U.S. patent application number 14/876786 was filed with the patent office on 2016-04-07 for prediction of atrial wall electrical reconnection based on contact force measured during rf ablation.
The applicant listed for this patent is St. Jude Medical Luxembourg Holding S. .R.L.. Invention is credited to Olivier B. Fremont, Hendrik Lambert, Stuart J. Olstad.
Application Number | 20160095653 14/876786 |
Document ID | / |
Family ID | 45561085 |
Filed Date | 2016-04-07 |
United States Patent
Application |
20160095653 |
Kind Code |
A1 |
Lambert; Hendrik ; et
al. |
April 7, 2016 |
Prediction of atrial wall electrical reconnection based on contact
force measured during RF ablation
Abstract
A method and device for determining the transmurality and/or
continuity of an isolation line formed by a plurality of point
contact ablations. In one embodiment, a method for determining the
size of a lesion (width, depth and/or volume) is disclosed, based
on contact force of the ablation head with the target tissue, and
an energization parameter that quantifies the energy delivered to
the target tissue during the duration time of the lesion formation.
In another embodiment, the sequential nature (sequence in time and
space) of the ablation line formation is tracked and quantified in
a quantity herein referred to as the "jump index," and used in
conjunction with the lesion size information to determine the
probability of a gap later forming in the isolation line.
Inventors: |
Lambert; Hendrik; (Deinze,
BE) ; Olstad; Stuart J.; (Plymouth, MN) ;
Fremont; Olivier B.; (Annecy le Vieux, FR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
St. Jude Medical Luxembourg Holding S. .R.L. |
Luxembourg |
|
LU |
|
|
Family ID: |
45561085 |
Appl. No.: |
14/876786 |
Filed: |
October 6, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13337896 |
Dec 27, 2011 |
9149327 |
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14876786 |
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61427425 |
Dec 27, 2010 |
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61427423 |
Dec 27, 2010 |
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Current U.S.
Class: |
606/41 |
Current CPC
Class: |
A61B 18/082 20130101;
A61B 2034/301 20160201; A61B 2018/00702 20130101; A61B 2018/00375
20130101; A61B 2018/0072 20130101; A61B 2018/00875 20130101; A61B
2018/00357 20130101; A61B 2090/065 20160201; A61B 18/10 20130101;
A61B 2018/00767 20130101; A61B 5/1076 20130101; A61B 2018/00577
20130101; A61B 2218/002 20130101; A61B 2018/00351 20130101; A61B
18/1492 20130101; A61B 34/30 20160201 |
International
Class: |
A61B 18/14 20060101
A61B018/14 |
Claims
1. A method for automatically controlling an ablation catheter,
comprising: providing an elongate flexible catheter, said catheter
including a distal portion having an ablation head and a force
sensor and operatively coupled with an energy source and an
energization parameter measuring device; providing instructions for
introducing said catheter into a patient during a medical procedure
and guiding said distal portion of said catheter so said ablation
head of said catheter is exerted against a first target tissue
location; automatically energizing said ablation head with said
energy source over a period of time while said ablation head is
exerted against said first target tissue location; measuring a
sequence of energization parameters with said energization
parameter measuring device while said ablation head is energized;
measuring a sequence of contact forces with said force sensor while
said ablation head is energized, said contact forces being in
reaction to said ablation head exerted against said target tissue;
automatically determining a lesion size based on said sequence of
contact forces and said sequence of said energization parameters
over said period of time; and automatically generating control
information based on said lesion size for use in guiding said
ablation head to a second and subsequent target tissue
location.
2. The method of claim 1, wherein said determination of lesion size
includes determining a joule heating component and a diffusive
heating component.
3. The method of claim 1, wherein the energization parameter of
said sequence of energization parameters is electrical current.
4-11. (canceled)
12. A method for determining the continuity of an isolation line
formed by point contact ablation in a region of the human heart,
comprising: providing an elongate flexible catheter, said catheter
including a distal portion having an ablation head operatively
coupled with an energy source, a force sensor and a position
sensing device, said energy source, said force sensor and said
position sensing device being operatively coupled with a processor;
configuring said processor to: provide instructions for forming a
plurality of lesions substantially along a desired ablation line
with said ablation head, sense the location of each of said
plurality of lesions with said position sensing device during the
forming of said plurality of lesions, determine if a jump occurred
between each consecutively formed pair of lesions of said plurality
of lesions, said jump being defined by a predetermined criteria of
spatial separation between the lesions of said consecutively formed
pairs of lesions, and increment a jump index for each jump detected
in the formation of said plurality of lesions; and determining a
probability of gap formation along said isolation line based on
said Limp index and said force data.
13. The method of claim 12, wherein said predetermined criteria for
determining if a jump occurred is based on a zoned accounting
method wherein said isolation line is divided into adjacent zones
and said jump is established when consecutively formed lesions are
created in non-adjacent zones.
14. A method of forming an isolation line in a region of a human
heart, comprising: providing an elongate flexible catheter adapted
to be introduced into a patient during a medical procedure, said
catheter including a distal portion having an ablation head
operatively coupled with a force sensor, a position sensing device
and a control system having a processor, said processor being
operatively coupled with said force sensor, said position sensing
device and a receiving device, said processor having access to a
storage medium that contains programming instructions to be
executed by said processor, said programming instructions
including: determining an actual location of a first lesion of said
isolation line; calculating a desired location for a second lesion,
said desired location of said second lesion being proximate to and
based on said actual location of said first lesion; generating an
instruction to position said ablation head at said desired location
of said second lesion; and sending said instruction to position
said ablation head at said desired location of said second lesion
to said receiving device.
15. The method of claim 14, further comprising: providing an energy
source operatively coupled with an energization parameter measuring
device, said energy source further being operatively coupled with
said ablation head and said processor; and additional programming
instructions contained on said storage medium to be executed by
said processor, said additional programming instructions including:
energizing said ablation head with said energy source for formation
of said second lesion; acquiring position data from said position
sensing device during formation of said second lesion; acquiring
force data from said force sensor during formation of said second
lesion; acquiring energization parameter data from said
energization parameter measuring device during formation of said
second lesion; and acquiring duration time data for formation of
said second lesion.
16. The method of claim 15, wherein said first and second lesions
are formed sequentially in time, without formation of other lesions
therebetween.
17. The method of claim 15, further comprising storing said
position data, said force data, said energization parameter data
and said duration time data to a data storage device.
18. The method of claim 15, wherein said energization parameter
measuring device provided in said step of providing said energy
source is adapted to measure electrical current.
19. The method of claim 15, wherein said programming instructions
further comprise: determining an actual location of said second
lesion from said position data acquired during formation of said
second lesion, calculating a desired location for a third lesion,
said desired location of said third lesion being proximate to and
based on said actual location of said second lesion; generating an
instruction to position said ablation head at said desired location
of said third lesion; and sending to said receiving device said
instruction to position said ablation head at said desired location
of said third lesion.
20. The method of claim 19, further comprising inferring an
estimated size of said second lesion based on said force data and
said duration time data; and calculating said desired location for
said third lesion based on said estimated size of said second
lesion.
21. The method of claim 14, wherein said location of said first
lesion is proximate a desired ablation line, and wherein said
desired location for said second lesion is based on a location of
said desired ablation line.
22. The method of claim 14, further comprising: after said step of
sending said instruction, monitoring said position sensing device
to track movement of said distal portion of said elongate flexible
catheter; and from said monitoring of said position sensing device,
determining when said ablation head is within a predetermined
distance of said desired location of said second lesion.
23. The method of claim 14, wherein said receiving device is a
robotic manipulator.
24. The method of claim 14, wherein said desired location of said
second lesion is sufficiently close to said first lesion for
continuity between said first and second lesions if said second
lesion is formed at said desired location.
25. The method of claim 24, wherein said second lesion physically
overlaps said first lesion if said second lesion is formed at said
desired location.
26. The method of claim 14, wherein said first lesion is located
proximate a pulmonary vein.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 13/337,896, filed 27 Dec. 2011 (the '896
application), now U.S. Pat. No. 9,149,327, issued 6 Oct. 2015,
which claims the benefit of U.S. provisional patent application
Nos. 61/427,423 (the '423 application) and 61/427,425 (the '425
application), both filed on 27 Dec. 2010. The '896, '423, and '425
applications are hereby incorporated by reference in their entirety
as though fully set forth herein.
BACKGROUND
[0002] a. Field
[0003] The field of the invention relates generally to the
treatment of organic tissues using ablation therapy, and more
specifically to the prediction and display of lesion sizes using
catheter-based contact ablation delivery systems.
[0004] b. Background Art
[0005] Atrial fibrillation is a common cardiac arrhythmia involving
the two upper chambers (atria) of the heart. In atrial
fibrillation, disorganized electrical impulses that originate in
the atria and pulmonary veins overwhelm the normal electrical
impulses generated by the sinoatrial node, leading to conduction of
irregular impulses to the ventricles that generate the heartbeat.
Atrial fibrillation can result in poor contraction of the atria
that can cause blood to recirculate in the atria and form clots.
Thus, individuals with atrial fibrillation have a significantly
increased risk of stroke. Atrial fibrillation can also lead to
congestive heart failure or, in extreme cases, death.
[0006] Common treatments for atrial fibrillation include
medications or synchronized electrical cardioversion that convert
atrial fibrillation to a normal heart rhythm. Surgical-based
therapies have also been developed for individuals who are
unresponsive to or suffer serious side effects from more
conventional treatments. The surgical techniques include making
incisions in the right and left atria to block propagation of the
abnormal electrical impulse around the atrial-chamber.
