U.S. patent application number 09/854982 was filed with the patent office on 2002-04-11 for method for non-synchronous laser-assisted myocardial revascularization.
Invention is credited to Giba, Jeffrey J., Harman, Stuart D., Kinley, Fred G., Mueller, Richard L., Murphy-Chutorian, Douglas, Roush, Mark.
Application Number | 20020042639 09/854982 |
Document ID | / |
Family ID | 27111852 |
Filed Date | 2002-04-11 |
United States Patent
Application |
20020042639 |
Kind Code |
A1 |
Murphy-Chutorian, Douglas ;
et al. |
April 11, 2002 |
Method for non-synchronous laser-assisted myocardial
revascularization
Abstract
A method of selecting laser parameters for performing
laser-assisted myocardial revascularization to avoid inducing
undesired cardiac arrhythmia without synchronization of delivery of
laser energy and the patient's cardiac cycle, the method comprising
the steps of minimizing the power level of laser energy used,
thereby decreasing the overall trauma to the heart, selecting a
pulse frequency as great as possible while avoiding adverse
summation effects, selecting a pulse width as wide as possible to
prevent excessively high peak powers per pulse and not so wide as
to cause undesired thermal damage, selecting an energy flux rate,
shaping the front end of each pulse of laser energy to provide
efficient, non-explosive TMR channeling, and correcting the
selected power level, pulse width, pulse frequency and energy flux
rate for mechanical events, including method of access to the
heart, position of selected portions of myocardium in the heart,
temporal duration of the procedure, natural movement of the heart,
specific heart geometry, pre-existing heart arrhythmia and other
factors causing a predisposition to heart arrhythmia. A method for
performing laser-assisted transmyocardial revascularization (TMR)
and percutaneous transluminal myocardial revascularization (PTMR)
using such laser energy with parameters selected to avoid inducing
undesired cardiac arrhythmia, the method comprising the steps of
generating laser energy having a predetermined non-square wave
shape, a predetermined wavelength, a predetermined energy flux and
a predetermined power level, and delivering the laser energy in a
plurality of pulses, the plurality of pulses having a predetermined
pulse frequency and a predetermined pulse width, to selected
portions of myocardium to form TMR and PTMR channels without
synchronizing delivery of the laser beam with the cardiac
cycle.
Inventors: |
Murphy-Chutorian, Douglas;
(Palo Alto, CA) ; Mueller, Richard L.; (Byron,
CA) ; Harman, Stuart D.; (San Jose, CA) ;
Giba, Jeffrey J.; (Marina Valley, CA) ; Roush,
Mark; (Los Gatos, CA) ; Kinley, Fred G.;
(Mission Viejo, CA) |
Correspondence
Address: |
ILENE JANOFSKY
Eclipse Surgical Technologies
1049 Kiel Court
Sunnyvale
CA
94085
US
|
Family ID: |
27111852 |
Appl. No.: |
09/854982 |
Filed: |
May 14, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09854982 |
May 14, 2001 |
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09312128 |
May 14, 1999 |
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09312128 |
May 14, 1999 |
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08904222 |
Jul 31, 1997 |
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5931834 |
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08904222 |
Jul 31, 1997 |
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08729325 |
Oct 15, 1996 |
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5785702 |
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Current U.S.
Class: |
607/89 |
Current CPC
Class: |
A61B 18/24 20130101;
A61B 2017/00703 20130101; A61B 2018/00392 20130101; A61B 2017/00247
20130101 |
Class at
Publication: |
607/89 |
International
Class: |
A61N 001/00 |
Claims
We claim:
1. A method for performing transmyocardial revascularization (TMR)
with laser energy having parameters selected to avoid cardiac
arrhythmia, the method comprising the following steps, in
combination: determining a wavelength of the laser energy from a
laser selected to perform transmyocardial revascularization; using
the wavelength determination to select parameters for the laser
energy to produce a non-square wave shape; generating the laser
energy at the determined wavelength with the selected parameters to
produce the non-square wave shape; and delivering the generated
laser energy in one or more pulses to selected portions of heart
tissue to perform transmyocardial revascularization in myocardium
without inducing cardiac arrhythmia and without synchronizing
delivery of the laser energy to a cardiac cycle.
2. The method of claim 1 wherein the selected parameters are power
level, energy flux, pulse width, and pulse frequency.
3. The method of claim 2 wherein the laser energy has a wavelength
of between about 1.8 and about 2.2 microns, an energy flux of about
1.78 J/square millimeter and a power level of at least about 6
watts, the laser energy being delivered with a pulse frequency of
at least about 5 Hertz and a pulse width of between about 150 and
about 350 millisecond, the laser energy as delivered causing about
5 millimeters or less lateral necrosis surrounding a
transmyocardial treatment site.
4. The method of claim 3 wherein the laser energy is generated by a
Holmium:YAG laser.
5. The method of claim 2 wherein the laser energy has a wavelength
of about 0.308 microns, a power level of about 2 watts and an
energy flux of between about 2 and about 8 J/square millimeter, and
is delivered with a pulse frequency of between about 5 and about 25
Hertz and a pulse width of about between about 10 and about 200
microseconds, and causes about 5 microns lateral necrosis
surrounding the TMR channel produced thereby.
6. The method of claim 5 wherein the laser energy is generated by a
Xe:CI excimer laser.
7. The method of claim 2 wherein the laser energy has a wavelength
of about 10.6 microns, an energy flux of about 51 J/square
millimeter and a power level at least about 800 watts, is delivered
in a single pulse about of 0.03 seconds and can be gated, and
causes between about 0.05 to about 0.2 millimeters lateral necrosis
surrounding a TMR channel produced thereby.
8. The method of claim 7 wherein the laser energy is generated by a
CO.sub.2 laser.
9. The method of claim 2 wherein the laser energy has a wavelength
of between about 0.488 and about 0.514 microns, an energy flux of
between about 1.3 and about 12.74 J/square millimeter and a power
level at least between about 1 and about 10 watts, is delivered in
a single pulse, and causes approximately 4 millimeters lateral
necrosis surrounding a TMR channel produced thereby.
10. The method of claim 9 wherein the laser energy is generated by
an Argon laser.
11. The method of claim 2 wherein the laser energy has a wavelength
of about 1.06 microns, an energy flux of between about 9.5 and
about 13 J/square centimeter and a power level at least between
about 2 and about 100 watts, is delivered with a pulse frequency of
between about 1 and about Hertz and a pulse width of about 10
nanoseconds, and causes at least about 15 millimeters lateral
necrosis surrounding a TMR channel produced thereby.
12. The method of claim 11 wherein the laser energy is generated by
an Nd:YAG laser.
13. The method of claim 2 wherein the laser energy has a wavelength
of about 2.94 microns, an energy flux of between about 50 and about
500 J/square millimeter, is delivered with a pulse frequency of
between about 1 and about 15 Hertz and a pulse width of between
about 1 and about 250 microseconds, and causes about 0.1
millimeters lateral necrosis surrounding a TMR channel produced
thereby.
14. The method of claim 13 wherein the laser energy is generated by
an Er:YAG laser.
15. The method of claim 1 wherein the laser energy is delivered to
the selected portions of heart tissue using a catheter apparatus
with laser delivery means, the method further comprising the
following steps: introducing the catheter apparatus with laser
delivery means percutaneously into the vasculature of the patient;
and positioning the laser delivery means at the endocardial surface
of the selected portions of heart tissue.
16. The method of claim 1 wherein the laser energy is delivered to
the selected portions of heart tissue in a surgical procedure using
laser delivery means, the method further comprising the following
steps: surgically accessing the selected portions of heart tissue;
and positioning the laser delivery means at an epicardial surface
of the heart tissue.
17. The method of claim 1 wherein assess to the selected portions
of heart tissue is achieved from inside a coronary artery.
18. The method of claim 15 further including the following step:
mechanically piercing the endocardial surface adjacent the selected
portions of heart tissue prior to delivering the laser energy into
the myocardium.
19. The method of claim 16 further including the following step:
mechanically piercing the epicardial surface adjacent the selected
portions of heart tissue prior to delivering the laser energy into
the myocardium.
20. A method of percutaneous transluminal myocardial
revascularization of a patient's myocardium for relieving angina
and avoiding cardiac arrhythmia, comprising the steps of: inserting
a guidable elongated flexible lasing apparatus encompassing an
optical fiber into a patient's vasculature; guiding a distal end of
the lasing apparatus to an area within the patient's heart;
directing the distal end of the lasing apparatus to an area within
the heart to be revascularized; providing a Holmium:YAG laser
source operating at around 5 Hz with a laser pulse width between
100-250 milliseconds, and approximately 0.7-0.9 Joules/mm.sup.2
emitted from the distal end of the lasing apparatus; and
irradiating an inner wall of the heart with the no more than five
laser pulses to form at least one channel by the method whereby the
channel extends into myocardial tissue without epicardial
penetration.
21. The method of claim 20 wherein the step of irradiating the
inner wall of the heart uses the optical fiber that is between
600-1100 microns in diameter and pulsation number is between 2-5
pulses with an average laser power of 3.5 watts.
