DEVICES AND METHODS FOR CARDIOVASCULAR REFLEX CONTROL

Abstract: Devices, systems and methods by which the blood pressure, nervous system activity and neurohormonal activity may be selectively and controllably reduced by activating baroreceptors. A baroreceptor activation device (70) is positioned near a baroreceptor, preferably a baroreceptor located in the carotid sinus. A control system (60) may be used to modulate the baroreceptor activation device (70). The control system (60) may utilize an algorithm defining a stimulus regimen which promotes long term efficacy and reduces power requirements/consumption. A mapping method permits the baroreceptor activation device (70) to be precisely located to maximize therapeutic efficacy. The baroreceptor activation device (70) may utilize electrodes to activate the baroreceptors. The electrodes may be adapted for connection to the carotid arteries at or near the carotid sinus, and may be designed to minimize extraneous tissue stimulation.

Description: DEVICESAND METHODS FORCARDIOVASCULARREFLEXCONTROL Field 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. Background of the invention 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 may occur 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 (NAD) 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. Patent 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. Situations may also arise in which it would be beneficial to raise the blood pressure of a patient. For example, the patient may be experiencing a period of reduced blood pressure, or hypotension. Conditions associated with symptomatic hypotension include vasovagal reactions, orthostatic hypotension and dysautonomia. Alternatively, it may be advantageous to augment the blood pressure of a patient in whom the blood pressure may be normal or near normal, for example in claudication syndromes. Therefore, a also need exists for a therapy that can acutely increase the blood pressure in a patient. 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 affect 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 affect 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 mechanical, electrical, thermal, chemical, biological, or other means to activate the baroreceptor. The baroreceptor may be activated directly, or activated indirectly via the adjacent vascular tissue. The baroreceptor activation device may be positioned inside the vascular lumen (i.e., intravascularly), outside the vascular wall (i.e., extravascularly) or within the vascular wall (i.e., intramurally). To maximize therapeutic efficacy, a mapping method may be employed to precisely locate or position the baroreceptor activation device. For embodiments utilizing electrical means to activate the baroreceptor, 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. A control system may be used to generate a control signal which activates, deactivates or otherwise modulates the baroreceptor activation device. The control system may operate in an open-loop or a closed-loop mode. For example, in the open-loop mode, the patient and/or physician may directly or remotely interface with the control system to prescribe the control signal. In the closed-loop mode, the control signal may be responsive to feedback from a sensor, wherein the response is dictated by a preset or programmable algorithm defining a stimulus regimen. The stimulus regimen is preferably selected to promote long term efficacy and to minimize power requirements. It is theorized that uninterrupted activation of the baroreceptors may result in the baroreceptors and or baroreflex system becoming less responsive over time, thereby diminishing the effectiveness of the therapy. Therefore, the stimulus regimen may be selected to modulate, for example, the baroreceptor activation device in such a way that the baroreceptors maintain their responsiveness over time. Specific examples of stimulus regimens which promote long term efficacy are described in more detail hereinafter. To address low blood pressure and other conditions requiring blood pressure augmentation, the present invention provides a number of devices, systems and methods 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 Figure 1 is a schematic illustration of the upper torso of a human body showing the major arteries and veins and associated anatomy; Figure 2A is a cross-sectional schematic illustration of the carotid sinus and baroreceptors within the vascular wall; Figure 2B is a schematic illustration of baroreceptors within the vascular wall and the baroreflex system; Figure 3 is a schematic illustration of a baroreceptor activation system in accordance with the present invention; Figures 4 A and 4B are schematic illustrations of a baroreceptor activation device in the form of an internal inflatable balloon which mechanically induces a baroreceptor signal in accordance with an embodiment of the present invention; Figures 5A and 5B are schematic illustrations of a baroreceptor activation device in the form of an external pressure cuff which mechanically induces a baroreceptor signal in accordance with an embodiment of the present invention; Figures 6A and 6B are schematic illustrations of a baroreceptor activation device in the form of an internal deformable coil structure which mechanically induces a baroreceptor signal in accordance with an embodiment of the present invention; Figures 6C and 6D are cross-sectional views of alternative embodiments of the coil member illustrated in Figures 6A and 6B; Figures 7A and 7B are schematic illustrations of a baroreceptor activation device in the form of an external deformable coil structure which mechanically induces a baroreceptor signal in accordance with an embodiment of the present invention; Figures 7C and 7D are cross-sectional views of alternative embodiments of the coil member illustrated in Figures 7A and 7B; Figures 8A and 8B are schematic illustrations of a baroreceptor activation device in the form of an external flow regulator which artificially creates back pressure to induce a baroreceptor signal in accordance with an embodiment of the present invention; Figures 9A and 9B are schematic illustrations of a baroreceptor activation device in the form of an internal flow regulator which artificially creates back pressure to induce a baroreceptor signal in accordance with an embodiment of the present invention; Figures 10A and 10B are schematic illustrations of a baroreceptor activation device in the form of a magnetic device which mechanically induces a baroreceptor signal in accordance with an embodiment of the present invention; Figures 11A and 11B are schematic illustrations of a baroreceptor activation device in the form of a transducer which mechanically induces a baroreceptor signal in accordance with an embodiment of the present invention; Figures 12A and 12B are schematic illustrations of a baroreceptor activation device in the form of a fluid delivery device which may be used to deliver an agent which chemically or biologically induces a baroreceptor signal in accordance with an embodiment of the present invention; Figures 13A and 13B are schematic illustrations of a baroreceptor activation device in the form of an internal conductive structure which electrically or thermally induces a baroreceptor signal in accordance with an embodiment of the present invention; Figures 14A and 14B are schematic illustrations of a baroreceptor activation device in the form of an internal conductive structure, activated by an internal inductor, which electrically or thermally induces a baroreceptor signal in accordance with an embodiment of the present invention; Figures 15A and 15B are schematic illustrations of a baroreceptor activation device in the form of an internal conductive structure, activated by an internal inductor located in an adjacent vessel, which electrically or thermally induces a baroreceptor signal in accordance with an embodiment of the present invention; Figures 16A and 16B are schematic illustrations of a baroreceptor activation device in the form of an internal conductive structure, activated by an external inductor, which electrically or thermally induces a baroreceptor signal in accordance with an embodiment of the present invention; Figures 17A and 17B are schematic illustrations of a baroreceptor activation device in the form of an external conductive structure which electrically or thermally induces a baroreceptor signal in accordance with an embodiment of the present invention; Figures 18A and 18B are schematic illustrations of a baroreceptor activation device in the form of an internal bipolar conductive structure which electrically or thermally induces a baroreceptor signal in accordance with an embodiment of the present invention; Figures 19A and 19B are schematic illustrations of a baroreceptor activation device in the form of an electromagnetic field responsive device which electrically or thermally induces a baroreceptor signal in accordance with an embodiment of the present invention; Figures 20A and 20B are schematic illustrations of a baroreceptor activation device in the form of an external Peltier device which thermally induces a baroreceptor signal in accordance with an embodiment of the present invention; Figures 21A- 21C are schematic illustrations of a preferred embodiment of an inductively activated electrically conductive structure; Figures 22A - 22F are schematic illustrations of various possible arrangements of electrodes around the carotid sinus for extravascular electrical activation embodiments; Figure 23 is a schematic illustration of a serpentine shaped electrode for extravascular electrical activation embodiments; Figure 24 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; Figures 25 - 28 are schematic illustrations of various multi-channel electrodes for extravascular electrical activation embodiments; Figure 29 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; Figure 30 is a schematic illustration of an alternative extravascular electrical activation device including a plurality of ribs and a spine; Figure 31 is a schematic illustration of an electrode assembly for extravascular electrical activation embodiments; Figure 32 is a schematic illustration of a fragment of an alternative cable for use with an electrode assembly such as shown in Figure 31; Figure 33 is a schematic illustration of the right carotid artery showing a bulge in the vascular wall which is a landmark of the carotid sinus; Figure 34 is a schematic illustration of a baroreceptor activation device disposed about the right carotid artery which may be used for mapping baroreceptors therein; Figure 35 is a schematic cross-sectional view taken along line 35-35 in Figure 34, showing a mapping coordinate system for the left and right carotid arteries; Figures 36 and 37 are graphs illustrating the variability of baroreceptor responsiveness around the right and left carotid arteries, respectively; Figure 38 illustrates an arterial pressure signal, an ECG signal and a control/output signal as a function of time; and Figures 39 - 41 are graphs which illustrate the effectiveness of various stimulus regimens in accordance with various embodiments of the present invention. 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 to Figure 1 which is a schematic illustration of the upper torso of a human body 10 showing some of the major arteries and veins of the cardiovascular system. The left ventricle of the heart 11 pumps oxygenated blood up into the aortic arch 12. The right subclavian artery 13, the right common carotid artery 14, the left common carotid artery 15 and the left subclavian artery 16 branch off the aortic arch 12 proximal of the descending thoracic aorta 17. Although relatively short, a distinct vascular segment referred to as the brachiocephalic artery 22 connects the right subclavian artery 13 and the right common carotid artery 14 to the aortic arch 12. The right carotid artery 14 bifurcates into the right external carotid artery 18 and the right internal carotid artery 19 at the right carotid sinus 20. Although not shown for purposes of clarity only, the left carotid artery 15 similarly bifurcates into the left external carotid artery and the left internal carotid artery at the left carotid sinus. From the aortic arch 12, oxygenated blood flows into the carotid arteries 18/19 and the subclavian arteries 13/16. From the carotid arteries 18/19, oxygenated blood circulates through the head and cerebral vasculature and oxygen depleted blood returns to the heart 11 by way of the jugular veins, of which only the right internal jugular vein 21 is shown for sake of clarity. From the subclavian arteries 13/16, 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 right subclavian vein 23 is shown, also for sake of clarity. The heart 11 pumps the oxygen depleted blood through the pulmonary system where it is re-oxygenated. The re-oxygenated blood returns to the heart 11 which pumps the re-oxygenated blood into the aortic arch as described above, and the cycle repeats. Within the arterial walls of the aortic arch 12, common carotid arteries 14/15 (near the right carotid sinus 20 and left carotid sinus), subclavian arteries 13/16 and brachiocephalic artery 22 there are baroreceptors 30. For example, as best seen in Figure 2 A, baroreceptors 30 reside within the vascular walls of the carotid sinus 20. Baroreceptors 30 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. Because baroreceptors 30 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. The baroreceptors 30 located in the right carotid sinus 20, the left carotid sinus and the aortic arch 12 play the most significant role in sensing blood pressure that affects the baroreflex system 50, which is described in more detail with reference to Figure 2B. Refer now to Figure 2B, which shows a schematic illustration of baroreceptors 30 disposed in a generic vascular wall 40 and a schematic flow chart of the baroreflex system 50. Baroreceptors 30 are profusely distributed within the arterial walls 40 of the major arteries discussed previously, and generally form an arbor 32. The baroreceptor arbor 32 comprises a plurality of baroreceptors 30, each of which transmits baroreceptor signals to the brain 52 via nerve 38. The baroreceptors 30 are so profusely distributed and arborized within the vascular wall 40 that discrete baroreceptor arbors 32 are not readily discernable. To this end, those skilled in the art will appreciate that the baroreceptors 30 shown in Figure 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 the baroreflex system 50. Baroreceptors 30 are connected to the brain 52 via the nervous system 51. Thus, the brain 52 is able to detect changes in blood pressure, which is indicative of cardiac output. If cardiac output is insufficient to meet demand (i.e., the heart 11 is unable to pump sufficient blood), the baroreflex system 50 activates a number of body systems, including the heart 11, kidneys 53, vessels 54, and other organs/tissues. Such activation of the baroreflex system 50 generally corresponds to an increase in neurohormonal activity. Specifically, the baroreflex system 50 initiates a neurohormonal sequence that signals the heart 11 to increase heart rate and increase contraction force in order to increase cardiac output, signals the kidneys 53 to increase blood volume by retaining sodium and water, and signals the vessels 54 to constrict to elevate blood pressure. The cardiac, renal and vascular responses increase blood pressure and cardiac output 55, and thus increase the workload of the heart 11. 