U.S. patent application number 12/705801 was filed with the patent office on 2010-06-10 for method to sense temperature in an implantable pump.
This patent application is currently assigned to Medtronic, Inc.. Invention is credited to Jerome T. Hartlaub, James M. Olsen.
Application Number | 20100145271 12/705801 |
Document ID | / |
Family ID | 23168075 |
Filed Date | 2010-06-10 |
United States Patent
Application |
20100145271 |
Kind Code |
A1 |
Hartlaub; Jerome T. ; et
al. |
June 10, 2010 |
METHOD TO SENSE TEMPERATURE IN AN IMPLANTABLE PUMP
Abstract
An implantable drug infusion pump for delivering drug therapy is
made more reliable and its performance improved by monitoring drug
pump temperature. Monitoring pump temperature can also provide for
temperature-related drug therapy modification. A pump temperature
sensor is read by the infusion pump's microprocessor. Pump
temperature data is stored in pump memory for later access by a
remote controller. A simple thermistor or semiconductor temperature
sensor can provide fast and reliable temperature monitoring of the
pump and/or of a patient by reading the temperature sensor's value
and calculating a temperature therefrom.
Inventors: |
Hartlaub; Jerome T.; (New
Brighton, MN) ; Olsen; James M.; (Plymouth,
MN) |
Correspondence
Address: |
MEDTRONIC, INC.
710 MEDTRONIC PARKWAY NE
MINNEAPOLIS
MN
55432-9924
US
|
Assignee: |
Medtronic, Inc.
Minneapolis
MN
|
Family ID: |
23168075 |
Appl. No.: |
12/705801 |
Filed: |
February 15, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11775490 |
Jul 10, 2007 |
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12705801 |
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09950154 |
Sep 10, 2001 |
7658737 |
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11775490 |
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09302517 |
Apr 30, 1999 |
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09950154 |
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Current U.S.
Class: |
604/111 ;
374/45 |
Current CPC
Class: |
A61M 2005/14264
20130101; A61M 2205/3372 20130101; A61M 2230/50 20130101; A61M
2205/52 20130101; F04B 13/00 20130101; A61M 5/14276 20130101; A61M
5/1723 20130101; A61M 2205/3368 20130101; F04B 2201/0403
20130101 |
Class at
Publication: |
604/111 ;
374/45 |
International
Class: |
A61M 5/50 20060101
A61M005/50; G01N 25/00 20060101 G01N025/00 |
Claims
1. A method of evaluating an implantable device, comprising: (A)
automatically storing in a memory of the implantable device
information corresponding to sensed temperatures associated with
the implantable device; and (B) determining whether to implant the
implantable device based on the information stored in the
memory.
2. The method of claim 1, wherein the storing in step (A)
comprises: (i) obtaining from a sensor a value corresponding to a
temperature that the implantable device is exposed to; and (ii)
storing the value in the memory.
3. The method of claim 2, wherein the storing in step (ii)
comprises: (1) determining whether the value is associated with a
temperature outside a normal temperature range; and (2) if the
value is associated with a temperature outside the normal
temperature range, storing the value and a time corresponding to
when the value was obtain in the memory.
4. The method of claim 2, wherein the sensor is positioned inside
the implantable device.
5. The method of claim 1, wherein the determining in step (B)
comprises: (i) retrieving the information from the memory; (ii)
determining an amount of time that the implantable device was
exposed to temperatures outside the normal range; and (iii) if the
amount of time indicates a reliability of the implantable device
was affected by the exposure to temperatures outside the normal
range, not implanting the implantable device.
6. The method of claim 1, wherein the information stored relates
the period of time that the sensed temperature was not within a
normal temperature range.
7. The method of claim 6, wherein the information relates to the
period of time that the sensed temperature was below the normal
temperature range.
8. The method of claim 1, wherein the determining in step (B)
comprises: (i) downloading the information from the memory; and
(ii) rendering the information on a display.
9. The method of claim 8, wherein the rendering only disclosed a
period of time that the implantable device was exposed to
temperatures outside the normal temperature range.
10. The method of claim 1, wherein the information stored in the
memory corresponds to an accumulated time that the implantable
device has been exposed to a range of temperatures.
11. A method of evaluating an implantable pump, comprising: (A)
storing in a memory a plurality of values corresponding to a
temperature associated with the implantable pump; (B) retrieving a
plurality of values stored in the memory; and (C) determining
whether to implant the implantable pump based on the retrieved
plurality of values.
