U.S. patent application number 13/281244 was filed with the patent office on 2012-05-10 for microwave catheter apparatuses, systems, and methods for renal neuromodulation.
This patent application is currently assigned to Medtronic Ardian Luxembourg S.a.r.l.. Invention is credited to Mark Gelfand, Karun D. Naga, Arye Rosen, Roman Turovskiy, Denise Zarins.
Application Number | 20120116486 13/281244 |
Document ID | / |
Family ID | 44906469 |
Filed Date | 2012-05-10 |
United States Patent
Application |
20120116486 |
Kind Code |
A1 |
Naga; Karun D. ; et
al. |
May 10, 2012 |
MICROWAVE CATHETER APPARATUSES, SYSTEMS, AND METHODS FOR RENAL
NEUROMODULATION
Abstract
Microwave catheter apparatuses, systems, and methods for
achieving renal neuromodulation by intravascular access are
disclosed herein. One aspect of the present application, for
example, is directed to apparatuses, systems, and methods that
incorporate a catheter treatment device comprising an elongated
shaft. The elongated shaft is sized and configured to deliver a
microwave transmission element to a renal artery via an
intravascular path. Renal neuromodulation may be achieved via
dielectric heating in the presence of microwave irradiation that
modulates neural fibers that contribute to renal function or alters
vascular structures that feed or perfuse the neural fibers.
Inventors: |
Naga; Karun D.; (Los Altos,
CA) ; Turovskiy; Roman; (San Francisco, CA) ;
Zarins; Denise; (Saratoga, CA) ; Gelfand; Mark;
(New York, NY) ; Rosen; Arye; (Cherry Hill,
NJ) |
Assignee: |
Medtronic Ardian Luxembourg
S.a.r.l.
Luxembourg
LU
|
Family ID: |
44906469 |
Appl. No.: |
13/281244 |
Filed: |
October 25, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61406534 |
Oct 25, 2010 |
|
|
|
Current U.S.
Class: |
607/102 ;
607/101 |
Current CPC
Class: |
A61B 18/1815 20130101;
A61B 2018/00511 20130101; A61B 2018/00577 20130101; A61B 2018/00023
20130101; A61B 2018/00434 20130101; A61B 2018/00404 20130101; A61B
2018/0022 20130101; A61B 2018/00791 20130101; A61B 2018/1861
20130101 |
Class at
Publication: |
607/102 ;
607/101 |
International
Class: |
A61N 5/02 20060101
A61N005/02 |
Claims
1. A catheter apparatus, comprising: an elongated shaft having a
proximal portion, a distal portion, and a central lumen; a
therapeutic assembly at the distal portion of the elongated shaft,
wherein the therapeutic assembly is configured for intravascular
delivery to a renal artery of a human patient; and a microwave
transmitting element carried by the therapeutic assembly, wherein
the microwave transmitting element is configured to radiate
microwaves through a wall of the renal artery to modulate a renal
nerve at a treatment site within the renal artery.
2. The catheter apparatus of claim 1 wherein the therapeutic
assembly comprises a tubular inner conductor, an outer conductor,
and an insulator separating at least a portion of the inner
conductor and the outer conductor.
3. The catheter apparatus of claim 1, further comprising a
radiating element and a shield at least parially surrounding the
radiating element, wherein the shield is configured to
preferentially direct the radiated microwaves.
4. The catheter apparatus of claim 1, further comprising a radiator
positioned in the central lumen.
5. The catheter apparatus of claim 4 wherein: the therapeutic
assembly comprises a tubular inner conductor, an outer conductor,
and an insulator separating at least a portion of the inner
conductor and the outer conductor, and the radiator comprises
protrusions configured to contact an inner wall of the inner
conductor to electrically couple the radiator to the inner
conductor.
6. The catheter apparatus of claim 4 wherein the radiator extends
distally beyond a distal end of the elongated shaft.
7. The catheter apparatus of claim 1 wherein the central lumen
comprises a coolant supply channel configured to deliver coolant in
a vicinity of the microwave transmitting element.
8. The catheter apparatus of claim 7, further comprising a
temperature sensor coupled to the elongated shaft and configured to
sense the temperature in a vicinity of the microwave transmitting
element.
9. The catheter apparatus of claim 1, further comprising a guide
wire positioned in the central lumen, wherein the therapeutic
assembly is configured to be delivered over the guidewire for
placement within the renal artery.
10. A method for treatment of a human patient via renal
denervation, the method comprising: intravascularly positioning a
catheter having a microwave transmitting element within a renal
artery of the patient; generating microwaves with a microwave
generator positioned externally to the patient and transferring the
microwaves through the catheter to the microwave transmitting
element; and radiating the microwaves from the microwave
transmitting element through a wall of the renal artery to modulate
neural function of a renal nerve of the patient.
11. The method of claim 10 wherein modulating neural function of
the renal nerve comprises dielectrically heating the renal
nerve.
12. The method of claim 11 wherein dielectrically heating the renal
nerve comprises inducing necrosis in the renal nerve.
13. The method of claim 10 wherein intravascularly positioning a
catheter having a microwave transmitting element within a renal
artery of the patient comprises centering the microwave
transmitting element within the renal artery.
14. The method of claim 13 wherein centering the microwave
transmitting element within the renal artery comprises centering
the microwave transmitting element within the artery without fully
occluding blood flow through the renal artery.
15. The method of claim 10 wherein generating microwaves with a
microwave generator comprises generating microwaves with a cavity
magnetron.
16. The method of claim 10 wherein intravascularly positioning a
catheter having a microwave transmitting element comprises
positioning a catheter having a microwave antenna.
17. The method of claim 16 wherein intravascularly positioning a
catheter having a microwave antenna comprises positioning a
catheter having a coaxial antenna.
18. The method of claim 17 wherein transferring the microwaves
through the catheter to the microwave transmitting element
comprises transferring the microwaves through a coaxial cable to
the coaxial antenna.
19. The method of claim 17 wherein the coaxial antenna includes a
radiating element, and wherein radiating the microwaves from the
microwave transmitting element comprises dynamically varying a
length of the radiating element.
20. The method of claim 17 wherein the coaxial antenna includes a
radiating element, and wherein radiating the microwaves from the
microwave transmitting element further comprises shielding a
portion of the radiating element of the coaxial antenna to
preferentially direct the radiated microwaves.
21. The method of claim 10 wherein radiating the microwaves from
the microwave transmitting element comprises preferentially
directing the radiated microwaves.
22. The method of claim 10 wherein intravascularly positioning a
catheter having a microwave transmitting element within the renal
artery comprises advancing the catheter to the renal artery over a
guide wire via an intravascular path.
23. The method of claim 10, further comprising redirecting arterial
blood flow within the renal artery of the patient while radiating
the microwaves.
24. The method of claim 23 wherein redirecting arterial blood flow
comprises increasing a velocity of the blood flow near a wall of
the renal artery in order to enhance a rate of heat transfer
between the wall and the blood flow.
25. The method of claim 10, further comprising actively cooling the
microwave transmitting element while radiating the microwaves.
26. The method of claim 25 wherein actively cooling the microwave
transmitting element comprises circulating a coolant in a vicinity
of the microwave transmitting element.
27. The method of claim 25, further comprising monitoring a
temperature of the microwave transmitting element while radiating
the microwaves.
28. The method of claim 27, further comprising altering the active
cooling or the microwave radiation in response to the monitored
temperature of the microwave transmitting element.
29. The method of claim 10, further comprising monitoring a
temperature of the microwave transmitting element.
Description
CROSS-REFERENCE TO RELATED APPLICATION(S)
[0001] This application claims the benefit of U.S. Provisional
Application No. 61/406,534, filed Oct. 25, 2010, and incorporated
herein by reference in its entirety.
TECHNICAL FIELD
[0002] The technologies disclosed in the present application
generally relate to catheter apparatuses, systems, and methods for
intravascular neuromodulation. More particularly, the technologies
disclosed herein relate to microwave catheter apparatuses, systems,
and methods for achieving intravascular renal neuromodulation via
dielectric heating.
BACKGROUND
[0003] The sympathetic nervous system (SNS) is a primarily
involuntary bodily control system typically associated with stress
responses. Fibers of the SNS innervate tissue in almost every organ
system of the human body and can affect characteristics such as
pupil diameter, gut motility, and urinary output. Such regulation
can have adaptive utility in maintaining homeostasis or in
preparing the body for rapid response to environmental factors.
Chronic activation of the SNS, however, is a common maladaptive
response that can drive the progression of many disease states.
Excessive activation of the renal SNS in particular has been
identified experimentally and in humans as a likely contributor to
the complex pathophysiology of hypertension, states of volume
overload (such as heart failure), and progressive renal disease.
For example, radiotracer dilution has demonstrated increased renal
norepinephrine (NE) spillover rates in patients with essential
hypertension.
[0004] Cardio-renal sympathetic nerve hyperactivity can be
particularly pronounced in patients with heart failure. For
example, an exaggerated NE overflow from the heart and kidneys to
plasma is often found in these patients. Heightened SNS activation
commonly characterizes both chronic and end stage renal disease. In
patients with end stage renal disease, NE plasma levels above the
median have been demonstrated to be predictive for cardiovascular
diseases and several causes of death. This is also true for
patients suffering from diabetic or contrast nephropathy. Evidence
suggests that sensory afferent signals originating from diseased
kidneys are major contributors to initiating and sustaining
elevated central sympathetic outflow.
[0005] Sympathetic nerves innervating the kidneys terminate in the
blood vessels, the juxtaglomerular apparatus, and the renal
tubules. Stimulation of the renal sympathetic nerves can cause
increased renin release, increased sodium (Na.sup.+) reabsorption,
and a reduction of renal blood flow. These neural regulation
components of renal function are considerably stimulated in disease
states characterized by heightened sympathetic tone and likely
contribute to increased blood pressure in hypertensive patients.
