.S. Pat. No. 5,722,637; Ser. No. 08/481,606, entitled “Intravenous-Line Air-Elimination System” for an invention by Manning, Larkins, Houle, Kamen and Faust, which issued on Feb. 3, 1998 as U.S. Pat. No. 5,713,865; and Ser. No. 08/477,380, entitled “Intravenous-Line Air-Detection System” for an invention by Larkins, Beavis and Kamen, which issued on Jun. 24, 1997 as U.S. Pat. No. 5,641,892. All of these related applications are hereby incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to apparatus and methods for controlling flow through an intravenous line. 
     SUMMARY OF THE INVENTION 
     The invention is directed to a cassette for controlling the flow of IV fluid from a source to a patient. The cassette preferably includes, along the fluid passage through the cassette, first and second membrane-based valves on either side of a pressure-conduction chamber, and a stopcock-type valve. The stopcock valve is preferably located downstream of the second membrane-based valve, which is preferably located downstream of the pressure-conduction chamber. 
     In a preferred version of the cassette, which is primarily made out of rigid material, the membrane for the second membrane-based valve is disposed adjacent the housing, such that the rigid housing and the membrane define a valving chamber. One passage enters the valving chamber at a first mouth located at the end of a protrusion of the rigid housing into the valving chamber towards the membrane, and the valve may prevent the flow of fluid therethrough when the membrane is forced against the first mouth, by the control unit. The control valve restricts the flow of intravenous fluid from the valving chamber to the patient, since it is located downstream of the valving chamber. The membrane defining the valving chamber is preferably large and resilient, so that the valving chamber may provide a supply of pressurized intravenous fluid to the patient, when the first mouth is sealed closed and when there is a restriction downstream of the valving chamber. 
     For the pressure-conduction chamber, a membrane is preferably disposed adjacent the rigid housing, so as to define a pressure-conduction chamber, wherein the rigid housing portion that defines the pressure-conduction chamber is generally dome-shaped. The membrane has a filled-chamber position, in which position the pressure-conduction chamber is substantially at its greatest volume, and an empty-chamber position, in which position the pressure-conduction chamber is at its smallest volume, and in which position the membrane rests against the rigid housing and assumes the dome shape of the rigid housing. The membrane preferably has a structure for creating instability in the membrane in the filled-chamber position. Preferably, this structure may be actuated to create instability in the membrane in the empty-chamber position. The rigid housing and the second membrane in the empty-chamber position preferably define an unobstructed fluid passageway through the pressure-conduction chamber from the first to the second pressure-conduction chamber mouth. Preferably, the structure for creating instability in the membrane causes the membrane, when its at its full-chamber position, to collapse in the region of the pressure-conduction chamber&#39;s outlet mouth before collapsing nearer the inlet mouth. This structure helps force bubbles in the fluid upward toward the inlet mouth and the IV fluid source during a bubble-purge cycle. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a top view of a cassette according to a preferred embodiment of the present invention. 
     FIGS. 2 and 3 show front and bottom views respectively of the cassette of FIG.  1 . 
     FIG. 4 shows a control unit for receiving and controlling a cassette, such as the cassette of FIGS. 1-3. 
     FIG. 5 shows a cross-section of the cassette of FIGS. 1-3. 
     FIG. 6 shows a rear view of the cassette and shows the fluid paths through the cassette. 
     FIG. 7 shows a front view of the middle rigid panel of the cassette of FIGS. 1-3. 
     FIGS. 8 and 9 show side and rear views respectively of the middle panel of FIG.  7 . 
     FIG. 10 shows a partial cross-section of the middle panel of FIG.  7 . 
     FIG. 11 is a cross-sectional detail of the control valve of the cassette according to a preferred embodiment of the invention. 
     FIG. 12 shows a side view of an outer cylinder (a valve-seat member) having rigid and resilient elements that may be used in the control valve. 
