U.S. patent application number 13/460900 was filed with the patent office on 2013-11-07 for systems and methods for detecting tissue contact during ablation.
This patent application is currently assigned to MEDTRONIC ABLATION FRONTIERS LLC. The applicant listed for this patent is Catherine R. CONDIE, Marshall L. SHERMAN, Mark T. STEWART. Invention is credited to Catherine R. CONDIE, Marshall L. SHERMAN, Mark T. STEWART.
Application Number | 20130296840 13/460900 |
Document ID | / |
Family ID | 49513127 |
Filed Date | 2013-11-07 |
United States Patent
Application |
20130296840 |
Kind Code |
A1 |
CONDIE; Catherine R. ; et
al. |
November 7, 2013 |
SYSTEMS AND METHODS FOR DETECTING TISSUE CONTACT DURING
ABLATION
Abstract
Systems and methods for assessing tissue contact, including
positioning an electrode adjacent a tissue region; delivering
radiofrequency energy to the electrode, wherein the delivered
radiofrequency energy is sufficient to ablate at least a portion of
the tissue region; obtaining a plurality of impedance measurements
from the electrode during the delivery of the radiofrequency
energy; calculating a change in the impedance measurements over a
pre-determined time period; and generating an alert if the
calculated change is less than a pre-defined value.
Inventors: |
CONDIE; Catherine R.;
(Shoreview, MN) ; SHERMAN; Marshall L.; (Cardiff
by the Sea, CA) ; STEWART; Mark T.; (Lino Lakes,
MN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
CONDIE; Catherine R.
SHERMAN; Marshall L.
STEWART; Mark T. |
Shoreview
Cardiff by the Sea
Lino Lakes |
MN
CA
MN |
US
US
US |
|
|
Assignee: |
MEDTRONIC ABLATION FRONTIERS
LLC
Minneapolis
MN
|
Family ID: |
49513127 |
Appl. No.: |
13/460900 |
Filed: |
May 1, 2012 |
Current U.S.
Class: |
606/33 |
Current CPC
Class: |
A61B 2018/00285
20130101; A61B 2018/00863 20130101; A61B 2018/0022 20130101; A61B
2018/00898 20130101; A61B 2018/00875 20130101; A61B 2018/00577
20130101; A61B 18/1492 20130101; A61B 2018/00642 20130101; A61B
2018/00345 20130101; A61B 2018/00791 20130101; A61B 2018/0262
20130101; A61B 90/06 20160201; A61B 2018/0212 20130101; A61B
2018/00708 20130101; A61B 2018/1467 20130101; A61B 2090/065
20160201 |
Class at
Publication: |
606/33 |
International
Class: |
A61B 18/18 20060101
A61B018/18 |
Claims
1. A method of operating a medical device having an electrode,
comprising: positioning the electrode adjacent a tissue region;
delivering radiofrequency energy to the electrode, wherein the
delivered radiofrequency energy is sufficient to ablate at least a
portion of the tissue region; obtaining a plurality of impedance
measurements from the electrode during the delivery of the
radiofrequency energy; calculating a change in the impedance
measurements over a pre-determined time period; and generating an
alert if the calculated change is less than a pre-defined
value.
2. The method of claim 1, wherein the pre-determined time period is
between approximately 5 seconds and approximately 20 seconds.
3. The method of claim 1, further comprising terminating delivery
of radiofrequency energy to the electrode.
4. The method of claim 1, wherein the generated alert includes
visually displaying at least one of 1) a measured impedance value,
2) a calculated difference between a measured impedance value and a
predetermined impedance value, and 3) a calculated difference
between at least two measured impedance values.
5. The method of claim 1, further comprising attributing the
calculated change in the impedance measurements to a tissue contact
condition, wherein the generated alert signals the tissue contact
condition.
6. A method of operating a medical device having a plurality of
electrodes, comprising: positioning the plurality of electrodes
adjacent a tissue region; delivering radiofrequency energy to each
electrode, wherein the delivered radiofrequency energy is
sufficient to ablate at least a portion of the tissue region;
obtaining a plurality of impedance measurements from each of the
plurality of electrodes during the delivery of the radiofrequency
energy; calculating a change in the impedance measurements for each
of the plurality of electrodes over a pre-determined time period;
comparing the calculated change for at least one of the plurality
of electrodes to the calculated change in at least one of the other
plurality of electrodes; and generating an alert based at least in
part on the comparison.
7. The method of claim 6, wherein the pre-determined time period is
between approximately 5 seconds and approximately 20 seconds.