[0007] Catheter-based contact ablation techniques have evolved as a
minimally invasive alternative to surgical-based techniques, and
also as an alternative for individuals who are unresponsive to or
suffer serious side effects from more conventional treatments
(e.g., medications). Contact ablation techniques involve the
ablation of groups of cells near the pulmonary veins where atrial
fibrillation is believed to originate, or the creation of extensive
lesions to break down the electrical pathways from the pulmonary
veins located on the posterior wall of the left atrium. Methods of
energy delivery include radiofrequency, microwave, cryotherapy,
laser, and high intensity ultrasound. The contacting probe is
placed into the heart via a catheter that enters veins in the groin
or neck and is routed to the heart, thus negating the need for an
incision in the heart wall from the outside. The probe is then
placed in contact with the posterior wall of the left atrium and
energized to locally ablate the tissue and electrically isolate the
pulmonary veins from the left atrium. The advantages of
catheter-based contact ablation techniques have been recognized to
include a minimally invasive surgical access, thus reducing risks
of infection, and reduced recuperation times.
[0008] Where complete electrical isolation is desired, the
objective of the contact ablation technique is to form a continuous
"ablation line" or "isolation line" of ablated tissue between the
left atrium and the pulmonary veins. Two different approaches for
achieving an isolation line have been developed: point contact
ablation where the energy delivery is from a head end of the
contacting probe generally in line with a longitudinal axis of the
contacting probe; and linear contact ablation where the energy
delivery is from a side of the contacting probe and generally
transverse to the longitudinal axis of the contacting probe.
[0009] A concern with catheter-based contact ablation techniques is
the post-operative recurrence of atrial fibrillation, believed to
be caused by electrical reconnection of pulmonary veins across the
isolation line. The sites along the isolation line where this type
of electrical reconnection occurs are referred to as "isolation
gaps" or simply "gaps." Gaps can occur due to suboptimal catheter
contact force during ablation for either point contact ablation or
linear contact ablation techniques. The left anterior wall is often
a difficult area to achieve stable contact during pulmonary vein
isolation resulting in higher incidence of local isolation
gaps.
[0010] One approach to identifying or predicting possible isolation
gaps has been to make electrical continuity measurements across the
isolation line after the isolation line has been created. While
this approach may work in some cases for linear contact ablation
techniques, it is generally not effective for point contact
ablation techniques because it requires too much time and too many
continuity measurements in order to establish a relatively high
confidence in the ability to predict whether there will or will not
be isolation gaps as a result of incomplete lesion formations
during the ablation process of creating the isolation. In addition,
it has been found that intra-operative continuity measurements of
the isolation line may not be an accurate predictor of the
recurrence of atrial fibrillation as the tissue properties of the
lesion just after ablation can change over time and may not be
representative of the final lesions associated with the isolation
line.
[0011] The predictability of lesion formation in the context of
point contact ablation techniques has been enhanced with the advent
of force sensing ablation catheters. The ability to incorporate the
contact forces utilized in point-to-point ablation procedures has
led to new systems and processes directed to the prediction of
ablation size. U.S. patent application publication no. 2010/0298826
to Leo et al. (Leo), assigned to the assignee of the instant
application, discloses the use of a force-time integral for real
time estimation of lesion size in catheter-based ablation
systems.
[0012] Further improvements in the application of force sensing
catheter-based contact ablation devices and methods to mitigate the
occurrence of electrical reconnection across isolation lines after
ablation treatments for atrial fibrillation would be a useful and
welcomed development.
BRIEF SUMMARY
[0013] A device and method for prediction of successful isolation
and/or the occurrence of gap formation in the contact of
catheter-based point contact ablation techniques is presented. In
one embodiment, the size of a lesion is predicted on the basis of
the contact force between the ablation head and a target tissue, an
energization parameter applied to the ablation head during contact,
and the time duration of the ablation. In another aspect of the
invention, the integrity of the isolation line can be enhanced as
well as predicted by tracking and quantifying the sequential nature
(sequence in time and space) of the ablation line formation. The
ability to better predict the temporal and spatial vicinity of a
pair of consecutively formed lesions without the need for repeated
post-ablation measurement is then utilized by various embodiments
of the invention to dynamically determine aspects of subsequent
contact point lesion to create a more effective isolation line.
[0014] With respect to the prediction of lesion size, various
embodiments of the invention predict the lesion size based on a
quantity referred to herein as the "lesion size index," or "LSI."
The LSI is a parameter that can be used to evaluate the lesion size
during an ablation in real time. More specific forms of the lesion
size index include a "lesion width index" (LWI) for estimating the
maximum width or diameter of a lesion, a "lesion depth index" (LDI)
for estimating the maximum and/or effective depth of the lesion,
and the "lesion volume index" (LVI) for estimating the total volume
of the lesion.
[0015] In one embodiment, the LSI is derived from a mathematical
expression that incorporates a contact force F between the ablation
head and the target tissue, an energization parameter E applied to
target the tissue (e.g., power, current or voltage), and a duration
time t of the energization. These indexes are based on an empirical
model developed from a series of experiments where lesion sizes
were formed on the beating hearts of canines and the lesions
subsequently measured.
[0016] The LSI represents an improvement over the force-time
integral in several respects. For instance, the LSI incorporates
the energization parameter E directly. Also, the LSI is based on a
model that utilizes both a joule heating component (i.e., heating
by the passage of electrical current) and a diffusive heating
component. The LSI model can also account for more subtle,
non-linear characteristics of lesion formation, such as the delay
between the variation of force and/or current and the change of
lesion growth rate due to thermal latency, and the discovery that
lesions rapidly grow to a certain depth (typically about 3 mm),
beyond which the depth parameter continues to grow at a slower
rate. Furthermore, the LSI model can account for the different
results from different energizations. For example, an increase in
the energization and/or contact force will cause the growth rate of
the lesion to increase. A moderate drop in energization and/or
contact force causes the growth rate of lesions to slow, while a
dramatic drop in energization and/or contact force causes the
growth rate to stop altogether. Embodiments of the LSI model can
account for these various characteristics of lesion formation.
Thus, the combination of the aforementioned aspects of the LSI
enable a robust and refined prediction of lesion size.
[0017] With respect to the sequential aspects of line formation,
the temporal and spatial vicinity of two consecutive lesions has
been found to be a factor in the quality of isolation line
continuity. Spatial vicinity (i.e., formation of adjacent lesions
in sequence) is advantageous because of the limited reproducibility
of the catheter positioning systems and limitations with respect to
catheter maneuverability. Temporal vicinity (i.e., formation of the
lesions in a time efficient manner) is also advantageous because,
within approximately a minute after ablation, edema is formed. The
onset of edema can vitiate the formation of lesions in the adjacent
area.
[0018] Accordingly, various embodiments of the invention track and
quantify the sequential characteristics of the isolation line
formed by the ablation process using a parameter herein referred to
as the "jump index," or "JI." In one embodiment, a zoned-based
accounting of the jump index is utilized. For zone-based
accounting, the isolation line to be formed is divided into a
series of ablation zones. The jump index JI can be a cumulative sum
of the number of ablation zones that are passed over or "jumped"
between two consecutive but non-adjacent lesion formations during
the formation of the isolation line. That is, if a pair of lesions
consecutively formed are centered within ablation zones that are
adjacent each other, the jump index JI is not incremented because
no ablation zones were passed over between the formation of the
consecutive lesions. However, if two consecutively formed lesions
are in non-adjacent zones, the JI is incremented by the number of
ablation zones that were passed over between the two ablation
sites. Treatment of the carina between two ipsilateral veins before
a full isolation around the veins is completed is also considered a
jump. The incrementing of the jump index II is tracked until at
least one lesion has been formed in all designated zones of the
desired isolation line, at which time the incrementing of the JI
ceases.
[0019] In another embodiment, a distance-based detection of jump is
utilized. With distance-based methods, a "jump" occurs whenever the
distance between consecutively formed lesions along a desired
isolation line exceeds a predetermined arc length. Here, the
incrementing of the jump index can remain active, for example,
until the maximum arc length between any two lesions is less than
the predetermined arc length.
[0020] Accumulation of a low jump index JI during the formation of
an isolation line results in a statistically significant increase
in the success of the isolation line long term (3 months or more).
That is, a low JI results in an enhanced, statistically significant
chance that no post-operative gaps will form, at least within the
first 3 months after ablation.
[0021] The jump index JI not only demonstrates the superior
effectiveness of constructing an isolation line in a substantially
consecutive manner, but can also be implemented as a predictor of
gap formation in procedures where isolation line formation did not
occur in a substantially sequential manner. Accordingly, in certain
embodiments, the probability of gap prediction is based on 1) the
lesion size index LSI or the force-time integral FTI and 2) the
jump index JI. The LSI and/or FTI is believed to be an indicator of
lesion transmurality, and the jump index JI is believed to be an
indicator of the continuity of the isolation line.
[0022] In various embodiments, a method of forming an isolation
line in a region of a human heart, is described. The method
comprises providing an elongate flexible catheter adapted to be
introduced into a patient during a medical procedure, the catheter
including a distal portion having an ablation head operatively
coupled with a force sensor, a position sensing device and a
control system. The control system can include a processor
operatively coupled with the force sensor, the position sensing
device and a receiving device (such as a robotic manipulator or a
display), the processor having access to a storage medium that
contains programming instructions to be executed by the processor.