     FIG. 13 shows a cross-sectional view of the cylinder of FIG.  12 . 
     FIG. 14 depicts the relationship between the aperture of the FIG. 12 cylinder and the groove used in the control valve. 
     FIG. 15 shows a cross-sectional view of the membrane that may be used in the pressure-conduction chamber of the cassette shown in FIG.  1 . 
     FIGS. 16 and 17 show front and rear views respectively of the FIG. 15 membrane. 
     FIG. 18 shows a front view of the membrane used in the valve located downstream of the pressure-conduction chamber and upstream of the control valve. 
     FIG. 19 shows a cross-section of the FIG. 18 membrane. 
     FIG. 20 is a schematic representing how the compliant membrane of FIG. 18 may be used to regulate the pressure of fluid to the patient. 
     FIG. 21 is a graph depicting the advantage of using a compliant membrane such as that shown in FIG.  18 . 
     FIGS. 22 and 23 depict the preferred shape of the inlet valve to the pressure conduction chamber. 
     FIG. 24 shows a cross-sectional view of the inlet valve to the pressure conduction chamber. 
     FIG. 25 shows a preferred arrangement of teeth around the circumference of the control wheel. 
     FIG. 26 shows a front view of a cassette according to an alternative preferred embodiment of the present invention. 
     FIG. 27 shows a front view of the membrane that may be used in the pressure-conduction chamber of the cassette shown in FIG.  26 . 
     FIG. 28 shows a cross-sectional view of the membrane shown in FIG. 27 along line B—B. 
     FIG. 29 shows a cross-sectional view of the membrane shown in FIG. 27 along line  29 ; 
     FIG. 30 shows a cross-sectional view of the membrane shown in FIG. 27 along line A—A. 
     FIG. 31 shows a perspective view of an alternative cassette which may use the membrane shown in FIGS.  27 - 30 . 
    
    
     DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
     The present invention includes a cassette for use in a system for controlling the flow of IV fluid to a patient, along the lines of the cassettes disclosed in U.S. Pat. Nos. 5,088,515 and 5,195,986. A preferred embodiment of the cassette is depicted in FIGS. 1-3, which respectively depict top, front and bottom views of the cassette. The cassette is used in a control unit, such as that described in above-referenced U.S. Pat. No. 5,772,637, entitled “Intravenous-Line Flow-Control System,” which is similar to the control unit described in U.S. Pat. No. 5,088,515, which describe the use of pressure, preferably pneumatic pressure, for controlling the actuation of valves and the urging of fluid into and out of a pressure-conduction chamber. In addition to performing the function of a pump urging fluid through the IV line, the pressure-conduction chamber can measure the amount of IV fluid being delivered to the patient as well as detect the presence of bubbles in the IV fluid in the pressure-conduction chamber. Preferred methods of detecting and eliminating air bubbles from the IV fluid are discussed in the above-referenced patent applications for “Intravenous-Line Air-Detection System” and “Intravenous-Line Air-Elimination System,” now U.S. Pat. Nos. 5,641,982 and 5,713,865, respectively. FIG. 4 depicts a preferred version of a control unit  10 . Control unit  10 , which has a user-interface panel  103  containing a key pad and a display so that the status of the IV fluid delivery may be monitored and modified by medical personnel. The cassette is slipped behind door  102 , and by turning handle  101  the door is pressed against the cassette, which in turn is then pressed against the main housing of the control unit  10 . The main housing  104  preferably includes mechanical means for actuating membrane-covered valves and for applying a pressure against the membrane of the pressure-conduction chamber. The main housing  104  also includes means for turning the control wheel of the cassette. 
