U.S. patent application number 13/096590 was filed with the patent office on 2011-08-25 for pressure sensing in implantable medical devices.
This patent application is currently assigned to Medtronic, Inc.. Invention is credited to Keith A. Miesel.
Application Number | 20110208163 13/096590 |
Document ID | / |
Family ID | 34396408 |
Filed Date | 2011-08-25 |
United States Patent
Application |
20110208163 |
Kind Code |
A1 |
Miesel; Keith A. |
August 25, 2011 |
Pressure Sensing in Implantable Medical Devices
Abstract
An implantable medical device for delivering a therapeutic
substance to a delivery site in a patient. A reservoir holds a
supply of the fluid therapeutic substance. A catheter has a
proximal end, a delivery region and a lumen extending from the
proximal end to the delivery region. The proximal end of the
catheter is operatively coupled to the reservoir. The delivery
region of the catheter is adapted to be placed proximate the
delivery site in the patient. The therapeutic substance is adapted
to be delivered through the lumen to the patient. A sensing device
is operatively coupled with the lumen of the catheter being capable
of detecting a pressure of the therapeutic substance in the lumen.
A controller is operatively coupled to the sensing device, the
controller being capable of taking an action in response to the
pressure in the lumen.
Inventors: |
Miesel; Keith A.; (St. Paul,
MN) |
Assignee: |
Medtronic, Inc.
Minneapolis
MN
|
Family ID: |
34396408 |
Appl. No.: |
13/096590 |
Filed: |
April 28, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11778400 |
Jul 16, 2007 |
7955319 |
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13096590 |
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10836115 |
Apr 30, 2004 |
7320676 |
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11778400 |
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60508020 |
Oct 2, 2003 |
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Current U.S.
Class: |
604/523 |
Current CPC
Class: |
A61M 2205/3355 20130101;
A61M 5/16854 20130101; A61M 2205/3523 20130101; A61M 5/14276
20130101 |
Class at
Publication: |
604/523 |
International
Class: |
A61M 25/00 20060101
A61M025/00 |
Claims
1. A sensor for a medical device for detecting pressure in a lumen
of a catheter, comprising: a base having a first side and a second
side; a first pressure sensing diaphragm operatively coupled to
said first side of said base; a second pressure sensing diaphragm
operatively coupled to said second side of said base; a connector
having a first end operatively coupled to a movable portion of said
first pressure sensing diaphragm and a second end operatively
coupled to a movable portion said second pressure sensing
diaphragm, said connector moving with said first pressure sensing
diaphragm and said second pressure sensing diaphragm in response to
a change in pressure; and an electrical element responsive to
movement of said first pressure sensing diaphragm and said second
pressure sensing diaphragm.
2. A sensor as in claim 1 wherein said electrical element
comprises: a first electrical sensor producing a first output in
response to movement of said first pressure sensing diaphragm; and
a second electrical sensor producing a second output in response to
movement of said second pressure sensing diaphragm; wherein said
first output and said second output are combined to produce a
pressure output.
3. A sensor as in claim 2 wherein said first output and said second
output are additive resulting in a doubled electrical output.
4. A sensor as in claim 3 wherein said pressure output comprises a
ratio of said first output and said second output.
5. A sensor as in claim 3 wherein said pressure output comprises a
duty cycle of one of said first output and said second output
divided by a sum of both of said first output and said second
output.
Description
RELATED APPLICATION
[0001] This application is a continuation of U.S. patent
application Ser. No. 11/778,400, filed Jul. 16, 2007, now allowed,
which is a continuation of U.S. patent application Ser. No.
10/836,115, filed Apr. 30, 2004, now U.S. Pat. No. 7,320,676, which
claims the benefit of U.S. Provisional Application No. 60/508,020,
filed Oct. 2, 2003. The entire contents of each of these
applications is incorporated herein by reference.
FIELD OF THE INVENTION
[0002] This application is a continuation of The present invention
relates generally to pressure sensing in implantable medical
devices and, more particularly, to pressure sensing in implantable
medical devices delivering a therapeutic substance to a
patient.
BACKGROUND OF THE INVENTION
[0003] Implantable drug delivery or infusion devices and/or systems
are commonly used, for example when chronic administration of a
pharmaceutically active agent or therapeutic substance to a patient
is required. An implantable infusion pump-catheter delivery system
may be preferred when it is important to deliver the agent to a
specific site or when the agent must be administered to spaced
sites in tightly controlled, yet minute dosages.
[0004] Typically, an implantable therapeutic substance delivery
device has a reservoir for holding a supply of therapeutic
substance awaiting delivery to a delivery site in the patient. A
pump may be fluidly coupled to the reservoir for creating fluidic
pressure to facilitate delivery of the therapeutic substance to the
patient. A catheter provides a pathway for delivering the
therapeutic substance to the delivery site in the patient.
[0005] All parts of the therapeutic substance delivery
device/system need to operate adequately to ensure proper
functioning of the device/system. While perhaps the least complex,
catheters can have and can develop operational problems.
[0006] Sometimes catheters in such delivery systems can become
obstructed or clogged. A partial or complete blockage could prevent
the therapeutic substance from reaching the delivery site in the
patient or, in the case of a partial obstruction, could prevent an
adequate supply of the therapeutic substance from reaching the
delivery site in the patient.
[0007] Catheters can also leak due to cuts, tears, etc. A leak,
small or large, can also prevent the therapeutic substance from
reaching the delivery site in the patient. A leak can result in a
double problem. In addition to the lack of therapeutic substance
supplied to the delivery site of the patient, the therapeutic
substance could be dispersed elsewhere in the body of the patient
which may create further issues.