In one embodiment, the programming instructions include: [0023]
determining an actual location of a first lesion of the isolation
line; [0024] calculating a desired location for a second lesion,
the desired location of the second lesion being proximate to and
based on the actual location of the first lesion; [0025] generating
an instruction to position the ablation head at the desired
location of the second lesion; and [0026] sending the instruction
to position the ablation head at the desired location of the second
lesion to the receiving device. The method can also comprise
providing an energy source operatively coupled with an energization
parameter measuring device, the energy source also being
operatively coupled with the ablation head and the processor.
Additional programming instructions contained on the storage medium
to be executed by the processor can include: [0027] energizing the
ablation head with the energy source for formation of the second
lesion; [0028] acquiring position data from the position sensing
device during formation of the second lesion; [0029] acquiring
force data from the force sensor during formation of the second
lesion; [0030] acquiring energization parameter data from the
energization parameter measuring device during formation of the
second lesion; and [0031] acquiring duration time data for
formation of the second lesion.
[0032] In another embodiment of the invention, the programming
instructions can further comprise: [0033] determining an actual
location of the second lesion from the position data acquired
during formation of the second lesion, [0034] calculating a desired
location for a third lesion, the desired location of the third
lesion being proximate to and based on the actual location of the
second lesion; [0035] generating an instruction to position the
ablation head at the desired location of the third lesion; [0036]
sending to the receiving device the instruction to position the
ablation head at the desired location of the third lesion; and
[0037] calculating the desired location for the third lesion based
on the estimated size of the second lesion. The desired location of
the second lesion can be sufficiently close to the first lesion for
continuity between the first and second lesions if the second
lesion is formed at the desired location, and, in some embodiments,
the second lesion physically overlaps the first lesion if the
second lesion is formed at the desired location.
[0038] In another embodiment of the invention, a method for
automatically controlling an ablation catheter comprises providing
an elongate flexible catheter, the catheter including a distal
portion having an ablation head and a force sensor and operatively
coupled with an energy source. Instructions are provided for
introducing the catheter into a patient during a medical procedure
and guiding the distal portion of the catheter so the ablation head
of the catheter is exerted against a first target tissue location.
The ablation head is automatically energized with the energy source
over a period of time while the ablation head is exerted against
the first target tissue location. A sequence of energization
parameters (e.g., electrical current) can also be measured with the
energization parameter measuring device, as well as a sequence of
contact forces with the force sensor, while the ablation head is
energized, the contact forces being in reaction to the ablation
head exerted against the target tissue. A lesion size can be
automatically determined based on the sequence of contact forces
and the sequence of the energization parameters over the selected
period of time. In one embodiment, the determination of lesion size
includes determining a joule heating component and a diffusive
heating component. Also, control information can be automatically
generated based on the lesion size for use in guiding the ablation
head to a second and subsequent target tissue location.
[0039] In another embodiment, a method for automatically
controlling an ablation catheter includes providing an elongate
flexible catheter with a distal portion having an ablation head
operatively coupled with an energy source and a position sensing
device, and also providing instructions for introducing the
catheter into a patient during a medical procedure and guiding the
distal portion of the catheter so the ablation head of the catheter
is exerted against a first target tissue location. The ablation
head can be automatically energized with the energy source over a
period of time while the ablation head is exerted against the first
target tissue location. A sequence of locations of the distal
portion of the elongate flexible catheter can then be measured with
the position sensing device while the ablation head is energized. A
location of a lesion created during the energizing of the ablation
head can be automatically inferred from the sequence of locations,
and control information automatically generated based on the
location of the lesion for use in guiding the ablation head to a
second and subsequent target tissue location. In addition, this
method can further comprise measuring a sequence of contact forces
with the force sensor, and measuring a sequence of energization
parameters with the energization parameter measuring device, all
while the ablation head is energized. A lesion size can then be
determined based on the sequence of contact forces and the sequence
of the energization parameters measured over the period of time.
Control information can be automatically generated based on the
lesion size, for use in guiding the ablation head to the second and
subsequent target tissue location.
[0040] In another embodiment, a method for determining the
continuity of an isolation line formed by point contact ablation in
a region of the human heart is disclosed. The method includes
providing an elongate flexible catheter, the catheter including a
distal portion having an ablation head operatively coupled with an
energy source, a force sensor and a position sensing device, the
energy source, the force sensor and the position sensing device
being operatively coupled with a processor. A processor can be
configured to: [0041] provide instructions for forming a plurality
of lesions substantially along a desired ablation line with the
ablation head; [0042] sense the location of each of the plurality
of lesions with the position sensing device during the forming of
the plurality of lesions; [0043] determine if a jump occurred
between each consecutively formed pair of lesions of the plurality
of lesions, the jump being defined by a predetermined criteria of
spatial separation between the lesions of the consecutively formed
pairs of lesions, and [0044] increment a jump index for each jump
detected in the formation of the plurality of lesions. A
probability of gap formation along the isolation line can also be
determined based on the jump index and the force data. In one
embodiment, the predetermined criteria for determining if a jump
occurred is based on a zoned accounting method wherein the
isolation line is divided into adjacent zones and the jump is
established when consecutively formed lesions are created in
nonadjacent zones.
[0045] In another aspect of the invention, a method for predicting
the depth of lesions formed during RF ablation therapy is developed
and presented based on the force-time integral (FTI). In one
embodiment, lesion depth predictions utilizing the FTI are based on
two parameters: (1) the contact force between the RF ablation head
and the target tissue, and (2) the power delivered to the RF
ablation head. In still another aspect of the invention, a
relationship between contact force and the formation of gaps in the
isolation line is established. A prospective study was performed
for an evaluation of electrical reconnections at three months after
the ablation procedure. The objective of the study was to identify
parameters correlating to gaps in the isolation line and to predict
the likelihood of failure of the isolation treatment.
[0046] The foregoing and other aspects, features, details,
utilities, and advantages of the present disclosure will be
apparent from reading the following description and claims, and
from reviewing the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0047] FIG. 1 depicts a schematic of a contact ablation system in
an embodiment of the invention;
[0048] FIG. 2 depicts parameters of the point contact ablation
lesion of FIG. 1;
[0049] FIG. 3 is a graph of lesion depths as a function of RF
ablation power and contact force in an embodiment of the
invention;
[0050] FIGS. 4A through 4F are graphical representations of data
used in embodiments of the invention;
[0051] FIG. 5 is a graphical representation of the correlation
between the lesion width and the lesion depth parameters used in
embodiments of the invention;
[0052] FIGS. 6A through 6D are perspective views of a human heart,
showing typical preferred locations of isolation lines for various
embodiments of the invention;
[0053] FIGS. 7A through 7C depict a zone accounting method for
tracking the jump index in embodiments of the invention;
[0054] FIGS. 8A and 8B depict gap formation rates for a zone-based
jump index (JI) vs. minimum force-time integral (FTI) and for jump
index (JI) vs. minimum lesion width index (LWI), respectively, in
embodiments of the invention;
[0055] FIGS. 9A and 9B depict a distance-based method for tracking
the jump index in an embodiment of the invention, distinguishing it
from zone accounting methods;
[0056] FIG. 10 depicts a schematic of a contact ablation system in
an embodiment of the invention;
[0057] FIGS. 11A through 11C depict aspects of a variable reference
line method in an embodiment of the invention;
[0058] FIG. 12A through 12D depict aspects of a fixed reference
line method in an embodiment of the invention; and
[0059] FIG. 13 is a flow chart depicting certain aspects of the
variable reference line method and the fixed reference line method
in embodiments of the invention.
DETAILED DESCRIPTION OF EMBODIMENTS
[0060] Referring to FIG. 1, a contact ablation system 30 is
depicted in an embodiment of the invention. The contact ablation
system 30 includes a catheter 32 having a distal portion 34
comprising an ablation head 36 operatively coupled with a force
sensor 38, the ablation head 36 arranged for contact with a target
tissue 40. The catheter 32 is operatively coupled with a power
source 42 that provides and measures the delivered energy to the
ablation head 36. A measurement device 44 is also depicted, capable
of sourcing the force sensor 38 and measuring an output signal from
the force sensor 38. The contact ablation system 30 can also
include a central controller 45 such as a computer or
microprocessor operatively coupled with the power source 42 and the
measurement device 44 for control thereof and for processing
information received therefrom.
[0061] In operation, the ablation head 36 is brought into contact
with the target tissue 40 and energized to create a lesion 46 on
and within the target tissue 40. The force sensor 38 is configured
to generate an output from which a magnitude of a contact force
vector 48 can be inferred. Generally, the contact force is
time-variant, particularly when the target tissue 40 is subject to
motion (e.g., the wall of a beating heart). The energy flow (e.g.,
current or power) through the ablation head 36 can also be time
variant, as the energy flow may depend on the contact resistance
between the ablation head 36 and the target tissue 40, which in
turn can vary with the contact force and the changing properties of
the lesion 46 during ablation.
[0062] Referring to FIG. 2, typical characteristics of the lesion
46 are depicted. The lesion 46 can be 15 characterized as having a
maximum depth 22, a maximum width 24 and a volume 26. An effective
depth 28 can also be characterized as the maximum depth 22 divided
by the square root of two ( 2).
[0063] Various embodiments of the invention implement a "force-time
integral" (FTI), broadly defined herein as a measured quantity that
involves the measurement of force over time. The force-time
integral can be defined one of several ways, all involving the
measurement of force over time. One example of a force-time
integral is, of course, the numerical integration of the force over
time (FOT):
FOT=.intg.F(t)dt Eqn. (1)
where F(t) is the contact force measured over time between a target
tissue and a distal portion of an ablation head. The parameter t
designates time, indicating that the contact force can be time
variant.
