US20090131863A1 - Volume Measurement Using Gas Laws - Google Patents

Abstract

A system and method for measuring fluid volume and changes in fluid volume over time, using a simple, low cost architecture is described.

Description

BACKGROUND
• The present disclosure relates to fluid flow control devices and more particularly to feedback control 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 clinical practice, however, 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. 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 nurse 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 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 pressures 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 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 for determining the volume of fluid remaining in an infusion is provided.
• In another aspect, a method for determining fluid flow rate over an extended period of time is provided.
• In another aspect, a method for determining fluid flow rate over a relatively short period of time is provided.
• One advantage of the present disclosure is that long term doses can be delivered on time, because the remaining fluid volume can measured, so that flow rate errors do not accumulate over time.
• Another advantage of the present disclosure is that nurses or other users of the infusion system will not have to estimate, enter, and re-enter the volume to be infused. This will reduce the workload for the user and will eliminate opportunities for programming error.
• Another advantage is found in that volume measurements made over time can be used to accurately compute fluid flow rate.
• Another advantage is found in that volume measurements may be made using an inexpensive and simple pumping mechanism.
• Another advantage is found in that volume measurements may be made without significant loss of energy.
• Another advantage is found in that volume measurements may be made over a wide range of volumes.
• Another advantage of the present disclosure is that its simplicity, along with feedback control, makes for a reliable architecture.
• Other benefits and advantages of the present disclosure will become apparent to those skilled in the art upon a reading and understanding of the preferred embodiments.
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.
• FIGS. 1 and 2 are perspective and side views of an infusion pump in accordance with an exemplary embodiment.
• FIG. 3 is a functional block diagram showing the fluidic connections of a volume measurement system according to an exemplary embodiment.
• FIG. 4 is a functional block diagram showing the control elements of a volume measurement system according to an exemplary embodiment.
• FIG. 5 is a functional block diagram showing the sensing elements of the system.
• FIG. 6 is a flow chart diagram outlining an exemplary method of volume measurement.
• FIG. 7 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 numerals reference numerals are used to indicate like or analogous components throughout the several views, FIG. 1 depicts an exemplary volume and flow measurement system in accordance with an exemplary embodiment of the present invention. The system includes a pressure frame 10 that is of known total volume and contains within it an air bladder 20, and a flexible bag 30 that contains within it a liquid to be infused 40.
• Referring now to FIG. 2, the air bladder 20 is connected to an air pump 50 via a bladder connection line 608, a bladder valve 106, and a bladder valve line 606. The air bladder 20 may be vented to atmosphere via a bladder vent valve 108.
• A calibration tank 60 of known volume is connected to the air pump 50 via a tank connection line 604, a tank valve 102, and a tank valve line 602. The tank 60 may be vented to atmosphere via a tank vent valve 104.
• The liquid 40 is fluidically coupled to an output 500 via a liquid drain line 610, going through a fluid flow resistor 400 and through an output line 612. The liquid 40 may be, for example, a medication fluid, intravenous solution, or the like, and the output 500 may be, for example, a patient or subject in need thereof.
• The tank 60 is connected to a tank pressure sensor 204 and an optional tank temperature sensor 304. The bladder 20 is connected to a bladder pressure sensor 202 and an optional bladder temperature sensor 302.
• Referring now to FIG. 4, an electronic module includes a processing unit 700 such as a microprocessor, microcontroller, controller, embedded controller, or the like, and is preferably a low cost, high performance processor designed for consumer applications such as MP3 players, cell phones, and so forth. More preferably, the processor 700 is a modern digital signal processor (DSP) chip that offers low cost and high performance. Such processors are advantageous in that they support the use of a 4th generation programming environment that may substantially reduce software development cost. It also provides an ideal environment for verification and validation of design. It will be recognized that the control logic of the present development may be implemented in hardware, software, firmware, or any combination thereof, and that any dedicated or programmable processing unit may be employed. Alternately the processing unit 700 may be a finite state machine, e.g., which may be realized by a programmable logic device (PLD), field programmable gate array (FPGA), field programmable object arrays (FPOAs), or the like. Well-known internal components for processor 700, such as power supplies, analog-to-digital converters, clock circuitry, etc, are not shown in FIG. 3 for simplicity, and would be understood by persons skilled in the art. Advantageously, the processing module may employ a commercially available embedded controller, such as the BLACKFIN® family of microprocessors available from Analog Devices, Inc., of Norwood, Mass.