     Referring to FIG. 2, the main components of the preferred embodiment of the cassette are a first membrane-based valve  6 , a pressure-conduction chamber  50 , a second membrane based valve  7  and a stopcock-type control valve  20 . Valve  6  controls the flow to the pressure-conduction chamber  50  from the inlet  31  to the cassette, which is connected to an IV line, which in turn is connected to a source of IV fluid. The second membrane-based valve  7  and the control valve  20  together are used to control the flow of fluid from the pressure-conduction chamber  50  to the outlet to the cassette  33 , which is connected to the IV line leading to the patient. 
     The rigid housing  15  of the cassette is made primarily from three rigid panels. A front panel  17 , a middle panel  18 , and a rear panel  16 , all three of which can be seen in FIGS. 1 and 3. The front panel is preferably molded integrally with the outer collar  21  of the control valve  2 . The wheel  20  of the control valve  2  preferably includes ribs  281  and/or teeth mounted along the circumference  29  of the knob  20 . (FIG. 25 shows a preferred arrangement of teeth around the circumference  29  of the control knob  20 .) The teeth and/or ribs  281  may be engaged by the main housing  104  of the control unit  10 , so that the control unit  10  may change the resistance that the control valve  2  exerts on the IV fluid passing through the valve. 
     The cassette may also be used without the control unit  10 . In that case, the control wheel  20  may be turned by hand. When disengaged from the control unit  10 , the membrane of the pressure-conduction chamber  50  is preferably collapsed so that it rests against the rigid rear wall  59  of the pressure-conduction chamber  50 . With the membrane in this collapsed state, IV fluid may still easily flow through the pressure-conduction chamber  50  through a raised portion  35  of the rear wall  59 . This raised portion  35  defines a conduit  36  leading from the inlet mouth of the pressure-conduction chamber  50  to the outlet mouth of the pressure-conduction chamber, as can be seen in FIG.  6 . FIG. 6 shows the fluid paths leading through the cassette. As noted above, fluid enters the cassette through the inlet  31 , whence it flows through a fluid path to valve  6 . The fluid then enters the valving chamber of valve  6  through an inlet port  62 . An outlet port  61  is preferably mounted on a protrusion so that pressure from the pressure-conduction chamber  50  is less likely to force the membrane to lift from the outlet port  61 . From valve  6  the fluid passes to the inlet mouth  56  of the pressure-conduction chamber  50 . The pressure-conduction chamber is seen in the cross-sectional view of FIG. 5. A membrane  41  allows pressure from the control unit  10  to be applied to the fluid in the pressure-conduction chamber  50  without the fluid coming into contact with the control unit  10 . When the membrane  41  is in its collapsed position resting against rigid wall  59 , as shown in FIG. 5, fluid can still pass from inlet valve  56  through conduit  36  to the outlet valve  57 . After passing through the pressure-conduction chamber  50 , the fluid flows to the second membrane-based valve  7 , which included an inlet mouth  73 , which is mounted on a protrusion  72  in similar fashion to the outlet port  61  of the first membrane-based valve  6 . The second membrane-based valve&#39;s inlet mouth  73  and the protrusion  72  on which it is mounted can be seen in the cross-sectional view of FIG.  5 . Like the outlet port  61  of the first membrane-based valve, the inlet mouth  73  may be closed by the application of pressure by the control unit on a membrane; a first portion  71  of the membrane that closes off the inlet mouth  73  can be seen in FIG.  5 . After passing through the outlet mouth  76  of the second membrane-based valve  7 , the fluid passes to the inlet  77  of the stopcock-type control valve, which inlet can be seen in both FIGS. 5 and 6. After passing through the control valve and the fluid path  78  exiting from the control valve, the fluid passes to the outlet of the cassette  33  and to the IV line leading to the patient. 