[0064] The force-time integral can also be expressed a force-time
product (FTP), given by
FTP= F.DELTA.t Eqn. (2)
where F is a representative value of F(t) over a time period
.DELTA.t.
[0065] Another expression of a force-time integral comprises a
force-energization over time (FEOT) integral or a
force-energization-time product (FETP), given respectively as
FEOT=.intg.F(t)E(t)dt Eqn. (3)
FETP= F .DELTA.t Eqn. (4)
where E(t) is the measured energization indicative of the energy
flow delivered to the ablation head (e.g., power or electrical
current) and is a representative value of the measured energization
E(t) over the time period .DELTA.t (for example a time-averaged
energization value). The measured energization E(t) can also be
time-variant, as noted above. The force-time-energization product
(FETP) can include combinations of the above parameters, for
example:
FETP= .intg.F(t)dt Eqn. (5)
FETP= F.intg.E(t)dt Eqn. (6)
[0066] In another embodiment, a normalized force over time (NFOT)
integration that is normalized with respect to the energization
levels can also be implemented:
NFOT = .intg. F ( t ) E ( t ) t .intg. E ( t ) t .DELTA. t Eqn . (
7 ) ##EQU00001##
Such an approach may be useful for enhanced accuracy where only FOT
or FTP calibrations are available.
[0067] It is further noted that with respect to the present
invention the measurement of "force" per se is not necessary to
infer or derive a force-time integral. Although force and strain or
pressure may not be equivalent in other contexts, other parameters
that have a relationship with force (e.g., strain, pressure) can be
substituted for the force component of the force-time integral in
the present invention and still reliably predict lesion size.
Likewise, it is understood that other references to "force" herein
(including, but not limited to, force sensor, force signal, force
conversion, force set point, force interval, force values, force
measurement, force level, force limits, contact force and reaction
force) are intended to be broadly construed to include other
parameters such as pressure and strain that have a relationship
with force.
[0068] Patients with paroxysmal atrial fibrillation received
pulmonary vein (PV) isolation in accordance with standard ablation
procedures using an irrigated RF ablation catheter that provides
tip-to-tissue contact force information (TACTICATH, Endosense,
Switzerland). The operator was blinded to contact force, which was
recorded for later analysis. Pulmonary vein antra were each divided
into 8 segments of interest. For each ablation, the catheter
position, contact force, RF power and the force-time integral (FTI)
were collected. The FTI is a useful parameter for expressing the
accumulated energy delivered in an ablation (i.e. the energy
delivered during the formation of a lesion), with unstable contact
resulting in low FTI.
[0069] The initial application of RF at each segment was separately
analyzed to give insight into early tissue changes with focus on
low FTI. Patients underwent a 2.sup.nd interventional diagnostic
procedure at three months to evaluate gap occurrence in each
segment of interest for each pulmonary vein. Incidence of gaps per
segment of interest at three months was correlated with contact
force and FTI during the pulmonary vein isolation procedure, and a
method for gap prediction developed based on these parameters.
[0070] Twenty-seven patients having an age span of 58+/-11 years
(nineteen males and eight females) were treated at two centers by
nine different operators. Thirteen patients were subject to a three
month interventional follow up, of which five gaps on the left
anterior wall were detected. There was no measurable difference in
contact force for sections with gap or without gap (13.4.+-.4.7 gmf
vs. 13.1.+-.7.5 gmf, p=0.2727, where "gmf" is the force equivalent
of the weight of one gram at standard gravity). However, for the
first RF application at each segment of interest, the FTI was
significantly lower in segments with gaps than with no gap
(79.0.+-.68.2 gmf-sec vs. 364.8.+-.568.4 gmf-sec, p=0.0006).
Probability for gap occurrence at the left atrial wall increases by
18% for every first RF application where the FTI is less than 250
gmf-sec per segment.
[0071] Combining gap probability per segment of interest provides a
method for predicting the likelihood of reconnection at the left
atrium wall per patient. The method is able to discriminate between
patients with gaps versus patients with no gaps at the left atrium
wall (45% vs 24%, p=0.0015).
[0072] The general methodology is as follows:
[0073] Assumptions [0074] First ablation is determinant, too low
(<250) FTI induces an edema and cannot be catched again. [0075]
2 ablations are required to isolate a segment. [0076] Each bad
event induces a certain probability of getting a gap. [0077] The
probability to have a gap at the patient level is the product of
the probability to have a gap at the segment level.
[0078] Determination of the probability of success [0079] Counting
the number of ablation within the 2 first for each position where
FTI has been lower than 250 gmf-sec (defined as a "bad event").
[0080] the probability that such an event is followed by a gap is
p.sub.position=(#ablation|FTI<250 gmf-sec. at position i &
gap at this position)/(#ablation|FTI<250 gmf-sec at position i)
[0081] For each position the probability of success after such a
bad event occurs is
P.sub.success.sub._.sub.position=1-p.sub.position
[0082] Computing the predictive probability for each patient.
[0083] Counting for a patient when a "bad ablation" has been done
within the 2 first ablations for each position: N.sub.bad [0084]
Probability of success for a certain number of position is
P.sub.success=(P.sub.success.sub._.sub.position).sup.Nbad
[0085] On the left atrium wall, a low initial FTI is a predictive
parameter for early gap 15 occurrence following pulmonary vein
isolation. The probability of gap occurrence can be quantified.
This allows a prediction of the probability of success per patient
already during pulmonary vein isolation and has the potential to
adapt ablation strategy during the procedure. In one embodiment of
the invention, lesion depth predictions were correlated from
ablation studies, based on pre-clinical ablation studies involving
a total of 31 animals and 218 measures. A lesion depth (D) was
found to correlate a general data form as follows
D=(A1F2+A2F+A3)(B1P2+B2P) Eq. (8)
where F is contact force (e.g., gmf), P is power delivered to the
ablation head (e.g., watts), and A1, A2, A3 and B1, B2 are
coefficients based on curve fits to the animal study data. A "gmf"
is the force equivalent to the weight of 1 gram of mass at standard
gravity.
[0086] An example and non-limiting graph predicting lesion depth D
is presented at FIG. 3. The predictions take the form of Eq. (8)
and are based on three sets of data from ablation studies on
animals involving a total of 31 animals and 218 measures. The
least-squares values of the coefficients A1, A2, A3 and B1, B2 for
the curve fits presented in FIG. 3 are:
[0087] A1=-0.29E-05 mm/gmf.sup.2
[0088] A2=1.41E-02 mm/gmf
[0089] A3=0.559 mm
[0090] B1=-3.81E-03 W.sup.2
[0091] B2=0.409
[0092] In another embodiment of the invention, a lesion size index
(LSI) is related to the contact force F between the ablation head
36 and the target tissue 40, an energization parameter E applied to
target the tissue (e.g., power, voltage, current), and the duration
time t of the ablation. The effect of these parameters have been
modeled and correlated with ablation data from numerous clinical
studies to arrive at an equation set based on the model. The LSI
can thus be expressed as a retrospective equation or set of
equations that can be programmed into the central controller
45.
[0093] Each of the F, E and t parameters is taken into account
through an exponential term that models saturation effects. The
saturation effect takes into account the asymptotic nature of
lesion formation, wherein lesion growth approaches a size limit at
infinite time. Also, because the modeling of the present work is
based on real data, changes in the material properties of the
tissue under ablation are accounted for (e.g., changes in the
electrical resistivity, which affects the quantity of the heat
generated by the joule heating effect).
[0094] Referring to FIGS. 4A-4F, data showing the exponential form
of the LSI is depicted in an embodiment of the invention,
demonstrating similar forms of the various lesion width and the
lesion depth parameters. For these data, the energization parameter
E is electrical current. Referring to FIG. 5, the correlation
between the lesion width and the lesion depth parameters is
observable. For the data presented in FIG. 5, a correlation of
R=0.91 is realized. The high correlation confirms that the same
model can be applied to calculate both the lesion depth index (LDI)
and the lesion width index (LWI).
[0095] The retrospective equation that describes the LSI model can
be of the following general form
LSI ( F , I , t ) = k 1 * ( f 2 ( 1 - - F / f 1 ) + f 0 ) * i 2 ( 1
- - ( I / i 1 ) 2 ) * ( ( 1 - k 0 ) + k 0 1 - - t / .tau. 1 - - T /
.tau. ) Eq . ( 9 ) ##EQU00002##
where f.sub.0, f.sub.1, and f.sub.2 are force parameter
coefficients, i.sub.1 and i.sub.2 are electrical current
coefficients, k.sub.0 is a diffusive heating coefficient, k.sub.1
is a rescaling coefficient and .tau. is a characteristic time
value. The input units for the LSI are gmf for the force F,
milliamps (mA) for the current I and seconds (sec) for the duration
time t. The resulting output of Eq. (9) correlates with a length
that is expressed in millimeters.
[0096] The LSI model reflected in Eq. (9) comprises a joule heating
component (1-k.sub.0) that is independent of time and a diffusive
heating component k.sub.0
1 - - t / .tau. 1 - - T / .tau. ##EQU00003##
that is a function of time. The joule heating and diffusive heating
components are multiplied by the lesion depth estimated for an
ablation lasting a time period of T, with the averaged force F and
electrical current I over the time period T. Data analyzed for this
work was generated for a time period T of 60 seconds. It is noted
that the baseline time of 60 seconds was a result of the
availability of lesion data that was based on 60 second ablation
times. Data from ablations of different durations (e.g., 30 sec.,
45 sec.) can also be utilized in a form similar to Eq. (9) by
substitution of the appropriate time for the "60) found in the
numerator of the diffusive heating component.
