US20080177364A1 - Self-locking electrode assembly usable with an implantable medical device - Google Patents

Abstract

An electrode assembly for an implantable medical device, including a spine, and a plurality of electrodes protruding from the spine. At least two electrodes protrude from the spine in opposing directions and define a nerve-receiving channel. When the electrode assembly is not coupled to a nerve and the electrodes are in a relaxed state position, the nerve receiving channel comprises a cross-sectional area that is substantially less than a cross-sectional area of a nerve to which the electrode assembly is adapted to be coupled. And when coupled to the nerve, each electrode wraps around and directly contacts at least 60% of the circumference of the nerve.

Description

CROSS-REFERENCES TO RELATED APPLICATIONS
• This application is a continuation of U.S. patent application Ser. No. 10/402,911, filed on Mar. 27, 2003, which is: (i) a continuation-in-part of U.S. patent application Ser. No. 09/963,777, filed on Sep. 26, 2001, which is a continuation-in-part of U.S. patent application Ser. No. 09/671,850, filed on Sep. 27, 2000, now issued as U.S. Pat. No. 6,522,926; and (ii) claims the benefit of U.S. Provisional Patent Application No. 60/368,222, filed on Mar. 27, 2002, the disclosures of each of the above being hereby incorporated by reference in their entirety. The parent application for this application has incorporated by reference the disclosures of the following U.S. patent applications: U.S. patent application Ser. No. 09/964,079, filed on Sep. 26, 2001, now issued as U.S. Pat. No. 6,985,774, and U.S. patent application Ser. No. 09/963,991, filed on Sep. 26, 2001, now issued as U.S. Pat. No. 6,850,801, now issued as U.S. Pat. No. 6,850,801, the disclosures of which are also effectively incorporated by reference herein.
BACKGROUND OF THE INVENTIONField of the Invention
• The present invention generally relates to medical devices and methods of use for the treatment and/or management of cardiovascular and renal disorders. Specifically, the present invention relates to devices and methods for controlling the baroreflex system for the treatment and/or management of cardiovascular and renal disorders and their underlying causes and conditions.
• Cardiovascular disease is a major contributor to patient illness and mortality. It also is a primary driver of health care expenditure, costing more than $326 billion each year in the United States. Hypertension, or high blood pressure, is a major cardiovascular disorder that is estimated to affect over 50 million people in the United Sates alone. Of those with hypertension, it is reported that fewer than 30% have their blood pressure under control. Hypertension is a leading cause of heart failure and stroke. It is the primary cause of death in over 42,000 patients per year and is listed as a primary or contributing cause of death in over 200,000 patients per year in the U.S. Accordingly, hypertension is a serious health problem demanding significant research and development for the treatment thereof.
• Hypertension occurs when the body's smaller blood vessels (arterioles) constrict, causing an increase in blood pressure. Because the blood vessels constrict, the heart must work harder to maintain blood flow at the higher pressures. Although the body may tolerate short periods of increased blood pressure, sustained hypertension may eventually result in damage to multiple body organs, including the kidneys, brain, eyes and other tissues, causing a variety of maladies associated therewith. The elevated blood pressure may also damage the lining of the blood vessels, accelerating the process of atherosclerosis and increasing the likelihood that a blood clot may develop. This could lead to a heart attack and/or stroke. Sustained high blood pressure may eventually result in an enlarged and damaged heart (hypertrophy), which may lead to heart failure.
• Heart failure is the final common expression of a variety of cardiovascular disorders, including ischemic heart disease. It is characterized by an inability of the heart to pump enough blood to meet the body's needs and results in fatigue, reduced exercise capacity and poor survival. It is estimated that approximately 5,000,000 people in the United States suffer from heart failure, directly leading to 39,000 deaths per year and contributing to another 225,000 deaths per year. It is also estimated that greater than 400,000 new cases of heart failure are diagnosed each year. Heart failure accounts for over 900,000 hospital admissions annually, and is the most common discharge diagnosis in patients over the age of 65 years. It has been reported that the cost of treating heart failure in the United States exceeds $20 billion annually. Accordingly, heart failure is also a serious health problem demanding significant research and development for the treatment and/or management thereof.
• Heart failure results in the activation of a number of body systems to compensate for the heart's inability to pump sufficient blood. Many of these responses are mediated by an increase in the level of activation of the sympathetic nervous system, as well as by activation of multiple other neurohormonal responses. Generally speaking, this sympathetic nervous system activation signals the heart to increase heart rate and force of contraction to increase the cardiac output; it signals the kidneys to expand the blood volume by retaining sodium and water; and it signals the arterioles to constrict to elevate the blood pressure. The cardiac, renal and vascular responses increase the workload of the heart, further accelerating myocardial damage and exacerbating the heart failure state. Accordingly, it is desirable to reduce the level of sympathetic nervous system activation in order to stop or at least minimize this vicious cycle and thereby treat or manage the heart failure.
• A number of drug treatments have been proposed for the management of hypertension, heart failure and other cardiovascular disorders. These include vasodilators to reduce the blood pressure and ease the workload of the heart, diuretics to reduce fluid overload, inhibitors and blocking agents of the body's neurohormonal responses, and other medicaments.
• Various surgical procedures have also been proposed for these maladies. For example, heart transplantation has been proposed for patients who suffer from severe, refractory heart failure. Alternatively, an implantable medical device such as a ventricular assist device (VAD) may be implanted in the chest to increase the pumping action of the heart. Alternatively, an intra-aortic balloon pump (IABP) may be used for maintaining heart function for short periods of time, but typically no longer than one month. Other surgical procedures are available as well.
• It has been known for decades that the wall of the carotid sinus, a structure at the bifurcation of the common carotid arteries, contains stretch receptors (baroreceptors) that are sensitive to the blood pressure. These receptors send signals via the carotid sinus nerve to the brain, which in turn regulates the cardiovascular system to maintain normal blood pressure (the baroreflex), in part through activation of the sympathetic nervous system. Electrical stimulation of the carotid sinus nerve (baropacing) has previously been proposed to reduce blood pressure and the workload of the heart in the treatment of high blood pressure and angina. For example, U.S. Pat. No. 6,073,048 to Kieval et al. discloses a baroreflex modulation system and method for stimulating the baroreflex arc based on various cardiovascular and pulmonary parameters.
• Although each of these alternative approaches is beneficial in some ways, each of the therapies has its own disadvantages. For example, drug therapy is often incompletely effective. Some patients may be unresponsive (refractory) to medical therapy. Drugs often have unwanted side effects and may need to be given in complex regimens. These and other factors contribute to poor patient compliance with medical therapy. Drug therapy may also be expensive, adding to the health care costs associated with these disorders. Likewise, surgical approaches are very costly, may be associated with significant patient morbidity and mortality and may not alter the natural history of the disease. Baropacing also has not gained acceptance. Several problems with electrical carotid sinus nerve stimulation have been reported in the medical literature. These include the invasiveness of the surgical procedure to implant the nerve electrodes, and postoperative pain in the jaw, throat, face and head during stimulation. In addition, it has been noted that high voltages sometimes required for nerve stimulation may damage the carotid sinus nerves. Accordingly, there continues to be a substantial and long felt need for new devices and methods for treating and/or managing high blood pressure, heart failure and their associated cardiovascular and nervous system disorders.
• U.S. Pat. No. 6,522,926, signed to the Assignee of the present application, describes a number of systems and methods intended to activate baroreceptors in the carotid sinus and elsewhere in order to induce the baroreflex. Numerous specific approaches are described, including the use of coil electrodes placed over the exterior of the carotid sinus near the carotid bifurcation. While such electrode designs offer substantial promise, there is room for improvement in a number of specific design areas. For example, it would be desirable to provide designs which permit electrode structures to be closely and conformably secured over the exterior of a carotid sinus or other blood vessels so that efficient activation of the underlying baroreceptors can be achieved. It would be further desirable to provide specific electrode structures which can be variably positioned at different locations over the carotid sinus wall or elsewhere. At least some of these objectives will be met by these inventions described hereinbelow.
BRIEF SUMMARY OF THE INVENTION
• To address hypertension, heart failure and their associated cardiovascular and nervous system disorders, the present invention provides a number of devices, systems and methods by which the blood pressure, nervous system activity, and neurohormonal activity may be selectively and controllably regulated by activating baroreceptors. By selectively and controllably activating baroreceptors, the present invention reduces excessive blood pressure, sympathetic nervous system activation and neurohormonal activation, thereby minimizing their deleterious effects on the heart, vasculature and other organs and tissues.
• The present invention provides systems and methods for treating a patient by inducing a baroreceptor signal to effect a change in the baroreflex system (e.g., reduced heart rate, reduced blood pressure, etc.). The baroreceptor signal is activated or otherwise modified by selectively activating baroreceptors. To accomplish this, the system and method of the present invention utilize a baroreceptor activation device positioned near a baroreceptor in the carotid sinus, aortic arch, heart, common carotid arteries, subclavian arteries, and/or brachiocephalic artery. Preferably, the baroreceptor activation device is located in the right and/or left carotid sinus (near the bifurcation of the common carotid artery) and/or the aortic arch. By way of example, not limitation, the present invention is described with reference to the carotid sinus location.
• Generally speaking, the baroreceptor activation device may be activated, deactivated or otherwise modulated to activate one or more baroreceptors and induce a baroreceptor signal or a change in the baroreceptor signal to thereby effect a change in the baroreflex system. The baroreceptor activation device may be activated, deactivated, or otherwise modulated continuously, periodically, or episodically. The baroreceptor activation device may comprise a wide variety of devices which utilize electrodes to directly or indirectly activate the baroreceptor. The baroreceptor may be activated directly, or activated indirectly via the adjacent vascular tissue. The baroreceptor activation device will be positioned outside the vascular wall. To maximize therapeutic efficacy, mapping methods may be employed to precisely locate or position the baroreceptor activation device.
• The present invention is directed particularly at electrical means and methods to activate baroreceptors, and various electrode designs are provided. The electrode designs may be particularly suitable for connection to the carotid arteries at or near the carotid sinus, and may be designed to minimize extraneous tissue stimulation. While being particularly suitable for use on the carotid arteries at or near the carotid sinus, the electrode structures and assemblies of the present invention will also find use for external placement and securement of electrodes about other arteries, and in some cases veins, having baroreceptor and other electrically activated receptors therein.
