Patent Publication Number: US-6981960-B2

Title: Closed-loop IV fluid flow control

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of U.S. patent application Ser. No. 09/628,846, filed Jul. 31, 2000, now U.S. Pat. No. 6,685,668 B1. 
    
    
     FIELD OF THE INVENTION 
     The invention generally concerns control of fluid flow rates, and more particularly concerns the control of fluid flow rate in intravenous fluid delivery systems. 
     BACKGROUND OF THE INVENTION 
     Intravenous (IV) fluid delivery systems are used to deliver fluids and medicines to patients at controlled rates. To more accurately control IV fluid delivery, an open-loop control system is typically used. A processor included in the open-loop control system varies the speed of a relatively accurate fluid pump used to infuse a medicinal fluid into a patient, based on a predefined algorithm and as a function of various parameters, such as temperature, fluid type, and desired flow rate. These open-loop processor-controlled pumping systems are generally expensive and complex. Usually, compensation for variations in pump accuracy must be employed in such systems to achieve an acceptable accuracy. The rate of fluid delivery is also affected by the precision of disposable components used in the fluid path that conveys a medicinal fluid to a patient. However, variations in the internal diameter and material hardness of fluid lines and pumping component comprising the disposable components, both initially, and as a result of changes over their period of use, cannot readily be compensated in an open-loop control algorithm. As a result, higher cost disposable components that are guaranteed to meet tight tolerance specifications must be used in such systems to avoid loss of accuracy. 
     Accordingly, it will be apparent that it would be desirable to provide a relatively low cost, low complexity system for delivery of medicinal fluids. A closed-loop system in which a desired parameter is measured to control the system can provide the required accuracy. For example, in a closed-loop system, it would be preferable to measure flow with a low cost flow sensor and to control an inexpensive fluid delivery pump based upon the measured flow rate, so as to achieve a desired flow rate. Previously, measurement of fluid flow has generally been prohibitively expensive in medicinal fluid infusion systems. However, the development of low cost flow sensors have made it much more practical and economical to monitor fluid flow in order to control a medical infusion system. 
     Low cost pumps can be used in a closed-loop system medicinal fluid infusion system, since the accuracy of the pump is not important in achieving a desired delivery rate. Similarly, the tolerance specifications for the disposable components used in the system can be greatly relaxed, because the precision of these components will no longer be of much concern. Also, most of the variables that must be considered in algorithms currently employed for open-loop control can be ignored in a closed-loop controlled infusion system. Consequently, the process control logic used in a closed-loop infusion system is relatively simple. 
     SUMMARY OF THE INVENTION 
     In accord with the present invention, a fluid delivery system is defined for infusing a medicinal fluid supplied from a reservoir into a patient at a desired rate. The fluid delivery system includes a fluid line through which the medicinal fluid is conveyed from the reservoir to a patient, and a flow controller that selectively varies a rate of flow of the medicinal fluid through the fluid line. A processor is controllably coupled to the flow controller and to a flow sensor that monitors a rate of flow of the medicinal fluid through the fluid line, producing an output signal that is indicative thereof. The processor responds to the output signal and operates the flow controller in a closed-loop process, to achieve the desired rate of infusion of the medicinal fluid into a patient. 
     In one preferred form of the invention, the flow sensor includes an orifice disposed in a fluid path through which the medicinal fluid flows in the fluid line, and the orifice has a cross-sectional size that is substantially less than that of the fluid line. A pressure-sensing module in the fluid line is configured to sense a pressure drop across the orifice, producing the signal indicative of flow rate. In one embodiment, the pressure sensing module includes a distal pressure sensor and a proximal pressure sensor, the distal pressure sensor being used for monitoring a distal pressure of the medicinal fluid, downstream of the orifice, and the proximal pressure sensor being used for monitoring a proximal pressure of the medicinal fluid, upstream of the orifice. A difference between the distal pressure and the proximal pressure signals is indicative of the rate of flow of the medicinal fluid through the fluid line. 
     In another embodiment, the pressure sensing module includes a differential pressure sensor that monitors a differential pressure across the orifice and in response thereto, produces the signal supplied to the processor, which is indicative of the rate of flow of medicinal fluid through the fluid line. 
