Abstract:
A portable infusion apparatus and method are provided for controlling the delivery of medicinal fluid to a patient. A fluid delivery system receives control input to control a setting of a variable fluid flow resistor. The variable fluid flow resistor resists passage of fluid through a fluid pathway between a fluid source and a recipient. The fluid delivery system; produces a control signal indicative of the setting of the variable fluid flow resistor; derives a fluid flow rate value from the control signal; and applies pressure to the fluid source to deliver the fluid from the fluid source to the recipient through the variable fluid flow resistor at a rate as specified by the derived fluid flow rate value.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of priority under 35 U.S.C. §119(e) based on U.S. Provisional Application No. 61/372,557 filed Aug. 11, 2010. The aforementioned application is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     The present disclosure relates to fluid flow control devices and more particularly to portable infusion pumps. 
     The primary role of an intravenous (IV) infusion device has been traditionally viewed as a way of delivering IV fluids at a certain flow rate. In emergency situations, such as natural disasters, industrial accidents, traffic accidents, police or battlefield actions, the best outcomes for trauma victims occur when an IV can be administered to prevent shock and stabilize the patient for transport to a medical facility. In practice, the limited battery function, the large size and the complexity of modern pumps has required first responders to rely on gravity infusions or delay in starting an infusion until the patient is safely inside a secure care area. This delay in providing fluids and medicine increases the mortality risk for those needing care, and gravity infusions are inconsistent at best in transport situations, sometimes running faster, sometimes slower, and sometimes not at all depending on the relative height of the fluid source and the patient&#39;s systolic blood pressure. Clearly then, there is a need for a portable pump that can be used by people with only limited training in trauma situations. Such a pump would allow greater control of infusions that are currently delayed or set and run as gravity infusions and subject to the variability of gravity infusion during patient transport and would add a significant level of safety and benefit to trauma patients, helping to stabilize them during transport to a secure care area. 
     In clinical practice, it is common to have fluid delivery goals other than flow rate. For example, it may be important to deliver a certain dose over an extended period of time, even if the starting volume and the actual delivery rate are not specified. This scenario of “dose delivery” is analogous to driving an automobile a certain distance in a fixed period of time by using an odometer and a clock, without regard to a speedometer reading. The ability to perform accurate “dose delivery” would be augmented by an ability to measure the volume of liquid remaining in the infusion. 
     Flow control devices of all sorts have an inherent error in their accuracy. Over time, the inaccuracy of the flow rate is compounded, so that the actual fluid volume delivered is further and further from the targeted volume. If the volume of the liquid to be infused can be measured, then this volume error can be used to adjust the delivery rate, bringing the flow control progressively back to zero error. The ability to measure fluid volume then provides an integrated error signal for a closed feedback control infusion system. 
     In clinical practice, the starting volume of an infusion is not known precisely. This is especially true with first responders arriving in a disaster or battlefield situation. The original contained volume is not a precise amount and then various concentrations and mixtures of medications are added. The result is that the actual volume of an infusion may range, for example, from about 5% below to about 20% above the nominal infusion volume. The EMT or other user of an infusion control device is left to play a game of estimating the fluid volume, so that the device stops prior to completely emptying the container, otherwise generating an alarm for air in the infusion line or the detection of an occluded line. This process of estimating often involves multiple steps to program the “volume to be infused.” This process of programming is time consuming and presents an unwanted opportunity for programming error. Therefore, it would be desirable if the fluid flow control system could measure fluid volume accurately and automatically. 
     If the fluid volume can be measured then this information could be viewed as it changes over time, providing information related to fluid flow rates. After all, a flow rate is simply the measurement of volume change over time. 
     The formulation of the ideal gas law, PV=nRT, has been commonly used to measure gas volumes. One popular method of using the gas law theory is to measure the pressure in two chambers, one of known volume and the other of unknown volume, and then to combine the two volumes and measure the resultant pressure. This method has two drawbacks. First the chamber of known volume is a fixed size, so that the change in pressure resultant from the combination of the two chambers may be too small or too large for the measurement system in place. In other words, the resolution of this method is limited. Second, the energy efficiency of this common measurement system is low, because the potential energy of pressurized gas in the chambers is lost to the atmosphere during the testing. The present invention contemplates an improved volume measurement system and method and apparatus that overcome the aforementioned limitations and others. 
     SUMMARY 
     In one aspect, a method and apparatus for determining fluid flow rate over an extended period of time are provided. 
