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 . fig1 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 when the cassette piston 60 is fully withdrawn , shuttle 40 disengages from cam 160 and motor 140 , so that spring 120 pushes the cassette piston 60 into the cassette central chamber 80 via shuttle 40 to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 . fig2 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 fig3 ). 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 fig4 ). 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 fig3 ). 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 : 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 : 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 : 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 . 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 .
methods and apparatus for air content and pressure measurement of sample fluid , especially sample fluid in association with an infusion pump . 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 . the outlet pressure of the cassette central chamber , e . g ., blood pressure , can be monitored based on the cassette central chamber pressure .