U.S. patent application number 12/280869 was filed with the patent office on 2009-05-21 for volume measurement using gas laws.
This patent application is currently assigned to FLUIDNET CORPORATION. Invention is credited to Jeffrey A. Carlisle, John M. Kirkman Jr., Lawrence M. Kuba.
Application Number | 20090131863 12/280869 |
Document ID | / |
Family ID | 38509960 |
Filed Date | 2009-05-21 |
United States Patent
Application |
20090131863 |
Kind Code |
A1 |
Carlisle; Jeffrey A. ; et
al. |
May 21, 2009 |
Volume Measurement Using Gas Laws
Abstract
A system and method for measuring fluid volume and changes in
fluid volume over time, using a simple, low cost architecture is
described.
Inventors: |
Carlisle; Jeffrey A.;
(Stratham, NH) ; Kuba; Lawrence M.; (Nashua,
NH) ; Kirkman Jr.; John M.; (Trumansburg,
NY) |
Correspondence
Address: |
SCOTT C. RAND, ESQ.;MCLANE, GRAF, RAULERSON & MIDDLETON, PA
900 ELM STREET, P.O. BOX 326
MANCHESTER
NH
03105-0326
US
|
Assignee: |
FLUIDNET CORPORATION
Portsmouth
NH
|
Family ID: |
38509960 |
Appl. No.: |
12/280869 |
Filed: |
January 23, 2007 |
PCT Filed: |
January 23, 2007 |
PCT NO: |
PCT/US07/02039 |
371 Date: |
August 27, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60777193 |
Feb 27, 2006 |
|
|
|
Current U.S.
Class: |
604/67 ;
73/861.49 |
Current CPC
Class: |
G01F 22/02 20130101;
A61M 5/16886 20130101; A61M 5/16804 20130101; A61M 5/1483 20130101;
A61M 2205/3306 20130101 |
Class at
Publication: |
604/67 ;
73/861.49 |
International
Class: |
A61M 5/168 20060101
A61M005/168; G01F 1/34 20060101 G01F001/34 |
Claims
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.
Description
BACKGROUND
[0001] The present disclosure relates to fluid flow control devices
and more particularly to feedback control infusion pumps.
[0002] 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.
[0003] 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.
[0004] 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.
[0005] 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.
[0006] 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
[0007] In one aspect, a method for determining the volume of fluid
remaining in an infusion is provided.
[0008] In another aspect, a method for determining fluid flow rate
over an extended period of time is provided.
[0009] In another aspect, a method for determining fluid flow rate
over a relatively short period of time is provided.
[0010] 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.
[0011] 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.
[0012] Another advantage is found in that volume measurements made
over time can be used to accurately compute fluid flow rate.
[0013] Another advantage is found in that volume measurements may
be made using an inexpensive and simple pumping mechanism.
[0014] Another advantage is found in that volume measurements may
be made without significant loss of energy.
[0015] Another advantage is found in that volume measurements may
be made over a wide range of volumes.
[0016] Another advantage of the present disclosure is that its
simplicity, along with feedback control, makes for a reliable
architecture.
[0017] 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
[0018] 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.
[0019] FIGS. 1 and 2 are perspective and side views of an infusion
pump in accordance with an exemplary embodiment.
[0020] FIG. 3 is a functional block diagram showing the fluidic
connections of a volume measurement system according to an
exemplary embodiment.
[0021] FIG. 4 is a functional block diagram showing the control
elements of a volume measurement system according to an exemplary
embodiment.
[0022] FIG. 5 is a functional block diagram showing the sensing
elements of the system.
[0023] FIG. 6 is a flow chart diagram outlining an exemplary method
of volume measurement.
[0024] FIG. 7 is a flow chart outlining an exemplary method of
calculating flow rate based on pressure decay.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0025] 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.
[0026] 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.
[0027] 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.
[0028] 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.
[0029] 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.
[0030] 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.RTM. family of microprocessors available from Analog
Devices, Inc., of Norwood, Mass.
[0031] 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.
[0032] 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
[0033] 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.
[0034] The pressure frame 10 defines a rigid container of known
volume, V.sub.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.sub.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.sub.tbi, is equal to
V.sub.frame, less the fixed and known volume of the bladder 20
itself, V.sub.blad, less any incompressible materials of the bag
30, V.sub.bag, and less the volume of gas in bladder 20, V.sub.gas.