[0097] The retrospective equation of Eq. (9) is a separable
variable function of contact force F, electrical current I and
duration time t of the ablation. The parameters of this equation
were obtained by best fit of experimental data acquired during
preclinical studies. The same general form was utilized to
calculate both the LDI and the LWI. Only the best fit coefficients
differ between the equations. The various coefficients are
presented in Table 1:
TABLE-US-00001 TABLE 1 Best fit coefficients for LDI and LWI
equations f2 f1 f0 i2 i1 k0 k1 r Lesion 4.36 20.67 2.17 2.57 630.75
0.578 1.22/ 2 38.57 Depth Index Lesion 3.74 18.20 1.99 3.29 525.85
0.481 1.10 29.23 Width Index
[0098] The k.sub.0 for the LDI includes a separate 2 factor in the
denominator for conversion from maximum depth to effective depth.
That is, if the LDI of the effective depth is desired, the 2
[0099] factor should be included in the calculation.
[0100] By implementation of Eq. (9), the central controller 45 can
apprise operators of the estimated lesion growth in essentially
real time, as the ablation is in progress.
[0101] Development of the lesion width index (LWI) is now
described. The LWI model considers two aspects of lesion
development when computing the lesion width in real time: the
completed portion of the growth of the lesion width and the
uncompleted portion of the growth of the lesion width, based on a
total time T. As explained above, the total time T for this work is
60 seconds because that was the total time of the ablations for the
data analyzed for the modeling. Based on observations of the data
and the exponential behaviour attributed to saturation, the LWI
uses the exponential functions of time. The exponential function
can be function of previous time step exponential plus an
increment:
f(t.sub.1)=A(1-e.sup.-t1/.tau.)=f(t.sub.0)+(A-f(t.sub.0))(1-e.sup.-.DELT-
A.t/.tau.), .DELTA.t=t.sub.1-t.sub.0 Eq. (10)
Calculations can be gated to be performed only at the time step
.DELTA.t (1 second, for example) in the interest of computational
economy.
[0102] In one embodiment, calculations are made with force and
current averaged over a migrating averaging window, i.e. over the
last n seconds. The migrating averaging window helps account for
the phenomena of thermal latency, as explained in S. K. S. Huang
and M. A. Wood, Catheter Ablation of Cardiac Arrhythmia, Elsevier,
2006, chapter 1, which is hereby incorporated by reference in its
entirety except for express definitions contained therein. Thermal
latency is the mechanism by which the temperature and growth of the
lesion continue to rise that after energization ceases. Huang and
Wood, for example, report that the temperature of the lesion
continues to rise for an additional 6 seconds after cessation of
energization. Accordingly, in one embodiment, the time period for
the migrating averaging window is 6 seconds.
[0103] In part because of the thermal latency effect, the evolution
of the lesion is not well known for the first 6 seconds of
ablation. Lesions are analyzed post-ablation, and the size of the
lesions for short duration ablations is dwarfed by the thermal
latency effect. Accordingly, in one embodiment, the LWI is
calculated within the first 6 seconds of ablation as a linear
interpolation between the origin and the value expected at 6
seconds.
[0104] The estimation of what the lesion width would be at time t=T
of ablation (LWI.sub.T) is the width that the lesion would reach if
constant current and force were applied during the whole time
period T:
LWI.sub.T(F, I)=LWI(F, I,
t=T)=k.sub.1*(f.sub.2(1-e.sup.-F/f1)+f.sub.0)*i.sub.2(1-e.sup.-(I/i.sup.1-
.sup.).sup.2) Eq. (11)
[0105] The joule heating component of the lesion width index
(LWI.sub.JH) accounts for the tissue that is heated directly by
passage of electrical current applied by the catheter. In one
embodiment, LWI.sub.JH is thus assumed as the source of heat which
then diffuses in the tissue. The LWI.sub.JH can also be defined as
a constant ratio of the LWI at the total time T (i.e.,
LWI.sub.T):
LWI.sub.JH=LWI.sub.T Eq. (12)
That is, in one embodiment, the LWI.sub.JH component of the lesion
formation is constant with respect to time, but is still variable
with respect to the energization parameter E and the applied
contact force F.
[0106] The complete portion of the growth of the lesion width is
taken as the LWI at the last time step t0 (LWI.sub.t0), or the
lesion size due to new joule heating LWI.sub.JH if it exceeds the
lesion at LWI.sub.t0.
max {LWI.sub.t0, LWI.sub.JH} Eq. (13)
[0107] The uncompleted portion of the growth of the lesion is
driven by the LWI.sub.T and the LWI.sub.JH (both using average
force and current on the previous 6 seconds).
[0108] The actual LWI at time t.sub.1 (LWI.sub.t1) is the
LWI.sub.t0 plus an incremental lesion .DELTA.LWI.
.DELTA. LWI = ( LWI T * k 0 k 3 - [ max { LWI t 0 , LWI JH } - LWI
JH ] ) ( 1 - - .DELTA. t / .tau. ) .DELTA. t = t 1 - t 0 , k 3 = 1
- - T / .tau. Eq . ( 14 ) LWI t 1 = max { LWI t 0 , LWI JH } +
.DELTA. LWI Eq . ( 15 ) ##EQU00004##
Subtracting the LWI.sub.JH from the completed portion of the growth
of the lesion demonstrates that the exponential characteristics of
the LWI and the .DELTA.LWI only applies on the diffusive
component.
[0109] It is noted that the development of the lesion depth index
(LDI) is the same as the development of the LWI because both
indexes have the same form and are driven by the same physics.
Accordingly, the derivation of LDI is the same as for the LWI,
albeit using different data (i.e., depth data).
[0110] The lesion volume can be inferred from the lesion width by
multiplying a cubic of the maximum width of the lesion by a
constant. In one embodiment, the equation is for converting from
maximum lesion width to lesion volume is given by
Lesion volume=0.125167*.pi.*[MAX WIDTH].sup.3 Eq. (16)
Based on data analyzed for this work, Eq. (16) has a correlation
coefficient of R=0.99. Because LWI is based on the maximum width of
a lesion, the LVI is related to the LWI in the same way:
Lesion Volume Index=0.125167*.pi.*LWI.sup.3 Eq. (17)
[0111] Referring to FIGS. 6A-6D, depictions of perspective views of
the human heart 50 are presented, showing typical preferred
locations of isolation lines for various embodiments of the
invention. The depictions present the left atrium in the forefront,
including the left superior pulmonary vein (LSPV) 54, the left
inferior pulmonary vein (LIPV) 56, the right superior pulmonary
vein (RSPV) 58 and the right inferior pulmonary vein (RIPV) 60. In
FIG. 6A, desired isolation lines 62 and 64 encircle the left
pulmonary veins (LSPV 54, LIPV 56) and the right pulmonary veins
(RSPV 58, RIPV 60), respectively. Carinas 66 and 68 are located
between the left superior and inferior veins 54, 56 and the right
superior and inferior veins 58, 60, respectively. In FIG. 6B,
additional desired isolation lines 70 and 72 traverse the
respective interiors of the desired isolation lines 62 and 64, each
substantially along the respective carina 66, 68. In FIG. 6C, an
additional isolation line 73 is defined that connects the desired
isolation lines 62 and 64 line along the roof of the left atrium.
In FIG. 6D, desired isolation lines 74, 76, 78 and 80 surround the
base of each pulmonary vein separately, i.e., LSPV 54, LIPV 56,
RSPV 58 and RIPV 60, respectively. While the depictions represent
full ablation lines, partial ablation lines (i.e., ablation lines
that do not form a closed loop) can also be utilized.
[0112] Referring to FIG. 7A, a schematic of the left atrium with
left veins 84 and right veins 86 is presented, along with the
desired isolation lines 62 and 64, for use in a "zoned" accounting
method in an embodiment of the invention. In the zoned accounting
method, the desired isolation lines 62 and 64 can be divided into
ablation zones. In one embodiment, the desired isolation lines 62
and 64 are divided into eight ablation zones each (ablation zones
I-VIII and ablation zones IX-XVI). This corresponds to
approximately two or three lesions per zone for nominal lesion
sizes on the order of 6 to 10 mm diameter each. A plurality of
lesions 87a through 87h (referred to collectively as lesions 87)
are also schematically depicted in FIG. 7A, each denoted by a
"circle-x" symbol ({circle around (.times.)}). The lesions 87
depict the start of an isolation line. A trace line 90 depicts the
order in which the lesions 87 were formed, starting at lesion 87a
and stepping through lesion 87h.
[0113] Lesions 87a, 87b and 87c were formed consecutively in both
time and space, with each successive lesion overlapping the lesion
formed immediately prior. Accordingly, no ablation zones were
passed over in the creation of lesions 87a, 87b and 87c. However,
lesion pair 87c and 87d, while created consecutively in time, are
not adjacent in space. Instead, between the creation of lesion 87c
and lesion 87d, two ablation zones were passed over, as denoted in
FIG. 7A as segment 90a of trace line 90. Lesion 87c, being located
in ablation zone II, and lesion 87d, being located in ablation zone
V, means that ablation zones III and IV were passed over between
their respective formations. In one embodiment of the invention,
there is said to be a "jump" between lesions 87c and 87d because
entire zones were passed over. According to one embodiment of the
invention, the jump index JI is incremented by the number of
ablation zones passed over in a jump. By this convention, because
jump 90a passes over two ablation zones, the jump index JI is
incremented by two.
