Abstract:
A fluid delivery system includes a first fluid container having infusion fluid for a patient. The first fluid container has an outlet and a conduit attached to the outlet. A second fluid container applies pressure to the first fluid container to force fluid out of the first container and into the conduit at a desired flow rate. A pressure sensor is coupled to the second fluid container and a flow sensor is coupled to the conduit. The processor monitors the flow rate and controls the duty cycle of a downstream flow control actuator to maintain the desired flow rate. The processor determines the downstream resistance of the system by varying the flow rate about the selected flow rate, receiving pressure signals from the pressure sensor and flow signals from the flow sensor, and calculating the change in pressure over the change in flow rate to produce the fluid resistance measurement.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates generally to fluid delivery systems, and more particularly, to an intravenous (IV) infusion system having a positive-pressure based mechanism for inducing fluid flow and a monitoring system for measuring the downstream resistance of the IV infusion system based on changes in pressure and flow rate. 
     IV infusion systems for infusing fluid to a patient typically include a supply of fluid for administration, an infusion needle or cannula, an administration set connecting the fluid supply to the cannula, and a flow control device. The administration set typically includes a flexible IV tube and a drip chamber. The cannula is mounted at the distal end of the flexible IV tubing for insertion into a patient&#39;s blood vessel or other body location to deliver the fluid to the patient. 
     The flow control device may be either gravity-pressure based or positive-pressure based. Gravity-pressure based flow control devices rely on the force of gravity for fluid flow. These devices may include an “IV controller” which interfaces with the IV tube. An IV controller is a device that automatically controls the flow rate of fluid through the IV tube by use of a pinching device that pinches the tube more or less to control the flow of fluid therethrough. The IV controller is usually responsive to a control signal which is typically generated by a flow sensor attached to the drip chamber. The flow sensor senses fluid drops falling in the drip chamber. The number of drops per unit time is counted and a flow rate calculated. If the calculated flow rate is greater than a desired flow rate, the controller adjusts the pinching device to lower the flow rate by pinching the tube further. Advantages of gravity administration sets include their relative simplicity and low cost. Relatively inexpensive tubing may be used such as polyvinyl chloride (“PVC”) tubing or similar type tubing. The pinching device comprises a relatively simple mechanical device under electrical control. IV controllers, however, are limited to gravity pressure, dependent upon the “head height” or “head pressure” of the administration fluid, which can be under 1 psi. 
     In certain situations the amount of pressure provided by a gravity-pressure based flow control device may be insufficient. In other situations, greater accuracy and precision of flow rates are required. In these situations a positive-pressure based flow control device is necessary. Positive-pressure based flow control devices exert a mechanical force on the fluid to establish fluid flow. One commonly used positive-pressure based flow control device is a linear peristaltic pump. A linear peristaltic pump is a complex device comprising several cams and cam-actuated fingers that sequentially occlude portions of the flexible tubing along a specially designed pumping segment to create a moving zone of occlusion. The peristaltic action forces the fluid through the tubing of the administration set to the cannula and into the patient. Because of its complexity and number of components, a linear peristaltic type pump is relatively expensive and may be undesirable in situations where cost containment is a factor. The pumping segment is also typically part of a disposable administration set and thus is relatively expensive. 
     Another type of positive-pressure based flow control device is a piston-and-valve-type device that uses a specially designed plastic cassette or cylinder device that interfaces with the piston and valve to control fluid flow. The cassette or cylinder is small in size and has precise dimensional requirements so as to provide accurate fluid flow control. Due to such requirements these devices are expensive to manufacture. The cassette or cylinder is also typically part of a disposable administration set and thus have an increased cost. 
     Another type of positive-pressure based flow control device includes a collapsible fluid treatment bag and an inflatable bladder. A fluid pump or other pressure source provides fluid, typically air, to the bladder. As the bladder inflates, pressure is applied to the collapsible fluid treatment bag. This pressure forces fluid through the tubing of the administration set to the cannula and into the patient. 
     During infusion events may occur that interfere with the proper administration of fluid to the patient, such as an occlusion of the administration line. It is desirable to detect these conditions as soon as possible so that they can be remedied. A commonly used technique for detecting such conditions and for evaluating the operating status of the IV infusion system is to monitor the pressure in the downstream portion of the fluid delivery tube. The “downstream” portion of the tube is typically thought of as the portion between the flow control device, such as the pinching device in a controller or the peristaltic fingers in a linear peristaltic pump, and the patient&#39;s blood vessel. An increase in the downstream pressure may be caused by an occlusion. 