8. The method of claim 6, further comprising terminating delivery
of radiofrequency energy to at least one of the plurality of
electrodes.
9. The method of claim 6, wherein the generated alert includes
visually displaying at least one of 1) a measured impedance value,
2) a calculated difference between a measured impedance value and a
predetermined impedance value, and 3) a calculated difference
between at least two measured impedance values.
10. The method of claim 5, further comprising attributing the
calculated change in the impedance measurements to a tissue contact
condition, wherein the generated alert signals the tissue contact
condition.
11. A medical system, comprising: a medical device having a
plurality of electrodes; and a control unit in communication with
the plurality of electrodes, the control unit programmed to:
deliver radiofrequency energy to the plurality of electrodes,
wherein the delivered radiofrequency energy is sufficient to ablate
tissue; obtain a plurality of impedance measurements from the
plurality of electrodes during the delivery of the radiofrequency
energy; calculate a change in the impedance measurements over a
pre-determined time period; and generate an alert if the calculated
change is less than a pre-defined value.
12. The system of claim 11, wherein the pre-determined time period
is between approximately 5 seconds and approximately 20
seconds.
13. The system of claim 11, wherein the control unit is programmed
to terminate delivery of radiofrequency energy to the
electrode.
14. The system of claim 11, wherein the generated alert includes
visually displaying at least one of 1) a measured impedance value,
2) a calculated difference between a measured impedance value and a
predetermined impedance value, and 3) a calculated difference
between at least two measured impedance values.
15. The system of claim 11, wherein the control unit is programmed
to attribute the calculated change in the impedance measurements to
a tissue contact condition, and wherein the generated alert signals
the tissue contact condition.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] n/a
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] n/a
FIELD OF THE INVENTION
[0003] The present invention relates to a method and system for the
detection of tissue contact during treatment or diagnostic
procedures.
BACKGROUND OF THE INVENTION
[0004] Radiofrequency (RF) and cryogenic ablation procedures are
well recognized treatments for vascular and cardiac diseases such
as atrial fibrillation. The application of either RF or cryogenic
treatment is usually based on the preference of the physician or
the specific tissue to be treated. In either RF or cryogenic
ablation, however, the location and quality of the lesion produced
is a primary concern. The clinical success of cardiac tissue
ablation to treat arrhythmias depends on efficacy and safety of the
application of the selected energy. In particular, successful
treatments may involve successfully ablating problematic tissue
while also preventing the transmission of ablative energy to
surrounding healthy or otherwise non-targeted tissue.
[0005] For example, during procedures such as pulmonary vein
ablation or pulmonary vein antrum isolation (PVAI), an ablation
device may be used to ablate cardiac tissue surrounding one or more
pulmonary veins in the left atrium of the heart. However, ablation
extending into the pulmonary veins can contribute to serious
complications for the patient, including pulmonary vein stenosis,
phrenic nerve injury, and embolic events. Medical imaging
modalities provide some aid in assessing the particular location of
the ablation device within the patient. However, minimally-invasive
ablation procedures are typically performed within a beating heart,
and the tortuous, moving cardiac environment presents challenges in
accurately locating a targeted tissue and positioning a medical
device in place to treat the tissue, even with external imaging
systems.
[0006] Accordingly, it is desirable to provide systems and methods
of use thereof that facilitates accurate positioning of a medical
device within a patient for subsequent treatment while avoiding
damaging surrounding structures or tissue regions that are not the
target of a therapeutic or diagnostic procedure.
SUMMARY OF THE INVENTION
[0007] The present invention advantageously provides systems and
methods for assessing placement or positioning of one or more
electrodes within the body. For example, a method of assessing
electrode position is disclosed, including positioning a plurality
of electrodes within a heart and proximate a pulmonary vein;
obtaining an impedance measurement from each of the plurality of
electrodes; determining whether any of the plurality of electrodes
is located within the pulmonary vein based at least in part on the
obtained impedance measurements; and generating an indication if at
least one of the plurality of electrodes is determined to be
located within the pulmonary vein. The impedance measurements may
be between each of the plurality of electrodes and a ground
electrode external to the heart. Determining whether any of the
plurality of electrodes is located within the pulmonary vein may
include comparing an obtained impedance measurement to a predefined
impedance value and/or comparing one of the obtained impedance
measurements to another one of the obtained impedance measurements.