[0008] When catheters become clogged or leak and the infusion pump
continues to deliver drug, a patient's well being may be placed in
danger.
[0009] However, it has been difficult to detect the malfunction of
a catheter. For example, if the catheter has a leakage, the
implantable drug delivery device could continue to delivery
therapeutic substance and there may be no way to know that the
therapeutic substance was not reaching the desired delivery site.
The patient may not receive the benefit of the therapeutic
substance but might not know why. As another example, if the
catheter has an obstruction, the implantable drug delivery device
might cease to deliver the therapeutic substance. But it may be
difficult to know why the failure occurred. The failure to deliver
might have been caused by other factors, such as power failure,
pump failure or an empty reservoir.
[0010] If a catheter malfunctions, it is desirable to know so that
appropriate corrective action can be taken.
BRIEF SUMMARY OF THE INVENTION
[0011] The present invention can detect a malfunction in a catheter
and take appropriate action if and when the malfunction occurs. By
the sensing of pressure in the lumen of the catheter, cuts and
leaks might result in lower than normal pressure, the lack of
appreciable pressure or the lack of a pressure increase. An
obstruction might result in higher than normal pressure or a slower
than normal pressure decay.
[0012] In one embodiment, the present invention provides an
implantable medical device for delivering a therapeutic substance
to a delivery site in a patient. A reservoir holds a supply of the
fluid therapeutic substance. A catheter has a proximal end, a
delivery region and a lumen extending from the proximal end to the
delivery region. The proximal end of the catheter is operatively
coupled to the reservoir. The delivery region of the catheter is
adapted to be placed proximate the delivery site in the patient.
The therapeutic substance is adapted to be delivered through the
lumen to the patient. A sensing device is operatively coupled with
the lumen of the catheter being capable of detecting a pressure of
the therapeutic substance in the lumen. A controller is operatively
coupled to the sensing device, the controller being capable of
taking an action in response to the pressure in the lumen.
[0013] In another embodiment, the present invention provides a
method of delivering a therapeutic substance to a delivery site in
a patient. The therapeutic substance is pumped under pressure from
a reservoir through a catheter fluidly coupled to the reservoir.
The catheter has a proximal end, a delivery region and a lumen
extending between the proximal end and the delivery region. The
delivery region of the catheter is placed in proximity to the
delivery site in the patient. A pressure of the therapeutic
substance is detected in the lumen. An action is taken in response
to the pressure in the lumen.
[0014] In a preferred embodiment, the therapeutic substance is a
fluid.
[0015] In a preferred embodiment, the therapeutic substance is a
liquid.
[0016] In another embodiment, the present invention provides a drug
delivery system for delivering a liquid therapeutic substance to a
delivery site in a patient. An implantable medical device has a
reservoir holding a supply of the fluid therapeutic substance and a
pump fluidly coupled to the reservoir, the pump being capable of
fluidly driving the therapeutic substance to the delivery site
under pressure. A catheter has a proximal end, a delivery region
and a lumen extending from the proximate end to the delivery
region. The proximal end of the catheter is operatively coupled to
the pump. The delivery region of the catheter is adapted to be
placed proximate the delivery site in the patient. The therapeutic
substance is adapted to be delivered through the lumen to the
patient. A sensing device is operatively coupled with the lumen of
the catheter being capable of detecting a pressure of the
therapeutic substance in the lumen. A controller is operatively
coupled to the sensing device, the controller being capable of
taking an action in response to the pressure in the lumen.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 illustrates an implantable medical device in
accordance with an embodiment of the present invention;
[0018] FIG. 2 is a block diagram the implantable medical device of
FIG. 1;
[0019] FIG. 3 is a block diagram of a catheter containing a
restriction in accordance with a preferred embodiment of the
present invention;
[0020] FIG. 4 is a drawing illustrating an exterior view of a drug
delivery system of an embodiment of the invention;
[0021] FIG. 5 is a drawing illustrating a pressure sensor of an
embodiment of the invention;
[0022] FIG. 6 is a drawing illustrating a cross section of the
pressure sensor of FIG. 5;
[0023] FIG. 7A is a drawing illustrating a cross section of an
alternative pressure sensor of an embodiment of the invention;
[0024] FIG. 7B is a schematic illustrating a circuit diagram of an
LVDT pickoff;
[0025] FIG. 8 is a graph of relative catheter pressure versus time
illustrating alert pressures and pressure limits according to an
embodiment of the invention;
[0026] FIG. 9 is a flow chart illustrating how pressure may be used
to control a pump's delivery of a therapeutic substance;
[0027] FIG. 10A is a prior art drug delivery system without a
pressure sensor;
[0028] FIG. 10B is a drug delivery system with a pressure sensor in
accordance with an embodiment of the invention; and
[0029] FIG. 11 is drawing illustrating coupling of sensors and/or
sensors to electronics according to an embodiment of the
invention;
[0030] FIG. 12 illustrates a preferred embodiment of a capacitive
pressure sensor; and
[0031] FIG. 13 illustrates a preferred embodiment of an inductive
pressure sensor.
DETAILED DESCRIPTION OF THE INVENTION
[0032] FIG. 1 shows implantable medical device 16, for example, a
drug pump, implanted in patient 18. The implantable medical device
16 is typically implanted by a surgeon in a sterile surgical
procedure performed under local, regional, or general anesthesia.
Before implanting the medical device 16, a catheter 22 is typically
implanted with the distal end position at a desired therapeutic
delivery site 23 and the proximal end tunneled under the skin to
the location where the medical device 16 is to be implanted.