     One measurement of downstream infusion system parameters that has proved useful is a measurement of resistance. Downstream resistance may be affected by a downstream occlusion, an infiltration of the cannula into the patient&#39;s tissue surrounding the blood vessel, a cannula that has become removed from the blood vessel, or others. By monitoring downstream resistance, an operator may be able to determine if any of the above events has occurred. Appropriate steps may be taken to remedy the situation sooner than with other monitoring approaches. It should be noted that when the cannula is in place in a patient&#39;s blood vessel, that blood vessel also contributes an effect to the flow and pressure in the tubing and is therefore considered part of the downstream resistance. 
     Sophisticated flow control devices monitor the downstream resistance of the infusion system by altering the flow rate through the tube and measuring the corresponding change in downstream pressure. The change in pressure over the change in the flow rate has been found to accurately indicate the resistive part of the downstream fluid impedance. In these systems, a pressure sensor is coupled to the infusion tube. The pressure sensor monitors the pressure existing in the downstream portion of the tube and produces pressure signals representing the detected pressure. 
     A disadvantage of these existing systems for detecting downstream resistance is that the pressure sensor must be coupled to the IV tube. Because of this, the pressure sensors must be capable of accurately detecting fluid pressure through an IV tube. Such sensors tend to be complex and expensive. 
     Hence, those skilled in the art have recognized a need for a simpler and less expensive positive-pressure based IV infusion system. Those skilled in the art have also recognized the need for an administration set using standard tubing and a standard flow monitoring system, such as a drip chamber, with a standard collapsible administration fluid container which may be accurately used with a positive pressure based IV infusion system. A need has also been recognized for a closed-loop positive-pressure based IV infusion system wherein flow rate and pressure can be monitored using standard administration set tubing and drip chamber devices for lowered cost. A further need has been recognized for a single integrated package design containing fluid source, flow sensing, flow control, pressure source, pressure sensing, and pressure control. The present invention fulfills these needs and others. 
     SUMMARY OF THE INVENTION 
     Briefly, and in general terms, the invention is directed to an apparatus and a method for controlling fluid flow through an IV infusion system and, in another aspect, for monitoring the downstream resistance of the IV infusion system. 
     In a first aspect, the invention is an apparatus for pumping infusion fluid from a first fluid container having an output communicating with a conduit. The apparatus includes a second fluid container for applying pressure to the first fluid container to expel fluid from the first fluid container, and a pressure sensor coupled to the second fluid container adapted to sense pressure in the second fluid container and provide pressure signals in response to the pressure sensed. The apparatus also includes a flow sensor coupled to the conduit and adapted to sense fluid flow in the conduit and provide flow signals in response to the fluid flow sensed, and a processor responsive to the pressure signals and flow signals for determining the downstream resistance of the conduit. 
     By providing a second fluid container, e.g., a fluid bladder, which applies pressure to a first fluid container to expel fluid from the first fluid container, a pressure sensor coupled to the second fluid container, and a flow sensor coupled to the conduit the present invention provides a pressure-based flow control device that is free of complicated and costly peristaltic or cassette pumping means. The flow control device also has the capability of determining the downstream resistance in the conduit communicating with the output of the first container through the use of a standard, readily available pressure sensor at the input of the second fluid container, as opposed to a complicated and costly pressure sensor attached to the downstream region of the conduit as is common in current flow control devices. Furthermore, since there are no cassette pumping means, a very simple and inexpensive straight line gravity IV set may be used. 
     In a more detailed aspect, the apparatus further includes a pump coupled to the second fluid container for controlling the pressure within the second fluid container. In another aspect, the apparatus further includes a flow control actuator coupled to the conduit downstream from the flow sensor. In still another facet, the processor determines the downstream resistance by setting a plurality of target flow rates, measuring the pressure applied to the fluid in the first container to cause each of the plurality of target flow rates to exist at the output of the first fluid container, receiving the flow data resulting from each target flow rate and determining the impedance. In yet another facet, the processor determines the downstream resistance by controlling the pressure applied to the first container to cause a plurality of different flow rates to exist at the output of the first fluid container, receiving the flow data resulting from each flow rate and determining the impedance by processing changes in applied pressure and changes in flow together. 
