Patent Publication Number: US-7904175-B2

Title: Trans-esophageal vagus nerve stimulation

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a related application to U.S. patent application Ser. No. 11/796,158, entitled “Nonsurgical Device and Methods for Trans-Esophageal Vagus Nerve Stimulation,” which is filed on the same day as the present application and in the name of the same inventors. 
     BACKGROUND 
     1. Technical Field 
     The subject matter of this disclosure generally relates to the field of medical devices. More specifically, the present disclosure relates to non-surgically implantable medical devices and methods for implementing vagus nerve therapy. 
     2. Background Information 
     Many advancements have been made in treating medical conditions involving or mediated by the neurological systems and structures of the human body. In addition to drugs and surgical intervention, therapies using electrical signals for modulating the electrical activity of the body have been found to be effective. In particular, medical devices have been effectively used to deliver therapeutic electrical signals to various portions of a patient&#39;s body (e.g., the vagus nerve) for treating a variety of medical conditions. Electrical signal therapy may be applied to a target portion of the body by an implantable medical device (IMD) that is located inside the patient&#39;s body or, alternatively, may be applied by devices located external to the body. In addition, some proposed devices include a combination of implanted and external components. 
     The use of medical devices to provide electrical signal therapy has increased rapidly in recent decades. Such devices include pacemakers and defibrillators, which provide electrical signal therapy to heart tissue, as well as spinal cord stimulators for treatment of pain. In addition, devices have also been approved to provide electrical signal therapy to the vagus nerve (the 10 th  cranial nerve) to treat epilepsy and depression. Additional medical devices for providing electrical signal therapy have been proposed for stimulation of other nerves, such as the sympathetic nerve, the phrenic nerve and the occipital nerve. 
     The vagus nerve (cranial nerve X) is the longest nerve in the human body. It originates in the brainstem and extends, through the jugular foramen, down below the head, to the abdomen. Branches of the vagus nerve innervate various organs of the body, including the heart, the stomach, the lungs, the kidneys, the pancreas, and the liver. In view of the vagus nerve&#39;s many functions, a medical device such as an electrical signal generator has been coupled to a patient&#39;s vagus nerve to treat a number of medical conditions. In particular, electrical signal therapy for the vagus nerve, often referred to as vagus nerve stimulation (VNS), has been approved in the United States and elsewhere to treat epilepsy and depression. Application of an electrical signal to the vagus nerve is thought to affect some of its connections to areas in the brain that are prone to seizure activity. Because of the vagus nerve&#39;s innervation of the stomach, stimulating the vagus nerve may also be therapeutically beneficial to treating eating disorders such as bulimia nervosa, as well as treating morbid obesity. 
     Current and proposed VNS treatments have involved surgically coupling electrodes to the left and/or right vagus nerves in the neck. Other treatments involve surgically implanting electrodes to one or more surfaces in the abdomen, such as by laparoscopic surgery through the patient&#39;s abdominal wall. Electrical signal therapies in addition to VNS have been proposed to treat eating disorders such as obesity. These techniques include coupling electrodes to the exterior and/or interior of the stomach, duodenum, or intestinal walls. However, all of the aforementioned therapies either do not provide stimulation directed specifically to the vagus nerve, substantially interfere with normal gastrointestinal function, require some type of invasive surgery, or all of the above. 
     Consequently, there is a need for non-surgical devices and methods for treating medical conditions. There is also a need for providing an easily implanted device that provides little or no interference with normal gastrointestinal function. There is also a need to provide improved methods and devices for vagus nerve stimulation, and to avoid undesired side effects associated with conventional surgical vagus nerve stimulation. 
     BRIEF SUMMARY 
     The present invention provides devices and methods for non-surgically providing vagus nerve therapy to treat a variety of medical conditions. Embodiments of an IMD provide electrical stimulation to the vagus nerve through the wall of the esophagus (i.e. trans-esophageally) via a device coupled to the inner surface of the esophagus. Furthermore, implantation of the IMD may be accomplished non-surgically by oral insertion, precluding the need for incisions or even laparoscopic surgery. 
     In an embodiment, an implantable medical device for providing a therapeutic electrical signal to a vagus nerve of a patient comprises a support member having an outer surface. The support member is adapted to engage the inner wall of an esophagus. The IMD also comprises at least one electrode disposed on the outer surface of the support member. The at least one electrode is adapted to apply a trans-esophageal electrical signal to a vagus nerve through the wall of the esophagus from the inner lumen thereof. The implantable medical device further comprises a signal generator coupled to the support member and to the at least one electrode. The signal generator generates the electrical signal for application to the vagus nerve to treat a medical condition. 
     In another embodiment, an implantable medical device comprises a support member having an outer surface adapted to engage an inner surface of an esophagus of a patient. The implantable medical device comprises at least one electrode coupled to the outer surface of the support member. The at least one electrode is adapted to apply an electrical signal to a vagus nerve through the wall of the esophagus. Additionally, the implantable medical device comprises a signal generator coupled to the support member and to the at least one electrode. The signal generator is capable of generating an electrical signal for application to a vagus nerve of the patient. Furthermore, the implantable medical device comprises a power supply coupled to said support member and to the signal generator. The power supply is capable of providing electrical power to the signal generator. 
     In yet another embodiment, a medical device system comprises a support member having an outer surface. The outer surface is adapted to engage an inner surface of an esophagus of a patient. The system also comprises at least one electrode coupled to the outer surface of the support member and adapted to apply a therapeutic electrical signal trans-esophageally to a vagus nerve of the patient. Furthermore, the system comprises a signal generator coupled to the support member and to the at least one electrode. The signal generator is capable of generating said electrical signal for application to a vagus nerve. In addition, the system comprises a power supply for providing electrical power to said signal generator. The system also comprises an external programming system for programming one or more parameters defining said therapeutic electrical signal. 
     In an embodiment, a method of providing electrical signal therapy to a vagus nerve of a patient comprises implanting a medical device which includes at least one electrode in the lumen of the abdominal portion of the esophagus of the patient such that the at least one electrode is in contact with the surface of the esophagus wall. The method further comprises applying an electrical signal to at least one vagus nerve of the patient through the wall of the esophagus to treat a medical condition of the patient. 
     In another embodiment, a method of providing electrical signal therapy to a vagus nerve of a patient comprises using an ambulatory medical device to apply an electrical signal trans-esophageally to a vagus nerve of the patient from inside the abdominal portion of an esophagus. 