22. The method of claim 20 wherein the step of directing the distal
end of the lasing apparatus to an area within the patient's heart
comprises placement of the optical fiber end at the endocardial
wall prior to the step of laser irradiation.
23. The method of claim 21 wherein the step of directing the distal
end of the lasing apparatus to an area within the patient's heart
comprises placement of the optical fiber end at the endocardial
wall prior to the step of laser irradiation.
24. The method of claim 20 wherein the step of directing the distal
end of the lasing apparatus to an area within the patient's heart
comprises the step of piercing the optical fiber end partially
through the endocardial wall prior to the step of laser
irradiation.
25. The method of claim 21 wherein the step of directing the distal
end of the lasing apparatus to an area within the patient's heart
comprises the step of piercing the optical fiber end partially
through the endocardial wall prior to the step of laser
irradiation.
26. The method of claim 20 wherein the step of directing the distal
end of the lasing apparatus to an area within the patient's heart
comprises having the optical fiber end pierce totally through the
endocardial layer prior to the step of laser irradiation.
27. The method of claim 21 wherein the step of directing the distal
end of the lasing apparatus to an area within the patient's heart
comprises having the optical fiber end pierce totally through the
endocardial wall prior to the step of laser irradiation.
28. The method of claim 20 wherein the step of directing the distal
end of the lasing apparatus to an area within the patient's heart
comprises having the optical fiber end pierce totally through the
endocardial layer and part way into the myocardial tissue and the
average power level setting is reduced prior to the step of laser
irradiation.
29. The method of claim 21 wherein the step of directing the distal
end of the lasing apparatus to an area within the patient's heart
comprises having the optical fiber end pierce totally through the
endocardial layer and part way into the myocardial tissue and the
average power level setting is reduced prior to the step of laser
irradiation.
30. The method of claim 22 wherein the method further includes a
step of progressively advancing the optical fiber end between
successive laser pulses.
31. The method of claim 24 wherein the method further includes a
step of progressively advancing the optical fiber end between
successive laser pulses.
32. The method of claim 26 wherein the method further includes a
step of progressively advancing the optical fiber end between
successive laser pulses.
33. The method of claim 20 wherein the method further includes a
step of providing an ultrasound device for determining a position
of the fiber optical end with respect to a ventricle wall prior to
each laser pulse emission.
34. The method of claim 20 wherein the step of directing the distal
end of the lasing apparatus to an area within the heart to be
revascularized comprises engaging the optical fiber end by piercing
the endocardial wall surface with the optical fiber end.
35. A method of percutaneous transluminal myocardial
revascularization of a patient for relief from angina while
avoiding cardiac arrhythmia during the procedure, comprising the
steps of: inserting a guidable elongated flexible lasing apparatus
encompassing an optical fiber into the patient's vasculature;
guiding the distal end of the lasing apparatus to an area within
the patient's heart; directing the distal end of the lasing
apparatus to an area within the heart to be revascularized; and
irradiating an inner wall of the patient's heart using a
Holmium:YAG laser source with a single pulse of laser energy with a
laser pulse width between 100-250 milliseconds and approximately
1.8 Joules/mm.sup.2 emitted from the optical fiber's end of the
lasing apparatus and the optical fiber that is between 600-1100
microns in diameter; whereby a formed channel by the method extends
no more than approximately 3 mm into myocardial tissue without
epicardial penetration.
36. The method of claim 35 wherein the step of directing the distal
end of the lasing apparatus to an area within the heart to be
revascularized comprises engaging the optical fiber=s end by
piercing the endocardial wall surface with the optical fiber's
end.
37. A method of percutaneous transluminal myocardial
revascularization of a patient's heart for relieving angina and
avoiding cardiac arrhythmia, comprising the steps of: a) inserting
a guidable elongated flexible lasing apparatus encompassing an
optical fiber with a distal end into a patient's vasculature; b)
guiding the distal end of the lasing apparatus to an area within
the patient's heart; c) directing the distal end of the lasing
apparatus to an area within the heart to be revascularized; d)
controlling a distance the lasing apparatus may be advanced into
heart tissue; e) providing a Holmium; YAG laser source operating at
around 5 Hz with a laser pulse width between 100-250 milliseconds;
f) selecting a laser power level emitted from the distal end
dependent upon a controlled distance of the distal end of the laser
apparatus in step d); g) selecting a number of laser pulses based
upon the controlled distance in step d); and h) irradiating an
inner wall of the heart and forming a channel into myocardial
tissue without epicardial wall penetration.
38. The method of claim 37 wherein the method further includes a
step of providing an ultrasound device for determining a position
of the distal end with respect to a ventricle wall prior to each
laser pulse.
39. A method of percutaneous transluminal myocardial
revascularization of a patient's heart for relieving angina and
avoiding cardiac arrhythmia, comprising the steps of: a) inserting
a guidable elongated flexible lasing apparatus encompassing an
optical fiber with optical distal end into the patient's
vasculature; b) guiding the distal end of the lasing apparatus to
an area within the patients heart; c) directing the distal end of
the lasing apparatus to an area within the heart to be
revascularized; d) controlling a distance the distal end may be
advanced into heart tissue; e) providing an excimer laser source
operating at around 15 Hz with a laser pulse width where the distal
end can emit radiation of at least about 50 mJ/mm.sup.2, f)
selecting a laser power level emitted from the distal end of the
lasing apparatus dependent upon a controlled distance of the laser
apparatus in step d); g) selecting a number of laser pulses based
upon the controlled distance in step d); and h) irradiating the
heart's inner wall to form a channel into myocardial tissue without
epicardial wall penetration.
40. The method of claim 38 wherein the number of pulses is less
than ten and the optical fiber's distal end is approximately 1 mm
in diameter.
41. A method of selecting laser parameters for performing
laser-assisted percutaneous transluminal revascularization (PTMR)
to avoid cardiac arrhythmia and without synchronization of delivery
of laser energy to a patient's cardiac cycle, the method comprising
the following steps, in combination: selecting a minimum power
level of laser energy to be used, the minimum power level being
sufficient to ablate heart tissue; setting a pulse frequency as
great as possible and selected to avoid summation effects; setting
a pulse width as long as possible and selected to prevent
excessively high peak power without causing undesired levels of
thermal damage during PTMR; shaping a front end of each pulse of
laser energy to provide non-linear pulses to avoid cardiac
arrhythmia during PTMR; and correcting the selected power level,
pulse width, pulse frequency, and shaping for limiting the depth of
a formed channel to no more than half the wall thickness of a
patient's myocardium.
Description
RELATED APPLICATIONS
[0001] This application is a continuation-in-part of U.S. Ser. No.
08/729,325 entitled A METHOD FOR NON-SYNCHRONOUS LASER-ASSISTED
TRANSMYOCARDLAL REVASCULARIZATION, filed Oct. 15, 1996,
incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention relates to a procedure known as
laser-assisted myocardial revascularization, and more particularly,
to improved methods for revascularization of the heart by creating
a plurality of small pathways or channels through predetermined
portions of the heart using laser energy delivered via a laser
delivery means according to specific parameters, including variable
frequency, and without requiring synchronization of laser energy
delivery with the beating of the heart.
BACKGROUND OF THE INVENTION
[0003] Much of the heart consists of a special type of muscle
called myocardium. The myocardium requires a constant supply of
oxygen and nutrients to allow it to contract and pump blood
throughout the vasculature. One method of improving reduced
myocardial blood supply is called transmyocardial revascularization
(TMR), the creation of pathways or channels into the myocardium
generally from either an outer epicardial surface of the heart in a
surgical procedure or from an inner endothelium cell covered
surface of a heart's endocardium chamber in a percutaneous
transluminal myocardial revascularization (PTMR).
[0004] A procedure using needles in a form of myocardial
acupuncture was used clinically in the 1960s. Deckelbaum. L. I.,
Cardiovascular Applications of Laser Technology, Lasers in Surgery
and Medicine 15:315-341 (1994). The technique was said to relieve
ischemia by allowing blood to pass from the ventricle through the
channels either directly into other vessels perforated by the
channels or into myocardial sinusoids which connect to the
myocardial microcirculation. These sinusoidal communications vary
in size and structure, but represent a network of direct
arterial-luminal, arterial-arterial, arterial-venous, and
venous-luminal connections. Interest in myocardial acupuncture or
boring, which mechanically displaces or removes tissue, decreased
when it was discovered that the mechanically created channels
closed because of acute thrombosis followed by organization and
fibrosis of clots.