Catheter 22 may disgorge therapeutic substance at other than at its
distal end. For example, catheter 22 may intentionally have a
delivery region that is not proximate its distal, e.g., a hole or
valve positioned somewhere before reaching the distal end of the
catheter 22. Thus, catheter 22 may be placed in patient 18 with a
delivery region of catheter 22 placed in or near to, generally
proximate to, delivery site 23.
[0033] Implantable medical device 16 is generally implanted
subcutaneously at depths, depending upon application and device 16,
of from 1 centimeter (0.4 inches) to 2.5 centimeters (1 inch) where
there is sufficient tissue to support the implanted system. Once
medical device 16 is implanted into the patient 18, the incision
can be sutured closed and medical device 16 can begin
operation.
[0034] Implantable medical device 16 operates to infuse a
therapeutic substance into patient 18 through catheter 22 to
delivery site 23. Implantable medical device 16 can be used for a
wide variety of therapies such as pain, spasticity, cancer, and
many other medical conditions.
[0035] The therapeutic substance contained in implantable medical
device 16 is a substance intended to have a therapeutic effect such
as pharmaceutical compositions, genetic materials, biologics, and
other substances. Pharmaceutical compositions are chemical
formulations intended to have a therapeutic effect such as
intrathecal antispasmodics, pain medications, chemotherapeutic
agents, and the like. Pharmaceutical compositions are often
configured to function in an implanted environment with
characteristics such as stability at body temperature to retain
therapeutic qualities, concentration to reduce the frequency of
replenishment, and the like. Genetic materials are substances
intended to have a direct or indirect genetic therapeutic effect
such as genetic vectors, genetic regulator elements, genetic
structural elements, DNA, and the like. Biologics are substances
that are living matter or derived from living matter intended to
have a therapeutic effect such as stem cells, platelets, hormones,
biologically produced chemicals, and the like. Other substances may
or may not be intended to have a therapeutic effect and are not
easily classified such as saline solution, fluoroscopy agents,
disease diagnostic agents and the like. Unless otherwise noted in
the following paragraphs, a drug is synonymous with any
therapeutic, diagnostic, or other substance that is delivered by
the implantable infusion device.
[0036] If catheter 22 malfunctions, i.e., has or develops a leak or
an obstruction, that malfunction may be detected by analyzing the
pressure of the therapeutic substance, typically a fluid and more
typically a liquid, in a lumen of catheter 22. FIG. 2 illustrates,
in block diagram form, an implantable medical device 16.
Implantable medical device is also shown in FIG. 10A and FIG. 10B.
Therapeutic substance is stored in reservoir 24 in housing 26. Pump
28 is fluidly coupled to reservoir 24 gaining access to therapeutic
substance. The output of pump 28 is coupled to catheter 22 through
a check valve 30. Pump 28 and check valve 30 are controllable by
electronics module 32. Pressure sensor 34 is operatively coupled to
detect/sense pressure in a lumen of catheter 22. If the pressure
sensed by pressure sensor 34 is not appropriate, then electronics
module 32 may take appropriate action such as by sounding alarm 36.
Refill port (see FIG. 4) may be used to refill reservoir 24 without
explanting implantable medical device 16.
[0037] To detect pressure within catheter 22, pressure sensor 34
may be placed in fluid contact with a lumen of catheter 22.
Pressure sensor 34 may be placed in fluid contact with a lumen of a
catheter 22 anywhere along the lumen of the catheter 22. In an
embodiment, where catheter 22 is coupled to implantable pump 28,
pressure sensor 34 may be contained within housing 26. Pressure
sensor 34 could also be located external to housing 26. Pressure
sensor 34 may be coupled to electronics module 32. For ease of
coupling pressure sensor 34 to electronics module 32, it may be
preferred to locate pressure sensor 34 within housing 26.
Electronics module 32 may also be located in housing 26.
Electronics module 32 may control pump 28 and may be coupled to
pressure sensor 34. Electronics module 32 may stop pump 28 from
continuing to deliver therapeutic substance when a predetermined
pressure is detected in catheter 22, which pressure is indicative
of a leak in or obstruction of catheter 22.
[0038] In certain circumstances it may be desirable to obtain a
relative pressure within lumen of catheter 22. That is, it may be
preferable to compare a pressure within catheter 22 to a pressure
not in catheter 22 to avoid false indications of a leaky or
obstructed catheter 22. For example, a pressure reading falsely
indicating a leaky catheter may be obtained when catheter 22 is
subjected to decreasing atmospheric pressure (e.g., when a subject
having an implanted catheter with a pressure sensor goes up an
elevator or flies in an airplane). Similarly, a pressure reading
falsely indicating that an obstruction exists within catheter 22
may result when catheter 22 is subject to increasing atmospheric
pressure (e.g., when a subject having an implanted catheter with a
pressure sensor goes scuba diving). To avoid false indications of
an obstructed or leaky catheter, it may be desirable to compare a
pressure within catheter 22 to a pressure not within catheter 22,
preferably within the vicinity of catheter 22. Thus, the pressure
not within catheter 22 may be used as a reference pressure.
[0039] A reference pressure may be detected within a patient's 18
body in which catheter 22 is implanted or may be detected outside
of patient's 18 body. When detected within a patient's body, a
reference pressure may be detected in a location near catheter 22
or a location in a separate area of the patient's 18 body. A
reference pressure may be obtained in any location capable of
providing a pressure indicative of the external environment of
implanted catheter 22.
[0040] For example, a reference pressure may be taken in or around
(in the proximity of) implantable medical device 16, i.e., in a
region of a subject's body cavity where a pump system is implanted.