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 the baroreflex system 50 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 which baroreceptors 30 may be activated, thereby indicating an increase in blood pressure and signaling the brain 52 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 to Figure 3, the present invention generally provides a system including a control system 60, a baroreceptor activation device 70, and a sensor 80 (optional), which generally operate in the following manner. The sensor 80 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. The control system 60 generates a control signal as a function of the received sensor signal. The control signal activates, deactivates or otherwise modulates the baroreceptor activation device 70. Typically, activation of the device 70 results in activation of the baroreceptors 30. Alternatively, deactivation or modulation of the baroreceptor activation device 70 may cause or modify activation of the baroreceptors 30. The baroreceptor activation device 70 may comprise a wide variety of devices which utilize mechanical, electrical, thermal, chemical, biological, or other means to activate baroreceptors 30. Thus, when the sensor 80 detects a parameter indicative of the need to modify the baroreflex system activity (e.g., excessive blood pressure), the control system 60 generates a control signal to modulate (e.g. activate) the baroreceptor activation device 70 thereby inducing a baroreceptor 30 signal that is perceived by the brain 52 to be apparent excessive blood pressure. When the sensor 80 detects a parameter indicative of normal body function (e.g., normal blood pressure), the control system 60 generates a control signal to modulate (e.g., deactivate) the baroreceptor activation device 70. As mentioned previously, the baroreceptor activation device 70 may comprise a wide variety of devices which utilize mechanical, electrical, thermal, chemical, biological or other means to activate the baroreceptors 30. Specific embodiments of the generic baroreceptor activation device 70 are discussed with reference to Figures 4 - 21. In most instances, particularly the mechanical activation embodiments, the baroreceptor activation device 70 indirectly activates one or more baroreceptors 30 by stretching or otherwise deforming the vascular wall 40 surrounding the baroreceptors 30. In some other instances, particularly the non-mechanical activation embodiments, the baroreceptor activation device 70 may directly activate one or more baroreceptors 30 by changing the electrical, thermal or chemical environment or potential across the baroreceptors 30. It is also possible that changing the electrical, thermal or chemical potential across the tissue surrounding the baroreceptors 30 may cause the surrounding tissue to stretch or otherwise deform, thus mechanically activating the baroreceptors 30. In other instances, particularly the biological activation embodiments, a change in the function or sensitivity of the baroreceptors 30 may be induced by changing the biological activity in the baroreceptors 30 and altering their intracellular makeup and function. All of the specific embodiments of the baroreceptor activation device 70 are suitable for implantation, and are preferably implanted using a minimally invasive percutaneous translumenal approach and/or a minimally invasive surgical approach, depending on whether the device 70 is disposed intravascularly, extravascularly or within the vascular wall 40. The baroreceptor activation device 70 may be positioned anywhere baroreceptors 30 affecting the baroreflex system 50 are numerous, such as in the heart 11, in the aortic arch 12, in the common carotid arteries 18/19 near the carotid sinus 20, in the subclavian arteries 13/16, or in the brachiocephalic artery 22. The baroreceptor activation device 70 may be implanted such that the device 70 is positioned immediately adjacent the baroreceptors 30. Alternatively, the baroreceptor activation device 70 may be outside the body such that the device 70 is positioned a short distance from but proximate to the baroreceptors 30. Preferably, the baroreceptor activation device 70 is implanted near the right carotid sinus 20 and/or the left carotid sinus (near the bifurcation of the common carotid artery) and/or the aortic arch 12, where baroreceptors 30 have a significant impact on the baroreflex system 50. For purposes of illustration only, the present invention is described with reference to baroreceptor activation device 70 positioned near the carotid sinus 20. The optional sensor 80 is operably coupled to the control system 60 by electric sensor cable or lead 82. The sensor 80 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, the sensor 80 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, O 2 or CO 2 content, mixed venous oxygen saturation (SVO 2 ), vasoactivity, nerve activity, tissue activity or composition. Examples of suitable transducers or gauges for the sensor 80 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 (SVO 2 ) or a strain gage. Although only one sensor 80 is shown, multiple sensors 80 of the same or different type at the same or different locations may be utilized. The sensor 80 is preferably positioned in a chamber of the heart 11, or in/on a major artery such as the aortic arch 12, a common carotid artery 14/15, a subclavian artery 13/16 or the brachiocephalic artery 22, such that the parameter of interest may be readily ascertained. The sensor 80 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. The sensor 80 may be separate from the baroreceptor activation device 70 or combined therewith. For purposes of illustration only, the sensor 80 is shown positioned on the right subclavian artery 13. By way of example, the control system 60 includes a control block 61 comprising a processor 63 and a memory 62. Control system 60 is connected to the sensor 80 by way of sensor cable 82. Control system 60 is also connected to the baroreceptor activation device 70 by way of electric control cable 72. Thus, the control system 60 receives a sensor signal from the sensor 80 by way of sensor cable 82, and transmits a control signal to the baroreceptor activation device 70 by way of control cable 72. The memory 62 may contain data related to the sensor signal, the control signal, and/or values and commands provided by the input device 64. The memory 62 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. As mentioned previously, the baroreceptor activation device 70 may activate baroreceptors 30 mechanically, electrically, thermally, chemically, biologically or otherwise. In some instances, the control system 60 includes a driver 66 to provide the desired power mode for the baroreceptor activation device 70. For example if the baroreceptor activation device 70 utilizes pneumatic or hydraulic actuation, the driver 66 may comprise a pressure/vacuum source and the cable 72 may comprise fluid line(s). If the baroreceptor activation device 70 utilizes electrical or thermal actuation, the driver 66 may comprise a power amplifier or the like and the cable 72 may comprise electrical lead(s). If the baroreceptor activation device 70 utilizes chemical or biological actuation, the driver 66 may comprise a fluid reservoir and a pressure/vacuum source, and the cable 72 may comprise fluid line(s). In other instances, the driver 66 may not be necessary, particularly if the processor 63 generates a sufficiently strong electrical signal for low level electrical or thermal actuation of the baroreceptor activation device 70. The control system 60 may operate as a closed loop utilizing feedback from the sensor 80, or as an open loop utilizing commands received by input device 64. The open loop operation of the control system 60 preferably utilizes some feedback from the transducer 80, but may also operate without feedback. Commands received by the input device 64 may directly influence the control signal or may alter the software and related algorithms contained in memory 62. The patient and/or treating physician may provide commands to input device 64. Display 65 may be used to view the sensor signal, control signal and/or the software/data contained in memory 62. The control signal generated by the control system 60 may be continuous, periodic, episodic or a combination thereof, as dictated by an algorithm contained in memory 62. The algorithm contained in memory 62 defines a stimulus regimen which dictates the characteristics of the control signal as a function of time, and thus dictates the stimulation of baroreceptors as a function of time. Continuous control signals include a pulse, a train of pulses, a triggered pulse and a triggered train of pulses, all of which are generated continuously. 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 minute, hour or day) and a designated duration (e.g., 1 second, 1 minute, 1 hour). 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 patient/physician, an increase in blood pressure above a certain threshold, etc.). The stimulus regimen governed by the control system 60 may be selected to promote long term efficacy. It is theorized that uninterrupted or otherwise unchanging activation of the baroreceptors 30 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 may be selected to activate, deactivate or otherwise modulate the baroreceptor activation device 70 in such a way that therapeutic efficacy is maintained long term. In addition to maintaining therapeutic efficacy over time, the stimulus regimens of the present invention may be selected reduce power requirement/consumption of the system 60. As will be described in more detail hereinafter, the stimulus regimen may dictate that the baroreceptor activation device 70 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 level after the desired therapeutic effect is initially attained, the power required or consumed by the activation device 70 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). Another advantage of the stimulus regimens of the present invention is the reduction of unwanted collateral tissue stimulation. As mentioned above, the stimulus regimen may dictate that the baroreceptor activation device 70 be initially activated at a relatively higher energy and/or power level to attain the desired effect, and subsequently activated at a relatively lower energy and/or power level to maintain the desired effect. By reducing the output energy and/or power level, the stimulus may not travel as far from the target site, thereby reducing the likelihood of inadvertently stimulating adjacent tissues such as muscles in the neck and head. Such stimulus regimens may be applied to all baroreceptor activation embodiments described herein. In addition to baroreceptor activation devices 70, such stimulus regimens may be applied to the stimulation of the carotid sinus nerves or other nerves affecting the baroreflex system. In particular, the stimulus regimens described herein may be applied to baropacing (i.e., electrical stimulation of the carotid sinus nerve), which has been proposed to reduce blood pressure and the workload of the heart in the treatment of high blood pressure and angina. For example, the stimulus regimens of the present invention may be applied to the baropacing system disclosed in U.S. Patent No. 6,073,048 to Kieval et al., the entire disclosure of which is incorporated herein by reference. The stimulus regimen may be described in terms of the control signal and/or the output signal from the baroreceptor activation device 70. Generally speaking, changes in the control signal result in corresponding changes in the output of the baroreceptor activation device 70 which affect corresponding changes in the baroreceptors 30. The correlation between changes in the control signal and changes in the baroreceptor activation device 70 may be proportional or disproportional, direct or indirect (inverse), or any other known or predictable mathematical relationship. For purposes of illustration only, the stimulus regimen may be described herein in such a way that assumes the output of the baroreceptor activation device 70 is directly proportional to the control signal. 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 the baroreceptor activation device 70 to have a first output level of relatively higher energy and/or power, and subsequently changing the control signal to cause the baroreceptor activation device 70 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 X ls the second output level may have a power and/or energy value of X 2 , wherein X 2 is less than Xi. In some instances, X 2 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, but may, at least in some contexts, be used interchangeably. Generally speaking, power is a time derivative of energy. Thus, 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 or episodic as discussed previously. The output (power or energy) level of the baroreceptor activation device 70 may be changed in a number of different ways depending on the mode of activation utilized. For example, in the mechanical activation embodiments described herein, the output level of the baroreceptor activation device 70 may be changed by changing the output force/pressure, tissue displacement distance, and/or rate of tissue displacement. In the thermal activation embodiments described herein, the output level of the baroreceptor activation device 70 may be changed by changing the temperature, the rate of temperature increase, or the rate of temperature decrease (dissipation rate). In the chemical and biological activation embodiments described herein, the output level of the baroreceptor activation device 70 may be changed by changing the volume/concentration of the delivered dose and/or the dose delivery rate. In electrical activation embodiments using a non-modulated signal, the output (power or energy) level of the baroreceptor activation device 70 may be changed by changing the voltage, current and/or signal duration. The output signal of the baroreceptor activation device 70 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 embodiments wherein the output signal comprises a pulse train, several other signal characteristics may be changed in addition to the pulse characteristics described above. As illustrated in Figure 38, the control or output signal 410 may comprise a pulse train 412 which generally includes a series of pulses 414 occurring in bursts 416. Pulse train 412 characteristics which may be changed include, but are not limited to: burst amplitude ( equal