The reduction of renal blood flow and glomerular filtration rate as
a result of renal sympathetic efferent stimulation is likely a
cornerstone of the loss of renal function in cardio-renal syndrome
(i.e., renal dysfunction as a progressive complication of chronic
heart failure). Pharmacologic strategies to thwart the consequences
of renal efferent sympathetic stimulation include centrally acting
sympatholytic drugs, beta blockers (intended to reduce renin
release), angiotensin converting enzyme inhibitors and receptor
blockers (intended to block the action of angiotensin II and
aldosterone activation consequent to renin release), and diuretics
(intended to counter the renal sympathetic mediated sodium and
water retention). These pharmacologic strategies, however, have
significant limitations including limited efficacy, compliance
issues, side effects, and others. Accordingly, there is a strong
public-health need for alternative treatment strategies.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] FIG. 1 is a conceptual illustration of the sympathetic
nervous system (SNS) and how the brain communicates with the body
via the SNS.
[0007] FIG. 2 is an enlarged anatomic view of nerves innervating a
left kidney to form the renal plexus surrounding the left renal
artery.
[0008] FIGS. 3A and 3B provide, respectively, anatomic and
conceptual views of a human body, depicting neural efferent and
afferent communication between the brain and kidneys.
[0009] FIGS. 4A and 4B are, respectively, anatomic views of the
arterial and venous vasculatures of a human.
[0010] FIG. 5 is a perspective view of a microwave system for
achieving intravascular renal neuromodulation, comprising a
treatment device and a microwave generator.
[0011] FIGS. 6A and 6B are, respectively, a schematic view
illustrating placement of a distal region of the treatment device
of FIG. 5 within a renal artery via an intravascular path, and a
detailed schematic view of an embodiment of the distal region
within the renal artery for delivery of microwave energy.
[0012] FIG. 7 is a schematic side-sectional view of the distal
region of an embodiment of the microwave treatment device of FIG. 5
comprising a coaxial cable feed line and a coaxial antenna
microwave transmission element.
[0013] FIGS. 8A and 8B are schematic side-sectional views of the
distal region of an embodiment of the microwave treatment device of
FIG. 5 in a low-profile delivery configuration and in an expanded
deployed configuration, including a multi-filament centering
element.
[0014] FIG. 9 is a schematic side-sectional view of the distal
region of an alternative embodiment of the microwave treatment
device of FIGS. 8A and 8B comprising an alternative multi-filament
centering element.
[0015] FIGS. 10A and 10B are, respectively, a schematic
side-sectional view and a schematic cross-sectional view along
section line 10B-10B of FIG. 10A of the distal region of an
embodiment of the microwave treatment device of FIGS. 8A and 8B
comprising an expandable balloon centering element.
[0016] FIGS. 11A and 11B are schematic cross-sectional views of the
distal region of alternative embodiments of the microwave treatment
device of FIGS. 10A and 10B comprising an alternative expandable
balloon centering element.
[0017] FIG. 12 is a schematic side-sectional view of the distal
region of another alternative embodiment of the microwave treatment
device of FIGS. 10A and 10B comprising another alternative
expandable balloon centering element having proximal and distal
balloons.
[0018] FIG. 13 is a schematic side-sectional view of the distal
region of another alternative embodiment of the microwave treatment
device of FIGS. 10A and 10B comprising another alternative
expandable balloon centering element having conductive electrode
traces applied on the surface of the balloon.
[0019] FIGS. 14 and 15 are schematic side-sectional views of the
distal regions of other alternative embodiments of the microwave
treatment device of FIGS. 10A and 10B comprising other alternative
expandable balloon centering elements having one or more shields
applied on the surface of the balloon.
[0020] FIG. 16 is a schematic side-sectional view of the distal
region of another alternative embodiment of the microwave treatment
device of FIGS. 10A and 10B comprising another alternative
expandable balloon that places the microwave antenna off-center
within a vessel.
[0021] FIG. 17 is a schematic view of the distal region of an
embodiment of the microwave treatment device of FIG. 5 comprising a
flow directing element.
[0022] FIG. 18A is a schematic side-sectional view of the distal
region of an embodiment of the microwave treatment device of FIG. 5
configured for delivery over a guide wire.
[0023] FIG. 18B is a schematic side-sectional view of the distal
region of an embodiment of the microwave treatment device of FIG. 5
configured for rapid exchange delivery.
[0024] FIGS. 19A and 19B are schematic side-sectional views of the
distal region of an embodiment of the microwave treatment device of
FIG. 5 illustrating dynamic variation of the coaxial antenna's
radiating element.
[0025] FIG. 20 is schematic side-sectional view of the distal
region of an alternative embodiment of the microwave treatment
device of FIGS. 19A and 19B configured for dynamic variation of the
coaxial antenna's radiating element.
[0026] FIGS. 21A and 21B are schematic side-sectional views of the
distal region of another alternative embodiment of the microwave
treatment device of FIGS. 19A and 19B configured for dynamic
variation of the coaxial antenna's radiating element and for
over-the-wire delivery.
[0027] FIGS. 22A and 22B are schematic side-sectional views of the
distal regions of embodiments of the microwave treatment device of
FIG. 5 configured for active cooling.
[0028] FIG. 23A is a schematic side-sectional view of the distal
region of an embodiment of a microwave treatment device having a
shielding component.
[0029] FIGS. 23B and 23C are cross-sectional views of the microwave
treatment device of FIG. 23A.
[0030] FIGS. 23D and 23E are schematic side views of the distal
region of an embodiment of a microwave treatment device having a
shielding component and deflection capabilities.
DETAILED DESCRIPTION
[0031] The present technology is directed to apparatuses, systems,
and methods for achieving electrically- and/or thermally-induced
renal neuromodulation (i.e., rendering neural fibers that innervate
the kidney inert or inactive or otherwise completely or partially
reduced in function) by percutaneous transluminal intravascular
access. In particular, embodiments of the present technology relate
to microwave apparatuses, systems, and methods that incorporate a
catheter treatment device. The catheter treatment device may
comprise an elongated shaft sized and configured to deliver at
least one microwave transmission element within a renal artery via
an intravascular path (e.g., a femoral artery, an iliac artery and
the aorta, a transradial approach, or another suitable
intravascular path). In one embodiment, for example, the microwave
transmission element comprises an antenna that radiates microwaves
within the renal artery in order to induce dielectric heating of
target renal nerves or of vascular structures that perfuse target
renal nerves. Microwaves are generated by a microwave generator,
transferred along a feed or transmission line, and then radiated by
the antenna. The microwave generator may be positioned along or
near a proximal region of the elongated shaft external to the
patient, while the feed line extends along or within the elongated
shaft, and the antenna is positioned along a distal region of the
shaft configured for placement in the renal artery via the
intravascular path.
[0032] Specific details of several embodiments of the technology
are described below with reference to FIGS. 1-23E. Although many of
the embodiments are described below with respect to devices,
systems, and methods for intravascular modulation of renal nerves
using microwave apparatuses, other applications and other
embodiments in addition to those described herein are within the
scope of the technology. Additionally, several other embodiments of
the technology can have different configurations, components, or
procedures from those described herein. A person of ordinary skill
in the art, therefore, will accordingly understand that the
technology can have other embodiments with additional elements, or
the technology can have other embodiments without several of the
features shown and described below with reference to FIGS.
1-23E.
[0033] As used herein, the terms "distal" and "proximal" define a
position or direction with respect to the treating clinician or the
clinician's control device (e.g., a handle assembly). "Distal" or
"distally" are a position distant from or in a direction away from
the clinician or clinician's control device. "Proximal" and
"proximally" are a position near or in a direction toward the
clinician or the clinician's control device.
I. PERTINENT ANATOMY AND PHYSIOLOGY
[0034] The following discussion provides various details regarding
pertinent patient anatomy and physiology. This section is intended
to provide additional context regarding the disclosed technology
and the therapeutic benefits associated with renal denervation, and
to supplement and expand upon the disclosure herein regarding the
relevant anatomy and physiology. For example, as mentioned below,
several properties of the renal vasculature may inform the design
of treatment devices and associated methods for achieving renal
neuromodulation via intravascular access, and impose specific
design requirements for such devices. Specific design requirements
may include accessing the renal artery, facilitating stable contact
between the energy delivery elements of such devices and a luminal
surface or wall of the renal artery, and/or effectively modulating
the renal nerves with the neuromodulatory apparatus.
[0035] A. The Sympathetic Nervous System
[0036] The Sympathetic Nervous System (SNS) is a branch of the
autonomic nervous system along with the enteric nervous system and
parasympathetic nervous system. It is always active at a basal
level (called sympathetic tone) and becomes more active during
times of stress. Like other parts of the nervous system, the
sympathetic nervous system operates through a series of
interconnected neurons. Sympathetic neurons are frequently
considered part of the peripheral nervous system (PNS), although
many lie within the central nervous system (CNS). Sympathetic
neurons of the spinal cord (which is part of the CNS) communicate
with peripheral sympathetic neurons via a series of sympathetic
ganglia. Within the ganglia, spinal cord sympathetic neurons join
peripheral sympathetic neurons through synapses. Spinal cord
sympathetic neurons are therefore called presynaptic (or
preganglionic) neurons, while peripheral sympathetic neurons are
called postsynaptic (or postganglionic) neurons.
[0037] At synapses within the sympathetic ganglia, preganglionic
sympathetic neurons release acetylcholine, a chemical messenger
that binds and activates nicotinic acetylcholine receptors on
postganglionic neurons. In response to this stimulus,
postganglionic neurons principally release noradrenaline
(norepinephrine). Prolonged activation may elicit the release of
adrenaline from the adrenal medulla.