[0005] By contrast, recent histological evidence of patent,
endothelium-lined tracts within pathways created with laser energy
supports the assumption that the lumen of the laser pathways is or
can become hemocompatible and resist occlusion caused by
thrombo-activation and/or fibrosis. A thin zone of charring occurs
on the periphery of the laser-created transmyocardial channels
through the well-known thermal effects of optical radiation on
cardiovascular tissue. This type of interface may inhibit the
immediate activation of the intrinsic clotting mechanisms because
of the inherent hemocompatibility of carbon. In addition, the
precise cutting action that results from the high absorption and
low scattering of laser energy (CO.sub.2, Ho, etc.) may minimize
structural damage to collateral tissue, thus limiting the tissue
thromboplastin-mediated activation of extrinsic coagulation. Recent
histological studies show that both patent and non-patent channels
promote growth of an alternate circulation, one of the mechanisms
believed to be beneficial following the procedure.
[0006] Despite the creation of patent channels and pathways with
lasers, there are reported problems associated with laser TMR
procedures. Such problems can include channel closure which may be
caused by selection and use of TMR laser parameters which do not
produce channels with the characteristics detected in the
histological evidence discussed above. An additional reported
problem encountered in TMR procedures is adverse effects created by
the laser on the diseased hearts of TMR patient's.
[0007] U.S. Pat. No. 4,658,817 issued Apr. 21, 1987 to Hardy
teaches a method and apparatus for TMR using a surgical CO.sub.2
laser including a handpiece for directing a laser beam to a desired
location. Hardy suggests that the creation of TMR channels using a
laser may affect contractility of the heart and states that the
number of perforations may have to be limited accordingly.
[0008] Two subsequent patents, U.S. Pat. Nos. 5,380,316 issued Jan.
10, 1995 and 5,389,096 issued Feb. 14, 1995 both to Aita et al.,
discuss in general methods for intra-operative and percutaneous
myocardial revascularization, respectively. Both patents suggest
synchronization of the laser with the heart beat is necessary to
avoid arrhythmias. PCT WO 96/35469 issued Nov. 14, 1996 to Aita et
al. also discusses apparatus and general methods for percutaneous
myocardial revascularization synchronized with the heart beat to
avoid arrhythmias.
[0009] Synchronization of the laser energy delivery with the
beating of the heart was also considered an important tool in U.S.
Pat. No. 5,125,926 issued Jun. 30, 1992 to Rudko et al., reportedly
to reduce the chance of laser induced fibrillation. Rudko et al
teaches a heart-synchronized pulsed laser system for TMR. Utilizing
electrical sensing, the heart beat is monitored using an EKG
device. The device automatically delivers what appears to be a
square pulse of laser energy to the heart only in response to
electrical detection of a predetermined portion of the heartbeat
cycle.
[0010] The prior art discussed above suggests that at least some
pulsed laser systems and parameters are potentially damaging to the
beating heart or its action and may induce fibrillation or
arrhythmia, hence the need for heart synchronization to minimize
such effects.
[0011] U.S. Serial No. 08/729,325 entitled A METHOD FOR
NON-SYNCHRONOUS LASER-ASSISTED TRANSMYOCARDIAL REVASCULARIZATION,@
filed Oct. 15, 1996. The prior art discussed above suggests that at
least some pulsed laser systems and parameters are potentially
damaging to the beating heart or its action and may induce
fibrillation or arrhythmia, hence the need for heart
synchronization to minimize such effects.
[0012] An arrhythmia is a disturbed heart rhythm which often takes
over as the primary rhythm of the heart, as evidenced by a rapid
flutter or other rhythm of the heart muscle, which renders it
ineffective at pumping blood through the vasculature. The process
of delivering laser energy to tissue results in polarization of
individual cells of the heart in the area of delivery of the laser
energy. Polarization of the specialized conducting cells as well as
myocardial cells drives the action potential of cells resulting in
responsive contractile motion. Delivering laser energy can disrupt
the normal rhythm of the heartbeat since the cardiac rhythm can be
side-tracked to that of the polarized cells as opposed to
propagating through the heart along the normal path of the
impulse.
[0013] The heart's natural, primary pacemaker is found in a group
of cells called the sinoatrial or sinus node located near the
junction of the superior vena cava and. the right atrium. The
electrical impulse originates in the endocardium and propagates
through the myocardium to the epicardial surface. The electrical
impulse is conducted out of the sinus node to the atria, where it
stimulates atrial muscle cells to contract, and to the
atrioventricular node. Upon leaving the atrioventricular node, the
electrical impulse continues to propagate down the conducting
system to the bundle of His, into right and left branches thereof.
The right bundle spreads the electrical impulse to the right
ventricle and the left bundle branch propagates the impulse to
anterior and posterior positions in the left ventricle to reach the
Purkinje fibers. These small fibers form a rapid conduction network
through the myocardium to deliver the impulse to all of the
individual contractile muscle cells of the myocardium. The
electrical signal travels at different speeds at different parts of
the network. While electrical signals on the portion of the network
extending through the atria have been found to travel at velocities
of about 1 meter per second, these signals slow to about 0.2 m/s as
they pass through the atrioventricular node. Signal propagation
through the ventricular Purkinje network, however, is much
faster--approximately 4 m/s. Thus, the sinus node is responsible
for producing a repeating electrical impulse which ultimately
causes the muscle cells of the heart to contract in repetitive,
wave-like convulsions.
[0014] The synchronization solutions proposed in the prior art
discussed above do not address methods for detecting and
compensating for hard to detect, abnormal conduction patterns or
rhythms which may occur in damaged hearts. Additionally, EKG
monitoring may not detect and allow compensation for localized or
isolated areas of heart tissue which may not be synchronized with
other areas of heart tissue. Excitation of such isolated areas may
cause arrhythmias. In addition to the problems discussed above,
heart synchronization as described in the prior art limits the
amount of time the laser can be activated during a heart cycle,
thereby increasing the time of a TMR procedure.
[0015] A need exists in the prior art for a method and apparatus
for performing TMR and PTMR procedures quickly using specified
laser parameters selected to minimize possible cardiac arrhythmias
without the need for monitoring the heart beat.
ADVANTAGES AND SUMMARY OF THE INVENTION
[0016] Thus, it is an advantage of the present invention to provide
a method for performing both transmyocardial revascularization
(TMR) and percutaneous transluminal myocardial revascularization
(PTMR) with laser energy having parameters selected to avoid
cardiac arrhythmia.
[0017] A method for TMR and PTMR with laser energy having
parameters selected to avoid cardiac arrhythmia comprises the
following steps, in combination of: determining a wavelength of the
laser energy from a laser selected to perform either TMR or PTMR;
using the wavelength determination to select parameters for the
laser energy to produce a non-square wave shape; generating the
laser energy at the determined wavelength with the selected
parameters to produce the non-square wave shape; and delivering the
generated laser energy in one or more pulses to selected portions
of heart tissue to perform either TMR or PTMR in the myocardium
without inducing cardiac arrhythmia and without synchronizing
delivery of the laser energy to a cardiac cycle. site, When using a
Holmium:YAG laser in either a TMR or PTMR procedure, the selected
parameters are power level, energy flux, pulse width, and pulse
frequency. In a TMR or PTMR procedure, the laser energy has a
wavelength of between about 1.8 and about 2.2 microns, an energy
flux of between 0.7-1.78 J/mm.sup.2 and a power level of at least
about 3 watts, the laser energy being delivered with a pulse
frequency of at least about 5 Hertz and a pulse width of between
about 150 and about 350 millisecond, the laser energy as delivered
causing about 5 millimeters or less lateral necrosis surrounding a
TMR treatment. When using a Xe:Cl excimer laser in either a TMR or
PTMR procedure, the laser energy has a wavelength of about 0.308
microns, a power level of between 0.3-2.0 watts and an energy flux
of between about 25-80 mJ/mm.sup.2, and is delivered with a pulse
frequency of between about 5-25 Hz and a pulse width of between
about 20-200 microseconds, and causes about 5 microns lateral
necrosis surrounding the TMR channel produced thereby. When using a
CO.sub.2 laser for the TMR procedure, the laser energy has a
wavelength of about 10.6 microns, an energy flux of about 51
J/mm.sup.2 and a power level at least about 800 W, is delivered in
a single pulse about of 0.05 seconds and can be gated, and causes
between about 0.03 to about 0.2 millimeters lateral necrosis
surrounding a TMR channel produced. When using an Argon laser for
TMR, the laser energy has a wavelength of between about 0.488 and
about 0.514 microns, an energy flux of about 1.3-12.74 J/mm.sup.2
and a power level at least about 1-10 W, is delivered in a single
pulse, and causes approximately 4 mm lateral necrosis surrounding a
TMR channel produced thereby, and is generated by an Argon laser.
When using a Nd:YAG laser in a TMR procedure, the laser energy has
a wavelength of about 1.06 microns, an energy flux of about 9.5-13
J/cm.sup.2 and a power level at least about 2-100 W, is delivered
with a pulse frequency of about 1-10 Hz and a pulse width of about
10 nanoseconds, and causes at least about 15 mm lateral necrosis
surrounding a TMR channel produced thereby. When using an Er: YAG
laser for the TMR procedure, the laser energy has a wavelength of
about 2.94 microns, an energy flux of about 50-500 J/mm.sup.2, is
delivered with a pulse frequency of about 1-15 Hertz and a pulse
width of about 1-250 microseconds, and causes about 0.1 millimeters
lateral necrosis surrounding a TMR channel produced by an Er:YAG
laser.