     Preferably, the flow sensor is disposed in a “Y” fitting in the fluid line. In one embodiment, the flow sensor is removably coupled to the processor through a connector. In another embodiment, the flow sensor is removably coupled to the processor. 
     In some cases, it will occasionally be desirable to provide a substantially greater flow of medicinal fluid that can be achieved through the orifice of the flow sensor, e.g., to prime the fluid line before connecting it to a patient. In this case, a bypass channel is provided within the fitting, generally in parallel with the orifice. The bypass channel is then selectively opened to enable the medicinal fluid to substantially bypass the orifice when a greater rate of flow of the medicinal fluid than the desired rate is required through the fluid line. 
     One preferred form of the invention employs a pump for the flow controller, and the pump forces the medicinal fluid through the fluid line and into a patient. Alternatively, an electronically controlled valve is employed for the flow controller, the medicinal fluid flowing through the fluid line under the force of gravity. 
     A user interface is preferably included to enable input by a user of the desired rate of medicinal fluid flow through the fluid line. 
     Another aspect of the present invention is directed to a method for controlling a rate of infusion of a medicinal fluid into a patient through a fluid path. The method includes steps that are generally consistent with the functions performed by the elements discussed above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
       The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is an elevational view of a portion of IV tube set and a first embodiment of the present invention, showing a cross-sectional view of a Y site that is provided with a flow sensor, which produces a signal for use in controlling a pump in a closed-loop process; 
         FIG. 2  is an elevational view of a portion of an IV tube set much like that of  FIG. 1 , but showing an embodiment that includes a connector for coupling a flow sensor to a controller; 
         FIG. 3A  is an elevational view of an embodiment that includes an electronically controlled valve for varying fluid flow rate and which includes a bypass around a flow sensor in a Y site; 
         FIG. 3B  is a cross-sectional view of the flow sensor, showing the bypass path around the flow sensor, in the Y site shown in  FIG. 3A ; 
         FIG. 4  is an enlarged elevational view of a flow sensor having proximal and distal pressure sensors for sensing proximal and distal pressures across an orifice; 
         FIG. 5  is a cross-sectional view of the flow sensor of  FIG. 4 , taken along section line  5 — 5  in  FIG. 4 ; 
         FIG. 6  is a cross-sectional view of the flow sensor of  FIG. 4 , taken along section line  6 — 6  in  FIG. 4 ; 
         FIG. 7  is a cross-sectional view of yet another embodiment of the Y site for the present invention, which includes a bypass channel; 
         FIG. 8  is a cross-sectional view of the embodiment of the Y site, taken along section line  8 — 8  in  FIG. 7 , and illustrating the bypass channel in its open state; 
         FIG. 9  is a cross-sectional view of the Y site shown in  FIGS. 7 and 8 , illustrating the use of a clamp that includes electrical contact on one jaw and which is employed for closing the bypass flow channel and for electrically connecting to a pressure sensor in the Y site; 
         FIG. 10  is an elevational view of an end portion of one of the jaws of the clamp shown in  FIG. 9 , illustrating the electrical contacts and leads provided thereon; and 
         FIG. 11  is a functional block diagram of the controller, illustrating the components included therein. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Several different embodiments of systems suitable for administering a medicinal fluid at a desired rate are illustrated in the Figures and are described below. A first such embodiment of a system  10  is shown in FIG.  1 . System  10  includes a fluid line  12  that extends from a reservoir (not shown in this FIGURE) through a peristaltic pump  14 . Peristaltic pump  14  comprises a plurality of rollers  18  that are driven along in a circular path by an electric motor  16  (or other suitable prime mover) in a rotational direction as indicated by the curved arrow. As is common in most such peristaltic pumps, rollers  18  periodically contact and compress fluid line  12 , as the rollers move along the circular path, forcing successive boluses of a medicinal fluid through the fluid line for infusion into a patient (not shown). Fluid line  12  extends within the concave portion of a curved guide  20  against which rollers  18  act to compress the fluid line in pumping the medicinal fluid. However, it should be pointed out that many other types of pumps can be used in connection with the present invention. 