     In another embodiment, a method of providing an electrical signal therapy to a vagus nerve of a patient comprises providing an ambulatory medical device, which in turn comprises a support member having an outer surface, at least one electrode on the outer surface of the support member, and an electrical signal generator coupled to the support member and to the at least one electrode. The method further comprises non-surgically implanting the ambulatory medical device, generating a therapeutic electrical signal using the electrical signal generator, and applying the therapeutic electrical signal trans-esophageally to a vagus nerve of a patient from inside the abdominal portion of the patient&#39;s esophagus. 
     The foregoing has broadly outlined certain features and technical advantages of the invention in order that the detailed description of embodiments of the invention that follows may be better understood. Additional features and advantages of embodiments of the invention will be described hereinafter that form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a detailed description of the preferred embodiments of the invention, reference will now be made to the accompanying drawings in which: 
         FIG. 1  depicts, in schematic form, an embodiment of an implantable medical device (IMD) system comprising an IMD implanted within a patient and an external device for programming the IMD and, in an alternative embodiment, providing power to the IMD; 
         FIG. 2  illustrates an enlarged view of an embodiment of the IMD implanted inside the abdominal portion of the esophagus; 
         FIGS. 3A-D  illustrate embodiments of an IMD according to the present invention; 
         FIG. 4  is a block diagram of an IMD for use in an embodiment of the present invention, and an external unit for communication and programming of the IMD; 
         FIGS. 5A-B  illustrate embodiments of implantable medical devices according to the present invention; 
         FIG. 6  illustrates an embodiment of an IMD according to the present invention; 
         FIG. 7  is a schematic view of an IMD and a programming system that may be used in conjunction with embodiments of the IMD; and implantable medical device according to the present invention; 
         FIGS. 8A-D  illustrate an embodiment of a method of non-surgically implanting an IMD for providing trans-esophageal VNS to a patient to treat a medical condition. 
     
    
    
     NOTATION AND NOMENCLATURE 
     Certain terms are used throughout the following description and claims to refer to particular system components. This document does not intend to distinguish between components that differ in name but not function. In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . ”. Also, the term “couple” or “couples” is intended to mean either an indirect or direct electrical connection. Thus, if a first device couples to a second device, that connection may be through a direct electrical connection, or through an indirect electrical connection via other devices and connections. 
     The term “implantable medical device” refers to any medical device placed inside the human body. The placement of such a device may occur in a body lumen of the patient, such as the esophagus or a blood vessel, or may involve surgical implantation, such as when a conventional VNS system is implanted in the patient&#39;s chest and neck area. Furthermore, as defined herein, the term “non-surgical” is used to describe methods that do not require incision or dissection of body tissues to effect implantation of an IMD. In one embodiment, non-surgical implantation may involve placing a medical device in the esophagus of a patient using, e.g., laparoscopic instruments extending from the patient&#39;s mouth, through the throat, and into the esophagus. 
     A “therapeutic signal” refers to a stimulation signal delivered to a patient&#39;s body with the intent of treating a medical condition by providing a modulating effect to neural tissue. “Stimulate” or “stimulation signal” refers to the application of an electrical, mechanical, magnetic, electro-magnetic, photonic, audio and/or chemical signal to a neural structure in the patient&#39;s body. The signal is an exogenous signal that is distinct from the endogenous electrical, mechanical, and chemical activity (e.g., afferent and/or efferent electrical action potentials) generated by the patient&#39;s body and environment. In other words, the stimulation signal (whether electrical, mechanical, magnetic, electro-magnetic, photonic, audio or chemical in nature) applied to the nerve in the present invention is a signal applied from an artificial source, e.g., a neurostimulator. 
     The effect of a stimulation signal on neural activity is termed “modulation”; however, for simplicity, the terms “stimulating” and “modulating”, and variants thereof, may be used interchangeably. In general, the application of an exogenous signal to a nerve refers to “stimulation” of the neural structure, while the effects of that signal, if any, on the electrical activity of the neural structure are properly referred to as “modulation.” 
     As defined herein, the term “ambulatory” may describe implantable medical devices, portions of which a patient may carry around with minimal effect or interruption to his or her daily lifestyle. 
     In some medical devices, electrical neurostimulation may be provided by implanting an electrical device (e.g., a pulse generator) underneath the skin of a patient and delivering an electrical signal to a nerve such as a cranial nerve. In one embodiment, the electrical neurostimulation involves sensing or detecting a body parameter, with the electrical signal being delivered in response to the sensed body parameter. This type of stimulation is generally referred to as “active,” “feedback,” or “triggered” stimulation. In another embodiment, the system may operate without sensing or detecting a body parameter once the patient has been diagnosed with a medical condition that may be treated by neurostimulation. In this case, the system may apply a series of electrical pulses to the nerve (e.g., a cranial nerve such as a vagus nerve) periodically, intermittently, or continuously throughout the day, or over another predetermined time interval. This type of stimulation is generally referred to as “passive,” “non-feedback,” or “prophylactic,” stimulation. The electrical signal may be applied by an IMD that is implanted within the patient&#39;s body. In other cases, the signal may be generated by an external pulse generator outside the patient&#39;s body, coupled by an RF or wireless link to an implanted electrode. 
     Generally, neurostimulation signals that perform neuromodulation are delivered by the IMD via one or more leads. The leads generally terminate at their distal ends in one or more electrodes, and the electrodes, in turn, are electrically coupled to tissue in the patient&#39;s body. For example, a number of electrodes may be attached to various points of a nerve or other tissue inside a human body for delivery of a neurostimulation signal. In some types of neurostimulation devices, electrodes are coupled to the electrical signal generator without the use of leads. In the present invention, some embodiments include leads and some embodiments do not incorporate leads. 
     While feedback stimulation schemes have been proposed, conventional vagus nerve stimulation (VNS) usually involves non-feedback stimulation characterized by a number of parameters. Specifically, convention vagus nerve stimulation usually involves a series of electrical pulses in bursts defined by an “on-time” and an “off-time.” During the on-time, electrical pulses of a defined electrical current (e.g., 0.5-2.0 milliamps) and pulse width (e.g., 0.25-1.0 milliseconds) are delivered at a defined frequency (e.g., 20-30 Hz) for the on-time duration, usually a specific number of seconds, e.g., 10-100 seconds. The pulse bursts are separated from one another by the off-time, (e.g., 30 seconds-5 minutes) in which no electrical signal is applied to the nerve. The on-time and off-time parameters together define a duty cycle, which is the ratio of the on-time to the combination of the on-time and off-time, and which describes the percentage of time that the electrical signal is applied to the nerve. 