[0018] In a preferred embodiment, the laser energy is delivered to
the selected portions of heart tissue using a catheter apparatus
with laser delivery means, the method further comprising the
following steps of introducing the catheter apparatus with laser
delivery means percutaneously into the vasculature of the patient;
and positioning the laser delivery means at the endocardial surface
of the selected portions of heart tissue. In a preferred
embodiment, the laser energy is delivered to the selected portions
of heart tissue in a procedure using laser delivery means where the
revasculature site is accessed by positioning the laser delivery
means at an endocardial surface of the heart tissue through inside
a patient's coronary artery. In a preferred embodiment, the method
further includes the step of mechanically piercing the endocardial
and/or myocardial layer heart tissue prior to delivering the laser
energy into the myocardium thereby creating a welling affect by the
surrounding endocardial tissue. In a preferred embodiment, the
method further includes mechanically piercing the endocardial
surface adjacent the selected portions of heart tissue prior to
delivering the laser energy into the myocardium and penetrating no
more than half the wall thickness of the myocardium.
[0019] The TMR method comprises the following steps of generating
laser energy having a non-square wave shape, a selected wavelength,
a selected energy flux and a selected power level; and delivering
the laser energy in a plurality of pulses, the plurality of pulses
having a selected pulse frequency and a selected pulse width, to
selected portions of myocardium without cardiac arrhythmia and
without synchronizing delivery of the laser beam with the cardiac
cycle. In a preferred embodiment, a variable number of pulses of
laser energy is delivered with a variable pulse frequency between
5-20 Hz. In a preferred embodiment, the laser energy is delivered
with a variable pulse repetition rate of between about 1 and 10
pulses. In a preferred embodiment, the laser energy is delivered
with a constant pulse frequency of between about 5 and 20 Hertz and
a variable pulse preset limit of between about 1-10 pulses. In a
preferred embodiment, the laser energy is delivered in a pulsed
mode at a high repetition rate of fixed frequency, the method using
a laser with an optical shutter and in which the shutter of the
laser is opened and closed in response to a random sequence of
commands. In a preferred embodiment, the pulsed laser energy is
delivered in a pulsed mode pulsed at a high repetition rate of
fixed frequency, the method using a laser with a controllable
flashlamp and in which the flashlamp is allowed to fire only during
certain pulses within the fixed frequency laser operation in
response to a random sequence of commands. In a preferred
embodiment, the laser energy is delivered in a pulsed mode pulsed
at a random, variable frequency rate.
[0020] It is a further advantage of the present invention to
provide a method of selecting laser parameters for performing
laser-assisted TMR or PTMR procedure to avoid cardiac arrhythmia
and without synchronization of delivery of laser energy to a
patient's cardiac cycle. The method comprises the following steps,
in combination: selecting a minimum power level of laser energy to
be used, the minimum power level being sufficient to ablate heart
tissue; setting a pulse frequency as great as possible and selected
to avoid summation effects; setting a pulse width as long as
possible and selected to prevent excessively high peak power
without causing undesired levels of thermal damage during TMR or
PTMR; shaping a front end of each pulse of laser energy to provide
non-linear pulses to avoid cardiac arrhythmia during TMR; and
correcting the selected power level, pulse width, pulse frequency,
pulse width, and shaping for mechanical events. In a preferred
embodiment, the selected parameters are a single pulse, power
level, energy flux, and pulse width. Numerous other advantages and
features of the present invention will become readily apparent from
the following detailed description of the invention and the
embodiments thereof, from the claims and from the accompanying
drawings in which the details of the invention are fully and
completely disclosed as a part of this specification.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIGS. 1A-1C are graphical representations of the process of
summation.
[0022] FIGS. 2A and 2B are graphical comparisons of the resultant
difference between ablation with a square wave versus ablation with
a non-square wave.
[0023] FIG. 3 is a flow chart demonstrating a method of selecting
and setting variable laser parameters.
[0024] FIG. 4 is a representative example of variable laser
parameters.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0025] This invention teaches laser parameters which, in optimized
combinations, reduce or eliminate the risk of inducing arrhythmia
while performing a laser-assisted TMR and PTMR. The methods
described herein do not require cardiac monitoring or any other
form of synchronization of laser delivery with the natural cardiac
rhythm. The present invention is intended for use with any medical
laser. In particular, excimer and Holmium lasers, including many of
various different types known and available now or at any time, are
particularly suited to the present invention. However, any suitable
laser source, pulsed or otherwise, may be used to provide laser
energy to the laser delivery means of the present invention for
performing the method of the present invention. Other laser sources
include but are not limited to CO.sub.2, argon, neodymium: yttrium
aluminum garnet (Nd:YAG) as well as erbium: yttrium aluminum garnet
(Er:YAG). The following laser operating parameters have been
determined to be optimal parameters for performing laser-assisted
revascularization without causing arrhythmias.
1 Laser Operating Parameters Xe: Cl Type excimer Ho: YAG CO.sub.2
Argon Nd: YAG Er: YAG Wavelength 0.308 .mu.m 2.1 .mu.m 10.6 .mu.m
0.488-.514 .mu.m 1.06 .mu.m 2.94 .mu.m Laser Pulse 5-25 Hz Greater
than Single CW- 1-10 Hz; CW 1-15 Hz Frequency Approx. pulse, CW
multimode, or pulsed; CW 5 Hz or super- can be can be "gated"
pulsed q-switched or gated Energy About About 0.7- About About.
About. About Flux 25-80 1.78 J/mm.sup.2 51 J/mm.sup.2 1.3-12.75
9.5-13 J/cm.sup.2 50-500 J/mm mJ/mm.sup.2 J/mm.sup.2 pulsed; About.
8-27 J/mm.sup.2 CW Laser Pulse About About About N/A 10 ns, CW or
About Width 20-200 ns 150-350 .mu.s 0.05 s pulsed 1-250 .mu.s Wave
Non-square Non-square Non-square N/A Non-square Shape Ave. Power
0.3-2.0 W Greater 800 W 1-10 W 2-100 W than 3 W Lateral About 5
.mu.m About About Greater than Greater than About Necrosis 0.5 mm
0.05-0.2 about 4 mm about 15 mm 025-0.1 mm mm q-switched 10-15
.mu.m
[0026] The above laser operating parameters for various types of
lasers were selected to enable laser energy to smoothly and
gradually interact with the tissue of the heart thereby avoiding,
where possible, sharp and sudden tissue interaction. A contoured,
smooth, gradual application of energy will provide less shock to
the heart itself. A non-square wave with a relatively shorter pulse
width generally is preferred to achieve such smooth and gradual
tissue interaction. It will be recognized by those skilled in the
art that achieving the desire effect requires variation of the
individual elements of the laser operating parameters depending
upon the type of laser selected. Each laser operating parameter is
discussed separately below.
[0027] Wavelength
[0028] It is recognized that numerous medical lasers having
different wavelengths are currently available. Selection of an
appropriate wavelength depends at least in part upon the tissue to
be treated with the laser. For TMR, the mechanism of delivering
laser energy to the water component of heart tissue to effect
ablation and channeling highly efficient. Mid-infrared lasers, such
as the 2.1 micron wavelength Holmium:YAG laser, are well suited for
cutting and ablating heart tissue because, in general, wavelengths
longer than approximately 1.4 microns are highly absorbed by water.
It is by means of this strong absorption in water that mid-infrared
laser energy is converted to heat energy in tissue.
[0029] If the energy density in the tissue is high enough, the
tissue vaporizes. Since water is the primary constituent of most
soft tissue, about 80% or more, the correlation is fairly accurate,
although there will be minor differences between absorption of
mid-infrared energy in water and in tissue, as well as between
different types of tissue. The mid-infrared absorption spectrum of
water is well known and absorption coefficients over a larger range
are available.
[0030] The absorption coefficient a of the 2.1 micron Holmium:YAG
laser is about 26 cm.sup.-1. This modality is highly efficacious
for cutting and ablating. For comparison, the absorption
coefficients of the 1.06 micron wavelength energy created by the
Neodymium:YAG laser and the 10.6 micron radiation produced by a
CO.sub.2 laser are 0.13 and 823 cm.sup.-1, respectively. The
inverse of the absorption-coefficient is directly correlated with
energy absorption depth d:
d=1/.alpha.
[0031] A large absorption coefficient implies a short penetration
depth. Penetration is short because the energy is immediately
absorbed by the cells closest to the source and does not extend
into the tissue.
[0032] Other lasers include the Xe:Cl excimer laser which delivers
energy at 0.308 .mu.m, the argon laser which delivers energy at
between about 0.488-0.514 Fm, the CO.sub.2 laser which delivers
energy at 10.6 Fm, and the Nd:YAG laser which delivers energy at
1.06 .mu.m.