[0114] Lesions 87d and 87e are a pair of consecutively formed
lesions that, like lesions 87a, 87b and 87c, were formed
consecutively and overlap adjacently. Therefore, no incrementing of
the jump index JI is incurred between lesions 87d and 87e.
[0115] Between lesions 87e and 87f, ablation sites are passed over.
However, lesions 87e and 87f are formed in adjacent ablation zones
(ablation zones V and VI). Therefore, there is no "jump" and thus
no incrementing of the jump index due to the sequence of lesions
87e and 87f.
[0116] In the depiction of FIG. 7A, a plurality of lesions 92 are
depicted as formed along the desired ablation line 64 about the
right veins 86 and in a sequence depicted by a trace line 94. Here,
however, all of the lesions are formed consecutively in both time
and space, with each successive lesion overlapping the lesion
formed immediately prior. If this pattern were to continue all the
way around the desired ablation line 64 until the isolation line
fully surrounds the RSPV 58 and the RIPV 60, no incrementing of the
jump index JI would occur, and the jump index JI would equal
zero.
[0117] Referring to FIG. 7B, the left veins 84 are depicted, again
in the zoned accounting method, with the desired isolation line 62
having been fully formed with a plurality of lesions 96, as well as
the additional desired isolation line 70 having been formed along
the carina 66 with a plurality of lesions 98. In FIG. 7B, a
sequence line 100 depicts the order in which the lesions 96 were
formed, starting with lesion 96a. Likewise, a sequence line 102
depicts the order in which the lesions 98 were formed, starting
with lesion 98a. Assuming that the lesions 96 were formed first, it
would be necessary to jump to the middle of zone III to begin the
lesions 98. However, because all of the ablation zones received at
least one ablation during the formation of the lesions 96,
incrementing of the jump index JI is terminated. Therefore, the
jump index would not be incremented as a result of the treatment of
the carina in this instance. If, however, the carina were treated
prior to the formation of at least one lesion in all the zones,
movement of the ablation head to the carina from a non-adjacent
zone (i.e., from zones other than zones III or VII) would cause the
jump index JI to be incremented.
[0118] Referring to FIG. 7C, the right veins 86 are depicted in a
zoned accounting method with the desired ablation lines 78 and 80
of FIG. 6D in an embodiment of the invention. A plurality of
lesions 104 are depicted as being formed in contact with the
desired ablation line 78. A trace line 106 depicts the order in
which the lesions 104 are formed, starting with lesion 104a, with
the remaining lesions 104 being formed consecutively to overlap
with the lesion formed immediately prior thereto. In one
embodiment, treatment of the carina before completion of all the
zones of the isolation line is treated as a "jump," and the jump
index is therefore incremented by 1 in forming the lesions 104,
even though the lesions 104 of FIG. 7C are depicted as being formed
consecutively and in an overlapping manner.
[0119] A plurality of lesions 108 are depicted as being formed in
contact with the desired ablation line 80. A trace line 110 depicts
the order in which the lesions 108 are formed, starting with lesion
108a, with the remaining of the plurality of lesions 108 being
formed consecutively to overlap with the lesion formed immediately
prior thereto. Assuming the lesions 104 are formed first and in the
order depicted in FIG. 7C, one would have to reposition the
ablation catheter from zone IX to zone XI in order to form the
first lesion 108a in the line. The attendant jump would pass over
zone X. Because some of the ablation zones remain untreated (i.e.,
zones XII, XIII and XIV), incrementing of the jump index JI is
still active and the passing over of zone X would thus causing the
jump index JI to be incremented by 1.
[0120] Any number of ablation zones can be utilized for defining
the resolution of the jump index JI. For example, the number of
zones could be doubled, thus providing a jump index JI having a
higher resolution. Or the number of ablation zones could be reduced
to provide a jump index having a coarser resolution. Furthermore,
the ablation zones do not have to be of equal tangential dimension.
For example, ablation zones II, III and IV could be combined into
one ablation zone. This provides greater resolution for the jump
index JI about the smaller ablation zones. In this way, the jump
index can be tailored for greater sensitivity in regions more prone
to gap formation.
[0121] In various embodiments of the invention, the jump index JI
can be used in combination with either the minimum force-time
integral FTI or the minimum lesion size index LSI utilized in the
formation of the isolation line for predicting the post-operative
formation of gaps. Empirical relationships have been developed to
quantify this effect, using a total of 3164 ablations performed in
the formation of a total of 99 PV lines over a total of 50
patients, and using eight zones per pair of ipsilateral veins as
depicted in FIGS. 7A and 7B. The data is presented in Tables 2 and
3 below for JI vs. minimum FTI and JI vs. minimum LSI,
respectively. The data from Tables 2 and 3 are also presented in
FIGS. 8A and 8B, respectively. The specific form of the FTI used
for the data of Table 2 is the force over time (FOT) form of Eq.
(1) above.
TABLE-US-00002 TABLE 2 Gap formation rate as a function of FTI and
JI Jump Index FTI .ltoreq.5 6 .ltoreq. JI .ltoreq. 11 .gtoreq.12
>400 1% 5% 20% 200-400 2% 17% 15% <200 7% 14% 30%
TABLE-US-00003 TABLE 3 Gap formation rate as a function of LWI and
JI Jump Index LWI .ltoreq.5 6 .ltoreq. JI .ltoreq. 11 .gtoreq.12
>6 0% 6% 20% 4-6 2% 10% 22% .ltoreq.4 7% 14% 22%
[0122] Both sets of data show the same trends. Specifically, the
chance of gap formation increases generally with jump index JI and
decreases generally for increasing minimum LWI values and for
increasing minimum FTI values. The minimum LWI and JI data of Table
3/FIG. 8B indicate an increase in the chance of gap formation that
is substantially monotonic with both increasing jump index and
decreasing minimum LWI.
[0123] The data of Tables 2 and 3/FIGS. 8A and 8B support the
conclusion that formation of lesions sequentially in both time and
space is advantageous. Furthermore, the tables provide a way to
predict the likelihood of gap formation based on the jump index JI,
minimum FTI and/or minimum LWI. For example, if, during the course
of forming an isolation line a jump index JI of 7 was accrued and a
minimum lesion width index LWI of 8 was observed, there would be a
6% chance that a gap would develop along the isolation line. In
terms of minimum FTI, the same JI=7 in conjunction with a minimum
FTI of 350 gmf-sec during the ablation line formation would equate
to a 17% chance of gap formation.
[0124] Referring to FIGS. 9A and 9B, a distance-based accounting
technique for computing the jump index JI is depicted in an
embodiment of the invention, and distinguished from the zoned
accounting method. The right veins 58, 60 are again depicted with
the desired ablation line 64 of FIG. 6A. A plurality of lesions
112a through 112e are depicted as having been formed along the
desired ablation line 64 and in a sequence depicted by a trace line
114. Trace segments 114a and 114b, depicted between lesions
112c/112d and 112d/112e, respectively, depict that lesions 112c,
112d and 112e were formed sequentially but not with continuity.
Measured arc lengths 116, depicted individually as 116a and 116b,
represent the distances between the centers of lesions 112c/112d
and 112d/112e, respectively, along the desired ablation line 64.
Reference arc lengths 118, depicted individually as 118a and 118b,
are also depicted adjacent the measured arc lengths 116a and 116b,
respectively.
[0125] With distance-based accounting of the jump index JI, a
"jump" occurs when the arc distance between consecutively formed
lesions along a desired isolation line exceeds a predetermined
distance. In the depiction of FIG. 9A, the reference arc length 118
represents predetermined lengths upon which incrementing of the
jump index JI is based. If the measured arc length 116 between
consecutive lesions exceeds the respective reference arc length
118, the jump index JI is incremented. If the measured arc length
116 between consecutive lesions does not exceed the respective
reference arc length 118, the jump index JI is not incremented. By
this methodology, the trace segment 114a represents a jump that
causes the jump index JI to be incremented because measured arc
length 116a is greater than reference arc length 118a. In contrast,
the trace segment 114b does not represent a jump that would cause
the jump index JI to be incremented because measured arc length
116b is less than reference arc length 118b. The reference arc
lengths 118a and 118b can be of varying length dependent on
location on the desired ablation line 64, or they can be of the
same length.
[0126] In one embodiment, the jump index JI can incur multiple
increments from a single jump according to a ratio of the measured
arc length 116 to the respective reference arc length 118. For
example, if arc length 116a is 1.6 times longer than reference arc
length 118a, the jump index could be simply the ratio (i.e., 1.6),
or rounded down to the base integer (i.e., 1) or rounded to the
nearest integer (i.e., 2). Other incrementing schemes can be
developed based on the length of the measured arc lengths 116,
location relative to the pulmonary veins, or other observations
garnered from lesion formation data.
[0127] In one embodiment of distanced-based accounting, the jump
index JI is incremented until the maximum arc length between any
two lesions along the desired isolation line is less than the
predetermined arc length. In another embodiment, a hybrid between
the zone-based and the distance-based accounting techniques can be
implemented. For example, jumps can be detected in accordance with
distance-based accounting until at least one lesion is formed in
all of the zones of a zone-based segmentation.
