Abstract:
Apparatus is provided for treating hypertension of a subject. The apparatus includes an implantable element which has a non-circular shape and which is configured to reduce the hypertension by facilitating an assumption of a non-circular shape by a blood vessel in a vicinity of a baroreceptor of the subject, during diastole of the subject. Other embodiments are also described.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
       [0001]     The present patent application is a continuation-in-part of International Patent Application. PCT/IL2006/000856 to Gross (WO 07/013065), filed Jul. 25, 2006, entitled, “Electrical stimulation of blood vessels,” which claims the benefit of (a) U.S. Provisional Application 60/702,491, filed Jul. 25, 2005, entitled, “Electrical stimulation of blood vessels,” and (b) U.S. Provisional Application 60/721,728, filed Sep. 28, 2005, entitled, “Electrical stimulation of blood vessels.” All of the above applications are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention generally relates to implanted medical apparatus. Specifically, the present invention relates to apparatus and methods for reducing blood pressure.  
       BACKGROUND OF THE INVENTION  
       [0003]     Hypertension is a condition from which many people suffer. It describes a constant state of elevated blood pressure which can be caused by a number of factors, for example, genetics, obesity or diet. Baroreceptors located in the walls of blood vessels act to regulate blood pressure. They do so by sending information to the central nervous system (CNS) regarding the extent to which the blood vessel walls are stretched by the pressure of the blood flowing therethrough. In response to these signals, the CNS adjusts certain parameters so as to maintain a stable blood pressure.  
         [0004]     US Patent Application Publication 2003/0060858 to Kieval et al., which is incorporated herein by reference, describes 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 is positioned near a baroreceptor, for example a baroreceptor in the carotid sinus. A control system may be used to modulate the baroreceptor activation device. The control system may utilize an algorithm defining a stimulus regimen which promotes long term efficacy and reduces power requirements/consumption.  
         [0005]     US Patent Application Publication 2005/0154418 to Kieval et al., which is incorporated herein by reference, describes systems and methods to provide baroreflex activation to treat or reduce pain and/or to cause or enhance sedation or sleep. Methods involve activating the baroreflex system to provide pain reduction, sedation, improved sleep or some combination thereof. Systems include at least one baroreflex activation device, at least one sensor for sensing physiological activity of the patient, and a processor coupled with the baroreflex activation device(s) and the sensor(s) for processing sensed data received from the sensor and for activating the baroreflex activation device. In some embodiments, the system is described as being fully implantable within a patient, such as in an intravascular, extravascular or intramural location.  
         [0006]     US Patent Application Publication 2006/0074453 to Kieval et al., which is incorporated herein by reference, describes a method for treating heart failure in a patient which involves activating a baroreflex system of the patient with at least one baroreflex activation device and resynchronizing the patient&#39;s heart with a cardiac resynchronization device. Activating the baroreflex system and resynchronizing the heart may be performed simultaneously or sequentially, in various embodiments. In some embodiments, one or more patient conditions are sensed and such condition(s) may be used for setting and/or modifying the baroreflex activation and/or heart resynchronization. A device for treating heart failure includes a baroreflex activation member coupled with a cardiac resynchronization member. Some embodiments further include one or more sensors and a processor. In some embodiments, the device is fully implantable.  
         [0007]     US Patent Application Publication 2005/0027346 to Arkusz et al., which is incorporated herein by reference, describes a tubular vascular stent graft with a passively pulsating midsection where the difference between the cross-sectional areas of the lumen under the systolic and diastolic pressures after the implantation is 10% or more. The pulsating stent graft accumulates blood during the systolic pressure wave thus lowering the peak value of the tugging force at the proximal attachment site.  
         [0008]     PCT Publication WO 03/076008 to Shalev, which is incorporated herein by reference, describes an implantable device which uses the carotid baroreflex in order to control systemic blood pressure. The implant includes sampling and pulse stimulation electrodes preferably located on the carotid sinus nerve branch of the glossopharyngeal nerve, adjacent and distal to the carotid sinus baroreceptors. The stimulators have an external control unit, which communicates with the implant for determining appropriate operational parameters, and for retrieving telemetry information from the device&#39;s data bank. Typically, two internal devices are implanted, one at each side of the patient&#39;s neck.  