     In a second aspect, the invention is a fluid delivery system having a first fluid container with an outlet and a fluid conduit attached to the outlet. The system includes an input device for selecting a flow rate, a second fluid container for applying pressure to the first fluid container to expel the fluid from the first fluid container into the conduit, a pressure sensor coupled to the second fluid container adapted to sense pressure in the second fluid container and provide pressure signals in response to the pressure sensed, and a flow sensor coupled to the conduit and adapted to sense fluid flow in the conduit and provide flow signals in response to the fluid flow sensed. The fluid delivery system also includes a processor that varies the flow rate about the selected flow rate by varying the pressure applied to the first fluid container, receives the pressure signals and the flow signals, and calculates the downstream resistance of the system. 
     In a third facet, the invention is a fluid delivery system having a first fluid container with an outlet and a fluid conduit attached to the outlet. The system includes an input device for selecting a flow rate, a second fluid container for applying pressure to the first fluid container, and a pressure sensor coupled to the second fluid container and adapted to sense pressure and provide pressure signals in response to the pressure sensed. The system also includes a flow sensor coupled to the conduit and adapted to sense fluid flow in the conduit and provide flow signals in response to the fluid flow sensed, and a processor for maintaining the flow rate at a value substantially equal to the selected flow rate. 
     In a more detailed facet the flow rate is maintained by varying the pressure applied to the first fluid container. In another facet, the system includes a pump coupled to the second fluid container and the flow rate is maintained by varying the output of the pump. In yet another aspect, the flow rate is maintained by compressing the conduit. In still another facet, the system includes a flow control actuator coupled to the conduit and the flow rate is maintained by varying the flow of fluid through the conduit using the flow control actuator. 
     In a fourth aspect, the invention is an apparatus for delivering infusion fluid including a fluid container for holding infusion fluid that has an outlet and a tube attached to the outlet that is in fluid communication with the fluid container. The apparatus also includes a pressure bladder proximal the fluid container, for applying pressure to the fluid container to expel fluid from the fluid container into the tube, a pressure device for varying the pressure within the pressure bladder, a pressure sensor for monitoring the pressure within the pressure bladder and providing pressure data, a flow sensor for monitoring the flow of fluid from the fluid container and providing flow data, and a flow control actuator for controlling the flow of fluid through the tube. The apparatus further includes a processor responsive to the pressure data and flow data for determining the downstream resistance of the tube and for providing pressure control data to the pressure device and flow control data to the flow control actuator. 
     In a fifth facet, the invention is related to a method for pumping infusion fluid from a first fluid container having an outlet communicating with a conduit. The method includes the step of positioning a second fluid container proximal the first fluid container, applying mean pressure within the second container such that pressure is applied to the first fluid container to expel the fluid from the first container into the conduit; and determining the downstream resistance of fluid flow through the conduit. 
     In another aspect, the step of determining the downstream resistance comprises the steps of setting a first target flow rate from the first container, applying a first pressure within the second fluid container to obtain the first target flow rate, setting a second target flow rate from the first container different than the first target flow rate, applying a second pressure within the second fluid container to obtain the second target flow rate and processing the changes in pressure and changes in target flow rate together. In a more detailed facet, the step of determining the downstream resistance comprises the steps of applying a first pressure within the second fluid container, sensing a first fluid-flow rate at the outlet, applying a second pressure within the second fluid container wherein the second pressure is different than the first pressure, sensing a second fluid-flow rate at the outlet, and processing the changes in pressure and changes in flow together. 
     In a sixth aspect, the invention is related to a method for pumping infusion fluid from an infusion pump having a first fluid container with an outlet communicating with a conduit, and a second container positioned proximal the first container. The method includes the steps of applying pressure within the second container such that pressure is applied to the first fluid container whereby fluid is expelled from the first container into the conduit and determining the downstream resistance of fluid flow through the conduit. 
     In a seventh facet, the invention is related to an apparatus for infusing medical fluid into a patient from a first collapsible fluid container. The apparatus includes a standard conduit coupled to the first container to conduct medical fluid from the first container to the patient. An expandable pressure-control container is positioned so as to apply mechanical force against the first container to controllably cause it to collapse and expel its contents to the conduit. A pump is coupled to the pressure-control container to provide fluid under pressure to the pressure-control container to control the amount of expansion of the pressure-control container. This, in turn, controls the mechanical force applied to the first container. The apparatus also includes a pressure sensor coupled to the pressure-control container to sense the pressure of the fluid in the container and to provide pressure signals representative thereof. A flow sensor is coupled to the conduit downstream of the first container to sense the flow of medical fluid through the conduit from the first container and to provide flow signals. A flow control actuator is located downstream of the flow sensor to control the amount of fluid flowing through the conduit. The actuator is responsive to actuator control signals. The apparatus further includes a processor that is coupled to the pump, to the pressure sensor, to the flow sensor, and to the flow control actuator. The processor monitors the flow signals, the pressure signals, and controls the pump and the flow control actuator to achieve a desired flow of fluid through the conduit. 
