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BACKGROUND OF THE INVENTION
This invention concerns surgical instruments and procedures and, in
particular, systems, devices and methods for performing laser surgery
(e.g., angioplasty) to treat atherosclerosis and the like.
Atherosclerosis is a disease which causes the thickening and hardening of
the arteries, particularly the larger artery walls. It is characterized by
lesions of raised fibrous plaque which form within the arterial lumen. The
plaques are most prevalent in abdominal aorta, coronary arteries and
carotid arteries and increase progressively with age. They commonly
present dome-shaped, opaque, glistening surfaces which bulge into the
lumen. A lesion typically will consist of a central core of lipid and
necrotic cell debris, capped by a collagen fibromuscular layer.
Complicated lesions will also include calcific deposits and various
degrees of necrosis, thrombosis and ulceration.
The deformities of the arterial lumen presented by the plaque and
associated deposits result in occluded blood flow, higher blood pressure
and ultimately ischemic heart disease, if untreated. In 1984, coronary
atherosclerosis was still the leading cause of death in the United States,
claiming the lives of over a half million Americans annually, roughly
twice as many as are killed by cancer.
The treatment of coronary atherosclerosis presently consists of drug
therapy, thoracic surgery and percutaneous angioplasty. Drug therapy is
primarily directed to the control of hypertension with diuretics,
antiadrenergic agents, vasodilators and angiotension blockers. The goal of
the drug therapy is to return the arterial pressure to normal levels and
thereby reduce the stress on the patient's heart, kidneys and other
organs. Unfortunately, drug therapy is not without side effects and cannot
be relied upon to control progressive or acute atherosclerosis.
In the more serious instances of coronary atherosclerosis, thoracotomies
are typically performed and so called "bypass" operations are conducted.
In the bypass procedure, a vein (usually from the patient's leg) is
utilized to construct a detour around the occluded coronary artery. One
end of the vein is attached to the aorta, and the other end is attached to
occluded vessel just beyond the obstruction. Although bypass surgery has
become an accepted surgical procedure, it presents substantial morbidity
risks, involves costs ranging from $25,000 to $40,000 in 1984 dollars, and
generally requires extended hospitalization. Moreover, the procedure is
often limited to arteries proximal to the heart and the long-term
prognosis is less than satisfactory. Roughly 5 percent of the bypass
grafts can be expected to occlude with each year following the operation;
during this time it is not uncommon for the native artery to also become
completed occluded as well, necessitating repeated procedures.
Recently, small balloon-tipped catheters have been developed which can be
passed percutaneously into various arteries and inflated to dilate areas
of partial obstruction. While this procedure has gained a measure of
acceptance as a less invasive alternative to bypass surgery, balloon
angioplasty simply redistributes the atherosclerosis stenoses; the
frequency of reoccurence may be as high as 30 percent and such
reoccurences further increase both as a function of the number of lesions
treated and the time post-angioplasty.
Laser therapy has been suggested as another approach to angioplasty. For
example, in a proposed procedure a catheter carrying a fiber optic
waveguide is passed into an occluded blood vessel, positioned proximal to
an atherosclerstic lesion and activated to decompose the plaque. Devices
along these lines have been disclosed, for example, in U.S. Pat. No.
4,207,874 issued to Choy on June 17, 1980; and U.S. Pat. No. 4,448,188
issued to Loeb on May 15, 1984. See also generally, Marcruz et al.,
"Possibilidades Terapeuticas do Raio Laser em Ateromas," Vol 34, No. 9,
Arq Bras Cardiol, (1980); Lee et al., "Laser Dissolution of Coronary
Atherosclerosis Obstruction, Vol. 102, Amer Heart J, pp. 1074-1075 (1980);
Abela et al., "Effects of Carbon Dioxide, Nd:YAG, and Argon Laser
Radiation on Coronary Atherosclerosis Plaques,", Vol. 5, Amer J Cardiol,
pp. 1199-1205 (1982); Choy et al., "Transluminal Laser Catheter
Angioplasty" Vol. 50, Amer J Cardiol, pp. 1206-1208 (December 1982); Choy
et al " Laser Coronary Angioplasty: Experience with 9 cadaver hearts,"
Amer J Cardiol, Vol. 50, pp. 1209-1211, (1982); Ginsberg et al., "Salvage
of An Ischemic Limb by Laser Angioplasty, Description of a New Technique,"
Vol. 7, Clin Cardiol, pp. 54-58 (1984); Isner and Clark, "The Current
Status of Lasers in the Treatment of Cardiovascular Disease", Vol. QE-20,
No. 12, IEEE J Quantum Electronics, pp. 1406-1414 (1984); and Abela et
al., "Laser Recanalization of Occluded Atherosclerotic Arteries In Vivo
and In Vitro," Vol. 71, Circulation pp. 403-422 (1985), the teachings of
which are incorporated herein by reference.