     One of the advantages of the present invention is that it enables a relatively inexpensive peristaltic pump or other type of pump, which may be of a relatively low accuracy in maintaining a desired rate of delivery, to be used, since the pump is directly controlled in a closed-loop process to achieve the desired delivery rate of the medicinal fluid to the patient. To control the rate at which peristaltic pump  14  infuses a medicinal fluid, the speed of electric motor  16  is varied so as to achieve the desired rate for delivery of the medicinal fluid by the pump. Further details of system  10  that enable the pump (i.e., its prime mover) to be controlled in this manner to achieve the desired rate of fluid flow are described below. 
     Fluid line  12  connects to an upper arm  22  of a Y site  24 . The outlet of the Y site is connected to a fluid line  28  that conveys the medicinal fluid flowing under the urging of peristaltic pump  14  into the body of a patient at an infusion site. It should be noted, however, that in the present invention, peristaltic pump  14  (or other low cost pump) can be disposed either proximal or distal to the Y site. The medicinal fluid flows through a cavity  23  formed within the Y site to reach fluid line  28 . 
     A flow-sensing module  36  is disposed within an upper arm  26  of Y site  24  and extends into the lower portion of the Y site. Fluid-sensing module  36  includes a solid state flow sensor  30  that comprises a proximal pressure sensor  32  and a distal pressure sensor  34 . The proximal and distal pressure sensors are disposed on opposite sides of a restriction or orifice (shown more clearly in FIG.  5 ). By monitoring the proximal and distal pressure at points on opposite sides of the restriction or orifice of known cross-sectional size, flow sensor  30  determines the rate of flow of medicinal fluid through Y site  24 , and thus through the fluid path into the patient. Flow-sensing module  36  is retained within Y site  24  by a flange  38 , which sealingly engages a lip  40  formed on the upper end of arm  26 . 
     A cable  42  connects the signal produced by flow sensor  30  to a controller  44 . Controller  44  includes a display  46  on which either the volume or the rate of medicinal fluid infusion is displayed. Details of the controller are discussed below, in connection with FIG.  11 . The user interface on controller  44  includes a switch  48  that switches between a display of the rate of fluid delivery in ml/hr and the volume to be infused (VTBI) in ml. Also provided on the controller are start and stop buttons  50  and  52 , a button  54  for silencing alarms such as occur when an out-of-fluid condition or air bubble is detected in the fluid line, and buttons  55  and  57  for enabling a user to respectively increase and decrease displayed values being input for the desired VTBI and the desired rate of fluid delivery. 
     It should be noted that the flow-sensing module can be disposed in elements of the fluid line other than a Y site. For example, a portion of the fluid line can simply include a flow monitoring module that is sufficiently low in cost to be disposed of after use with a single patient. Several different techniques are shown herein for electrically connecting the flow sensing module to controller  44  or its equivalent. 
     Controller  44  responds to the proximal pressure and distal pressure signals received from flow sensor  30 , deriving a flow signal therefrom corresponding to their difference, and the difference in pressures sensed on opposite sides of the restriction or orifice is indicative of the rate of flow of medicinal fluid through Y site  24  and into the patient. Based upon the monitored rate of flow of the medicinal fluid, which comprises a feedback signal, controller  44  implements a closed-loop control process by varying the speed of motor  16 , and thus, the speed of peristaltic pump  14  to achieve the desired rate of flow of the medicinal fluid being infused. If the monitored rate of flow exceeds the desired rate of flow of the medicinal fluid, controller  44  causes motor  16  driving peristaltic pump  14  to slow sufficiently to the desired rate of infusion. Conversely, if the monitored rate of flow is less than the desired rate of flow of the medicinal fluid, the controller causes the motor to speed up, thereby increasing the rate at which peristaltic pump  14  is infusing the medicinal fluid sufficiently to achieve the desired rate. 