• With continued reference to FIG. 4, the processing unit 700 controls the air pump 50 via a pump control line 750. The processor 700 controls the tank vent valve 104 via a tank vent valve control line 704. The processor 700 controls the tank valve 102 via a tank valve control line 702. The processor 700 controls the bladder vent valve 108 via a bladder vent valve control line 708. The processor 700 controls the bladder valve 106 via a bladder valve control line 706.
• With reference now to FIG. 5, the processor 700 can measure pressure and temperature from the bladder 20 and tank 60. The processor 700 reads the pressure in the tank 60 via a tank pressure sensor 204, which is coupled to the via tank pressure line 724. The processor 700 reads the pressure in the bladder 20 via a bladder pressure sensor 202, which is coupled to the processor 700 via a tank pressure line 722. The processor 700 reads temperature of the gas in the tank 60 via a tank temperature sensor 304, which is coupled to the processor 700 via a tank temperature line 714. The processor 700 reads the temperature of the gas in the bladder 20 via a bladder temperature sensor 302, which is coupled to the processor 700 via a bladder temperature line 712.
Volume Measurement
• Ultimately, the objective of volume measurement is to know the quantity of liquid 40 remaining in an infusion and how that quantity changes over time.
• The pressure frame 10 defines a rigid container of known volume, V_frame. This volume is known by design and is easily verified by displacement methods. Within the pressure frame 10, there is the air bladder 20, which has a nominal capacity greater than the volume V_frame. When expanded, the bladder must conform to the geometry of the rigid container and its contents. The volume of liquid 40 to be infused, V_tbi, is equal to V_frame, less the fixed and known volume of the bladder 20 itself, V_blad, less any incompressible materials of the bag 30, V_bag, and less the volume of gas in bladder 20, V_gas. Once the value V_gas is computed, then it is trivial to compute V_tbi.
• 
  V _tbi =V _frame −V _blad −V _bag −V _gas
• With the following method, at any given point in time, the volume of air contained in the bladder, V_gas, can be measured and V_tbi can be subsequently computed.
• For purposes of economy and flexibility, the pump 50 may be an imprecise air pump, such as that of a rolling diaphragm variety, although other types of pumps are also contemplated. The output of such a pump may vary significantly with changes in back pressure, temperature, age of the device, power supply variation, etc. One advantage of the device and method disclosed herein is that they allow an imprecise pump to be used in a precision application, by calibrating the pump in situ.
• FIG. 6 shows the steps leading to computation of V_tbi. Shown as step 802, the first step is to find an optimum amount of air mass, N_pump, to add to the bladder to effect a significant pressure change, for example, on the order of about 10%. If the amount of air mass added to the bladder is too small, then the pressure change will not be measurable with accuracy. If the amount of the air mass is too great, then pressure in the bladder will increase more than necessary and energy will be wasted.
• The initial pressure in the bladder 20, P_bladder1, is measured using the bladder pressure sensor 202. The tank valve 102 is set to a closed state via the tank control valve line 702 from the processor 700. The bladder valve 106 is set to an open state via the tank control valve line 706 from the processor 700. The pump 50 is activated by the processor 700 via the pump control line 750 for a period of time, S_test, nominally, for example, about 250 milliseconds. A new measurement of the pressure in the bladder 20 is made, P_bladder2. Based on the percent of pressure change from this pumping action, a new pump activation time, S_pump, will be computed. This calculation needs no precision; it is only intended to find an amount of pumping that provides a significant change in pressure, P_deltatarget, in bladder 20, for example, on the order of about 10%.
• $S_{\mathrm{pump}}=S_{\mathrm{test}}*\frac{P_{\mathrm{deltatarget}}}{\left(P_{\mathrm{bladder}2}-P_{\mathrm{bladder}1}\right)/P_{\mathrm{bladder}1}}$
• In step 804, the pump 50 or the tank vent valve 104 are activated to increase or decrease, respectively, the pressure, P, in the tank 60, so that it approximately equals the pressure, P_bladder, in bladder 20. The combination of valve and pump settings required for such adjustments are shown in the table below:

• 		Bladder	Bladder	Tank
  	Pump	Valve	Vent	Valve	Tank Vent
  	10	106	Valve 108	102	Valve 104

  Increase Pbladder	ON	OPEN	CLOSED	CLOSED	CLOSED
  Decrease Pbladder	OFF	CLOSED	OPEN	CLOSED	CLOSED
  Increase Ptank	ON	CLOSED	CLOSED	OPEN	CLOSED
  Decrease Ptank	OFF	CLOSED	CLOSED	CLOSED	OPEN

• Adjustments made in step 804 can be made iteratively until P_tank is roughly equal to P_bladder, for example, within about 5% of the relative pressure measured in P_bladder. This does not need to be a precise process. Following the adjustment, the pressure in tank 60, P_tank2, is recorded.
• In step 806, the system is configured to increase the pressure in tank 60, as shown in the above table. The pump 50 is activated for a time period equal to S_pump After a delay of approximately five seconds, the pressure in the tank 60 is measured, P_tank3. This delay is to reduce the effect of an adiabatic response from the increase in pressure in the tank 60.
• In step 808, the system is configured to increase the pressure in bladder 20, as shown in the above table. The pump 50 is activated for a period equal to S_pump. After a delay of approximately five seconds, the pressure in the bladder 20 is measured, P_bladder3. This delay is to reduce the effect of an adiabatic response from the increase in pressure in the bladder 20.
• Because the initial pressures in the bladder 20 and the tank 60 were approximately equal, the quantity of air mass injected into tank 60 in step 806 and into bladder 20 in step 808 will be roughly equal, even though the pump 50 need not be a precise metering device.
• We take advantage of several simplifications. First, the ambient temperature for sequential steps 806 and 808 is unchanged. Second, the atmospheric pressure during sequential steps 806 and 808 is unchanged. These conditions simplify the ideal gas law formula and allow the use of gauge pressure measurements, rather than absolute pressure.
• In step 810, the volume of gas in the bladder 20, V_gas, can be calculated with a reduced form of PV=nRT:
• $V_{\mathrm{gas}}=\frac{V_{\mathrm{tank}}*\left(P_{\mathrm{tank}3}-P_{\mathrm{tank}2}\right)}{\left(P_{\mathrm{bladder}3}-P_{\mathrm{bladder}2}\right)}$
• As examples of this calculation, if the pressure change were the same in the bladder 20 and the tank 60, then V_gas would be equal to V_tank. If the pressure change in the bladder 20 were 20% as large as that in the tank 60, then V_gas would be 5 times greater than V_tank.
• Step 812 derives the value for V_tbi from V_gas, using known values for V_frame Vblad, and V_bag and using the calculated value of V_gas, from step 810.
• 
  V _tbi =V _frame −V _blad −V _bag −V _gas
• The valves 102, 106, 104, and 108 can be configured in many ways, including multiple function valves and or manifolds that toggle between distinct states. The depiction herein is made for functional simplicity and ease of exposition, not necessarily economy or energy efficiency.
Flow Rate Calculation
• Once the fluid volume has been computed, multiple measurements made over time will yield knowledge of fluid flow rate, which is, by definition, fluid volume changing over time. Repeated measurements of volume over time provided more and more resolution of average flow rate. The average flow rate and the volume of liquid 40 remaining to be infused can be used to estimate the time at which the fluid volume will be delivered. If the infusion is to be completed within some specified period of time, any error between the specified time and the estimated time can be calculated and the flow rate can be adjusted accordingly.
• There are situations where the short-term flow rate is of interest. Rather than make repeated volume measurements over a short period of time, there is an alternative approach. Once the gas volume in bladder 20 is known, then the observation of pressure decay in the bladder can be converted directly to a flow rate. It is important to know that the measurement of pressure decay, by itself, is not adequate to compute flow rate. For example, if the pressure were decaying at a rate of 10% per hour, this information cannot be converted into flow rate, unless the starting gas volume is known. As an example, if V_gas has been measured to be 500 ml and the absolute pressure is decaying at a rate of 5% per hour, then the flow rate is 5% of 500 ml per hour or 25 ml per hour. The knowledge of the initial volume is critical to compute fluid flow rate.
• The measurement of pressure decay is a simple procedure of observing the time the absolute pressure of P_bladder to drop by a small, but significant, amount, preferably for example about 2%. Because the processor 700 is capable of measuring times from microseconds to years, this measurement carries a very wide dynamic range. By observing a 2% drop, the change in pressure is well above the noise floor of the pressure measurement system.
• A flow chart outlining an exemplary process 900 for calculating flow rate by monitoring the rate of pressure decay in the bladder 20 is shown in FIG. 7. At step 904, the volume of gas in the bladder 20 is calculated as detailed above. At step 908, the pressure in the bladder 20, P_bladder1 is measured using the sensor 202 at time T1, which is recorded in step 912. The pressure in the bladder 20 is measured again at step 916 and the time T2 is recorded at step 920. The change in pressure, ΔP, between the time T1 and the time T2 is calculated in step 924 as P_bladder1−P_bladder2 and the change in time, ΔT is calculated as T2-T1 at step 928. At step 932, it is determined whether ΔP is greater than some predetermined or prespecified threshold value, e.g., about 2% with respect to P_bladder1 If ΔP has not reached the threshold value at step 932, the process returns to step 916 and continues as described above. If ΔP has reached the threshold value at step 932, the rate of pressure decay is calculated as ΔP/ΔT at step 936. The flow rate is then calculated as ΔP/ΔT×V_gas−P_bladder1 at step 940.
• The invention has been described with reference to the preferred embodiments. Modifications and alterations will occur to others upon a reading and understanding of the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.