     In conventional VNS, the on-time and off-time may be programmed to define an intermittent pattern in which a repeating series of electrical pulse bursts are generated and applied to the vagus nerve. Each sequence of pulses during an on-time may be referred to as a “pulse burst.” The burst is followed by the off-time period in which no signals are applied to the nerve. The off-time is provided to allow the nerve to recover from the stimulation of the pulse burst, and to conserve power. If the off-time is set at zero, the electrical signal in conventional VNS may provide continuous stimulation to the vagus nerve. Alternatively, the idle time may be as long as one day or more, in which case the pulse bursts are provided only once per day or at even longer intervals. Typically, however, the ratio of “off-time” to “on-time” may range from about 0.5 to about 10. 
     In addition to the on-time and off-time, the other parameters defining the electrical signal in conventional VNS may be programmed over a range of values. The pulse width for the pulses in a pulse burst of conventional VNS may be set to a value not greater than about 1 msec, such as about 250-500 μsec, and the number of pulses in a pulse burst is typically set by programming a frequency in a range of about 20-150 Hz (i.e., 20 pulses per second to 150 pulses per second). A non-uniform frequency may also be used. Frequency may be altered during a pulse burst by either a frequency sweep from a low frequency to a high frequency, or vice versa. Alternatively, the timing between adjacent individual signals within a burst may be randomly changed such that two adjacent signals may be generated at any frequency within a range of frequencies. 
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The disclosed systems, devices and methods are susceptible to implementation in various embodiments. The disclosure of specific embodiments, including preferred embodiments, is not intended to limit the scope of the invention as claimed unless expressly specified. In addition, persons skilled in the art will understand that the invention has broad application. Accordingly, the discussion of particular embodiments is meant only to be exemplary, and does not imply that the scope of the disclosure, including the claims, is limited to specifically disclosed embodiments. 
     In some embodiments of the present invention, an implantable medical device may be implanted within a patient&#39;s body without surgery. The IMD generates and applies an electrical signal to the vagus nerve of the patient to treat a medical condition. The invention is possible because of the unique anatomy of the left and right vagus nerves. From the neck region, the left vagus nerve rotates anteriorly and becomes attached to the exterior surface of the abdominal or lower esophagus (i.e. the abdominal portion of the esophagus) as the anterior vagus nerve. As used herein, “abdominal esophagus” and “lower esophagus” may be used interchangeably to refer to the portion of the esophagus running from the thorax to the esophagogastric junction of a patient. The right vagus nerve, on the other hand, rotates posteriorly and becomes attached to the exterior surface of the lower esophagus as the posterior vagus nerve. As the anterior and posterior vagus nerves travel down the esophagus, the nerves separate into a number of branches at the esophagus/stomach junction, and the branches remain attached to the outer wall of the stomach. The close proximity of the nerves to the surfaces of the abdominal esophagus and the stomach permit the application of an electrical signal to the vagus nerve from the interior of the abdominal esophagus using a medical device coupled to the esophagus&#39;s interior wall. 
     In alternative embodiments, the invention may include devices and methods for providing electrical signal therapy across the wall of the upper portion of the stomach rather than the abdominal portion of the esophagus. However, the greater accessibility of the vagus nerves at the lower esophagus/stomach junction makes delivery of the signal across the wall of the lower esophagus a preferred embodiment. Application of a therapeutic electrical signal to the stomach is more difficult than the esophagus for a number of reasons. First, the motility of the esophagus is less than that of the stomach, thus making the esophagus a more stable structure. Second, the anterior and posterior vagus nerves are essentially a unitary structure (i.e., a single nerve bundle—albeit with thousands of individual nerve fibers within it—traveling down the length of the lower esophagus), with limited branching until passing onto the surface of the stomach. Accordingly, stimulation of the vagus at the lower esophagus level facilitates modulation of a larger population of nerves than stimulation of only some of the vagus nerve branches on the upper portion of the stomach. 
     In one embodiment, an ambulatory implantable medical device may be non-surgically implanted within the abdominal esophagus of a patient to deliver an electrical signal to at least one of the anterior or posterior vagus nerves through the wall of the esophagus. 
       FIGS. 1 and 2  depict an IMD  100  implanted in a patient. In one embodiment, IMD  100  comprises a support member  101 . Support member  101  comprises a flexible, biocompatible material and is adapted to be coupled to the inner wall of the lower esophagus of the patient. Moreover, support member  101  has one or more electrodes  103  coupled to its outer surface. An electrical signal generator  122  generates a therapeutic electrical signal to apply to the vagus nerve  133 . Signal generator  122  is electrically coupled to electrodes  103 . In some embodiments, the electrodes  103  are coupled to the electrical signal generator  122  by leads, while in alternative embodiments the electrodes are directly connected to the electrical signal generator without the use of leads. In addition, programming system  120  may be coupled to electrical signal generator  122 . Programming system  120  may generally control and monitor IMD  100  from outside the body as IMD  100  provides stimulation to the vagus nerve to provide therapy to a patient. 
     In one embodiment, the support member  101  has at least one unipolar electrode coupled to its outer surface. In another embodiment, the support member  101  has a plurality of unipolar electrodes on its outer surface. The plurality of electrodes may include a first plurality on a first side of the support member  101 , and a second plurality on a second side of the support member  101 . The first plurality may be adapted to provide a trans-esophageal therapeutic electrical signal to an anterior vagus nerve of the patient, and the second plurality of electrodes may be adapted to provide a trans-esophageal therapeutic electrical signal (which may be the same as or different from the signal provided by the first plurality) to a posterior vagus nerve of the patient. Electrodes  103  are used to apply an electrical signal to the vagus nerve  133  through the wall of the lower esophagus  131 . In one embodiment, the electrodes  103  contact or engage the inner wall of the esophagus  131 . In another embodiment, the electrodes are adapted to partially penetrate the wall of the lower esophagus  131 . In a still further embodiment, the electrodes are adapted to completely penetrate the wall of the lower esophagus  131 , and in yet another embodiment, the electrodes may comprise multiple electrode elements, some of which contact the inner wall of the esophagus, some of which partially penetrate the esophagus wall, and some of which completely penetrate the wall. Without being bound by theory, it is believed that greater penetration of the wall of the esophagus corresponds with more efficient stimulation of the vagus nerve on the outer wall of the esophagus, since the signal from electrode penetrating the wall in whole or in part will be less attenuated than a signal from electrodes that contact the inner wall of the esophagus without penetration of the same. 
     In one embodiment, support member  101  has at least one electrode pair (e.g., a cathode and an anode) coupled to its outer surface. In another embodiment, support member  101  has a plurality of electrode pairs on its outer surface. In a further embodiment, support member  101  has at least one unipolar electrode as well as at least one electrode pair on its outer surface. In general, the disposition of electrodes (whether unipolar, bipolar, or a combination of unipolar and bipolar electrodes) may be used to steer the electrical current of the therapeutic electrical signal to effectively modulate the anterior and/or posterior vagus nerves of the patient. 
