Patent Document

This application is a divisional of U.S. patent application Ser. No. 09/899,300 filed on Jul. 6, 2001, entitled “Air-In-Line and Pressure Detection,” now issued as U.S. Pat. No. 7,232,430, which is based on and claims priority from U.S. Provisional Patent Application No. 60/216,772, filed on Jul. 7, 2000, the entire subject matter of these applications being incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a system and methods of determining air content and pressure in fluid, especially in association with an infusion pump. 
     BACKGROUND OF THE INVENTION 
     There is a need in the field for methods of measuring air content and pressure of a sample fluid. Particularly in an infusion pump, it is critical to determine the air content of the infusion fluid and the fluid pressure down stream of the outlet valve of the infusion pump. 
     SUMMARY OF THE INVENTION 
     The present invention is directed in part to unique methods of content measurement of a sample fluid. The present invention is based on the discovery that volume change in a chamber, as the chamber transitions between negative and positive pressure relates to the air content in the chamber. In particular, in an infusion pump, the volume change of infusion fluid as it transitions between being under negative pressure and positive pressure within a cassette central chamber, e.g., pumping chamber, relates to the air content in the infusion fluid. In addition, the present invention provides methods for determining pressure of a sample fluid. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objects and advantages of the invention will become more apparent and more readily appreciated from the following detailed description of an exemplary embodiment of the invention taken in combination with the accompanying drawings, of which: 
         FIG. 1  is a block diagram illustrating the fluid delivery system topology of an infusion pump. 
         FIG. 2  is a block diagram illustrating a cross-sectional view of the cassette assembly of an infusion pump. 
         FIG. 3  illustrates changes in cassette central chamber volume as a function of pressure. 
         FIG. 4  illustrates changes in piston position as a function of time. 
     
    
    