[0033] CO.sub.2 lasers used in a fluid environment in near-contact
procedures, despite a relatively short penetration depth, have the
disadvantage in that even a thin film of water between the fiber
and the tissue will greatly diminish the effective laser power
because of the large absorption coefficient. Hence, the use of gas
insufflation is generally required with the CO.sub.2 laser.
Furthermore, CO.sub.2 lasers generally require further manipulation
of parameters to create TMR channels.
[0034] In selecting a particular wavelength for a laser, and
adjusting parameters to compensate for differences in absorption by
the tissue, it is important to consider the need to achieve an end
result of a non-square wave.
[0035] Pulse With and Frequency
[0036] To prevent electrical arrhythmia of the heart during TMR or
PTMR, it has been found that certain pulse widths not only assure
minimal and predictable thermal damage to surrounding tissue but
are less likely to interfere with the cardiac signal which may lead
to arrhythmia. By reducing the interval between pulses to
significantly shorter than the interval of a beating heart at
approximately 60 beats per minute, or 1 beat per second (1 hertz),
there is much less chance that a single laser pulse will side track
the electrical rhythm and take over propagation of the electrical
impulse throughout the heart. Furthermore, higher frequencies
create a hole faster thereby reducing the probability that a laser
polarized group of cells in the area of channel creation will short
circuit a heartbeat signal. In a preferred embodiment of the method
of the present invention, pulse widths less than about 300
milliseconds, and preferably lower than about 200 milliseconds,
have been found to be most effective. Thus, in a preferred
embodiment of the method of the present invention, frequency rates
greater than 5 hertz and preferably between about 5 hertz and about
25 hertz are least likely to induce electrical arrhythmia during
TMR.
[0037] The Holmium:YAG or other comparable pulsed laser can be
pulsed at a certain rate or within a range of rates. Pulsed
application of laser energy to effect vaporization and ablation of
the tissue has been demonstrated to be preferable in some instances
to application of laser energy in a single pulse. In delivering
energy to tissue, a given volume of tissue will absorb laser energy
converting it into heat. To ablate a given target volume of tissue
it is necessary to put enough heat energy into the volume to
vaporize it. A certain minimum rate of delivery of energy is
required to counteract the effect of thermal relaxation which is
the phenomenon by which heat diffuses out of the heated volume of
tissue. The thermal relaxation time .tau. is defined as the amount
of time required for a given amount of heat to diffuse out of a
given volume of tissue. Based upon an estimation that the thermal
diffusivity of tissue is very close to that of water, and using
classical heat transfer theory, the thermal relaxation time period
for a spot size of tissue using a 400 micron diameter fiber in
contact or very close proximity to the tissue has been calculated
to be between approximately 57 and 286 milliseconds using the
following equation:
1/.tau.-4.alpha..sup.2K+16K/d.sup.2
[0038] where K=thermal diffusivity of tissue, d=diameter of the
illuminated spot. The calculation is based on the estimate that
K-1.4.times.10.sup.-3cm.sup.-2/s.
[0039] In general, the pulse width must be optimized to prevent a
peak power spike while achieving a predetermined energy flux for
ablation or any other procedure. Too short a duration will require
too sharp an energy spike with a very high peak power level. On the
other hand, too long a pulse will result in summation of energy
effects which result in overheating, an increased zone of thermal
necrosis, and possible other coagulation type effects.
[0040] It will be understood that the Nd:YAG, Ho:YAG, Xe:Cl excimer
and Er:YAG lasers are all pulsed lasers in at least preferred
forms, and therefore optimum pulse widths may be selected.
Appropriate pulsing will avoid summation effects and will provide a
contoured, gentle leading edge of the energy delivery profile.
[0041] Use of a continuous wave laser, such as argon, some Nd:YAG
and some CO.sub.2 lasers, typically can be enhanced by mechanically
chopping the continuous wave to reduce unfavorable thermal effects.
The advantage to chopping such laser beams is to reduce or
eliminate adverse summation effects on the tissue. However,
mechanically chopping a continuous wave does make it more difficult
to achieve a non-square or otherwise less assaultive delivery of
laser energy. In any event, as discussed below, whether delivering
pulsed energy or continuous wave energy, the rate at which it is
delivered is also important. Too narrow of a pulse width at too low
of a power level will fail to create patent channels. Thus, the
power of the laser beam, or the amount of energy delivered to the
tissue in terms of Joules per second, is also an important factor
to consider.
[0042] In general, optimal pulse frequencies and pulse widths to
avoid arrhythmias combine to create a relatively narrow pulse width
delivered at a relatively high pulse frequency.
[0043] Energy Flux and Power
[0044] Energy flux E.sub.pulse or fluence, also referred to as
radiant exposure or energy density, is expressed in units of pulse
energy per area, or joules per square centimeter. The threshold
radiant exposure F.sub.th is defined as the single pulse threshold
for the ablation of biologic tissue. For radiant exposures less
than this threshold, tissue is not vaporized or ablated, but,
instead, is heated. For radiant exposures higher than this
threshold, tissue is vaporized. Thus, for effective ablation, the
energy per pulse divided by the area of the spot size must be
greater than the threshold radiant exposure.
E.sub.pulse/A>F.sub.th
[0045] As an approximation, the threshold energy per unit volume
for water is known to be 2500 joules per cubic centimeter. This is
also the heat of vaporization, or the heat required to raise one
cubic centimeter from about body temperature to steam at 100
degrees centigrade. This threshold energy per unit volume is given
by the equations:
F.sub.th.times..alpha. or F.sub.th/d
[0046] Depth of penetration, discussed above, is equal to the
inverse of the absorption coefficient and the threshold radiant
exposure for biologic tissue is about 100 joules per square
centimeter. An effective energy flux must be at least this great.
Experimentally, threshold energy flux rates of between about 5 and
75 joules per square centimeter are found.
[0047] The average power P.sub.av of a repetitively pulsed laser is
equal to the energy per pulse times the number of pulses per unit
time (P.sub.av=joules/pulse.times.pulses/second, in watts). The
number of tissue parcels ablated per second is equal to the
repetition rate of the laser in pulses per second, or hertz. Thus,
increasing the pulse repetition rate to increase the average power
delivered to the tissue linearly increases the rate of tissue
removal.
[0048] If a region of tissue is illuminated by more than one pulse,
an excess of energy accumulates in the tissue. For example, if the
individual pulse energy and spot size provide an energy flux below
the single pulse threshold radiant exposure F.sub.th, but the
interval between pulses at a particular repetition rate is shorter
than the characteristic thermal relaxation time .tau., a parcel or
given volume of tissue may be ablated after a number of pulses has
impinged the tissue. This process is referred to as summation.
[0049] FIGS. 1A-1C are graphical representations of the process of
summation. In FIG. 1A, an initial pulse of energy 80 increases the
temperature (T) of the tissue locally. However, given a large
amount of time (t) between pulses, the temperature of the tissue
drops rapidly and immediately at 82 during the beginning of the
delay period (D) following irradiation with no net result.
Referring now to FIG. 1B, decreasing the time between pulses
results in a positive net result and tissue ablation. An initial
pulse of energy 84 increases the local temperature to a certain
point 86, and then the temperature begins to drop during the time
period 88 immediately following the first irradiation step. At an
intermediate temperature level 90, another pulse of laser energy is
delivered to impinge upon the tissue. The additional pulse elevates
the temperature from temperature level 90 to a temperature 91 which
is high enough to cause ablation. Repeated cycling as in FIG. 1B is
very effective at achieving ablation without causing excess thermal
damage in the surrounding tissue. During this mode of operation,
the system can be described with the following equation:
(P.sub.av/A)-(F.sub.th/.tau.)>0
[0050] (It should be noted that Fth may not remain constant in this
regime: only the first pulse encounters fully hydrated, native
tissue at body temperature.) Effectively, this criterion indicates
that ablation can generally take place only if energy is supplied
faster than it diffuses away. Because the thermal relaxation time
.tau. is between 57 and 286 milliseconds, the interval between
pulses can be as short as between 20 and 50 milliseconds, which is
shorter than the shortest thermal relaxation time that arises from
laser interaction with biologic tissue. A repetition rate greater
than 3.5 hertz will permit entry into this FIG. 1B regime, however,
as described above, pulse frequencies of between 5 and 15 hertz are
optimum for laser-assisted TMR in which the risk of electrical
arrhythmia is optimally reduced. Using these parameters, an average
power delivery will be greater than 6 watts, or 6 joules per
second.
[0051] By further decreasing the period between laser pulses, an
excess of heat builds up in the tissue as shown in FIG. 1C. After
an initial temperature rise 92 and a brief period of thermal
diffusion 94, a further pulse of energy 96 will continue to drive
the temperature of the tissue upwards. Not only is this an
inefficient modality for revascularization, but the risk of thermal
runaway and associated thermal damage to surrounding tissue is very
great. Additionally, an elevated temperature and excessive tissue
damage will both tend to increase the risk of arrhythmia. Not only
are the cells which are elevated in temperature more easily
depolarized, as they lie in a period of relative refractivity, but
thermal damage to cells also interferes with their normal firing
and propagation of the electrical impulse. Therefore, in general,
the power of the laser used, or the rate of delivery of energy to
the tissue, can be tailored to avoid collateral damage of
subsequent pulses. Though summation and overheating are more of a
problem with continuous wave lasers, too small a pulse width with
too high a peak-power pulse with lasers such as the Xe:Cl excimer,
though at relatively similar powers as other pulsed lasers, may
also have a harmful, explosive result.