12. The method of claim 11, wherein the storing in step (A)
comprises: (i) determining a first value associated with a sensor;
(ii) storing the first value in memory; and (iii) repeating steps
(i)-(ii) after a predetermined period of time.
13. The method of claim 12, wherein the first and second value are
selected from the list consisting of a voltage value and a
resistance value.
14. The method of claim 11, wherein the temperature is associated
with a temperature sensor positioned inside the implantable
pump.
15. The method of claim 11, wherein the determining in step (C) is
based on whether the implantable pump has been exposed to a
temperature outside a safe range of temperatures associated with a
medicament being stored within the implantable pump.
16. The method of claim 15, further comprising: (D) if the
medicament has been exposed to temperatures outside the safe range,
replacing the medicament in the implantable pump.
17. The method of claim 11, wherein the retrieved plurality of
values provides a history of temperatures the implantable pump has
been exposed to.
18. The method of claim 11, wherein the values being stored
correspond to temperatures outside a normal temperature range.
19. The method of claim 11, wherein the retrieving in step (B) is
done wirelessly.
20. A method of evaluating an implantable device, comprising: (A)
determining a period of time that the implantable device was
exposed to a temperature outside a normal range; and (B)
determining whether to implant the implantable device based on the
determined period of time.
21. The method of claim 20, wherein determining in step (A)
comprises: (i) periodically determining a temperature that is
associated with the implantable device; (ii) if the temperature is
outside the normal range, storing information in memory
corresponding to the temperature and a time the temperature was
sensed; and (iii) using the stored information to determine the
period of time that the implantable device was exposed to
temperatures outside the normal temperature range.
22. The method of claim 20, wherein the determining to implant in
step (B) is based on a component of the implantable device that is
susceptible to damage when exposed to temperature outside the
normal range.
23. The method of claim 20, wherein the determining in step (A) is
based on sensed temperature of at least a portion of the
implantable device.
Description
[0001] This is a continuation of U.S. patent application Ser. No.
11/775,490, filed Jul. 10, 2007, which is a continuation of U.S.
patent application Ser. No. 09/950,154, filed Sep. 10, 2001, both
of which are incorporated herein by reference in their entirety,
which is a continuation-in-part application of U.S. patent
application Ser. No. 09/302,517, filed Apr. 30, 1999, to which
priority is claimed.
FIELD OF THE INVENTION
[0002] This invention relates to implantable drug infusion pumps.
In particular, this invention relates to a method and apparatus for
continuously sensing and recording temperature of an implantable
infusion pump.
BACKGROUND OF THE INVENTION
[0003] Implanted infusion pumps deliver therapeutic drugs to a
patient according to a computer program executed by a processor
that is programmed with drug dosing parameters. Some infusion pumps
use a microprocessor to control a small, positive displacement pump
according to programming instructions delivered to the
microprocessor through an RF programming link so as to permit the
implantable pump to be remotely programmed and operated. Other
infusion pumps use compressed-gas propellants instead of a pump to
deliver a drug.
[0004] Most medical devices, including infusion pumps, are
specified to be stored in a particular not-to-be-exceeded
temperature range. Storage temperatures outside the manufacturer's
specified storage temperature range can damage implantable infusion
pumps and for this reason, precautions are normally taken to insure
that an implantable infusion pump is not inadvertently subjected to
adversely high or low temperatures. Monitoring a pump's temperature
over time would provide a mechanism by which damaging temperature
extremes could be identified prior to implantation.
[0005] In addition, a pump that includes a mechanism by which the
pump's temperature can be monitored might provide drug-delivery
performance improvements. The flow characteristics of mechanical
pumps are often temperature sensitive. Temperature compensation of
undesirable flow changes can be achieved using the electrical
temperature signal to adjust the flow via the internal
controller.
[0006] Furthermore, monitoring patient temperature by an infusion
pump, either remotely, for example at the distal end of a catheter
connected to the pump, or at the pump, might allow for drug therapy
delivery to be modified according to the patient's measured
temperature, improving the effectiveness of the therapy.
