MICROWAVE CATHETER APPARATUSES, SYSTEMS, AND METHODS FOR RENAL NEUROMODULATION
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.
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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
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
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.
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.
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+) 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.
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.
Specific details of several embodiments of the technology are described below with reference to
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
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.
A. The Sympathetic Nervous System
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.
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.
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.
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.
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.
1. The Sympathetic Chain
As shown in
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.
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.
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).
2. Innervation of the Kidneys
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.
3. Renal Sympathetic Neural Activity
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.
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.
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.
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.
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.
(i) Renal Sympathetic Efferent Activity
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.
(ii) Renal Sensory Afferent Nerve Activity
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
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.
B. Additional Clinical Benefits of Renal Denervation
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
C. Achieving Intravascular Access to the Renal Artery
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
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.
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.
D. Properties and Characteristics of the Renal Vasculature
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.
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.
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.
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.
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.
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.
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.
As noted above, an apparatus positioned within a renal artery should conform to the geometry of the artery. Renal artery vessel diameter, DRA, typically is in a range of about 2-10 mm, with most of the patient population having a DRA of about 4 mm to about 8 mm and an average of about 6 mm. Renal artery vessel length, LRA, 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.
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°-135°.
E. Achieving Renal Denervation via an Intravascularly Delivered Microwave Field
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.
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.
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.
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.
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
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.
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
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.
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
As shown in
As shown in
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
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.
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° C.) but less than about 45° C. for non-ablative thermal alteration, or the target temperature can be about 45° C. or higher for ablative thermal alteration. Desired non-thermal neuromodulation effects may include altering the electrical signals transmitted in a nerve.
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
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
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.
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.
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 ½ 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 ½ 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
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
As seen in
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.
With reference now to
As seen in
As shown in
In another embodiment as shown in
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.
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.
4. Flow Directing Elements
In the expanded deployed configuration of
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,
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.
Referring now to
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
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
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
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.
In some embodiments, it may also be desirable for the elongated shaft 16 (
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
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.
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.
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.
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.
Filed: Oct 25, 2011
Publication Date: May 10, 2012
Applicant: Medtronic Ardian Luxembourg S.a.r.l. (Luxembourg)
Inventors: Karun D. Naga (Los Altos, CA), Roman Turovskiy (San Francisco, CA), Denise Zarins (Saratoga, CA), Mark Gelfand (New York, NY), Arye Rosen (Cherry Hill, NJ)
Application Number: 13/281,244