• In a first aspect of the present invention, a baroreceptor activation device or other electrode useful for a carotid sinus or other blood vessel comprises a base having one or more electrodes connected to the base. The base has a length sufficient to extend around at least a substantial portion of the circumference of a blood vessel, usually an artery, more usually a carotid artery at or near the carotid sinus. By “substantial portion,” it is meant that the base will extend over at least 25% of the vessel circumference, usually at least 50%, more usually at least 66%, and often at least 75% or over the entire circumference. Usually, the base is sufficiently elastic to conform to said circumference or portion thereof when placed therearound. The electrode connected to the base is oriented at least partly in the circumferential direction and is sufficiently stretchable to both conform to the shape of the carotid sinus when the base is conformed thereover and accommodate changes in the shape and size of the sinus as they vary over time with heart pulse and other factors, including body movement which causes the blood vessel circumference to change.
• Usually, at least two electrodes will be positioned circumferentially and adjacent to each other on the base. The electrode(s) may extend over the entire length of the base, but in some cases will extend over less than 75% of the circumferential length of the base, often being less than 50% of the circumferential length, and sometimes less than 25% of the circumferential length. Thus, the electrode structures may cover from a small portion up to the entire circumferential length of the carotid artery or other blood vessel. Usually, the circumferential length of the elongate electrodes will cover at least 10% of the circumference of the blood vessel, typically being at least 25%, often at least 50%, 75%, or the entire length. The base will usually have first and second ends, wherein the ends are adapted to be joined, and will have sufficient structural integrity to grasp the carotid sinus.
• In a further aspect of the present invention, an extravascular electrode assembly comprises an elastic base and a stretchable electrode. The elastic base is adapted to be conformably attached over the outside of a target blood vessel, such as a carotid artery at or near the carotid sinus, and the stretchable electrode is secured over the elastic base and capable of expanding and contracting together with the base. In this way, the electrode assembly is conformable to the exterior of the carotid sinus or other blood vessel. Preferably, the elastic base is planar, typically comprising an elastomeric sheet. While the sheet may be reinforced, the reinforcement will be arranged so that the sheet remains elastic and stretchable, at least in the circumferential direction, so that the base and electrode assembly may be placed and conformed over the exterior of the blood vessel. Suitable elastomeric sheets may be composed of silicone, latex, and the like.
• To assist in mounting the extravascular electrode over the carotid sinus or other blood vessel, the assembly will usually include two or more attachment tabs extending from the elastomeric sheet at locations which allow the tabs to overlap the elastic base and/or be directly attached to the blood vessel wall when the base is wrapped around or otherwise secured over a blood vessel. In this way, the tabs may be fastened to secure the backing over the blood vessel.
• Preferred stretchable electrodes comprise elongated coils, where the coils may stretch and shorten in a spring-like manner. In particularly preferred embodiments, the elongated coils will be flattened over at least a portion of their lengths, where the flattened portion is oriented in parallel to the elastic base. The flattened coil provides improved electrical contact when placed against the exterior of the carotid sinus or other blood vessel.
• In a further aspect of the present invention, an extravascular electrode assembly comprises a base and an electrode structure. The base is adapted to be attached over the outside of a carotid artery or other blood vessel and has an electrode-carrying surface formed over at least a portion thereof. A plurality of attachment tabs extend away from the electrode-carrying surface, where the tabs are arranged to permit selective ones thereof to be wrapped around a blood vessel while others of the tabs may be selectively removed. The electrode structure on or over the electrode-carrying surface.
• In preferred embodiments, the base includes at least one tab which extends longitudinally from the electrode-carrying surface and at least two tabs which extend away from the surface at opposite, transverse angles. In an even more preferred embodiment, the electrode-carrying surface is rectangular, and at least two longitudinally extending tabs extend from adjacent corners of the rectangular surface. The two transversely angled tabs extend at a transverse angle away from the same two corners.
• As with prior embodiments, the electrode structure preferably includes one or more stretchable electrodes secured to the electrode-carrying surface. The stretchable electrodes are preferably elongated coils, more preferably being “flattened coils” to enhance electrical contact with the blood vessel to be treated. The base is preferably an elastic base, more preferably being formed from an elastomeric sheet. The phrase “flattened coil,” as used herein, refers to an elongate electrode structure including a plurality of successive turns where the cross-sectional profile is non-circular and which includes at least one generally flat or minimally curved face. Such coils may be formed by physically deforming (flattening) a circular coil, e.g., as shown inFIG. 24 described below. Usually, the flattened coils will have a cross-section that has a width in the plane of the electrode assembly greater than its height normal to the electrode assembly plane. Alternatively, the coils may be initially fabricated in the desired geometry having one generally flat (or minimally curved) face for contacting tissue. Fully flattened coils, e.g., those having planar serpentine configurations, may also find use, but usually it will be preferred to retain at least some thickness in the direction normal to the flat or minimally curved tissue-contacting surface. Such thickness helps the coiled electrode protrude from the base and provide improved tissue contact over the entire flattened surface.
• In a still further aspect of the present invention, a method for wrapping an electrode assembly over a blood vessel comprises providing an electrode assembly having an elastic base and one or more stretchable electrodes. The base is conformed over an exterior of the blood vessel, such as a carotid artery, and at least a portion of an electrode is stretched along with the base. Ends of the elastic base are secured together to hold the electrode assembly in place, typically with both the elastic backing and stretchable electrode remaining under at least slight tension to promote conformance to the vessel exterior. The electrode assembly will be located over a target site in the blood vessel, typically a target site having an electrically activated receptor. Advantageously, the electrode structures of the present invention when wrapped under tension will flex and stretch with expansions and contractions of the blood vessel. A presently preferred target site is a baroreceptor, particularly baroreceptors in or near the carotid sinus.
• In a still further aspect of the present invention, a method for wrapping an electrode assembly over a blood vessel comprises providing an electrode assembly including a base having an electrode-carrying surface and an electrode structure on the electrode-carrying surface. The base is wrapped over a blood vessel, and some but not all of a plurality of attachment tags on the base are secured over the blood vessel. Usually, the tabs which are not used to secure an electrode assembly will be removed, typically by cutting. Preferred target sites are electrically activated receptors, usually baroreceptors, more usually baroreceptors on the carotid sinus. The use of such electrode assemblies having multiple attachment tabs is particularly beneficial when securing the electrode assembly on a carotid artery near the carotid sinus. By using particular tabs, as described in more detail below, the active electrode area can be positioned at any of a variety of locations on the common, internal, and/or external carotid arteries.
• In another aspect, the present invention comprises pressure measuring assemblies including an elastic base adapted to be mounted on the outer wall of a blood vessel under circumferential tension. A strain measurement sensor is positioned on the base to measure strain resulting from circumferential expansion of the vessel due to a blood pressure increase. Usually, the base will wrap about the entire circumference of the vessel, although only a portion of the base need be elastic. Alternatively, a smaller base may be stapled, glued, clipped or otherwise secured over a “patch” of the vessel wall to detect strain variations over the underlying surface. Exemplary sensors include strain gauges and micro machined sensors (MEMS).
• In yet another aspect, electrode assemblies according to the present invention comprise a base and at least three parallel elongate electrode structures secured over a surface of the base. The base is attachable to an outside surface of a blood vessel, such as a carotid artery, particularly a carotid artery near the carotid sinus, and has a length sufficient to extend around at least a substantial portion of the circumference of the blood vessel, typically extending around at least 25% of the circumference, usually extending around at least 50% of the circumference, preferably extending at least 66% of the circumference, and often extending around at least 75% of or the entire circumference of the blood vessel. As with prior embodiments, the base will preferably be elastic and composed of any of the materials set forth previously.
• The at least three parallel elongate electrode structures will preferably be aligned in the circumferential direction of the base, i.e., the axis or direction of the base which will be aligned circumferentially over the blood vessel when the base is mounted on the blood vessel. The electrode structures will preferably be stretchable, typically being elongate coils, often being flattened elongate coils, as also described previously.
• At least an outer pair of the electrode structures will be electrically isolated from an inner electrode structure, and the outer electrode structures will preferably be arranged in a U-pattern in order to surround the inner electrode structure. In this way, the outer pair of electrodes can be connected using a single conductor taken from the base, and the outer electrode structures and inner electrode structure may be connected to separate poles on a power supply in order to operate in the “pseudo” tripolar mode described hereinbelow.
• To address low blood pressure and other conditions requiring blood pressure augmentation, the present invention provides electrode designs and methods utilizing such electrodes by which the blood pressure may be selectively and controllably regulated by inhibiting or dampening baroreceptor signals. By selectively and controllably inhibiting or dampening baroreceptor signals, the present invention reduces conditions associated with low blood pressure.
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 is a schematic illustration of the upper torso of a human body showing the major arteries and veins and associated anatomy.
• FIG. 2A is a cross-sectional schematic illustration of the carotid sinus and baroreceptors within the vascular wall.
• FIG. 2B is a schematic illustration of baroreceptors within the vascular wall and the baroreflex system.
• FIG. 3 is a schematic illustration of a baroreceptor activation system in accordance with the present invention.
• FIGS. 4A and 4B are schematic illustrations of a baroreceptor activation device in the form of an implantable extraluminal conductive structure which electrically induces a baroreceptor signal in accordance with an embodiment of the present invention.
• FIGS. 5A-5F are schematic illustrations of various possible arrangements of electrodes around the carotid sinus for extravascular electrical activation embodiments.
• FIG. 6 is a schematic illustration of a serpentine shaped electrode for extravascular electrical activation embodiments.
• FIG. 7 is a schematic illustration of a plurality of electrodes aligned orthogonal to the direction of wrapping around the carotid sinus for extravascular electrical activation embodiments.