to pulse amplitude if constant within burst packet 416), burst waveform (i.e., pulse amplitude variation within burst packet 416), burst frequency (BF), and burst width or duration (BW). The signal 410 or a portion thereof (e.g., burst 416 within the pulse train 412) may be triggered by any of the events discussed previously, or by a particular portion of the arterial pressure signal 450 or the ECG signal 460 (e.g., R-wave as shown in Figure 38), or another physiologic timing indicator. If the signal 410 or a portion thereof is triggered, the triggering event may be changed and/or the delay from the triggering event may be changed. A second general approach for a stimulus regimen which promotes long term efficacy and reduces power requirements/consumption involves the use of one baroreceptor activation device 70 having multiple output means (e.g., electrodes) or the use of multiple baroreceptor activation devices 70 each having a single or multiple output means. Basically, the stimulus regimen according to this approach calls for alternating activation of two or more devices 70 or output means, which are positioned at different anatomical locations. Alternating activation may be accomplished by alternating the control signal between the devices or output means. As used in this context, switching or alternating activation includes switching between individual output means, switching between sets of output means and individual output means, and switching between different sets of output means. By alternating activation between two or more different anatomical locations, the exposure of any single anatomical location to an output signal is reduced. More specifically, a first device 70 or output means may be connected to a first baroreceptor location, and a second device 70 or output means may be connected to a second baroreceptor location, wherein the first location is different from the second location, and the control signal alternates activation of the first and second devices or output means. Although described with reference to two (first and second) devices 70 or output means, more than two may be utilized. By way of example, not limitation, a first device 70 or output means may be connected to the right carotid sinus, and a second device 70 or output means may be connected to the left carotid sinus. Alternatively, a first device 70 or output means may be connected to the left internal carotid artery, and a second device 70 or output means may be connected to the right internal carotid artery. As yet another alternative, first and second devices 70 or output means may be disposed next to each other but separated by a small distance (e.g., electrodes with multiple contact points). In each instance, the control signal alternates activation of the first and second devices or output means to reduce the signal exposure for each anatomical location. Those skilled in the relevant art will recognize that there are many possible anatomical combinations within the scope of this approach which are not specifically mentioned herein for sake of simplicity only. A third general approach for a stimulus regimen which promotes long term efficacy and reduces power requirements/consumption involves changing the time domain characteristics and/or the triggering event characteristics of the therapy. For example, a periodic control signal which has a designated start time (e.g., beginning of each minute, hour or day; specific time of day) and a designated duration (e.g., 1 second, 1 minute, 1 hour) may have a change in the designated start time and/or duration. Alternatively, an episodic control signal which is triggered by an episode (e.g., activation by the patient/physician, a particular part of the ECG signal, an increase in blood pressure above a certain threshold, specific time of day, etc.) may have a change in the delay from the triggering event or a change in the triggering event itself. For this latter alternative, the triggering event may be provided by feedback control utilizing sensor 80. As a further alternative, the control signal may be asynchronous, wherein the start time, duration or delay from a base line event is asynchronous (e.g., random). Any of the foregoing approaches may be utilized alone or in combination. The use of a combination of approaches may further promote long term efficacy and may further reduce power requirements/consumption. To demonstrate the effectiveness of the first approach described above, an animal experiment was performed utilizing a baroreceptor activation device 70 in the form of an extravascular electrical activation device bilaterally (right and left) applied to the carotid sinus 20. The results of this experiment are illustrated in Figure 39, which shows the control signal (volts) 410, heart rate (beats per minute) 420, mean arterial pressure (mmHg) 430 and arterial pressure (mmHg) 440 plotted as a function of time. The control signal 410 comprised a pulse train having a pulse amplitude of 2.5 volts, an initial pulse width or duration of 1.0 millisecond, and a pulse frequency of 100 Hz applied for a first period 401 of approximately 4 minutes. During this first period 401, the blood pressure 430/440 and heart rate 420 were significantly reduced. Subsequently, the pulse duration was changed to a 0.25 milliseconds and applied for a second period 402 of approximately 2 minutes, while the other parameters remained unchanged. During this second period 402, the reduced blood pressure 430/440 and heart rate 420 were sustained. After the second period 402, the pulse train was turned off for a period of time 403. During this third period 403, the blood pressure 430/440 and heart rate 420 began to gradually rise to their pretest values. This stimulus regimen demonstrated that the desired therapeutic effect may be sustained after reducing the pulse width of the stimulus device. Another demonstration of the effectiveness of the first approach is illustrated in Figure 40, which shows the results of an animal experiment utilizing a baroreceptor activation device 70 in the form of an extravascular electrical activation device bilaterally (right and left) applied to the carotid sinus 20. Figure 40 shows the control/output signal (volts) 410, heart rate (beats per minute) 420, mean arterial pressure (mmHg) 430 and arterial blood pressure (mmHg) 440 plotted as a function of time. The control signal 410 comprised a pulse train having a pulse amplitude of 2.5 volts, a pulse duration of 1.0 millisecond, and an initial pulse frequency of 100 Hz applied for a first period 401 of approximately 1 minute. During this first period 401, the heart rate 420 and blood pressure 430/440 were significantly reduced. The control signal 410 was changed to a pulse frequency of 10 Hz, while the pulse amplitude and duration remained unchanged, for a second period 402 of approximately 4 minutes. During this second period 402, the reduced heart rate and blood pressure were substantially sustained. The control signal 410 was changed back to a pulse frequency of 100 Hz, while the pulse amplitude and duration remained unchanged, for a third period 403 of approximately 40 seconds. During this third period 403, the heart rate 420 and blood pressure 430/440 were further reduced. The control signal 410 was changed again to a pulse frequency of 10 Hz, while the pulse amplitude and duration remained unchanged, for a fourth period 404 of approximately 1.5 minutes. During this fourth period 404, the reduced heart rate 420 and blood pressure 430/440 were substantially sustained. After the fourth period 404, the pulse train was turned off for a fifth period of time 405. During this fifth third period 405, the blood pressure 430/440 and heart rate 420 began to gradually rise to their pretest values. A demonstration of effectiveness of the second approach is illustrated in Figure 41, which shows the results of an animal experiment wherein a baroreceptor activation device 70 in the form of an extravascular activation device having two electrodes was utilized. A first electrode was connected to the right carotid sinus and a second electrode was connected to the left carotid sinus. Figure 41 shows the control signal (volts) 410, heart rate (beats per minute) 420, mean arterial pressure (mmHg) 430 and arterial blood pressure (mmHg) 440 plotted as a function of time. The control/output signal 410 comprised a pulse train having a pulse amplitude of 4.0 volts, a pulse duration of 1.0 millisecond, and a pulse frequency of 100 Hz applied for a 60 second period 406 to the left side. During this period 406, the heart rate 420 and blood pressure 430/440 were significantly reduced. The control signal 410 was switched to the right side for a 60 second period 408, while sustaining the reduced heart rate and blood pressure. The control signal 410 was switched between the left and right sides for a total period of 10.5 minutes, during which the reduced heart rate and blood pressure were substantially sustained. These experiments demonstrate the effectiveness of the general approaches described previously, each of which involve a stimulus regimen to promote long term efficacy. These stimulus regimens generally involve reducing the output level of the baroreceptor activation device after the desired initial effect is established (first approach), alternating activation between two or more devices or output means positioned at different anatomical locations (second approach), and/or changing the time domain characteristics and/or the triggering event characteristics of the therapy (third approach). All of these approaches have the common objective of promoting long term efficacy by maintaining baroreflex responsiveness. The control system 60 may be implanted in whole or in part. For example, the entire control system 60 may be carried externally by the patient utilizing transdermal connections to the sensor lead 82 and the control lead 72. Alternatively, the control block 61 and driver 66 may be implanted with the input device 64 and display 65 carried externally by the patient utilizing transdermal connections therebetween. As a further alternative, the transdermal connections may be replaced by cooperating transmitters/receivers to remotely communicate between components of the control system 60 and/or the sensor 80 and baroreceptor activation device 70. With general reference to Figures 4 - 21, schematic illustrations of specific embodiments of the baroreceptor activation device 70 are shown. The design, function and use of these specific embodiments, in addition to the control system 60 and sensor 80 (not shown), are the same as described with reference to Figure 3, unless otherwise noted or apparent from the description. In addition, the anatomical features illustrated in Figures 4 - 20 are the same as discussed with reference to Figures 1, 2A and 2B, unless otherwise noted. In each embodiment, the connections between the components 60/70/80 may be physical (e.g., wires, tubes, cables, etc.) or remote (e.g., transmitter/receiver, inductive, magnetic, etc.). For physical connections, the connection may travel intraarterially, intravenously, subcutaneously, or through other natural tissue paths. Refer now to Figures 4A and 4B which show schematic illustrations of a baroreceptor activation device 100 in the form of an intravascular inflatable balloon. The inflatable balloon device 100 includes a helical balloon 102 which is connected to a fluid line 104. An example of a similar helical balloon is disclosed in U.S. Patent No. 5,181,911 to Shturman, the entire disclosure of which is hereby incorporated by reference. The balloon 102 preferably has a helical geometry or any other geometry which allows blood perfusion therethrough. The fluid line 104 is connected to the driver 66 of the control system 60. In this embodiment, the driver 66 comprises a pressure/vacuum source (i.e., an inflation device) which selectively inflates and deflates the helical balloon 102. Upon inflation, the helical balloon 102 expands, preferably increasing in outside diameter only, to mechanically activate baroreceptors 30 by stretching or otherwise deforming them and/or the vascular wall 40. Upon deflation, the helical balloon 102 returns to its relaxed geometry such that the vascular wall 40 returns to its nominal state. Thus, by selectively inflating the helical balloon 102, the baroreceptors 30 adjacent thereto may be selectively activated. As an alternative to pneumatic or hydraulic expansion utilizing a balloon, a mechanical expansion device (not shown) may be used to expand or dilate the vascular wall 40 and thereby mechanically activate the baroreceptors 30. For example, the mechanical expansion device may comprise a tubular wire braid structure that diametrically expands when longitudinally compressed as disclosed in U.S. Patent No. 5,222,971 to Willard et al., the entire disclosure of which is hereby incorporated by reference. The tubular braid may be disposed intravascularly and permits blood perfusion through the wire mesh. In this embodiment, the driver 66 may comprise a linear actuator connected by actuation cables to opposite ends of the braid. When the opposite ends of the tubular braid are brought closer together by actuation of the cables, the diameter of the braid increases to expand the vascular wall 40 and activate the baroreceptors 30. Refer now to Figures 5A and 5B which show schematic illustrations of a baroreceptor activation device 120 in the form of an extravascular pressure cuff. The pressure cuff device 120 includes an inflatable cuff 122 which is connected to a fluid line 124. Examples of a similar cuffs 122 are disclosed in U.S. Patent No. 4,256,094 to Kapp et al. and U.S. Patent No. 4,881,939 to Newman, the entire disclosures of which are hereby incorporated by reference. The fluid line 124 is connected to the driver 66 of the control system 60. In this embodiment, the driver 66 comprises a pressure/vacuum source (i.e., an inflation device) which selectively inflates and deflates the cuff 122. Upon inflation, the cuff 122 expands, preferably increasing in inside diameter only, to mechanically activate baroreceptors 30 by stretching or otherwise deforming them and/or the vascular wall 40. Upon deflation, the cuff 122 returns to its relaxed geometry such that the vascular wall 40 returns to its nominal state. Thus, by selectively inflating the inflatable cuff 122, the baroreceptors 30 adjacent thereto may be selectively activated. The driver 66 may be automatically actuated by the control system 60 as discussed above, or may be manually actuated. An example of an externally manually actuated pressure/vacuum source is disclosed in U.S. Patent No. 4,709,690 to Haber, the entire disclosure of which is hereby incorporated by reference. Examples of transdermally manually actuated pressure/vacuum sources are disclosed in U.S. Patent No. 4,586,501 to Claracq, U.S. Patent No. 4,828,544 to Lane et al., and U.S. Patent No. 5,634,878 to Grundei et al., the entire disclosures of which are hereby incorporated by reference. Those