     An advantage of the invention is that a relatively inexpensive administration set may be manufactured for use in the disclosed infusion system. A standard straight line infusion set may be used with a computer controlled pressure source and flow restriction as opposed to a peristaltic or other mechanical pumping means to regulate fluid flow under positive pressure, resulting in a less expensive system. The system disclosed and illustrated herein provides the performance and features of a pump without the associated cost of specially designed pumping segments. The system takes advantage of the characteristics of an IV controller system but overcomes the head height pressure limitations by delivering from a pressurized source. 
     Such an administration set may have a conduit communicating with a collapsible bag, i.e., first container. The conduit may take the form of any well known spike. The administration set may further include a drip chamber, standard PVC tubing, a connector and a patient cannula communicating with the connector. In another approach, the administration set may comprise a collapsible bag with an integral drip chamber and tubing coupled to a patient cannula. The bladder as well may be disposable and come equipped with a simple fluid connection to the pump and pressure sensor combination. 
     Accordingly, a new and useful infusion system has been provided. The system described and illustrated is closed-loop for more accurate control over the infusion process. Additionally, standard, low cost tubing and administration equipment is used for the disposable part of the system. More expensive parts, such as the processor, display, keyboard, pump and sensors may be reused thus lowering the expense. The ability to determine downstream resistance is provided thus giving the care giver more complete information about the infusion process. Problems that may arise during the infusion process can be detected more readily. 
     These and other aspects and advantages of the invention will become apparent from the following detailed description and the accompanying drawings, which illustrate by way of example the features of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic block diagram of an IV infusion system incorporating aspects of the present invention; 
         FIG. 2  is a graph depicting pressure verses flow rate relationships for various levels of resistance; 
         FIG. 3   a  is a graph depicting small positive and negative pressure deviations from a mean pressure which occur during resistance calculations performed in accordance with the one embodiment of the present invention; 
         FIG. 3   b  is a graph depicting small positive and negative flow rate deviations from a mean flow rate which occur during the positive and negative pressure deviations of  FIG. 3   a.    
         FIG. 4   a  is a graph depicting small positive and negative pressure deviations from a mean pressure which occur during resistance calculations performed in accordance with another embodiment of the present invention; and 
         FIG. 4   b  is a graph depicting small positive and negative flow rate deviations from a mean flow rate which occur during the positive and negative pressure deviations of  FIG. 4   a;    
         FIG. 5  is a flow chart of the downstream resistance measurement process in accordance with one embodiment of the present invention; and 
         FIG. 6  is a flow chart of the downstream resistance measurement process in accordance with another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Turning now to the drawings, in which like reference numerals are used to designate like or corresponding elements among the several figures, in  FIG. 1  there is shown an IV infusion system  10  that utilizes a collapsible fluid container  12  and a pressure bladder  16 . The fluid container  12  and the pressure bladder  16  are placed in a housing  14  having a rear panel  40  and a front door  38  with a transparent window (not shown) to allow the user to view the fluid container  12 . The bladder  16  can be inflated by use of a pump  18 , such as an air or other fluid pump. The bladder  16  is positioned relative to the fluid container  12  such that when the bladder is pressurized, the pressure from the bladder is transferred to the fluid container  12 . Thus, the fluid container  12  is placed under pressure by the bladder  16  and the fluid within the container is forced into an IV tubing  20  communicating at one end with the outlet  42  of the container and at the other end with a catheter  44 . The catheter  44 , in turn, communicates through a cannula  46  with a patient (not shown). 