         [0009]     PCT Publication WO 04/073484 to Gross et al., which is incorporated herein by reference, describes apparatus which includes an inflatable bladder, adapted to be coupled to a blood vessel of a subject carrying oxygenated blood, such that an interior of the bladder is in fluid communication with the blood. The apparatus also includes a piston in mechanical communication with the bladder; a motor, adapted to synchronize contraction and expansion of the bladder with a cardiac cycle of the subject by applying a motor force to the piston; and a spring, adapted to apply a spring force to the piston. In some embodiments of the invention, a counterpulsation system comprises one or more springs, which are adapted to be inserted into an artery of a subject, such as a descending aorta. Typically, each of the springs is planar, i.e., flat rather than helical, and has a generally sinusoidal shape. For applications comprising more than one spring, the plurality of springs are arranged in substantially a single plane. The counterpulsation system causes the artery to have a cross-sectional area during diastole that is less than the cross-sectional area would be during diastole without use of the counterpulsation system. For example, the counterpulsation system may cause the artery to have a cross-sectional shape during diastole that generally resembles an ellipse. Use of the counterpulsation system is described as thus typically increasing diastolic blood pressure and decreasing systolic blood pressure, thereby providing counterpulsation treatment to the circulation of the subject.  
         [0010]     CVRx (Minneapolis, Minn.) manufactures the CVRx® Rheos Baroreflex Hypertension Therapy System, an implantable medical device for treating patients with high blood pressure. The product, which is under clinical investigation, works by electrically activating the baroreceptors, the sensors that regulate blood pressure. These baroreceptors are located on the carotid artery and in the carotid sinus. CVRx states that when the baroreceptors are activated by the Rheos System, signals are sent to the central nervous system and interpreted as a rise in blood pressure. The brain works to counteract this perceived rise in blood pressure by sending signals to other parts of the body to reduce blood pressure, including the heart, kidneys and blood vessels.  
         [0011]     The following patents and patent applications, which are incorporated herein by reference, may be of interest:  
         [0012]     US Patent Application Publication 2005/0033407 to Weber et al.  
         [0013]     European Patent 0,791,341 to Demeyere et al.  
         [0014]     PCT Publication WO 06/032902 to Caro et al.  
         [0015]     U.S. Pat. No. 7,044,981 to Liu et al.  
         [0016]     US Patent Application Publication 2005/0203610 to Tzeng  
         [0017]     US Patent Application Publication 2004/0193092 to Deal  
         [0018]     U.S. Pat. No. 6,575,994 to Marin et al.  
         [0019]     US Patent Application Publication 2005/0232965 to Falotico  
         [0020]     US Patent Application Publication 2004/0106976 to Bailey et al.  
         [0021]     U.S. Pat. No. 4,938,766 to Jarvik  
         [0022]     U.S. Pat. No. 4,201,219 to Bozal Gonzalez  
         [0023]     U.S. Pat. No. 3,650,277 to Sjostrand et al.  
         [0024]     U.S. Pat. No. 4,791,931 to Slate  
       SUMMARY OF THE INVENTION  
       [0025]     As people age, their blood vessels become more rigid, and, as a result, the baroreceptor response to changes in blood pressure decreases. The CNS interprets the low baroreceptor response as resulting from a low blood pressure, and responds by increasing blood pressure. This phenomenon can cause or exacerbate hypertension. Embodiments of the present invention reduce hypertension by increasing the changes in shape of given arteries during the cardiac cycle. Doing so increases the baroreceptor signaling to the CNS, and the CNS interprets the increased baroreceptor signaling as having resulted from elevated blood pressure. In response, the CNS acts to lower blood pressure.  
         [0026]     In some embodiments of the present invention, an element having an elliptical or other non-circular cross-section is placed near a baroreceptor in a blood vessel of a subject who has hypertension. The elliptical element changes the shape of the blood vessel such that the blood vessel is generally elliptical during diastole and less elliptical (e.g., generally circular) during systole.  