This location may be preferred because the reference pressure may
be taken without transporting (either before or after measurement)
the pressure from a distant location back to implantable medical
device 16. For example, FIG. 4 illustrates an implantable pump
system having a housing 26 with a vent 38 through which a reference
pressure may be obtained.
[0041] It is preferred to take a reference in or around (in the
proximity of or in the proximate area) delivery site 23. This is
the best reference location because therapeutic substance is to be
dispensed at this location. Any elevation difference between the
delivery site 23 and the reference location would be eliminated by
having the reference location at the delivery site 23. Of course, a
reference pressure could be taken outside of the patient 18. This
may be preferred, for example, when implantable medical device 16
reports the pressure taken in catheter to an external device for
adjustment to relative and, perhaps, subsequent appropriate action.
This location would eliminate the need for an implanted pressure
sensor for a relative pressure measurement and would still account
for changes in atmospheric pressure.
[0042] Alternatively, a drug delivery system (implantable medical
device 16) contains catheter 22 having a lumen for delivering a
pharmacological agent (therapeutic substance) and a second lumen
through which no pharmacological agent (therapeutic substance) is
delivered. A reference pressure may then be detected in the second
lumen. The second lumen in catheter 22 can easily be used to obtain
a reference pressure from a distal end of catheter 22, from a
delivery region of catheter 22 and/or from delivery site 23.
[0043] Any means capable of comparing an intracatheter pressure to
a reference pressure may be used. Overviews of how such a
comparison may be made are shown in FIG. 11. For example, a first
sensor 42 for detecting an intracatheter pressure may be coupled to
a second sensor 44 for detecting a reference pressure. The coupled
first and second sensors (42 and 44) may then be coupled to
electronics 46 that may interpret a signal regarding the compared
pressures from the coupled first and second sensors (42 and 44).
Electronics 46 may compare the pressures. The first and second
sensors (42 and 44) may communicate with the electronics 46 through
electrical means or through other means, e.g., telemetry. As shown
in FIG. 1, the electronics 46 may be part of electronics module 32
and contained within housing 26. When the reference pressure is
detected in an area of the body away from the catheter 22 or
external to the body, telemetry is the preferred means of
communicating the reference pressure to the electronics 46.
[0044] If catheter 22 has a leak, it is difficult to detect because
the back pressure against a normally flowing catheter is not very
high. Therefore, detecting an even lower pressure indicative of a
leak is extremely difficult. It may be preferred to introduce a
partial restriction into catheter 22 in order to create a higher
back pressure than would otherwise be encountered. The partial
restriction would, of course, significantly limit the delivery of
therapeutic substance to delivery site 23. Pressure sensor 34 is
placed between pump 28 and restrictor 48 in order to be able to
detect the increased back pressure. If catheter 22 then has or
develops a leak before the location of restrictor 48, a significant
pressure drop or lack of pressure rise in a transient condition can
be detected and a leak can more easily be detected.
[0045] Preferably, catheter 22 of implantable medical device 16
contains restriction 48 (see FIG. 3) to create back pressure within
a lumen of catheter 22. Flow restrictor 48 may be placed in
catheter 22 to impede the flow of fluid therapeutic substance
through catheter 22. Pressure sensor 34 is in fluid communication
with a lumen of catheter 22 but upstream of flow restrictor 48,
i.e., between pump 28 and restrictor 48, may sense backpressure
within catheter 22 resulting from restrictor 48. Creating
backpressure in catheter 22 may be desirable when, for example, a
leak in catheter 22 is to be detected. When back pressure is
created due to restrictor 48, a leak in catheter 22 will result in
a more substantial drop in intracatheter pressure than when no
restrictor is present. Thus, a leak may be more easily and
accurately detected when backpressure is created in the
catheter.
[0046] Restrictor 48 may be any restrictor capable of creating
backpressure within catheter 22 while allowing sufficient amounts
of therapeutic substance (pharmacological agent) to be delivered
from catheter 22. Examples of suitable flow restrictors include a
valve, a tortuous path, and a permeable membrane. A preferred
restrictor 48 is shown and described in U.S. Pat. No. 7,217,251,
entitled "Pressure Relief Methods in a Medical Catheter System,"
which is hereby incorporated by reference.
[0047] Any means for detecting pressure within a catheter or a
reference pressure may be used. One suitable means for detecting
pressure is a diaphragm.
[0048] FIG. 5 shows an overview of an exterior view of a pressure
sensor 34, where a first sensor 42 and a second sensor 44 are
housed within a sensor casing 50. The first sensor 42, which is
adapted to detect a pressure within a catheter 22, is attached to
the sensor casing 50 in a manner such that no or minimal fluid from
the lumen of the catheter 22 may penetrate to the interior of the
casing 50. The second sensor 44, which is adapted to detect a
reference pressure, is also attached to the sensor casing 50 in a
manner such that no or minimal fluid from may penetrate the
interior of the casing 50 from the second sensor 44. The sensor
casing 50 may contain an opening 52 such that wires or other
objects capable of carrying a signal to electronics 46 may exit and
enter the casing 50. As shown in FIG. 4, housing 26 of implantable
medical device 16 may contain a vent 38 through which the second
sensor 44 may detect a pressure in a body cavity of patient 18 in
which implantable medical device 16 is implanted.
[0049] FIG. 6 illustrates a cross section of a pressure sensor 34
of FIG. 5. First sensor 42 may be coupled through physical coupler
54 to a second sensor 44. The first and second sensors (42 and 44)
may be diaphragms.
[0050] The coupler 54 and a portion of the diaphragms may move in
relation to a change in pressure (reference or intracatheter). The
relative movement of the coupler 54 or a diaphragm may be used to
transmit information regarding a relative intracatheter pressure.