The method may include comparing an impedance measurement obtained
from the plurality of electrodes to impedance measured from an
electrode or a plurality of electrodes located on a guide wire or
other auxiliary device positioned within the pulmonary vein
delivering a non-ablative radiofrequency energy. The guide wire or
auxiliary device may be independently delivered to the anatomical
region or may be passed through a guide wire lumen provided by a
medical device containing the plurality of electrodes.
[0008] The method may include preventing ablative energy delivery
to an electrode determined to be located within the pulmonary vein.
The method may include conducting non-ablative radiofrequency
energy through the plurality of electrodes while positioning the
electrodes, where the conduction of non-ablative radiofrequency
energy may be substantially continuous and/or duty cycled. The
non-ablative radiofrequency energy may be conducted between each of
the plurality of electrodes and a ground electrode external to the
heart and/or conducted between the plurality of electrodes. The
method may include conducting non-ablative radiofrequency energy
between a plurality of electrodes and skin patches placed on a
patient's skin in anterior, posterior, and bilateral positions,
allowing differentiation of relative impedances between the
plurality of electrodes and the plurality of skin patches. The
method may include determining whether any of the plurality of
electrodes is located on an antrum of the pulmonary vein based at
least in part on the obtained impedance measurements; and
generating an indication if at least one of the plurality of
electrodes is determined to be located on the antrum of the
pulmonary vein, where determining whether any of the plurality of
electrodes is located on an antrum of the pulmonary vein may
include comparing an obtained impedance measurement to a predefined
impedance value. The method may include conducting ablative
radiofrequency energy through at least one of the plurality of
electrodes, where obtaining an impedance measurement is performed
during the conduction of the ablative radiofrequency signal. The
generated indication may include visually displaying at least one
of 1) a measured impedance value, 2) a calculated difference
between a measured impedance value and a predetermined impedance
value, and 3) a calculated difference between at least two measured
impedance values.
[0009] A medical system is disclosed, including a medical device
having a plurality of electrodes; a control unit coupled to the
medical device, the control unit programmed to: obtain an impedance
measurement from each of the plurality of electrodes; determine
whether any of the plurality of electrodes is located within the
pulmonary vein based at least in part on the obtained impedance
measurements; and generate an indication if at least one of the
plurality of electrodes is determined to be located within the
pulmonary vein. The control unit may determine whether any of the
plurality of electrodes is located within the pulmonary vein by
comparing an obtained impedance measurement to a predefined
impedance value and/or by comparing one of the obtained impedance
measurements to another one of the obtained impedance measurements.
The control unit may be programmed to prevent ablative energy
delivery through an electrode determined to be located within the
pulmonary vein.
[0010] A method of operating a medical device having an electrode
is disclosed, including positioning the electrode adjacent a tissue
region; delivering radiofrequency energy to the electrode, wherein
the delivered radiofrequency energy is sufficient to ablate at
least a portion of the tissue region; obtaining a plurality of
impedance measurements from the electrode during the delivery of
the radiofrequency energy; calculating a change in the impedance
measurements over a pre-determined time period; and generating an
alert if the calculated change is less than a pre-defined value.
The pre-determined time period may be between approximately 5
seconds and approximately 20 seconds. The method may include
terminating delivery of radiofrequency energy to the electrode. The
method may include attributing the calculated change in the
impedance measurements to a tissue contact condition, where the
generated alert signals the tissue contact condition to a user.
[0011] A method of operating a medical device having a plurality of
electrodes is disclosed, including positioning the plurality of
electrodes adjacent a tissue region; delivering radiofrequency
energy to each electrode, wherein the delivered radiofrequency
energy is sufficient to ablate at least a portion of the tissue
region; obtaining a plurality of impedance measurements from each
of the plurality of electrodes during the delivery of the
radiofrequency energy; calculating a change in the impedance
measurements for each of the plurality of electrodes over a
pre-determined time period; comparing the calculated change for at
least one of the plurality of electrodes to the calculated change
in at least one of the other plurality of electrodes; and
generating an alert based at least in part on the comparison. The
pre-determined time period may be between approximately 5 seconds
and approximately 20 seconds, and/or the method may include
terminating delivery of radiofrequency energy to at least one of
the plurality of electrodes.
[0012] A method of operating a medical device having an electrode
is disclosed, including delivering radiofrequency energy to the
electrode, wherein the radiofrequency energy is delivered in a
plurality of successive time fields each having a period and a duty
cycle comprising a first portion of the period when ablative
radiofrequency energy is being delivered to the electrode and a
second portion of the period when ablative radiofrequency energy is
not being delivered to the electrode; measuring a first impedance
from the electrode during the first portion of the period;
measuring a second impedance from the electrode during the second
portion of the period; comparing the first and second impedance
measurements; and modulating a subsequent duty cycle duration based
at least in part on the comparison. The method may include
generating an alert based at least in part on the comparison and/or
terminating delivery of radiofrequency energy to the electrode
based at least in part on the comparison.