     IMD  100  is preferably implanted non-surgically, as described below in more detail. However, in general, IMD  100  is implanted in the lower or abdominal portion of the esophagus via oral insertion of the IMD  100  through the patient&#39;s mouth and passage through the throat and the upper esophagus. Since IMD  100  is implanted within the lower esophagus, no incisions are necessary to position the IMD  100  at the desired implantation site. The IMD  100  may be retained in place and migration prevented by suitable anchors to engage the tissue of the esophagus  131 . The anchors may comprise any of a number of medical anchors known in the art, e.g., sutures, barbs, staples, adhesives, pins, etc. The IMD  100  may further comprise a retainment element to facilitate maintaining the device in position in the esophagus and to prevent migration. In one embodiment, the support member  101  comprises a ring or partial ring structure with a retainment element comprising a resilient spring made of metal or other suitable material. The resilient spring may be embedded in or coupled to support member  101  (e.g., by mechanical affixation). Support member  101  ring or partial ring may be sized to be slightly larger than the diameter of the patient&#39;s esophagus, such that the resilient spring retainment element coupled thereto provides a radially outwardly directed force against the lower esophagus to maintain the device in a fixed position in the esophagus, while allowing the IMD  100  to flex with normal movement of the esophagus. 
     Once implanted, IMD  100  provides therapy to the patient by generating and applying an electrical signal to the vagus nerve across the wall of the esophagus  131 . As used herein, the terms “trans-esophageal” or “trans-esophageally” refer to or describe delivery of a therapy (i.e. vagus nerve stimulation) across the esophageal wall. The signal preferably comprises an electronic signal generated by the electronic signal generator  122  in the inner lumen of the esophagus and applied trans-esophageally to the vagus nerve on the outer surface of the esophagus wall. The electrical signal provided by the electronic signal generator  122  may be defined by programming system  120 . In some embodiments, the patient may control and/or alter the therapy by a user signal, e.g., a magnet, a tap sensor, or a patient module for noninvasively transmitting and/or receiving data from the IMD  100 , e.g., by RF signals. 
     Generally, as mentioned above, neurostimulation signals that perform neuromodulation are delivered by the IMD  100  via one or more electrodes  103  as shown in  FIG. 2 . The modulating effect of the neurostimulation signal upon the neural tissue may be excitatory or inhibitory, and may potentiate acute and/or long-term changes in neuronal activity. For example, the “modulating” effect of the stimulation signal to the neural tissue may comprise one more of the following effects: (a) initiation of an action potential (afferent and/or efferent action potentials); (b) inhibition or blocking of the conduction of action potentials, whether endogenous or exogenously induced, including hyperpolarizing and/or collision blocking, (c) affecting changes in neurotransmitter/neuromodulator release or uptake, and (d) changes in neuro-plasticity or neurogenesis of brain tissue. 
     In a preferred embodiment, the support member  101  is a ring-shaped or tube-shaped member defining a circular cross-section as shown in  FIG. 3 . The diameter of the support member  101  may be sized to match or slightly exceed the diameter of the esophagus  131 . Alternatively, support member  101  may be semi-circular, i.e., comprising an incomplete circle or a C-shape as shown in  FIG. 5B . More generally, support member  102  may be of any suitable geometry that conforms to at least a portion of the inner surface of an organ. 
     Referring to  FIG. 5A , in one embodiment, the support member  101  comprises a generally planar, flexible member that is capable of bending to conform to a portion of the inner wall of the lower esophagus  131 . In such an embodiment, support member  101  may be comprised of a biocompatible polymer such as silicone rubber in a suitable shape, i.e., a circular shape, a rectangular shape, or any desired shape, and may have an electrical signal generator  122  coupled thereto (for example by embedding the electrical signal generator in the biocompatible polymer). Furthermore, in such an embodiment, support member  101  may be attached to inner wall of esophagus by using a tissue adhesive or biocompatible glue. 
     In another embodiment, the support member may include a retainment element comprising an expandable stent, which may be introduced into the lumen of the esophagus  131  in a compressed state via conventional balloon catheters, and then expanded by inflating the balloon to conform the device to the size of the esophagus. 
     According to another embodiment of the invention, the support member  101  has a contracted position to facilitate introduction and implantation of the device in the esophagus, and an expanded position engaging the inner wall of the esophagus to maintain the device in position and reduce or eliminate migration. In one aspect, the support member  101  has a fixed expanded position to hold the device in place within the esophagus. As described in detail above, support member  101  may comprise a retainment element that may be a resilient spring member or a stent to provide a radially outwardly directed force against the inner wall of the esophagus. In addition, support member  101  and the retainment element may be mounted on a catheter in the first, contracted position to facilitate introduction of the IMD  100  into the patient&#39;s mouth and passage through the upper esophagus into the lower esophagus, whereupon the IMD  100  may be deployed in the expanded position and optionally further retained in position with one or more anchor members. 
     In one embodiment, support member  101  may further comprise one or more anchor members (not shown) for attaching the IMD  100  to the inner wall of the esophagus  131 . For example, support member  101  may comprise a plurality of suture elements such as a suture ring or apertures so that a surgeon may suture the IMD  100  to the inner esophageal wall to prevent displacement of the IMD  100 . In another aspect, the anchor members may comprise a plurality of barbs or protrusions which are capable of grasping or engaging the inner surface of the esophagus, and/or of penetrating the wall of the esophagus, to prevent movement or displacement. The barbs or protrusions may be disposed in close proximity to the electrodes  103  so as to stabilize the IMD  100  in a fixed position relative to the anterior and/or posterior vagus nerves. In a further embodiment, the electrodes  103  may serve as anchor members. For example, electrodes  103  themselves may comprise protrusions which project radially from the support member  101  to engage the wall of the esophagus  131 . The electrodes  103  may contact the inner surface of the esophagus wall, and/or may comprise barbs to penetrate the wall in whole or part. Another means of engaging the inner wall of the esophagus comprises a tissue adhesive or biocompatible glue coated on to the outer surface of support member  101 . 
     The support member  101  may be made of any suitable biocompatible material. The materials used in fabricating the IMD  100  are preferably non-toxic such that if the IMD  100  is dislodged into the stomach, it may easily pass through the digestive system. In a preferred embodiment, support member  101  is made of a resilient or flexible biocompatible material such as a polymer. Suitable polymers include without limitation, hydrogels, silicone, polyethylene, polypropylene, polyurethane, polycaprolactone, polytetrafluoroethylene (PTFE), copolymers and combinations of the foregoing, and the like. In another embodiment, the support member is made of a silicone polymer. In alternative embodiments, support member  101  is made of biocompatible metals such as without limitation, titanium, silver, gold, alloys, nitinol, shape-memory metals, or combinations thereof. Furthermore, support member  101  may be made of biological materials such as tissue-engineered esophageal tissue from an animal donor, bovine pericardium, and the like. Where human or animal tissue from a donor is used, the materials are preferably decellularized and cross-linked to minimize adverse immunological reactions and prevent the immune system of the patient from degrading the materials. 