At present the use of laser angioplasty almost entirely has been restricted
to animal studies and in vitro experiments on vessels obtained from human
donors post mortem. The few reports of human therapy appear to confirm the
feasibility of the procedure but the patency of arteries recannalized
using present laser therapy techniques remains to be proven. A number of
difficulties and adverse side effects also have emerged from the studies
to date. Many of the attempts on excised blood vessels have resulted in
charred tissue, coagulation necrosis and/or polymorphous lacunae. These
pathological injuries suggest that blood vessels treated by laser
angioplasty will require significant healing time periods and may be left
with scarred, thromobogenic surfaces. Moreover, two other serious problems
with present techniques are thermal and mechanical perforation. These
perforations occur most commonly in connection with calcific deposits,
branch points and torturous coronary arterial segments. Branch points and
torturous coronary segments lead to mechanical and thermal perforations
when they cause the optical fiber not to be coaxial with the artery.
Several histopathologic features of atherosclerotic arterial segments
contribute directly to the problem of mechanical-thermal perforations.
First, collagen is a principal component of atherosclerotic plaque.
Because collagen typically imparts a white hue to the intimal surface of
the plaque, the output of at least one commercially available laser
system, the argon laser with its 454.514 nm blue-green light, is not
preferentially absorbed. Second, calcification of the plaque further
diminishes absorbance. Consequently, when the optical fiber initiates
vaporization of plaque, the fiber will often "track" away from the
calcified, severely fibrotic portions of the plaque toward the "softer"
portions of the plaque, such as foci of yellow pultaceous debris or well
developed (red) vascularity. These sites constitute a potential path of
least resistance, which not infrequently promotes eccentric
fiber-penetration of plaque into the highly absorbant (red) media and
then, the outer adventitia. The result is perforation. Third, because the
principal component of the media, smooth muscle, is characteristically
depleted or attentuated in segments of atherosclerotic coronary arteries,
a limited margin for error exists between the target of vaporization
(i.e., plaque) and the underlying arterial wall.
There exists a need for better methods and devises for performing
angioplasty. A system which could selectively remove complicated plaque
lesions and associated materials from the arterial lumen with minimal
injury to the underlying tissue and less risk of thermal or mechanical
perforation would represent a substantial improvement. A system suitable
for use in a surgical environment, with its components sealed to patient
exposure and set to operate within a predefined range of optimal
conditions, would satisfy a significant need in the art.
SUMMARY OF THE INVENTION
It has been discovered that a laser therapy system having improved
effectiveness in surgical use, particularly in laser angioplasty, can be
formed by employing a pulsed source of radiation. The pulsed energy source
is preferably an excimer laser having a coherent beam of ultraviolet
radiation and preferably is employed in conjunction with a dye laser to
produce an output beam which is tunable over a wide portion of the
ultraviolet and visible spectrum The output beam is coupled via an optical
fiber to the surgical instrument, for example, a percutaneous catheter In
operation, a pulsed, high energy beam of extremely short duration is
available to remove atherosclerotic plague with less damage to the
underlying tissue and less chance of perforating the blood vessel wall.