[0013] A medical system is disclosed, including a medical device
having a plurality of electrodes; and a control unit in
communication with the plurality of electrodes, the control unit
programmed to: deliver radiofrequency energy to the plurality of
electrodes, wherein the delivered radiofrequency energy is
sufficient to ablate tissue; obtain a plurality of impedance
measurements from the plurality of electrodes during the delivery
of the radiofrequency energy; calculate a change in the impedance
measurements over a pre-determined time period; and generate an
alert if the calculated change is less than a pre-defined value.
The control unit may be programmed to terminate delivery of
radiofrequency energy to the electrode, the generated alert may
include visually displaying at least one of 1) a measured impedance
value, 2) a calculated difference between a measured impedance
value and a predetermined impedance value, and 3) a calculated
difference between at least two measured impedance values; and/or
the control unit may be programmed to attribute the calculated
change in the impedance measurements to a tissue contact condition,
and the generated alert may signal the tissue contact
condition.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] A more complete understanding of the present invention, and
the attendant advantages and features thereof, will be more readily
understood by reference to the following detailed description when
considered in conjunction with the accompanying drawings
wherein:
[0015] FIG. 1 is an illustration of an example of a medical system
constructed in accordance with the principles of the present
invention;
[0016] FIG. 2 is an illustration of an example of a medical device
assembly constructed in accordance with the principles of the
present invention;
[0017] FIG. 3 is another illustration of an example of a medical
device assembly constructed in accordance with the principles of
the present invention;
[0018] FIG. 4 is still another illustration of an example of a
medical device assembly constructed in accordance with the
principles of the present invention;
[0019] FIG. 5 is yet another illustration of an example of a
medical device assembly constructed in accordance with the
principles of the present invention;
[0020] FIG. 6 is an illustration of an example of using a medical
system in accordance with the principles of the present invention;
and
[0021] FIG. 7 is a flow chart of an exemplary method of operating a
medical device in accordance with the principles of the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0022] The present disclosure advantageously provides a system and
methods of use thereof that facilitate accurate positioning of a
medical device within a patient for subsequent treatment while
avoiding damaging surrounding structures or tissue regions that are
not the target of a therapeutic or diagnostic procedure. Referring
now to the drawing figures in which like reference designations
refer to like elements, an embodiment of a medical system
constructed in accordance with principles of the present invention
is shown in FIG. 1 and generally designated as "10." The system 10
generally includes a medical device 12 that may be coupled to a
control unit 14 or operating console. The medical device 12 may
generally include one or more diagnostic or treatment regions for
energetic, therapeutic and/or investigatory interaction between the
medical device 12 and a treatment site. The treatment region(s) may
deliver, for example, cryogenic therapy, radiofrequency energy, or
other energetic transfer with a tissue area in proximity to the
treatment region(s), including cardiac tissue.
[0023] Now referring to FIG. 1, the medical device 12 may include
an elongate body 16 passable through a patient's vasculature and/or
proximate to a tissue region for diagnosis or treatment, such as a
catheter, sheath, or intravascular introducer. The elongate body 16
may define a proximal portion 18 and a distal portion 20, and may
further include one or more lumens disposed within the elongate
body 16 thereby providing mechanical, electrical, and/or fluid
communication between the proximal portion of the elongate body 16
and the distal portion of the elongate body 16, as discussed in
more detail below.
[0024] The medical device 12 may include a shaft 22 at least
partially disposed within a portion of the elongate body 16. The
shaft 22 may extend or otherwise protrude from a distal end of the
elongate body 16, and may be movable with respect to the elongate
body 16 in longitudinal and rotational directions. That is, the
shaft 22 may be slidably and/or rotatably moveable with respect to
the elongate body 16. The shaft 22 may further define a lumen 24
therein for the introduction and passage of a guide wire. The shaft
22 may include or otherwise be coupled to a distal tip 26 that
defines an opening and passage therethrough for the guide wire.