BRIEF SUMMARY OF THE INVENTION
[0007] An implantable drug infusion pump is made more reliable and
its performance is improved by inclusion of a temperature sensor in
the pump, which monitors the pump's temperature. Undesirable
temperature dependencies in an infusion pump's performance can be
reduced or eliminated by measuring the pump's actual temperature
using a separate temperature sensor and adjusting the pump's
operation accordingly by way of a computer program designed to
modify pump performance according to temperature variations. Drug
therapy administered by an infusion pump can be automatically or
manually adjusted according to the pump's actual temperature.
[0008] In the preferred embodiment, a thermistor, embedded within a
pump at an empirically determined optimum location to monitor the
overall temperature of the pump's constituent mechanisms, is
operatively coupled to the pump's control microprocessor. The
microprocessor's control program is written to read the
thermistor's resistance and from the temperature-dependent
resistance of the thermistor, calculate the pump's temperature.
[0009] In at least one alternate embodiment, a temperature sensor
external to the infusion pump can be used to measure a patient's
temperature. Such an embodiment would include using a temperature
sensing device, on the distal end of a catheter for example,
providing a faster temperature sensor and a temperature more
closely similar to the core temperature of a patient.
[0010] EEPROM or battery-powered RAM, on-board the microprocessor
or in an external device, can be used to store the date and time at
which a microprocessor controlling the pump and also monitoring a
temperature probe, read the pump's temperature. The microprocessor
can correlate an electrically measurable parameter, such as a
temperature-dependent resistance of a thermistor for example, to a
real temperature. The pump's temperature history since manufacture
and prior to implant into a patient can be stored in memory and
subsequently read from memory thereby providing a complete history
of the pump's temperature. Historical temperatures stored and read
prior to installation might help insure that the pump will not fail
due to having been frozen or fail because of exposure to abnormally
high temperatures since manufacture, causing a possible electrical
or mechanical failure.
[0011] Pump temperature data values stored in memory can be read
from the pump prior to installation using a
direct-connect-programming link or through a RF programming link,
which is commonly used to transfer data to and from implantable
infusion pumps and described elsewhere in the literature. See e.g.
U.S. Pat. No. 4,676,248, "Circuit for Controlling a Receiver in an
Implanted Device" by Berntson.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 discloses a simplified block diagram of an
implantable, software controlled infusion pump that also includes a
built-in temperature sensor.
[0013] FIG. 2 shows an alternate embodiment wherein a catheter
includes a temperature sensor coupled to a pump, which might
include an external controller.
[0014] FIG. 3 shows a flow chart illustrating the steps to
compensate drug delivery flow in an infusion pump.
[0015] FIG. 4A shows a graph of an uncompensated flow rate of an
implantable pump versus temperature.
[0016] FIG. 4B shows a graph of a temperature compensation
algorithm's programmed relationship between pump revolutions per
hour for an implantable pump and temperature.
[0017] FIG. 4C shows a graph of a temperature compensated flow rate
of an implantable pump.
[0018] FIG. 5A shows the temperature profile of an implantable pump
over time.
[0019] FIG. 5B shows the histogram output of the total time an
implantable pump was exposed to various temperature ranges.
[0020] FIG. 6 shows a flow chart depicting the steps needed to
produce a histogram output of the total time an implantable pump
was exposed to various temperature ranges.
[0021] FIG. 7A shows a graph of an implantable pump's pump cycles
verses a patient's body temperature.
[0022] FIG. 7B shows a graph of flow rate of an implantable pump
versus a patient's body temperature.
[0023] FIG. 8 shows a flow chart depicting the steps needed to
produce a flow from an implantable pump dependent on a patient's
body temperature.
DETAILED DESCRIPTION OF THE INVENTION
[0024] FIG. 1 shows a simplified block diagram of the functional
elements of an implantable and programmable drug infusion pump 100
having a programmable microprocessor 108 and a temperature sensor
150 for monitoring the pump temperature.
[0025] The functional elements of the infusion pump 100 are shown
in FIG. 1 are small, such that the pump can be readily implanted
into the abdomen of a patient for purposes of treating chronic
diseases, such as diabetes. An implanted infusion pump might also
be used for acute treatment regimens, e.g. to administer
chemotherapy drugs or morphine, for example. A reservoir 102
contains a volume of drug to be administered to the patient by a
pump 104, preferably a precision positive displacement pump
controlled by the microprocessor 108 and drawing drug material from
the reservoir 102.