     A standard, off-the-shelf, pressure sensor  22  is connected at the inlet end of the bladder  16  to monitor bladder pressure. A drip chamber  26  is located in the IV tubing  20  near the outlet  42  of the fluid container  12 . A drop-detecting flow sensor  24  is positioned about the drip chamber  26  to detect fluid drops falling in the drip chamber. The flow sensor  24  may be any well known optical type or capacitive type sensor. Drip chambers and drop-detecting flow sensors are well known to those skilled in the art and no further details with regard to these devices are provided here. A flow control actuator  28  is placed about the IV tubing  20  below the drip chamber  26  to control flow. The actuator  28 , in this embodiment, comprises a standard pinching device that subjects the infusion tubing  20  to a degree of pinching, thereby altering the inner diameter or inner opening of the tubing and thereby controlling the amount of flow through the tubing. The flow control actuator  28  may also be a pulse-width-modulation type rather than a degree-of-pinch type. 
     A processor  30  is provided to accept user input from a keypad  32  and to display pertinent information including downstream resistance, infusion rate, time and volume on an information display  34 . The processor  30  also receives pressure and flow-rate data from the pressure sensor  22  and the flow sensor  24  to determine the pressure, the flow rate and the downstream resistance. Based on the signals received from the sensors  22 ,  24 , the processor  30  commands the flow control actuator  28  and the bladder pump  18  in such a way as to regulate the fluid-flow rate to the desired value entered by a user. The pressure bladder  16  establishes the pressure within the IV tubing  20 , the drip chamber  26 , the catheter  44  and the cannula  46 . The flow control actuator  28  applies the appropriate degree of pinch to the IV tubing  20  to regulate the fluid flow rate through the downstream portion of the system, i.e., the portion of the system downstream from the flow control actuator  28 , including a portion of the IV tubing  20 , the catheter  44  and the cannula  46 . In one embodiment, the actuator  28  is a pinching-type valve that is either entirely open with no significant contact with the tubing  20 , or entirely closed wherein the tubing  20  is clamped shut. The amount of time that the pinching valve is in the entirely open position and the amount of time that the pinching valve is in the entirely closed position in a predetermined time frame is the duty cycle. Typically, the duty cycle is the comparison of the time the tubing is open to the predetermined time frame. For example, if the tubing is left open for 100 milliseconds out of each 200 milliseconds of time, the duty cycle is 50%. 
     In operation, the user enters the desired infusion rate into the controller  30  using the keypad  32 . The processor  30  determines the pressure P 0 , i.e., the “mean pressure”, and the flow control actuator duty cycle D 0 , necessary to establish the desired flow rate. The processor  30  controls the pump  18  to inflate the bladder  16  to the mean pressure. The bladder  16  pressure is monitored by the processor  30  through the pressure sensor  22  while the flow rate is monitored through the flow sensor  24 . If the sensed flow rate is greater than the desired flow rate, the processor  30  provides flow control data to the flow control actuator  28  to set the duty cycle of the actuator to maintain the flow rate in the downstream portion of the IV tubing  20  at the desired rate. For example, if an infusion rate of 100 ml/hr is desired, but the mean pressure is set at that pressure which establishes a flow rate of 110 ml/hr, the duty cycle would be set at 91% to establish a 100 ml/hr downstream flow rate. If, however, the sensed flow rate is less than the desired flow rate, the processor  30  may either increase the mean pressure at the bladder  16  or decrease the duty cycle of the flow control actuator  28 . Naturally, if the duty cycle is set at 100%, i.e., the IV tubing  20  is completely open at all times, the only way to adjust the flow rate is to increase the mean pressure. 
     The system  10  is capable of calculating the downstream resistance of the IV tubing  20  either on a periodic basis or as desired intermittently by the operator. To determine the downstream resistance, the processor  30  changes the pressure within the bladder  16  by small positive deviations, ΔP, and negative deviations −ΔP, about the mean pressure and measures the flow rate changes resulting from the pressure changes. As shown in  FIG. 2 , the flow rate is a function of both pressure and downstream resistance. At a nominal downstream resistance R 1 , the relationship between pressure and flow rate is rather predictable, that is, an increase or decrease in pressure produces an expected corresponding increase or decrease in flow rate. At a high resistance R 2 , a significant increase or decrease in pressure produces only a slight increase or decrease in flow rate. In the extreme situation where the IV tubing  20  is completely occluded, the resistance line would be substantially horizontal and no amount of pressure change will cause fluid flow change. In this situation, the flow rate will probably be zero. At a low resistance R 3 , a slight increase or decrease in pressure produces a significant increase or decrease in flow rate. In the extreme situation where the IV tubing  20  is completely unobstructed, e.g., the IV needle is no longer in the patient, the resistance line may have a large slope depending on the sizes of the tubing, catheter  44  and the cannula  46  and the change in flow rates will be large in response to pressure changes. 
     To measure the downstream resistance, i.e., the resistance downstream of the flow control actuator  28 , the flow rate F 0  through the drip chamber  26  at pressure P 0  is first measured using data from the flow sensor  24 . The pressure within the bladder  16  is then increased above the mean pressure P 0  by ΔP to pressure P 1 , as shown in  FIG. 3   a . The flow rate F 1 , as shown in  FIG. 3   b , is then measured using data from the flow sensor  24 . The downstream resistance is than calculated using the following equation: 
     