     FIG. 7 shows a front view of the rigid middle portion of the cassette, and FIG. 8 shows a side view of the middle rigid panel  18 . The middle rigid panel  18  defines the cassette inlet  31  and outlet  33 , a circumferential portion of the pressure-conduction chamber  50 , and port  62 , outlet port  61 , inlet mouth  73 , and outlet mouth  76  of the two membrane-based valves  6  and  7 . The protrusions  63  and  72  of the outlet port  61  and inlet mouth  73  can also be seen in FIG.  7 . FIG. 9 shows a rear view of the middle rigid panel  18  shown in FIGS. 7 and 8. The ports/mouths  61 ,  62 ,  73 ,  76  can also be seen in FIG.  9 . FIG. 10 shows a partial cross-section of the middle rigid portion. The cross-section shows the outer collar  21  of the control valve, which is integrally molded with the rest of the middle rigid portion. The outer collar  21  defines a hollow area  22  and a fluid path  23  leading from the hollow area  22 . 
     FIG. 11 shows a cross-section of an assembled control valve  2  that may be used in a cassette according to the present invention. Just inside of the outer collar  21  is a valve-seat member  22  fixedly attached to the outer collar  21  so that the valve-seat member  21  does not rotate with respect to the rest of the cassette. The valve-seat member  21  is depicted in greater detail in FIG.  12  and in cross-section in FIG.  13 . The valve-seat member  22  also defines a hollow area, which accepts the shaft  220  of the control wheel  20 , so that the control wheel&#39;s shaft  220  rotates with the control wheel  20 . The valve-seat member  22  is comprised mostly of rigid material, but importantly it also includes molded-over resilient material, which is used to form sealing O-rings. This resilient material forms an O-ring  26  around the base of the valve-seat member  22 ; the rigid portion of the base defines a passage  222 , connecting the valve inlet  77  to passage  24 . The resilient material  25  also provides a seal around an aperture  251  in the circumferential surface of the member  22 . At the end of the member  22  opposite the inlet passage  222  is an inner O-ring  27  which forms the seal between the control wheel&#39;s shaft  220  and the valve-seat member  22 . The O-ring  26  around the exterior circumference of the base provides a seal between the outer circumferential wall of the valve-seat member  22  and the inner circumferential wall of the outer collar  21 . Likewise, the O-ring  25  around the circumferential port  251  may provide a seal between the outer circumferential wall of the valve-seat member  22  and the inner circumferential wall of the outer collar  21 . Together, O-rings  25 ,  26  prevent fluid from leaking between the valve-seat member  22  and the outer collar  21 . Importantly, the O-ring  25  of port  251  also provides a seal between the valve-seat member  22  and the shaft  220 , so that when the valve is in the fully closed position no flow is permitted between passageway  24  of shaft  220  and the port  251  of the valve-seat member  22 . 
     The advantage of this design over previous stopcock valves is that the outer diameter of the shaft  220  may be slightly less than the inner diameter of the valve-seat member  22 , whereas previous stopcock valves required an interference fit between the inner and outer components. It will be appreciated that the stopcock valve of the present invention may use frusto-conical-shaped members instead of cylindrical members. The interference fit of prior-art devices created a great deal of resistance when the stopcock valves were turned. The use of O-rings in the stopcock valve of the present invention avoids the need for this interference fit and the greater torque required for turning the valve resulting from the interference fit. O-ring  27  prevents leaking from the space between the valve-seat member  22  and the shaft of the control wheel  20 . 