skilled in the art will recognize that other external compression devices may be used in place of the inflatable cuff device 120. For example, a piston actuated by a solenoid may apply compression to the vascular wall. An example of a solenoid actuated piston device is disclosed in U.S. Patent No. 4,014,318 to Dokum et al, and an example of a hydraulically or pneumatically actuated piston device is disclosed in U.S. Patent No. 4,586,501 to Claracq, the entire disclosures of which are hereby incorporated by reference. Other examples include a rotary ring compression device as disclosed in U.S. Patent No. 4,551,862 to Haber, and an electromagnetically actuated compression ring device as disclosed in U.S. Patent No. 5,509,888 to Miller, the entire disclosures of which are hereby incorporated by reference. Refer now to Figures 6A and 6B which show schematic illustrations of a baroreceptor activation device 140 in the form of an intravascular deformable structure. The deformable structure device 140 includes a coil, braid or other stent- like structure 142 disposed in the vascular lumen. The deformable structure 142 includes one or more individual structural members connected to an electrical lead 144. Each of the structural members forming deformable structure 142 may comprise a shape memory material 146 (e.g., nickel titanium alloy) as illustrated in Figure 6C, or a bimetallic material 148 as illustrated in Figure 6D. The electrical lead 144 is connected to the driver 66 of the control system 60. In this embodiment, the driver 66 comprises an electric power generator or amplifier which selectively delivers electric current to the structure 142 which resistively heats the structural members 146/148. The structure 142 may be unipolar as shown using the surrounding tissue as ground, or bipolar or multipolar using leads connected to either end of the structure 142. Electrical power may also be delivered to the structure 142 inductively as described hereinafter with reference to Figures 14-16. Upon application of electrical current to the shape memory material 146, it is resistively heated causing a phase change and a corresponding change in shape. Upon application of electrical current to the bimetallic material 148, it is resistively heated causing a differential in thermal expansion and a corresponding change in shape. In either case, the material 146/148 is designed such that the change in shape causes expansion of the structure 142 to mechanically activate baroreceptors 30 by stretching or otherwise deforming them and/or the vascular wall 40. Upon removal of the electrical current, the material 146/148 cools and the structure 142 returns to its relaxed geometry such that the baroreceptors 30 and/or the vascular wall 40 return to their nominal state. Thus, by selectively expanding the structure 142, the baroreceptors 30 adjacent thereto may be selectively activated. Refer now to Figures 7A and 7B which show schematic illustrations of a baroreceptor activation device 160 in the form of an extravascular deformable structure. The extravascular deformable structure device 160 is substantially the same as the intravascular deformable structure device 140 described with reference to Figures 6A and 6B, except that the extravascular device 160 is disposed about the vascular wall, and therefore compresses, rather than expands, the vascular wall 40. The deformable structure device 160 includes a coil, braid or other stent-like structure 162 comprising one or more individual structural members connected to an electrical lead 164. Each of the structural members may comprise a shape memory material 166 (e.g., nickel titanium alloy) as illustrated in Figure 7C, or a bimetallic material 168 as illustrated in Figure 7D. The structure 162 may be unipolar as shown using the surrounding tissue as ground, or bipolar or multipolar using leads connected to either end of the structure 162. Electrical power may also be delivered to the structure 162 inductively as described hereinafter with reference to Figures 14-16. Upon application of electrical current to the shape memory material 166, it is resistively heated causing a phase change and a corresponding change in shape. Upon application of electrical current to the bimetallic material 168, it is resistively heated causing a differential in thermal expansion and a corresponding change in shape. In either case, the material 166/168 is designed such that the change in shape causes constriction of the structure 162 to mechanically activate baroreceptors 30 by compressing or otherwise deforming the baroreceptors 30 and/or the vascular wall 40. Upon removal of the electrical current, the material 166/168 cools and the structure 162 returns to its relaxed geometry such that the baroreceptors 30 and/or the vascular wall 40 return to their nominal state. Thus, by selectively compressing the structure 162, the baroreceptors 30 adjacent thereto may be selectively activated. Refer now to Figures 8A and 8B which show schematic illustrations of a baroreceptor activation device 180 in the form of an extravascular flow regulator which artificially creates back pressure adjacent the baroreceptors 30. The flow regulator device 180 includes an external compression device 182, which may comprise any of the external compression devices described with reference to Figures 5 A and 5B. The external compression device 182 is operably connected to the driver 66 of the control system 60 by way of cable 184, which may comprise a fluid line or electrical lead, depending on the type of external compression device 182 utilized. The external compression device 182 is disposed about the vascular wall distal of the baroreceptors 30. For example, the external compression device 182 may be located in the distal portions of the external or internal carotid arteries 18/19 to create back pressure adjacent to the baroreceptors 30 in the carotid sinus region 20. Alternatively, the external compression device 182 may be located in the right subclavian artery 13, the right common carotid artery 14, the left common carotid artery 15, the left subclavian artery 16, or the brachiocephalic artery 22 to create back pressure adjacent the baroreceptors 30 in the aortic arch 12. Upon actuation of the external compression device 182, the vascular wall is constricted thereby reducing the size of the vascular lumen therein. By reducing the size of the vascular lumen, pressure proximal of the external compression device 182 is increased thereby expanding the vascular wall. Thus, by selectively activating the external compression device 182 to constrict the vascular lumen and create back pressure, the baroreceptors 30 may be selectively activated. Refer now to Figures 9A and 9B which show schematic illustrations of a baroreceptor activation device 200 in the form of an intravascular flow regular which artificially creates back pressure adjacent the baroreceptors 30. The intravascular flow regulator device 200 is substantially similar in function and use as extravascular flow regulator 180 described with reference to Figures 8 A and 8B, except that the intravascular flow regulator device 200 is disposed in the vascular lumen. Intravascular flow regulator 200 includes an internal valve 202 to at least partially close the vascular lumen distal of the baroreceptors 30. By at least partially closing the vascular lumen distal of the baroreceptors 30, back pressure is created proximal of the internal valve 202 such that the vascular wall expands to activate the baroreceptors 30. The internal valve 202 may be positioned at any of the locations described with reference to the external compression device 182, except that the internal valve 202 is placed within the vascular lumen. Specifically, the internal compression device 202 may be located in the distal portions of the external or internal carotid arteries 18/19 to create back pressure adjacent to the baroreceptors 30 in the carotid sinus region 20. Alternatively, the internal compression device 202 may be located in the right subclavian artery 13, the right common carotid artery 14, the left common carotid artery 15, the left subclavian artery 16, or the brachiocephalic artery 22 to create backpressure adjacent the baroreceptors 30 in the aortic arch 12. The internal valve 202 is operably coupled to the driver 66 of the control system 60 by way of electrical lead 204. The control system 60 may selectively open, close or change the flow resistance of the valve 202 as described in more detail hereinafter. The internal valve 202 may include valve leaflets 206 (bi-leaflet or tri- leaflet) which rotate inside housing 208 about an axis between an open position and a closed position. The closed position may be completely closed or partially closed, depending on the desired amount of back pressure to be created. The opening and closing of the internal valve 202 may be selectively controlled by altering the resistance of leaflet 206 rotation or by altering the opening force of the leaflets 206. The resistance of rotation of the leaflets 206 may be altered utilizing electromagnetically actuated metallic bearings carried by the housing 208. The opening force of the leaflets 206 may be altered by utilizing electromagnetic coils in each of the leaflets to selectively magnetize the leaflets such that they either repel or attract each other, thereby facilitating valve opening and closing, respectively. A wide variety of intravascular flow regulators may be used in place of internal valve 202. For example, internal inflatable balloon devices as disclosed in U.S. Patent No. 4,682,583 to Burton et al. and U.S. Patent No. 5,634,878 to Grundei et al., the entire disclosures of which is hereby incorporated by reference, may be adapted for use in place of valve 202. Such inflatable balloon devices may be operated in a similar manner as the inflatable cuff 122 described with reference to Figure 5. Specifically, in this embodiment, the driver 66 would comprises a pressure/vacuum source (i.e., an inflation device) which selectively inflates and deflates the internal balloon. Upon inflation, the balloon expands to partially occlude blood flow and create back pressure to mechanically activate baroreceptors 30 by stretching or otherwise deforming them and/or the vascular wall 40. Upon deflation, the internal balloon returns to its normal profile such that flow is not hindered and back pressure is eliminated. Thus, by selectively inflating the internal balloon, the baroreceptors 30 proximal thereof may be selectively activated by creating back pressure. Refer now to Figures 10A and 10B which show schematic illustrations of a baroreceptor activation device 220 in the form of magnetic particles 222 disposed in the vascular wall 40. The magnetic particles 222 may comprise magnetically responsive materials (i.e., ferrous based materials) and may be magnetically neutral or magnetically active. Preferably, the magnetic particles 222 comprise permanent magnets having an elongate cylinder shape with north and south poles to strongly respond to magnetic fields. The magnetic particles 222 are actuated by an electromagnetic coil 224 which is operably coupled to the driver 66 of the control system 60 by way of an electrical cable 226. The electromagnetic coil 224 may be implanted as shown, or located outside the body, in which case the driver 66 and the remainder of the control system 60 would also be located outside the body. By selectively activating the electromagnetic coil 224 to create a magnetic field, the magnetic particles 222 may be repelled, attracted or rotated. Alternatively, the magnetic field created by the electromagnetic coil 224 may be alternated such that the magnetic particles 222 vibrate within the vascular wall 40. When the magnetic particles are repelled, attracted, rotated, vibrated or otherwise moved by the magnetic field created by the electromagnetic coil 224, the baroreceptors 30 are mechanically activated. The electromagnetic coil 224 is preferably placed as close as possible to the magnetic particles 222 in the vascular wall 40, and may be placed intravascularly, extravascularly, or in any of the alternative locations discussed with reference to inductor shown in Figures 14-16. The magnetic particles 222 may be implanted in the vascular wall 40 by injecting a ferro-fluid or a ferro-particle suspension into the vascular wall adjacent to the baroreceptors 30. To increase biocompatibility, the particles 222 may be coated with a ceramic, polymeric or other inert material. Injection of the fluid carrying the magnetic particles 222 is preferably performed percutaneously. Refer now to Figures 11A and 11B which show schematic illustrations of a baroreceptor activation device 240 in the form of one or more transducers 242. Preferably, the transducers 242 comprise an array surrounding the vascular wall. The transducers 242 may be intravascularly or extravascularly positioned adjacent to the baroreceptors 30. In this embodiment, the transducers 242 comprise devices which convert electrical signals into some physical phenomena, such as mechanical vibration or acoustic waves. The electrical signals are provided to the transducers 242 by way of electrical cables 244 which are connected to the driver 66 of the control system 60. By selectively activating the transducers 242 to create a physical phenomena, the baroreceptors 30 may be mechanically activated. The transducers 242 may comprise an acoustic transmitter which transmits sonic or ultrasonic sound waves into the vascular wall 40 to activate the baroreceptors 30. Alternatively, the transducers 242 may comprise a piezoelectric material which vibrates the vascular wall to activate the baroreceptors 30. As a further alternative, the transducers 242 may comprise an artificial muscle which deflects upon application of an electrical signal. An example of an artificial muscle transducer comprises plastic impregnated with a lithium-perchlorate electrolyte disposed between sheets of polypyrrole, a conductive polymer. Such plastic muscles may be electrically activated to cause deflection in different directions depending on the polarity of the applied current. Refer now to Figures 12A and 12B which show schematic illustrations of a baroreceptor activation device 260 in the form of a local fluid delivery device 262 suitable for delivering a chemical or biological fluid agent to the vascular wall adjacent the baroreceptors 30. The local fluid delivery device 262 may be located mtravascularly, extravascularly, or intramurally. For purposes of illustration only, the local fluid delivery device 262 is positioned extravascularly. The local fluid delivery device 262 may include proximal and distal seals 266 which retain the fluid agent disposed in the lumen or cavity 268 adjacent to vascular wall. Preferably, the local fluid delivery device 262 completely surrounds the vascular wall 40 to maintain an effective seal. Those skilled in the art will recognize that the local fluid delivery device 262 may comprise a wide variety of implantable drug