         [0027]     In some embodiments of the invention, the non-circular element comprises a stent. Alternatively or additionally, the non-circular element comprises a ring, or a plurality of rings. For some applications, the one or more rings are used as the non-circular element in order to reduce the total surface contact between the element and the blood vessel, which, in turn, limits fibrosis between the element and the blood vessel. Alternatively, as when the element comprises a stent, the contact surface area is not necessarily minimized, and the one or more rings are used as the non-circular element for a different purpose.  
         [0028]     In an embodiment, the ring is flexible and flexes in coordination with the cardiac cycle of the subject. In a further embodiment, a control unit is configured to detect the real-time blood pressure of the subject and to drive current, via the element, toward the baroreceptor, responsively to the detected blood pressure. Alternatively or additionally, the apparatus comprises a dedicated electrode, and current is driven toward the baroreceptor, via the dedicated electrode, responsively to the detected blood pressure.  
         [0029]     In an embodiment, the cross-section of the ring is altered in response to the detection of real-time blood pressure of the subject. For example, if the blood pressure of the subject increases as a result of the subject undergoing a stressful experience, a blood pressure detector detects the increase. The detected increase in blood pressure results in the eccentricity of the ring being increased.  
         [0030]     In some patients, the baroreceptor adapts to the presence of the ring within the blood vessel, and reverts toward its original firing rate. In some embodiments of the invention, the eccentricity of the ring is modified periodically, in response to measurements of resting blood pressure of the subject. For example, a balloon may be transcatheterally inserted into the inside of the ring. The balloon is inflated to modify the cross-section of the ring.  
         [0031]     In some embodiments, an embolic protection device is inserted into the blood vessel during the implantation of the non-circular element. Typically, the embolic protection device comprises a mesh, and the mesh is placed distal to the non-circular element. The mesh is typically inserted into the blood vessel transcatheterally.  
         [0032]     There is therefore provided, in accordance with an embodiment of the invention, apparatus for treating hypertension of a subject, including an implantable element which has a non-circular shape and which is configured to reduce the hypertension by facilitating an assumption of a non-circular shape by a blood vessel in a vicinity of a baroreceptor of the subject, during diastole of the subject.  
         [0033]     In an embodiment, the element includes a non-circular stent.  
         [0034]     In an embodiment, the element includes a single non-circular ring.  
         [0035]     In an embodiment, the element includes a plurality of non-circular rings.  
         [0036]     In an embodiment, the element includes a plurality of non-circular rings which are not connected to each other.  
         [0037]     In an embodiment, the element includes a plurality of non-circular rings which are not rigidly connected to each other.  
         [0038]     In an embodiment, the element is rigid.  
         [0039]     In an embodiment, the apparatus includes a control unit configured to detect real-time blood pressure of the subject.  
         [0040]     In an embodiment, the control unit is configured to be implantable in a body of the subject.  
         [0041]     In an embodiment, the control unit is configured to drive current, via the element, toward the baroreceptor, in response to the detected blood pressure.  
         [0042]     In an embodiment, the apparatus includes an electrode, and the control unit is configured to drive current, via the electrode, toward the baroreceptor, in response to the detected blood pressure.  
         [0043]     In an embodiment, the control unit is configured to change the cross-section of the element in response to the detected blood pressure.  
         [0044]     In an embodiment, the element includes a plurality of rings which are coupled to each other.  
         [0045]     In an embodiment, the apparatus includes a single rod, and the rings are coupled to each other by the single rod.  
         [0046]     In an embodiment, the apparatus includes exactly two rods, and the rings are coupled to each other by the exactly two rods.  
         [0047]     In an embodiment, the apparatus includes three or more rods, and the rings are coupled to each other by the three or more rods.  
         [0048]     In an embodiment, the element includes two rings which are coupled to each other and which are separated from each other by a distance that is between 5 mm and 20 mm.  
         [0049]     In an embodiment, the element includes two rings which are coupled to each other and which are separated from each other by a distance that is between 20 mm and 50 mm.  
         [0050]     In an embodiment, the element is flexible.  