     The valve-seat member is preferably made in a two-part molding process, wherein the rigid portion is first molded and then the softer resilient material is over-molded onto the rigid portion. Channels may be provided in the initially molded rigid portion so that the resilient material may flow to all the desired locations; this results in columns of resilient material  28  connecting the areas of resilient material through these channels. The valve-seat member  22  is preferably molded separately from the rest of the cassette, and when the cassette is assembled the valve-seat member  22  is placed in the hollow defined by the outer collar  21  of the middle panel  18 , and aligned so that aperture  251  lines up with passageway  23 . (The shape of the outer diameter of the valve-seat member  22  and the inner diameter of the outer collar  21  may be complementarily shaped so that the valve-seat member must align properly with the aperture  251  and the passageway  23  lined up.) Then, the front rigid panel  17  is ultrasonically welded (along with the rear rigid panel  16 ) to the middle rigid panel  18 , and the valve-seat member  22  is then held in place in the hollow area defined by the outer collar  21 . The outer circumference of the valve-seat member  22  may be a bit smaller than the inner diameter of the outer collar  21 ; O-rings  25 ,  26  prevent fluid from flowing from the passages  77  or  23  to point  19 . This design of the valve-seat member  22  avoids the need for tight tolerances in the various components of the valve  2 . The control wheel&#39;s shaft  220  may be inserted into the hollow area defined by valve-seat member  22  after the rest of the valve has been assembled. The shaft  22 ( 0  is held in place by a lip  161 around the inner circumference of the hollow area defined by the rear rigid panel  16   
     When the valve  2  is fully opened, the circumferential aperture  251  is lined up with the fluid passage  24  in the shaft  220 . When the valve is fully closed there is no fluid communication between the aperture  251  and the fluid passage  24 . The outer circumferential surface of the shaft  220  preferably includes a groove extending circumferentially around the shaft&#39;s outer circumferential wall from the terminus of the fluid passage  24  at the outer circumferential wall; the groove tapers in cross-sectional area and does not extend all the way around the outer circumference of the shaft  220 . The groove provides greater control of the flow rate. FIG. 14 shows the respective locations of the groove  231 , which is located on the outer circumference of the shaft  220  and the circumferential aperture  251  of the valve seat member  22 . As the aperture  251  rotates to the right, in the FIG. 14 perspective, the resistance to flow increases, until the groove  231  tends and the aperture  251  loses fluid communication with the groove  231 , at which point flow is completely shut off through the control valve  2 . As the aperture  251  rotates to the left, in the FIG. 14 perspective, the resistance to flow decreases. Preferably, the groove  231  is longer than the diameter of the aperture  251 , so that the flow rate may be controlled more finely. 
     As noted above, the cassette may be used independently of the control unit  10 . When the cassette is used in this manner it is preferable that the membrane  41  rest against the rigid back  59  of the pressure-conduction chamber  50  so as to minimize the volume of the conduit  36  for fluid passing through the pressure conduction chamber  50 . If the membrane  41  were too flexible and the volume of the pressure-conduction chamber  50  varied widely, medical personnel would be unable to rely on a quick visual inspection of the rate of dripping in the drip chamber to indicate a steady, desired flow rate through the IV line. Thus, it is desired that the structure of the membrane  41  be such that it tends to rest against wall  59  unless and until a sufficient pressure differential is created across the diaphragm  41 . This pressure differential is preferably caused by a negative gas pressure caused by the control unit  10 . Although it is desired to manufacture the diaphragm  41  so that it has some tendency to rest against wall  59 , it is desired to make the diaphragm  41  floppy in the other direction so that less pressure is required to move it from its position when the pressure-conduction chamber  50  is full, the “filled-chamber” position. It is also desired that the measurement gas provided by the control unit  10  against the outer face of the membrane  41  be at substantially the same pressure as the fluid on the inner side of the membrane  41  in the pressure-conduction chamber  50 . 