[0038] Once released, norepinephrine and epinephrine bind
adrenergic receptors on peripheral tissues. Binding to adrenergic
receptors causes a neuronal and hormonal response. The physiologic
manifestations include pupil dilation, increased heart rate,
occasional vomiting, and increased blood pressure. Increased
sweating is also seen due to binding of cholinergic receptors of
the sweat glands.
[0039] The sympathetic nervous system is responsible for up- and
down-regulating many homeostatic mechanisms in living organisms.
Fibers from the SNS innervate tissues in almost every organ system,
providing at least some regulatory function to things as diverse as
pupil diameter, gut motility, and urinary output. This response is
also known as sympatho-adrenal response of the body, as the
preganglionic sympathetic fibers that end in the adrenal medulla
(but also all other sympathetic fibers) secrete acetylcholine,
which activates the secretion of adrenaline (epinephrine) and to a
lesser extent noradrenaline (norepinephrine). Therefore, this
response that acts primarily on the cardiovascular system is
mediated directly via impulses transmitted through the sympathetic
nervous system and indirectly via catecholamines secreted from the
adrenal medulla.
[0040] Science typically looks at the SNS as an automatic
regulation system, that is, one that operates without the
intervention of conscious thought. Some evolutionary theorists
suggest that the sympathetic nervous system operated in early
organisms to maintain survival as the sympathetic nervous system is
responsible for priming the body for action. One example of this
priming is in the moments before waking, in which sympathetic
outflow spontaneously increases in preparation for action.
[0041] 1. The Sympathetic Chain
[0042] As shown in FIG. 1, the SNS provides a network of nerves
that allows the brain to communicate with the body. Sympathetic
nerves originate inside the vertebral column, toward the middle of
the spinal cord in the intermediolateral cell column (or lateral
horn), beginning at the first thoracic segment of the spinal cord
and are thought to extend to the second or third lumbar segments.
Because its cells begin in the thoracic and lumbar regions of the
spinal cord, the SNS is said to have a thoracolumbar outflow. Axons
of these nerves leave the spinal cord through the anterior
rootlet/root. They pass near the spinal (sensory) ganglion, where
they enter the anterior rami of the spinal nerves. However, unlike
somatic innervation, they quickly separate out through white rami
connectors which connect to either the paravertebral (which lie
near the vertebral column) or prevertebral (which lie near the
aortic bifurcation) ganglia extending alongside the spinal
column.
[0043] In order to reach the target organs and glands, the axons
should travel long distances in the body, and, to accomplish this,
many axons relay their message to a second cell through synaptic
transmission. The ends of the axons link across a space, the
synapse, to the dendrites of the second cell. The first cell (the
presynaptic cell) sends a neurotransmitter across the synaptic
cleft where it activates the second cell (the postsynaptic cell).
The message is then carried to the final destination.
[0044] In the SNS and other components of the peripheral nervous
system, these synapses are made at sites called ganglia. The cell
that sends its fiber is called a preganglionic cell, while the cell
whose fiber leaves the ganglion is called a postganglionic cell. As
mentioned previously, the preganglionic cells of the SNS are
located between the first thoracic (T1) segment and third lumbar
(L3) segments of the spinal cord. Postganglionic cells have their
cell bodies in the ganglia and send their axons to target organs or
glands.
[0045] The ganglia include not just the sympathetic trunks but also
the cervical ganglia (superior, middle and inferior), which sends
sympathetic nerve fibers to the head and thorax organs, and the
celiac and mesenteric ganglia (which send sympathetic fibers to the
gut).
[0046] 2. Innervation of the Kidneys
[0047] As FIG. 2 shows, the kidney is innervated by the renal
plexus RP, which is intimately associated with the renal artery.
The renal plexus RP is an autonomic plexus that surrounds the renal
artery and is embedded within the adventitia of the renal artery.
The renal plexus RP extends along the renal artery until it arrives
at the substance of the kidney. Fibers contributing to the renal
plexus RP arise from the celiac ganglion, the superior mesenteric
ganglion, the aorticorenal ganglion and the aortic plexus. The
renal plexus RP, also referred to as the renal nerve, is
predominantly comprised of sympathetic components. There is no (or
at least very minimal) parasympathetic innervation of the
kidney.
[0048] Preganglionic neuronal cell bodies are located in the
intermediolateral cell column of the spinal cord. Preganglionic
axons pass through the paravertebral ganglia (they do not synapse)
to become the lesser splanchnic nerve, the least splanchnic nerve,
first lumbar splanchnic nerve, second lumbar splanchnic nerve, and
travel to the celiac ganglion, the superior mesenteric ganglion,
and the aorticorenal ganglion. Postganglionic neuronal cell bodies
exit the celiac ganglion, the superior mesenteric ganglion, and the
aorticorenal ganglion to the renal plexus RP and are distributed to
the renal vasculature.
[0049] 3. Renal Sympathetic Neural Activity
[0050] Messages travel through the SNS in a bidirectional flow.
Efferent messages may trigger changes in different parts of the
body simultaneously. For example, the sympathetic nervous system
may accelerate heart rate; widen bronchial passages; decrease
motility (movement) of the large intestine; constrict blood
vessels; increase peristalsis in the esophagus; cause pupil
dilation, piloerection (goose bumps) and perspiration (sweating);
and raise blood pressure. Afferent messages carry signals from
various organs and sensory receptors in the body to other organs
and, particularly, the brain.
[0051] Hypertension, heart failure and chronic kidney disease are a
few of many disease states that result from chronic activation of
the SNS, especially the renal sympathetic nervous system. Chronic
activation of the SNS is a maladaptive response that drives the
progression of these disease states. Pharmaceutical management of
the renin-angiotensin-aldosterone system (RAAS) has been a
longstanding, but somewhat ineffective, approach for reducing
over-activity of the SNS.
[0052] As mentioned above, the renal sympathetic nervous system has
been identified as a major contributor to the complex
pathophysiology of hypertension, states of volume overload (such as
heart failure), and progressive renal disease, both experimentally
and in humans. Studies employing radiotracer dilution methodology
to measure overflow of norepinephrine from the kidneys to plasma
revealed increased renal norepinephrine (NE) spillover rates in
patients with essential hypertension, particularly so in young
hypertensive subjects, which in concert with increased NE spillover
from the heart, is consistent with the hemodynamic profile
typically seen in early hypertension and characterized by an
increased heart rate, cardiac output, and renovascular resistance.
It is now known that essential hypertension is commonly neurogenic,
often accompanied by pronounced sympathetic nervous system
overactivity.
[0053] Activation of cardiorenal sympathetic nerve activity is even
more pronounced in heart failure, as demonstrated by an exaggerated
increase of NE overflow from the heart and the kidneys to plasma in
this patient group. In line with this notion is the recent
demonstration of a strong negative predictive value of renal
sympathetic activation on all-cause mortality and heart
transplantation in patients with congestive heart failure, which is
independent of overall sympathetic activity, glomerular filtration
rate, and left ventricular ejection fraction. These findings
support the notion that treatment regimens that are designed to
reduce renal sympathetic stimulation have the potential to improve
survival in patients with heart failure.
[0054] Both chronic and end stage renal disease are characterized
by heightened sympathetic nervous activation. In patients with end
stage renal disease, plasma levels of norepinephrine above the
median have been demonstrated to be predictive for both all-cause
death and death from cardiovascular disease. This is also true for
patients suffering from diabetic or contrast nephropathy. There is
compelling evidence suggesting that sensory afferent signals
originating from the diseased kidneys are major contributors to
initiating and sustaining elevated central sympathetic outflow in
this patient group; this facilitates the occurrence of the well
known adverse consequences of chronic sympathetic over activity,
such as hypertension, left ventricular hypertrophy, ventricular
arrhythmias, sudden cardiac death, insulin resistance, diabetes,
and metabolic syndrome.
[0055] (i) Renal Sympathetic Efferent Activity
[0056] Sympathetic nerves to the kidneys terminate in the blood
vessels, the juxtaglomerular apparatus and the renal tubules.
Stimulation of the renal sympathetic nerves causes increased renin
release, increased sodium (Na+) reabsorption, and a reduction of
renal blood flow. These components of the neural regulation of
renal function are considerably stimulated in disease states
characterized by heightened sympathetic tone and clearly contribute
to the rise in blood pressure in hypertensive patients. The
reduction of renal blood flow and glomerular filtration rate as a
result of renal sympathetic efferent stimulation is likely a
cornerstone of the loss of renal function in cardio-renal syndrome,
which is renal dysfunction as a progressive complication of chronic
heart failure, with a clinical course that typically fluctuates
with the patient's clinical status and treatment. Pharmacologic
strategies to thwart the consequences of renal efferent sympathetic
stimulation include centrally acting sympatholytic drugs, beta
blockers (intended to reduce renin release), angiotensin converting
enzyme inhibitors and receptor blockers (intended to block the
action of angiotensin II and aldosterone activation consequent to
renin release) and diuretics (intended to counter the renal
sympathetic mediated sodium and water retention). However, the
current pharmacologic strategies have significant limitations
including limited efficacy, compliance issues, side effects and
others.