[0128] In FIG. 9B, the same lesions 112a-112e and trace line 114 is
overlaid on the zone-segmented scheme of FIG. 7A to contrast the
distance-based accounting technique with the zoned accounting
technique. The reference arc lengths 118a and 118b represent the
same length as one of the zones X and XI of FIG. 9B. Yet the zoned
accounting method would not result in an incrementing of the jump
index JI because none of the zones are entirely passed over between
lesions.
[0129] Referring to FIG. 10, a force sensing catheter-based point
contact ablation system 120 is depicted in an embodiment of the
invention. The system 120 comprises a force sensing catheter
assembly 122 operatively coupled to a data acquisition and
processing unit or control system 124, a power source 126 and an
infusion pump 128. The catheter assembly 122 may include a handle
portion 132 operatively coupled with an elongate, flexible catheter
134 having a proximal portion 136 and a distal portion 138. The
catheter assembly 122 may also include a digital memory device 154
for storage of calibration parameters specific to the force sensor
142 and coupled to the control system 124 via a computer cable
156.
[0130] The distal portion 138 includes a contact ablation probe or
ablation head 144 operatively coupled with a force sensor 142 and a
position sensor/emitter 143. The ablation head 144 may comprise one
or more electrodes operatively coupled to the power source 126 via
a power cable 146. The ablation head 144 may also include one or
more temperature sensors 150. The force sensor 142 is adapted to
output a signal in response to a contact force exerted on the
ablation head 144. Signals from the force sensor 142 and
temperature sensor 150 (when present) may be routed to the control
system 124 via instrumentation cabling 152.
[0131] The position sensor/emitter 143 represents various forms of
three-dimensional position sensing available in the art. Examples
of such sensing and/or emitting devices that are operatively
coupled to the ablation head 144 includes: electromagnetic mapping,
such as the Aurora system marketed and sold by NDI of Waterloo,
Ontario, Canada; electric mapping, such as the EnSite Velocity
system marketed by St. Jude Medical of St. Paul, Minn., U.S.A.;
fluoroscopic imaging; ultrasound echo techniques; magnetic
resonance imaging (MRI) techniques; fiber optic shape and position
sensing. Such systems are known in art and provide the capability
of locating the position of the ablation head in three-dimensional
space. Certain positioning systems (e.g., fiber optic shape and
position systems) can provide three-dimensional position
information from the position sensor 143 to the control system 124
via the instrumentation cabling 152 (see, e.g., U.S. patent
application Ser. No. 12/127,657, filed 27 May 27 2008, now U.S.
Pat. No. 8,622,935, issued 7 Jan. 2014, assigned to the assignee of
the instant application, and hereby incorporated by reference
herein in its entirety except for express definitions contained
therein). Other systems (e.g., MRI and fluoroscopic imaging) may
require a receiver 145 operatively coupled to receive signals
actively emanating from the position emitter 143, or a receiver 145
responding to signals passively reflected from or transmitted
through or past the position emitter 143 (e.g., transesophageal
echo). In such systems, the receiver 145 is configured to send
information regarding the spatial position of the ablation head 144
to the control system 124.
[0132] The control system 124 may include an analog-to-digital
(A/D) converter 160, a force conversion module or force signal
conditioning system 162 and a controller or processor 164, all of
which may be operatively coupled to an interface 166. In other
embodiments, communication with the control system can be done
through a communication bus such as a RS-485 bus, an Ethernet bus
or a wireless connection. The interface 166 may include connection
for the various cabling 146, 152, 156 from the force sensing
catheter assembly 122, and may also be operatively coupled to a
tare or zero reset for zeroing the force sensor 142. The processor
164 may include or have access to a storage medium 168 that
contains programming instructions 170 to be carried out by the
processor 164. The processor 164 may also control and log data from
the force signal conditioning system 162, and may also communicate
with the A/D converter 160 via a communications cable 172, such as
a RS-422 cable. In one embodiment, the power source 126 is equipped
with an output controller 173 operatively coupled to the processor
164 via a control line 174 for computer control of the power
output. One or more displays 176 can act as a receiving device(s)
that receives instructions and other real time information from the
processor 164, for example for conveying the information to an
operator controlling the flexible catheter 134. A non-limiting
example of the rate at which information is logged by the processor
164 is approximately 60-Hz. A non-limiting example of the rate at
which the displays are updated is approximately 10-Hz.
[0133] Force sensing can be achieved with strain sensors or
distance/displacement sensors that sense the movement of a
deformable body. Strain sensors include common resistive strain
sensors, piezoelectric and piezoresistive elements and MEMS
sensors. Distance sensors include capacitive, inductive and optical
sensor technologies. For example, certain distance sensors utilize
a single magnetic emitter opposite three pickup coils to measure
the local intensity changes at each coil and therefore the strain
on the body.
[0134] Generally, the force signal conditioning system 162
comprises equipment for driving or sourcing the sensing element or
elements of the force sensor 142 and/or digitizing or monitoring an
output of the force sensor 142. For example, if the force sensor
142 implements foil-type strain gauges in a Wheatstone bridge
configuration, the force signal conditioning system 162 may include
an excitation source, a signal conditioner for conditioning and
amplification of the output of the Wheatstone bridge, and an A/D
converter (not depicted). The force signal conditioning system 162
may also include firmware that converts the digitized output into
engineering units (e.g., newtons, pounds-force or grams-force).
Alternatively, the digital signal may be converted to engineering
units by the processor 164.
[0135] In one embodiment, the force sensor 142 comprises one or
more fiber optic strain elements, such as fiber Bragg grating(s) or
Fabry-Perot resonator(s). In this embodiment, the instrumentation
cabling 152 includes fiber optic cables and the force signal
conditioning system 162 comprises a fiber optic interrogator, such
as the Micron Optics model is SM125 (for fiber Bragg grating
interrogation) and the FISO model FCM (for Fabry-Perot
interrogation).
[0136] A current detector 180 may be operatively coupled with the
power cable 146 for detection of the electrical current flowing to
the ablation head 144. The current detector 180 may be operatively
coupled to the A/D converter 160 for processing by the processor
164. In one embodiment, the current detector 180 comprises a
conductive coil surrounding the power cable 146 which produces an
output signal 182 proportional to the magnetic field generated by
the AC current passing through the power cable 146.
[0137] In one embodiment, a robotic manipulator 184 can be
operatively coupled with the force sensing catheter assembly 122.
The robotic manipulator 184 acts as a receiving device for
controlling the flexible catheter 134. In one embodiment, the
robotic manipulator 184 is a stand-alone device operatively coupled
to a local microprocessor controller 186, which receives
instructions from a user via a local interface 187, and/or from the
processor 164 (FIG. 10). Alternatively, the robotic manipulator 184
can be integrated with the system 120, responding to instructions
directly from the processor 164, which may eliminate the need for a
separate microprocessor controller and attendant interface.
[0138] Functionally, the force sensor 142 and the current detector
180 and/or the output controller 173 can provide contact force F,
energization parameter E and time duration t information that can
be utilized by the processor 164 to calculate the lesion size index
LSI (i.e., the LDI, LWI and/or LVI), from which lesion size
information can be calculated and displayed on the display(s) 176.
The three-dimensional position information provided to the control
system 124 to calculate the position of the next ablation for
display on the display(s) 176. The three-dimensional position
information can also be utilized when tracking the jump index JI.
In one embodiment, the display(s) 176 can include both output from
a particular visualization system being utilized during the
procedure (e.g., fluoroscopy or transesophageal echo) along with a
computer-generated three-dimensional image reflecting the position
and control information determined by various embodiments of the
present invention. In another embodiment, a display 176 can present
a combined or overlaid set of images of the visualization system
output together with the positional and control information
provided by various embodiments of the present invention.
[0139] The robotic manipulator 184 can be made to respond to the
commands of the local microprocessor controller 186 to control the
movement of the catheter 134 and the magnitude of any subsequent
reaction force exerted on the ablation head 144. The movement may
be the controlled parameter in a closed loop control scheme, and
the force measured by the force sensor 142 the feedback
measurement. A desired force set point or desired force interval
set point may be provided to the local microprocessor controller
186 by an operator via the local interface 187 or via the processor
164.
[0140] Referring to FIGS. 11A and 11B, a variable reference line
method for forming an isolation line 202 is depicted and described
in an embodiment of the invention. In FIG. 11A, the left pulmonary
veins (LSPV 54, LIPV 56) are depicted as surrounded by a
predetermined desired ablation line 204 and a plurality of lesions
206. The variable reference line method involves establishing a
desired location for a first lesion 206a that is on the desired
ablation line 204. However, for a variety of reasons, the actual
location of the lesion 206a may not be in perfect alignment with
the prescribed location or be centered on the desired ablation line
204. These reasons include the dynamic nature of the target tissue
(a beating heart), operator experience, etc.
[0141] After formation of the first lesion, a desired location for
each subsequent lesion of the plurality of lesions 206 can be
determined by extrapolating from the actual location of a center
210 of the most recently formed lesion 206i (rather than along the
desired ablation line 204), as depicted in FIG. 11B. The
extrapolation can be performed by locating where the most recently
formed lesion 206i is with respect to the desired ablation line
204. This can be done by determining an intersection point 212 of a
line 214 that passes through the center 210 of the most recently
formed lesion 206i and intersects the desired ablation line 204 at
a right angle 216. A slope 220 of the desired ablation line 204 at
the intersection point 212 can then be determined. A projection
line 222 can then be extrapolated from the center 210 of the most
recently formed lesion 206i at the same slope 220, along which a
center 224 of a desired location of the next lesion 206j to be
formed is located. A distance 226 between the center 210 of the
most recently formed lesion 206i and the center 224 of the desired
location of the next lesion 206j can be established that provides
reasonable assurance that the next lesion 206j will overlap with
the most recently formed lesion 206i. For example, the distance 226
can be set at some fraction f of the expected diameter D of the
lesions being formed (e.g., f=0.75).