delivery devices or pumps known in the art. The local fluid delivery device 260 is connected to a fluid line 264 which is connected to the driver 66 of the control system 60. In this embodiment, the driver 66 comprises a pressure/vacuum source and fluid reservoir containing the desired chemical or biological fluid agent. The chemical or biological fluid agent may comprise a wide variety of stimulatory substances. Examples include veratridine, bradykinin, prostaglandins, and related substances. Such stimulatory substances activate the baroreceptors 30 directly or enhance their sensitivity to other stimuli and therefore may be used in combination with the other baroreceptor activation devices described herein. Other examples include growth factors and other agents that modify the function of the baroreceptors 30 or the cells of the vascular tissue surrounding the baroreceptors 30 causing the baroreceptors 30 to be activated or causing alteration of their responsiveness or activation pattern to other stimuli. It is also contemplated that injectable stimulators that are induced remotely, as described in U.S. Patent No. 6,061,596 which is incorporated herein by reference, may be used with the present invention. As an alternative, the fluid delivery device 260 may be used to deliver a photochemical that is essentially inert until activated by light to have a stimulatory effect as described above. In this embodiment, the fluid delivery device 260 would include a light source such as a light emitting diode (LED), and the driver 66 of the control system 60 would include a pulse generator for the LED combined with a pressure/vacuum source and fluid reservoir described previously. The photochemical would be delivered with the fluid delivery device 260 as described above, and the photochemical would be activated, deactivated or modulated by activating, deactivating or modulating the LED. As a further alternative, the fluid delivery device 260 may be used to deliver a warm or hot fluid (e.g. saline) to thermally activate the baroreceptors 30. In this embodiment, the driver 66 of the control system 60 would include a heat generator for heating the fluid, combined with a pressure/vacuum source and fluid reservoir described previously. The hot or warm fluid would be delivered and preferably circulated with the fluid delivery device 260 as described above, and the temperature of the fluid would be controlled by the driver 66. Refer now to Figures 13 A and 13B which show schematic illustrations of a baroreceptor activation device 280 in the form of an intravascular electrically conductive structure or electrode 282. The electrode structure 282 may comprise a self-expanding or balloon expandable coil, braid or other stent-like structure disposed in the vascular lumen. The electrode structure 282 may serve the dual purpose of maintaining lumen patency while also delivering electrical stimuli. To this end, the electrode structure 282 may be implanted utilizing conventional intravascular stent and filter delivery techniques. Preferably, the electrode structure 282 comprises a geometry which allows blood perfusion therethrough. The electrode structure 282 comprises electrically conductive material which may be selectively insulated to establish contact with the inside surface of the vascular wall 40 at desired locations, and limit extraneous electrical contact with blood flowing through the vessel and other tissues. The electrode structure 282 is connected to electric lead 284 which is connected to the driver 66 of the control system 60. The driver 66, in this embodiment, may comprise a power amplifier, pulse generator or the like to selectively deliver electrical control signals to structure 282. As mentioned previously, the electrical control signal generated by the driver 66 may be continuous, periodic, episodic or a combination thereof, as dictated by an algorithm contained in memory 62 of the control system 60. Continuous control signals include a constant pulse, a constant train of pulses, a triggered pulse and a triggered train of pulses. Periodic control signals include each of the continuous control signals described above which have a designated start time and a designated duration. Episodic control signals include each of the continuous control signals described above which are triggered by an episode. By selectively activating, deactivating or otherwise modulating the electrical control signal transmitted to the electrode structure 282, electrical energy may be delivered to the vascular wall to activate the baroreceptors 30. As discussed previously, activation of the baroreceptors 30 may occur directly or indirectly. In particular, the electrical signal delivered to the vascular wall 40 by the electrode structure 282 may cause the vascular wall to stretch or otherwise deform thereby indirectly activating the baroreceptors 30 disposed therein. Alternatively, the electrical signals delivered to the vascular wall by the electrode structure 282 may directly activate the baroreceptors 30 by changing the electrical potential across the baroreceptors 30. In either case, the electrical signal is delivered to the vascular wall 40 immediately adjacent to the baroreceptors 30. It is also contemplated that the electrode structure 282 may delivery thermal energy by utilizing a semi-conductive material having a higher resistance such that the electrode structure 282 resistively generates heat upon application of electrical energy. Various alternative embodiments are contemplated for the electrode structure 282, including its design, implanted location, and method of electrical activation. For example, the electrode structure 282 may be unipolar as shown in Figures 13A and 13B using the surrounding tissue as ground, or bipolar using leads connected to either end of the structure 282 as shown in Figures 18A and 18B. In the embodiment of Figures 18A and 18B, the electrode structure 282 includes two or more individual electrically conductive members 283/285 which are electrically isolated at their respective cross-over points utilizing insulative materials. Each of the members 283/285 is connected to a separate conductor contained within the electrical lead 284. Alternatively, an array of bipoles may be used as described in more detail with reference to Figure 21. As a further alternative, a multipolar arrangement may be used wherein three or more electrically conductive members are included in the structure 282. For example, a tripolar arrangement may be provided by one electrically conductive member having a polarity disposed between two electrically conductive members having the opposite polarity. In terms of electrical activation, the electrical signals may be directly delivered to the electrode structure 282 as described with reference to Figures 13A and 13B, or indirectly delivered utilizing an inductor as illustrated in Figures 14-16 and 21. The embodiments of Figures 14 - 16 and 21 utilize an inductor 286 which is operably connected to the driver 66 of the control system 60 by way of electrical lead 284. The inductor 286 comprises an electrical winding which creates a magnetic field 287 (as seen in Figure 21) around the electrode structure 282. The magnetic field 287 may be alternated by alternating the direction of current flow through the inductor 286. Accordingly, the inductor 286 may be utilized to create current flow in the electrode structure 282 to thereby deliver electrical signals to the vascular wall 40 to directly or indirectly activate the baroreceptors 30. In all embodiments, the inductor 286 may be covered with an electrically insulative material to eliminate direct electrical stimulation of tissues surrounding the inductor 286. A preferred embodiment of an inductively activated electrode structure 282 is described in more detail with reference to Figures 21 A - 21C. The embodiments of Figures 13-16 may be modified to form a cathode/anode arrangement. Specifically, the electrical inductor 286 would be connected to the driver 66 as shown in Figures 14-16 and the electrode structure 282 would be connected to the driver 66 as shown in Figure 13. With this arrangement, the electrode structure 282 and the inductor 286 may be any suitable geometry and need not be coiled for purposes of induction. The electrode structure 282 and the inductor 286 would comprise a cathode/anode or anode/cathode pair. For example, when activated, the cathode 282 may generate a primary stream of electrons which travel through the inter-electrode space (i.e., vascular tissue and baroreceptors 30) to the anode 286. The cathode is preferably cold, as opposed to thermionic, during electron emission. The electrons may be used to electrically or thermally activate the baroreceptors 30 as discussed previously. The electrical inductor 286 is preferably disposed as close as possible to the electrode structure 282. For example, the electrical inductor 286 may be disposed adjacent the vascular wall as illustrated in Figures 14 A and 14B. Alternatively, the inductor 286 may be disposed in an adjacent vessel as illustrated in Figures 15A and 15B. If the electrode structure 282 is disposed in the carotid sinus 20, for example, the inductor 286 may be disposed in the internal jugular vein 21 as illustrated in Figures 15A and 15B. In the embodiment of Figures 15A and 15B, the electrical inductor 286 may comprise a similar structure as the electrode structure 282. As a further alternative, the electrical inductor 286 may be disposed outside the patient's body, but as close as possible to the electrode structure 282. If the electrode structure 282 is disposed in the carotid sinus 20, for example, the electrical inductor 286 may be disposed on the right or left side of the neck of the patient as illustrated in Figures 16A and 16B. In the embodiment of Figures 16A and 16B, wherein the electrical inductor 286 is disposed outside the patient's body, the control system 60 may also be disposed outside the patient's body. In terms of implant location, the electrode structure 282 may be intravascularly disposed as described with reference to Figures 13A and 13B, or extravascularly disposed as described with reference to Figures 17A and 17B, which show schematic illustrations of a baroreceptor activation device 300 in the form of an extravascular electrically conductive structure or electrode 302. Except as described herein, the extravascular electiode structure 302 is the same in design, function, and use as the intravascular electrode structure 282. The electrode structure 302 may comprise a coil, braid or other structure capable of surrounding the vascular wall. Alternatively, the electrode structure 302 may comprise one or more electrode patches distributed around the outside surface of the vascular wall. Because the electrode structure 302 is disposed on the outside surface of the vascular wall, intravascular delivery techniques may not be practical, but minimally invasive surgical techniques will suffice. The extravascular electrode structure 302 may receive electrical signals directly from the driver 66 of the control system 60 by way of electrical lead 304, or indirectly by utilizing an inductor (not shown) as described with reference to Figures 14-16. Refer now to Figures 19A and 19B which show schematic illustrations of a baroreceptor activation device 320 in the form of electrically conductive particles 322 disposed in the vascular wall. This embodiment is substantially the same as the embodiments described with reference to Figures 13-18, except that the electrically conductive particles 322 are disposed within the vascular wall, as opposed to the electrically conductive structures 282/302 which are disposed on either side of the vascular wall. In addition, this embodiment is similar to the embodiment described with reference to Figure 10, except that the electrically conductive particles 322 are not necessarily magnetic as with magnetic particles 222, and the electrically conductive particles 322 are driven by an electromagnetic filed rather than by a magnetic field. In this embodiment, the driver 66 of the control system 60 comprises an electromagnetic transmitter such as an radiofrequency or microwave transmitter. Electromagnetic radiation is created by the transmitter 66 which is operably coupled to an antenna 324 by way of electrical lead 326. Electromagnetic waves are emitted by the antenna 324 and received by the electrically conductive particles 322 disposed in the vascular wall 40. Electromagnetic energy creates oscillating current flow within the electrically conductive particles 322, and depending on the intensity of the electromagnetic radiation and the resistivity of the conductive particles 322, may cause the electrical particles 322 to generate heat. The electrical or thermal energy generated by the electrically conductive particles 322 may directly activate the baroreceptors 30, or indirectly activate the baroreceptors 30 by way of the suπounding vascular wall tissue. The electromagnetic radiation transmitter 66 and antenna 324 may be disposed in the patient's body, with the antenna 324 disposed adjacent to the conductive particles in the vascular wall 40 as illustrated in Figures 19A and 19B. Alternatively, the antenna 324 may be disposed in any of the positions described with reference to the electrical inductor shown in Figures 14-16. It is also contemplated that the electromagnetic radiation transmitter 66 and antenna 324 may be utilized in combination with the intravascular and extravascular electrically conductive structures 282/302 described with reference to Figures 13-18 to generate thermal energy on either side of the vascular wall. As an alternative, the electromagnetic radiation transmitter 66 and antenna 324 may be used without the electrically conductive particles 322. Specifically, the electromagnetic radiation transmitter 66 and antenna 324 may be used to deliver electromagnetic radiation (e.g., RF, microwave) directly to the baroreceptors 30 or the tissue adjacent thereto to cause localized heating, thereby thermally inducing a baroreceptor 30 signal. Refer now to Figures 20A and 20B which show schematic illustrations of a baroreceptor activation device 340 in the form of a Peltier effect device 342. The Peltier effect device 342 may be extravascularly positioned as illustrated, or may be mtravascularly positioned similar to an intravascular stent or filter. The Peltier effect device 342 is operably connected to the driver 66 of the control system 60 by way of electrical lead 344. The Peltier effect device 342 includes two dissimilar metals or semiconductors 343/345 separated by a thermal transfer junction 347. In this particular embodiment, the driver 66 comprises a power source which delivers electrical energy to the dissimilar metals or semiconductors 343/345 to create current flow across the thermal junction 347. When current is delivered in an appropriate direction, a cooling effect is created at the thermal junction 347. There is also a heating effect created at the junction between the