• FIGS. 8-11 are schematic illustrations of various multi-channel electrodes for extravascular electrical activation embodiments.
• FIG. 12 is a schematic illustration of an extravascular electrical activation device including a tether and an anchor disposed about the carotid sinus and common carotid artery.
• FIG. 13 is a schematic illustration of an alternative extravascular electrical activation device including a plurality of ribs and a spine.
• FIG. 14 is a schematic illustration of an electrode assembly for extravascular electrical activation embodiments.
• FIG. 15 is a schematic illustration of a fragment of an alternative cable for use with an electrode assembly such as shown inFIG. 14.
• FIG. 16 illustrates a foil strain gauge for measuring expansion force of a carotid artery or other blood vessel.
• FIG. 17 illustrates a transducer which is adhesively connected to the wall of an artery.
• FIG. 18 is a cross-sectional view of the transducer ofFIG. 17.
• FIG. 19 illustrates a first exemplary electrode assembly having an elastic base and plurality of attachment tabs.
• FIG. 20 is a more detailed illustration of the electrode-carrying surface of the electrode assembly ofFIG. 19.
• FIG. 21 is a detailed illustration of electrode coils which are present in an elongate lead of the electrode assembly ofFIG. 19.
• FIG. 22 is a detailed view of the electrode-carrying surface of an electrode assembly similar to that shown inFIG. 20, except that the electrodes have been flattened.
• FIG. 23 is a cross-sectional view of the electrode structure ofFIG. 22.
• FIG. 24 illustrates the transition between the flattened and non-flattened regions of the electrode coil of the electrode assemblyFIG. 20.
• FIG. 25 is a cross-sectional view taken along the line25-25 ofFIG. 24.
• FIG. 26 is a cross-sectional view taken along the line26-26 ofFIG. 24.
• FIG. 27 is an illustration of a further exemplary electrode assembly constructed in accordance with the principles of the present invention.
• FIG. 28 illustrates the electrode assembly ofFIG. 27 wrapped around the common carotid artery near the carotid bifurcation.
• FIG. 29 illustrates the electrode assembly ofFIG. 27 wrapped around the internal carotid artery.
• FIG. 30 is similar toFIG. 29, but with the carotid bifurcation having a different geometry.
DETAILED DESCRIPTION OF THE INVENTION
• The following detailed description should be read with reference to the drawings in which similar elements in different drawings are numbered the same. The drawings, which are not necessarily to scale, depict illustrative embodiments and are not intended to limit the scope of the invention.
• To better understand the present invention, it may be useful to explain some of the basic vascular anatomy associated with the cardiovascular system. Refer toFIG. 1 which is a schematic illustration of the upper torso of ahuman body10 showing some of the major arteries and veins of the cardiovascular system. The left ventricle of theheart11 pumps oxygenated blood up into theaortic arch12. The rightsubclavian artery13, the right commoncarotid artery14, the left commoncarotid artery15 and the leftsubclavian artery16 branch off theaortic arch12 proximal of the descendingthoracic aorta17. Although relatively short, a distinct vascular segment referred to as thebrachiocephalic artery22 connects the rightsubclavian artery13 and the right commoncarotid artery14 to theaortic arch12. The rightcarotid artery14 bifurcates into the right externalcarotid artery18 and the right internalcarotid artery19 at the rightcarotid sinus20. Although not shown for purposes of clarity only, the leftcarotid artery15 similarly bifurcates into the left external carotid artery and the left internal carotid artery at the left carotid sinus.
• From theaortic arch12, oxygenated blood flows into thecarotid arteries18/19 and thesubclavian arteries13/16. From thecarotid arteries18/19, oxygenated blood circulates through the head and cerebral vasculature and oxygen depleted blood returns to theheart11 by way of the jugular veins, of which only the right internaljugular vein21 is shown for sake of clarity. From thesub clavian arteries13/16, oxygenated blood circulates through the upper peripheral vasculature and oxygen depleted blood returns to the heart by way of the subclavian veins, of which only the rightsubclavian vein23 is shown, also for sake of clarity. Theheart11 pumps the oxygen depleted blood through the pulmonary system where it is reoxygenated. The re-oxygenated blood returns to theheart11 which pumps the re-oxygenated blood into the aortic arch as described above, and the cycle repeats.
• Within the arterial walls of theaortic arch12, commoncarotid arteries14/15 (near the rightcarotid sinus20 and left carotid sinus),subclavian arteries13/16 andbrachiocephalic artery22 there arebaroreceptors30. For example, as best seen inFIG. 2A,baroreceptors30 reside within the vascular walls of thecarotid sinus20.Baroreceptors30 are a type of stretch receptor used by the body to sense blood pressure. An increase in blood pressure causes the arterial wall to stretch, and a decrease in blood pressure causes the arterial wall to return to its original size. Such a cycle is repeated with each beat of the heart. Becausebaroreceptors30 are located within the arterial wall, they are able to sense deformation of the adjacent tissue, which is indicative of a change in blood pressure. Thebaroreceptors30 located in the rightcarotid sinus20, the left carotid sinus and theaortic arch12 play the most significant role in sensing blood pressure that affects thebaroreflex system50, which is described in more detail with reference toFIG. 2B.
• Refer now toFIG. 2B, which shows a schematic illustration ofbaroreceptors30 disposed in a genericvascular wall40 and a schematic flow chart of thebaroreflex system50.Baroreceptors30 are profusely distributed within thearterial walls40 of the major arteries discussed previously, and generally form anarbor32. Thebaroreceptor arbor32 comprises a plurality ofbaroreceptors30, each of which transmits baroreceptor signals to thebrain52 vianerve38. Thebaroreceptors30 are so profusely distributed and arborized within thevascular wall40 thatdiscrete baroreceptor arbors32 are not readily discernable. To this end, those skilled in the art will appreciate that thebaroreceptors30 shown inFIG. 2B are primarily schematic for purposes of illustration and discussion.
• Baroreceptor signals are used to activate a number of body systems which collectively may be referred to as thebaroreflex system50.Baroreceptors30 are connected to thebrain52 via thenervous system51. Thus, thebrain52 is able to detect changes in blood pressure, which is indicative of cardiac output. If cardiac output is insufficient to meet demand (i.e., theheart11 is unable to pump sufficient blood), thebaroreflex system50 activates a number of body systems, including theheart11,kidneys53,vessels54, and other organs/tissues. Such activation of thebaroreflex system50 generally corresponds to an increase in neurohormonal activity. Specifically, thebaroreflex system50 initiates a neurohormonal sequence that signals theheart11 to increase heart rate and increase contraction force in order to increase cardiac output, signals thekidneys53 to increase blood volume by retaining sodium and water, and signals thevessels54 to constrict to elevate blood pressure. The cardiac, renal and vascular responses increase blood pressure andcardiac output55, and thus increase the workload of theheart11. In a patient with heart failure, this further accelerates myocardial damage and exacerbates the heart failure state.
• To address the problems of hypertension, heart failure, other cardiovascular disorders and renal disorders, the present invention basically provides a number of devices, systems and methods by which thebaroreflex system50 is activated to reduce excessive blood pressure, autonomic nervous system activity and neurohormonal activation. In particular, the present invention provides a number of devices, systems and methods by whichbaroreceptors30 may be activated, thereby indicating an increase in blood pressure and signaling thebrain52 to reduce the body's blood pressure and level of sympathetic nervous system and neurohormonal activation, and increase parasypathetic nervous system activation, thus having a beneficial effect on the cardiovascular system and other body systems.
• With reference toFIG. 3, the present invention generally provides a system including acontrol system60, abaroreceptor activation device70, and a sensor80 (optional), which generally operate in the following manner. The sensor(s)80 optionally senses and/or monitors a parameter (e.g., cardiovascular function) indicative of the need to modify the baroreflex system and generates a signal indicative of the parameter. Thecontrol system60 generates a control signal as a function of the received sensor signal. The control signal activates, deactivates or otherwise modulates thebaroreceptor activation device70. Typically, activation of thedevice70 results in activation of thebaroreceptors30. Alternatively, deactivation or modulation of thebaroreceptor activation device70 may cause or modify activation of thebaroreceptors30. Thebaroreceptor activation device70 may comprise a wide variety of devices which utilize electrical means to activatebaroreceptors30. Thus, when thesensor80 detects a parameter indicative of the need to modify the baroreflex system activity (e.g., excessive blood pressure), thecontrol system60 generates a control signal to modulate (e.g. activate) thebaroreceptor activation device70 thereby inducing abaroreceptor30 signal that is perceived by thebrain52 to be apparent excessive blood pressure. When thesensor80 detects a parameter indicative of normal body function (e.g., normal blood pressure), thecontrol system60 generates a control signal to modulate (e.g., deactivate) thebaroreceptor activation device70.
• As mentioned previously, thebaroreceptor activation device70 may comprise a wide variety of devices which utilize electrical means to activate thebaroreceptors30. Thebaroreceptor activation device70 of the present invention comprises an electrode structure which directly activates one ormore baroreceptors30 by changing the electrical potential across thebaroreceptors30. It is possible that changing the electrical potential across the tissue surrounding thebaroreceptors30 may cause the surrounding tissue to stretch or otherwise deform, thus mechanically activating thebaroreceptors30, in which case the stretchable and elastic electrode structures of the present invention may provide significant advantages.
• All of the specific embodiments of the electrode structures of the present invention are suitable for implantation, and are preferably implanted using a minimally invasive surgical approach. Thebaroreceptor activation device70 may be positioned anywhere baroreceptors30 are present. Such potential implantation sites are numerous, such as theaortic arch12, in the commoncarotid arteries18/19 near thecarotid sinus20, in thesubclavian arteries13/16, in thebrachiocephalic artery22, or in other arterial or venous locations. The electrode structures of the present invention will be implanted such that they are positioned on or over a vascular structure immediately adjacent thebaroreceptors30. Preferably, the electrode structure of thebaroreceptor activation device70 is implanted near the rightcarotid sinus20 and/or the left carotid sinus (near the bifurcation of the common carotid artery) and/or theaortic arch12, wherebaroreceptors30 have a significant impact on thebaroreflex system50. For purposes of illustration only, the present invention is described with reference tobaroreceptor activation device70 positioned near thecarotid sinus20.
• Theoptional sensor80 is operably coupled to thecontrol system60 by electric sensor cable or lead82. Thesensor80 may comprise any suitable device that measures or monitors a parameter indicative of the need to modify the activity of the baroreflex system. For example, thesensor80 may comprise a physiologic transducer or gauge that measures ECG, blood pressure (systolic, diastolic, average or pulse pressure), blood volumetric flow rate, blood flow velocity, blood pH, O2 or CO2 content, mixed venous oxygen saturation (SVO2), vasoactivity, nerve activity, tissue activity, body movement, activity levels, respiration, or composition. Examples of suitable transducers or gauges for thesensor80 include ECG electrodes, a piezoelectric pressure transducer, an ultrasonic flow velocity transducer, an ultrasonic volumetric flow rate transducer, a thermodilution flow velocity transducer, a capacitive pressure transducer, a membrane pH electrode, an optical detector (SVO2), tissue impedance (electrical), or a strain gauge. Although only onesensor80 is shown,multiple sensors80 of the same or different type at the same or different locations may be utilized.