[0052] In summary, depending upon the laser selected, the energy
flux and power parameters should be adjusted to ensure ablation
without deleterious summation effects. The net result of such
optimization is a reduction in the incidence of arrhythmia during
TMR or other laser procedure.
[0053] Wave Shape
[0054] As has been stated, adjustment of the parameters discussed
above is designed to create a non-square wave for TMR and other
laser applications. There is a direct correlation between
likelihood of arrhythmia and wave shape. When TMR is performed
using a wave with a square shape, that is, a wave which immediately
and linearly increases to a near-maximum level, continues at that
level until dropping sharply back to the lower level. A non-square
wave would be one that is non-linear and had a more gradual
increase in amplitude, or curve with a relatively lower slope, with
a subsequent gradual decrease. Past experiences with square wave
lasers in PTMR and TMR applications clearly demonstrate that the
square waveform is more explosive and traumatic to the heart. A
comparable energy wave with a Gaussian, bell-shaped or other shaped
waveform does not have a tendency to create arrhythmias whereas
square waveforms have a greater tendency to induce arrhythmia due
to a resulting shock wave which adversely impacts the entire
heart.
[0055] FIGS. 2A and 2B are graphical comparisons of the resultant
difference between ablation with a square wave versus ablation with
a non-square wave. In both plots, the vertical axis corresponds to
both the temperature of individual cells in the volume of tissue
heated by a single pulse as well as the energy level of those
individual cells. In FIG. 2A, when a square-wave pulse is applied
to the tissue, the temperature of the tissue rises sharply during
the period between t.sub.0 and t.sub.1. As the population density
of individual cells heated above the threshold temperature T.sub.th
up to a maximum temperature T.sub.max is relatively large, the
chance that one individual cell might depolarize and take over or
capture the rhythm of the heart is increased. These cells at risk
of taking over the heart rate and causing an arrhythmia are
identified by the shaded portion of the graph 100. A large
population of cells rapidly depolarize and any one can serve as a
situs of arrhythmic activity. When the pulse of laser energy is
non-square shaped as in FIG. 2B, the statistical probability that a
single cell in the population of cells between T.sub.th and
T.sub.max might capture the electrical cycle and become arrhythmic
is much less. Assuming that a threshold temperature T.sub.th exists
above which temperature individual cells may become depolarized
(but below which the risk is very small), the area 102 under the
non-square, generally bell-shaped curve in FIG. 2B is much smaller
than area 100. It will be understood that the shaded areas under
the curves above the threshold temperature levels generally are
proportional to the probability that arrhythmia will be
produced--the smaller the area the less likelihood of inducing an
arrhythmic cardiac cycle. Thus, the areas 104 and 106 can be said
to be proportional in magnitude to the decrease in likelihood that
an errant depolarized cell will capture the cardiac cycle and cause
arrhythmia.
[0056] The net effect of providing a non-square shaped pulse wave
is to cause the same amount of ablation, perhaps over a slightly
longer period of time, with a decreased risk of inducing
arrhythmia. Bell or Gaussian-shaped waveforms have are highly
effective at channeling in the TMR procedure and the risk of
inducing arrhythmia is optimally minimized.
[0057] It is demonstrated that a non-square, contoured wave shape
will tend to reduce the risk of causing arrhythmia in TMR and PTMR
patient's. This is generally difficult to achieve with continuous
wave lasers since chopping of a CW laser beam will not avoid a
square or otherwise fairly sharp wave front. However, using pulsed
lasers, optimization of a contoured wave will further eliminate a
high peak-power spike at or near the center of the pulse. Such
optimization will include adjustment of the other above
parameters.
[0058] Zone of Lateral Necrosis
[0059] While the precise influence of thermal injury on TMR channel
patency, or other desirable TMR result, is unclear, the extent of
lateral thermal necrosis should be controlled by careful selection
of the laser and its operating parameters. In general, minimizing
lateral tissue necrosis will result in more efficient tissue
removal from the channel itself, and will also result in minimal
trauma to surrounding tissue. Although lateral tissue damage may be
sought in some applications, in general, minimal trauma to
surrounding tissues is a desirable goal.
[0060] CO.sub.2 lasers have been found to produce an intense
inflammatory response which may be inconsistent with provision and
promotion of an alternative circulation, one of the therapeutic
mechanisms believed to be associated with TMR. Although
mechanically-formed channels were completely occluded within 2 days
of formation by cellular infiltrate, eventually forming scar
tissue, many laser created channels remain patent for a longer
period of time, but too may become occluded with fibroblasts,
macrophages and subsequently collagen.
[0061] Thermal injury to myocardium surrounding channels may delay
healing and thus increase duration of patency, though the lack of
an obvious, visibly patent channel may not preclude blood flow in
vivo via the channels. However, it has been shown that it is
possible to alter the degree of thermal injury, for example with
the Ho:YAG laser, by changing pulse energy or repetition rate. It
has also been shown that with laser created channels, the extent of
tissue damage associated with the creation of the channels can be
reflected in the degree of fibrosis produced. Fibrosis associated
with the initial injury results in disorganization of adjacent
myocytes. The observed disarray is similar to that found in viable
muscle adjacent to a healed infarct. Hence the degree of muscle
disorganization may be determined by the amount of
channel-associated fibrosis.
[0062] The above described structural changes due to lateral
thermal necrosis result in diminished heart function and also
provide a substrate for abnormal electrical conduction. This may
increase the chance of inducing arrhythmia during such procedure.
An increase in interstitial collagen can be expected to affect
heart function by decreasing contractility, elasticity and pumping
strength, and can also be expected to decrease cell-to-cell
contact.
[0063] Thus, in general, at least a minimum degree of thermal
necrosis will be present and is considered beneficial. An excess of
lateral necrosis, however, should be avoided since too great an
amount of thermal injury will cause other complications
non-beneficial to the TMR patient.
[0064] Miscellaneous Parameters
[0065] Another cause of tissue damage is the production of vapor
bubbles in the tissue being ablated with lasers. The degree of
myocardial disruption by such acoustic injury is slight at
repetition rates of between about 2 and 3 Hertz, but vapor bubble
effect may increase tissue injury in excess of that caused by
temperature increases.
[0066] An additional consideration will be the temperature of the
heart itself. In TMR procedures, cooling the heart will help
prevent an accumulation of undesirable, potentially harmful heat.
In other words, summation effects can be minimized by applying
external cooling to the heart itself, or portions thereof,
selectively or over large areas or otherwise, to effectively
increase, by as much as several fold or more, the thermal
relaxation time for the tissue.
[0067] Variable Parameters
[0068] The above discussion demonstrates various parameters for
different laser energies chosen to minimize the possibility of
arrhythmias and is based upon a generally constant, regular
delivery of laser energy using those parameters. Arrhythmias also
may be prevented by providing variable, or non-synchronous,
delivery of laser energy. Non-synchronous TMR and PTMR procedures
lessen the chance of capture of the heartbeat because the either of
these procedures do not use a predictable, constant firing sequence
for the heart to follow. FIG. 3 is a block diagram of a preferred
embodiment of a method for non-synchronous laser-assisted TMR using
variable parameters. Providing variable laser parameters further
decreases the risk that surrounding tissue will abandon the regular
pattern of the heart beat to follow the particular laser parameters
used in the laser TMR procedure. The following table illustrates
several variable laser parameters in TMR procedures with decreased
risks of arrhythmias.
2 Variable Parameters CONSTANT NUMBER OF PULSES FIXED PULSE
VARIABLE NUMBER DELIVERED/ REPETITION RATE, MODE OF RANDOMLY
VARIABLE PULSE PULSES GATED PULSE TRAIN CHANGING PULSES REPETITION
RATE RANDOMLY PULSE TRAIN #1 2 pulses 2 pulses 2 pulses 5 hertz 5
hertz 15 hertz 7 watts 7 watts 7 watts PULSE TRAIN #2 3 pulses 2
pulses 3 pulses 15 hertz 10 hertz 15 hertz 7 watts 7 watts 0 watts
= OFF PULSE TRAIN #3 1 pulse 2 pulses 3 pulses 8 hertz 15 hertz 15
hertz 7 watts 7 watts 7 watts PULSE TRAIN #4 2 pulses 2 pulses 1
pulses 6 hertz 8 hertz 15 hertz 7 watts 7 watts 0 watts = OFF PULSE
TRAIN #5 6 pulses 15 hertz 7 watts
[0069] The variable parameters may be preset at the laser console
control panel. As shown, the laser energy applied to the heart
muscle in a TMR procedure is varied by providing (1) a variable
number of pulses at a randomly changing pulse repetition rate, (2)
a constant number of pulses delivered at variable pulse repetition
rates, or (3) randomly gated pulse delivery at a fixed repetition
rate. Mechanical or direct modulation may be used to vary the
gating. Mechanical modulation is preferred using a mechanical
device such as an automatic shutter or beam chopper.