individual leads 344 connected to the dissimilar metals or semiconductors 343/345. This heating effect, which is proportional to the cooling effect, may be utilized to activate the baroreceptors 30 by positioning the junction between the electrical leads 344 and the dissimilar metals or semiconductors 343/345 adjacent to the vascular wall 40. Refer now to Figures 21A-21C which show schematic illustrations of a preferred embodiment of an inductively activated electrode structure 282 for use with the embodiments described with reference to Figures 14-16. In this embodiment, current flow in the electrode structure 282 is induced by a magnetic field 287 created by an inductor 286 which is operably coupled to the driver 66 of the control system 60 by way of electrical cable 284. The electrode structure 282 preferably comprises a multi-filar self-expanding braid structure including a plurality of individual members 282a, 282b, 282c and 282d. However, the electrode structure 282 may simply comprise a single coil for purposes of this embodiment. Each of the individual coil members 282a-282d comprising the electrode structure 282 consists of a plurality of individual coil turns 281 connected end to end as illustrated in Figures 21B and 21C. Figure 21C is a detailed view of the connection between adjacent coil turns 281 as shown in Figure 21B. Each coil turn 281 comprises electrically isolated wires or receivers in which a current flow is established when a changing magnetic field 287 is created by the inductor 286. The inductor 286 is preferably covered with an electrically msulative material to eliminate direct electrical stimulation of tissues surrounding the inductor 286. Current flow through each coil turn 281 results in a potential drop 288 between each end of the coil turn 281. With a potential drop defined at each junction between adjacent coil turns 281, a localized cuπent flow cell is created in the vessel wall adjacent each junction. Thus an array or plurality of bipoles are created by the electrode structure 282 and uniformly distributed around the vessel wall. Each coil turn 281 comprises an electrically conductive wire material 290 surrounded by an electrically insulative material 292. The ends of each coil turn 281 are connected by an electrically insulated material 294 such that each coil turn 281 remains electrically isolated. The insulative material 294 mechanically joins but electrically isolates adjacent coil turns 281 such that each turn 281 responds with a similar potential drop 288 when cuπent flow is induced by the changing magnetic field 287 of the inductor 286. An exposed portion 296 is provided at each end of each coil turn 281 to facilitate contact with the vascular wall tissue. Each exposed portion 296 comprises an isolated electrode in contact with the vessel wall. The changing magnetic field 287 of the inductor 286 causes a potential drop in each coil turn 281 thereby creating small current flow cells in the vessel wall coπesponding to adjacent exposed regions 296. The creation of multiple small cuπent cells along the inner wall of the blood vessel serves to create a cylindrical zone of relatively high cuπent density such that the baroreceptors 30 are activated. However, the cylindrical current density field quickly reduces to a negligible cuπent density near the outer wall of the vascular wall, which serves to limit extraneous cuπent leakage to minimize or eliminate unwanted activation of extravascular tissues and structures such as nerves or muscles. Refer now to Figures 22 A - 22F which show schematic illustrations of various possible aπangements of electrodes around the carotid sinus 20 for extravascular electrical activation embodiments, such as baroreceptor activation device 300 described with reference to Figures 17A and 17B. 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 extianeous tissue stimulation. In Figures 22A - 22F, the carotid arteries are shown, including the common 14, the external 18 and the internal 19 carotid arteries. The location of the carotid sinus 20 may be identified by a landmark bulge 21, which is typically located on the internal carotid artery 19 just distal of the bifurcation, or extends across the bifurcation from the common carotid artery 14 to the internal carotid artery 19. The carotid sinus 20, and in particular the bulge 21 of the carotid sinus, may contain a relatively high density of baroreceptors 30 (not shown) in the vascular wall. For this reason, it may be desirable to position the electrodes 302 of the activation device 300 on and/or around the sinus bulge 21 to maximize baroreceptor responsiveness and to minimize extraneous tissue stimulation. It should be understood that the device 300 and electiodes 302 are merely schematic, and only a portion of which may be shown, for purposes of illustrating various positions of the electrodes 302 on and/or around the carotid sinus 20 and the sinus bulge 21. In each of the embodiments described herein, the electrodes 302 may be monopolar (electrodes are cathodes, suπounding tissue is anode or ground), bipolar (cathode-anode pairs), or tripolar (anode-cathode-anode sets). Specific extravascular electrode designs are described in more detail hereinafter. In Figure 22 A, the electrodes 302 of the extravascular electrical activation device 300 extend around a portion or the entire circumference of the sinus 20 in a circular fashion. In Figure 22B, the electrodes 302 of the extiavascular electrical activation device 300 extend around a portion or the entire circumference of the sinus 20 in a helical fashion. In the helical aπangement shown in Figure 22B, the electrodes 302 may wrap around the sinus 20 any number of times to establish the desired electrode 302 contact and coverage. In the circular aπangement shown in Figure 22A, a single pair of electrodes 302 may wrap around the sinus 20, or a plurality of electrode pairs 302 may be wrapped around the sinus 20 as shown in Figure 22C to establish more electrode 302 contact and coverage. The plurality of electrode pairs 302 may extend from a point proximal of the sinus 20 or bulge 21, to a point distal of the sinus 20 or bulge 21 to ensure activation of baroreceptors 30 throughout the sinus 20 region. The electrodes 302 may be connected to a single channel or multiple channels as discussed in more detail hereinafter. The plurality of electrode pairs 302 may be selectively activated for purposes of targeting a specific area of the sinus 20 to increase baroreceptor responsiveness, or for purposes of reducing the exposure of tissue areas to activation to maintain baroreceptor responsiveness long term. In Figure 22D, the electiodes 302 extend around the entire circumference of the sinus 20 in a criss-cross fashion. The criss-cross aπangement of the electrodes 302 establishes contact with both the internal 19 and external 18 carotid arteries around the carotid sinus 20. Similarly, in Figure 22E, the electrodes 302 extend around all or a portion of the circumference of the sinus 20, including the internal 19 and external 18 carotid arteries at the bifurcation, and in some instances the common carotid artery 14. In Figure 22F, the electrodes 302 extend around all or a portion of the circumference of the sinus 20, including the internal 19 and external 18 carotid arteries distal of the bifurcation. In Figures 22E and 22F, the extiavascular electrical activation devices 300 are shown to include a substrate or base structure 306 which may encapsulate and insulate the electrodes 302 and may provide a means for attachment to the sinus 20 as described in more detail hereinafter. From the foregoing discussion with reference to Figures 22A - 22F, it should be apparent that there are a number of suitable aπangements for the electrodes 302 of the activation device 300, relative to the carotid sinus 20 and associated anatomy. In each of the examples given above, the electrodes 302 are wrapped around a portion of the carotid structure, which may require deformation of the electiodes 302 from their relaxed geometry (e.g., straight). To reduce or eliminate such deformation, the electrodes 302 and/or the base structure 306 may have a relaxed geometry that substantially conforms to the shape of the carotid anatomy at the point of attachment. In other words, the electrodes 302 and the base structure 306 may be pre-shaped to conform to the carotid anatomy in a substantially relaxed state. Alternatively, the electrodes 302 may have a geometry and/or orientation that reduces the amount of electrode 302 strain. For example, in Figure 23, the electrodes 302 are shown to have a serpentine or wavy shape. The serpentine shape of the electrodes 302 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 the electrode 302 with the carotid tissue. As an alternative, the electrodes 302 may be aπanged to be substantially orthogonal to the wrap direction (i.e., substantially parallel to the axis of the carotid arteries) as shown in Figure 24. In this alternative, the electrodes 302 each have a length and a width or diameter, wherein the length is substantially greater than the width or diameter. The electrodes 302 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 the device 300 is wrapped. As with the multiple electrode embodiments described previously, the electiodes 302 may be connected to a single channel or multiple channels as discussed in more detail hereinafter. Refer now to Figures 25 - 28 which schematically illustrate various multichannel electrodes for the extravascular electrical activation device 300. Figure 25 illustrates a six (6) channel electrode assembly including six (6) separate elongate electrodes 302 extending adjacent to and parallel with each other. The electrodes 302 are each connected to multi-channel cable 304. Some of the electiodes 302 may be common, thereby reducing the number of channels necessary in the cable 304. Base structure or substrate 306 may comprise a flexible and electrically msulative material suitable for implantation, such as silicone, perhaps reinforced with a flexible material such as polyester fabric. The base 306 may have a length suitable to wrap around all (360°) or a portion (i.e., less than 360°) of the circumference of one or more of the carotid arteries adjacent the carotid sinus 20. The electrodes 302 may extend around a portion (i.e., less than 360° such as 270°, 180° or 90°) of the circumference of one or more of the carotid arteries adjacent the carotid sinus 20. To this end, the electrodes 302 may have a length that is less than (e.g., 75%, 50% or 25%) the length of the base 206. The electrodes 302 may be parallel, orthogonal or oblique to the length of the base 306, which is generally orthogonal to the axis of the carotid artery to which it is disposed about. The electrodes 302 may comprise round wire, rectangular ribbon or foil formed of an electrically conductive and radiopaque material such as platinum. The base structure 306 substantially encapsulates the electrodes 302, leaving only an exposed area for electrical connection to extravascular carotid sinus tissue. For example, each electrode 302 may be partially recessed in the base 206 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 the elongate electrodes 302. In all embodiments described with reference to Figures 25 - 28, the multichannel electrodes 302 may be selectively activated for purposes of mapping and targeting a specific area of the carotid sinus 20 to determine the best combination of electrodes 302 (e.g., individual pair, or groups of pairs) to activate for maximum baroreceptor responsiveness, as described elsewhere herein. In addition, the multichannel electrodes 302 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, the electrodes 302 may be connected to a single channel whereby baroreceptors are uniformly activated throughout the sinus 20 region. An alternative multi-channel electiode design is illustrated in Figure 26. In this embodiment, the device 300 includes sixteen (16) individual electrode pads 302 connected to 16-channel cable 304 via 4-channel connectors 303. In this embodiment, the circular electrode pads 302 are partially encapsulated by the base structure 306 to leave one face of each button electrode 302 exposed for electrical connection to carotid tissues. With this aπangement, electrical paths through the carotid tissues may be defined by one or more pairs (bipolar) or groups (tripolar) of electrode pads 302. A variation of the multi-channel pad-type electrode design is illustrated in Figure 27. In this embodiment, the device 300 includes sixteen (16) individual circular pad electrodes 302 suπounded by sixteen (16) rings 305, which collectively may be refeπed to as concentric electrode pads 302/305. Pad electrodes 302 are connected to 17-charmel cable 304 via 4-channel connectors 303, and rings 305 are commonly connected to 17-channel cable 304 via a single channel connector 307. In this embodiment, the circular shaped electiodes 302 and the rings 305 are partially encapsulated by the base structure 306 to leave one face of each pad electiode 302 and one side of each ring 305 exposed for electrical connection to carotid tissues. As an alternative, two rings 305 may suπound each electrode 302, with the rings 305 being commonly connected. With these aπangements, electrical paths through the carotid tissues may be defined between one or more pad electrode 302 / ring 305 sets to create localized electrical paths. Another variation of the multi-channel pad electrode design is illustrated in Figure 28. In this embodiment, the device 300 includes a control IC chip 310 connected to 3 -channel cable 304. The control chip 310 is also connected to sixteen (16) individual pad electrodes 302 via 4-channel connectors 303. The control chip 310 permits the number of channels in cable 304 to be reduced by utilizing a coding system. The control system 60 sends a coded control signal which is received by chip 310. The chip 310 converts the code and enables or disables selected electiode 302 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 the chip 310 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) coπesponding to the code sent with that pulse. Each subsequent pulse would have a different code than the preceding pulse, such that the chip 310 enables and disables a different set of electrodes 302 coπesponding to the different code. Thus, virtually any number of electrode pairs may be selectively activated using control chip 310, without the need for a separate channel in cable 304 for each electrode 302. By reducing the number of channels in cable 304, the size and cost thereof may be reduced. Optionally, the IC chip 310 may be connected to feedback sensor 80, taking advantage of the same functions as described with reference to Figure 3. In addition, one or more of the electrodes 302 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 electiode 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 to Figure 29 which schematically illustrates an extravascular electrical activation device 300 including a