• An example of an implantable blood pressure measurement device that may be disposed about a blood vessel is disclosed in U.S. Pat. No. 6,106,477 to Miesel et al., the entire disclosure of which is incorporated herein by reference. An example of a subcutaneous ECG monitor is available from Medtronic under the trade name REVEAL ILR and is disclosed in PCT Publication No. WO 98/02209, the entire disclosure of which is incorporated herein by reference. Other examples are disclosed in U.S. Pat. Nos. 5,987,352 and 5,331,966, the entire disclosures of which are incorporated herein by reference. Examples of devices and methods for measuring absolute blood pressure utilizing an ambient pressure reference are disclosed in U.S. Pat. No. 5,810,735 to Halperin et al., U.S. Pat. No. 5,904,708 to Goedeke, and PCT Publication No. WO 00/16686 to Brockway et al., the entire disclosures of which are incorporated herein by reference. Thesensor80 described herein may take the form of any of these devices or other devices that generally serve the same purpose.
• Thesensor80 is preferably positioned in a chamber of theheart11, or in/on a major artery such as theaortic arch12, a commoncarotid artery14/15, asubclavian artery13/16 or thebrachiocephalic artery22, such that the parameter of interest may be readily ascertained. Thesensor80 may be disposed inside the body such as in or on an artery, a vein or a nerve (e.g. vagus nerve), or disposed outside the body, depending on the type of transducer or gauge utilized. Thesensor80 may be separate from thebaroreceptor activation device70 or combined therewith. For purposes of illustration only, thesensor80 is shown positioned on the rightsubclavian artery13.
• By way of example, thecontrol system60 includes acontrol block61 comprising aprocessor63 and amemory62.Control system60 is connected to thesensor80 by way ofsensor cable82.Control system60 is also connected to thebaroreceptor activation device70 by way ofelectric control cable72. Thus, thecontrol system60 receives a sensor signal from thesensor80 by way ofsensor cable82, and transmits a control signal to thebaroreceptor activation device70 by way ofcontrol cable72.
• Thesystem components60/70/80 may be directly linked viacables72/82 or by indirect means such as RF signal transceivers, ultrasonic transceivers or galvanic couplings. Examples of such indirect interconnection devices are disclosed in U.S. Pat. No. 4,987,897 to Funke and U.S. Pat. No. 5,113,859 to Funke, the entire disclosures of which are incorporated herein by reference.
• Thememory62 may contain data related to the sensor signal, the control signal, and/or values and commands provided by theinput device64. Thememory62 may also include software containing one or more algorithms defining one or more functions or relationships between the control signal and the sensor signal. The algorithm may dictate activation or deactivation control signals depending on the sensor signal or a mathematical derivative thereof The algorithm may dictate an activation or deactivation control signal when the sensor signal falls below a lower predetermined threshold value, rises above an upper predetermined threshold value or when the sensor signal indicates a specific physiologic event. The algorithm may dynamically alter the threshold value as determined by the sensor input values.
• As mentioned previously, thebaroreceptor activation device70 activatesbaroreceptors30 electrically, optionally in combination with mechanical, thermal, chemical, biological or other co-activation. In some instances, thecontrol system60 includes adriver66 to provide the desired power mode for thebaroreceptor activation device70. For example, thedriver66 may comprise a power amplifier or the like and thecable72 may comprise electrical lead(s). In other instances, thedriver66 may not be necessary, particularly if theprocessor63 generates a sufficiently strong electrical signal for low level electrical actuation of thebaroreceptor activation device70.
• Thecontrol system60 may operate as a closed loop utilizing feedback from thesensor80, or other sensors, such as heart rate sensors which may be incorporated or the electrode assembly, or as an open loop utilizing reprogramming commands received byinput device64. The closed loop operation of thecontrol system60 preferably utilizes some feedback from thetransducer80, but may also operate in an open loop mode without feedback. Programming commands received by theinput device64 may directly influence the control signal, the output activation parameters, or may alter the software and related algorithms contained inmemory62. The treating physician and/or patient may provide commands to inputdevice64.Display65 may be used to view the sensor signal, control signal and/or the software/data contained inmemory62.
• The control signal generated by thecontrol system60 may be continuous, periodic, alternating, episodic or a combination thereof, as dictated by an algorithm contained inmemory62. Continuous control signals include a constant pulse, a constant train of pulses, a triggered pulse and a triggered train of pulses. Examples of periodic control signals include each of the continuous control signals described above which have a designated start time (e.g., beginning of each period as designated by minutes, hours, or days in combinations of) and a designated duration (e.g., seconds, minutes, hours, or days in combinations of). Examples of alternating control signals include each of the continuous control signals as described above which alternate between the right and left output channels. Examples of episodic control signals include each of the continuous control signals described above which are triggered by an episode (e.g., activation by the physician/patient, an increase/decrease in blood pressure above a certain threshold, heart rate above/below certain levels, etc.).
• The stimulus regimen governed by thecontrol system60 may be selected to promote long term efficacy. It is theorized that uninterrupted or otherwise unchanging activation of thebaroreceptors30 may result in the baroreceptors and/or the baroreflex system becoming less responsive over time, thereby diminishing the long term effectiveness of the therapy. Therefore, the stimulus regimen maybe selected to activate, deactivate or otherwise modulate thebaroreceptor activation device70 in such a way that therapeutic efficacy is maintained preferably for years.
• In addition to maintaining therapeutic efficacy over time, the stimulus regimens of the present invention may be selected reduce power requirement/consumption of thesystem60. As will be described in more detail hereinafter, the stimulus regimen may dictate that thebaroreceptor activation device70 be initially activated at a relatively higher energy and/or power level, and subsequently activated at a relatively lower energy and/or power level. The first level attains the desired initial therapeutic effect, and the second (lower) level sustains the desired therapeutic effect long term. By reducing the energy and/or power levels after the desired therapeutic effect is initially attained, the energy required or consumed by theactivation device70 is also reduced long term. This may correlate into systems having greater longevity and/or reduced size (due to reductions in the size of the power supply and associated components).
• A first general approach for a stimulus regimen which promotes long term efficacy and reduces power requirements/consumption involves generating a control signal to cause thebaroreceptor activation device70 to have a first output level of relatively higher energy and/or power, and subsequently changing the control signal to cause thebaroreceptor activation device70 to have a second output level of relatively lower energy and/or power. The first output level may be selected and maintained for sufficient time to attain the desired initial effect (e.g., reduced heart rate and/or blood pressure), after which the output level may be reduced to the second level for sufficient time to sustain the desired effect for the desired period of time.
• For example, if the first output level has a power and/or energy value of X1, the second output level may have a power and/or energy value of X2, wherein X2 is less than X1. In some instances, X2 may be equal to zero, such that the first level is “on” and the second level is “off”. It is recognized that power and energy refer to two different parameters, and in some cases, a change in one of the parameters (power or energy) may not correlate to the same or similar change in the other parameter. In the present invention, it is contemplated that a change in one or both of the parameters may be suitable to obtain the desired result of promoting long term efficacy.
• It is also contemplated that more than two levels may be used. Each further level may increase the output energy or power to attain the desired effect, or decrease the output energy or power to retain the desired effect. For example, in some instances, it may be desirable to have further reductions in the output level if the desired effect may be sustained at lower power or energy levels. In other instances, particularly when the desired effect is diminishing or is otherwise not sustained, it may be desirable to increase the output level until the desired effect is reestablished, and subsequently decrease the output level to sustain the effect.
• The transition from each level may be a step function (e.g., a single step or a series of steps), a gradual transition over a period of time, or a combination thereof. In addition, the signal levels may be continuous, periodic, alternating, or episodic as discussed previously.
• In electrical activation using a non modulated signal, the output (power or energy) level of thebaroreceptor activation device70 may be changed by adjusting the output signal voltage level, current level and/or signal duration. The output signal of thebaroreceptor activation device70 may be, for example, constant current or constant voltage. In electrical activation embodiments using a modulated signal, wherein the output signal comprises, for example, a series of pulses, several pulse characteristics may be changed individually or in combination to change the power or energy level of the output signal. Such pulse characteristics include, but are not limited to: pulse amplitude (PA), pulse frequency (PF), pulse width or duration (PW), pulse waveform (square, triangular, sinusoidal, etc.), pulse polarity (for bipolar electrodes) and pulse phase (monophasic, biphasic).
• In electrical activation wherein the output signal comprises a pulse train, several other signal characteristics may be changed in addition to the pulse characteristics described above, as described in copending application Ser. No. 09/964,079, the full disclosure of which is incorporated herein by reference.
• FIGS. 4A and 4B show schematic illustrations of abaroreceptor activation device300 in the form of an extravascular electrically conductive structure orelectrode302. Theelectrode structure302 may comprise a coil, braid or other structure capable of surrounding the vascular wall. Alternatively, theelectrode structure302 may comprise one or more electrode patches distributed around the outside surface of the vascular wall. Because theelectrode structure302 is disposed on the outside surface of the vascular wall, intravascular delivery techniques may not be practical, but minimally invasive surgical techniques will suffice. Theextravascular electrode structure302 may receive electrical signals directly from thedriver66 of thecontrol system60 by way ofelectrical lead304, or indirectly by utilizing an inductor (not shown) as described in copending commonly assigned application Ser. No. 10/402,393 (Attorney Docket No. 21433-000420), filed on the same day as the present application, the full disclosure of which is incorporated herein by reference.
• Refer now toFIGS. 5A-5F which show schematic illustrations of various possible arrangements of electrodes around thecarotid sinus20 for extravascular electrical activation embodiments, such asbaroreceptor activation device300 described with reference toFIGS. 4A and 4B. The electrode designs illustrated and described hereinafter may be particularly suitable for connection to the carotid arteries at or near the carotid sinus, and may be designed to minimize extraneous tissue stimulation.
• InFIGS. 5A-5F, the carotid arteries are shown, including the common14, the external18 and the internal19 carotid arteries. The location of thecarotid sinus20 may be identified by alandmark bulge21, which is typically located on the internalcarotid artery19 just distal of the bifurcation, or extends across the bifurcation from the commoncarotid artery14 to the internalcarotid artery19.