       FIG. 6  illustrates an embodiment of an IMD  100 . IMD  100  may comprise a retainment element  142  comprising a skeleton or frame embedded within a support member  101 . The support member may in some embodiments comprise a polymer exterior or coating  144  (e.g., a fabric exterior or engineered polymer to provide improved biocompatibility). The retainment element  142  (depicted in a grid-like configuration although many other geometries may be employed) may be made of a flexible metal or polymer, and has sufficient stiffness when support member  101  is in an expanded position to maintain the position of the IMD  100  in the esophagus. The support member  101  serves as a substrate to which to mount the electrodes  103  and other components of the IMD  100  (e.g., a pulse generator and a controller). In a specific embodiment, the exterior/coating  144  comprises a fatty acid derivative covalently bonded to a silicone polymer comprising the support member  101 . 
     According to an embodiment, when the support member  101  and the retainment element  142  are expanded, they typically have a diameter or size that corresponds to the inner dimensions of the esophagus. For example, the esophagus has an inner diameter ranging from approximately 10 mm to approximately 30 mm. As such, in an embodiment, the support member  101  has an expanded size ranging from approximately 8 mm to about 50 mm, preferably from about 10 mm to about 40 mm, more preferably from about 10 mm to about 35 mm in diameter. In other embodiments, different support members  101  may be fabricated with different sizes such that a surgeon may have the option of selecting the most appropriate size depending on the patient. 
     The exterior or outer surface of the support member  101  comprises one or more electrodes  103  which apply a therapeutic electrical signal to the wall of the lower esophagus  131 . As noted, in alternative embodiments the electrodes  103  may provide an electrical signal therapy across the wall of the upper portion of the stomach instead of or in addition to the lower portion of the esophagus. At any rate, electrodes  103  are disposed on support member  101  such that they will be in close proximity to the vagus nerve when the IMD  100  is non-surgically implanted in the esophagus. Preferably, the electrodes comprise a conductive material such as platinum, iridium, platinum-iridium alloys, gold, silver, copper, aluminum, and/or oxides of the foregoing. The electrodes  103  may be arranged in any suitable pattern on the exterior of the ring. The pattern is preferably optimized to deliver electrical stimulation to the anterior and/or posterior vagus nerves regardless of the location of the IMD  100  within the esophagus. According to one embodiment, electrodes  103  are arranged as parallel bands or rings around all or part of the outer surface of the IMD  100  as shown in  FIG. 3A . In another embodiment, electrodes  103  are arranged as rows of circular or hemispherical contacts as shown in  FIGS. 3B and 3D , respectively. In still another embodiment, electrodes  103  are arranged in vertical columns of electrodes, with each column acting as a unipolar electrode, a cathode or an anode ( FIG. 3C ).  FIG. 3D  also depicts other components of the IMD  100  embedded in support member  101 , including a power supply  430 , a signal generation unit  420 , and a controller  410 , as more fully discussed with regard to  FIG. 4 , below. In some embodiments, apertures  104  may be provided as shown in  FIG. 3D  for use in conjunction with an anchor member (not shown) such as a suture, barb, pin, etc. This may facilitate modulation of the vagus nerve if rows of electrodes are placed on either side of the descending vagus nerve bundle(s). 
       FIG. 4  illustrates a block diagram depiction of the IMD  400  of  FIG. 1 , in accordance with one illustrative embodiment of the present invention. The IMD  400  may be used for electrical signal therapy to treat various disorders, such as (without limitation) epilepsy, depression, bulimia, heart rhythm disorders, gastric-related disorder, a hormonal disorder, a reproductive disorder, a metabolic disorder, a hearing disorder, and/or a pain disorder. The IMD  400  may be coupled to various leads, if necessary, and one or more electrodes for applying a therapeutic electrical signal to an anterior and/or posterior vagus nerve of the patient. Therapeutic electrical signals may be transmitted from the IMD  400  to a lower esophagus wall of the patient, and across the wall of the esophagus to the patient. The electrodes may be positioned either in contact with the inner wall of the esophagus, or may penetrate the wall of the esophagus in whole or in part. The net effect of the IMD  400  and the electrodes is to generate an electrical signal in the inner lumen of the esophagus and to transmit that signal from the inner lumen to a vagus nerve located outside the wall of the esophagus. Therapeutic electrical signals from the IMD  400  may be generated by a signal generation unit  420  and transmitted to the electrode(s) either directly or via one or more leads (not shown). Further, signals from sensor electrodes, which may also have corresponding leads, may also be coupled to the IMD  400 . In one embodiment, the electrodes applying the signal to the vagus nerve may also function as sensing electrodes. Sensing electrodes may be used to trigger feedback stimulation routines based upon, e.g., body temperature, EEG recordings, heart rate, vagus nerve activity, etc. 
     The IMD  400  may comprise a controller  410  capable of controlling various aspects of the operation of the IMD  400 . The controller  410  is capable of receiving internal data and/or external data and controlling the generation and delivery of a therapeutic electrical signal to a vagus nerve of the patient&#39;s body. For example, the controller  410  may receive manual instructions from an operator externally, or may generate and apply a therapeutic electrical signal based on internal calculations and programming. The controller  410  is capable of affecting substantially all functions of the IMD  400 . 
     The controller  410  may comprise various components, such as a processor  415 , a memory  417 , etc. The processor  415  may comprise one or more microcontrollers, microprocessors, etc., that are capable of executing a variety of software components. The memory  417  may comprise various memory portions, where a number of types of data (e.g., internal data, external data instructions, software codes, status data, diagnostic data, etc.) may be stored. The memory  417  may store various tables or other database content that could be used by the IMD  400  to implement the override of normal operations. The memory  417  may comprise random access memory (RAM) dynamic random access memory (DRAM), electrically erasable programmable read-only memory (EEPROM), flash memory, etc. 