Once the value V.sub.gas is computed, then it is trivial to compute
V.sub.tbi.
V.sub.tbi=V.sub.frame-V.sub.blad-V.sub.bag-V.sub.gas
[0035] With the following method, at any given point in time, the
volume of air contained in the bladder, V.sub.gas, can be measured
and V.sub.tbi can be subsequently computed.
[0036] 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.
[0037] FIG. 6 shows the steps leading to computation of V.sub.tbi.
Shown as step 802, the first step is to find an optimum amount of
air mass, N.sub.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.
[0038] The initial pressure in the bladder 20, P.sub.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.sub.test, nominally, for example, about
250 milliseconds. A new measurement of the pressure in the bladder
20 is made, P.sub.bladder2. Based on the percent of pressure change
from this pumping action, a new pump activation time, S.sub.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.sub.deltatarget, in bladder 20, for example,
on the order of about 10%.
S pump = S test * P deltatarget ( P bladder 2 - P bladder 1 ) / P
bladder 1 ##EQU00001##
[0039] 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.sub.bladder, in bladder 20. The combination of valve and pump
settings required for such adjustments are shown in the table
below:
TABLE-US-00001 Bladder Bladder Tank Pump Valve Vent Valve Tank Vent
10 106 Valve 108 102 Valve 104 Increase P.sub.bladder ON OPEN
CLOSED CLOSED CLOSED Decrease P.sub.bladder OFF CLOSED OPEN CLOSED
CLOSED Increase P.sub.tank ON CLOSED CLOSED OPEN CLOSED Decrease
P.sub.tank OFF CLOSED CLOSED CLOSED OPEN
[0040] Adjustments made in step 804 can be made iteratively until
P.sub.tank is roughly equal to P.sub.bladder, for example, within
about 5% of the relative pressure measured in P.sub.bladder. This
does not need to be a precise process. Following the adjustment,
the pressure in tank 60, P.sub.tank2, is recorded.
[0041] 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.sub.pump After a delay of
approximately five seconds, the pressure in the tank 60 is
measured, P.sub.tank3. This delay is to reduce the effect of an
adiabatic response from the increase in pressure in the tank
60.
[0042] 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.sub.pump. After a delay of
approximately five seconds, the pressure in the bladder 20 is
measured, P.sub.bladder3. This delay is to reduce the effect of an
adiabatic response from the increase in pressure in the bladder
20.
[0043] 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.
[0044] 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.
[0045] In step 810, the volume of gas in the bladder 20, V.sub.gas,
can be calculated with a reduced form of PV=nRT:
V gas = V tank * ( P tank 3 - P tank 2 ) ( P bladder 3 - P bladder
2 ) ##EQU00002##
[0046] As examples of this calculation, if the pressure change were
the same in the bladder 20 and the tank 60, then V.sub.gas would be
equal to V.sub.tank. If the pressure change in the bladder 20 were
20% as large as that in the tank 60, then V.sub.gas would be 5
times greater than V.sub.tank.
[0047] Step 812 derives the value for V.sub.tbi from V.sub.gas,
using known values for V.sub.frame Vblad, and V.sub.bag and using
the calculated value of V.sub.gas, from step 810.
V.sub.tbi=V.sub.frame-V.sub.blad-V.sub.bag-V.sub.gas
[0048] 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
[0049] 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.
[0050] 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.sub.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.
[0051] The measurement of pressure decay is a simple procedure of
observing the time the absolute pressure of P.sub.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.
[0052] 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.sub.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, .DELTA.P, between the
time T1 and the time T2 is calculated in step 924 as
P.sub.bladder1-P.sub.bladder2 and the change in time, .DELTA.T is
calculated as T2-T1 at step 928. At step 932, it is determined
whether .DELTA.P is greater than some predetermined or prespecified
threshold value, e.g., about 2% with respect to P.sub.bladder1 If
.DELTA.P has not reached the threshold value at step 932, the
process returns to step 916 and continues as described above. If
.DELTA.P has reached the threshold value at step 932, the rate of
pressure decay is calculated as .DELTA.P/.DELTA.T at step 936. The
flow rate is then calculated as
.DELTA.P/.DELTA.T.times.V.sub.gas-P.sub.bladder1 at step 940.
[0053] 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.
* * * * *