[0026] The pump 104 shown in FIG. 1 is operatively coupled to and
responsive to electrical signals delivered to it from a radio
frequency (RF) interface unit 106. Electrical signals from the
interface unit 106 might, for example, start and stop the pump 104
and including its delivery rate so as to modulate the delivery of
drugs from the reservoir 102 to the patient. Control circuitry
within the microprocessor unit 108 would typically include
appropriate electronic drive circuits, the essential function of
which is to couple a central processor 108 to the pump 104 through
appropriate interface circuitry well know to those skilled in the
art. Alternate embodiments of the invention would of course include
implementing any required pump/CPU interface directly into the
microprocessor, or selecting and/or designing the pump 104 to
eliminate the need for an interface between it and the low power
circuits of the microprocessor. Many commercial grade
microprocessors include a plethora of ancillary circuitry on a
single substrate including analog-to-digital converters,
digital-to-analog converters, counters, timers, clocks and so
forth.
[0027] The central processor unit 108 controls the amount of drug
treatment administered to the patient according to program
instructions stored in a program memory 110. In the case of a
displacement pump mechanism, the microprocessor might control a
drive motor's speed as well as its "on" time.
[0028] A temperature sensor 150 is operatively coupled to at least
one input of the microprocessor 108. The preferred embodiment of
the invention contemplates that the temperature sensor is a
thermistor, the resistance of which varies with the temperature of
the thermistor. Many single-chip microcontrollers are fabricated to
include an analog-to-digital converter which might be employed to
measure the resistance of the thermistor by the microcontroller
thereby reducing parts count. In using an on-chip circuit to
measure the thermistor's temperature, the control program of the
microcontroller can correlate the thermistor's resistance to a
temperature, indirectly measuring temperature by the thermistor's
resistance. Alternate embodiments of the invention would include
using a temperature sensor that is a semiconductor for it is well
known that semiconductor performance characteristics are affected
by temperature. A semiconductor temperature sensor might be
fabricated directly on the same die as the microprocessor.
[0029] Still other embodiments of the invention would include a
pump that senses temperature through a remote temperature probe.
FIG. 2 shows an alternate embodiment of a temperature-sensing
infusion pump 200 wherein a temperature sensor 202 is affixed to
the distal end of a catheter 204 and electrically coupled to a pump
or its microprocessor 206 through appropriate-small gauge wire 208.
Such a device might be used to sense a patient's temperature,
separate and apart from the pump's temperature remotely from the
pump but still within the patient's body, or in addition to the
pump's temperature for purposes of varying drug dosage according to
the patient's temperature.
[0030] The pump 200 with the catheter 204 connected are implanted
in the patient's body 216 under the skin 214. For remote
programming purposes, RF energy 212 flows bidirectionally between
the pump 200 and the external controller 210 as is commonly done in
the art.
[0031] Monitoring the pump's temperature over time means that the
microprocessor's 108 control program might periodically scan or
read the resistance of the thermistor or other temperature sensing
device. Temperature data values read from the temperature sensor
might be stored in memory to be read out or analyze at a later
time. Alternatively, temperatures that are read and which are
outside an acceptable temperature range limit can be selectively
stored reducing the amount of data that might need subsequent
analysis. In other words, only temperatures that are too high or
too low might be stored in memory for later analysis.
[0032] Data read from the temperature sensor can be stored in EPROM
114. EEPROM 114 is particularly useful in the invention as it
readily lends itself as a repository for long-term data storage
regardless of whether or not power to the memory device has been
supplied continuously or interrupted. Many commercially available
microprocessors include addressable EEPROM directly on the
substrate comprising the CPU further simplifying the implementation
of a software-limited dosage implantable drug infusion device.
Alternate embodiments of the invention for storing temperature data
include internal RAM memory or would external EEPROM, such as the
memory device identified by reference numeral 115.
[0033] Historical data of the pump's temperature might be read from
the pump using the RF programming link 212. Appropriate instruction
to the microprocessor would cause the microprocessor to read and
transfer for uploading one or more of the data values stored in
EEPROM, RAM or other data storage device. A complete record of the
pump's temperature from its manufacture could be re-created
providing some assurance that the pump had not been subjected to a
damaging temperature extreme.
[0034] By use of the invention disclosed herein, the storage
temperature history of an implantable infusion pump over time might
help identify pumps that are likely to fail after installation.