       
         
           
             
               
                 
                   R 
                   = 
                   
                     
                       
                         P 
                         1 
                       
                       - 
                       
                         P 
                         0 
                       
                     
                     
                       
                         F 
                         1 
                       
                       - 
                       
                         F 
                         0 
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     Eq 
                     . 
                     
                         
                     
                     ⁢ 
                     1 
                   
                   ) 
                 
               
             
           
         
       
     
     A subsequent resistance measurement is obtained by decreasing the pressure within the bladder  16  below the mean pressure P 0  by −ΔP to pressure P 2 . The flow rate F 2  is then measured using data from the flow sensor  24 . The downstream resistance is than calculated using the following equation: 
                   R   =         P   1     -     P   2           F   1     -     F   2                 (     Eq   .           ⁢   2     )               
Once the resistance measurement is complete, the bladder pressure is reset to the initial mean pressure P 0 .
 
     In an alternate embodiment, as shown in  FIGS. 4   a  and  4   b , the subsequent resistance measurement is determined by resetting the pressure to the initial pressure P 0  measuring the flow rate F 0  and then decreasing the pressure within the bladder  16  below the mean pressure P 0  by −ΔP to pressure P 2 . The downstream resistance is than calculated using the following equation: 
     
       
         
           
             
               
                 
                   R 
                   = 
                   
                     
                       
                         P 
                         0 
                       
                       - 
                       
                         P 
                         2 
                       
                     
                     
                       
                         F 
                         0 
                       
                       - 
                       
                         F 
                         2 
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     Eq 
                     . 
                     