     DETAILED DESCRIPTION 
     Infusion pumps are widely used for administering medications to patients over an extended time period. During an infusion of medication, it is critical to monitor the air content of the fluid medication administered to a patient. In addition, it is often convenient/helpful to measure the pressure on the patient side of the pump, e. g. measure the blood pressure of the patient. One of the applications of the methods for air content measurement of a sample fluid is to measure the air content in a cassette central chamber in an infusion pump. In addition, the mechanism for air content measurement also provides means to monitor the blood pressure of a patient connected to an infusion pump. 
       FIG. 1  is a block diagram illustrating one embodiment of the present invention. The fluid delivery system  100  includes a cassette assembly  20  and a shuttle mechanism  40 . A suitable cassette assembly is described in patent application No. 60/216,658, filed Jul. 7, 2000, entitled “Cassette”, to Carlisle, Costa, Holmes, Kirkman, Thompson and Semler, the entire contents of which are incorporated herein by reference. Within the cassette assembly  20  is a cassette piston  60  and a cassette central chamber  80 . A spring  120  biases shuttle mechanism  40  which is connected to the cassette piston  60 . Piston  60  slides freely in the cassette central chamber  80  to draw fluid into central chamber  80  and pump fluid out of central chamber  80 . A motor  140  is activated in one direction to draw the cassette piston  60  out of cassette central chamber  80  via cam  160  and shuttle  40 . When the cassette piston  60  is fully withdrawn, shuttle  40  disengages from cam  160  and motor  140 , so that spring  120  pushes the cassette piston  60  into the cassette central chamber  80  via shuttle  40  to apply positive pressure to the fluid in the cassette central chamber  80 . The shuttle mechanism  40  is also operably linked to an optical position sensor  180 . A suitable position sensor is described in patent application No. 60/217,885, filed Jul. 7, 2000, entitled “Optical Position Sensor and Position Determination Method”, to Carlisle, Kaplan and Kirkman, the entire contents of which are incorporated herein by reference. A processor  220  is connected to motor  140  and the position sensor  180 . 
       FIG. 2  is a diagram illustrating a cross-sectional view of a cassette assembly  20 . The cassette assembly  20  contains an inlet valve  200 , an outlet valve  210 , a cassette central chamber  80 , and a cassette piston  60 . Cassette piston  60  is connected to shuttle  40 . 
     In operation, the motor  140  is activated in one direction to withdraw the cassette piston  60  against the force of spring  120  via cam  160 , creating a relative vacuum in the cassette central chamber  80  and pulling fluid through a one-way passive inlet valve  200  into the cassette central chamber  80 . During this fill stroke, the pressure in the cassette central chamber  80  is negative, e.g., between 0 and −10 psi. The amount of negative pressure depends on the withdrawal speed of the piston, fluid resistance, fluid viscosity, etc. Once the cassette piston  60  has been withdrawn, cam  160  disengages from the shuttle  40 , enabling the spring mechanism  120  to urge shuttle  40  to drive piston  60  into the cassette central chamber  80 . The pressure in the chamber then moves from a negative value through zero to a positive value. The one-way passive inlet valve  200  is now fully closed. The positive pressure in the cassette central chamber  80  is typically between +2 and +7 psi depending on the spring force applied to the cassette piston  60  through the shuttle  40  which is directly related to the length of the withdrawal stroke, e.g., the further the withdrawal stroke the stronger the spring force. 
     In a closed cassette central chamber, the volume changes as a function of cassette central chamber pressure. In theory, when the cassette central chamber  80  is closed and contains only liquid, i.e., air free fluid, the cassette central chamber is not compressible, thus no volume change occurs. Nevertheless in practice, a “base volume change” exists when the chamber contains just air free fluid (as shown in  FIG. 3 ). Such “base volume change” is irrelevant to the air content in the fluid and is mostly due to system designs such as the shape of a sealing member of the cassette piston  60  or the flexing of elastomeric inlet and outlet valve elements connected to the cassette central chamber  80 . 
     For example, the cassette piston  60  in the cassette central chamber  80  acts as a nearly ideal piston when under positive pressure from the spring mechanism  120 ; thus a change in the axial position of the piston represents a fluid volume change in the cassette central chamber. Nevertheless, when cassette central chamber pressure changes from negative to positive, the shape of a sealing member of the cassette piston changes and results in piston travel without any change in central chamber fluid volume. This amount of travel contributes to the “base volume change”; it is significant, however, that this travel is a relative constant of the system design and does not change over time. In addition, the elastomeric valve elements connected to the cassette central chamber have some inherent displacement determined by their geometry and material properties. When under pressure, these elements move and to an insignificant degree, continue to move (creep) over time. The movement of these elements also contributes to the “base volume change”. 
     The “base volume change” of the cassette central chamber  80  can be determined by detecting the volume change of a control fluid under the pressure change of the cassette central chamber. The volume change can be measured by determining the change of shuttle position, i.e., the piston travel position when the cassette central chamber pressure changes from negative to positive. The change of shuttle position is determined by the precision position sensor  180  linked to the shuttle mechanism  60 . For example, one can compare the shuttle position in two states: the peak position during piston withdrawal and the shuttle position after the spring pressure is applied (as shown in  FIG. 4 ). This net displacement change of the shuttle position as a result of the pressure change in the cassette central chamber  80  filled with control fluid is a measure of the “base volume change” of the system  100 , e.g., the volume change that is inherent in the system  100 . 
     In one embodiment, the “base volume change” of a control fluid is determined more than once and statistically conservative limits of “base volume change”, e.g., lower than average, is selected as the “base volume change” for calculating the fluid air content. In another embodiment, the base volume change of a control fluid for a sample infusion fluid is determined by measuring the median volume change of more than one sample of the same infusion fluid, e.g., over more than one fill stroke, resulting from the pressure change of the cassette central chamber. In yet another embodiment, the base volume change is the median volume change of an infusion fluid over eleven (11) contiguous fill strokes, and is updated or modified periodically throughout an infusion therapy; and such base volume change is used to measure the sample fluid air content of the same infusion fluid. 
     In a closed cassette central chamber, e.g., both inlet and outlet valves are closed, any cassette central chamber volume-versus-pressure changes above the “base volume change” are interpreted as volume changes in the cassette central chamber due to the presence of air (as shown in  FIG. 3 ). The air content of fluid contributes to the total volume change of the cassette central chamber and is proportional to the total volume change, e.g., sample volume change minus the base volume change. 
     For example, one can fill the cassette central chamber  80  with a sample fluid until the volume (gVolMax) in the chamber is greater than the desired volume of fluid to be delivered in a single pump stroke (gVdue) plus the “base volume change” (Vbase). During the fill cycle, the fill volume can be monitored through the piston position, i.e., the shuttle position which is determined by the optical position sensor  180 . As a result of the geometry and design of the cassette assembly  20 , there is a linear relationship between the shuttle position and the fill volume. Once the fill volume (gVolMax) is achieved, the motor direction is reversed so that the shuttle  40  falls off the cam  160  and rides freely on the spring mechanism  120 . Similarly the end-diastolic volume (gVolEnd) can be determined from the stabilized shuttle position after the cam  160  releases shuttle  40 . The sample volume change is the difference between the gVolMax and gVolEnd. The air content of the sample fluid is calculated in processor  220  as follows:
 