Implantable pumps have a specified storage temperature range over
which the implantable pump can be stored safely and continue to be
suitable for patient implantation. If the pump is exposed to a
higher or lower temperature than the storage temperature limit
permits, it is possible for the pump to be damaged and not function
as designed. Thus, it is desirable that the healthcare provider be
aware of historical temperatures that an implantable pump has been
exposed to prior to implant. If the implantable pump has been
exposed to temperature damaging extremes, the healthcare provider
can decide not to implant the pump into the patient.
[0035] The temperature sensor 150 of FIG. 1 may be used to
continuously monitor the temperature of the implantable pump.
Alternatively, a temperature sensor placed in the packaging of the
pump prior to shipment could be used to monitor the temperature of
the implantable pump. FIG. 6 illustrates a flow chart describing a
preferred embodiment of the steps for producing a histogram output
of the total time an implantable pump is exposed to various
temperature ranges. The histogram is one visual form of output a
healthcare provider can quickly examine to determine if the
implantable pump was exposed to pump damaging temperature extremes.
Those skilled in the art will appreciate that other forms of output
may also be provided and still be considered within the scope of
the invention. For example, the output may be in the form of an
audio and/or visual signal. Such a signal may provide a ready
indicator to the patient's health care provider (such as a red
light or a green light) as to whether the pump is suitable for
implant. In this embodiment, the microprocessor within the pump
would make the determination based on the data histogram
output.
[0036] Referring still to FIG. 6, the temperature 600 of the
implantable pump as sensed by temperature sensor 602 could be read
by a temperature sampler 608 or microprocessor, in the form of an
electrical voltage or other electrically measurable quantity such
as the resistance of a thermistor. The temperature sampler 608 may
continuously monitor the implantable pumps temperature or may be
programmed to sample the temperature at predetermined intervals of
time. The predetermined sampling interval could be determined by
the pump manufacturer and vary depending upon the available memory
capacity of the pump. The temperature sampler signals 609 would be
input to a temperature discriminator 610 that would separate the
signals into three general temperature ranges: high temperature
612, normal temperature 614, and low temperature 616. The
temperature ranges could be determined based upon the specified
storage temperature range over which the implantable pump can be
stored safely and continue to be suitable for patient implantation.
Those skilled in the art will understand that numerous temperature
ranges could be determined. For example, the temperature ranges
could vary in value and number according to the type of drug being
used or stored in the pump.
[0037] The actual temperatures along with the time, date, and range
classifications are stored in histogram memory 618 for later
retrieval. The time and date could be recorded with a clock
operatively coupled to the temperature sampler 608 or
microprocessor. Additionally, the histogram memory 618 may save the
accumulated time the temperature has been in the three
predetermined ranges. FIG. 5A, illustrates the type of information
that is stored in histogram memory 618. This graph shows the
temperature profile of the implantable pump over time as monitored
by the temperature sensor 602. Prior to pump implant, the
healthcare provider receives the stored accumulated time in each of
the three temperature ranges, for example, via telemetry from the
pump. Optionally, the healthcare provider may display the data for
reading as shown in FIG. 5B. The histogram format shown in FIG. 5B
is one of several possible data display formats that a healthcare
provider can use to assist in interpreting the data.
[0038] In another embodiment of the invention, the pump reservoir
may be filled with a medicament that is temperature sensitive. For
example, the medicament may have a narrow storage temperature range
prior to implant of the pump. In this case, the healthcare provider
may program the upper and lower temperature limits to a narrower
range for the medicament monitoring. If the temperature exceeds the
acceptable or normal range, the damaged medicament could be
replaced as necessary.
[0039] As an additional advantage, manufacturing the pump to
monitor its temperature provides another quantum of data that might
be useful in the patient's treatment regimen. After installation
into a patient, the temperature of the pump 100 will quickly adjust
to match the temperature of the body into which it is implanted.
Therefore, the pump 100 can also function as a patient temperature
probe which tracks patient temperature. Infused medication dosage
might be modulated according to a patient's temperature such that
as the microprocessor noticed the pump's temperature steadily
rising the microprocessor might modulate dosages and/or initiate a
communication via the RF link to a health-care provider.
Alternatively, as discussed earlier in FIG. 2, a temperature sensor
could be placed at the distal tip of the infusion catheter with
sensor electrical wire(s) running the length of the catheter and
interfacing to the pump via an electrical connector. If the distal
tip of the catheter which includes the temperature sensor would be
near the body surface it may detect surface or patient-ambient
temperature which may not be as therapeutically useful. Therefore,
the distal tip of the catheter should be positioned so it could
detect the core temperature of a patient. The distal tip position
may also be determined by the need to provide a localized infusion
of a therapeutic medicament. Advantageously, the sensed temperature
of the patient may be used to adaptively administer a drug therapy
regimen based on patient temperature.