     In another aspect, a method and apparatus for determining fluid flow rate over a relatively short period of time are provided. 
     One advantage of the presently disclosed portable infusion device over most alternate infusion means is that it does not require being hung above the patients head. Most alternate infusion means rely at least in part on the pressure head created by suspending the fluid source above the patient. Suspending the fluid a distance above the device and the patient helps fill the pumping segment or chamber or alternate embodiments and deviations from the expected pressure head generated by suspending the fluid above the patient affects the flow rate accuracy of the device. In situations such as would be experienced by a first responder to an accident or disaster or a medic attending to and evacuating wounded, suspending the fluid source above the patient is often inconvenient or impossible. In those situations, delivery of fluid to stabilize and maintain patients in transit can be variable, putting patients at risk. Since the preferred embodiment of the device herein described generates all the pressure required for delivering the fluid, the system is indifferent to the relative height of the device to the patient. An infusion can be started with this device and then the device can be secured to the patient, which makes transporting the patient in difficult circumstances simpler. 
     Another advantage of the presently disclosed portable infusion device is found in that the pressure measurements made over time can be used to accurately compute fluid flow rate. 
     Another advantage of the present portable infusion device and method is found in that pressure measurements may be made using an inexpensive and simple pumping mechanism. 
     Another advantage of the present portable infusion device and method is found in that pressure measurements may be made without significant loss of energy. 
     Another advantage of the present portable infusion device and method is found in that pressure measurements may be made over a wide range of volumes. 
     Another advantage of the present portable infusion device and method of this disclosure is that its simplicity, along with feedback control, makes for a reliable architecture. 
     Still further features and advantages of the present portable infusion device in accordance with the present disclosure will become apparent to those skilled in the art upon a reading and understanding of the following detailed description, which serves to illustrate by way of example the principles of the invention and is not intended to limit the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating preferred embodiments and are not to be construed as limiting the invention. 
         FIG. 1  is a perspective view of an infusion pump in accordance with an exemplary embodiment. 
         FIG. 2  is a perspective view of an infusion pump in accordance with the exemplary embodiment appearing in  FIG. 1 , with the housing cover removed for ease of exposition. 
         FIG. 3  is a top perspective view of the infusion pump embodiment of  FIG. 1 , wherein the housing cover has been removed. 
         FIG. 4  is a flow chart outlining an exemplary method of calculating flow rate based on pressure decay. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings, wherein like reference numerals are used to indicate like or analogous components throughout the several views,  FIG. 1  depicts an exemplary infusion system  1  in accordance with an exemplary embodiment of the present invention. The system includes a housing  2  that contains a power supply  21  and associated electronics. The housing  2  connects to an administration cuff  6  and interfaces with a variable fluid flow resistor  5 . The variable flow fluid resistor  5  includes a cap  12 , which snaps into a pocket containing a potentiometer (not shown), such that the cap  12  and potentiometer turn together. An actuator  13  is rotatably coupled to the housing  2  and includes a shaft which interfaces with a potentiometer. Rotation or twisting of the actuator  13  rotates the cap  12  of the variable fluid flow resistor  5  and the potentiometer provides an indication of the rotational position of the resistor cap  12 . 
     The administration cuff  6  includes an inflatable bladder  16 . The administration cuff  6  is wrapped around a fluid source  7  comprising a flexible bag containing the fluid to be infused. It will be recognized that the IV fluid to be delivered may be any medicinal fluid, intravenous solution, blood product, or the like, and the term medicinal fluid, IV fluid, etc., may be used herein interchangeably without regard to nuances in meaning. The cuff  6  also includes a structural element  15  disposed about the inflatable bladder  16 . Similar to a blood pressure cuff, the administration cuff  6  can be wrapped around flexible fluid source bags  7  of a variety of sizes. The flap  26  is securely closed via one or more fasteners (not shown), e.g., hook and loop fastener, incorporated into the structural element  15  of the cuff  6 . One skilled in the art can envision variations on this theme and cuffs made from woven fabric as well as from flexible plastic film. Closures allowing adjustment of the cuff  6  can include the mentioned hook and loop as well as, snaps, post and eye fasteners, and a variety of other embodiments. The fasteners may allow for the adjustment of the cuff  6  and may be incorporated into the outer, structural portion  15  of the cuff  6 . These variations are all within the spirit of the invention and included herein. 