[0070] FIG. 4 is a representation of beam patterns which are
possible using the variable laser parameter method of the present
invention. One method of creating a random frequency of pulses is
to provide an intra-cavity beam shutter. The laser is pulsed at a
high repetition rate of fixed interval, for example 15-20 hertz.
The system's central processing unit (CPU) or computer contains a
pre-programmed random sequence of commands to open the shutter for
a random period of time, and close the shutter for a random period
of time until the foot switch is released. Thus, line 200
represents the fixed interval pulse rate of the laser. This is a
constant rate, for example 15-20 hertz. Line 210 represents the
controller's shutter operation. It is seen that the actual laser
output during this time will be controlled in an on and off manner
automatically by the operating program of the system CPU or other
processor. Line 220 is representative of the actual output of the
laser, allowing pulses of laser energy to be emitted during the Aon
periods and suspending laser emission past the shutter during the
Aoff periods.
[0071] A second novel method for producing a variable laser output
pattern is to control the flash-lamp of the laser as opposed to a
mechanical shutter. As with the mechanical intra-cavity shutter,
the laser is pulsed at a fixed repetition rate of, typically, 15-20
hertz. The system's CPU contains a pre-programmed randomized
sequence of commands to allow the flash-lamp driver to fire only
during certain pulses with the fixed repetition rate. A pattern
similar to that shown in line 220 is possible in this manner.
[0072] A third method for producing a variable laser output is to
control the flash-lamp directly. Instead of maintaining a
continuous fixed repetition rate, of which only certain pulses are
allowed to occur, this method uses a pre-programmed memory of
variable repetition rates between about 5 and 20 hertz continuously
delivered to tissue until the foot switch is released. This
resultant beam pattern is depicted in line 230, a random
pattern.
[0073] Yet a fourth method for producing a variable laser output is
to change the repetition rate after a predetermined set number of
pulses have been delivered. For example, the pattern shown in line
240 is a set of 2 individual pulses per each pulse repetition rate
used. The random variation of repetition rate changes after each 2
pulse set, as shown in line 250 where the number of pulses per
second is shown. The resultant beam pattern is depicted in line
260, a random pattern.
[0074] Percutaneous Transluminal Myocardial Revascularization
(PTMR)
[0075] Selection of laser parameters for percutaneous transluminal
myocardial revascularization (PTMR) procedures requires
consideration of a number of difficulties encountered in most
percutaneous ventricle procedures and consideration of factors
particularly associated with PTMR. U.S. Pat. No. 5,389,096 issued
Feb. 14, 1995, as discussed above and PCT WO 96/35469 published
Nov. 14, 1996 to Aita et al., both discuss apparatus and general
methods for percutaneous myocardial revascularization synchronized
with the heart beat to avoid arrhythrnias. Like Aita's U.S. Pat.
No. 5,380,316 entitled "Method for Intra-Operative Myocardial
Device Revascularization" which proposed that surgical TMR should
be performed by lasing through the epicardium, these teachings
suggest that conventional channels should be formed in a
percutaneous procedure by firing laser energy from the endocardial
surface, thereby destroying viable endocardial tissue. The
percutaneous teachings briefly suggest in general terms that
channels should extend into the myocardium at depths up to 10 mm.
In particular, the >096 patent preferably suggests that channels
are 1.5-2.0 mm in diameter with 10-30 mm depths from the
endocardium into the myocardium. Both references are drawn
particularly to methods and apparatus which attempt to stabilize
the optical fiber device within the ventricle , and although both
references note the obvious need to avoid perforation of the
epicardium, neither reference addresses a number of problems
particular to formation of laser revascularization channels in a
PTMR setting.
[0076] Access to the ventricle for PTMR procedures is discussed in
U.S. patent application Ser. No. 08/ entitled ASteerable Catheter,@
by Giba et al. filed Apr. 3, 1997 which is hereby incorporated by
reference. This application teaches of a percutaneous steerable
laser delivery catheter that is positionable near the heart=s
endocardium for PTMR treatment. Pulsed laser energy that avoids
cardiac arrhythmia is delivered to sections of heart tissue through
an optical fiber disposed within the catheter. The catheter is
inserted through the femoral artery in the groin region and passes
into the inner heart over the aortic arch into the left ventricle.
Channel formation in a PTMR procedure requires a catheter of
lengths up to 120 cm which poses a number of problems which must be
solved by a cardiologist controlling the channel forming device
from a remote position outside of the body. In particular, many
dynamic forces occur during optical fiber advancement through a
catheter=s lumen for correct positioning thereof in relation to the
endocardial wall.
[0077] These forces include frictional forces within the catheter=s
lumen as well as forces encountered because of the patient's
cardiac activity. For example, these variable forces are caused by
1) the overall length of the catheter and the attendant frictional
forces caused by optical fiber advancement within the catheter 2)
movement of the heart wall as the heart beats; and 3) variable
tissue resistance to the optical fiber advancement through the
spongy variable density endocardium with subsequent advancement
through the denser myocardium. The endocardium, a porous variable
density tissue layer, moves towards and away from a positioned
catheter delivery system tip as the heart contracts and expands.
This motion is resisted by the resilient nature of the catheter tip
which attempts to bias the tip against the endocardial wall. The
dynamic encounter between the endocardium and the distal tip of the
catheter also poses potentially undesirable heart arrythmias due to
mechanical forces created by optical fiber placement against the
endocardium and subsequent advancement through the endocardial
layer into the myocardium. During a PTMR procedure, the
cardiologist must monitor these cumulative variable forces
occurring at the catheter tip and perform laser tissue ablations
using the catheter=s proximal hand piece for both positioning the
optical fiber tip and advancing the optical fiber as channel
formation occurs.
[0078] PTMR channels must be created in tissue which, in addition
to having variable densities, may vary in depth. The average
thickness of the endocardium is approximately one mm or less and
the thickness of the myocardium typically is approximately several
cms or less. However, coronary anatomical irregularities may result
in sections of the myocardium which are less than 10 mm in
thickness. Unintended tissue ablations in thin areas of the
myocardium may cause perforations through the epicardial wall
leading to cardiac tamponade. Failure to account for differences in
tissue density also may cause irregular channel formation with
unwanted lateral necrosis. Laser parameters should be selected to
take into consideration the relative ease in penetrating and
advancing through the less dense endocardium as opposed to the
relatively more difficult penetration and advancement into the
denser myocardium. Laser power level(s) and/or pulse rate(s) must
be correct to ensure proper fiber advancement to avoid irregular
channel formation with unwanted lateral necrosis. Most important,
procedural steps must be observed to select proper laser power
level(s) and/or pulse rate(s) to ensure a patient's safety and well
being.
[0079] Generally, PTMR laser parameters should be adjusted to
compensate for several possible fiber optic delivery system
positions, i.e. 1) juxtaposed or resting lightly against the
endocardium without urging, 2) urged against the endocardium, and
3) pierced into the endocardium and juxtaposed to the myocardium.
This is particularly true with smaller fiber optic delivery
systems. In particular fibers smaller than 1 mm in diameter,
piercing can be performed readily through the endocardium with the
forces described. Any one of these three scenarios can occur while
the heart is beating thereby causing variable density of the
endocardium and myocardium. If the position of the optical fiber
optic delivery system is not known or cannot be determined with
certainty, prior to delivery of laser energy, laser parameters
should be selected to ensure no harm would occur given any possible
position of the fiber optic delivery system. For instance,
delivering relatively high powered laser energy in scenario one
without fiber advancement may cause cratering and destruction of
the viable endocardium layer, or delivering such energy in scenario
three with significant fiber advancement may cause perforation of
the epicardium. Thus, the encountered catheter forces and
uncertainties as to positioning the optical fiber tip in
relationship to variable tissue densities bear upon the nature and
quality of PTMR channel formation. Recent clinical studies have
shown that PTMR procedures using laser parameters selected to form
shallower channels will alleviate angina, create channels without
significant damage to the endocardium or cratering, and incur
minimal risk to a patient.
[0080] PTMR laser parameters are selected to provide a generally
low power approach using a limited number of pulses for confined
myocardium penetration to provide a safe mode to perform PTMR.
Percutaneous laser revascularization procedures at lower power
levels with limited number of pulses per channel formed relieve a
patient's angina symptoms while ensuring optimal patient safety.