In one aspect of the invention, the unique properties of excimer energy
sources are exploited. "Excimer" is a coined word, used to describe the
physical operation of certain gas lasers which typically contain noble
gas-halide combinations as the active medium. In operation, an electrical
discharge or ionizing field is applied to the medium and energy is
absorbed by the individual atoms, thereby raising them to a higher energy
state. The electrical excitation of one of the atoms (i.e. the halogen)
initiates bonding with the other atomic species (i.e. xenon or krypton)
resulting in the formation of an electronically "excited dimer" or
"excimer". As the molecule returns to its ground state, short wavelength
(and correspondingly high energy) ultraviolet radiation is emitted. In one
preferred embodiment, a Xenon-Hydrogen Chloride-Neon excimer medium is
employed with suitable buffer gases in a mode-locked or Q-switched
configuration. Another useful medium is Xenon-Fluoride-Neon. Various other
combinations can also be employed and may be preferred for particular
applications. The precise wavelength emitted by the excimer energy source
can be varied by choice of the gas mixture.
In particular applications it may also be preferred to employ pulsed energy
sources other than excimer lasers. The pulsed laser medium can be gaseous,
liquid or solid state. Rare earth-doped solid state lasers, ruby lasers,
alexandrite lasers, carbon dioxide lasers, Nd: YAG lasers and Ho:YLF
lasers are all examples of lasers that can be operated in a pulsed mode
and used in the present invention.
The term "pulsed" is used herein to describe lasers which generate peak
powers on the order of 100 kilowatts per square centimeter or greater
(with peak power being defined in terms of pulse energy over pulse length)
Preferably the peak power of the pulsed radiation source is at least 500
kilowatts per square centimeter. Also preferably, the pulse length of the
output pulse is about 1 microsecond or less. Typically, the laser medium
is excited by a capacitive discharging flash lamp or similar fast
excitation source.
The operation of the pulsed laser provides a substantial improvement to
laser angioplasty. Until now researchers have been applying continuous
wave ("CW") radiation to atherosclerotic plague with limited success.
Thermal injury of the laser irradiated surfaces has been a consistent
consequence of CW laser irradiation. Grossly thermal injury is manifested
by charring that is easily recognized by non-magnified visual inspection.
Under the microscope, the effects of CW irradiation are characteristically
evidenced by coagulation necrosis and polymorphous lacunae. In contrast,
similar experiments on excised blood vessels with pulsed radiation reveals
no gross or microscopic evidence of thermal injury. The high peak energy
density and extremely short duration of excimer radiation appears to
greatly reduce the thermal effects outside of the path of the laser beam.
Thus, the invention can be employed in angioplasty to avoid necrosis and
lacunae, facilitate a more benign healing process and less thrombogenic
surfaces, and in general better preserve structure and tissue integrity.
Moreover, by accomplishing lesion vaporization without non-target heating,
the invention also can be expected to reduce the principal complication of
laser angioplasty, blood vessel wall perforation.
In another aspect of the invention, a tunable dye laser is pumped by the
pulsed laser to produce an output beam which can be tuned, for example,
from about 300 nanometers to about 1000 nanometers in wavelength. In a
preferred embodiment, an eximer laser and dye laser combination are
designed to operate at about 307 nanometers or greater to avoid the
potential mutagenic or tumorigenic risks that have been suggested may
result from exposure to shorter wavelength UV radiation. A dye laser of
open cavity design is preferable (i.e., no intra cavity tuning elements)
to promote high conversion efficiency. The dye laser can consist of a
rectangular flowing dye cell within a kinematic mount to facilitate easy
changes of the dye, making both tuning and degraded dye replacement rapid
and safe.
Suitable dyes for use in the dye laser components of the invention include,
for example, P-terphenyl (peak wavelength 339); BiBuQ (peak wavelength:
385); DPS (peak wavelength: 405); and Coumanin 2 (peak wavelength: 448).
In addition, the excimer-dye laser system is preferably designed to allow
the user to bypass the dye cell and extract the excimer pump beam directly
for particular applications. Thus, for example, a Xenon-Hydrogen
Chloride-Neon excimer laser can be applied without the dye laser to
provide a high energy radiation source at about 307-308 nanometers.