[0057] (ii) Renal Sensory Afferent Nerve Activity
[0058] The kidneys communicate with integral structures in the
central nervous system via renal sensory afferent nerves. Several
forms of "renal injury" may induce activation of sensory afferent
signals. For example, renal ischemia, reduction in stroke volume or
renal blood flow, or an abundance of adenosine enzyme may trigger
activation of afferent neural communication. As shown in FIGS. 3A
and 3B, this afferent communication might be from the kidney to the
brain or might be from one kidney to the other kidney (via the
central nervous system). These afferent signals are centrally
integrated and may result in increased sympathetic outflow. This
sympathetic drive is directed towards the kidneys, thereby
activating the RAAS and inducing increased renin secretion, sodium
retention, volume retention and vasoconstriction. Central
sympathetic over activity also impacts other organs and bodily
structures innervated by sympathetic nerves such as the heart and
the peripheral vasculature, resulting in the described adverse
effects of sympathetic activation, several aspects of which also
contribute to the rise in blood pressure.
[0059] The physiology therefore suggests that (i) modulation of
tissue with efferent sympathetic nerves will reduce inappropriate
renin release, salt retention, and reduction of renal blood flow,
and that (ii) modulation of tissue with afferent sensory nerves
will reduce the systemic contribution to hypertension and other
disease states associated with increased central sympathetic tone
through its direct effect on the posterior hypothalamus as well as
the contralateral kidney. In addition to the central hypotensive
effects of afferent renal denervation, a desirable reduction of
central sympathetic outflow to various other sympathetically
innervated organs such as the heart and the vasculature is
anticipated.
[0060] B. Additional Clinical Benefits of Renal Denervation
[0061] As provided above, renal denervation is likely to be
valuable in the treatment of several clinical conditions
characterized by increased overall and particularly renal
sympathetic activity such as hypertension, metabolic syndrome,
insulin resistance, diabetes, left ventricular hypertrophy, chronic
end stage renal disease, inappropriate fluid retention in heart
failure, cardio-renal syndrome, and sudden death. Since the
reduction of afferent neural signals contributes to the systemic
reduction of sympathetic tone/drive, renal denervation might also
be useful in treating other conditions associated with systemic
sympathetic hyperactivity. Accordingly, renal denervation may also
benefit other organs and bodily structures innervated by
sympathetic nerves, including those identified in FIG. 1. For
example, as previously discussed, a reduction in central
sympathetic drive may reduce the insulin resistance that afflicts
people with metabolic syndrome and Type II diabetics. Additionally,
patients with osteoporosis are also sympathetically activated and
might also benefit from the down regulation of sympathetic drive
that accompanies renal denervation.
[0062] C. Achieving Intravascular Access to the Renal Artery
[0063] In accordance with the present technology, neuromodulation
of a left and/or right renal plexus RP, which is intimately
associated with a left and/or right renal artery, may be achieved
through intravascular access. As FIG. 4A shows, blood moved by
contractions of the heart is conveyed from the left ventricle of
the heart by the aorta. The aorta descends through the thorax and
branches into the left and right renal arteries. Below the renal
arteries, the aorta bifurcates at the left and right iliac
arteries. The left and right iliac arteries descend, respectively,
through the left and right legs and join the left and right femoral
arteries.
[0064] As FIG. 4B shows, the blood collects in veins and returns to
the heart, through the femoral veins into the iliac veins and into
the inferior vena cava. The inferior vena cava branches into the
left and right renal veins. Above the renal veins, the inferior
vena cava ascends to convey blood into the right atrium of the
heart. From the right atrium, the blood is pumped through the right
ventricle into the lungs, where it is oxygenated. From the lungs,
the oxygenated blood is conveyed into the left atrium. From the
left atrium, the oxygenated blood is conveyed by the left ventricle
back to the aorta.
[0065] As will be described in greater detail later, the femoral
artery may be accessed and cannulated at the base of the femoral
triangle just inferior to the midpoint of the inguinal ligament. A
catheter may be inserted percutaneously into the femoral artery
through this access site, passed through the iliac artery and
aorta, and placed into either the left or right renal artery. This
comprises an intravascular path that offers minimally invasive
access to a respective renal artery and/or other renal blood
vessels.
[0066] The wrist, upper arm, and shoulder region provide other
locations for introduction of catheters into the arterial system.
For example, catheterization of either the radial, brachial, or
axillary artery may be utilized in select cases. Catheters
introduced via these access points may be passed through the
subclavian artery on the left side (or via the subclavian and
brachiocephalic arteries on the right side), through the aortic
arch, down the descending aorta and into the renal arteries using
standard angiographic technique.
[0067] D. Properties and Characteristics of the Renal
Vasculature
[0068] Since neuromodulation of a left and/or right renal plexus RP
may be achieved in accordance with the present technology through
intravascular access, properties and characteristics of the renal
vasculature may impose constraints upon and/or inform the design of
apparatus, systems, and methods for achieving such renal
neuromodulation. Some of these properties and characteristics may
vary across the patient population and/or within a specific patient
across time, as well as in response to disease states, such as
hypertension, chronic kidney disease, vascular disease, end-stage
renal disease, insulin resistance, diabetes, metabolic syndrome,
etc. These properties and characteristics, as explained herein, may
have bearing on the efficacy of the procedure and the specific
design of the intravascular device. Properties of interest may
include, for example, material/mechanical, spatial, fluid
dynamic/hemodynamic and/or thermodynamic properties.
[0069] As discussed previously, a catheter may be advanced
percutaneously into either the left or right renal artery via a
minimally invasive intravascular path. However, minimally invasive
renal arterial access may be challenging, for example, because as
compared to some other arteries that are routinely accessed using
catheters, the renal arteries are often extremely tortuous, may be
of relatively small diameter, and/or may be of relatively short
length. Furthermore, renal arterial atherosclerosis is common in
many patients, particularly those with cardiovascular disease.
Renal arterial anatomy also may vary significantly from patient to
patient, which further complicates minimally invasive access.
Significant inter-patient variation may be seen, for example, in
relative tortuosity, diameter, length, and/or atherosclerotic
plaque burden, as well as in the take-off angle at which a renal
artery branches from the aorta. Apparatus, systems and methods for
achieving renal neuromodulation via intravascular access should
account for these and other aspects of renal arterial anatomy and
its variation across the patient population when minimally
invasively accessing a renal artery.
[0070] In addition to complicating renal arterial access, specifics
of the renal anatomy also complicate establishment of stable
contact between neuromodulatory apparatus and a luminal surface or
wall of a renal artery. When the neuromodulatory apparatus includes
an energy delivery element, such as an electrode, consistent
positioning and appropriate contact force applied by the energy
delivery element to the vessel wall are important for
predictability. However, navigation is impeded by the tight space
within a renal artery, as well as tortuosity of the artery.
Furthermore, establishing consistent contact is complicated by
patient movement, respiration, and/or the cardiac cycle because
these factors may cause significant movement of the renal artery
relative to the aorta, and the cardiac cycle may transiently
distend the renal artery (i.e., cause the wall of the artery to
pulse.
[0071] Even after accessing a renal artery and facilitating stable
contact between neuromodulatory apparatus and a luminal surface of
the artery, nerves in and around the adventia of the artery should
be safely modulated via the neuromodulatory apparatus. Effectively
applying thermal treatment from within a renal artery is
non-trivial given the potential clinical complications associated
with such treatment. For example, the intima and media of the renal
artery are highly vulnerable to thermal injury. As discussed in
greater detail below, the intima-media thickness separating the
vessel lumen from its adventitia means that target renal nerves may
be multiple millimeters distant from the luminal surface of the
artery. Sufficient energy should be delivered to or heat removed
from the target renal nerves to modulate the target renal nerves
without excessively cooling or heating the vessel wall to the
extent that the wall is frozen, desiccated, or otherwise
potentially affected to an undesirable extent. A potential clinical
complication associated with excessive heating is thrombus
formation from coagulating blood flowing through the artery. Given
that this thrombus may cause a kidney infarct, thereby causing
irreversible damage to the kidney, thermal treatment from within
the renal artery should be applied carefully. Accordingly, the
complex fluid mechanics and thermodynamic conditions present in the
renal artery during treatment, particularly those that may impact
heat transfer dynamics at the treatment site, may be important in
applying energy (e.g., heating thermal energy) and/or removing heat
from the tissue (e.g., cooling thermal conditions) from within the
renal artery.
[0072] The neuromodulatory apparatus should also be configured to
allow for adjustable positioning and repositioning of the energy
delivery element within the renal artery since location of
treatment may also impact clinical efficacy. For example, it may be
tempting to apply a full circumferential treatment from within the
renal artery given that the renal nerves may be spaced
circumferentially around a renal artery. In some situations,
full-circle lesion likely resulting from a continuous
circumferential treatment may be potentially related to renal
artery stenosis. Therefore, the formation of more complex lesions
along a longitudinal dimension of the renal artery via the mesh
structures described herein and/or repositioning of the
neuromodulatory apparatus to multiple treatment locations may be
desirable. It should be noted, however, that a benefit of creating
a circumferential ablation may outweigh the potential of renal
artery stenosis or the risk may be mitigated with certain
embodiments or in certain patients and creating a circumferential
ablation could be a goal. Additionally, variable positioning and
repositioning of the neuromodulatory apparatus may prove to be
useful in circumstances where the renal artery is particularly
tortuous or where there are proximal branch vessels off the renal
artery main vessel, making treatment in certain locations
challenging. Manipulation of a device in a renal artery should also
consider mechanical injury imposed by the device on the renal
artery. Motion of a device in an artery, for example by inserting,
manipulating, negotiating bends and so forth, may contribute to
dissection, perforation, denuding intima, or disrupting the
interior elastic lamina.
[0073] Blood flow through a renal artery may be temporarily
occluded for a short time with minimal or no complications.
However, occlusion for a significant amount of time should be
avoided because to prevent injury to the kidney such as ischemia.
It could be beneficial to avoid occlusion all together or, if
occlusion is beneficial to the embodiment, to limit the duration of
occlusion, for example to 2-5 minutes.