     A disposable administration set  8  connects to the outlet of the fluid source  7  and includes an air eliminating filter  9 , the variable fluid flow resistor  5 , and the line to the patient  11 , e.g., which in turn may be coupled to or include an IV catheter or cannula (not shown), as generally known in the art. The administration cuff  6  also includes a calibration tank  14  of known volume, a temperature sensor such as thermistor  17 , and the plumbing and electrical means to connect the calibration tank  14  to the pressure source within the housing  2  and to the bladder  16 ; and to connect the thermistor  17  to the controls within the housing  2 . The housing  2  includes a user interface, such as an ON/OFF button  3  and a user display  4 . The display  4  may be an LCD display, LED display, etc. The housing  2  has two end caps  10  and  18 , which connect securely to the housing  2  and act to contain and protect the electronics, power supply  21 , and pressure system enclosed within the housing  2 . 
     Referring now to  FIG. 2 ; in this view, the housing  2  is shown transparent to aid in the description of the device, and the tubes  8  and  11  and the air eliminating filter  9  have been omitted for clarity. In this view, the relation of the variable fluid flow resistor  5  to the potentiometer is more clearly seen. The variable flow resistor  5  snaps into the housing  2 , the cap  12  of the variable flow resistor  5  being captured by a pocket in the potentiometer actuator  13 . A protrusion such as a fin on the cap  12  of the variable flow resistor  5  fits into a complimentary, aligned slot or opening in the potentiometer actuator  13 , assuring that the resistor  5  can only be inserted when the resistor is closed position (preventing unintended flow), and enabling the cap and the potentiometer to turn together. The potentiometer actuator  13  may have a fluted exterior to enable the element to be turned like a thumb wheel. It is recessed in the housing to prevent inadvertent adjustment. 
     Also seen in this view are the batteries  21 , the electronic control board  22  and the hanging hole  23  that is part of the administration cuff  6 . The plumbing connection  19  from the known volume  14  to the plumbing manifold with the housing  2  and the electrical connection  20  from the thermistor  14  to the controls within the housing  2  are also more clearly seen in this view. The portable infusion device  1  is powered by a power supply  21  and is controlled by an electronic control board  22 . In the illustrated embodiment the power supply  21  consists of two batteries, however other power supplies such as one or more batteries or battery packs are contemplated. The hanging hole  23  on the cuff  6  provides a means for hanging the infusion device  1  when necessary such as during an infusion or for storage. The calibration tank  14  is connected to the plumbing manifold (not shown) within the housing  2  by a connection  19 . The thermistor  17  is connected to control board  22  within the housing  2  by an electrical connection  20 . 
     The control board  22  includes a processing unit, such as a microprocessor, microcontroller, an (optionally programmable) controller, embedded controller, or alternately (and providing the equivalent functionality) a finite state machine, e.g., which may be realized by a programmable logic device (PLD), field programmable gate array (FPGA) or field programmable object array (FPOA). 
     Referring now to  FIG. 3 ; in this view, the housing  2  is shown transparent to aid in the description of the device, and the tubes  8  and  11  and the air eliminating filter  9  have been omitted for clarity. In this view, a pump  24  and an internal plumbing manifold  25  can be seen within the housing  2 . 
     In preparation for using a device according to the preferred embodiment to deliver an infusion to a patient or subject, a user would connect the disposable tube set  8  to the flexible bag  7  containing the fluid to be infused. Typically, this is accomplished by including a spike fitting (not shown) on the end of the disposable tube set  8  and pushing that through a pierceable membrane that is molded into the flexible bag for this purpose. There are other configurations that would also work to connect the tube set  8  to the bag, as are known to persons skilled in the art, such as the use of male and female luer connectors on the bag and the tube set. The user would then open the variable flow fluid resistor by turning the cap  12  counter clockwise and prime the tube set  8  with fluid from the flexible bag, being careful to fully wet the air elimination filter  9  and displace all air from the tube set  8  with the fluid to be infused. Closing the variable flow resistor—e.g., by turning the cap  12  clockwise in the illustrated preferred embodiment—is sufficient to keep fluid from flowing from the primed tube set  8  in an uncontrolled manner. 
     Subsequent to connecting and priming the tube set  8 , the user places the flexible fluid source  7  in the administration cuff  6 , then pulls on the flap  26  of the cuff  6  to remove all slack, and closes the open end of the cuff with the hook and loop (or other) fastener. Friction between the flexible bag  7  and the inside of the administration cuff  6  prevents bag movement. The variable resistor  5  is snapped into the potentiometer actuator  13 , and the cap  12  is captured by the actuator such that the cap  12  can turn and open the variable fluid flow resistor  5  only by rotating the actuator  13  of the potentiometer. 