[0025] The medical device 12 may further include a fluid delivery
conduit 28 traversing at least a portion of the elongate body and
towards the distal portion. The delivery conduit 28 may be coupled
to or otherwise extend from the distal portion of the elongate body
16, and may further be coupled to the shaft 22 and/or distal tip of
the medical device 12. The fluid delivery conduit 28 may define a
lumen therein for the passage or delivery of a fluid from the
proximal portion of the elongate body 16 and/or the control unit 14
to the distal portion and/or treatment region of the medical device
12. The fluid delivery conduit 28 may further include one or more
apertures or openings therein, to provide for the dispersion or
directed ejection of fluid from the lumen to an environment
exterior to the fluid delivery conduit 28.
[0026] The medical device 12 may further include one or more
expandable elements 30 at the distal portion of the elongate body
16. The expandable element 30 may be coupled to a portion of the
elongate body 16 and also coupled to a portion of the shaft 22
and/or distal tip 26 to contain a portion of the fluid delivery
conduit 28 therein. The expandable element 30 defines an interior
chamber or region that contains coolant or fluid dispersed from the
fluid delivery conduit 28, and may be in fluid communication with
an exhaust lumen 32 defined by or included in the elongate body 16
for the removal of dispersed coolant from the interior of the
expandable element 30. The expandable element 30 may further
include one or more material layers providing for puncture
resistance, radiopacity, or the like.
[0027] The medical device 12 may further include one or more
electrically-conductive segments or electrodes 34 positioned on or
about the elongate body for conveying an electrical signal,
current, or voltage to a designated tissue region and/or for
measuring, recording, or otherwise assessing one or more electrical
properties or characteristics of surrounding tissue. The electrodes
34 may be configured in a myriad of different geometric
configurations or controllably deployable shapes, and may also vary
in number to suit a particular application, targeted tissue
structure or physiological feature. For example, as shown in FIG.
1, the electrodes 34 may include a first pair proximate to the
expandable element and a second electrode pair distal to the
expandable element. Alternative electrode configurations of the
medical device 12 are illustrated in FIGS. 2-5. FIG. 2 includes an
electrode array 36 configurable into a looped or substantially
circular configuration. The electrode array 36 in FIG. 3 includes a
plurality of arms 38, with the electrodes 34 positioned in a
proximal-facing direction or orientation on the arms 38. FIG. 4
also includes a plurality of extendable or deployable arms 38
having a plurality of electrodes 34 in a square-like or "X"-shaped
configuration. Turning to FIG. 5, a plurality of electrodes 34 are
shown in a substantially linear array 36 extending along a portion
of the elongate body 16 of the medical device 12. Additional
details related to the configurations, manipulation, and exemplary
uses of the electrode configurations shown in FIGS. 2-5 are
provided in U.S. patent application Ser. No. 12/116,753, filed on
May 7, 2008, entitled "Ablation Therapy System and Method for
Treating Continuous Atrial Fibrillation," the entirety of which is
hereby incorporated by reference.
[0028] Each electrode 34 may be electrically coupled to an output
portion of a radiofrequency signal generator, and each electrode 34
may also include a sensor, such as a thermocouple, an electrical
conductivity sensor, a spectrometer, a pressure sensor, a fluid
flow sensor, a pH sensor, and/or a thermal sensor (not shown)
coupled to or in communication with the electrodes. The sensors may
also be in communication with a feedback portion of the control
unit 14 to trigger or actuate changes in operation when
predetermined sequences, properties, or measurements are attained
or exceeded.
[0029] Referring again to FIG. 1, the medical device 12 may include
a handle 40 coupled to the proximal portion of the elongate body
16. The handle 40 can include circuitry for identification and/or
use in controlling of the medical device 12 or another component of
the system. Additionally, the handle 40 may be provided with a
fitting 42 for receiving a guide wire that may be passed into the
guide wire lumen 24. The handle 40 may also include connectors 44
that are matable to the control unit 14 to establish communication
between the medical device 12 and one or more components or
portions of the control unit 14.
[0030] The handle 40 may also include one or more actuation or
control features that allow a user to control, deflect, steer, or
otherwise manipulate a distal portion of the medical device 12 from
the proximal portion of the medical device 12. For example, the
handle 40 may include one or more components such as a lever or
knob 46 for manipulating the elongate body 16 and/or additional
components of the medical device 12. For example, a pull wire 48
with a proximal end and a distal end may have its distal end
anchored to the elongate body 16 at or near the distal portion 20.
The proximal end of the pull wire 48 may be anchored to an element
such as a cam in communication with and responsive to the lever 46.
The medical device 12 may include an actuator element 50 that is
movably coupled to the proximal portion of the elongate body 16
and/or the handle 40 for the manipulation and movement of a portion
of the medical device 12, such as the shaft 22, and/or one or more
portions of the electrode assemblies described above, for
example.