• Thecarotid sinus20, and in particular thebulge21 of the carotid sinus, may contain a relatively high density of baroreceptors30 (not shown) in the vascular wall. For this reason, it may be desirable to position theelectrodes302 of theactivation device300 on and/or around thesinus bulge21 to maximize baroreceptor responsiveness and to minimize extraneous tissue stimulation.
• It should be understood that thedevice300 andelectrodes302 are merely schematic, and only a portion of which may be shown, for purposes of illustrating various positions of theelectrodes302 on and/or around thecarotid sinus20 and thesinus bulge21. In each of the embodiments described herein, theelectrodes302 may be monopolar, bipolar, or tripolar (anode-cathode-anode or cathode-anode-cathode sets). Specific extravascular electrode designs are described in more detail hereinafter.
• InFIG. 5A, theelectrodes302 of the extravascularelectrical activation device300 extend around a portion or the entire circumference of thesinus20 in a circular fashion. Often, it would be desirable to reverse the illustrated electrode configuration in actual use. InFIG. 5B, theelectrodes302 of the extravascularelectrical activation device300 extend around a portion or the entire circumference of thesinus20 in a helical fashion. In the helical arrangement shown inFIG. 5B, theelectrodes302 may wrap around thesinus20 any number of times to establish the desiredelectrode302 contact and coverage. In the circular arrangement shown inFIG. 5A, a single pair ofelectrodes302 may wrap around thesinus20, or a plurality of electrode pairs302 may be wrapped around thesinus20 as shown inFIG. 5C to establishmore electrode302 contact and coverage.
• The plurality of electrode pairs302 may extend from a point proximal of thesinus20 orbulge21, to a point distal of thesinus20 orbulge21 to ensure activation ofbaroreceptors30 throughout thesinus20 region. Theelectrodes302 may be connected to a single channel or multiple channels as discussed in more detail hereinafter. The plurality of electrode pairs302 may be selectively activated for purposes of targeting a specific area of thesinus20 to increase baroreceptor responsiveness, or for purposes of reducing the exposure of tissue areas to activation to maintain baroreceptor responsiveness long term.
• InFIG. 5D, theelectrodes302 extend around the entire circumference of thesinus20 in a criss cross fashion. The criss cross arrangement of theelectrodes302 establishes contact with both the internal19 and external18 carotid arteries around thecarotid sinus20. Similarly, inFIG. 5E, theelectrodes302 extend around all or a portion of the circumference of thesinus20, including the internal19 and external18 carotid arteries at the bifurcation, and in some instances the commoncarotid artery14. InFIG. 5F, theelectrodes302 extend around all or a portion of the circumference of thesinus20, including the internal19 and external18 carotid arteries distal of the bifurcation. InFIGS. 5E and 5F, the extravascularelectrical activation devices300 are shown to include a substrate orbase structure306 which may encapsulate and insulate theelectrodes302 and may provide a means for attachment to thesinus20 as described in more detail hereinafter.
• From the foregoing discussion with reference toFIGS. 5A-5F, it should be apparent that there are a number of suitable arrangements for theelectrodes302 of theactivation device300, relative to thecarotid sinus20 and associated anatomy. In each of the examples given above, theelectrodes302 are wrapped around a portion of the carotid structure, which may require deformation of theelectrodes302 from their relaxed geometry (e.g., straight). To reduce or eliminate such deformation, theelectrodes302 and/or thebase structure306 may have a relaxed geometry that substantially conforms to the shape of the carotid anatomy at the point of attachment. In other words, theelectrodes302 and the base structure or backing306 may be pre shaped to conform to the carotid anatomy in a substantially relaxed state. Alternatively, theelectrodes302 may have a geometry and/or orientation that reduces the amount ofelectrode302 strain. Optionally, as described in more detail below, the backing orbase structure306 may be elastic or stretchable to facilitate wrapping of and conforming to the carotid sinus or other vascular structure.
• For example, inFIG. 6, theelectrodes302 are shown to have a serpentine or wavy shape. The serpentine shape of theelectrodes302 reduces the amount of strain seen by the electrode material when wrapped around a carotid structure. In addition, the serpentine shape of the electrodes increases the contact surface area of theelectrode302 with the carotid tissue. As an alternative, theelectrodes302 may be arranged to be substantially orthogonal to the wrap direction (i.e., substantially parallel to the axis of the carotid arteries) as shown inFIG. 7. In this alternative, theelectrodes302 each have a length and a width or diameter, wherein the length is substantially greater than the width or diameter. Theelectrodes302 each have a longitudinal axis parallel to the length thereof, wherein the longitudinal axis is orthogonal to the wrap direction and substantially parallel to the longitudinal axis of the carotid artery about which thedevice300 is wrapped. As with the multiple electrode embodiments described previously, theelectrodes302 may be connected to a single channel or multiple channels as discussed in more detail hereinafter.
• Refer now toFIGS. 8-11 which schematically illustrate various multi-channel electrodes for the extravascularelectrical activation device300.FIG. 8 illustrates a six (6) channel electrode assembly including six (6) separateelongate electrodes302 extending adjacent to and parallel with each other. Theelectrodes302 are each connected tomulti-channel cable304. Some of theelectrodes302 may be common, thereby reducing the number of conductors necessary in thecable304.
• Base structure orsubstrate306 may comprise a flexible and electrically insulating material suitable for implantation, such as silicone, perhaps reinforced with a flexible material such as polyester fabric. The base306 may have a length suitable to wrap around all (360.degree.) or a portion (i.e., less than 360.degree.) of the circumference of one or more of the carotid arteries adjacent thecarotid sinus20. Theelectrodes302 may extend around a portion (i.e., less than 360.degree. such as 270.degree., 180.degree. or 90.degree.) of the circumference of one or more of the carotid arteries adjacent thecarotid sinus20. To this end, theelectrodes302 may have a length that is less than (e.g., 75%, 50% or 25%) the length of the base206. Theelectrodes302 may be parallel, orthogonal or oblique to the length of thebase306, which is generally orthogonal to the axis of the carotid artery to which it is disposed about. Preferably, the base structure or backing will be elastic (i.e., stretchable), typically being composed of at least in part of silicone, latex, or other elastomer. If such elastic structures are reinforced, the reinforcement should be arranged so that it does not interfere with the ability of the base to stretch and conform to the vascular surface.
• Theelectrodes302 may comprise round wire, rectangular ribbon or foil formed of an electrically conductive and radiopaque material such as platinum. Thebase structure306 substantially encapsulates theelectrodes302, leaving only an exposed area for electrical connection to extravascular carotid sinus tissue. For example, eachelectrode302 may be partially recessed in the base206 and may have one side exposed along all or a portion of its length for electrical connection to carotid tissue. Electrical paths through the carotid tissues may be defined by one or more pairs of theelongate electrodes302.
• In all embodiments described with reference toFIGS. 8-11, themulti-channel electrodes302 may be selectively activated for purposes of mapping and targeting a specific area of thecarotid sinus20 to determine the best combination of electrodes302 (e.g., individual pair, or groups of pairs) to activate for maximum baroreceptor responsiveness, as described elsewhere herein. In addition, themulti-channel electrodes302 may be selectively activated for purposes of reducing the exposure of tissue areas to activation to maintain long term efficacy as described, as described elsewhere herein. For these purposes, it may be useful to utilize more than two (2) electrode channels. Alternatively, theelectrodes302 may be connected to a single channel whereby baroreceptors are uniformly activated throughout thesinus20 region.
• An alternative multi-channel electrode design is illustrated inFIG. 9. In this embodiment, thedevice300 includes sixteen (16)individual electrode pads302 connected to 16channel cable304 via 4channel connectors303. In this embodiment, thecircular electrode pads302 are partially encapsulated by thebase structure306 to leave one face of eachbutton electrode302 exposed for electrical connection to carotid tissues. With this arrangement, electrical paths through the carotid tissues may be defined by one or more pairs (bipolar) or groups (tripolar) ofelectrode pads302.
• A variation of the multi-channel pad type electrode design is illustrated inFIG. 10. In this embodiment, thedevice300 includes sixteen (16) individualcircular pad electrodes302 surrounded by sixteen (16) rings305, which collectively may be referred to asconcentric electrode pads302/305.Pad electrodes302 are connected to 17channel cable304 via 4channel connectors303, and rings305 are commonly connected to 17channel cable304 via asingle channel connector307. In this embodiment, the circular shapedelectrodes302 and therings305 are partially encapsulated by thebase structure306 to leave one face of eachpad electrode302 and one side of eachring305 exposed for electrical connection to carotid tissues. As an alternative, tworings305 may surround eachelectrode302, with therings305 being commonly connected. With these arrangements, electrical paths through the carotid tissues may be defined between one ormore pad electrode302/ring305 sets to create localized electrical paths.
• Another variation of the multi-channel pad electrode design is illustrated inFIG. 11. In this embodiment, thedevice300 includes acontrol IC chip310 connected to 3channel cable304. Thecontrol chip310 is also connected to sixteen (16)individual pad electrodes302 via 4channel connectors303. Thecontrol chip310 permits the number of channels incable304 to be reduced by utilizing a coding system. Thecontrol system60 sends a coded control signal which is received bychip310. Thechip310 converts the code and enables or disables selectedelectrode302 pairs in accordance with the code.
• For example, the control signal may comprise a pulse wave form, wherein each pulse includes a different code. The code for each pulse causes thechip310 to enable one or more pairs of electrodes, and to disable the remaining electrodes. Thus, the pulse is only transmitted to the enabled electrode pair(s) corresponding to the code sent with that pulse. Each subsequent pulse would have a different code than the preceding pulse, such that thechip310 enables and disables a different set ofelectrodes302 corresponding to the different code. Thus, virtually any number of electrode pairs may be selectively activated usingcontrol chip310, without the need for a separate channel incable304 for eachelectrode302. By reducing the number of channels incable304, the size and cost thereof may be reduced.
• Optionally, theIC chip310 may be connected tofeedback sensor80, taking advantage of the same functions as described with reference toFIG. 3. In addition, one or more of theelectrodes302 may be used as feedback sensors when not enabled for activation. For example, such a feedback sensor electrode may be used to measure or monitor electrical conduction in the vascular wall to provide data analogous to an ECG. Alternatively, such a feedback sensor electrode may be used to sense a change in impedance due to changes in blood volume during a pulse pressure to provide data indicative of heart rate, blood pressure, or other physiologic parameter.
• Refer now toFIG. 12 which schematically illustrates an extravascularelectrical activation device300 including a support collar oranchor312. In this embodiment, theactivation device300 is wrapped around the internalcarotid artery19 at thecarotid sinus20, and thesupport collar312 is wrapped around the commoncarotid artery14. Theactivation device300 is connected to thesupport collar312 bycables304, which act as a loose tether. With this arrangement, thecollar312 isolates the activation device from movements and forces transmitted by thecables304 proximal of the support collar, such as may be encountered by movement of thecontrol system60 and/ordriver66. As an alternative to supportcollar312, a strain relief (not shown) may be connected to thebase structure306 of theactivation device300 at the juncture between thecables304 and thebase306. With either approach, the position of thedevice300 relative to the carotid anatomy may be better maintained despite movements of other parts of the system.