                         
                     
                     ⁢ 
                     3 
                   
                   ) 
                 
               
             
           
         
       
     
     It is noted that in general there is a pressure differential between the pressure at the top of the fluid in the fluid container  12 , i.e., the bladder  16  pressure, and the pressure at the catheter  44 . This pressure differential is a result of the fluid column in the IV tubing  20  between the fluid container  12  and the catheter  44 . The fluid column causes the pressure at the catheter to be greater than the pressure at the fluid container  12 . The difference between the two pressures is a function of the vertical distance between the fluid container  12  and the catheter  44 , the greater the distance, the greater the pressure difference. As long as the vertical distance between the fluid container  12  and the catheter  44  remains relatively constant, this pressure differential remains constant. Because the resistance measurement itself is differential, this pressure differential cancels out and, in general, does not affect the accuracy of the resistance measurement. 
     The frequency of downstream resistance measurement may be set by the processor or set by the operator through the keypad  32 . For the pressure pattern depicted in  FIG. 3   a , two consecutive frequency measurements are calculated using Eqs. 1 and 2. These two measurements may be averaged to obtain a single resistance measurement. The time between the next pair of positive and negative pressure deviations defines the frequency. For the pressure pattern depicted in  FIG. 4   a , the positive pressure deviation provides a resistance measurement using Eq. 1. The negative pressure deviation provides a resistance measurement using Eq. 3. The time between the positive and negative deviations may define the frequency. In the alternative, if the positive and negative deviations occur substantially close to each other they may be considered a deviation pair, similar to the deviation pair of  FIG. 3   a , and the time between the next pair of positive and negative pressure deviations defines the frequency. A resistance measurement may also be received on command by the operator through the keypad  32 . 
     It is clear from equations 1 through 3 that a large change in pressure with a small change in flow results in a large number for the resistance, perhaps indicating an obstructed downstream line or an infiltration. Vice versa, a small change in pressure with a large change in flow results in a small number for resistance, perhaps indicating that the cannula has completely withdrawn from the patient for some reason. 
     During resistance measuring, the flow control actuator  28  operates at a fixed duty cycle, thereby maintaining the desired flow rate in the downstream portion of the system. The flow control actuator  28  compresses the IV tubing  20  in accordance with the fixed duty cycle selected to maintain the desired flow rate in the downstream portion of the system. The average flow rate within the drip chamber  26  is maintained by the small positive ΔP and small negative −ΔP pressure deviations from the mean P 0 , as shown in  FIGS. 3   a  and  4   a , which in the aggregate, cancel each other out to maintain an average flow rate within the drip chamber substantially equal to the desired flow rate. 
     It is significant that the pressure deviations from P 0  be both positive and negative, otherwise, were the pressure deviations only positive, the average flow rate would be higher than the desired flow rate. Conversely, were the pressure deviations only negative, the average flow rate would be lower than the desired flow rate. 
     A flow chart of one version of operation of the infusion system, including a downstream resistance measurement under the pressure deviation scenario of  FIG. 3   a , is provided in  FIG. 5 . In step S 1 , the pressure P of the bladder  16  is set to P 0  and the duty cycle D of the flow control actuator  28  is set to D 0  to establish a flow rate F 0 . In step S 2 , the flow rate F within the drip chamber  26  is measured. In step S 3 , it is determined whether the measured flow rate F equals the desired flow rate F 0 . If no, then the pressure P 0  and/or duty cycle D 0  are adjusted in step S 4  and the process returns to step S 2 . 