     In  FIG. 2 , a system  10 ′ is illustrated and is similar in most respects to system  10 . However, in system  10 ′, a cable  42 ′ includes a multi-pin connector  70  for electrically connecting to flow sensor  30 , which comprises a portion of a flow sensing module  36 ′ in which the flow sensor is connected through internal leads  78  to connector  70 . Cable  42 ′ and connector  70  are considered non-disposable and can be detached from flow sensor  30  and Y site  24 . In almost all other respects, system  10 ′ is identical to and includes equivalent elements to the embodiment shown in FIG.  1 . 
     Connector  70  includes a plurality of conductive pins  72  that are inserted into corresponding orifices  74  formed in the side of the upper tube of the Y site. Pins  72  make electrical contact with corresponding female receptacle  76 , which is connected to flow sensor  30  through internal leads  78  that extend through the interior of flow-sensing module  36 ′. The distal and proximal pressure signals determined by flow sensor  30  are conveyed through lead  78  and cable  42 ′ to controller  44  for use in controlling peristaltic pump  14  (or other device for varying the rate of flow of the medicinal fluid, as explained herein), to achieve the desired rate of flow of the medicinal fluid into a patient. 
       FIGS. 3A and 3B  illustrate further details of a system  10 ″, comprising yet another embodiment of the present invention. In system  10 ″, there are several differences compared to the previous two embodiments. For example, an electronically controlled valve  80  is used to vary the flow rate of a medicinal fluid  85  from a reservoir  83 , which is disposed at a substantially higher elevation than a patient&#39;s body (not shown). The pressure head thus developed is sufficient to infuse the medicinal fluid at more than the desired rate. However, electronically controlled valve  80  modulates the rate of flow of medicinal fluid  85  from reservoir  83  to achieve the desired rate. A controller  44 ′ provides a control signal that is conveyed to electronically controlled valve  80  through a cable  82 . The control signal causes the electronically controlled valve to adjust the flow of the medicinal fluid to achieve the desired rate of infusion. The controlled flow of medicinal fluid  85  flows through fluid line  12  into a Y site  24 ′, which includes an embedded differential pressure sensor  98  for monitoring the rate of flow of the medicinal fluid flow through the Y site. Differential pressure sensor  98  monitors the difference between a pressure at a distal point  102  and a proximal point  100 , producing a signal for the differential pressure that is indicative of the rate of flow of the medicinal fluid flow through a restriction or orifice, which is disposed between the points at which the distal and proximal pressures are measured. Further details of the differential pressure sensor and of a probe  92  are illustrated in FIG.  3 B. The power signal and the signal indicative of differential pressure are conveyed through a lead  84  that extends between controller  44 ′ and probe  92 , which has a plurality of spaced-apart contacts  86  that are sized and configured to couple with corresponding contacts (pads) on differential pressure sensor  98  when the probe is seated in an index notch  94  formed in the side of the Y-site adjacent to differential pressure sensor  98 , so that the signal indicative of flow through the differential pressure sensor is conveyed to controller  44 ′. 
     Also shown in  FIGS. 3A and 3B  are details of a bypass passage  104  that extends generally parallel to the fluid path through the restriction or orifice within differential sensor  98  and for receiving the signal that it produces corresponding to the differential pressure between the proximal and distal points. Normally, bypass passage  104  is clamped shut while Y site  24 ′ is being used for monitoring the flow of medicinal fluid  85  to a patient and is only opened in the event that a substantially greater rate of flow is required, for example, to flush the fluid line or to initially prime the fluid line, before connecting it to the patient.  FIGS. 3A and 3B  show bypass passage  104  open, but  FIG. 3A  also illustrates a dash line showing how the elastomeric material, i.e., a polymer of other plastic material, comprising Y site  24  is compressed with a suitable clamp (not shown) that holds probe  92  in place within index notch  94 , with contacts  86  electrically mating with the corresponding contacts on the differential pressure sensor. The clamp will thus close bypass passage  104  when the Y site is being used to monitor the rate of medicinal fluid flow into a patient. 