     Once the fluid flow resistor  5  is in place the user, e.g., a nurse, doctor, EMT, or other medically trained professional, turns the portable infusion system  1  on using the ON/OFF button  3  if the device were not already on. When the portable infusion system  1  is ready for programming, the volume rate visible on the display  4  will show display zero. The fluid outlet  11  of the disposable tube set  8  is then connected to the vasculature of the patient. The infusion is then started by the user rotating the potentiometer actuator  13  using a finger or thumb through the aligned opening in the housing  2  provided. The user turns the actuator  13  to the desired infusion rate. The infusion is then started by the user twisting the actuator  13 —where the variable flow resistor  5  is located—holding the fixed housing  2 . As the actuator  13  is rotated, the cap  12  of the variable resistor  5  is turned, opening the variable resistor  5 . The potentiometer is also rotated simultaneously, providing feedback to the control board  22 , e.g., by providing a control signal to the control board representative of the amount that the variable fluid flow resistor  5  has been opened. The display  4  will update as the actuator  13  is turned, showing an infusion rate. The user stops turning the potentiometer when the desired flow rate is displayed on the display  4 . The act of turning the actuator not only rotates the potentiometer to set the desired infusion rate, but also rotates the cap  12  to open the variable resistance valve  5  to the appropriate location to enable the infusion at the desired rate. When the potentiometer stops changing (i.e., when the user sees the desired flow rate displayed and stops turning the actuator  13 ), the controller initiates the pressure delivery cycle. 
     To initiate the pressure delivery cycle, the air pump  24  is turned on and pressurizes the bladder  16  surrounding the fluid  7  to be delivered. The bladder  16  is pressurized to an initial pressure. A pressure sensor, transducer, etc., is provided within the manifold  25  for indicating the actual pressure in the bladder  16 , e.g., by providing a voltage signal representative of the actual pressure in the bladder  16  to the processor on the electronic control board  22 . The pump  24  is again activated and the air from the pump  24  is used to pressurize the calibration tank  14  to a higher pressure than the bladder. Again, a pressure sensor within the manifold  25  indicating the actual pressure within the calibration tank  14 , and providing a voltage signal representative of the actual pressure to the processor on the electronic control board  22 . The pump  24  is then turned off and a valve in the manifold  25  is opened, fluidically connecting the calibration chamber  14  to the bladder  16 . The resulting pressure in the combined volume is measured by a pressure sensor within the manifold  25  indicating the actual pressure, and providing a voltage representative of the actual pressure of the combined volume to the processor on the electronic control board  22 . 
     The three pressure signals obtained as described above and the known volume of the calibration tank  14  provide sufficient information to determine the volume of air in the bladder  16 . As fluid flows from the fluid source  7 , the volume of the fluid leaving the fluid source  7  is filled by the bladder  16  resulting in a lower pressure within the bladder volume. Since the initial volume of air within the bladder  16  was known, measuring the pressure decay in the bladder  16  volume indicates the flow rate of the fluid from the fluid source  7 . If the flow from the fluid source  7  measured in this manner is not equal to the desired volume entered at the start of the infusion, the system increases the pressure in the bladder  16  for flow rates less than the desired flow rate, and vents pressure from the bladder  16  (through a valve located within the manifold assembly  25 ) for flow rates higher than the desired flow rate. Throughout the fluid delivery process, this sequence of events is repeated to verify the volume of air in the bladder  16  and monitor and adjust the pressure decay as a measure of flow rate.  FIG. 4  shows a flow chart illustrating this delivery process. 
     Since the Pressure/Volume curve of a gas is influenced by changes in temperature as described in Boyle&#39;s Laws, the processor on the electronic control board  22  monitors the temperature of the system as indicated by the thermistor  17  and adjusts the calculated flow rate accordingly. 
     It can be now be seen that the preferred embodiment described provides a simple device that can accurately deliver a fluid to a patient quickly and accurately, requiring few steps to start by first responders or medics, requires only minimal training and delivers fluid independently of the fluid source height in relation to the patient. 
     While there has been shown and described what is considered to be the presently preferred embodiments of the invention, it will of course, be understood that various modifications and changes in form or detail could readily be made without departing from the spirit of the invention. It is therefore intended that the invention not be limited to the exact forms described and illustrated, but should be construed to cover all such modifications and alterations that may fall within the scope of the appended claims and their legal equivalents.