     The IMD  400  may also comprise a signal generation unit or pulse generator  420 . The signal generation unit  420  is capable of generating and delivering a variety of electrical neurostimulation signals to one or more electrodes, e.g., via leads. The signal generation unit  420  is capable of generating a therapy portion, a ramping-up portion, and a ramping-down portion of the therapeutic electrical signal. In one embodiment, a number of leads may be used to electrically couple the one or more electrodes to the IMD  400 . In another embodiment, the electrodes may be directly coupled to the signal generation unit  420 . The therapeutic electrical signal may be delivered to the electrodes by the signal generation unit  420  based upon instructions from the controller  410 . The signal generation unit  420  may comprise various types of circuitry, such as pulse generators, impedance control circuitry to control the impedance “seen” by the leads, and other circuitry that receives instructions relating to the type of electrical signal therapy to be provided to the patient. The signal generation unit  420  is capable of delivering a controlled current therapeutic electrical signal to the electrodes. 
     The IMD  400  may also comprise a power supply  430 . The power supply  430  may comprise a battery, voltage regulators, capacitors, etc., to provide power for the operation of the IMD  400 , including delivering the therapeutic electrical signal. The power supply  430  provides power for the operation of the IMD  400 , including electronic operations and the electrical signal function. The power supply is typically a battery. The power supply  430  may comprise a lithium/thionyl chloride cell or a lithium/carbon monofluoride cell. Other battery types known in the art of implantable medical devices may also be used. Any suitable batteries may be used including without limitation, lithium ion, nickel cadmium alkaline, and the like. In a preferred embodiment, the power supply comprises a battery located inside support member  401  and coupled to the pulse generator  420 . In an alternative embodiment, the power supply is rechargeable. In one aspect, the power supply may be wirelessly rechargeable. A wirelessly rechargeable power supply may include an external power supply on the outside of the patient&#39;s body, inductively coupled to an implanted rectifier and/or capacitor attached to the support member  401 . Such a power supply may be charged from outside the patient&#39;s body without requiring removal of the medical device. In an alternative embodiment, the power supply  430  may comprise a power converter to convert energy from the patient&#39;s body into electrical power for the IMD  400 . The converter may use a temperature difference within the patient&#39;s body or between the patient&#39;s body and the environment to generate electrical power, or may convert body movement (e.g., gastric motility movement, arm movement, or leg movement) into electrical energy to provide power to the IMD  400 . In another embodiment, an external power supply may be part of an external unit  120  as will be described below. 
     The IMD  400  also comprises a communication unit  460  capable of facilitating communications between the IMD  400  and various devices. In particular, the communication unit  460  is capable of providing transmission and reception of electronic signals to and from an external unit  120 . In particular, communication unit  460  may be a wireless device capable of transmitting and receiving signals to and from IMD  400  without the use of wires. In some embodiments, the controller  410  and signal generator  420  may be part of an external unit or programming system  120 , described in more detail below. 
     The external unit  120  may be a device that is capable of programming various modules and electrical signal parameters of the IMD  400 . In one embodiment, the external unit  120  comprises a computer system that is capable of executing a data-acquisition program. The external unit  120  may be controlled by a healthcare provider, such as a physician, at a base station in, for example, a doctor&#39;s office. The external unit  120  may be a computer, preferably a handheld computer or PDA, but may alternatively comprise any other device that is capable of electronic communications and programming. The external unit  120  may download various parameters and program software into the IMD  400  for programming the operation of the implantable device. The external unit  120  may also receive and upload various status conditions and other data from the IMD  400 . The communication unit  460  may be hardware, software, firmware, and/or any combination thereof. Communications between the external unit  120  and the communication unit  460  may occur via a wireless or other type of communication, illustrated generally by line  475  in  FIG. 4 . 
     The IMD  400  is capable of delivering a therapeutic electrical signal that can be intermittent, periodic, random, sequential, coded, and/or patterned. The electrical signals may comprise an electrical signal frequency of approximately 0.1 to 1000 Hz. The electrical signals may comprise a pulse width of in the range of approximately 1-2000 microseconds. The electrical signals may comprise current amplitude in the range of approximately 0.1 mA to 10 mA. The therapeutic electrical signal may be delivered through either bipolar electrodes (i.e., a cathode (−) electrode and an anode (+) electrode), or may be delivered through one or more unipolar electrodes. In one embodiment, the various blocks illustrated in  FIG. 4  may comprise a software unit, a firmware unit, a hardware unit, and/or any combination thereof. 
     The IMD  400  may also comprise a magnetic field detection unit  490 . The magnetic field detection unit  490  is capable of detecting magnetic and/or electromagnetic fields of a predetermined magnitude. Whether the magnetic field results from a magnet placed proximate to the IMD  400 , or whether it results from a substantial magnetic field encompassing an area (such as an MRI machine), the magnetic field detection unit  490  is capable of informing the IMD of the existence of a magnetic field. 
     The magnetic field detection unit  490  may comprise various sensors, such as a Reed Switch circuitry, a Hall Effect sensor circuitry, and/or the like. The magnetic field detection unit  490  may also comprise various registers and/or data transceiver circuits that are capable of sending signals that are indicative of various magnetic fields, the time period of such fields, etc. In this manner, the magnetic field detection unit  490  is capable of deciphering whether the detected magnetic field relates to an inhibitory input or an excitatory input from an external source. The inhibitory input may refer to inhibition of, or deviation from, normal signal generation operation. The excitatory input may refer to additional electrical signal therapy or deviation from normal electrical signal therapy. 
     The IMD  400  may also include an electrical signal override unit  480 . The electrical signal override unit  480  is capable of overriding the reaction by the IMD to the detection of a magnetic signal provided by the magnetic field detection unit  490 . The electrical signal override unit  480  may comprise various software, hardware, and/or firmware units that are capable of determining an amount of time period in which to override the detection of a magnetic field. The signal override unit  480  may also contain safety features, such as returning to normal operation despite an override command after a predetermined period of time. The electrical signal override unit  480  is capable of preventing false interruption of normal operation due to false magnetic input signals or unintended magnetic input signals. The override unit  480  may receive an external indication via the communication unit  460  to engage in an override mode for a predetermined period of time. 
     In an embodiment, components of IMD  400  (i.e. controller  410 , signal generation unit  420 , etc.) are all coupled to support member  401  as described in detail above. Furthermore, support member  401  and components of IMD  400  may be completely integrated into a compact and unitary device. That is, each component of IMD  400  may be integral to the support member  401 . Thus, in such embodiments, the IMD  400  may be a complete standalone device without the need for any external devices to be carried around by a patient. As such, one of the many advantages of the IMD is that it may allow a patient to be completely ambulatory when the IMD is implanted, and providing maximum quality of life to the patient. 
