Patent Publication Number: US-2010114244-A1

Title: Electrical renal autonomic blockade

Description:
TECHNICAL FIELD 
     The disclosure relates to medical devices and, more particularly, medical devices that deliver electrical stimulation. 
     BACKGROUND 
     A wide variety of implantable medical devices for delivering a therapy or monitoring a physiologic condition of a patient have been clinically implanted or proposed for clinical implantation in patients. Some implantable medical devices may employ one or more elongated electrical leads and/or sensors. Such implantable medical devices may deliver therapy or monitor the heart, muscle, nerve, brain, stomach or other organs. In some cases, implantable medical devices may deliver electrical stimulation therapy and/or monitor physiological signals via one or more electrodes or sensor elements, which may be included as part of one or more elongated implantable medical leads. Implantable medical leads may be configured to allow electrodes or sensors to be positioned at desired locations for delivery of stimulation or sensing electrical signals. For example, electrodes or sensors may be located at a distal portion of the lead. A proximal portion of the lead may be coupled to an implantable medical device housing, which may contain electronic circuitry such as stimulation generation and/or sensing circuitry. 
     In patients with heart failure or hypertension, renal sympathetic activity has been shown to be markedly elevated. Elevated renal sympathetic activity may result in renal vasoconstriction, increased retention of sodium, as well as an increase in release of renin and angiotensin. Increased sodium, rennin and angiotensin in turn further exacerbate heart failure and hypertension by increasing blood volume and arterial hypertension, and triggering signs and symptoms of cardio-pulmonary congestion, such as edema or peripheral fluid accumulation. Furthermore, chronic elevation of renal sympathetic tone in various disease states, i.e., with our without heart failure, may play a role in the development of overt renal failure and end-stage renal disease. 
     Efforts to control renal sympathetic activity have included administration of medications such as angiotensin-converting enzyme inhibitors angiotensin II receptor blockers and beta-blockers. Such medications may have a broader effect than controlling the renin angiotensin aldosterone system (RAAS). Furthermore, symptoms associated with elevated renal sympathetic activity may persist despite such medications. 
     SUMMARY 
     In general, the disclosure relates to delivering electrical stimulation to decrease renal sympathetic activity. Renal sympathetic activity may worsen symptoms of heart failure, hypertension, and/or chronic renal failure. For example, renal sympathetic activity may increase fluid retention by the kidneys, which in turn increases blood volume, arterial hypertension, and pulmonary congestion. Electrical stimulation may be configured to decrease renal sympathetic activity by creating at least a partial functional conduction block in the efferent and/or afferent sympathetic nerve fibers that innervate the kidneys. 
     In some examples, a sensor may sense a physiological parameter of the patient, and the stimulation generator may activate, deactivate, or adjust the stimulation signal based on the physiological parameter. The physiological parameter may be indicative of sympathetic activity within the patient. Examples of physiological parameters that may indicate the level of sympathetic activity within the patient include blood pressure, blood flow, vascular tone, plasma renin level, or norepinephrine level. The parameters may be measured proximate to the renal system, such as a renal artery blood pressure or blood flow, or elsewhere within the patient. In some examples, information regarding the physiological parameter, or information derived therefrom, such as information regarding the progression or status of heart failure, renal failure, hypertension, or autonomic tone, may be transmitted to an external device, such as a programmer or server, for presentation to a clinician or other user. 
     In one aspect, the disclosure is directed to a method comprising sensing a physiological parameter of a patient, generating a stimulation signal via an implantable electrical stimulator based on the physiological parameter, delivering the stimulation signal from the implantable stimulator to a renal nerve of the patient, transmitting information regarding the physiological parameter to an external device outside of the patient, and presenting the information to a user. 
     In another aspect, the disclosure is directed to a system comprising an implantable sensor that senses a physiological parameter of a patient, an implantable electrical stimulator that communicates with the implantable sensor, wherein the implantable electrical stimulator comprises a stimulation generator that generates a stimulation signal based on the physiological parameter and delivers the stimulation signal to a renal nerve of the patient, and an external device that receives information regarding the physiological parameter and presents the information to a user. 
     In another aspect, the disclosure is directed to a method comprising sensing a physiological parameter indicative of sympathetic activity within a patient, identifying an increase in sympathetic activity based on the physiological parameter, and delivering a stimulation signal to a renal nerve of the patient in response to the increase in sympathetic activity. 
     In another aspect, the disclosure is directed to a system comprising a sensor that senses a physiological parameter indicative of sympathetic activity within a patient, a processor that identifies an increase in sympathetic activity based on the physiological parameter, and an electrical stimulator that delivers a stimulation signal to a renal nerve of the patient in response to the increase in sympathetic activity. 
     In another aspect, the disclosure is directed to a method for inhibiting renal sympathetic activity comprising generating a biphasic stimulation signal with a frequency of approximately 100 hertz to approximately 20 kilohertz and delivering the stimulation signal to a renal nerve of the patient. 