These studies used a non-synchronous based PTMR system using a
Holmium:YAG laser source, e.g. an Eclipse Surgical Technologies
TMR-2000 with a laser catheter delivery system whose internal
optical fiber was one mm in diameter. The catheter has a mechanism
for advancing the fiber for short penetrations up to 5 mm depending
upon left ventricle wall location. The patients who underwent this
treatment experienced good results when a Holmium:YAG laser was
operated at 5 Hz, in a preset pulse firing mode of up to five
pulses/cycle, having an output average power of 3-4 watts where the
laser pulse widths were between 100 and 250 ms, and the number of
pulses per formed channel was less than five. Parametrically, the
average power density/pulse in a pulse preset mode is approximately
1-4.5 watts/mm.sup.2, the energy density is approximately 0.9
joules/mm.sup.2, given an optical fiber diameter that varies
between 600 and 1100 microns respectively. Patient's experienced
relief from severe angina when myocardium penetration was less than
3 mm. Lower average power at the 3-4 watt range per pulse with 1 mm
diameter optical fiber and fiber advancement up to 5 mm compensated
for many variables discussed above.
[0081] At a 3.5 watt setting operating at 5 Hz, the laser readily
ablates approximately 1 mm sponge-like endocardial wall thickness
and an additional 0.5 mm of denser myocardial wall thickness during
the initial pulse, depending on tip location, with each subsequent
pulse ablating approximately 0.5 mm of myocardium tissue. This step
provides a welling action in the endocardial layer that assists in
oxygenated blood transportation into the channels formed in the
myocardium. Partial myocardial wall penetration for an average
patient requires three or four pulses to achieve a 2-3 mm channel
into the myocardium. For patient's with greater predetermined left
ventricle wall thickness, the fiber advance can penetrate up to 5
mm with five laser pulses being fired. The amount of energy
delivered using 3-4 watts and a maximum of five pulses is
insufficient to cause unwanted lateral necrosis or epicardium
penetration in any of the three possible catheter tip delivery
positions discussed above, but is sufficient to relieve angina. For
example with optical fiber advancement limited to 5 mm, when an
optical fiber tip is juxtaposed to the endocardium with a 3.5 watt
setting where 3-5 pulse are fired, the laser pulses typically would
form a channel approximately 2.5 to 3.5 mm depending upon whether
the optical fiber is stationary or advancing after each pulse
firing. If the fiber's optical tip pierces the endocardium prior to
initial pulse firing and the laser is at the same power level and a
pulse preset firing mode of 3-5 pulses, depth of the channel is
approximately approximately 3.5 to 4.5 mm depending upon whether
the optical fiber is stationary or advancing after each pulse
firing. Thus, the lower power settings provides a safety margin
that ensures patient's safety in a worst case scenario.
[0082] Although low power in the 3-4 watt range is preferred,
alternate parameters can be a single laser pulse with laser average
power settings in a 7 to 9 watt range when the heart is contracted.
Generally, the alternate parameter is particularly suitable when
the delivery system is urged against endocardium, and the energy is
delivered without fiber advancement. The single pulse prevents
cratering and the energy perforates approximately no more than 5
mm, or half the typical thickness of the myocardium.
[0083] If a Xe:Cl excimer laser is used as the laser source in the
PTMR procedure, the excimer laser operates at around a 50
mJ/mm.sup.2 active fluence energy setting at 15 Hz. The laser
ablates approximately 0.5 mm of the sponge-like endocardial wall
thickness during initial pulsation at the inner endocardial wall
where subsequent pulses in the myocardium ablate slightly less than
0.5 mm of myocardial tissue, these factors are dependent upon the
heart's state and position of the fiber prior to pulsing. A channel
in the myocardial wall for an average patient requires 8-10 pulses.
For patient's with greater left ventricle wall thickness, the fiber
advance can penetrate up to 5 mm using up to ten laser pulses.
[0084] If desired, the position of the catheter tip may be
positively determined by partial or total piercing of the
endocardial wall prior to delivery of a first laser pulse. A small
diameter, i.e. less than 1 mm, or a piercing tip type optical fiber
on a larger diameter fiber can be used as taught in U.S. patent
application Ser. No. 08/638,677 entitled AOptical Fiber Device for
Laser Surgery Procedures,@ which is incorporated by reference.
Piercing provides additional stability of the catheter tip to avoid
destruction of endocardial tissue during a PTMR procedure and allow
welling action to occur for oxygenated blood transport to the
myocardial layer. The pulses penetrate the myocardium with an
optical fiber advancement rate of approximately 0.5 mm after each
laser pulse so as to create nearly uniform channels of cylindrical
shape with minimal lateral necrosis. Differing heart anatomies may
require up to five pulses. PTMR channels of approximately the same
depth in the myocadium are created when delivering a single higher
power laser pulse when the heart is in a systolic state and heart
tissue density is greatest.
[0085] The cardiologist may observe the PTMR procedure on an
imaging screen, e.g. using well known fluoroscopic procedures to
observe the catheter=s positioning in relationship to the
endocardial wall. Also, an ultrasound imaging or ranging device in
conjunction with the PTMR catheter as taught in U.S. patent
application Ser. No. 08/7xxx entitled AUltrasound Device For Axial
Ranging,@ which is hereby incorporated by reference, may be used to
determine catheter tip position, particularly in the preferred low
power embodiment when the fiber is advanced during delivery of the
3-5 pulses. The ultrasound device would ideally be used
intermittently or in real time so that fiber advancement could be
assured concurrent with each pulse.
[0086] Fiber advancement for the laser delivery device can
typically be up to 5 mm. Compensation for greater depths of
controlled penetration can be accomplished by adjusting either
power and/or the number of pulses/formed channel. Any of the
parameters discussed above may be easily controlled using the laser
pulse preset mode which ensures that only the desired number of
pulses are delivered upon activation of the laser delivery
mechanism, typically a foot pedal with automatic laser shut-off
even with continued foot pedal activation to ensure safety and
controllability during either TMR and PTMR.
[0087] Method for Selecting Optimum Parameters
[0088] Based on the foregoing, a preferred method for determining
the optimum parameters for performing TMR utilizing any suitable
medical laser available will be apparent. At the outset, an initial
consideration is to achieve the minimum threshold for tissue
ablation and minimize the total amount of energy utilized to
complete the procedure. Adapting parameters to this consideration
will decrease the overall trauma to the heart and minimize the risk
of inducing arrhythmia. A power setting which eliminates potential
linear effects such as short, explosively high peak power pulses
will be more desirable.
[0089] Additionally, when using a pulsed laser, which is in general
more desirable than a continuous wave laser, increasing the pulse
width, to prevent an excessively high peak power, to deliver a
predetermined amount of energy in a given pulse, up to the point of
thermal damage caused by summation effects, is desirable. In other
words, the thermal relaxation time and factors which might affect
that value including, but not limited to, heart temperature, is
considered.
[0090] Furthermore, contouring or shaping the front end of the wave
form to provide an efficient cutting or ablation wave shape but to
prevent explosive, linear square wave shapes. Again, avoiding an
excessively high peak power spike within the pulse will be
advantageous.
[0091] Finally, correcting for mechanical events inherent in the
selected TMR system, including but not limited to the elected
access to the heart, duration of the entire procedure, manipulation
of the laser delivery means and movement of the beating heart; and,
mechanical events inherent in the individual patient, including but
not limited to heart geometry, pre-existing heart arrhythmia or
other factors causing a predisposition to such.
[0092] The present invention is intended for use with any medical
laser. In particular, the Holmium or excimer laser is particularly
suited to the present invention. However, any suitable laser
source, pulsed or otherwise, could provide laser energy to the
laser delivery means of the present invention for performing the
method of the present invention. Likewise, the catheter and
surgical equipment, including laser delivery means, referred to in
the present document as well as that known and used in medicine and
other disciplines today and in the future, will be included in the
scope of this disclosure. Such laser delivery means include, but
are not limited to, individual optical fibers as well as bundles of
fibers, rods, mirrors configurations and other laser delivery means
with and without focusing lens and the like. It will also be
understood that the apparatus and method of the present invention
as described herein, including the novel combinations or use with
any conventional mechanism or method which are known to those
skilled in the art, are included within the scope of this
invention.
[0093] It will further be understood that while the present
invention has been described for performing TMR on endocardial
surfaces in the left ventricle, the apparatus and methods described
herein are equally intended for use in any suitable procedure,
including but not limited to procedures where any device need be
extended through a guiding catheter to an opening or other point
within the body for other medical procedures including laser
treatment, visualization, biopsy, etc. Stimulation, for example, is
performed by using laser energy to create zones or pockets,
optionally interconnected at least initially by small channels
ablated through the tissue, for the introduction of blood born
growth and healing factors and stimulated capillary growth
surrounding the lased zones or pockets to create an increased
supply of oxygen to the tissue and thus a revitalization of the
heart muscle. Methods and apparatus for causing stimulation are
more fully described in co-pending U.S. patent application Ser. No.
08/664,956 filed Jun. 13, 1996.
[0094] While the principles of the invention have been made clear
in illustrative embodiments, there will be immediately obvious to
those skilled in the art many modifications of structure,
arrangement, proportions, the elements, materials, and components
used in the practice of the invention, and otherwise, which are
particularly adapted to specific environments and operative
requirements without departing from those principles. The appended
claims are intended to cover and embrace any and all such
modifications, with the limits only of the true spirit and scope of
the invention.
* * * * *