The coupler 54 may be placed in contact with the first sensor 42 or
second sensor 44 at any location capable of transmitting a pressure
signal. When the first and second sensors (42 and 44) are
diaphragms, it is preferred that the coupler 54 contact the
diaphragms at or near the center of the diaphragms. It is also
preferred that the area of the contact between the coupler 54 and
the diaphragms is small to avoid stiffening of the diaphragm and to
avoid potential gravitational influences.
[0051] FIG. 7A shows a sensor system 34 where a pick off means 56
is used to relay information to electronics 46 regarding a relative
intracatheter pressure. A pick off means 56 may be placed anywhere
in the sensor system 34 where the pickoff means 56 may detect a
relative intracatheter pressure. For example the pick off means 56
may be placed on a sensor diaphragm or on a coupler 54. Any pick
off means 56 capable of detecting a relative intracatheter pressure
where first and second pressure sensors (42 and 44) are attached to
a coupler 54 may be used. Suitable pick off means 56 include
optical, strain, inductive (such as LVDT), capacitive, ultrasound,
etc. An exemplary circuit diagram of a LVDT pick off 56 according
to an embodiment of the invention is shown in FIG. 7B. Such a dual
sensor 34 may simultaneously account for a reference pressure by
moving in relation to a reference pressure.
[0052] Alternatively, a combination of a dual sensor incorporating
a reference sensor and a mathematical adjustment of the measured
pressure may be used. In this case, the referenced pressure may not
be best reference pressure, or the best reference location. If a
better reference pressure is available from another source, the
relative pressure measured can be mathematically adjusted against
the better reference.
[0053] FIG. 10A and FIG. 10B shows a modification to an implantable
pump system (implantable medical device 16) that may be made to
accommodate a pressure sensor 34. FIG. 10A illustrates an
implantable pump system 16 without a pressure sensor 34. FIG. 10B
illustrates an implantable pump system 16 with a pressure sensor
system 34. With the pressure sensor system 34, the housing 26 has a
bulge 58. The bulge 58 results due to an accommodation made to fit
the pressure sensor 34 within the housing 26. The pressure sensor
34 may be placed in an area of the housing 26 such that it is in
close proximity to electronics module 32.
[0054] In various embodiments, the invention provides methods and
systems for controlling a pump 28 and presenting an alarm if an
intracatheter pressure indicative of an obstructed or leaky
catheter 22 is detected. An intracatheter or relative intracatheter
pressure indicative of a leak (lower limit 60 in FIG. 8) or an
obstruction (upper limit 62 in FIG. 8) or pressures nearing such
limits (alert pressures 64 in FIG. 8) transmitted from a sensor or
sensor system 34 to electronics 46 may be used to stop the pump 28
or sound an alert 36. When a pressure limit (60 or 62) is detected,
the pump 28 is stopped, or alternatively, the delivery rate of pump
28 may be reduced. It may be desirable to reduce the delivery rate,
rather than stop pump 28, in various situations, for example when
pump 28 is driving fluid through a bifurcated catheter and only one
of the bifurcated lumens is obstructed. When an alert pressure,
which may be a pressure limit (60 or 62), is detected an alert is
issued. An alert may comprise a warning to a patient, calling an
EMT, alarming a caregiver, etc.
[0055] Limits and alert pressures may be determined by a pump or
catheter manufacture, or they may be determined by a caregiver,
such as a physician, as experience dictates. Upper limits may be,
for example, the maximum pressure an implantable pump may be
capable of handling. Lower limits may be, for example, essentially
zero intracatheter pressure.
[0056] It may be preferable to introduce a transient, or a change
in the rate of delivery of therapeutic substance. Such a transient
can be an increase or a decrease in the delivery rate but,
typically, the preferred transient is an increase in the delivery
rate. An example of a transient in the delivery rate is a commonly
occurring bolus, whether programmed or initiated under patient
control. Alternatively, such a transient could be an intentional
introduced and specific change in delivery rate designed to more
easily detect a catheter abnormality. Such a transient could be a
dramatic increase in delivery rate but only for a short period of
time. While typically boluses may last for many minutes or hours,
such a transient may last only seconds or a few minutes. Such a
transient wouldn't substantially change the overall dosage of
therapeutic substance delivered to patient 18.
[0057] Implantable medical device 16 may look for a characteristic
signature from pressure sensor 34 upon initiation of, during or
following a transient in the delivery rate. It would be
characteristic of a normal catheter for the pressure to increase
upon and shortly following an increase in the delivery rate. It
would also be characteristic of a normal catheter for the pressure
to decrease over a decay time upon and following a decrease in
delivery rate.
[0058] If the delivery rate is increased and the pressure does not
correspondingly increase, the signature would be indicative of a
leak in catheter 22. Contrarily, if the delivery rate is decreased
and the pressure does not decay, the signature would be indicative
of an obstruction in catheter 22. A higher than normal decay rate
would be indicative of a slight leak in catheter 22. A slower than
normal decay rate would be indicative of a slight or partial
obstruction of catheter 22.
[0059] Pump 28 can be a peristaltic pump which operates with a
plurality of rollers squeezing a tube containing therapeutic
substance. It may be a characteristic signature of pressure
readings from a catheter coupled to a peristaltic pump to have the
pressure dip slightly as each of the plurality of rollers releases
the tubing. This characteristic signature of pressure in catheter
should occur in a normally functioning system using a peristaltic
pump. If this signature is absent, it is indicative of a
malfunction.
[0060] Further, the pressure occurring as each roller of a
peristaltic pump releases tubing can be considered to be a
transient inducing expected transient conditions in catheter 22 as
discussed above.