[0040] FIG. 8 depicts a flow chart illustrating the steps for
adaptively administering a drug regimen from an implantable pump
based on a patient's body temperature to maximize the therapeutic
effect of a drug therapy. As shown in FIG. 8, the temperature
sensor 802 senses a patient's body temperature 801. The temperature
signal 804 is read by the pump cycle controller 806 or
microprocessor, in the form of an electrical voltage or other
electrically measurable quantity such as the resistance of a
thermistor. The pump cycle controller 806 or microprocessor may
continuously monitor the patient's body temperature or may be
programmed to sample the patient's body temperature 801 at
predetermined intervals of time. After reading the temperature
signal 804, the pump cycle controller 806 may apply an algorithm
that contains a predetermined proportional relationship between
pump cycles and a patient's body temperature as depicted in FIG.
7A. Alternatively, the pump cycle controller 806 may apply an
algorithm that contains a predetermined proportional relationship
between pump flow rate and a patient's body temperature as depicted
in FIG. 7B. Based on the predetermined relationship, a pump cycle
signal 808 is generated and delivered to the pump mechanism 810 to
direct the pump to deliver the proper amount of drug flow 812. The
actual patient's body temperature may be stored in one or more
storage devices for later retrieval by a healthcare provider using
a data link coupled the pump cycle controller 806. Additionally, a
clock operatively coupled to the pump cycle controller 806 may be
used to generate a signal representative of the time and date of
the patient's body temperature.
[0041] For example, an implanted pump may continuously or at
predetermined intervals sense and store the core or body
temperature of a patient. When the sensed temperature increases to
a preset value, a low-grade fever is detected which may be
therapeutically undesirable. The pump may gradually or abruptly
increase the infusion rate, perhaps even providing a bolus
infusion, to counteract the low-grade fever. The infused medicament
could be a fever reducing medicament or perhaps an antibacterial
medicament. The low-grade fever may be a consequence of localized
infection or a systemic reason. When the low-grade fever has been
reduced or eliminated, the infusion rate may return to a basal rate
or cease. The time dependent temperature record could be sent to
the healthcare provider by telemetry on demand or automatically.
This record would be useful to monitor the effectiveness of the
therapy or to help the healthcare provider decide to adjust the
infusion rate dependence on a patient's body temperature.
[0042] In another embodiment of the invention, an infused drug
therapy regimen is adaptively administered according to a
temperature compensation algorithm that adjusts uncompensated flows
for temperature so that a constant flow rate can be achieved. As
illustrated in FIG. 3, a temperature sensor 302 would be read by
the microprocessor 306 in the form of an electrical voltage or
other electrically measurable quantity such as the resistance of a
thermistor. The temperature signal 304 would be read into the
controller 306 where an algorithm would be used to determine
whether the uncompensated flow should be temperature compensated.
If the controller 306 determines that a correction to the
uncompensated flow is needed, then a temperate compensated pump
drive signal 308 is delivered to the pump 310 to produce from the
pump a constant fluid flow rate 312. The temperature compensation
algorithm as illustrated in FIG. 4B shows a predetermined
relationship between pump cycles and temperature. As the
uncompensated flow rate changes with temperature, FIG. 4A, the
temperature compensation algorithm adjusts the flow rate to provide
a constant flow rate as illustrated in FIG. 4C.
[0043] An example of a temperature dependent flow rate can be found
in the propellant flow from a propellant pump. Monitoring a pump
propellant's temperature using a temperature probe allows the
controller to compensate drug delivery for the propellant's
pressure-temperature dependence and hence the propellant's
temperature. As gaseous propellant changes temperature, its
effectiveness in delivering drug therapy will also change.
Accordingly, by monitoring the propellant's temperature, the
microprocessor or other control circuitry can adjust the drug
delivery appropriately to provide for a constant fluid flow
delivery of the therapeutic.
[0044] While the invention has been described with respect to
specific examples including presently preferred mode of carrying
out the invention, those skilled in the art will appreciate that
there are numerous variation and permutations of the above
described systems and techniques that fall within the spirit an
scope of the invention as set forth in the appended claims and
their equivalents.
* * * * *