     In each of the preferred embodiments, including the one shown in  FIGS. 3A and 3B , the pressure sensors or differential pressure sensors can be fabricated as a capacitor, with one plate coupled to a substrate and an opposite, overlying plate supported in sealed relationship above the plate on the substrate, so that a vacuum exists between the two plates, enabling absolute pressure to be measured. In differential pressure sensor  98 , an orifice would be provided to couple the volume between the two plates to the point that is distal the orifice or restriction, while the plate overlying the plate supported by the substrate would be exposed to the pressure of the medicinal fluid proximate the orifice or restriction. Alternatively, piezoelectric type pressure sensors can be used for the two pressure sensors in flow sensor  30  and for differential pressure sensor  98 . 
     Further details of flow sensor  30  are illustrated in  FIGS. 4-6 . As will be evident particularly in  FIGS. 5 and 6 , flow sensor  30  includes a pair of glass slabs  124 , disposed on opposite sides of a silicon spacer  126  that defines the fluid path through the flow sensor. Furthermore, silicon spacer  126  forms a restriction or orifice  128  that separates proximal pressure sensor  32  from distal pressure sensor  34 , as shown in FIG.  4 . The substantially smaller cross-sectional area of the restriction or orifice within flow sensor  30  is shown in  FIG. 5 , in contrast to the much greater area of a fluid passage  130  on opposite sides of the restriction. Pressure sensors  32  and  34  are fabricated on the larger of the pair of glass slabs  124  using conventional lithographic techniques, as are often used in fabricating integrated circuits. Furthermore, proximal pressure sensor  32  is connected through leads  112  and  116  to pads  110  and  114  on the larger of the glass slabs  124 , pad  114  being a common terminal for both the proximal and distal pressure transducers. Likewise, distal pressure transducer  34  is connected through leads  118  and  122  to pads  114  and  120 , which are also disposed on the exposed portion of the larger of the pair of glass slabs  124 . While leads  112 ,  116 ,  118 , and  122  are shown as discrete wires to simplify the drawings, it will be understood that these “wires” preferably comprise conductive traces applied to the larger one of glass slabs  124  using a conventional photolithographic technique, which is also employed to form pads  110 ,  114 ,  120 . It will be understood that other suitable materials can be employed in fabricating proximal, distal, or differential pressure sensors, using much the same configuration disclosed above. 
     In a preferred embodiment, restriction or orifice  128  within pressure sensor  30  and in differential pressure sensor  98  is substantially smaller in cross-sectional area that that of fluid paths  130  on both the distal and proximal sides of the orifice or restriction. Those of ordinary skill in the art will appreciate that the dimensions used for the orifice and fluid paths can readily be varied, so long as the restriction provided by the orifice is substantially less than the cross-sectional areas of the proximal and distal fluid passages on opposite sides of the orifice, to ensure that a sufficiently great differential pressure is monitored as a result of the pressure drop of medicinal fluid flowing through the restriction or orifice to enable accurate control of the pump or electronically controlled valve that varies the flow rate of the medicinal fluid. 
     Another embodiment of a Y site  24 ″ is illustrated in  FIGS. 7-9 . Y site  24 ″ also includes bypass passage  104 , but includes flow sensor  30  with the two separate pressure sensors, instead of differential pressure sensor  98 . To connect to flow sensor  30 , a clamp  139  is provided as shown in  FIG. 9. A  series of three spaced-apart electrical contacts  141  are included on the end of a jaw  140  on clamp  139  and the spacing between contacts  141  and their disposition correspond to the spacing between pads  110 ,  114 , and  120  on flow sensor  30 . Thus, each of electrical contacts  141  can readily make electrical connection with a different one of the pads. Connected to each of contacts  141  is a different one of a plurality of leads  42 ′. Leads  42 ′ extend to controller  44  and convey the signals produced by the proximal and distal pressure sensors in flow sensor  30  to the controller. 