[0031] The system 10 may include one or more treatment sources
coupled to the medical device for use in an operative procedure,
such as tissue ablation, for example. The control unit 14 may
include a fluid supply 52 including a coolant, cryogenic
refrigerant, or the like, an exhaust or scavenging system (not
shown) for recovering or venting expended fluid for re-use or
disposal, as well as various control mechanisms. In addition to
providing an exhaust function for the fluid or coolant supply 52,
the control unit 14 may also include pumps, valves, controllers or
the like to recover and/or re-circulate fluid delivered to the
handle 40, the elongate body 16, and/or the fluid pathways of the
medical device 12. A vacuum pump 54 in the control unit 14 may
create a low-pressure environment in one or more conduits within
the medical device 12 so that fluid is drawn into the
conduit(s)/lumen(s) of the elongate body 16, away from the distal
portion 20 and towards the proximal portion 18 of the elongate body
16.
[0032] The control 14 unit may include a radiofrequency generator
or power source 56 as a treatment or diagnostic mechanism in
communication with the electrodes 34 of the medical device 12. The
radiofrequency generator 56 may have a plurality of output
channels, with each channel coupled to an individual electrode 34.
The radiofrequency generator 56 may be operable in one or more
modes of operation, including for example: (i) bipolar energy
delivery between at least two electrodes on the medical device
within a patient's body, (ii) monopolar or unipolar energy delivery
to one or more of the electrodes 34 on the medical device 12 within
a patient's body and through a patient return or ground electrode
57 spaced apart from the electrodes 34 of the medical device 14,
such as on a patient's skin or on an auxiliary device (such a guide
wire or secondary catheter) within the patient, for example, and
(iii) a combination of the monopolar and bipolar modes.
[0033] The signal provided by the radiofrequency generator or power
source 56 may be substantially continuous or include duty cycles of
on/off periods of energy delivery. As used herein, the term "duty
cycle" refers to the proportion of time during which a component,
device or system is operated to achieve a desired effect. The duty
cycle can be expressed as a ratio or as a percentage, e.g., 1/10
denotes 1 "on" period per ten total periods, or a 10% duty cycle.
As used herein, the term "field" refers to a single period of a
duty cycle. Each field includes an "on" time in which energy is
delivered to achieve a desired effect (such as ablating tissue) and
an "off" time in which little or no energy is delivered. Of note,
the "off" period may include some conduction or transfer of energy,
but not in sufficient amounts to achieve the effect of the "on"
period. In a particular example, the radiofrequency generator 56
may have a plurality of output channels each having a sequential
set of fields (e.g. 2, 4, 8) providing a customized power delivery
scheme which repeats and/or dynamically changes over time in
response to one or more monitored or measured parameters of the
system, a patient, and/or procedure being performed. Additional
details related to the configurations and exemplary uses of the
system 10 and radiofrequency generator or power source 56 discussed
herein are provided in U.S. patent application Ser. No. 12/117,596,
filed on May 8, 2008, entitled "RF Energy Delivery System 10 and
Method," the entirety of which is hereby incorporated by
reference.
[0034] The control unit 14 may include an impedance measurement
module or signal processing unit 58 to measure one or more
impedance characteristics between the electrodes of the medical
device. As used herein, the term impedance refers to the generally
accepted definition of the term: a complex ratio of sinusoidal
voltage to current in an electric circuit or component, except that
as used herein, impedance shall apply to any region or space
through which some electrical field is applied and current flows.
The impedance, Z, may be expressed as a complex number, Z=R+jX,
wherein R is the resistance in real number ohms, X is the reactance
in imaginary number ohms, and j is a multiplier that is the
positive square root of negative one (-1). Impedance may also be
expressed in polar form as Z=|Z|e.sup.j.theta., where |Z| is the
magnitude (e.g., the ratio of the voltage difference amplitude to
the current amplitude), .theta. is the phase difference between
voltage and current, and j is the imaginary unit. An excitation
current may be applied between one or more of the electrodes 34 on
the medical device 12 and/or a patient return or ground electrode
57, and the resulting impedance may be monitored or measured, as
described in more detail below. Alternatively, a plurality of
electrodes may be situated about the patient (such as on skin
patches or otherwise) placed on the patient's skin in anterior,
posterior, and bilateral positions, allowing differentiation of
relative impedances between the plurality of electrodes on the
medical device and the plurality of electrodes/skin patches.
Excitation and the resulting measurements for impedance parameters
may continue or otherwise be repeated until impedance measurements
are calculated for various combinations of electrodes before,
during and/or after a designated procedure.