     By molding the diaphragm  41  in the shape of a dome corresponding to that of the rigid wall  59 , the diaphragm will have a tendency to remain in its position, as shown in FIG. 5, resting against wall  59  when the chamber  50  is at its lowest volume, the “empty-chamber” position. However, when the diaphragm  41  is molded in this way, it also tends to remain in the filled-chamber position, in other words, when the diaphragm  41  is bulging convexly outward from the cassette. This convex, filled-chamber position can be made unstable by adding additional material on the outer, usually concave surface of the diaphragm  41 . This additional material  43  can be seen in the cross-section of a preferred embodiment of the diaphragm as shown in FIG.  15 . The diaphragm  41  shown in FIG. 15 is molded in the position shown and has a tendency to remain in that position. When the chamber is filled with fluid, the normally concave side of the diaphragm becomes convex, and the additional material  43  is subject to an additional amount of strain since it is at the outer radius of this convex, filled-chamber position. The diaphragm  41  shown in FIG. I 5  also includes an integrally molded O-ring  44  around its circumference for mounting and sealing the diaphragm  41  in the cassette. FIG. 16 shows a view of the exterior side of the diaphragm  41  of FIG.  15 . This surface of the diaphragm  41  is normally concave when the diaphragm is in the empty-chamber position. The additional material  43  can be seen in the view of FIG.  16 . FIG. 17 shows the interior side of the diaphragm  41  of FIG.  15 . This side is normally convex when the diaphragm  41  is in the empty-chamber position. Thus, as a result of molding the diaphragm so that its inner surface has a smooth constant radius and the outer surface has additional material, which thereby interrupts the smoothness and constant radius of the rest of the outer face of the diaphragm, the diaphragm  41  has the desired tendency to remain in the empty-chamber position while being unstable in the filled-chamber position. 
     By positioning this additional material  43  near the outlet mouth  57  of the pressure-conduction chamber  50 , the collapse of the diaphragm  41  from its filled-chamber can be somewhat controlled so that the diaphragm tends to collapse first in the lower portion of the pressure-conduction chamber near the outer mouth  57  before further collapsing in the upper region of the pressure conduction chamber nearer the inlet mouth  56 . The cassette is preferably mounted in the control unit with a slight tilt so that the passage  36  is vertical and the inlet mouth  56  is at the very top of the chamber  50  and the outlet mouth  57  is at the very bottom of the chamber  50 . This orientation permits the bubbles that may be present in the chamber  50  to gravitate towards the inlet mouth  56 , which is at the top of the chamber. In a preferred method of eliminating the bubbles from the IV fluid, as described in the above-referenced, concurrently filed application for “Intravenous-Line Air-Elimination System,” any bubbles that are detected by the control unit in the pressure conduction chamber  50  are forced by pressure from the control unit against the external surface of the membrane  41  up to the inlet mouth  56  to the cassette inlet  31  up the IV line to the fluid source, sometimes after several purging and filling cycles. When purging the bubbles from the chamber  50  through the inlet mouth  56  it is preferred that the chamber collapse at its bottom first so that the membrane does not interfere with bubbles moving upwards through the chamber  50 . 
     Thus, the additional material  43  creates an instability in the membrane  41  when the membrane is in the filled-chamber position, thereby making the membrane more likely to collapse from the filled-chamber position than a membrane that did not have the additional material. The additional material  43 , however, does not create an instability in the membrane  41  when the membrane is in the empty-chamber position. In many situations it is desirable to be able to introduce some instability into the membrane when the membrane is in the empty-chamber position. By introducing such instability into the membrane, less negative pressure is needed to move the membrane from its empty-chamber position. 
     To create an instability in the empty-chamber position, a pressure-relief tab  143  may be added to the membrane  141  as shown in FIG.  26 . The pressure-relief tab  143  extends from the exterior surface  145  near the edge of the membrane  141 , as can be seen in the cross-sectional view of FIG.  29 . FIG. 28 shows another cross-sectional view, which view does not pass through the pressure-relief tab  143 , and FIG. 27 shows a front view of the membrane  141 . The tab  143  may be actuated by an actuator  149  (shown in FIG. 30) mounted in the control unit. When it is desired to introduce instability into the membrane—which will typically be whenever it is desired to fill a previously empty chamber  50 —the actuator  143  forces the tab  143  towards the O-ring  144 . This action pulls the portion of the membrane  141  near the tab  143  away from the cassette&#39;s rigid wall, which partially defines the pressure-conduction chamber  50 . The tab  143  is located near the inlet mouth of the chamber  50  so that, when the actuator  149  pulls a portion of the membrane  141  away from the rigid wall, a pocket of space is formed into which the fluid can flow. By supplying a negative pressure to the exterior surface  145  of the membrane  141 , the control unit may cause more liquid to be drawn into the pressure-conduction chamber  50 . Less negative pressure is needed to move the membrane  141  out of the empty-chamber position, when the actuator  149  has urged the tab  143  towards the O-ring  144  and the rigid portion  117  of the cassette adjacent the tab  143 . 