In a further aspect of the invention, the operative components of the
invention are sealed in a gas tight, liquid tight housing to insure
against patient or user exposure in the surgical setting. Nonetheless, the
system is designed for ready manipulation by practitioners and technicians
during use. In a preferred embodiment, an electronic controller monitors
and adjusts the output energy density, duration, and pulse repetition
rate. Similarly, the controller can monitor the state of the excimer gas
and the laser dye to indicate when replacement is necessary. The
controller can also be connected to manual or foot controls, adjustment
knobs, visual displays and paper printouts. Preferably, the housing also
includes a filter to trap waste gases and fittings to receive sealed
canisters or cartridges for dye replacements.
The output beam from the system preferably has a peak energy density
ranging from about 0.2 to about 20 Joules per square centimeter,
preferably from about 0.5 to about 10 Joules per square centimeter. The
pulse duration preferably ranges from about 5 nanoseconds to about 100
nanosecond and the period between pulses can vary from about 1 millisecond
to about 1 second.
The output beam is coupled to a catheter to deliver laser therapy through a
receptacle, hermetically sealed to the housing of the laser system, having
an opening adapted to receive a optical waveguide (i.e., a
light-transmitting fiber) which passes into the catheter. The coupling
device also includes a fastening means for releasably fastening the
waveguide in a precise position in the receptacle during use and for
releasing the waveguide once the laser therapy is completed. Moreover, the
coupling device can include a focusing means formed within the receptacle
for receiving the beam of radiation from the laser system and focusing the
radiation into the waveguide fiber. The receptacle preferably is designed
such that the circumferential periphery of the waveguide fiber is
protected from the incident laser radiation by a mask or the like in order
to avoid chipping or otherwise damaging the fiber when high energy,
radiation pulses are focussed into the waveguide fiber.
Catheters, useful in practicing laser angioplasty with the laser system of
the present invention, can take various forms. For example, one embodiment
can consist of a catheter having an outer diameter of 3.5 millimeters or
less, preferably 2.5 millimeters or less. Disposed within the catheter is
the optical fiber for delivery of the laser therapy which can be a 100-200
micron diameter silica (fused quartz) fiber such as the model SG 800 fiber
manufactured by Spectran, Inc. of Sturbridge, Masss. The catheter is
preferably multi-lumen to provide flushing and suction ports. In one
embodiment the catheter tip can be constructed of radio-opaque and heat
resistant material, incorporating a transducer for either an ultrasound or
microwave source of imaging. The radio-opaque tip can be used to locate
the catheter under fluoroscopy. The catheter can also include an
inflatable balloon, such as that described in U.S. Pat. No. 4,448,188
issued to Loeb on May 15, 1984 or in International patent application No
PCT/US82/01669 by G. Lee filed on Nov. 30, 1982, or in an article by
Gruntzig et al., "Nonoperative Dilation of Coronary Artery Stenoses:
Percutaneous Transluminal Angioplasty", Vol. 301, New England Journal of
Medicine, pp. 66-68 (1979), each of which is incorporated herein by
reference. In use the balloon can be inflated periodically to stop blood
flow for viewing and/or laser therapy, to allow saline flushing or to
remove debris by suction. Additionally, the catheter can include a
steering means such as that disclosed in U.S. Pat. No. 3,470,876 issued to
Barchilon on Oct. 7, 1969, also incorporated herein by reference. The
catheter should be readily sterilizable and preferably is disposable.
The invention will next be described in connection with certain illustrated
embodiments. However, it should be clear that various changes and
modifications can be made by those skilled in the art without departing
from the spirit or scope of the invention. For example, various other
pulsed lasers can be substituted for the excimer source. Various gas
mixtures can be employed in the excimer laser. For examples of particular
excimer laser media and configurations, see U.S. Pat. Nos. 34,426,706
issued to Liu et al.; 4,393,505 issued to Fahlen; 4,348,647 issued to
Nigham et al.; and 4,340,968 issued to Willis et al., herein incorporated
by reference. Similarly, various dye materials can be used in the dye
laser. Configurations other than a free-flowing dye , such as
dye-impregnated plastic films or curvette-encased dyes, can be substituted
in the dye laser. The dye laser can also store a plurality of different
dyes and substitute one for another automatically in response to
user-initiated control signals or conditions encountered during
angioplasty (e.g. when switching from a blood-filled field to a saline
field or in response to calcific deposits).