     In another aspect, the disclosure is directed to a system comprising a signal generator that generates a biphasic stimulation signal with a frequency of approximately 100 hertz to approximately 20 kilohertz and an electrode configured to be positioned proximate to a renal nerve of the patient, wherein the signal generator delivers the stimulation signal to the renal nerve via the electrode. 
     The details of one or more examples of the disclosure are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a conceptual diagram illustrating an example therapy system that delivers stimulation therapy to decrease renal sympathetic activity. 
         FIG. 2  is a conceptual diagram illustrating the therapy system of  FIG. 1  in greater detail. 
         FIG. 3  is a functional block diagram of an example implantable medical device. 
         FIG. 4  is a block diagram of an example medical device programmer. 
         FIG. 5  is a flow diagram illustrating an example technique for delivering electrical stimulation to a patient to decrease renal sympathetic activity. 
         FIG. 6  is a flow diagram illustrating an example technique for modifying stimulation delivery based on a sensed physiological parameter. 
         FIG. 7  is a flow diagram illustrating an example technique for sensing physiological parameters. 
         FIG. 8  is a block diagram illustrating an example system that includes an external device, such as a server, and one or more computing devices that are coupled to the medical device and programmer shown in  FIG. 1  via a network. 
     
    
    
     DETAILED DESCRIPTION 
     In patients with heart failure, hypertension, and chronic renal failure, renal sympathetic activity may be elevated. As one example, the decreased cardiac output that results from heart failure may decrease circulation to the kidneys. The kidneys are responsible for maintaining blood volume and may perceive the decreased circulation as a decrease in blood volume. To counteract the perceived decrease in blood volume, the renal system may increase renal sympathetic activity. Renal sympathetic activity increases sodium retention to increase blood volume and increases the release of renin and angiotensin to increase blood pressure. The increased blood volume and blood pressure may further exacerbate heart failure and hypertension as well as trigger signs and symptoms of cardio-pulmonary congestion. As the symptoms of heart failure worsen, the renal system may respond by further increasing renal sympathetic activity. 
     Electrical stimulation may be configured to decrease renal sympathetic activity by creating at least a partial functional conduction block in the efferent and/or afferent sympathetic nerve fibers that innervate the kidneys. The blockade may reversibly interrupt neural signals between the central nervous system and the renal nerves that innervate the kidneys. For example, when an implantable medical device (IMD) delivers electrical stimulation, renal sympathetic tone may be reduced. Renal sympathetic tone may return when the IMD ceases stimulation delivery. In some examples, the conduction of sympathetic neural signals between the central nervous system and the renal nerves is substantially completely blocked when the IMD delivers electrical stimulation. Reducing renal sympathetic activity may increase renal blood flow, increase renal sodium excretion, and/or decrease renin release from the kidneys. 
       FIG. 1  is a conceptual diagram illustrating an example therapy system  10  that provides stimulation therapy to patient  12 . Patient  12  ordinarily, but not necessarily, will be a human. Therapy system  10  includes an IMD  14 , which is coupled to lead  16  and programmer  18 . In the example illustrated in  FIG. 1 , lead  16  is bifurcated into two distal segments, i.e., branches,  16 A and  16 B. In other examples, lead  16  may be unbranched. Although IMD  14  is illustrated in the example of  FIG. 1 , in other examples an external medical device positioned outside of patient  12  may provide the functionality of IMD  14 . 
     IMD  14  may generate and deliver electrical stimulation e.g., in the form of electrical pulses or a substantially continuous signal, to decrease renal sympathetic activity of patient  12 . For example, IMD  14  may generate and deliver stimulation to a nerve or other tissue site of patient  12 , e.g., proximate to kidneys  20 A and  20 B (collectively “kidneys  20 ”), via one or more electrodes (not shown in  FIG. 1 ) carried on distal segments  16 A and  16 B of lead  16  and/or one or more electrodes on an outer housing of IMD  14 . The renal nerves that innervate kidneys  20  may exit spinal cord  22  proximate to kidneys  20 . IMD  14  may generate and deliver stimulation to a renal nerve proximate to spinal cord  22  and/or kidneys  20  to decrease renal sympathetic activity. 
     In the example shown in  FIG. 1 , electrodes on distal portions  16 A and  16 B of lead  16  are positioned to deliver bilateral electrical stimulation, e.g., to nerves that innervate both kidneys  20 . IMD  14  may deliver the same or different therapy to kidneys  20 A and  20 B. For example, IMD  14  may sense parameters indicative of renal sympathetic activity proximate to each of kidneys  20 A and  20 B and separately control stimulation delivery via each of distal segments  16 A and  16 B of lead  16  based on the sensed parameters. In other example therapy systems, IMD  14  may be coupled to two or more leads, e.g., furcated and/or non-furcated leads, either directly or indirectly, e.g., via a lead extension. For example, IMD  14  may be coupled to two non-furcated leads such that a distal end of one lead is positioned proximate to kidney  20 A and the distal end of the other lead is positioned proximate to kidney  20 B. As another example, IMD  14  may be coupled to a single unbranched lead for applications in which IMD  14  delivers unilateral stimulation to a single side of patient  12 , e.g., to the nerves that innervate either kidney  20 A or kidney  20 B. 