     If it is desired to make the membrane  141  stable in the empty-chamber position, the control unit may cause the actuator  149  to be returned to the non-actuating position, so that the tab  143  may return to its normal position, extending outwardly from the cassette. As noted above, when the membrane is in the empty-chamber position, IV fluid may flow through the pressure-conduction chamber  50  through a conduit defined by raised portion of the rear wall (see FIGS. 3,  5  and  6 ) and leading from the inlet mouth of the pressure-conduction chamber  50  to the outlet mouth of the pressure-conduction chamber. 
     The pressure-reduction tab  143  also creates an instability in the filled-chamber position. When the pressure-conduction chamber  50  is filled with liquid, the exterior surface  145  of the membrane  141  becomes convex, rotating the tab  143  towards the O-ring  144 , so that the tab  143  is urged against the rigid portion  117  of the cassette. In this position, the tab  143  creates pressure on a portion of the membrane  141  so as to make the membrane less stable in the filled-chamber position so that the control unit needs to create less positive pressure to collapse the membrane  141  from its filled-chamber position. 
     FIG. 31 shows a cassette  215  that may be used in a bed-side pharmacy system, such as that described in the concurrently filed patent application for “System, Method and Cassette for Mixing and Delivering Intravenous Drugs” bearing assigned Ser. No. 08/916,890, and which lists Kamen, Grinnell, Mandro, Gilbreath, Grant, Demers, Larkins and Manning as inventors, now abandoned in favor of continuation-in-part application, assigned Ser. No. 09/137,025 which application is incorporated herein by reference. Such a cassette may also use a membrane  241  having a pressure-reduction tab  243 , which creates some instability in the filled-chamber position and which may be actuated to create some instability in the empty-chamber position. 
     Returning to the cassette  15  shown in FIGS. 1-3, a preferred membrane design for the second membrane-based valve  7  is shown in FIGS. 18 and 19. This membrane has an O-ring  78  for mounting and sealing the inlet; membrane onto the cassette (like the lip  44  on the membrane  41  for the pressure-conduction chamber, and like the circular membrane, which is not shown, for the first membrane-based valve  6 ). This membrane has a first portion  71 , which is used to seal off the inlet mouth  73  located on protrusion  72  (see FIG.  5 ). The control unit  10  exerts a pressure against this portion of the membrane  71  mechanically, in order to close off the valve  7 . A second compliant portion  74  of the membrane is sufficiently compliant so that when the control valve  2  is sufficiently restricting flow out of the outlet  76  of the second membrane-based valve  7 , the compliant portion  74  of the membrane will expand outwardly so as to hold, under pressure, a volume of IV fluid. This design is desirable so that when the inlet mouth  73  is closed, because the pressure-conduction chamber needs to be refilled, the fluid stored in the valving chamber (item  75  in FIG. 5) is available to be dispensed through the control valve  2 . 