         [0051]     In an embodiment, the element is configured to flex in coordination with a cardiac cycle of the subject.  
         [0052]     In an embodiment, the element is configured to flex passively in coordination with the cardiac cycle of the subject.  
         [0053]     In an embodiment, the apparatus includes a control unit configured to detect the cardiac cycle of the subject and to flex the element in coordination with the cardiac cycle.  
         [0054]     In an embodiment, the apparatus includes a shaping element configured to shape the non-circular element while the non-circular element is in the blood vessel.  
         [0055]     In an embodiment, the shaping element includes a balloon.  
         [0056]     In an embodiment, the shaping element includes an elliptical balloon.  
         [0057]     In an embodiment, the apparatus includes an embolic protection device configured to capture emboli during implanting of the element.  
         [0058]     In an embodiment, the embolic protection device includes a mesh.  
         [0059]     There is additionally provided, in accordance with an embodiment of the invention, a method for reducing hypertension of a subject, including:  
         [0060]     coupling an element having a non-circular cross-section to a blood vessel of the subject in a vicinity of a baroreceptor of the subject, by implanting the element; and  
         [0061]     reducing the hypertension by facilitating, with the element, an assumption of a non-circular shape by the blood vessel in the vicinity, during diastole of the subject.  
         [0062]     In an embodiment, implanting the element includes implanting in separate implantation steps, at respective longitudinal sites of the blood vessel in the vicinity of the baroreceptor, a plurality of rings having non-circular cross-sections.  
         [0063]     In an embodiment, implanting the element includes implanting, at respective longitudinal sites of the blood vessel in the vicinity of the baroreceptor, a plurality of rings which are coupled to each other, the rings having non-circular cross-sections.  
         [0064]     In an embodiment, implanting the element includes implanting the element during minimally-invasive surgery.  
         [0065]     In an embodiment, implanting the element includes placing a stent inside the blood vessel on one side of the baroreceptor, the stent having a non-circular cross-section.  
         [0066]     In an embodiment, implanting the element includes placing a ring inside the blood vessel on one side of the baroreceptor, the ring having a non-circular cross-section.  
         [0067]     In an embodiment, the method includes detecting blood pressure of the subject and changing the cross-section of the non-circular element in response to the detected blood pressure.  
         [0068]     In an embodiment, detecting the blood pressure includes detecting the blood pressure of the subject more frequently than once a week.  
         [0069]     In an embodiment, detecting the blood pressure includes detecting the blood pressure of the subject less frequently than once a week.  
         [0070]     In an embodiment, detecting the blood pressure includes detecting real time blood pressure of the subject, and changing the cross-section of the element includes changing the cross-section of the element in response to the detected real time blood pressure.  
         [0071]     In an embodiment, detecting the blood pressure includes detecting resting blood pressure of the subject, and changing the cross-section of the element includes changing the cross-section of the element in response to the detected resting blood pressure.  
         [0072]     In an embodiment, changing the cross-section of the element includes expanding a balloon within the element.  
         [0073]     In an embodiment, changing the cross-section of the element includes expanding an elliptical balloon within the element.  
         [0074]     In an embodiment, changing the cross-section of the element includes driving a current toward the element.  
         [0075]     In an embodiment, the element includes first and second rings having non-circular cross-sections, and implanting the element includes implanting the first ring on one side of the baroreceptor and implanting the second ring on another side of the baroreceptor.  
         [0076]     In an embodiment, implanting the first ring and the second ring includes implanting the first and second rings, the rings not being connected to each other.  
         [0077]     In an embodiment, implanting the first ring and the second ring includes implanting the first and second rings, the rings not being rigidly connected to each other.  
         [0078]     In an embodiment, implanting the first ring and the second ring includes implanting the first and second rings, the rings being coupled to each other.  
         [0079]     In an embodiment, implanting the first ring and the second ring includes implanting the first and second rings at a longitudinal distance from each other that is between 5 mm and 20 mm.  
         [0080]     In an embodiment, implanting the first ring and the second ring includes implanting the first and second rings at a longitudinal distance from each other that is between 20 mm and 50 mm.  