The invention can be used with various catheter devices, including devices
which operate under fluoroscopic guidance as well as devices which
incorporate imaging systems, such as echographic or photoacoustic imaging
systems or optical viewing systems. For one example of a photoacoustic
imaging system which can be specifically adapted for the catheter
environment, see U.S. Pat. No. 4,504,727 incorporated herein by reference.
When an optical viewing system is employed, the laser system can also
generate a beam of visible light for illumination purposes and the
catheter can also be designed to provide a focusing spot so that a user
viewing the field can precisely determine the focal point of the
therapeutic UV radiation beam. It may also be preferred to employ various
fluorogenic agents within the blood vessel for imaging purposes including
fluorogenic agents that are selectively absorbed by the plaque deposits.
Additionally, it may be preferred to practice the invention in conjunction
with the administration of hematorporphyrins or other agents that are
selectively taken up by plaque materials and aid in lyzing the lesion when
activated by the laser radiation.
Although the principal use described herein is laser angioplasty, it should
also be clear that the laser therapy system of the present invention can
also be coupled to other surgical instruments besides catheters designed
for percutaneous passage into an arterial lumen. The invention can also be
employed in conjunction with a laser scalpel for general operative
procedures or in conjunction with an endoscope for tracheal or gastric
operation. Moreover, the laser therapy system can be used with
intra-operative cardiac catheters to perform procedures such as myotomies
or myectomies within the heart or to debride calcific deposits on the
aortic valves. BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic block diagram of a laser therapy system according to
the invention;
FIG. 2 is a detailed schematic diagram of the excimer laser beam shaping
optics of the system shown in FIG. 1;
FIG. 3 is a detailed schematic diagram of the dye cell of the system shown
in FIG. 1;
FIG. 4 is a detailed schematic diagram of the coupling assembly of the
system shown in FIG. 1;
FIG. 5 is a detailed schematic diagram of a catheter instrument for use in
the system of FIG. 1.
DETAILED DESCRIPTION
FIG. 1 shows an overall block diagram of the laser therapy system 10 of the
present invention consisting of a housing 12 which encases a pulsed laser
(i.e., an excimer laser) 14 together with its high voltage power supply 16
and the excimer gas storage tank 18. Also disposed within the housing 12
is a dye laser cell 20 which is pumped by the output beam 30 of the
excimer laser 14. In addition to the dye laser cell 20 (discussed in more
detail below) the housing 12 also encloses a dye reservoir 22 and a dye
pump 24. The housing is designed to provide a sealed, gas-tight,
liquid-tight enclosure to ensure against patient or user exposure in the
surgical setting. The illustrated excimer laser is cooled by a heat
exchanger 32 external to the housing 12 via a circulating coolant. An
external vacuum source 36 is also provided to maintain the excimer medium
under the proper conditions for lasing. A trap filter 34 can be
incorporated into the vacuum line in order to remove any gas contaminants
and/or aid in gas replacement. The housing 12 is also fitted with a dye
substitution port 26 to allow rapid and safe dye replacement or
substitution.
In operation, the high voltage supply 16 under microprocessing control 46
excites the gas in the excimer laser 14 to yield an output beam 30. This
excimer output beam 30 is shaped by a lens system 50 and directed into the
dye laser cell 20 wherein it reacts with the flowing dye to yield a dye
laser output 40 which is focused on laser output port 70 for focusing into
the optical fiber 48 disposed within catheter 80. A foot pedal or manual
control (not shown) is connected to microprocessor 46. Shutter 42 is also
connected to microprocessor 46 and, when open, allows the output beam 40
to pass into the output port 70. The microprocessor 46 is also connected
to an output power monitor 47. Both the excimer laser 14 and the dye laser
20 as well as the dye pump 24 and the high voltage supply 16 are also
continuously monitored by the microprocessor 46 which can be programmed to
yield an output beam of a particular power, pulse duration, wavelength and
exposure time. Microprocessor 46 can also be connected to an imaging
system 110, such as an optical, echographic or photoacoustic imaging
system, and can automatically shut down the output beam 40 in response to
detected changes in the image signal, e.g., changes that would indicate a
danger of arterial wall perforation.