     It is also contemplated that various embodiments of the IMD may be temporary or permanent. That is, in some embodiments, IMD may be constructed to be inexpensive and easily disposable. For example, in particular embodiments, the IMD may be designed to be replaced once the power supply has been consumed. Such devices may be resident in the body of the patient for from several months up to several years, although shorter or longer residence times are possible. In other embodiments, IMD may be designed for permanent or long term operation such it will not need replacement for time periods of 10 years or more. Another advantage of the device is that it is easily removable. 
       FIGS. 2 and 7  illustrate an external unit or programming system  120  comprising a programming device  124  coupled to a wand  128  for transmitting and receiving signals to and from the IMD  100 . As mentioned above with respect to  FIG. 4 , the IMD  100  preferably comprises a signal or pulse generator  122 , and a controller to control the generation and delivery of the electrical signal by the pulse generator  122 . The controller preferably is, or includes a central processing unit (CPU) such as a low-power, mixed-signal microcontroller. In alternative embodiments, the controller and pulse generator  122  may be part of an external programming system  120 , described in more detail below. 
     In an additional embodiment, the IMD  100  comprises a power supply which is coupled to the support member  101 , and to the pulse generator  102 . The power supply is typically a battery, such as a lithium carbon monofluoride (LiCFx), lithium ion, nickel cadmium alkaline, and the like. In a preferred embodiment, the power supply comprises a battery located inside support member  101  and coupled to the pulse generator  102 . In an alternative embodiment, the power supply is rechargeable. In one aspect, the power supply may be wirelessly rechargeable. A wirelessly rechargeable power supply may include an external power supply on the outside of the patient&#39;s body, inductively coupled to an implanted rectifier and/or capacitor attached to the support member  101 . Such a power supply may be charged from outside the patient&#39;s body without requiring removal of the medical device. In another embodiment, an external power supply may be part of an external programming system  120  as will be described below. 
       FIGS. 2 and 7  further illustrate a programming system  120  comprising a programming device  124  coupled to a wand  128 , for transmitting and receiving wireless signals (e.g., RF signals) to and from the IMD  100 . The programming device  124  may comprise a personal computer, personal digital assistant (PDA) device, or other suitable computing device consistent with the description contained herein. Methods and apparatus for communication between an IMD  100  and an external programming system  120  are known in the art. In one embodiment, the IMD  100  includes a transceiver (such as a coil) that permits signals to be communicated wirelessly and non-invasively between the programming device  124  (via the external wand  128 ) and the implanted IMD  100 . Wand  128  facilitates wireless communication and may be placed on the skin of the patient&#39;s body overlying the implant site of the IMD  100 . The programming system  120  may also monitor the performance of the IMD  100  and download new programming parameters to define the therapeutic electrical signal (e.g., on-time, off-time, pulse width, current amplitude, frequency) into the IMD  100  to alter its operation as desired. In various embodiments, portions of the programming system may be integrated into the IMD  100 , itself, or may be external to the IMD  100 . 
       FIG. 7  shows a block diagram of one embodiment of the programming system  120 . As shown, the programming system  120  is external to the patient&#39;s body, and includes the programming device  124  and the wand  128 . Programming system  120  generally assists, controls, and/or programs the IMD  100  and may also receive data from the IMD referring to status and operational conditions stored in or generated by the IMD, such as battery and lead diagnostic information, remaining battery life calculations, current programming parameters, patient identification information, programming history information, etc. Thus, in an embodiment, the IMD  100  generates a therapeutic pulsed electrical signal for application to the vagus nerve  133  in a patient under the control of programming system  120 . 
     Programming device  124  preferably includes a central processing unit (CPU)  236  such as a low-power, mixed-signal microcontroller. In general, any suitable processor can be used to implement the functionality performed by the programming device  124  as explained herein. It will be appreciated that some features of the programming system  120  may also be provided in whole, or in part, by the IMD  100 , and vice versa. Thus, while certain features of the present invention may be described as part of the IMD  100 , it is not intended thereby to preclude embodiments in which the features are provided by the programming system  120 . Likewise, describing certain features herein as part of the programming system  120  does not preclude embodiments in which the features are included as part of the IMD  100 . 
     The CPU  236  of programming device  124  is preferably coupled to a memory  250 . The memory  250  may comprise volatile (e.g., random access memory) and/or non-volatile memory (e.g., read only memory (ROM), electrically-erasable programmable ROM (EEPROM), Flash memory, etc.). Memory  250  may comprise any suitable storage medium. Examples of suitable storage media include without limitation, USB flash drives, Compact Flash cards, memory sticks, Smart Media cards, Secure Digital (SD) cards, xD cards, CD-ROM, DVD-ROM, tape drives, Zip disks, floppy disk, RAM, hard drives, etc. The memory  250  may be used to store code (e.g., diagnostic software) that is executed by the CPU  236 . The executable code may be executed directly from the non-volatile memory or copied to a volatile memory for execution therefrom. 
     In addition, programming device  124  of programming system  120  may have a display or output  232  such that a user may monitor the functions or properties of the IMD  100 . In some embodiments, programming system  120  may have a graphical user interface. In preferred embodiments, a user may input parameter settings using an input device  238  through the graphical user interface on the display  232 , or other input means. Memory  250  may store data received from the IMD  100  or may be used to store software  380  (e.g., diagnostic software, therapy programs, code, etc.) that is executed by the CPU  236 . The executable code may be executed directly from the non-volatile memory or copied to the volatile memory for execution therefrom. 
     In one embodiment, the programming system  120  may be used by a physician as part of an office computer system, in which the IMD  100  is queried regularly during office visits by the patient. According to another embodiment, programming system  120  may be ambulatory, and wand  128  may be continuously in contact with the patient&#39;s skin near implant site (i.e., the abdominal region) to provide continuous control of IMD  100  in conjunction with a portable handheld device  124  that may comprise, e.g., a PDA. Wand  128  is coupled to the programming device  124  which may be conveniently worn by the patient on a belt, pocket, and the like. In another embodiment, programming device  124  and wand  128  are integrated into a single device which is kept in continuous proximity to the IMD  100  so as to maintain wireless communication with the IMD  100  through the skin. 
     In an embodiment of a method of providing an electrical signal to a vagus nerve, an IMD  100 , as described in detail above, is implanted in the esophagus  131  of a patient. See  FIGS. 8A-D . In at least one embodiment, the IMD  100  is implanted in the lower or abdominal portion of the esophagus  131  at or below the patient&#39;s diaphragm, e.g., where the anterior and posterior vagus nerves are attached to the exterior wall of the esophagus  131  near the esophagogastric junction. Preferably, the IMD  100  is delivered to the implantation site in the esophagus  131  orally i.e. through the mouth and esophagus as shown in  FIGS. 8A and 8B . In a particular embodiment, IMD  100  may be implanted via a catheter delivery system. Specifically, IMD  100  may be mounted at the end of a catheter  210 , e.g., at a balloon portion  211  of a balloon catheter in its contracted position. The catheter  210  and IMD  100  may then be inserted into the patient&#39;s mouth and throat (i.e. orally), and advanced into the esophagus as shown in  FIG. 8A . Preferably, the catheter  210  is inserted until the distal tip is near the esophagogastric junction (i.e., at the intersection of the esophagus and the stomach) as this portion of the esophagus  131  is where the vagus nerve  133  reforms from the esophageal plexus to form the anterior and posterior vagal trunks. See  FIG. 8B . 