         [0081]     There is additionally provided, in accordance with an embodiment of the invention, a method for reducing hypertension of a subject, including:  
         [0082]     coupling a ring having a non-circular cross-section to a blood vessel of the subject in a vicinity of a baroreceptor of the subject, by implanting the ring; and  
         [0083]     reducing the hypertension by facilitating, with the ring, an assumption of a non-circular shape by the blood vessel in the vicinity, during diastole of the subject.  
         [0084]     In some embodiments, implanting the ring includes implanting the ring during minimally-invasive surgery.  
         [0085]     In some embodiments, the ring includes a rigid ring, and implanting the ring includes implanting the rigid ring.  
         [0086]     In some embodiments, the method includes detecting blood pressure of the subject and changing the cross-section of the non-circular ring in response to the detected blood pressure.  
         [0087]     In some embodiments, detecting the blood pressure includes detecting the blood pressure of the subject more frequently than once a week.  
         [0088]     In some embodiments, detecting the blood pressure includes detecting the blood pressure of the subject less frequently than once a week.  
         [0089]     In some embodiments, detecting the blood pressure includes detecting real time blood pressure of the subject, and changing the cross-section of the ring includes changing the cross-section of the ring in response to the detected real time blood pressure.  
         [0090]     In some embodiments, detecting the blood pressure includes detecting resting blood pressure of the subject, and changing the cross-section of the ring includes changing the cross-section of the ring in response to the detected resting blood pressure.  
         [0091]     In some embodiments, changing the cross-section of the ring includes expanding a balloon within the ring.  
         [0092]     In some embodiments, changing the cross-section of the ring includes expanding an elliptical balloon within the ring.  
         [0093]     In some embodiments, changing the cross-section of the ring includes driving a current toward the ring.  
         [0094]     In some embodiments, the ring includes a flexible ring, and implanting the ring includes implanting the flexible ring.  
         [0095]     In some embodiments, the ring is configured to flex in response to a cardiac cycle of the subject, and implanting the ring includes implanting the ring that is configured to flex in response to the cardiac cycle.  
         [0096]     In some embodiments, the ring is configured to flex passively in coordination with the cardiac cycle of the subject, and implanting the ring includes implanting the ring that is configured to flex passively in coordination with the cardiac cycle.  
         [0097]     In some embodiments, the ring is coupled to a control unit, the control unit being configured to detect the cardiac cycle of the subject and to flex the ring in coordination with the cardiac cycle, and implanting the ring includes implanting the ring that is coupled to the control unit.  
         [0098]     In some embodiments, the method includes detecting real-time blood pressure of the subject and driving a current toward the baroreceptor responsively to the detected blood pressure.  
         [0099]     In some embodiments, driving the current includes driving the current via the ring.  
         [0100]     In some embodiments, driving the current includes driving the current via an electrode.  
         [0101]     In some embodiments, the method includes providing embolic protection during the implanting.  
         [0102]     In some embodiments, providing the embolic protection includes placing a mesh within the blood vessel.  
         [0103]     There is additionally provided, in accordance with an embodiment of the invention, apparatus for treating hypertension of a subject, including an implantable ring which has a non-circular shape and which is configured to reduce the hypertension by facilitating an assumption of a non-circular shape by a blood vessel in a vicinity of a baroreceptor of the subject, during diastole of the subject.  
         [0104]     In some embodiments, the ring is rigid.  
         [0105]     In some embodiments, the apparatus includes a control unit configured to detect real-time blood pressure of the subject.  
         [0106]     In some embodiments, the control unit is configured to be implantable in a body of the subject.  
         [0107]     In some embodiments, the control unit is configured to drive current, via the ring, toward the baroreceptor, in response to the detected blood pressure.  
         [0108]     In some embodiments, the apparatus includes an electrode, wherein the control unit is configured to drive current, via the electrode, toward the baroreceptor, in response to the detected blood pressure.  
         [0109]     In some embodiments, the control unit is configured to change the cross-section of the ring in response to the detected blood pressure.  
         [0110]     In some embodiments, the ring is flexible.  