[0074] Based on the above described challenges of (1) renal artery
intervention, (2) consistent and stable placement of the treatment
element against the vessel wall, (3) effective application of
treatment across the vessel wall, (4) positioning and potentially
repositioning the treatment apparatus to allow for multiple
treatment locations, and (5) avoiding or limiting duration of blood
flow occlusion, various independent and dependent properties of the
renal vasculature that may be of interest include, for example, (a)
vessel diameter, vessel length, intima-media thickness, coefficient
of friction, and tortuosity; (b) distensibility, stiffness and
modulus of elasticity of the vessel wall; (c) peak systolic,
end-diastolic blood flow velocity, as well as the mean
systolic-diastolic peak blood flow velocity, and mean/max
volumetric blood flow rate; (d) specific heat capacity of blood
and/or of the vessel wall, thermal conductivity of blood and/or of
the vessel wall, and/or thermal convectivity of blood flow past a
vessel wall treatment site and/or radiative heat transfer; (e)
renal artery motion relative to the aorta induced by respiration,
patient movement, and/or blood flow pulsatility: and (f) as well as
the take-off angle of a renal artery relative to the aorta. These
properties will be discussed in greater detail with respect to the
renal arteries. However, dependent on the apparatus, systems and
methods utilized to achieve renal neuromodulation, such properties
of the renal arteries, also may guide and/or constrain design
characteristics.
[0075] As noted above, an apparatus positioned within a renal
artery should conform to the geometry of the artery. Renal artery
vessel diameter, D.sub.RA, typically is in a range of about 2-10
mm, with most of the patient population having a D.sub.RA of about
4 mm to about 8 mm and an average of about 6 mm. Renal artery
vessel length, L.sub.RA, between its ostium at the aorta/renal
artery juncture and its distal branchings, generally is in a range
of about 5-70 mm, and a significant portion of the patient
population is in a range of about 20-50 mm. Since the target renal
plexus is embedded within the adventitia of the renal artery, the
composite Intima-Media Thickness, IMT, (i.e., the radial outward
distance from the artery's luminal surface to the adventitia
containing target neural structures) also is notable and generally
is in a range of about 0.5-2.5 mm, with an average of about 1.5 mm.
Although a certain depth of treatment is important to reach the
target neural fibers, the treatment should not be too deep (e.g.,
>5 mm from inner wall of the renal artery) to avoid non-target
tissue and anatomical structures such as the renal vein.
[0076] An additional property of the renal artery that may be of
interest is the degree of renal motion relative to the aorta,
induced by respiration and/or blood flow pulsatility. A patient's
kidney, which located at the distal end of the renal artery, may
move as much as 4'' cranially with respiratory excursion. This may
impart significant motion to the renal artery connecting the aorta
and the kidney, thereby requiring from the neuromodulatory
apparatus a unique balance of stiffness and flexibility to maintain
contact between the thermal treatment element and the vessel wall
during cycles of respiration. Furthermore, the take-off angle
between the renal artery and the aorta may vary significantly
between patients, and also may vary dynamically within a patient,
e.g., due to kidney motion. The take-off angle generally may be in
a range of about 30.degree.-135.degree..
[0077] E. Achieving Renal Denervation via an Intravascularly
Delivered Microwave Field
[0078] Microwave energy may be utilized to achieve renal
neuromodulation via at least partial denervation of the kidney. For
the purpose of this disclosure "microwave energy" and "microwave
field" may be equivalent and interchangeable. Microwave energy is
absorbed by tissue in a process called dielectric heating.
Molecules in the tissue, such as water molecules, are electric
dipoles that have a positive charge at one end and a negative
charge at the other. The microwave energy induces an alternating
electric field that causes the dipoles to rotate as they attempt to
align themselves with the field. This molecular rotation generates
heat as the molecules hit one another and cause additional motion.
The heating is particularly efficient with liquid water molecules,
which have a relatively high dipole moment. Tissue types that have
relatively low water content, such as fat, do not absorb microwave
energy as efficiently as other types of tissue.
[0079] The friction and heat produced through dipole rotation
increases tissue temperature in a process known as dielectric
heating, ultimately leading to cell death (i.e., necrosis) via
coagulation. Accordingly, one feature of microwave-induced
dielectric heating is that such an arrangement is expected to
provide therapeutically high temperatures in target tissue, large
and consistent ablation volumes, and/or relatively fast ablation
times.
[0080] The renal arterial wall is comprised of intima, media and
adventitia. Target renal nerves are positioned in and adjacent to
the adventitia, and connective tissue surrounds the adventitia and
nerves. Connective tissue is largely comprised of fat, which is a
poor absorber of microwave energy due to its low water content,
thereby reducing a risk of collateral damage to the fatty
connective tissue during microwave irradiation of the renal
nerves.
[0081] A microwave transmission element positioned within the renal
artery via intravascular access may deliver a microwave field
through the vessel wall and tissue, for example, omni-directionally
in a plane relatively perpendicular to the longitudinal axis of the
vessel. The microwave field may modulate (e.g., necrose), the
target renal nerves. The depth to which the microwave field
penetrates the wall and tissue is frequency dependent. Relatively
greater microwave frequencies will provide relatively lower tissue
penetration, while relatively lower microwave frequencies will
provide relatively greater tissue penetration.
[0082] When delivered intravascularly, preferentially heating the
adventitia while avoiding significant thermal exposure to the
intima/media may be challenging. However, renal arterial blood flow
may provide protective cooling of the intima/media. Alternatively,
open or closed circuit cooling may be utilized to remove excess
heat from the inner wall of the renal artery. Various methods,
systems and apparatuses for active and open- and closed-circuit
cooling have been described previously, for example, in U.S. patent
application Ser. No. 13/279,205, filed Oct. 21, 2011, and
International Patent App. No. PCT/US2011/033491, filed Apr. 21,
2011, both of which are incorporated herein by reference in their
entireties.
II. MICROWAVE CATHETER APPARATUSES, SYSTEMS, AND METHODS FOR RENAL
NEUROMODULATION
A. Overview
[0083] As just described, the left and/or right RP surrounds the
respective left and/or right renal artery. The RP extends in
intimate association with the respective renal artery into the
substance of the kidney. FIG. 5 shows a microwave system 10 for
inducing neuromodulation via dielectric heating of the left and/or
right RP by intravascular access into the respective left or right
renal artery.
[0084] The microwave system 10 includes an intravascular treatment
device 12 such as a catheter with an elongated shaft 16 having a
proximal end region 18 and a distal end region 20. The proximal end
region 18 of the elongated shaft 16 is connected to a handle
assembly 200. The distal end region 20 of the elongated shaft 16
carries at least one microwave transmission element 24, such as a
microwave antenna 100 (see FIG. 7). The elongated shaft 16 is sized
and configured for placement of its distal end region 20 within a
renal artery by intravascular access. The microwave transmission
element 24 is also specially sized and configured for manipulation
and use within a renal artery.
[0085] The microwave system 10 also includes a microwave source or
generator 26, such as a cavity magnetron, a klystron, a traveling
wave tube, etc. Under the control of the caregiver or automated
control algorithm 222, the microwave generator 26 generates a
selected form and magnitude of microwave energy. The generator
preferably generates microwaves at a medically acceptable
frequency, such as 915 MHz, 2.45 GHz, and/or 5.1 GHz. As discussed
previously, relatively greater microwave frequencies will provide
relatively lower tissue penetration, while relatively lower
microwave frequencies will provide relatively greater tissue
penetration.
[0086] A feed or transmission line 28, such as a coaxial cable or a
parallel wire, electrically transfers microwaves from the microwave
generator 26 to the microwave transmission element 24 (e.g.,
extends along or within the elongated shaft 16 from the generator
26 to the transmission element 24). A control mechanism, such as
foot pedal 110, can be connected (e.g., pneumatically or
electrically) to the generator 26 to allow the operator to
initiate, terminate and, optionally, adjust various operational
characteristics of the microwave generator 26, including, but not
limited to, microwave energy delivery. Optionally, one or more
sensors, such as one or more temperature (e.g., thermocouple,
thermistor, etc.), impedance, pressure, optical, flow, chemical or
other sensors, can be located proximate to or within the microwave
transmission element to monitor delivery of the microwave field
and/or to monitor dielectric heating in the vicinity of the
microwave transmission element (see FIG. 22B).
[0087] As shown in FIG. 6A, the treatment device 12 provides access
to the RP through an intravascular path 14 that leads to a
respective renal artery. The handle assembly 200 is sized and
configured to be securely or ergonomically held and manipulated by
a caregiver outside the intravascular path 14. By manipulating the
handle assembly 200 from outside the intravascular path 14, the
caregiver can advance the elongated shaft 16 through the sometimes
tortuous intravascular path 14 and remotely manipulate or actuate
the distal end region 20 if necessary. Image guidance (e.g., CT,
radiographic, IVUS, OCT, or another suitable guidance modality, or
combinations thereof), can be used to aid the caregiver's
manipulation. Further, in some embodiments, image guidance
components (e.g., IVUS, OCT) may be incorporated into the treatment
device 12 itself.