In FIG. 2, a more detailed schematic diagram of the excimer laser beam
shaping optics 50 is shown, consisting of a first negative lens 52 and a
subsequent condensing lens 54 which together form a Gallilean telescope.
In one preferred embodiment, the power of the telescope is 4 to 6 times
magnification. The effect of the two telescopic lenses 52, 54 is to take a
nominally square beam of about 1 cm .times.1 cm dimension and spread the
beam out to a width of 4 to 6 cm. In the illustrated embodiment, this
magnified beam is then reflected off of mirror 56 and passed through a
cylindrical lens 58 having a 6 to 10 cm focal length. The effect of the
cylindrical lens 58 is to focus the magnified beam to a line focus of
about 4-6 centimeters long and 0.5-1.0 mm high. This focussed line 60 acts
as the dye laser pump to produce dye laser output beams 40.
In FIG. 3, the details of one dye cell configuration are illustrated.
Although the illustration shows a vertically free-flowing dye 28 and a
horizontal output beam 40, it should be appreciated that other
configurations are equally feasible and that in particular applications it
may be preferable to pass the dye horizontally through the cell 20 and
produce a vertical output beam. In FIG. 3, a dye cell 20 is shown,
consisting of a hollow cell housing 61 and a cell insert 26. The cell
insert is designed with funnel-shaped or beveled inlets and outlets to
permit the flowing dye 28 to pass through the cell. The insert 26 also
includes general reflective coating 63 on its back wall, (i.e. an
aluminized coating), to reflect the focused excimer laser beam 60 back
into the dye for additional pumping. The housing 61 is designed to be
transmissive to excimer radiation in the ultraviolet region of the
spectrum (i.e. a quartz cell housing). As shown in FIG. 3, the effect of
the pump beam 60 is to induce lasing in the dye 28 within cell 20. A
reflective coating on 62 on the outside wall of housing 61 forms one
reflective surface of a resonant cavity and a partially transmissive
output coupler 64 forms the other wall. The resonant cavity defined by
these two reflectors yields a dye laser beam aligned along the Z axis. It
is also preferred to mount one end of the dye cell 20 on a translational
stage 66 such that motion in the X or Y direction of the stage will permit
slight tilting of the dye cell (i.e. in the XZ or YZ directions for
precise alignment). Similarly, the output coupler 64 is also mounted on a
translational stage 68 for alignment purposes. In one preferred
embodiment, the output coupler can include a spatial filter and thereby,
improve the spatial quality of the beam.
In FIG. 4, the output coupling port 70 is shown in more detail, consisting
of a receptacle 72 sealed to the housing 12. Disposed within the
receptacle is a lens 74, through which the laser output beam 40 is
focused. The receptacle also has a hollow external opening which includes
a fastening means 76 for releasably fastening a snap-in connector 78
carrying the optical fiber 48. In operation, the connector 78 is inserted
into the fastening means 76 and the lens 74 focuses the laser output beam
40 into the optical fiber 48. Also shown in FIG. 4 is mask 77, which is
disposed along the beam path to shield the peripheral region of the end
face of fiber 48 from incident laser radiation in order to avoid chipping
or otherwise damaging the fiber when the high energy pulsed radiation of
beam 40 is focused into the fiber 48.
In FIG. 5, a schematic illustration of one embodiment of a laser catheter
80 for performing laser angioplasty is shown. FIG. 5 shows the details of
the proximal end of the catheter 80 which is designed to be inserted into
the patient and located proximal to the atherosclerotic plaque. Catheter
80 includes an outer sheath 82 and a plurality of internal lumens: a laser
therapy transmitting fiber 84, an illuminating fiber 86, a viewing fiber
88, a flushing port 90 and a suction port 92. In the illustrated
embodiment, the catheter also includes a balloon 94 to allow the
practitioner to restrict blood flow in front of the catheter.
Additionally, the illustrated embodiment includes a radio-opaque tip
material 96 to aid the user in locating the catheter within the body by
fluoroscopy.
In use, the invention can be practiced by connecting a sterilized
disposable catheter, such as that illustrated in FIG. 5 or a similar
device, to the laser therapy delivery system 10 of FIG. 1. The system 10
is then programmed via controls 46 for a particular set of operating
conditions. For example, the ou | | |