• In this embodiment, thebase structure306 of theactivation device300 may comprise molded tube, a tubular extrusion, or a sheet of material wrapped into a tube shape utilizing asuture flap308 withsutures309 as shown. Thebase structure306 may be formed of a flexible and biocompatible material such as silicone, which may be reinforced with a flexible material such as polyester fabric available under the trade name DACRON® to form a composite structure. The inside diameter of thebase structure306 may correspond to the outside diameter of the carotid artery at the location of implantation, for example 6 to 8 mm. The wall thickness of thebase structure306 may be very thin to maintain flexibility and a low profile, for example less than 1 mm. If thedevice300 is to be disposed about asinus bulge21, a correspondingly shaped bulge may be formed into the base structure for added support and assistance in positioning.
• The electrodes302 (shown in phantom) may comprise round wire, rectangular ribbon or foil, formed of an electrically conductive and radiopaque material such as platinum or platinum iridium. The electrodes may be molded into thebase structure306 or adhesively connected to the inside diameter thereof, leaving a portion of the electrode exposed for electrical connection to carotid tissues. Theelectrodes302 may encompass less than the entire inside circumference (e.g., 300.degree.) of thebase structure306 to avoid shorting. Theelectrodes302 may have any of the shapes and arrangements described previously. For example, as shown inFIG. 12, tworectangular ribbon electrodes302 may be used, each having a width of 1 mm spaced 1.5 mm apart.
• Thesupport collar312 may be formed similarly tobase structure306. For example, the support collar may comprise molded tube, a tubular extrusion, or a sheet of material wrapped into a tube shape utilizing asuture flap315 withsutures313 as shown. Thesupport collar312 may be formed of a flexible and biocompatible material such as silicone, which may be reinforced to form a composite structure. Thecables304 are secured to thesupport collar312, leaving slack in thecables304 between thesupport collar312 and theactivation device300.
• In all embodiments described herein, it may be desirable to secure the activation device to the vascular wall using sutures or other fixation means. For example, sutures311 may be used to maintain the position of theelectrical activation device300 relative to the carotid anatomy (or other vascular site containing baroreceptors).Such sutures311 may be connected tobase structure306, and pass through all or a portion of the vascular wall. For example, thesutures311 may be threaded through thebase structure306, through the adventitia of the vascular wall, and tied. If thebase structure306 comprises a patch or otherwise partially surrounds the carotid anatomy, the corners and/or ends of the base structure may be sutured, with additional sutures evenly distributed therebetween. In order to minimize the propagation of a hole or a tear through thebase structure306, a reinforcement material such as polyester fabric may be embedded in the silicone material. In addition to sutures, other fixation means may be employed such as staples or a biocompatible adhesive, for example.
• Refer now toFIG. 13 which schematically illustrates an alternative extravascularelectrical activation device300 including one ormore electrode ribs316 interconnected byspine317. Optionally, asupport collar312 having one or more (non electrode)ribs316 may be used to isolate theactivation device300 from movements and forces transmitted by thecables304 proximal of thesupport collar312.
• Theribs316 of theactivation device300 are sized to fit about the carotid anatomy, such as the internalcarotid artery19 adjacent thecarotid sinus20. Similarly, theribs316 of thesupport collar312 may be sized to fit about the carotid anatomy, such as the commoncarotid artery14 proximal of thecarotid sinus20. Theribs316 may be separated, placed on a carotid artery, and closed thereabout to secure thedevice300 to the carotid anatomy.
• Each of theribs316 of thedevice300 includes anelectrode302 on the inside surface thereof for electrical connection to carotid tissues. Theribs316 provide insulating material around theelectrodes302, leaving only an inside portion exposed to the vascular wall. Theelectrodes302 are coupled to themulti-channel cable304 throughspine317.Spine317 also acts as a tether toribs316 of thesupport collar312, which do not include electrodes since their function is to provide support. Themulti-channel electrode302 functions discussed with reference toFIGS. 8-11 are equally applicable to this embodiment.
• The ends of theribs316 may be connected (e.g., sutured) after being disposed about a carotid artery, or may remain open as shown. If the ends remain open, theribs316 may be formed of a relatively stiff material to ensure a mechanical lock around the carotid artery. For example, theribs316 may be formed of polyethylene, polypropylene, PTFE, or other similar insulating and biocompatible material. Alternatively, theribs316 may be formed of a metal such as stainless steel or a nickel titanium alloy, as long as the metallic material was electrically isolated from theelectrodes302. As a further alternative, theribs316 may comprise an insulating and biocompatible polymeric material with the structural integrity provided by metallic (e.g., stainless steel, nickel titanium alloy, etc.) reinforcement. In this latter alternative, theelectrodes302 may comprise the metallic reinforcement.
• Refer now toFIG. 14 which schematically illustrates a specific example of an electrode assembly for an extravascularelectrical activation device300. In this specific example, thebase structure306 comprises a silicone sheet having a length of 5.0 inches, a thickness of 0.007 inches, and a width of 0.312 inches. Theelectrodes302 comprise platinum ribbon having a length of 0.47 inches, a thickness of 0.0005 inches, and a width of 0.040 inches. Theelectrodes302 are adhesively connected to one side of thesilicone sheet306.
• Theelectrodes302 are connected to a modified bipolar endocardial pacing lead, available under the trade name CONIFIX from Innomedica (now BIOMEC Cardiovascular, Inc.), model number 501112. The proximal end of thecable304 is connected to thecontrol system60 ordriver66 as described previously. The pacing lead is modified by removing the pacing electrode to form thecable body304. The MP35 wires are extracted from the distal end thereof to form twocoils318 positioned side by side having a diameter of about 0.020 inches. Thecoils318 are then attached to the electrodes utilizing 316 type stainless steel crimp terminals laser welded to one end of theplatinum electrodes302. The distal end of thecable304 and the connection between thecoils318 and the ends of theelectrodes302 are encapsulated by silicone.
• Thecable304 illustrated inFIG. 14 comprises a coaxial type cable including two coaxially disposed coil leads separated into twoseparate coils318 for attachment to theelectrodes302. Analternative cable304 construction is illustrated inFIG. 15.FIG. 15 illustrates analternative cable body304 which may be formed in a curvilinear shape such as a sinusoidal configuration, prior to implantation. The curvilinear configuration readily accommodates a change in distance between thedevice300 and thecontrol system60 or thedriver66. Such a change in distance may be encountered during flexion and/or extension of the neck of the patient after implantation.
• In this alternative embodiment, thecable body304 may comprise two or moreconductive wires304aarranged coaxially or collinearly as shown. Eachconductive wire304amay comprise a multifilament structure of suitable conductive material such as stainless steel or MP35N. An insulating material may surround thewire conductors304aindividually and/or collectively. For purposes of illustration only, a pair of electricallyconductive wires304ahaving an insulating material surrounding eachwire304aindividually is shown. Theinsulated wires304amay be connected by aspacer304bcomprising, for example, an insulating material. An additional jacket of suitable insulating material may surround each of theconductors304a. The insulating jacket may be formed to have the same curvilinear shape of theinsulated wires304ato help maintain the shape of thecable body304 during implantation.
• If a sinusoidal configuration is chosen for the curvilinear shape, the amplitude (A) may range from 1 mm to 10 mm, and preferably ranges from 2 mm to 3 mm. The wavelength (WL) of the sinusoid may range from 2 mm to 20 mm, and preferably ranges from 4 mm to 10 mm. The curvilinear or sinusoidal shape may be formed by a heat setting procedure utilizing a fixture which holds thecable304 in the desired shape while the cable is exposed to heat. Sufficient heat is used to heat set theconductive wires304aand/or the surrounding insulating material. After cooling, thecable304 may be removed from the fixture, and thecable304 retains the desired shape.
• Refer now toFIGS. 16-18 which illustrate various transducers that may be mounted to the wall of a vessel such as acarotid artery14 to monitor wall expansion or contraction using strain, force and/or pressure gauges. An example of an implantable blood pressure measurement device that may be disposed about a blood vessel is disclosed in U.S. Pat. No. 6,106,477 to Miesel et al., the entire disclosure of which is incorporated herein by reference. The output from such gauges may be correlated to blood pressure and/or heart rate, for example, and may be used to provide feedback to thecontrol system60 as described previously herein. InFIG. 16, an implantable pressure measuring assembly comprises a foil strain gauge or forcesensing resistor device740 disposed about an artery such as commoncarotid artery14. Atransducer portion742 may be mounted to a silicone base or backing744 which is wrapped around and sutured or otherwise attached to theartery14.
• Alternatively, thetransducer750 may be adhesively connected to the wall of theartery14 using a biologically compatible adhesive such as cyanoacrylate as shown inFIG. 17. In this embodiment, thetransducer750 comprises a micro machined sensor (MEMS) that measures force or pressure. TheMEMS transducer750 includes a micro arm752 (shown in section inFIG. 18) coupled to a silicon force sensor contained over anelastic base754. Acap756 covers the arm752 a top portion of thebase754. The base754 include an interior opening creating access from thevessel wall14 to thearm752. Anincompressible gel756 fills the space between thearm752 and thevessel wall14 such that force is transmitted to the arm upon expansion and contraction of the vessel wall. In both cases, changes in blood pressure within the artery cause changes in vessel wall stress which are detected by the transducer and which may be correlated with the blood pressure.
• Refer now toFIGS. 19-21 which illustrate an alternative extravascularelectrical activation device700, which, may also be referred to as an electrode cuff device or more generally as an “electrode assembly.” Except as described herein and shown in the drawings,device700 may be the same in design and function as extravascularelectrical activation device300 described previously.
• As seen inFIGS. 19 and 20, electrode assembly orcuff device700 includes coiledelectrode conductors702/704 embedded in aflexible support706. In the embodiment shown, anouter electrode coil702 and aninner electrode coil704 are used to provide a pseudo tripolar arrangement, but other polar arrangements are applicable as well as described previously. Thecoiled electrodes702/704 may be formed of fine round, flat or ellipsoidal wire such as 0.002 inch diameter round PtIr alloy wire wound into a coil form having a nominal diameter of 0.015 inches with a pitch of 0.004 inches, for example. The flexible support orbase706 may be formed of a biocompatible and flexible (preferably elastic) material such as silicone or other suitable thin walled elastomeric material having a wall thickness of 0.005 inches and a length (e.g., 2.95 inches) sufficient to surround the carotid sinus, for example.
• Each turn of the coil in the contact area of theelectrodes702/704 is exposed from theflexible support706 and any adhesive to form a conductive path to the artery wall. The exposedelectrodes702/704 may have a length (e.g., 0.236 inches) sufficient to extend around at least a portion of the carotid sinus, for example. Theelectrode cuff700 is assembled flat with the contact surfaces of thecoil electrodes702/704 tangent to the inside plane of theflexible support706. When theelectrode cuff700 is wrapped around the artery, the inside contact surfaces of the coiledelectrodes702/704 are naturally forced to extend slightly above the adjacent surface of the flexible support, thereby improving contact to the artery wall.