[0088] As shown in FIG. 6B, the distal end region 20 of the
elongated shaft 16 can flex in a substantial fashion to gain
entrance into a respective left/right renal artery by manipulation
of the elongated shaft 16. Optionally, the distal end region 20 of
the elongated shaft 16 can gain entrance to the renal artery
following a path defined by a guide catheter, a guide wire, or a
sheath (not shown). In such cases, the maximum outer dimension
(e.g., diameter) of any section of the elongated shaft 16,
including the microwave transmission element 24 it carries, may be
dictated by the inner diameter of the guide catheter through which
the elongated shaft 16 is passed. Assuming, for example, that an 8
French guide catheter (which has an inner diameter of approximately
0.091 inch (2.31 mm)) would likely be, from a clinical perspective,
the largest guide catheter used to access the renal artery, and
allowing for a reasonable clearance tolerance between the elongated
shaft 16 (i.e., microwave transmission element 24, centering
element 30, etc.) and the guide catheter, the maximum outer
dimension can realistically be expressed as being less than or
equal to approximately 0.085 inch (2.16 mm). However, use of a
smaller 5 French guide catheter may require the use of smaller
outer diameters along the elongated shaft 16. For example, an
elongated shaft 16 that is to be routed within a 5 French guide
catheter may have an outer dimension of no greater than 0.053 inch
(1.35 mm). In another example, an elongated shaft 16 that is to be
routed within a 6 French guide catheter may have an outer dimension
of no great than 0.070 inch (1.78 mm). In still further examples,
other suitable guide catheters may be used, and outer dimensions
and/or arrangements of the treatment device 12 can vary
accordingly.
[0089] Once entrance to a renal artery is gained, the microwave
transmission element 24 optionally may be aligned with tissue along
an interior wall of the respective renal artery. Optionally, the
microwave transmission element 24 also may be centered within the
renal artery via, for example, an expandable centering element 30,
such as a permeable centering element, an expandable braid or mesh,
a cage, a basket, a balloon, stabilizing members, prongs, etc.,
that may be remotely expanded and collapsed via the handle assembly
200. The centering element 30 has a low-profile delivery
configuration for intravascular delivery to, and retrieval from,
within the renal artery (e.g., through a guide catheter), and an
expanded deployed configuration (as seen in FIG. 6B) wherein the
centering element 30 contacts the internal luminal surface of the
renal artery and centers the microwave transmission element 24
within the artery.
[0090] Once the microwave transmission element 24 is positioned as
desired within the renal artery, the purposeful application of
microwave energy from the microwave generator 26 to tissue by
radiation from the microwave transmission element 24 induces one or
more desired neuromodulating effects on localized regions of the
renal artery and adjacent regions of the RP, which lay intimately
within or adjacent to the adventitia of the renal artery. The
purposeful application of neuromodulating effects can achieve
neuromodulation along all or a portion of the RP.
[0091] Neuromodulating effects can include thermal ablation,
non-ablative thermal alteration, coagulation or damage (e.g., via
sustained heating and/or dielectric heating), and electromagnetic
neuromodulation. Desired dielectric heating effects may include
raising the temperature of target neural fibers above a certain
threshold to achieve non-ablative thermal alteration, or above a
higher temperature to achieve ablative thermal alteration. For
example, the target temperature can be above body temperature
(e.g., approximately 37.degree. C.) but less than about 45.degree.
C. for non-ablative thermal alteration, or the target temperature
can be about 45.degree. C. or higher for ablative thermal
alteration. Desired non-thermal neuromodulation effects may include
altering the electrical signals transmitted in a nerve.
[0092] Specific embodiments of the microwave system 10, along with
associated methods and apparatuses, will now be described in
further detail. These embodiments are provided merely for the sake
of illustration and should in no way be construed as limiting.
B. Specific Embodiments
1. Coaxial Cable Feed Line and Coaxial Antenna Microwave
Transmission Element
[0093] As discussed previously, microwaves are transferred from the
microwave generator 26 to the microwave transmission element 24 via
the feed line 28. As seen, for example, in FIG. 7, the feed line 28
may comprise a coaxial cable 50, and the microwave transmission
element 24 may comprise, for example, a coaxial antenna 100.
Coaxial cables and antennas are described, for example, in U.S.
Pat. No. 2,184,729, to Bailey, which is incorporated herein by
reference in its entirety. Microwave catheters for use in cardiac
ablation procedures and comprising coaxial cables and antennas are
described, for example, in U.S. Pat. No. 4,641,649 to Walinsky et
al., which is incorporated herein by reference in its entirety.
[0094] Antennas convert electric current into electromagnetic
radiation (and vice versa). The coaxial antenna 100 is a type of
dipole antenna. The coaxial cable 50 connects the microwave
generator 26 to the coaxial antenna 100. The coaxial cable 50
comprises an inner conductor 52, insulation 54 coaxially disposed
about the inner conductor, and an outer conductor 56 comprising a
tubular metal braid or shield that is coaxially disposed about the
insulation 54. An outer delivery sheath or insulation 58 may cover
the outer conductor 56. Microwave energy is delivered down the
length of the coaxial cable 50 that terminates in the antenna 100
which is capable of radiating the energy into the surrounding
tissue. The microwave energy is delivered through the space between
the inner conductor 52 and the outer conductor 56. This space
serves as a conduit while the outer conductor 56 prevents energy
from escaping. Where a gap is encountered in the outer conductor 56
microwave energy is applied to the surrounding tissues. The gap may
be perceived as an aperture instrumental in directing the microwave
energy to the target in the controllable manner.
[0095] The insulation 54 between the inner and outer conductors 52,
56 electrically insulates and maintains a uniform distance between
the inner conductor 52 and the outer conductor 56. The insulation
54 may comprise a dielectric material, such as solid or foamed
PolyEthylene (PE) or PolyTetraFluoroEthylene (PTFE). The space
between the inner and outer conductors 52, 56 may have an impedance
that is approximately matched with the tissue impedance. An
impedance mismatch between the antenna 100 and surrounding tissue
may result in unbalanced currents on the inner and outer conductors
52, 56 of the feed line 28. In this case, a remainder current may
flow along the outside of the outer conductor 56 of the feed line
28.
[0096] To form the coaxial antenna 100, an exposed element that is
electrically connected to the inner conductor 52 is unshielded by
the outer conductor 56. For example, the exposed element may be an
extension of the inner conductor 52 or an electrically conductive
element that is electrically connected to the inner conductor 52.
The length of the exposed element may be a fraction of the
wavelength of the microwave radiation (i.e., a fractional length),
for example about 1/2 a wavelength. A distal region 102 of the
outer conductor 56 of the coaxial cable 50 optionally is exposed
(i.e., outer insulation or sheath 58 is optional) over a fractional
length, for example, about 1/2 a wavelength. The distal regions 102
and 104 form a radiating element 106 of dipole coaxial antenna 100.
When driven by a microwave signal generated by generator 26 and
transferred by coaxial cable 50, the radiating element 106 of
coaxial antenna 100 radiates microwaves outward in a torus or
toroidal pattern. Microwave emission E is maximal and
omni-directional in a plane perpendicular to the dipole (i.e.,
perpendicular to radiating element 106) and substantially reduced
in the direction of the dipole.
2. Expandable Multi-Filament Centering Elements
[0097] It may be desirable to heat the renal nerves disposed within
the adventitia while avoiding significant dielectric heating of the
intima/media during microwave irradiation. Renal arterial blood
flow may provide passive protective cooling of the intima/media.
Thus, the centering element 30 may be permeable (such as in the
expandable mesh/braid of FIG. 6B) and/or may not obstruct the
entire vessel lumen, in order to ensure continued blood flow
cooling of the renal artery intima/media during microwave-induced
dielectric heating of target renal nerves. The centering element
30, and/or some other element, also may increase the velocity of
blood flow at or near the vessel wall to enhance or accelerate the
transfer of heat from the wall to the blood.
[0098] In FIGS. 8A and 8B, an expandable centering element 830
comprises a plurality of resilient filaments 32 (e.g., fingers or
prongs), which are connected to the distal region 104 of inner
conductor 52 at a connector 34. The connector 34 may be conductive
or non-conductive, such that the centering element 830 may or may
not, respectively, comprise a portion of the radiating element of
an antenna 800. A nose cone 36 extends from connector 34, and all
or a portion of the nose cone 36 also may or may not be conductive
(i.e., may or may not comprise a portion of the radiating element),
as desired. In embodiments in which expandable centering element
830 is not a portion of the radiating element, it may be made from
a dielectric material such as a polymer or ceramic.
[0099] As seen in FIG. 8A, in the low-profile delivery
configuration, the outer sheath 58 serves as a delivery sheath that
extends and distally tapers to an attachment with the atraumatic
nose cone 36, thereby constraining filaments 32. As seen in FIG.
8B, upon proximal retraction of the delivery sheath (e.g., via
actuation of the handle assembly 200), the filaments 32 self-expand
into contact with the vessel wall, thereby centering and aligning
the coaxial antenna 800 with the longitudinal axis of the
vessel.
[0100] Microwave radiation from the microwave generator 26 then may
be transferred along the coaxial cable 50 to the antenna 800 and
radiated omni-directionally into the vessel wall to target renal
nerves. The microwaves dielectrically heat the target renal nerves,
as discussed previously, which induces neuromodulation (e.g.,
denervation). Filaments 32 of the centering element 830 do not
significantly obstruct blood flow, and thereby facilitate passive
blood flow cooling of non-target intima and media. Upon completion
of renal neuromodulation, microwave irradiation may be halted, and
the centering element 830 may be collapsed for retrieval within the
outer sheath 58 and/or within a guide catheter.
[0101] With reference now to FIG. 9, an alternative embodiment of a
centering element 930 is illustrated wherein filaments 32 are
distally coupled to the nose cone 36 to form an expandable basket
38. The basket 38 may self-expand into contact with the vessel wall
and/or may be actively expanded. FIG. 9 illustratively shows the
basket 38 in the deployed configuration. Embodiments comprising
expandable filaments or baskets for centering elements 930 may have
a multiple number of filaments 32 (e.g., two, three, four, five,
etc.), which may have variations of geometric shapes (e.g.,
straight, curved, helical, coiled, etc.).
3. Expandable Balloon Centering Elements
[0102] As seen in FIGS. 10A and 10B, an expandable centering
element 1030 optionally may comprise an expandable balloon 1040.