         [0111]     In some embodiments, the ring is configured to flex in coordination with a cardiac cycle of the subject.  
         [0112]     In some embodiments, the ring is configured to flex passively in coordination with the cardiac cycle of the subject.  
         [0113]     In some embodiments, the apparatus includes a control unit configured to detect the cardiac cycle of the subject and to flex the ring in coordination with the cardiac cycle.  
         [0114]     In some embodiments, the apparatus includes a shaping element configured to shape the non-circular ring while the non-circular ring is in the blood vessel.  
         [0115]     In some embodiments, the shaping element includes a balloon.  
         [0116]     In some embodiments, the shaping element includes an elliptical balloon.  
         [0117]     In some embodiments, the apparatus includes an embolic protection device configured to capture emboli during implanting of the ring.  
         [0118]     In some embodiments, the embolic protection device includes a mesh.  
         [0119]     The present invention will be more fully understood from the following detailed description of embodiments thereof, taken together with the drawings, in which:  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0120]      FIGS. 1A and 1B  are schematic illustrations of a non-circular rigid implant element inside a blood vessel during diastole and during systole, respectively, in accordance with an embodiment of the present invention;  
         [0121]      FIGS. 1C and 1D  are schematic illustrations of a non-circular flexible implant element inside a blood vessel during diastole and during systole, respectively, in accordance with another embodiment of the present invention;  
         [0122]      FIG. 2  is a schematic illustration of two non-circular rings which are coupled to each other, in accordance with an embodiment of the invention;  
         [0123]      FIG. 3  is a schematic illustration of two non-circular rings which are coupled to each other, in accordance with another embodiment of the invention;  
         [0124]      FIGS. 4A and 4B  are schematic illustrations of a balloon inside a non-circular ring, the balloon in deflated and inflated states thereof, respectively, in accordance with an embodiment of the present invention;  
         [0125]      FIGS. 5A and 5B  are schematic illustrations of apparatus for increasing the rate of firing of a baroreceptor, in accordance with respective embodiments of the invention; and  
         [0126]      FIG. 6  is a schematic illustration of an implant element having embolic protection, in accordance with an embodiment of the invention.  
     
    
     DETAILED DESCRIPTION OF EMBODIMENTS  
       [0127]     Reference is now made to  FIGS. 1A and 1B , which are schematic illustrations of a non-circular implant element  20  disposed within a blood vessel  30  of a subject, in accordance with an embodiment of the invention. Typically, the element is implanted into the aorta or the carotid artery of the subject.  FIG. 1A  shows the blood vessel during diastole and  FIG. 1B  shows the blood vessel during systole. Typically, the element includes one or more elliptical rigid rings, and/or an elliptical stent. The element is placed within the blood vessel in the vicinity of a baroreceptor and causes an increase in the change in shape which the blood vessel would in any case undergo during the cardiac cycle.  
         [0128]     In some embodiments, the element is placed as close as possible to the baroreceptor, e.g., within 1 cm or 2 cm of the baroreceptor. The implanting is typically performed during minimally-invasive surgery, e.g., using a transcatheter approach.  
         [0129]     Reference is now made to  FIGS. 1C and 1D , which are schematic illustrations of non-circular element  20 , in accordance with another embodiment of the present invention. In some embodiments (as shown), non-circular element  20  is flexible and flexes passively in coordination with the cardiac cycle. Blood vessel  30  changes the shape of element  20  from being non-circular during diastole ( FIG. 1C ), to being more circular during systole ( FIG. 1D ). For example, element  20  may be generally circular during systole, or generally elliptical, with lower eccentricity than during diastole. In all other aspects element  20  is generally the same as described hereinabove.  