[0061] FIG. 12 is a partial cross-sectional view of a preferred
embodiment of pressure sensor 34. Pressure sensor 34 in FIG. 12 is
a capacitive flow sensor utilizing two diaphragms (66 and 68).
Upper diaphragm 66 is mounted to sensor casing 50. Upper diaphragm
66 is made from or is coated, at least partially, with a conductive
material 70. Complementary conductive material 72 is coated on
stationary sapphire insulator 74. In a preferred embodiment, a
0.002 inch gap is created between conductive materials 70 and 72.
Using air as an insulator, conductive materials 70 and 72 form a
capacitor. As upper diaphragm 66 moves in response to pressure
changes, the capacitance created by conductive materials 70 and 72
also changes. A similar arrangement exists on the opposite end of
sensor casing 50 with lower diaphragm 68. Conductive materials 76
and 78 are coated on lower diaphragm 68 and sapphire insulator 80,
respectively, forming another capacitor. Coupler 54 is positioned
for relative movement with, preferably against, both upper
diaphragm 66 and lower diaphragm 68. Capacitive sensor 82 is
sensitive to changes in both capacitances and provides the sensing
output of sensor 34 illustrated in FIG. 12. Capacitive sensor 82 is
conventional. A preferred implementation for capacitive sensor 82
is described in U.S. Pat. No. 5,535,752, Halperin et al,
Implantable Capacitive Absolute Pressure and Temperature Monitor
System, the contents of which are hereby incorporated by reference.
It is worth noting that the above-described arrangement of dual
diaphragms and dual capacitors actually multiplies the amount of
change in capacitance with a given amount of movement in diaphragms
66 and 68. Since the pressure changes are small, the movement of
diaphragms 66 and 68 are small. The capacitance change is additive
resulting in twice the performance. It is preferred that coupler 54
contact diaphragms 66 and 68 in the center of the diaphragms in
order to obtain the maximum movement of diaphragms 66 and 68.
Coupler 54 should not significantly inhibit the movement of
diaphragms 66 and 68.
[0062] FIG. 13 illustrates an alternative embodiment of pressure
sensor 34 which operates on a change in inductance. Again, sensor
34 has two diaphragms 66 and 68 with coupler 54 mounted for
movement between them. A center primary coil 84 is excited with an
alternative current such as a sine wave. Upper and lower secondary
coils, 86 and 88 respectively, are mounted above and below primary
coil 84, respectively. Magnetic element 90 is mounted for movement
with coupler 54. As magnetic element 90 moves up and/or down in
response to changes in pressure, the inductance induced in
secondary coils 86 and 88 varies. An inductance sensor (not shown)
can detect the change in these inductances and provide an output
indicative of a change in pressure. Again, this arrangement doubles
the effectiveness of movement in diaphragms 66 and 68 by additively
combining the changes in inductance of each individual secondary
coil (86 or 88).
[0063] The following discussion is intended to put some of the
foregoing description in context with real world operating
conditions. It is to be recognized and understood that the specific
conditions, constructions and operations discussed herein may or
may not be preferred and are merely indicative of a possible
example of a construction, use, operating condition and the like
and should not be considered to limit the scope of the invention in
any way.
[0064] A pressure sensor may be placed in a drug flow path of an
implantable drug delivery system, which includes a pump and a
catheter. Pressure detected at a chosen location of the pressure
sensor could indicate catheter complications such as blockage or
leakage of the catheter. For purpose of this discussion, the sensor
is assumed to be placed in a casing of a drug pump, and,
preferably, a flow restriction is placed at the tip of the
catheter. However, it will be recognized that the sensor may be
placed anywhere in communication with the drug flow path and that
the flow restriction, if desired, may be placed anywhere downstream
of the sensor. Normal operation of the pump and catheter would
produce a detectable backpressure over all drug delivery flow
rates. A cut catheter would cause the pressure in the catheter to
drop to or about zero (vs. the pressure in the fluid outside the
catheter near the cut), and a blocked catheter would cause the
catheter backpressure to increase significantly relative to
cerebrospinal fluid (CSF) pressure, as an example of a bodily
location as a delivery site. A second assumption is that the
pressure drop at the catheter-tip or delivery region restriction is
large enough relative to other pressure changes between the tip or
region of the catheter and the sensor that other pressure changes
affecting the pressure sensor reading will be insignificant in
comparison, thus providing a robust measure of catheter status.
Thus the purpose of this analysis is to determine, for different
catheter pressure measurement approaches, what the minimum pressure
change would need to be to distinguish catheter complications from
background noise in the pressure signal.
[0065] The amount of pressure drop across a catheter-tip flow
restriction will depend to a great extent on the pressure reference
which is implemented, i.e., what is the pressure in the catheter
compared against. Laboratory tests of a catheter pressure
diagnostic have used a differential pressure sensor comparing
catheter backpressure against atmospheric pressure. This so-called
"gage" pressure reading provides a robust comparison of pressure
across the flow restriction, which automatically cancels out
potential reference pressure error sources such as atmospheric
pressure variation (upon which physiologic pressures float). An
atmospheric reference may not always be feasible in an implantable
device. While ways exist to achieve an atmospheric reference with
an implantable device such as plumbing a vent line across the
cutaneous boundary, it may be desirable to implement another
method. This discussion will look at implementations using a couple
types of in-vivo pressure references, and also consider the case
where there is a fixed reference, either by using a vacuum
reference or a reference consisting of a sealed cavity of gas.
[0066] Physiological pressure variation: In considering catheter
back pressure as a correlate for catheter flow status, typical
pressure variation in the body should be taken into account.