support collar or anchor 312. In this embodiment, the activation device 300 is wrapped around the internal carotid artery 19 at the carotid sinus 20, and the support collar 312 is wrapped around the common carotid artery 14. The activation device 300 is connected to the support collar 312 by cables 304, which act as a loose tether. With this aπangement, the collar 312 isolates the activation device from movements and forces transmitted by the cables 304 proximal of the support collar, such as may be encountered by movement of the control system 60 and/or driver 66. As an alternative to support collar 312, a strain relief (not shown) may be connected to the base structure 306 of the activation device 300 at the juncture between the cables 304 and the base 306. With either approach, the position of the device 300 relative to the carotid anatomy may be better maintained despite movements of other parts of the system. In this embodiment, the base structure 306 of the activation device 300 may comprise molded tube, a tubular extrusion, or a sheet of material wrapped into a tube shape utilizing a suture flap 308 with sutures 309 as shown. The base structure 306 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 the base structure 306 may coπespond to the outside diameter of the carotid artery at the location of implantation, for example 6 - 8 mm. The wall thickness of the base structure 306 may be very thin to maintain flexibility and a low profile, for example less than 1 mm. If the device 300 is to be disposed about a sinus bulge 21, a coπespondingly shaped bulge may be formed into the base structure for added support and assistance in positioning. The electrodes 302 (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 electiodes may be molded into the base structure 306 or adhesively connected to the inside diameter thereof, leaving a portion of the electrode exposed for electrical connection to carotid tissues. The electrodes 302 may encompass less than the entire inside circumference (e.g., 300°) of the base structure 306 to avoid shorting. The electrodes 302 may have any of the shapes and aπangements described previously. For example, as shown in Figure 29, two rectangular ribbon electrodes 302 may be used, each having a width of 1 mm spaced 1.5 mm apart. The support collar 312 may be formed similarly to base structure 306. For example, the support collar may comprise molded tube, a tubular extrusion, or a sheet of material wrapped into a tube shape utilizing a suture flap 315 with sutures 313 as shown. The support collar 312 may be formed of a flexible and biocompatible material such as silicone, which may be reinforced to form a composite structure. The cables 304 are secured to the support collar 312, leaving slack in the cables 304 between the support collar 312 and the activation device 300. In all extravascular embodiments described herein, including electrical activation embodiments, it may be desirable to secure the activation device to the vascular wall using sutures or other fixation means. For example, sutures 311 may be used to maintain the position of the electrical activation device 300 relative to the carotid anatomy (or other vascular site containing baroreceptors). Such sutures 311 may be connected to base structure 306, and pass through all or a portion of the vascular wall. For example, the sutures 311 may be threaded through the base structure 306, through the adventitia of the vascular wall, and tied. If the base structure 306 comprises a patch or otherwise partially suπounds 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 the base structure 306, 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 to Figure 30 which schematically illustrates an alternative extravascular electrical activation device 300 including one or more electrode ribs 316 interconnected by spine 317. Optionally, a support collar 312 having one or more (non-electiode) ribs 316 may be used to isolate the activation device 300 from movements and forces transmitted by the cables 304 proximal of the support collar 312. The ribs 316 of the activation device 300 are sized to fit about the carotid anatomy, such as the internal carotid artery 19 adjacent the carotid sinus 20. Similarly, the ribs 316 of the support collar 312 may be sized to fit about the carotid anatomy, such as the common carotid artery 14 proximal of the carotid sinus 20. The ribs 316 may be separated, placed on a carotid artery, and closed thereabout to secure the device 300 to the carotid anatomy. Each of the ribs 316 of the device 300 includes an electrode 302 on the inside surface thereof for electrical connection to carotid tissues. The ribs 316 provide insulative material around the electrodes 302, leaving only an inside portion exposed to the vascular wall. The electrodes 302 are coupled to the multi-channel cable 304 through spine 317. Spine 317 also acts as a tether to ribs 316 of the support collar 312, which do not include electrodes since their function is to provide support. The multi-channel electrode 302 functions discussed with reference to Figures 25 - 28 are equally applicable to this embodiment. The ends of the ribs 316 may be connected (e.g., sutured) after being disposed about a carotid artery, or may remain open as shown. If the ends remain open, the ribs 316 may be formed of a relatively stiff material to ensure a mechanical lock around the carotid artery. For example, the ribs 316 may be formed of polyethylene, polypropylene, PTFE, or other similar msulative and biocompatible material. Alternatively, the ribs 316 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 the electrodes 302. As a further alternative, the ribs 316 may comprise an insulative and biocompatible polymeric material with the structural integrity provided by metallic (e.g., stainless steel, nickel titanium alloy, etc.) reinforcement. In this latter alternative, the electrodes 302 may comprise the metallic reinforcement. Refer now to Figure 31 which schematically illustrates a specific example of an electrode assembly for an extravascular electrical activation device 300. In this specific example, the base structure 306 comprises a silicone sheet having a length of 5.0 inches, a thickness of 0.007 inches, and a width of 0.312 inches. The electrodes 302 comprise platinum ribbon having a length of 0.47 inches, a thickness of 0.0005 inches, and a width of 0.040 inches. The electiodes 302 are adhesively connected to one side of the silicone sheet 306. The electrodes 302 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 the cable 304 is connected to the control system 60 or driver 66 as described previously. The pacing lead is modified by removing the pacing electrode to form the cable body 304. The MP35 wires are extracted from the distal end thereof to form two coils 318 positioned side-by-side having a diameter of about 0.020 inches. The coils 318 are then attached to the electrodes utilizing 316 type stainless steel crimp terminals laser welded to one end of the platinum electrodes 302. The distal end of the cable 304 and the connection between the coils 318 and the ends of the electrodes 302 are encapsulated by silicone. The cable 304 illustrated in Figure 31 comprises a coaxial type cable including two coaxially disposed coil leads separated into two separate coils 318 for attachment to the electrodes 302. An alternative cable 304 construction is illustrated in Figure 32. Figure 32 illustrates an alternative cable body 304 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 the device 300 and the control system 60 or the driver 66. 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, the cable body 304 may comprise two or more conductive wires 304a aπanged coaxially or collinearly as shown. Each conductive wire 304a may comprise a multifilament structure of suitable conductive material such as stainless steel or MP35N. An msulative material may suπound the wire conductors 304a individually and/or collectively. For purposes of illustration only, a pair of electrically conductive wires 304a having an insulative material surrounding each wire 304a individually is shown. The insulated wires 304a may be connected by a spacer 304b comprising, for example, an insulative material. An additional jacket of suitable insulative material may suπound each of the conductors 304a. The msulative jacket may be formed to have the same curvilinear shape of the insulated wires 304a to help maintain the shape of the cable body 304 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 the cable 304 in the desired shape while the cable is exposed to heat. Sufficient heat is used to heat set the conductive wires 304a and/or the suπounding insulative material. After cooling, the cable 304 may be removed from the fixture, and the cable 304 retains the desired shape. To address low blood pressure and other conditions requiring blood pressure augmentation, some of the baroreceptor activation devices described previously may be used to selectively and controllably regulate blood pressure 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 as described previously. Specifically, the present invention would function to increase the blood pressure and level of sympathetic nervous . system activation by inhibiting or dampening the activation of baroreceptors. This may be accomplished by utilizing mechanical, thermal, electrical and chemical or biological means. Mechanical means may be triggered off the pressure pulse of the heart to mechanically limit deformation of the arterial wall. For example, either of the external compression devices 120/160 described previously may be used to limit deformation of the arterial wall. Alternatively, the external compression device may simply limit diametrical expansion of the vascular wall adjacent the baroreceptors without the need for a trigger or control signal. Thermal means may be used to cool the baroreceptors 30 and adjacent tissue to reduce the responsiveness of the baroreceptors 30 and thereby dampen baroreceptor signals. Specifically, the baroreceptor 30 signals may be dampened by either directly cooling the baroreceptors 30, to reduce their sensitivity, metabolic activity and function, or by cooling the suπounding vascular wall tissue thereby causing the wall to become less responsive to increases in blood pressure. An example of this approach is to use the cooling effect of the Peltier device 340. Specifically, the thermal transfer junction 347 may be positioned adjacent the vascular wall to provide a cooling effect. The cooling effect may be used to dampen signals generated by the baroreceptors 30. Another example of this approach is to use the fluid delivery device 260 to deliver a cool or cold fluid (e.g. saline). In this embodiment, the driver 66 would include a heat exchanger to cool the fluid and the control system 60 may be used to regulate the temperature of the fluid, thereby regulating the degree of baroreceptor 30 signal dampening. Electrical means may be used to inhibit baroreceptor 30 activation by, for example, hyperpolarizmg cells in or adjacent to the baroreceptors 30. Examples of devices and method of hyperpolarizmg cells are disclosed in U.S. Patent No. 5,814,079 to Kieval, and U.S. Patent No. 5,800,464 to Kieval, the entire disclosures of which are hereby incorporated by reference. Such electrical means may be implemented using any of the embodiments discussed with reference to Figures 13-18 and 21. Chemical or biological means may be used to reduce the sensitivity of the baroreceptors 30. For example, a substance that reduces baroreceptor sensitivity may be delivered using the fluid delivery device 260 described previously. The desensitizing agent may comprise, for example, tetiodotoxin or other inhibitor of excitable tissues. From the foregoing, it should be apparent to those skilled in the art that 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 or by inhibiting/dampening baroreceptor signals. Thus, the present invention may be used to increase or decrease blood pressure, sympathetic nervous system activity and neurohormonal activity, as needed to minimize deleterious effects on the heart, vasculature and other organs and tissues. The baroreceptor activation devices described previously may also be used to provide antiaπhythmic effects. It is well known that the susceptibility of the myocardium to the development of conduction disturbances and malignant cardiac aπhythmias is influenced by the balance between sympathetic and parasympathetic nervous system stimulation to the heart. That is, heightened sympathetic nervous system activation, coupled with decreased parasympathetic stimulation, increases the iπitability of the myocardium and likelihood of an aπhythmia. Thus, by decreasing the level of sympathetic nervous system activation and enhancing the level of parasympathetic activation, the devices, systems and methods of the cuπent invention may be used to provide a protective effect against the development of cardiac conduction disturbances. For each of these applications, it may be desirable to focus the output of the activation device 70 on portions of the carotid sinus 20 that are rich in baroreceptors 30, and minimize the output delivered to portions of the carotid sinus 20 with fewer or no baroreceptors 30. By focusing the output as such, baroreceptor activation may be maximized and the required device output (i.e., the required power or energy output of the baroreceptor activation device 70) may be minimized. In particular, the ratio of baroreceptor activation to device output (A/O) may be maximized. In addition, by focusing the output as such, extraneous tissue activation may be minimized, power consumption (by the device 70) may minimized, and the degradation rate of baroreceptor responsiveness may be minimized. It has been found that the A O ratio is a function of the position of the baroreceptor activation device. In particular, it has been found that the A/O ratio varies about the circumference of the carotid artery near the carotid sinus 20, perhaps due to variations in the location or density of baroreceptors. Although described herein with reference to the carotid sinus 20, it is also likely that the A O ratio varies at all of the anatomical locations which contain baroreceptors as described previously. In order to position the baroreceptor activation device 70 to maximize the A/O ratio, a mapping technique may be employed. For example, the device 70 may be oriented in two or more different positions and/or at two or more different anatomical locations. More specifically, the output means of the device 70 may be disposed in two or more different positions/locations. The output means generally refers to the structure through which the stimulus is transfeπed to the tissue suπounding the baroreceptors. In electrical activation embodiments, for example, the output means may comprise electrodes. At each position/location, the device 70 may be activated to a specified level, and the degree of baroreceptor activation may be observed or measured. The degree of