• The ratio of the diameter of the coiledelectrodes702/704 to the wire diameter is preferably large enough to allow the coil to bend and elongate without significant bending stress or torsional stress in the wire. Flexibility is a significant advantage of this design which allows theelectrode cuff700 to conform to the shape of the carotid artery and sinus, and permits expansion and contraction of the artery or sinus without encountering significant stress or fatigue. In particular, theflexible electrode cuff700 may be wrapped around and stretched to conform to the shape of the carotid sinus and artery during implantation. This may be achieved without collapsing or distorting the shape of the artery and carotid sinus due to the compliance of theelectrode cuff700. Theflexible support706 is able to flex and stretch with the conductor coils702/704 because of the absence of fabric reinforcement in the electrode contact portion of thecuff700. By conforming to the artery shape, and by the edge of theflexible support706 sealing against the artery wall, the amount of stray electrical field and extraneous stimulation will likely be reduced.
• The pitch of thecoil electrodes702/704 may be greater than the wire diameter in order to provide a space between each turn of the wire to thereby permit bending without necessarily requiring axial elongation thereof. For example, the pitch of the contact coils702/704 may be 0.004 inches per turn with a 0.002 inch diameter wire, which allows for a 0.002 inch space between the wires in each turn. The inside of the coil may be filled with a flexible adhesive material such as silicone adhesive which may fill the spaces between adjacent wire turns. By filling the small spaces between the adjacent coil turns, the chance of pinching tissue between coil turns is minimized thereby avoiding abrasion to the artery wall. Thus, the embeddedcoil electrodes702/704 are mechanically captured and chemically bonded into theflexible support706. In the unlikely event that acoil electrode702/704 comes loose from thesupport706, the diameter of the coil is large enough to be atraumatic to the artery wall. Preferably, the centerline of thecoil electrodes702/704 lie near the neutral axis ofelectrode cuff structure700 and theflexible support706 comprises a material with isotropic elasticity such as silicone in order to minimize the shear forces on the adhesive bonds between thecoil electrodes702/704 and thesupport706.
• The electrode coils702/704 are connected to correspondingconductive coils712/714, respectively, in anelongate lead710 which is connected to thecontrol system60. Anchoringwings718 may be provided on thelead710 to tether thelead710 to adjacent tissue and minimize the effects or relative movement between the lead710 and theelectrode cuff700. As seen inFIG. 21, theconductive coils712/714 may be formed of 0.003 MP35N bifilar wires wound into 0.018 inch diameter coils which are electrically connected to electrodecoils702/704 bysplice wires716. Theconductive coils712/714 may be individually covered by an insulatingcovering718 such as silicone tubing and collectively covered by insulatingcovering720.
• The conductive material of theelectrodes702/704 may be a metal as described above or a conductive polymer such as a silicone material filled with metallic particles such as Pt particles. In this latter embodiment, the polymeric electrodes may be integrally formed with theflexible support706 with the electrode contacts comprising raised areas on the inside surface of theflexible support706 electrically coupled to thelead710 by wires or wire coils. The use of polymeric electrodes may be applied to other electrode design embodiments described elsewhere herein.
• Reinforcement patches708 such as DACRON® fabric may be selectively incorporated into theflexible support706. For example,reinforcement patches708 may be incorporated into the ends or other areas of theflexible support706 to accommodate suture anchors. Thereinforcement patches708 provide points where theelectrode cuff700 may be sutured to the vessel wall and may also provide tissue in growth to further anchor thedevice700 to the exterior of the vessel wall. For example, thefabric reinforcement patches708 may extend beyond the edge of theflexible support706 so that tissue in growth may help anchor the electrode assembly orcuff700 to the vessel wall and may reduce reliance on the sutures to retain theelectrode assembly700 in place. As a substitute for or in addition to the sutures and tissue in growth, bioadhesives such as cyanoacrylate may be employed to secure thedevice700 to the vessel wall. In addition, an adhesive incorporating conductive particles such as Pt coated micro spheres may be applied to the exposed inside surfaces of theelectrodes702/704 to enhance electrical conduction to the tissue and possibly limit conduction along one axis to limit extraneous tissue stimulation.
• Thereinforcement patches708 may also be incorporated into theflexible support706 for strain relief purposes and to help retain thecoils702/704 to thesupport706 where theleads710 attach to theelectrode assembly700 as well as where theouter coil702 loops back around theinner coil704. Preferably, thepatches708 are selectively incorporated into theflexible support706 to permit expansion and contraction of thedevice700, particularly in the area of theelectrodes702/704. In particular, theflexible support706 is only fabric reinforced in selected areas thereby maintaining the ability of theelectrode cuff700 to stretch.
• Referring now toFIGS. 22-26, the electrode assembly ofFIGS. 19-21 can be modified to have “flattened” coil electrodes in the region of the assembly where the electrodes contact the extravascular tissue. As shown inFIG. 22, an electrode-carryingsurface801 of the electrode assembly, is located generally between parallel reinforcement strips ortabs808. The flattenedcoil section810 will generally be exposed on alower surface803 of the base806 (FIG. 23) and will be covered or encapsulated by a parylene or other polymeric structure ormaterial802 over anupper surface805 thereof. The coil is formed with a generallycircular periphery809, as best seen inFIGS. 24 and 26, and may be mechanically flattened, typically over a silicone or other supportinginsert815, as best seen inFIG. 25. The use of the flattened coil structure is particularly beneficial since it retains flexibility, allowing the electrodes to bend, stretch, and flex together with theelastomeric base806, while also increasing the flat electrode area available to contact the extravascular surface.
• Referring now toFIGS. 27-30, anadditional electrode assembly900 constructed in accordance with the principles of the present invention will be described.Electrode assembly900 comprises an electrode base, typically anelastic base902, typically formed from silicone or other elastomeric material, having an electrode-carryingsurface904 and a plurality of attachment tabs906 (906a,906b,906c,and906d) extending from the electrode-carrying surface. The attachment tabs906 are preferably formed from the same material as the electrode-carryingsurface904 of thebase902, but could be formed from other elastomeric materials as well. In the latter case, the base will be molded, stretched or otherwise assembled from the various pieces. In the illustrated embodiment, the attachment tabs906 are formed integrally with the remainder of thebase902, i.e., typically being cut from a single sheet of the elastomeric material.
• The geometry of theelectrode assembly900, and in particular the geometry of thebase902, is selected to permit a number of different attachment modes to the blood vessel. In particular, the geometry of theassembly902 ofFIG. 27 is intended to permit attachment to various locations on the carotid arteries at or near the carotid sinus and carotid bifurcation.
• A number of reinforcement regions910 (910a,910b,910c,910d,and910e) are attached to different locations on the base902 to permit suturing, clipping, stapling, or other fastening of the attachment tabs906 to each other and/or the electrode-carryingsurface904 of thebase902. In the preferred embodiment intended for attachment at or around the carotid sinus, afirst reinforcement strip910ais provided over an end of the base902 opposite to the end which carries the attachment tabs. Pairs of reinforcement strips910band910care provided on each of the axially alignedattachment tabs906aand906b,while similar pairs of reinforcement strips910dand910eare provided on each of the transversely angledattachment tabs906cand906d. In the illustrated embodiment, all attachment tabs will be provided on one side of the base, preferably emanating from adjacent corners of the rectangular electrode-carryingsurface904.
• The structure ofelectrode assembly900 permits the surgeon to implant the electrode assembly so that the electrodes920 (which are preferably stretchable, flat-coil electrodes as described in detail above), are located at a preferred location relative to the target baroreceptors. The preferred location may be determined, for example, as described in copending application Ser. No. 09/963,991, filed on Sep. 26, 2001, the full disclosure of which incorporated herein by reference.
• Once the preferred location for theelectrodes920 of theelectrode assembly900 is determined, the surgeon may position the base902 so that theelectrodes920 are located appropriately relative to the underlying baroreceptors. Thus, theelectrodes920 may be positioned over the common carotid artery CC as shown inFIG. 28, or over the internal carotid artery IC, as shown inFIGS. 29 and 30. InFIG. 28, theassembly900 may be attached by stretching thebase902 andattachment tabs906aand906bover the exterior of the common carotid artery. Thereinforcement tabs906aor906bmay then be secured to thereinforcement strip910a,either by suturing, stapling, fastening, gluing, welding, or other well-known means. Usually, thereinforcement tabs906cand906dwill be cut off at their bases, as shown at922 and924, respectively.
• In other cases, the bulge of the carotid sinus and the baroreceptors may be located differently with respect to the carotid bifurcation. For example, as shown inFIG. 29, the receptors may be located further up the internal carotid artery IC so that the placement ofelectrode assembly900 as shown inFIG. 28 will not work. Theassembly900, however, may still be successfully attached by utilizing the transversely angledattachment tabs906cand906drather than the central oraxial tabs906aand906b. As shown inFIG. 29, thelower tab906dis wrapped around the common carotid artery CC, while theupper attachment tab906cis wrapped around the internal carotid artery IC. Theaxial attachment tabs906aand906bwill usually be cut off (at locations926), although neither of them could in some instances also be wrapped around the internal carotid artery IC. Again, the tabs which are used may be stretched and attached toreinforcement strip910a,as generally described above.
• Referring toFIG. 30, in instances where the carotid bifurcation has less of an angle, theassembly900 may be attached using the upperaxial attachment tab906aand be lower transversely angledattachment tab906d.Attachment tabs906band906cmay be cut off, as shown atlocations928 and930, respectively. In all instances, the elastic nature of thebase902 and the stretchable nature of theelectrodes920 permit the desired conformance and secure mounting of the electrode assembly over the carotid sinus. It would be appreciated that these or similar structures would also be useful for mounting electrode structures at other locations in the vascular system.
• In most activation device embodiments described herein, it may be desirable to incorporate anti-inflammatory agents (e.g., steroid eluting electrodes) such as described in U.S. Pat. No. 4,711,251 to Stokes, U.S. Pat. No. 5,522,874 to Gates and U.S. Pat. No. 4,972,848 to Di Domenico et al., the entire disclosures of which are incorporated herein by reference. Such agents reduce tissue inflammation at the chronic interface between the device (e.g., electrodes) and the vascular wall tissue, to thereby increase the efficiency of stimulus transfer, reduce power consumption, and maintain activation efficiency, for example.
• Those skilled in the art will recognize that the present invention may be manifested in a variety of forms other than the specific embodiments described and contemplated herein. Accordingly, departures in form and detail may be made without departing from the scope and spirit of the present invention as described in the appended claims.