[0035] The system 10 may further include one or more sensors to
monitor the operating parameters throughout the system, including
for example, pressure, temperature, flow rates, volume, power
delivery, impedance, or the like in the control unit 14 and/or the
medical device 12, in addition to monitoring, recording or
otherwise conveying measurements or conditions within the medical
device 12 or the ambient environment at the distal portion of the
medical device 12. The sensor(s) may be in communication with the
control unit 14 for initiating or triggering one or more alerts or
therapeutic delivery modifications during operation of the medical
device 12. One or more valves, controllers, or the like may be in
communication with the sensor(s) to provide for the controlled
dispersion or circulation of fluid through the lumens/fluid paths
of the medical device 12. Such valves, controllers, or the like may
be located in a portion of the medical device 12 and/or in the
control unit 14. The control unit 14 may include one or more
controllers, processors, and/or software modules containing
instructions or algorithms to provide for the automated operation
and performance of the features, sequences, calculations, or
procedures described herein.
[0036] Now referring to FIGS. 6-7, exemplary methods of use of the
system described herein are disclosed. In particular, impedance may
be measured and/or monitored and used as a basis for determining
tissue type, electrode location, and/or contact between an
electrode and a tissue region. Measured impedances may also be used
to dynamically modulate or adjust energy delivery to one or more of
the electrodes during treatment.
[0037] For example, impedance across pulmonary vein tissue is
considerably higher than impedance across the cardiac tissue of the
antrum of a pulmonary vein within the left atrium of the heart.
Accordingly, measured impedance differences between one or more
electrodes 34 of the device 12 may be used as a basis to generate
an alert that at least a portion of the medical device is
undesirably placed too far within a pulmonary vein. Particularly
referring to FIGS. 6 and 7, one or more of the electrodes 34 the
medical device 12 may be positioned in proximity to a tissue region
60 targeted for a diagnostic or therapeutic procedure (Step 100),
which may include tissue ablation for the treatment of atrial
fibrillation or other condition. The tissue region 60 may include a
pulmonary vein opening and antrum in the left atrium, for example.
Once positioned, one or more impedance measurements may be taken or
obtained from each of the electrode(s) (Step 102). The impedance
measurements may be obtained by directing sufficient current from
the radiofrequency generator 56 to one or more of the electrodes 34
of the medical device 12 to obtain an impedance value between 1)
two or more electrodes on the medical device; 2) between one or
more of the electrodes on the medical device 12 and the ground
electrode 57; 3) between one or more of the electrodes on the
medical device 12 and one or more of a plurality of electrodes
external to the heart; and/or 4) between one or more of the
electrodes on the medical device 12 and one or more of a plurality
of electrodes of an auxiliary device, such as a guide wire or
secondary catheter. The energy conducted through the electrodes for
the impedance measurements may be significantly less than an energy
level that would ablate or damage the tissue and does not cause
cardiac stimulation. For example, the non-ablative energy may
include alternating current energy at a frequency of approximately
10 kHz having a voltage less than approximately 1 volt. The
delivery of such non-ablative energy may be substantially
continuous during placement of the medical device, or may be
duty-cycled. The non-ablative, non-stimulating energy may further
be delivered at a plurality of frequencies in sequential bursts or
duty-cycled durations of energy conductance. The impedance
measurements may be recorded, processed, and/or stored by the
impedance measurement module 58 in the control unit 14, for
example. Examples of multi-frequency energy delivery and
corresponding impedance measurements are provided in U.S. patent
application Ser. No. 13/017,122, filed on Jan. 31, 2011, the
entirety of which is hereby incorporated by reference.
[0038] Upon obtaining one or more of the impedance measurements,
the obtained measurements from each electrode may then be compared
to a pre-defined impedance value (which may include an expected
impedance value for atrial tissue and/or an expected value for
pulmonary vein tissue). Additionally or alternatively, the
measurements of each electrode may be compared to the measurements
from the other electrodes, and a difference or variation between
the electrodes may be processed or calculated (Step 104). A
determination whether any of the electrodes 34 are located within
the pulmonary vein may be made based at least in part on the
obtained impedance measurements and/or comparison of the obtained
values (Step 106). A determination whether any of the electrodes 34
are located on an antrum or atrial wall may be made based at least
in part on the obtained impedance measurements and/or comparison of
the obtained values. The determination may be performed by one or
more processors of the control unit 14 and/or the impedance
measurement module 58. If one or more of the electrodes 34 of the
medical device 12 are determined to extend into an undesirable
depth of the pulmonary vein, the control unit 14 may generate an
indication or alert of such positioning (Step 108). The generated
indication or alert may include, for example, visually displaying
at least one of 1) a measured impedance value, 2) a calculated
difference between a measured impedance value and a predetermined
impedance value, and/or 3) a calculated difference between at least
two measured impedance values. The generated indication or alert
may additionally or alternatively include one or more audible
indications as well.