baroreceptor activation may be inferentially determined by measuring changes in heart rate, blood pressure, and/or other physiological parameters indicative of baroreceptor activation. The resulting measurements may be used to generate an A/O ratio for each position/location. The A/O ratios for each location may be graphically plotted to generate a map. The A/O ratios may be compared, and the position/location having the most desirable A O ratio may be selected for the device 70. To illustrate this mapping method, reference may be made to Figures 33 - 35. By way of example, not limitation, the mapping method is described with specific reference to the arteries, but the method is equally applicable to all anatomical structures containing baroreceptors. Figure 33 shows the right carotid arteries including the common 14, internal 18, and external 19 carotid arteries. The carotid sinus 20 may be highlighted by a bulge 21, which typically extends from the common carotid artery 14 to the internal carotid artery 18 near the bifurcation. The carotid sinus 20 contains a significant number of baroreceptors, the number and density of which may vary around the circumference and along the length of the sinus 20. As such, it is desirable to determine the optimal position for the baroreceptor activation device 70, both in terms of circumferential and longitudinal position. The mapping method described herein is equally applicable to all baroreceptor activation devices 70, regardless of the mode of activation (mechanical, electrical, thermal, chemical, biological, or other means) and regardless of their invivo position (intravascular, extiavascular, intramural). By way of example, not limitation, the device 70 is shown in Figure 34 as an extiavascular electrical device 500 having two electrodes 520 which contact the outside wall of the carotid sinus 20 at two different locations. The device 500 includes a molded silicone housing 512. The housing 512 carries two metal strips 510 which are separated by approximately 4 mm and are formed of platinum ribbon (0.040 in. wide by 0.0005 in. thick by 10 mm long). The metal strips 510 are insulated by the housing 512 except at the 1 mm wide exposed area 516. The metal strips 510 in the exposed area 516 define two electrodes 520 that contact the outside surface of the carotid artery. Leads 514 couple the metal strips 510 to cable 502 which is connected to a control system 60 as described previously with reference to Figure 3. With the device 500 disposed about the carotid arteries as shown in Figure 34, the device 500 may be activated to produce an output signal from the electiodes 520, which in turn activates the baroreceptors, as evidenced by a change in heart rate and/or blood pressure. The position and/or location of the electrodes 520 is recorded along with the amount of output (e.g., power) and the coπesponding change in the heart rate, blood pressure and/or other physiological parameters indicative of baroreceptor activation. From this information, the A/O ratio may be determined for this particular position/location. The electrodes 520 of the device 500 are then oriented in a different position (e.g., rotated) and/or placed at a different anatomical location, and the same measurements are made. These steps are repeated to collect the desired amount of data, which may be graphically plotted to generate a map to determine an optimal position/location. The A/O ratios may be compared, and the position/location having the most desirable A/O ratio may be selected for the device 500. As an alternative to device 500, a hand held probe or similar device incorporating electrodes 520 may be used to permit easier manipulation and quicker changes between different locations/positions. To keep track of different circumferential positions around the carotid arteries, a coordinate system may be used as shown in Figure 35. Figure 35is a schematic cross-sectional view taken along line 35-35 in Figure 34, showing a mapping coordinate system for the left carotid artery 15 and right carotid artery 14. In this coordinate system, the left carotid artery 15 and right carotid artery 14 are viewed in cross-section looking from the head of the patient toward the feet, with 0° positioned anteriorly and 180° positioned posteriorly. The center or apex of the left bulge 21L which identifies the left carotid sinus 20L is typically located at 110° to 160°. The center or apex of the right bulge 21R which identifies the right carotid sinus 20R is typically located at 200° to 250°. This coordinate system is particularly useful for mapping the circumference of the carotid arteries, in addition to other arteries and tubular organs. To further illustrate this method, an animal experiment was performed utilizing device 500 on the left and right carotid arteries of an animal. The device 500 was wrapped around the carotid artery near the carotid sinus 20 at the bifurcation of the internal 18 and external 19 carotid arteries, substantially as shown in Figure 34. Using the coordinate system described with reference to Figure 35, the center of the bulge 21L of the left carotid sinus 20L for this animal was located at 120°, and the center of the bulge 21R of the right carotid sinus 20R was located at 200°. The electrodes 520 were rotated around the left carotid artery at 90°, 120°, 180°, and 270° positions. The electrodes 520 were rotated around the right carotid artery at 155°, 180°, 200°, 220°,and 255° positions. At each position, the electrodes 520 were activated with a 4 volt signal, and the mean arterial pressure (MAP) and heart rate (HR) were measured. The data from the right side is graphically illustiated in Figure 36, and the data from the left side is graphically illustrated in Figure 37. This data suggest that the responsiveness (i.e., the degree of baroreceptor activation) is non-homogenous and unpredictable around the circumference of the carotid arteries. However, from this data, it is possible to locate both hot spots (large A O ratio) and dead spots (low A/O ratio) around the circumference of the carotid arteries. For example, on the right side,there appears to be a dead zone between 155° and 180°, and a dead spot around 255°. Also on the right side, there appears to be a hot zone between 190° and 220°. On the left side, there appears to be a hot zone from 90° to 180°, and a dead spot near 270°. Thus, there is variability of the A/O ratio around the circumference of the carotid arteries, between the right and left sides, and probably between patients. Because of this variability, the mapping method described herein may be beneficial to positioning the baroreceptor activation device for optimal A/O ratio. It is also contemplated that the device 500 may have many individually controllable electrodes 520 disposed around a large area of the carotid sinus (e.g., around the entire circumference), as described elsewhere herein. The electrodes 520 may be individually activated, and the coπespondmg baroreceptor response for each electrode may be determined. The electrodes 520 having the most desirable A/O ratio(s) may then be selected for chronic use. This method negates the need to reposition or relocate the device to find the optimal A/O ratio. This method also enables the electiode selection to be changed after implantation without the need to change the position/location of the device 500 in a subsequent clinical procedure. 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: What is claimed is: 1. A baroreceptor activation device for activating a baroreceptor in a carotid sinus of a patient, the carotid sinus having a circumference, the device comprising: a base having a length sufficient to extend around at least a substantial portion of the circumference of the carotid sinus; and one or more electrodes connected to the base, the electrodes having a length less than the length of the base. 2. A baroreceptor activation device as in claim 1, comprising more than two electrodes positioned adjacent each other on the base. 3. A baroreceptor activation device as in claim 2, wherein the electrodes comprise electrode pads which are distributed about at least a portion of the base in a grid pattern. 4. A baroreceptor activation device as in claim 3, wherein each pad defines two concentric electrodes. 5. A baroreceptor activation device as in claim 2, wherein the electiodes comprise elongate electrodes which extend adjacent to and parallel with each other on the base. 6. A baroreceptor activation device as in claim 5, wherein the electrodes are positioned substantially parallel to the length of the base. 7. A baroreceptor activation device as in claim 5, wherein the electiodes are positioned substantially orthogonal to the length of the base. 8. A baroreceptor activation device as in claim 2, wherein the electrodes comprise non-linear elongate electiodes which extend adjacent to each other on the base. 9. A baroreceptor activation device as in claim 1, wherein the electrodes extend less than 75% of the length of the base. 10. A baroreceptor activation device as in claim 1, wherein the electrodes extend less than 50% of the length of the base. 11. A baroreceptor activation device as in claim 1, wherein the electiodes extend less than 25% of the length of the base. 12. A baroreceptor activation device as in claim 1, wherein the base has first and second ends, and wherein the ends are adapted to be joined. 13. A baroreceptor activation device as in claim 1, wherein the base has sufficient structural integrity to grasp the carotid sinus. 14. A baroreceptor activation device for activating a baroreceptor in a carotid sinus of a patient, the device comprising at least one electrode having a nonlinear shape along its length, the electrode assembly adapted to be placed on the carotid sinus such that the non-linear shaped length runs generally orthogonal to a longitudinal axis of the carotid sinus. 15. A baroreceptor activation device for activating a baroreceptor in a carotid sinus of a patient, the device comprising a plurality of electiodes each with a length and a width, the length being substantially greater then the width, the electrodes being adjacent and generally parallel to one another, the electrode assembly adapted to be placed on the carotid sinus such that the length of the electrodes runs generally parallel to a longitudinal axis of the carotid sinus. 16. A baroreceptor activation device for activating a baroreceptor in a carotid sinus of a patient, the device comprising a plurality of concentric electrodes, the electiode assembly adapted to be placed on the carotid sinus. 17. A baroreceptor activation device, comprising: a base; a plurality of electrodes connected to the base; and a means for chronically fixing the base to a carotid sinus vascular wall. 18. A system for inducing a baroreceptor signal to affect a change in the baroreflex system in a patient, the system comprising: a baroreceptor activation device positioned proximate a baroreceptor such that activation of the baroreceptor activation device induces a baroreceptor signal; and a contiol system connected to the baroreceptor activation device, the control system including a processor and a memory, wherein the memory includes software defining a stimulus regimen, the control system generating a contiol signal as a function of the stimulus regimen, the stimulus regimen causing a change in the control signal to promote long term efficacy of inducing baroreceptor signals. 19. A system as in claim 18, wherein the stimulus regimen dictates that the control signal have a first higher level delivered for a first period of time, and a second lower level delivered for a second period of time, wherein the first level is sufficient to attain a beneficial physiologic change and wherein the second level is sufficient to maintain the beneficial physiologic change. 20. A system as in claim 18, wherein baroreceptor activation device has a first electrode and a second electrode, and wherein the stimulus regimen dictates that the electiodes be alternatively activated, deactivated or otherwise modulated. 21. A system as in claim 18, wherein the stimulus regimen dictates a start time for the control signal, and wherein the start time varies. 22. A system for inducing a baroreceptor signal from a baroreceptor in a vascular wall of a patient to affect a change in the baroreflex system, the system comprising: a baroreceptor activation device adapted to be connected to the vascular wall, the baroreceptor activation device adapted to mechanically, thermally, chemically or biologically induce the baroreceptor signal. 23. A system as in claim 22, further comprising: a control system connected to the baroreceptor activation device, the contiol system generating a control signal to activate, deactivate or otherwise modulate the baroreceptor activation device. 24. A system as in claim 23, further comprising: a sensor for generating a signal indicative of a need to modify the patient's baroreflex system, the sensor connected to the control system, wherein the contiol signal is a function of the sensor signal. 25. A system as in claim 24, wherein the control system includes a processor and a memory, and wherein the memory includes software containing an algorithm defining a relationship between the control signal and the sensor signal, and wherein the baroreceptor activation device is activated in accordance with the algorithm. 26. A system as in claim 25, wherein the baroreceptor activation device comprises an expansion device which causes the vascular wall to expand. 27. A system as in claim 25, wherein the baroreceptor activation device comprises a compression device which causes the vascular wall to compress. 28. A system as in claim 25, wherein the baroreceptor activation device comprises a flow regulator positioned distal of the baroreceptor to create back pressure proximate the baroreceptor. 29. A system as in claim 25, wherein the baroreceptor activation device comprises a deformation device which causes the vascular wall to deform. 30. A system as in claim 25, wherein the baroreceptor activation device comprises a cyclic dynamic device which causes the vascular wall to vibrate. 31. A system as in claim 25, wherein the baroreceptor activation device comprises a localized fluid delivery device for delivering an agent which activates or increases the sensitivity of the baroreceptor. 32. A system for treating high blood pressure comprising: a signal generator for generating a control signal; and an intiavascular stent positioned proximate a baroreceptor, the signal generator transmitting the control signal to the stent to induce a baroreceptor signal.

Inventors: BURNS MATTHEW M, HANSEN GINA L, HEKTNER THOMAS R, IRWIN ERIC D, KEITH PETER T, KIEVAL ROBERT S, PERRSON BRUCE J, SERDAR DAVID J, BURNS, MATTHEW, M., HANSEN, GINA, L., HEKTNER, THOMAS, R., IRWIN, ERIC, D., KEITH, PETER, T., KIEVAL, ROBERT, S., PERRSON, BRUCE, J., SERDAR, DAVID, J.
Applicants: CVRX INC, CVRX, INC.
Application year: 2001
Main classification: A61N1
IPC codes: A61F2, A61N1