     Once the distal tip of catheter  210  is disposed at the desired implantation site for IMD  100 , the IMD  100  is deployed (for example, to an expanded position) and anchored to the inner wall of the esophagus. If a balloon catheter is used, the balloon portion may be inflated to expand the support member  101 . A balloon catheter may be used, for example, in conjunction with a support member comprising a stent or stent-like retainment element. Alternatively, the support member  101  may comprise a wire or band of a shape-memory metal such as Nitinol or a resilient polymer to provide strength and additional support to the support member  101 . The support member  101  (with the retainment element) may be wrapped around, folded, compressed or otherwise reduced to a contracted position and retained in position on the catheter by sutures, releasable wires, or other constraining means known in the art, where a constraint is needed. 
     Once in position,  FIG. 8C , the IMD  100  deployed into place against the inner wall of the esophagus,  FIG. 8D  by, for example, removing a the constraining means or expanding a balloon catheter. The IMD  100  may then be anchored in place using anchors known in the art, e.g., sutures, barbs, staples, adhesives, pins, etc. In one embodiment, the anchors are the electrodes, which comprise barbs to engage and penetrate into the wall of the esophagus. In another embodiment, anchors are not used, and the device may be retained in place solely by a retainment element, such as a C-shaped spring embedded in the support member  101 . 
     In one embodiment, IMD  100  is implanted such that electrodes  103  are in contact with the inner wall surface of the esophagus  131 . Once implanted, the IMD  100  provides electrical signal therapy (e.g., vagus nerve stimulation) to a vagus nerve by applying a therapeutic electrical signal through one or more electrodes  103  to the inner surface of the esophagus wall, through the wall of the esophagus, and to the vagus nerve  133  on the outer surface of the esophagus  131 . In another embodiment, at least one electrode penetrates into the wall of the esophagus, and the electrical signal is applied to the wall of the esophagus, penetrates the outer wall, and generates action potentials in the vagus nerve. The electrical signal may be defined an a variety of ways known in the art using a number of different parameters of the signal such as without limitation, pulse width, current amplitude, frequency, on-off time, duty cycle, number of pulses per burst, interburst period, interpulse interval, burst duration, or combinations thereof. In general, these parameters may be adjusted through the programming system  120 . 
     Vagus nerve stimulation (VNS) may involve non-feedback stimulation characterized by a number of parameters. Specifically, vagus nerve stimulation may involve a series of electrical pulses in bursts defined by an “on-time” and an “off-time.” During the on-time, electrical pulses of a defined electrical current (e.g., 0.5-10.0 milliamps) and pulse width (e.g., 0.25-1.5 milliseconds) are delivered at a defined frequency (e.g., 20-30 Hz) for the on-time duration, usually a specific number of seconds, e.g., 7-120 seconds. The pulse bursts are separated from one another by the off-time, (e.g., 30 seconds-5 minutes) in which no electrical signal is applied to the nerve. The on-time and off-time parameters together define a duty cycle, which is the ratio of the on-time to the combination of the on-time and off-time, and which describes the percentage of time that the electrical signal is applied to the nerve. In other embodiments, vagus nerve stimulation may comprise a feedback stimulation scheme. 
     In VNS, the on-time and off-time may be programmed to define an intermittent pattern in which a repeating series of electrical pulse bursts are generated and applied to the vagus nerve. Each sequence of pulses during an on-time may be referred to as a “pulse burst.” The burst is followed by the off-time period in which no signals are applied to the nerve. The off-time is provided to allow the nerve to recover from the stimulation of the pulse burst, and to conserve power. If the off-time is set at zero, the electrical signal in conventional VNS may provide continuous stimulation to the vagus nerve. Typically, however, the ratio of “off-time” to “on-time” may range from about 0.5 to about 10. 
     In addition to the on-time and off-time, the other parameters defining the electrical signal in conventional VNS may be programmed over a range of values. The pulse width for the pulses in a pulse burst of conventional VNS may be set to a value not greater than about 1 msec, such as about 250-500 μsec, and the number of pulses in a pulse burst is typically set by programming a frequency in a range of about 20-150 Hz (i.e., 20 pulses per second to 150 pulses per second). A non-uniform frequency may also be used. Frequency may be altered during a pulse burst by either a frequency sweep from a low frequency to a high frequency, or vice versa. Alternatively, the timing between adjacent individual signals within a burst may be randomly changed such that two adjacent signals may be generated at any frequency within a range of frequencies. 
     Accordingly, the electrical signal may be varied in a number of ways known in the art using a number of different parameters of the signal such as without limitation, pulse width, current amplitude, frequency, on-off time, duty cycle, number of pulses per burst, interburst period, interpulse interval, burst duration, or combinations thereof. In general, these parameters may be adjusted through the programming system  120 . 
     It is envisioned that the disclosed device and methods may have many applications associated with vagus nerve stimulation or VNS therapy. Embodiments of the IMD may be used to treat a wide variety of medical conditions without exposing the patient to surgical complications, including without limitation epilepsy, neuropsychiatric disorders (including but not limited to depression), eating disorders/obesity, traumatic brain injury/coma, addiction disorders, dementia, sleep disorders, pain, migraine, endocrine/pancreatic disorders (including but not limited to diabetes), motility disorders, hypertension, congestive heart failure/cardiac capillary growth, hearing disorders, angina, syncope, vocal cord disorders, thyroid disorders, pulmonary disorders, and reproductive endocrine disorders (including fertility) in a patient. The IMD may be easily deployed and may also avoid undesired side effects, in particular voice alteration, by providing a therapeutic electrical signal to the vagus nerve in a location remote from the larynx compared to conventional vagus nerve stimulation systems implanted in the neck area. 
     While embodiments of this invention have been shown and described, modifications thereof can be made by one skilled in the art without departing from the spirit or teaching of this invention. The embodiments described herein are exemplary only and are not limiting. Many variations and modifications of the system and apparatus are possible and are within the scope of the invention. Accordingly, the scope of protection is not limited to the embodiments described herein, but is only limited by the claims which follow, the scope of which shall include all equivalents of the subject matter of the claims.