Physiologic fluid pressures typically ride on top of atmospheric
pressure.
[0067] Atmospheric pressure can vary for a number of reasons.
[0068] Altitude is probably the most significant cause of
physiological absolute pressure variation. Atmospheric pressure
declines by approximately 1'' Hg (about 1/2 psi) per 1000 feet
increase in altitude. Putting it in perspective, one can expect
about 1/4 psi change in air pressure going from ground floor to the
top floor of the IDS building in downtown Minneapolis (The IDS
Center building is approximately 775 feet tall, with about 57
stories). Aircraft cabins are typically pressurized to 6000 feet
above sea level, although unpressurized commuter and charter
aircraft typically fly to 10,000 feet above sea level. Thus, the
ambient pressure variation during a flight from a sea level airport
could be as high as 5 psi in the case of the unpressurized aircraft
with a 10,000 foot cruise altitude. Ten thousand feet is also the
altitude of the highest mountain passes in the American Rocky
Mountains. Travelers in mountainous regions can also experience
significant variations in atmospheric pressure.
[0069] Weather is another cause of physiological pressure
variation. At a given location, weather-induced atmospheric
pressure variation can be on the order of 20 mm Hg (0.1 psi).
[0070] Cerebrospinal fluid (CSF) pressure, which for purpose of
this discussion is the pressure reading of interest, is roughly
equal to venous blood pressure, which in healthy individuals is
around 0 psi at the level of the heart. However, pressure within
the CSF volume can vary for a number of reasons.
[0071] Pressure may vary due to fluid column height. Pressure in
the spinal column while standing, being around zero psig at the
level of the heart, increases as one moves lower along the spinal
column The increase corresponds to the height of the column of salt
water between the point of interest and the heart level. Typical
variation is 0.04 psi/inch. The pressure difference between two
points is naturally affected by the posture, with a standing
posture producing maximum pressure gradient along the spinal
column, and a supine posture producing minimal or zero pressure
gradient.
[0072] Pressure may also vary due to disease state. Diseases such
as congestive heart failure (CHF) and hydrocephalus can cause
increase in CSF pressure of up to 100 mmHg (0.05 psi).
[0073] Pressure may also vary due to transient events. Straining,
coughing, and other such actions on the part of the patient can
cause momentary increases in physiologic pressures of up to 100
mmHg (0.05 psi). Submersion in water can cause external pressure
change (and subsequent physiological fluid pressure change) on the
order of 0.89 in-Hg (0.44 psi) per foot of submersion. Transient
pressure events can be filtered out using algorithms commonly used
to disregard outlying data, and thus will not be considered as part
of the range requirements for the sensor.
[0074] These and other pressure variations may affect a pressure
within a catheter. In addition, the location of a catheter within
the body may affect pressure within the catheter. It will be
recognized that the catheter may be placed at any location in a
body where delivery of a pharmacological agent is desired and is
not limited to positioning for delivery of an agent to the CSF.
[0075] Diagnostic catheter pressure change elements according to an
embodiment of the invention. In an absolute pressure system,
pressure in the catheter is referenced to pressure inside a sealed
chamber. The pressure in the reference chamber could be any value,
including a vacuum. Since there is no reference to either
atmospheric pressure or the body cavity, the sensor can detect all
of the pressure changes described in section 3. Range of absolute
pressures detected by the sensor would include: Atmospheric
pressure 10-15 psi; Weather: add 0.01 psi to top of scale, subtract
0.01 from bottom; Posture: add 0.5 psi to top end of absolute
pressure range; and Disease state: add 0.05 psi to top of pressure
range.
[0076] The total absolute pressure range requirement ranges from a
low end of 10 psi-0.01 psi=9.99 psi to a top end of 15 psi+0.01
psi+0.5 psi+0.05 psi=15.56 psi. Pressure ranges and recommended
pressure change to detect cut or leaky catheter: A catheter back
pressure of at least 15.56 psi-9.99 psi=5.57 psi is preferably
maintained in the catheter relative to the surrounding body fluids,
to decrease the chance that the catheter diagnostic would detect a
false positive due to such things as aircraft cabin pressure
fluctuations, etc. Thus the absolute pressure in the catheter
during normal operation may be as high as 15.56 psi+5.57 psi=21.13
psi. For the blocked catheter diagnostic, the pump would preferably
deliver absolute pumping pressure higher than 21.13 psi to decrease
the likelihood that catheter pressure would climb noticeably higher
than normal operating pressure when blocked if high ambient fluid
pressures are present.
[0077] Generally, smaller pressure changes may be used to detect a
cut in a catheter if the time course of the pressure changes is
taken into account in a diagnostic algorithm Since most transient
pressure events (coughing, submersion in water, etc.) tend to
increase pressure, and significant decreases in pressure (altitude
change) occur slowly, an abrupt decrease in pressure may indicate a
cut catheter. To detect such an abrupt decrease in pressure, a
pressure baseline is preferably established prior to the catheter
being cut.
[0078] Barometric Reference: A separate absolute pressure sensor
may be used to significantly reduce the pressure change and max
absolute pressure requirements of a diagnostic pressure sensor
since the largest error term, atmospheric pressure, could be
eliminated from the band of pressure uncertainty. A separate
absolute pressure sensor may be located, for example, either in the
drug pump sensing peritoneal pressure or carried by the patient and
sensing atmospheric.