     If the measured flow rate F equals the desired flow rate F 0 , then in step S 5  it is determined whether it is time to perform a resistance measurement. If it is not yet time, the process returns to step S 2 . If it is time to perform a resistance measurement, the duty cycle is fixed at D 0  in step S 6 . In step S 7 , the bladder pressure P is set to P 0 +ΔP=P 1 . In step S 8 , the flow rate F 1  within the drip chamber  26  is measured. In step S 9 , the downstream resistance is calculated using Eq. 1. In step S 10 , the bladder pressure P is set to P 0 −ΔP=P 2 . In step S 11 , the flow rate F 2  within the drip chamber  26  is measured. In step S 12 , the downstream resistance is calculated using Eq. 2. In step S 13 , the bladder pressure P is set to P 0  and the process returns to step S 2 . 
     A flow chart of another version of operation of the infusion system, also including a downstream resistance measurement under the pressure deviation scenario of  FIG. 3   a , is provided in  FIG. 6 . In step S 21 , the pressure P of the bladder  16  is set to P 0  and the duty cycle D of the flow control actuator  28  is set to D 0  to establish a flow rate F 0 . In step S 22 , the flow rate F within the drip chamber  26  is measured. In step S 23 , it is determined whether the measured flow rate F equals the desired flow rate F 0 . If no, then the pressure P 0  and/or duty cycle D 0  are adjusted in step S 24  and the process returns to step S 22 . 
     If the measured flow rate F equals the desired flow rate F 0 , then in step S 25  it is determined whether it is time to perform a resistance measurement. If it is not yet time, the process returns to step S 22 . If it is time to perform a resistance measurement, the duty cycle is fixed at D 0  in step S 26 . In step S 27 , a first target flow rate F 1  is set to F 0 +ΔF. In step S 28 , it is determined whether the measured flow rate F equals the first target flow rate F 1 . If no, then the pressure P 0  is increased by an incremental amount in step S 29  and the process returns to step S 28 . If yes, then in step S 30  the pressure P 1  is measured, where P 1  is the pressure required to achieve the first target flow rate F 1 . 
     In step S 31 , a second target flow rate F 2  is set to F 0 −ΔF. In step S 32 , it is determined whether the measured flow rate F equals the second target flow rate F 2 . If no, then the pressure P 0  is decreased by an incremental amount in step S 33  and the process returns to step S 32 . If yes, then in step S 34  the pressure P 2  is measured, where P 2  is the pressure required to achieve the second target flow rate F 2 . In step S 35 , the downstream resistance is calculated using Eq. 2. In step S 36 , the bladder pressure P is set to P 0  and the process returns to step S 22 . 
     In measuring the resistance by determining changes in pressure in response to changes in flow rate as opposed to determining changes in flow rate in response to changes in pressure, this version of operation is better able to maintain the average flow rate within the drip chamber near F 0 . 
     During operation, the processor monitors for alarm conditions. Such alarm conditions may include system occlusion conditions and system disconnect conditions. A system occlusion is detected when the flow data provided by the flow sensor  24  indicates a flow rate that is less than expected for a given applied pressure. The larger the flow-rate deviates from the expected flow rate the more significant the system occlusion. In the extreme case, if the flow rate is zero, a complete occlusion is likely. A system occlusion may occur in the downstream portion of the system or in the portion of the system between the fluid container  12  and the flow control actuator  28 . 
     A system disconnect is detected when the flow data provided by the flow sensor  24  indicates a flow rate that is significantly greater than expected for a given applied pressure. A system disconnect may occur when the tubing  20  disconnects from the catheter  44 , when the catheter disconnects from the cannula  46  or when the cannula disconnects from the patient. 
     In the event of an alarm condition, the processor  30  may, if desired, automatically activate a dump valve  36  to rapidly depressurize the bladder  16  thereby stopping further fluid flow from the fluid container  12  to the IV tubing  20 . The processor  30  may also stop signaling the flow control actuator  28  causing it to pinch the IV tubing  20  completely shut thereby stopping further fluid flow to the patient. 
     It will be apparent from the foregoing that while particular forms of the invention have been illustrated and described, various modifications can be made without departing from the spirit and scope of the invention. Accordingly, it is not intended that the invention be limited, except as by the appended claims.