Claims (16)

1. An electrode assembly for an implantable medical device, comprising: a spine; and a plurality of electrodes protruding from the spine, wherein at least two electrodes protrude from the spine in opposing directions and define a nerve-receiving channel; wherein, when said electrode assembly is not coupled to a nerve and said electrodes are in a relaxed state position, said nerve receiving channel comprises a cross-sectional area that is substantially less than a cross-sectional area of a nerve to which the electrode assembly is adapted to be coupled; and wherein, when coupled to said nerve, each electrode wraps around and directly contacts at least 60% of the circumference of the nerve.

2. The electrode assembly ofclaim 1 wherein, when said electrode assembly is not coupled to a nerve and said electrodes are in a relaxed state position, the cross-sectional area of the nerve receiving channel is less than 80% of the cross-sectional area of the nerve.

3. The electrode assembly ofclaim 2 wherein, when said electrode assembly is not coupled to a nerve and said electrodes are in a relaxed state position, the cross-sectional area of the nerve receiving channel is less than 60% of the cross-sectional area of the nerve.

4. The electrode assembly ofclaim 1 wherein said plurality of electrodes comprises at least three electrodes and at least two electrodes adjacent one another along the spine protrude from the spine in a common direction.

5. The electrode assembly ofclaim 1 wherein said spine includes a plurality of electrical conductors, and wherein each conductor is coupled to at least one electrode, and is electrically insulated from all other of said electrical conductors.

6. An implantable medical device, comprising: a pulse generator; a lead assembly coupled to said pulse generator; and an electrode assembly coupled to said lead assembly, wherein the electrode assembly comprises a spine and a plurality of electrodes protruding from the spine, and wherein at least two electrodes protrude from the spine in opposing directions and define a nerve-receiving channel; wherein, when said electrode assembly is not coupled to a nerve and said electrodes are in a relaxed state position, said nerve receiving channel comprises a cross-sectional area that is substantially less than a cross-sectional area of a nerve to which the electrode assembly is adapted to be coupled; and wherein, when coupled to said nerve, each electrode wraps around and directly contacts at least 70% of the circumference of the nerve.

7. The implantable medical device ofclaim 6 wherein, when said electrode assembly is not coupled to a nerve and said electrodes are in a relaxed state position, the cross-sectional area of the nerve receiving channel is less than 80% of the cross-sectional area of the nerve.

8. The implantable medical device ofclaim 7 wherein, when said electrode assembly is not coupled to a nerve and said electrodes are in a relaxed state position, the cross-sectional area of the nerve receiving channel is less than 60% of the cross-sectional area of the nerve.

9. The implantable medical device ofclaim 6 wherein said plurality of electrodes comprises at least three electrodes and at least two electrodes adjacent one another along the spine protrude from the spine in a common direction.

10. The implantable medical device ofclaim 6 wherein said spine includes a plurality of electrical conductors, and wherein each conductor is coupled to at least one electrode, and is electrically insulated from all other of said electrical conductors.

11. An electrode assembly usable with an implantable medical device, comprising: a spine; and a plurality of curved fingers protruding from said spine, wherein at least two fingers protrude from the spine in opposing directions and define a nerve-receiving channel, and wherein at least one of said fingers comprises an electrode that is adapted to electrically contact a nerve; wherein, when said electrode assembly is not coupled to the nerve and said fingers are in a relaxed state position, said nerve-receiving channel comprises a cross-sectional area that is substantially less than a cross-sectional area of a nerve to which the electrode assembly is adapted to be coupled; and wherein, when coupled to said nerve, all of said fingers contact the nerve on a partial outer surface of the nerve, said partial outer surface extending circumferentially at least approximately 40% of the circumference of the nerve.

12. The electrode assembly ofclaim 11 wherein each finger wraps around and directly contacts at least 70% of the circumference of the nerve.

13. The electrode assembly ofclaim 11 wherein at least two fingers comprise an electrode.

14. The electrode assembly ofclaim 11 wherein, when said electrode assembly is not coupled to a nerve and said fingers are in a relaxed state position, the cross-sectional area of the nerve receiving channel is less than 80% of the cross-sectional area of the nerve.

15. The electrode assembly ofclaim 14 wherein, when said electrode assembly is not coupled to a nerve and said fingers are in a relaxed state position, the cross-sectional area of the nerve receiving channel is less than 60% of the cross-sectional area of the nerve.

16. An electrode assembly for an implantable medical device, comprising: a spine; and a plurality of electrodes protruding from the spine, wherein at least two electrodes protrude from the spine in opposing directions and define a nerve-receiving channel; wherein, when said electrode assembly is not coupled to a nerve and said electrodes are in a relaxed state position, said nerve receiving channel comprises a cross-sectional area that is less than 80% of the cross-sectional area of a nerve to which the electrode assembly is adapted to be coupled.

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