[0079] An absolute pressure sensor implementation is believed to be
the most straightforward and least disruptive in terms of amount of
modification to the pump. A sensor similar to Medtronic, Inc.'s
Chronicle.TM. pressure sensor could be inserted in the drug path,
with electrical interface created to interface to the pump
electronics. Adding a second sensor in the implant for purpose of
subtracting peritoneal pressure may also be employed. With use of
an external barometer a type of low-power wireless short-medium
distance communications medium is preferred.
[0080] Differential Pressure Sensor: A differential pressure sensor
may be used. A differential pressure sensor may be designed to
measure the difference between pressures in two regions, typically
by applying the pressure from one region to one side of a sensor
diaphragm, and the pressure from the other region to the opposite
side of the sensing diaphragm. In this way the diaphragm deflection
is proportional to the difference in pressure between the two
regions.
[0081] One system uses as reference a pressure in the vicinity of
the pump (peritoneal pressure), and the other system uses a
pressure in the CSF at the tip of the catheter.
[0082] For a reference in the peritoneum, Error terms due to
atmospheric pressure variation and global physiological pressure
change would be expected to cancel. Some errors due to local
physiological pressure change may still exist, for instance if the
patient lies atop the pump and pressure increases locally due to
the weight of the patient being supported by tissue and fluid
around the pump. The magnitude of this local pressure may be taken
into consideration. Additional pressure error may be due to
postural effects on pressure difference between the pump implant
site and the tip of the catheter, which as described previously
could cause up to 0.5 psi max variation in differential pressure.
Thus a catheter backpressure of about 0.5 psi or greater is
preferred.
[0083] A vent port to the pump to access peritoneal pressure may be
included, and internal plumbing to conduct peritoneal fluid
pressure from the port to the backside of the sensor diaphragm may
be included in a pump system. Preferably, fluid would be in contact
with both sides of the sensing diaphragm in operation of such a
system. Pickoff methods or mechanical designs that allow fluid to
press against both sides of a diaphragm while keeping the pickoff
hardware from contacting fluids are preferred.
[0084] For a reference at the tip of the catheter or at a delivery
region of the catheter, it is believed that all error terms
described in this document would substantially cancel out. Any
means may be used to locate a pressure sensor at a catheter tip.
For example, a dual-lumen catheter or a separate catheter may be
employed to conduct fluid pressure from the tip of the catheter to
the sensor in the drug pump. A minimal pressure drop across
catheter tip may be sufficient for detection.
[0085] A dual lumen catheter, one lumen of which terminates with a
flow restriction, is preferred. A second catheter connector port
may be added on the pump and internal plumbing to conduct CSF
pressure from the connector port to the backside of the diaphragm
may be added. Preferably, a pressure sensor design that allows
fluid contact with both sides of the diaphragm is employed.
Further, it may be desirable to periodically flush the reference
lumen, as it is a stagnant column of CSF. One way to allow flushing
would be to connect the reference lumen to a catheter access port
of an implantable pump system. A second catheter access port may be
used to flush the main drug delivery line. Alternatively, a means
for accessing both lines from a single catheter access port
(without allowing pressure communication between the two lumens
during normal pump operation) was may be used.
[0086] So far this discussion has so far assumed a constant flow
rate of drug delivery producing a long-term pressure trend, and
technical issues in separating out changes in catheter back
pressure from pressure changes caused by other influences. If the
drug delivery method were changed from a constant flow to a pulsed
approach, then the time-course of pressure changed could be used as
an additional diagnostic. In this case, the buildup and dissipation
of pressure caused by flow pulsation could be distinguishable from
other pressure changes because of its characteristic signature, and
because the pump controls the timing of the pressure pulsations and
thus pressure changes which do not correspond with programmed drug
flow pulses may be largely ignored. A system could thus be
implemented using a single absolute pressure sensor largely
ignoring long-term pressure changes in favor of concentrating on
the pressure signature in the time window of pump pulsations.
[0087] Pulsed flow would create pressure spikes instead of a
constant back pressure at a given flow rate. An advantage of a
pulsed delivery is that the pressure spike amplitude may be easier
to detect than a steady-state pressure, since at a given average
flow rate the instantaneous flow rate during the pulse, assuming
the pulses are spaced apart in time, will be much higher than if
the flow rate was constant over the same time period. Also, a
diagnostic algorithm may look at the pressure signal only during
the pulse, decreasing the importance of a reference pressure to
compare against catheter backpressure since the pressure sensor can
obtain a baseline pressure reading just before the pulse. In other
words, the pressure change caused by the pulse relative to the
pressure just before the pulse may be used. Also, the pressure
decay characteristic after the pulse pressure peak may provide
additional information that a steady state pressure may not be able
to provide (because the time constant of the pressure decay is
defined by the capacity of the catheter and the forward resistance
to flow). A cut catheter would exhibit a very rapid pressure decay
(and in fact probably wouldn't show a substantial peak), since
there would likely be little or no forward resistance. A blocked
catheter may show a slow decay, or if completely blocked may show
no decay at all, and may just find a new steady-state pressure
level due to the increased volume of fluid trying to fill the
blocked catheter. The point along the catheter where the blockage
has occurred may be predicted by noting the pressure rise for a
given volume increase. A blockage nearer the pump would decrease
the capacity of the catheter to absorb additional fluid volume
behind the blockage, and thus would likely show a very large
pressure increase (relative to one where the blockage further away
from the pump). A blockage at the end of the catheter would be
expected conversely exhibit much lower pressure increase after a
pulse since there is more capacity to absorb the additional volume
of fluid.
[0088] Thus, embodiments of the invention are disclosed. One
skilled in the art will appreciate that the present invention can
be practiced with embodiments other than those disclosed. The
disclosed embodiments are presented for purposes of illustration
and not limitation, and the present invention is limited only by
the claims that follow.
* * * * *