Claims (14)

1. A method of measuring a volume of liquid in a flexible container, comprising:

placing the flexible container within a first rigid container of known volume, the first rigid container containing an inflatable bladder;

pressurizing the inflatable bladder with a gas;

pressurizing a second rigid container of known volume with a gas, the pressure of the gas in the bladder being approximately equal to the pressure of the gas in the second rigid container;

delivering a first molar quantity of gas to the bladder to cause a measurable increase in pressure in the bladder;

delivering a second molar quantity of gas to the second rigid container to cause a measurable increase in pressure in the second rigid container, the first molar quantity of gas being approximately equal to the second molar quantity of gas;

measuring the increase in pressure in the bladder;

measuring the increase in pressure in the second rigid container;

calculating the volume of gas in the bladder using the known volume of the second rigid container, the measured increase in pressure in the bladder, and the measured increase in pressure in the second rigid container; and

calculating the volume of liquid in the flexible container by subtracting the calculated volume of gas in the bladder from the known volume of the first rigid container.

2. The method of claim 1, further comprising:

subtracting the known volume of incompressible materials within the first rigid container from the known volume of the first rigid container.

3. The method of either one of claims 1 and 2, wherein said gas is air.

4. The method of any one of claims 1-3, wherein said gas is delivered to said first and second rigid containers with a pump.

5. The method of claim 5, wherein said pump is not a precise metering device.

6. The method of any one of claims 1-5, further comprising:

prior to delivering said first and second molar quantities of gas, adjusting the pressure in one or both of said bladder and said second rigid container.

7. The method of any one of claims 1-6, further comprising:

monitoring the pressure in said bladder; and

calculating a flow rate of liquid exiting the flexible container.

8. A method for calculating a flow rate of liquid exiting a flexible container contained within a rigid container of known volume, the rigid container containing an inflatable bladder, the inflatable bladder pressurized with a gas to urge the liquid out of the flexible container, said method comprising:

calculating the volume of gas in the inflatable bladder;

determining the initial pressure in the inflatable bladder;

monitoring the pressure decay in the inflatable bladder over time until the change in pressure reaches some preselected threshold value;

calculating the rate of pressure decay; and

calculating the flow rate using the rate of pressure decay, the calculated volume of gas in the inflatable bladder, and the initial pressure in the inflatable bladder.

9. A fluid delivery system, comprising:

a pressure frame (10) of known total volume;

an inflatable bladder (20) within said pressure frame;

said pressure frame adapted to receive a flexible bag (30) containing a liquid to be infused (40);

a calibration tank (60) of known volume;

a pump (50) fluidically coupled to said bladder and said calibration tank for selectively delivering a gas to said bladder and said calibration tank;

a first pressure sensor (202) coupled to said bladder for sensing the pressure of a gas in said bladder;

a second pressure sensor (204) coupled to said calibration tank for sensing the pressure of a gas in said calibration tank;

a processing unit (700) coupled to said first and second pressure sensors and said first and second temperature sensors for storing pressure and temperature information from said first and second pressure sensors and said first and second temperature sensors;

said processing unit coupled to said pump for controlling operation of said pump; and

said processing unit further including means for calculating one or both of:

a volume of liquid in the flexible container; and

a flow rate of fluid exiting the flexible container.

10. The fluid delivery system of claim 9, further comprising:

a first vent valve (108) fluidically coupled to said bladder for selectively venting gas within said bladder; and

a second vent valve (104) fluidically coupled to said calibration tank for selectively venting within said calibration tank.

11. The fluid delivery system of either one of claims 9 and 10, further comprising:

a first inlet valve (106) fluidically coupled to said bladder and said pump; and

a second inlet valve (102) fluidically coupled to said calibration tank and said pump.

12. The fluid delivery system of any one of claims 9-11, further comprising:

a first temperature sensor (302) coupled to said bladder for sensing the temperature of the gas in said bladder; and

a second temperature sensor (304) coupled to said calibration tank for sensing the temperature of a gas in said calibration tank.

13. The fluid delivery system of any one of claims 9-12, wherein the gas is air.

14. The fluid delivery system of any one of claims 9-13, wherein said pump is not a precise metering device.

Images