The balloon 1040 may be delivered in a low-profile configuration,
expanded within the renal artery prior to and during application of
the microwave field, and then collapsed for retrieval. As
illustrated in the side-sectional view of FIG. 10A, expansion of
the balloon 1040 into contact with the vessel wall may center an
antenna 1000 within the vessel and align it with the longitudinal
axis of the vessel to facilitate omni-directional microwave
radiation into the wall. The balloon 1040 may be expanded by
injecting a fluid/gas (e.g. nitrogen, carbon dioxide, saline, or
other suitable liquids or gases) through an injection lumen (see,
e.g., injection lumen 69 in FIG. 16). To contract the balloon the
injected fluid or gas can be extracted through the same injection
lumen, or through a separate lumen (not shown). A chilled fluid
(e.g., chilled saline) may be used to inflate the balloon 1040 and
may be circulated by injecting the fluid through an injection lumen
and extracting the fluid through a separate extraction lumen to
allow continuous or semi-continuous flow. Circulation of chilled
fluid may have an added benefit of cooling the surface layers of
the artery while allowing the deeper adventitia and renal nerves to
heat and become neuromodulated (e.g., ablated).
[0103] As shown in FIG. 10A, a balloon may inflate to about the
same diameter as the lumen of the artery and thereby occlude the
artery (as shown in FIG. 10B). Alternatively (as illustrated in the
cross-section of FIGS. 11A and 11B), the balloon 1040 may not
completely obstruct the lumen of the renal artery, thereby allowing
blood flow to cool and protect non-target intima and media during
microwave irradiation of target renal nerves. By reducing the
unobstructed cross-sectional area of the arterial lumen, the
balloon 1040 may increase the velocity of blood flow through the
unobstructed area, which may enhance the rate of heat transfer at
the vessel wall along unobstructed portions of the lumen.
[0104] In FIG. 11A, a centering element 1130a comprises a balloon
1140a having two opposed lobes 42 that contact the inner wall of
the renal artery. FIG. 11B illustrates an alternative embodiment of
a centering element 1130b comprising a balloon 1140b having three
lobes 42 spaced equidistantly about the circumference of the
vessel. As will be apparent, balloons 1140a, 1140b may comprise any
number of lobes, as desired.
[0105] FIG. 12 illustrates another embodiment of an antenna 1200
having a plurality of balloons (illustrated individually as 1240a
and 1240b) positioned proximal and distal of radiating section 106
of the antenna 1200. Optionally, the balloons 1240a and 1240b
comprise multiple lobes and do not fully obstruct the lumen of the
renal artery. Providing proximal and distal balloons may better
center the antenna 1200 within the renal artery.
[0106] In another embodiment as shown in FIG. 13, an expandable
balloon centering element 1340 and the distal region 104 of the
inner conductor 52 may be combined such that the distal region 104
comprises one or more conductive electrode traces 55 applied on the
surface of the balloon 1340 for improved microwave energy delivery.
In this example, the emitting antenna is the metallic spiral
pattern 55 applied or deposited on the inner or outer surface of
the balloon 1340. Balloons comprising electrodes have been
fabricated, for example, by MicroPen Technologies of Honeoye Falls,
N.Y. The metallic spiral pattern 55 is electrically connected to
the inner conductor 52 by a joint 57 that can be a weld or a solder
joint. The antenna is thus implemented by a metalized path printed,
deposited, or otherwise applied on the wall of the balloon 1340 and
can be joined to the inner conductor 52 of the coaxial cable by an
extension of the inner conductor 52 or by a separate
interconnecting wire (not shown).
[0107] As in previous examples to form the antenna, the distal
region 104 of the inner conductor 52 of the coaxial cable 50 is
exposed over a length equal to a fraction of the wavelength of the
microwave radiation. To achieve optimal length of the exposed inner
conductor 52 and at the same time achieve the desired dimensions of
the distal region 104 of the renal artery catheter, it may be
desired to have the exposed conductor longer than the length of the
balloon 1340. The proposed spiral pattern achieved when the balloon
is expanded and the antenna is unfurled, is one of the many
metallic surface patterns that can be used to achieve the desired
length of the antenna in a compact device suitable for renal artery
deployment. Alternative zigzag patterns can be proposed, for
example, instead of a spiral. Common to these embodiments, the
deposited metal forms an antenna that is electrically connected to
one of the conductors in the feed line and is longer than the
length of the balloon. The shape of the field expected from these
embodiments is toroidal and substantially similar to the microwave
field produced by the linear dipole antenna.
[0108] FIGS. 14 and 15 illustrate embodiments of antennas 1400 and
1500, respectively, where expandable balloon centering elements
1440 and 1540, respectively, comprise one or more conductive areas
applied on the surfaces of the balloons for improved microwave
energy delivery, where the conductive areas are not electrodes but
shields.
[0109] The inner conductor 52 is in electric connection with the
exposed segment of the antenna 1400, 1500 that is the emitter of
the microwave field and energy. The applied metallic patterns 1462,
1463, and 1564 are not intended to emit energy but to reshape the
microwave field and to improve the antennae matching network and
thus ultimately change the geometry of the periarterial lesion.
These designs can be useful when the toroidal shape of the
microwave field is not desired in order to preferentially treat one
area of the renal artery and spare another to reduce the risk of
stenosis or for other medical reasons. In other words, to prevent
direct penetration of microwaves into the tissue surrounding the
balloon catheter, the balloon envelope is provided with a metallic
coating in selected areas. The metallic patterns 1462, 1463, and
1564 and other possible patterns (e.g., helical shielding patterns
or shielding lines that are parallel to the axis of the vessel)
distort the microwave field and shield certain areas of the renal
artery in a predictable fashion. The unprotected areas receive a
relatively high concentration of microwave energy.
[0110] FIG. 15 also illustrates incorporation of a conductive
element 101 into an atraumatic tip 1536. Such an element can be
instrumental in tuning up the antenna. The nose cone 1536 can
incorporate a cap useful in the cap and choke antennae designs
intended to improve impedance matching. The choke is a conductive
sleeve placed above the coaxial feed assembly right before the area
where the emitter antenna emerges from the coaxial shield.
[0111] FIG. 16 illustrates an embodiment of an antenna 1600 where a
balloon 1640 is asymmetric relative to the catheter shaft and the
antenna 1600. The purpose of such a balloon 1640 is to increase the
delivery of microwave energy to one side of the internal lumen of
the renal artery and decrease the delivery to the opposite
side.
4. Flow Directing Elements
[0112] FIG. 17 illustrates an embodiment of a microwave system 1710
comprising an expandable centering element 1730 in combination with
a flow directing velocity enhancement element 1745. In FIG. 17, the
centering element 1730 illustratively comprises multiple resilient
filaments 32, as discussed previously. Flow directing element 1745
illustratively comprises an expandable balloon having a diameter
less than the diameter of the lumen of the renal artery in which
treatment is conducted. Microwave transmitting element 24/antenna
1700 may be positioned within the flow directing element 1745.
[0113] In the expanded deployed configuration of FIG. 17, the
centering element 1730 aligns and centers the antenna 1700 within
the renal artery, while the flow directing element 1745 obstructs
the center of the artery. This central obstruction directs blood
flow around the flow directing element 1745 and toward the luminal
wall of the renal artery, where protective cooling is desired
during microwave irradiation and dielectric heating of the target
renal nerves. The central obstruction also increases flow velocity,
which in turn increases the rate of protective heat transfer from
the vessel wall to the blood during such microwave irradiation.
5. Over-The-Wire and Rapid Exchange Microwave Catheters
[0114] It may be desirable for the microwave system 10 to comprise
an intravascular treatment device 12 configured for delivery over a
guide wire. In any of the previously described embodiments, the
coaxial cable 50 and coaxial antenna 100 may be modified such that
the inner conductor 52 comprises a tubular inner conductor with a
guide wire lumen (e.g., a coiled tube, a braided metal tube, or a
flexible polymer tube coated with a conductive material, such as
silver). For example, FIG. 18A illustrates an alternative
embodiment of the microwave system of FIG. 7 having an antenna 1800
wherein the inner conductor 52 comprises a tubular inner conductor
having a guide wire lumen 53. In the embodiment illustrated in FIG.
18A, the guide wire lumen 53 extends completely through the shaft
16 from the proximal opening of the shaft 16 at an adaptor (e.g.,
at the handle 200 shown in FIG. 5) to the distal opening of the
shaft 16 in an over-the-wire (OTW) configuration, whereas in the
embodiment illustrated in FIG. 18B, a guide wire 183 and the guide
wire lumen 53 extend through only a portion of the shaft 16 in a
rapid exchange (RX) configuration. Although the proximal end of the
guide wire lumen 53 is shown in FIG. 18B extending through the
sidewall of the shaft 16 at the distal region 20, in other
embodiments, the proximal end of the guide wire lumen 53 can be
accessible anywhere between the proximal and distal ends of the
shaft 16. The guide wire lumen 53 shown in FIGS. 18A and 18B, or
variations thereof, may be included in various embodiments
described herein to facilitate navigation through the vasculature.
Suitable OTW and RX guide wire configurations are disclosed in U.S.
Pat. No. 5,545,134, filed Oct. 27, 1994, U.S. Pat. No. 5,782,760,
filed May 23, 1995, U.S. Patent App. Publication No. US
2003/0040769, filed Aug. 23, 2001, and U.S. Patent App. Publication
No. US 2008/0171979, filed Oct. 17, 2006, each of which is
incorporated herein by reference in its entirety.