Air content (Volair)˜Sample volume change−Base volume change
 
˜(gVolmax−gVolEnd)−Base volume change
 
     The air content measured according to the present invention is independent of the size and shape of the air bubble contained in a sample fluid, e.g., the air content includes the content of big bubbles, small bubbles, integrated bubbles, and unintegrated bubbles. 
     In one embodiment, the effective amount of fluid that is pumped out of cassette central chamber  80  is calculated based on the air content of a sample fluid. For example, for a given stroke, the effective amount of fluid that is infused is calculated in processor  220  as follows:
 
effective amount of fluid infused˜gVolmax−Volair
 
     In another embodiment, the proportion of air content and volume content in a given stroke is calculated directly on the change of position of the piston. Specifically the proportion is calculated in processor  220  as follows:
 
proportion of air/fluid content=(position change due to pressure change)/(max. position under neg. pressure)
 
     The processor  220  adjusts subsequent fluid flow rate based on the air/fluid content proportion in a given stroke to compensation for the air content detected in a sample fluid. 
     In yet another embodiment, processor  220  compares the air content of a sample fluid to a predetermined value stored in the processor  220 ; the processor  220  activates an alarming device if the air content of the sample fluid is close to or beyond the predetermined value. Alternatively, the processor  220  activates an alarming device as well as shuts down the out flow of sample fluid from the cassette central chamber  80 , e.g., closes the outlet valve  210  of the cassette central chamber  80  and shuts down infusion process by the fluid delivery system  100 . 
     According to another feature of the invention, sample fluid continuously passes through cassette central chamber  80  and the air content of the sample fluid is determined at different time points and stored in processor  220 . The processor  220  calculates accumulated air content of the sample fluid by adding the air content values collected at different time points. Such accumulated air content over a period of time is compared to a threshold air content value stored in the processor  220 ; the processor  220  triggers a notifying device, e.g., an alarm, if the accumulated air content is close or beyond a predetermined limitation. Alternatively, the processor  220  activates a notifying device as well as shuts down the out flow of sample fluid from the cassette central chamber  80 , e.g., closes the outlet valve  210  of the cassette central chamber  80  and shuts down infusion process by the fluid delivery system  100 . 
     In one embodiment, the outlet pressure of the cassette central chamber, e.g., the blood pressure of a mammal such as a human connected to the infusion pump is monitored. For example, during the fluid displacement, the outlet valve  210  of the cassette central chamber  80  is opened via external actuation. Fluid then flows from the higher pressure in the cassette central chamber  80  to the outlet via the outlet valve  210 . If the outlet valve  210  remains open, the cassette piston  60  will stop when the cassette central chamber pressure equals the outlet pressure. The position of the piston on the spring load is associated with a known spring force. Processor  220  then calculates the outlet pressure from the fixed geometry of the cassette central chamber  80 . With the outlet valve  210  open, the outlet pressure including even rapid changes in arterial, vein, or capillary pressure of a patient can be measured. 
     For example, the spring rate, k (in units of force/distance), of the shuttle mechanism  40  and the piston cross-sectional area, A, are known system design constants. Such system design constants, i.e., k/A are pre-calculated and stored in the processor  220  as Design_Constant. During the empty cycle, the outlet valve  210  of the cassette central chamber remains open. Once the spring  120  reaches a stabilized position, the system reaches equilibrium, e.g., the outlet pressure equals the cassette central chamber pressure. Subsequently the shuttle position, i.e., X, is measured by the optical position sensor  180  and processor  220  calculates the outlet pressure as the following:
 
Outlet pressure=Design_Constant* X  
 
     In another embodiment, processor  220  monitors the outlet pressure of the cassette central chamber and compares it to a predetermined value over a period of time. An increase of the outlet pressure indicates a partial or complete blockage of the cassette central chamber outlet, i.e., blockage of the outlet pathway or a body fluid pathway receiving fluid displaced from the cassette central chamber  80 . Depending on the degree of outlet pressure increase, processor  220  generates a signal to either alert the pressure increase or stop the fluid displacement of the system  100 . 
     Other Embodiments 
     Although several exemplary embodiments of this invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiment without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the following claims.

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