         [0130]     Reference is now made to  FIGS. 2 and 3 , which are schematic illustrations of implant element  20 , comprising two non-circular rings  22  and  24 , which are coupled to each other by a single rod  26  ( FIG. 2 ) or two or more rods  26  and  28  ( FIG. 3 ), in accordance with respective embodiments of the invention. Typically, the width D 1  of each of the rings is between 2 mm and 6 mm, e.g., 4 mm, and the rings are implanted at a longitudinal separation D 2  from each other, along the blood vessel, which is between about 5 mm and 20 mm, or between about 20 mm and 50 mm. During diastole, the ratio of length D 4  of the major axis of the ellipse to length D 3  of the minor axis is typically between 1.5:1 and 2.5:1, e.g., 2:1. For some applications, the rings are implanted such that one ring is disposed within the blood vessel on one side of the baroreceptor and the second ring is disposed within the blood vessel on the other side of the baroreceptor. In some embodiments, the two rings are not connected to each other and are implanted in separate implantation steps. In alternative embodiments, the two rings are coupled to each other by three or more rods.  
         [0131]     Reference is now made to  FIGS. 4A and 4B , which are schematic illustrations of a shaping balloon  42 , inside non-circular ring  22 , in accordance with an embodiment of the present invention. In  FIG. 4A , the balloon is deflated, and in  FIG. 4B , the balloon is inflated. In some patients, baroreceptors adapt to a ring being deployed within a blood vessel (as described herein) and revert toward their original firing rate. In an embodiment of the invention, periodic measurements are made of the subject&#39;s resting blood pressure. If the blood pressure of the subject has increased, the eccentricity of the cross-section of the ring is increased by inflation of shaping balloon  42 . Typically, the balloon is inserted transcatheterally into the inside of the ring, and the balloon is inflated. The balloon expands and permanently increases the eccentricity of the cross-section of the implanted ring. Alternatively, if it is determined that the eccentricity of the ring is having too great an effect on resting blood pressure, the balloon is inflated in a manner to decrease eccentricity (e.g., by increasing the minor axis of the ring).  
         [0132]     Reference is now made to  FIGS. 5A and 5B , which are schematic illustrations of apparatus for increasing the rate of firing of a baroreceptor, in accordance with respective embodiments of the invention. The apparatus comprises elliptical ring  22 , which is implanted in blood vessel  30 , and control unit  52 . In  FIGS. 5A  and SB, blood vessel  30  is shown during systole. In  FIG. 5A  the control unit is coupled to the ring, and in  FIG. 5B , the control unit is coupled to an electrode  54 . In some embodiments, the control unit is configured to detect real-time blood pressure of the subject. The control unit is configured to drive a current into the blood vessel to excite the baroreceptor, in a transient manner, in response to real-time blood pressure measurements. For example, the control unit may detect a transient increase in blood pressure as a result of the subject undergoing a stressful experience. In response, the control unit excites the baroreceptor. The current is typically driven into the blood vessel via the ring ( FIG. 5A ) and/or via the electrode ( FIG. 5B ). Alternatively, or additionally, the control unit is configured to transiently modulate the eccentricity of the ring in response to the real-time blood pressure measurements. For example, the ring may comprise mechanical deforming elements (e.g., piezoelectric elements), and the control unit actuates the deforming elements to transiently alter the eccentricity of the ring.  
         [0133]     In some embodiments, the ring is flexible, and the control unit is configured to detect the cardiac cycle of the subject and to flex the ring in coordination with the cardiac cycle, to enhance baroreceptor firing and blood pressure reduction.  
         [0134]     Reference is now made to  FIG. 6 , which is a schematic illustration of an embolic protection device  60  disposed within the blood vessel during implantation of element  20 , in accordance with an embodiment of the present invention. The implant element and the embolic protection device are placed in blood vessel  30  in the vicinity of a baroreceptor. Typically, the embolic protection device comprises a mesh. During the implanting of the implant element, the embolic protection device is inserted into the blood vessel distal to the implant element. Embolic protection device  60  is typically inserted into the blood vessel via a catheter  40 . The protection device prevents embolisms, caused by the implanting of the implant element, from occluding blood vessels of the subject. Following implantation of element  20 , embolic protection device  60  is removed.  
         [0135]     It is to be understood that use of a non-circular plurality of rings is described herein by way of illustration and not limitation, and that the scope of the present invention includes the use of a plurality of rings that are circular in cross-section.  
         [0136]     It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof that are not in the prior art, which would occur to persons skilled in the art upon reading the foregoing description.