     FIG. 20 shows a schematic for an electrical model of the operation of the second membrane-based valve  7  working in conjunction with the stopcock-type control valve  20 . When the valve leading from the outlet  57  of the pressure-conduction chamber  50  is open, permitting flow from the pressure-conduction chamber through valve  7 , and if the stopcock valve  20  is set to provide a large amount of resistance to the flow from valve  7  to the patient, the valving chamber  75  and its corresponding compliant membrane portion  74  can accumulate a “charge” of fluid, much like a capacitor, as shown in FIG.  20 . When first portion  71  is then urged against inlet mouth  73  closing off flow from the pressure-conduction chamber  50 , the charge of fluid in the valving chamber  75  is urged by the compliant membrane portion  74  to continue flow through the stopcock valve  20 . As fluid exits the valving chamber  75 , the pressure of the fluid decreases as the compliant portion  74  of the membrane returns to its unstretched state. FIG. 21 shows a graph depicting the pressure of the IV fluid being delivered to a patient over time as outlet valve  71 ,  73  is closed at time t 1  and reopened at t 2 . A solid line depicts the pressure to the patient without a compliant membrane portion  74  design. With a compliant membrane portion  74 , the sharp drop off in pressure at t 1  is eliminated or ameliorated. If the stopcock valve is nearly closed so that only a small trickle of fluid is allowed to flow through it, the design of the compliant membrane portion  74  will greatly smooth out the delivery of fluid, as long as the time between t 1  and t 2  is not too long. When the stopcock valve  2  is fully open a sharp drop in pressure may still be expected at time t 1 . 
     As noted above (and as described in the above-referenced U.S. Pat. No. 5,713,865, entitled “Intravenous-Line Air-Elimination System”), when an air bubble is being purged from the pressure-conduction chamber  50 , it is preferably forced up through the chamber&#39;s inlet valve  56  (which in this air-elimination mode is acting as an outlet). Preferably, the inlet port  56  is shaped so that a small bubble will not tend to stick to an edge of the port while allowing liquid to flow past it. To prevent such sticking of a small bubble, the port  56  preferably flares out so that the corner where the port  56  meets the inner wall of the pressure-conduction chamber  50  is greater than 90°, making the corner less likely a place where the bubble will stick. However, the mouth of the port  56  cannot =be so large that liquid can easily flow by the bubble when fluid is exiting the pressure-conduction through the port  56 . In order to accomplish this, the port must be sized and shaped so that the surface tension of the IV fluid being forced upward from the pressure-conduction chamber  50  forces a bubble located at the port  56  up through the inlet valve  6 . It is also preferable that the port  56  be sized and shaped so that when liquid is pulled back into the pressure-conduction chamber  50 , the bubble can hover near the port as liquid passes around it. A preferred inlet port  56  shape is shown in FIGS. 22 and 23. The port&#39;s size increases from the end  57  that connects to the IV line&#39;s upper portion to the end  58  leading into the pressure-conduction chamber. FIG. 24 shows a cross-section of the inlet valve  56 . It has been found that providing an inlet port to the pressure-conduction chamber with this shape improves the air-elimination system&#39;s ability to purge bubbles from the chamber. Using a port such as that shown in FIGS. 22-24 in conjunction with the membrane  41  of FIGS. 15-17 helps force bubbles more quickly out of the pressure-conduction chamber when attempting to purge the bubbles back through the cassette&#39;s inlet  31  to the IV source. 
     FIG. 25 shows a preferred arrangement of teeth around the circumference  29  of the control wheel  20 . The teeth provide means for a gear in the control unit  10  to engage securely the control wheel&#39;s circumference-in particular, a gear that is used to prevent the free flow of fluid through the cassette when the cassette is removed from the control unit  10 . When the door  102  of the control unit  10  is being opened, the gear turns the control wheel  20  to close the stopcock-type valve  2 , thereby stopping all flow through the cassette and preventing free flow. To ensure that the gear does not continue turning the wheel  20  once the valve  2  has been closed off entirely, a sector  92  along the wheel&#39;s circumference is left free of teeth. When the wheel  20  is turned enough so that the gear is adjacent this toothless sector  92 , the valve  2  is fully closed. The lack of teeth prevents the gear from continuing to turn the wheel; thus, the wheel cannot be turned too much. 
     Although the invention has been described with reference to several preferred embodiments, it will be understood by one of ordinary skill in the art that various modifications can be made without departing from the spirit and the scope of the invention, as set forth in the claims hereinbelow.