[0039] The determination of whether any electrode is located within
the pulmonary vein may also result in deactivating or preventing
ablative energy form being transmitted or conducted through the
electrode(s) determined to be in the pulmonary vein (Step 110). One
or more electrode(s) not determined to be within the pulmonary vein
may then proceed to be used for the procedure at hand, which may
include delivering or conducting ablative energy through the
electrode(s) in a desired position or location on the antrum and/or
atrium wall. Additionally, impedance measurements may be obtained,
processed, and compared as described above during the conduction of
the ablative radiofrequency signal to prevent or signal an unwanted
change in electrode position during a procedure, e.g., if any of
the active electrodes migrate into the pulmonary vein while
ablating.
[0040] Changes in measured impedances may also be used to indicate
contact between one or more of the electrodes 34 and the targeted
tissue region 60. For example, impedance measurements are higher
when a treatment element is in contact with tissue than when a
treatment element is not in contact with tissue but rather
surrounded by blood within vasculature. Moreover, during energy
delivery to targeted tissue, electrodes lacking quality contact
with tissue will show a relatively stable impedance value, whereas
electrodes in good tissue contact will show an impedance reduction
during initial energy delivery to the tissue. Accordingly, in
addition to the impedance measurements providing positioning
information as described above, the measured impedance values may
also be used to indicate whether electrodes used in treating tissue
have maintained or lost their contact. In particular, upon
delivering ablative radiofrequency energy to one or more electrodes
(such as those found to be on an antrum and not in the pulmonary
vein, for example), the control unit 14 and/or impedance
measurement module 58 may continue obtaining a plurality of
impedance measurements from each electrode during the delivery of
the radiofrequency energy. The control unit 14 and/or impedance
measurement module 58 may calculate a change in the impedance
measurements over a pre-determined time period, which may be
between approximately 5 seconds and approximately 20 seconds, for
example. The calculated change may include an absolute value, or
the calculated change may include an average rate of change.
[0041] The control unit 14 and/or impedance measurement module 58
may then generate an alert or indication if the calculated change
(or rate of change) is less than (or greater than) a pre-defined
value or otherwise fails to fall within a defined, acceptable range
of values at any time during a procedure. Additionally or
alternatively, the control unit 14 and/or impedance measurement
module 58 may compare impedance changes between electrodes to
determine whether there is any significant variation in the
measured rates of change for each electrode, which indicates that
the electrodes are not in sufficient contact with the targeted
tissue. The control unit 14 and/or radiofrequency generator 56 may
subsequently terminate delivery of radiofrequency energy to the
electrode as a result of the calculation and comparison of
impedance changes.
[0042] Changes in measured impedances may also be used to modulate
or regulate duty cycles for power delivery to one or more of the
electrodes 34. For example, the control unit 14 and/or impedance
measurement module 58 may monitor the impedance between each
electrode of the medical device 12 and the ground electrode 57
during duty cycled energy delivery; monitor impedance to the ground
electrode 57 during the "on" periods of duty cycled delivery where
therapeutic or ablative energy levels are being delivered;
monitoring impedance to the ground electrode 57 during the "off"
periods of duty cycled delivery where during the off periods, the
driving voltage of the phased radiofrequency energy is not zero but
is at a level that allows impedance measurement without ablating
tissue; and/or monitor impedance during the off periods where the
non-zero level of energy is totally unipolar, regardless of the
mode of delivery during the on periods (of note, there may be
bipolar components delivered during the "on" periods due to out of
phase condition between individual electrodes). The control unit 14
and/or impedance measurement module 58 may compare impedance
measurements made during the "on" and "off" periods of duty cycled
energy delivery and modulate the duty cycle durations based at
least in part on measured changes in relative impedance.
[0043] It will be appreciated by persons skilled in the art that
the present invention is not limited to what has been particularly
shown and described herein above. In addition, unless mention was
made above to the contrary, it should be noted that all of the
accompanying drawings are not to scale. A variety of modifications
and variations are possible in light of the above teachings without
departing from the scope and spirit of the invention, which is
limited only by the following claims.
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