     To ensure that contacts  141  correctly meet and make contact with pads  110 ,  114 , and  120  on flow sensor  30 , clamp  139  also includes a jaw  142  shaped to fit within an index groove  134  provided on the side of Y site  24 ″, disposed adjacent flow sensor  30 , but opposite a recess  132 . Jaw  140  is thus indexed to fit within recess  132 , bringing contacts  141  into electrically conductive connection with pads  110 ,  114 , and  120 . Alternatively, the indexing function can be accomplished by providing the indexing geometry of jaw  142  and index groove  23  on jaw  140  and recess  132 . Furthermore, clamp  139  includes handles  136  and a torsion spring  138  that is enclosed therein and which extends around a pivot  146  that couples the handles together. Torsion spring  138  provides a biasing force sufficient to compress the elastomeric material comprising Y site  24 ″ so as to close bypass passage  104  as shown in FIG.  9 . 
     It will be understood that other techniques for providing a probe configured for making electrical contact with pads  110 ,  114 , and  120  on pressure sensor  30  can alternatively be used, and that such a probe or stylus can be held in place by a separate clamp that closes bypass passage  104 . As noted above, when bypass passage  104  is closed, fluid flows through the fluid path and orifice or restriction within flow sensor  30 , enabling a signal to be produced by the flow sensor indicative of the rate of the medicinal fluid flow therethrough, which is used by the controller in determining the rate at which the medicinal fluid is being infused into the patient. This feedback signal is used by the controller to achieve a desired rate of infusion, and for monitoring the total amount of medicinal fluid infused into a patient, to achieve a desired VTBI. 
       FIG. 11  illustrates internal functional components of controllers  44 / 44 ′. Flow sensor  30  or differential pressure sensor  98  are connected to an appropriate sensor measuring circuit  154  having an output coupled to an analog-digital (A-D) converter  152 . A-D converter  152  converts the analog signals supplied by the sensor measuring circuit into a digital signal that is input to a microcontroller  150 . As a further alternative, microcontroller  150  may include its own internal A-D converter, in which case A-D converter  152  can be omitted. 
     Microcontroller  150  is connected to a memory  156  (or may alternatively include an internal memory) that comprises both random access memory (RAM) and read only memory (ROM)—neither separately shown. Machine instructions stored within memory  156  are used to implement control functions when executed by microcontroller  150 . A keypad  158  comprising the buttons on the user interface of controllers  44 / 44 ′ enables user to control the microcontroller functions. The microcontroller drives display  46 , which indicates the values of the parameters selected by the user with keypad  158 . A radio frequency (RF) communication link  160  is optionally provided, enabling microcontroller  150  to communicate with external devices (not shown) via an RF transmission. The communication with such external devices is likely to be bi-directional, enabling input of desired parameters to alternatively be provided by an external device instead of via keypad  158 . A power supply  162  provides the appropriate voltage levels for each of the components comprising controller  44  or controller  44 ′. 
     Microcontroller  150  produces an output signal that is applied to a digital-to-analog (D-A) converter  164 . The D-A converter changes the digital signal from microcontroller  150  to a corresponding analog signal that is applied to a motor drive block  166 . It should also be noted that microcontroller  150  may include an internal D-A converter, enabling D-A converter  164  to be omitted. Also, it is contemplated that a motor drive  166  responsive to digital signals may be employed, also obviating the need for the D-A converter. As an alternative, if electrically-controlled valve  80  is used instead of peristaltic pump  14  to vary the flow of medicinal fluid through the fluid line to the patient, the digital signal from the microcontroller or the analog signal from D-A converter  164  may be used to control the electrically-controlled valve. When peristaltic pump  14  is used, motor drive  166  provides the drive signal to the electric motor that drives the pump to vary the rate at which the medicinal fluid is infused into the patient. 
     By monitoring the rate of flow of a medicinal fluid using flow sensor  30  or differential pressure sensor  98 , a feedback signal (i.e., the signal indicative of the current rate of flow of the medicinal fluid received from the Y site) is produced. Microcontroller  150  uses the feedback signal to control peristaltic pump  14  or electrically controlled valve  80  to achieve the desired rate selected by the user. 
     Although the present invention has been described in connection with the preferred form of practicing it and modifications thereto, those of ordinary skill in the art will understand that many other modifications can be made to the invention within the scope of the claims that follow. Accordingly, it is not intended that the scope of the invention in any way be limited by the above description, but instead be determined entirely by reference to the claims that follow.