6. Inner Conductors With Dynamically Variable Exposed Length
[0115] When utilizing a coaxial antenna, it may be desirable to
provide the inner conductor 52 with a dynamically variable exposed
length in order to better match the antenna to the surrounding
media (i.e., blood and tissue). Media matching depends on multiple
factors, including electrical characteristics of the media (e.g.,
dielectric properties), frequency of the microwave signal, and
power of the microwave signal, as well as geometrical parameters of
the radiating element 106 of the antenna. Dynamically varying the
exposed length of the inner conductor 52 dynamically varies the
geometrical parameters of the radiating element 106, which may be
utilized to facilitate better media matching.
[0116] Referring now to FIGS. 19A and 19B, in one embodiment, a
coaxial antenna 1900 may comprise an adjustable gap for dynamically
varying the exposed length of the inner conductor 52. As seen in
FIG. 19A, the distal region of outer conductor 56 may comprise a
first outer conductor 56a and a second outer conductor 56b spaced
longitudinally from the first outer conductor 56a. Likewise the
(dielectric) insulation 54 comprises a first insulation 54a and a
second insulation 54b spaced longitudinally from the first
insulation 54a. The outer sheath 58 is attached to the second outer
conductor 56b, but may freely slide relative to first outer
conductor 56a.
[0117] In FIG. 19A, radiating element 106 of antenna 1900 comprises
the exposed portion of inner conductor 52 positioned between the
first and second outer conductors. As seen in FIG. 19B, proximal
retraction of outer sheath 58 proximally retracts the second outer
conductor 56b and the second insulation 54b relative to the inner
conductor 52, thereby dynamically altering the radiating element
106 of the antenna 1900. As will be apparent, in an alternative
embodiment, the microwave system of FIGS. 19A and 19B may be
modified such that the inner conductor 52 comprises a tubular inner
conductor having a guide wire lumen to facilitate over-the-wire
delivery of the treatment device 12. The microwave system may
alternatively be modified for RX delivery of the treatment device
12.
[0118] FIG. 20 illustrates another embodiment of an antenna 2000
having an inner conductor 52 with a dynamically variable exposed
length distal region 104. In FIG. 20, the exposed length along the
distal region 104 of the inner conductor 52 includes a conductive
wire loop 60 that loops back and extends within a lumen 61 through
an elongated shaft 16 of an intravascular treatment device 12. As
illustrated by dotted lines in FIG. 20, the medical practitioner
may dynamically extend and retract the wire loop 60 to dynamically
vary the exposed length of the inner conductor 52.
[0119] FIGS. 21A and 21B illustrate an over-the-wire embodiment of
a microwave system having an inner conductor 52 with a dynamically
variable exposed length. As seen in FIG. 21A, the distal end region
20 of the elongated shaft 16 having a coaxial antenna 2100 with
tubular inner conductor 52 having a guide wire lumen 53 may be
advanced over a guide wire 108 into the renal artery. As seen in
FIG. 21B, the guide wire 108 then may be removed and replaced with
a radiator 70 that is electrically coupled to the tubular inner
conductor 52. The radiator 70 may, for example, comprise
protrusions that contact the inner wall of the inner conductor 52
to electrically couple to the tubular inner conductor 52.
[0120] The radiator 70 extends beyond the distal end region 20 of
the elongated shaft 16 to form the exposed distal region 104 of the
inner conductor 52, thereby forming a portion of the radiating
element 106 of the antenna 2100 during microwave irradiation of the
renal nerves. The length of the exposed distal region 104 of inner
conductor 52 may be varied by dynamically varying how far the
radiator 70 extends beyond distal end region 20 of elongated shaft
16.
7. Active Cooling
[0121] In addition to the passive cooling provided by blood flow,
active cooling may be provided in the vicinity of the microwave
transmission element via a coolant (e.g., a circulating coolant).
For example, as seen in FIG. 22A, a coolant 112 may be introduced
into an annular space between the coaxial cable 50 and dielectric
or insulator 2258. As seen in FIG. 22B, the annular space may
extend over a coaxial antenna 2200. Optionally, the coolant may be
circulated to enhance heat transfer. Optionally, a temperature
sensor, such as a thermistor or thermocouple 59, may be positioned
within the coolant in the vicinity of the radiating element of the
antenna 2200 to monitor temperature. Temperature data collected
with the temperature sensor may be utilized in a feedback loop to
control or alter delivery of the microwave field and/or the coolant
in response to the measured temperature (e.g., to maintain the
temperature within a desired range).
[0122] Additionally or alternatively, the inner layers of an artery
may be spared from heat if microwave energy is delivered in pulses.
During pauses of energy delivery, blood will flow away from the
area of energy application and be replaced by colder blood. At the
same time tissues surrounding the inner lumen of the artery will
continue to accumulate heat leading to the desired targeted tissue
destruction. Pulsed delivery of microwave energy can be achieved by
setting the duty cycle of the microwave energy generator. The
thermal inertia of the targeted tissues will ensure the desired
build up of heat while sparing the inner lumen of the blood
vessel.
8. Directed Application of a Microwave Field
[0123] The specific embodiments described above provide
toroidal-shaped, omni-directional emission of microwaves from a
microwave transmission element 24 (i.e., from the radiating element
106 of coaxial antenna 100). While such an omni-directional
microwave energy deposition delivered intravascularly may desirably
provide a circumferential treatment about the renal artery, it may
be desirable under certain circumstances to target a specific
non-circumferential area (e.g., to more narrowly direct application
of the microwave energy deposition to specific target renal
nerves). Thus, a microwave transmission element 24 may include
shielding or other means for directional application of microwave
energy.
[0124] For example, FIG. 23A illustrates a shielding 65
substantially surrounding the radiating element 106 of the coaxial
antenna 100. FIGS. 23B and 23C are cross-sectional views of the
antenna 100 along lines B-B and C-C, respectively. Referring to
FIGS. 23A-23C, the shielding 65 includes a window 21 through which
the microwave emissions E may be directed. The window 21 can take
up various proportions of the circumference of the shielding 65.
For example, in one embodiment, the window 21 comprises
approximately 30% of the circumference of the shielding 65 over the
length of the emitting portion of the antenna 100. In further
embodiments, the window 21 can comprise more or less of the
circumference of the shielding 65 and can extend only part of the
length of the emitting portion of the antenna 100. In some
embodiments, it may be desirable for the shielding 65 to possess
reflective properties such that a substantial portion of the
omni-directional field that encounters the shielding 65 is
redirected through the window 21. In further embodiments, the
shielding 65 can include more than one window (e.g., multiple
longitudinally offset windows facing opposite directions, or a
helical window, etc.). The lesion geometry will accordingly match
the window geometry.
[0125] In some embodiments, it may also be desirable for the
elongated shaft 16 (FIG. 5) to have deflection capability at or
near its distal region 20 proximal of the antenna 100 to facilitate
positioning of the shielding window 21 within the renal artery. For
example, as illustrated in FIGS. 23D and 23E, deflection capability
may be provided by a control wire 23 running through the catheter
from the handle to or near the distal region 20 proximal of the
antenna 100, where the catheter includes a flexibly biased
structure such as a laser cut tube 25. When the control wire 23 is
pulled or pushed by an actuator in the handle (not shown), the
flexibly biased structure is deflected in the flexibly biased
direction.
[0126] In one embodiment, pulling or pushing the control wire 23
can cause the distal region 20 to deflect in the direction of the
window 21 to facilitate positioning of the window 21 in substantial
contact with the vessel wall (as illustrated in FIG. 23E). In
another embodiment, pulling or pushing the control wire 23 can
cause the catheter to deflect in a direction opposite that of the
window 21 to ensure that there is sufficient space between the
window 21 and targeted area of the vessel wall to allow for blood
flow to cool non-target intima/media tissue. Other embodiments
disclosed herein (e.g., balloon-centering embodiments), can
similarly employ an elongated shaft 16 having deflection
capabilities to control positioning of the antenna 100 in the renal
artery.
III. CONCLUSION
[0127] Although the specific embodiments of a microwave system have
been described with a feed line comprising a coaxial cable and a
microwave transmission element comprising a coaxial antenna, it
should be understood that any alternative feed lines and microwave
transmission elements may be utilized. For example, a feed line may
comprise a parallel wire. Likewise, a microwave transmission
element may, for example, comprise a waveguide or an alternative
type of antenna, such as a patch antenna, a slot antenna, another
form of dipole antenna, a Yagi-Uda antenna, a parabolic antenna,
etc.
[0128] The above detailed descriptions of embodiments of the
technology are not intended to be exhaustive or to limit the
technology to the precise form disclosed above. Although specific
embodiments of, and examples for, the technology are described
above for illustrative purposes, various equivalent modifications
are possible within the scope of the technology, as those skilled
in the relevant art will recognize. For example, while steps are
presented in a given order, alternative embodiments may perform
steps in a different order. The various embodiments described
herein may also be combined to provide further embodiments.
[0129] Moreover, unless the word "or" is expressly limited to mean
only a single item exclusive from the other items in reference to a
list of two or more items, then the use of "or" in such a list is
to be interpreted as including (a) any single item in the list, (b)
all of the items in the list, or (c) any combination of the items
in the list. Where the context permits, singular or plural terms
may also include the plural or singular term, respectively.
Additionally, the term "comprising" is used throughout to mean
including at least the recited feature(s) such that any greater
number of the same feature and/or additional types of other
features are not precluded. It will also be appreciated that
specific embodiments have been described herein for purposes of
illustration, but that various modifications may be made without
deviating from the technology. Further, while advantages associated
with certain embodiments of the technology have been described in
the context of those embodiments, other embodiments may also
exhibit such advantages, and not all embodiments need necessarily
exhibit such advantages to fall within the scope of the technology.
Accordingly, the disclosure and associated technology can encompass
other embodiments not expressly shown or described herein.
* * * * *