[0142] The extrapolation technique of the variable reference line
method continues around the ipsilateral pulmonary veins until
hopefully an isolation line is formed. Preferably, the plurality of
lesions 206 remain in close proximity to the desired ablation line
However, there may be instances where the actual isolation line 202
is biased in one direction (e.g., radially outward, as depicted in
FIG. 11A) relative to the desired ablation line 204. In this case,
the lesions 206a-206z will not form a closed isolation line, but
instead forms an open isolation line, as depicted in FIG. 11A.
[0143] In FIG. 11A, it can be determined that lesion 206z would
have closed the isolation line if it, along with the other lesions
206, were in closer proximity to the desired ablation line 204.
That lesion 206z should have closed the line can be determined by
checking whether its respective intersection point 212 with the
desired ablation line 204 is within a diameter D of the first
lesion 206a. At that point, if the actual location of the center of
lesion 206z is more than a distance D away from the center of
lesion 206a, a straight line 230 is established between lesions
206z and 206a and the desired location of a supplemental lesion 207
is established along the straight line at a distance off D from the
center of lesion 206z. Lesions can prescribed along the straight
line 230 until closure of the isolation line is obtained.
[0144] It is noted that, herein, "206z" does not denote a certain
numbered lesion--e.g., lesion #26--but rather denotes the last
lesion formed before implementation of the straight line 230. Also,
lesion(s) 207 denote lesions that are formed that are targeted for
the straight line 230.
[0145] Referring to FIG. 11C, the calculation of the straight line
230 can be established before arriving at the location of expected
closure of the isolation line. That is, projection of the straight
line 230 and the formation of lesions thereabout can be started
upon reaching or first exceeding a predetermined location 244 on
the desired ablation line, thus making the transition to closure of
the isolation line 202 less tortuous.
[0146] To aid the operator in performing the various lesion
patterns depicted in FIG. 6 and FIG. 7, various steps of the
variable reference line method described above can be included in
the programming instructions 170 of the control system 124 for
access by the processor 164.
[0147] Referring to FIGS. 12A-12D, a fixed reference line method
for forming an isolation line is presented in an embodiment of the
invention. In FIG. 12A, a PV wall 252 is depicted upon which a
desired ablation line 254 is ascribed. A desired location 256 for a
first lesion 258a is established, centered on the desired ablation
line 254. An actual location 260 of the first lesion 258a is
measured during the ablation. The actual location 260 may differ
from the desired location 256.
[0148] In FIG. 12B, establishment of a desired location 264 for a
second lesion 258b is depicted. A desired location of a subsequent
lesion can be determined by calculating where a lesion of an
anticipated diameter centered on the desired ablation line 254
would overlap the previously formed lesion. In one embodiment, an
estimated width or diameter of the previously formed lesion can
also be inferred, for example using the lesion width index LWI of
Eq. (15), and the position along the desired ablation line 254
established based on the width estimate. By this methodology, the
desired location 264 is again centered on the desired ablation line
254, and centered so as to overlap with the first lesion 258a by a
predetermined amount if properly placed and formed to the desired
size.
[0149] In FIGS. 12C and 12D, the second lesion 258b is depicted
after formation as having an actual location 266 that is
substantially out of alignment with the desired location 264. A
desired location 270 of a third lesion is calculated based on the
actual location 266 of the second lesion 258b.
[0150] If a lesion is formed at an actual location that is centered
away from the desired ablation line 254 by a dimension that exceeds
the expected diameter of ablation, there is no calculated overlap
between that lesion and a subsequent lesion located on the desired
ablation line 254, and the continuity of the ablation line becomes
questionable. In one embodiment of the invention, the previously
formed lesion can be ignored and the desired location of the
previously formed lesion reestablished as the desired location of
the next subsequent lesion. In other embodiments, a line between
the previously formed lesion and the desired ablation line 254 can
be established, and lesions formed along this line until the lesion
pattern is again in contact with the desired ablation line 254.
[0151] Referring to FIG. 13, a flow chart 280 depicting certain
aspects of the variable reference line method and the fixed
reference line method is presented in embodiments of the invention.
In certain embodiments, the actual locations of the various lesions
(e.g., 260 and 266 in FIGS. 12A and 12C, respectively) are
measured, for example, using the position sensor/emitter 143 of
FIG. 10 to measure the location of the ablation head 144 during
energization. Other parameters (e.g., contact force F, energization
(e.g. electrical current I), and duration of time t) can also be
measured and utilized by the central control system 124. The
various steps for assisting an operator in performing the methods
can also be included as programming instructions 170 for access by
the processor 164.
[0152] Initially, an origin of the desired isolation line to be
formed (e.g., line 62, 64, 70, 72, 73, 74, 76, 78 and 80 of FIGS.
6A through 6D) is identified in three-dimensional space (step 282).
In one embodiment, the physician utilizes the visualization system
to identify a present location of the ablation head 144 relative to
the anatomy of the pulmonary vein (PV) or any other reference point
on the heart of the patient and then correlates that location with
a corresponding location in a suitable three-dimensional model. The
three-dimensional model can be utilized by the processor 164 to
determine the positional and control information for creating the
desired isolation line. In one embodiment, the three-dimensional
model is a generic model of a pulmonary vein that is maintained by
the processor 164 and memory 170. In another embodiment, the
three-dimensional model is an anatomical reconstruction of the
pulmonary vein of the particular patient that may be loaded into
the processor 164 and memory 170. In a further embodiment, a
four-dimensional animated version of the anatomical model may be
utilized to reflect positional movement of the PV in response to
heart beats. Optionally, the patient's ECG may be used as an input
for such a four-dimensional model to correlate the expected motion
of the PV in response to the heart beat of the patient.
[0153] The desired location of a first lesion (e.g., the desired
location of lesion 206a of FIG. 11A or desired location 256 of FIG.
12A), located on the respective desired isolation line, is
identified at step 284 in accordance with any of the various
embodiments previously described. The processor 164 then instructs
the operator/robotic manipulator 184 to move the distal portion 138
of the flexible catheter 134 to position the ablation head 144 at
the desired location (step 286).
[0154] During the positioning of the ablation head 144, the
position of the ablation head 144 can be tracked by actively
utilizing the position sensor/emitter 143 (step 288). The movement
of the position sensor/emitter 143 (and therefore the position of
the ablation head 144) can be tracked by the processor 164 and
updated to the display(s) 176 by the processor 164.
[0155] The instruction to the operator/robotic manipulator 184 as
well as the notifications and updates regarding movement or
positioning of the ablation head 144, can be presented on the
display 176 by the processor 164, for example, notifying the
operator/robotic manipulator 184 that the ablation head 144 is
within an acceptable range or tolerance of the desired location for
the lesion to be formed (step 288). Various visual presentations
can be utilized to convey the existing and desired locations of
lesions along the isolation line that display different information
in different colors and/or overlays of information. The instruction
and notification can also be performed audibly, such as by a voice
instruction or a beeping sound.
[0156] During the tracking of the movement of the ablation head,
the processor 164 can also continuously monitor whether
energization of the ablation head 144 has been initiated (steps 288
and 290 within loop 291). Upon energization, the processor 164 can
go into a data acquisition and display mode, represented by loop
293. In the data acquisition and display mode, the force F,
energization parameter E, duration time t of energization acquired
(step 292) and used in the calculation and display of the lesion
size index LSI and/or force-time integral FTI (step 294). The
processor 164 can also store the acquired information (e.g., F, E,
t and position) to electronic memory (step 296), such as (but not
limited to) storage medium 168. In one embodiment, the processor
164 remains in the data acquisition and display mode 293 until
energization of the ablation head ceases (step 298).
[0157] The data acquisition and display mode 293 is thus exited
after the lesion is formed. In one embodiment, the position of the
so-formed lesion can be determined (step 300), for example, by
averaging the position data acquired during the duration of the
lesion formation. In one embodiment, a determination is made
whether there was a "jump" between the last two consecutively
formed lesions (step 302). The step 302 can implement, for example,
the zoned accounting methods or the distance-based accounting
methods described above for tracking the jump index JI. In the
event that a jump occurred, the processor 164 can increment the
jump index and record it to the storage medium 168.
[0158] The processor 164 can also determine whether the isolation
line is complete (step 306), for example by implementation of the
variable or the fixed reference line methods outlined above. If it
is determined that the isolation line is not complete, the desired
position of the next lesion to be form can be determined (step
308). This determination can also be in accordance with the
methodology described in the variable or the fixed reference line
methods outlined above.
[0159] Upon completion of the prescribed ablation lines, the
automatic generation of control information can be ceased (step
310).
[0160] Each of the features and methods disclosed herein may be
used separately, or in conjunction with other features and methods,
to provide improved devices, systems and methods for making and
using the same. Therefore, combinations of features and methods
disclosed herein may not be necessary to practice the invention in
its broadest sense and are instead disclosed merely to particularly
describe representative embodiments of the invention.
[0161] For purposes of interpreting the claims for the present
invention, it is expressly intended that the provisions of Section
112, sixth paragraph of 35 U.S.C. are not to be invoked unless the
specific terms "means for" or "steps for" are recited in the
subject claim.
* * * * *