     IMD  14  may sense electrical signals associated with the sympathetic activity of the nerves that innervate kidneys  20  via electrodes carried by lead  16 . For example, IMD  14  may monitor an electrogram (EGM) signal of a renal nerve to determine the level of renal sympathetic activity within patient  12 . In some examples, IMD  14  may monitor separate signals from distal segments  16 A and  16 B of lead  16  to evaluate renal sympathetic activity proximate to kidneys  20 A and  20 B individually. The configurations of electrodes used by IMD  14  for sensing may be unipolar or bipolar. 
     Additionally or alternatively, therapy system  10  may include other sensors (not shown in  FIG. 1 ) to monitor sympathetic activity, e.g., systemic and/or renal sympathetic activity. For example, therapy system  10  may include one or more intravascular sensors in communication with IMD  14 . Intravascular sensors, such as chemical sensors, may monitor the blood chemistry of patient  12 , e.g., a plasma renin level or norepinephrine level within the blood of patient  12 . Other intravascular sensors, such as a strain gauge, capacitive pressure sensor, ultrasonic flow sensor, or electrodes for determining impedance, may monitor blood pressure, blood flow, and/or assess vascular tone in a blood vessel of patient  12 . 
     Although intravascular sensors are described primarily herein, therapy system  10  may include any appropriate intravascular or extravascular sensor to detect these or any other physiological parameters indicative of sympathetic activity. For example, one or more sensors may be implanted in extravascular spaces of patient  12 , such as the intraperitoneal space within the abdominal cavity of patient  12 . Sensors implanted in extravascular spaces may permit monitoring of blood parameters without requiring intravascular implantation. 
     The sensors of therapy system  10  may monitor systemic sympathetic activity and/or renal sympathetic activity. For example, a sensor may be implanted within the superior vena cava that supplies blood to the heart of patient  12  to monitor systemic sympathetic activity of patient  12 . In contrast, a sensor may be implanted within a renal vessel, e.g., a renal artery or renal vein, to monitor renal sympathetic activity. Sensors positioned proximate to a renal nerve of patient  12 , e.g., proximate to kidneys  20  in the abdomen of patient  12 , may monitor renal sympathetic tone. Sensors that monitor renal sympathetic tone may provide IMD  14  with more specific feedback than sensors that monitor systemic sympathetic tone. 
     In the example of  FIG. 1 , IMD  14  has been implanted in the chest cavity of patient  12 . Other implant locations are also contemplated, such as in the back or abdominal cavity of patient  12 . IMD  14  may be, for example, subcutaneously or submuscularly implanted in the body of patient  12  at any appropriate location. Upon implantation of IMD  14 , the proximal end of lead  16  may be both electrically and mechanically coupled to connector  24  of IMD  14  either directly or indirectly, e.g., via a lead extension. In particular, conductors disposed in the lead body of lead  16  may electrically connect stimulation electrodes (and sense electrodes, if present) of lead  16  to IMD  14 . 
     In some examples, external programmer  18  may be a handheld computing device or a computer workstation. Programmer  18  may include a user interface that receives inputs from a user. The user interface may include, for example, a keypad and a display, which may for example, be a cathode ray tube (CRT) display, a liquid crystal display (LCD) or light emitting diode (LED) display. The keypad may take the form of an alphanumeric keypad or a reduced set of keys associated with particular functions. Programmer  18  may additionally or alternatively include a peripheral pointing device, such as a mouse, via which a user may interact with the user interface. In some examples, a display of programmer  18  may include a touch screen display, and a user may interact with programmer  18  via the display. 
     A user, such as patient  12 , a physician, technician, or other clinician, may interact with programmer  18  to communicate with IMD  14 . The user may interact with programmer  18  to retrieve physiological or diagnostic information from IMD  14 . For example, the user may use programmer  18  to retrieve information from IMD  14  regarding sensed physiological parameters of patient  12  indicative of renal sympathetic activity, such as electrical signals, e.g., EGM signals, blood pressure, or the like. IMD  14  may transfer information to programmer  18  regarding diagnostic information determined based on the sensed physiological parameters, such as renal function and heart failure status, for view by a user, e.g., a clinician and/or patient  12 . A user may also interact with programmer  18  to program IMD  14 , e.g., select values for operational parameters of IMD  14  based on the sensed physiological parameters received from IMD  14 . 
     The user may use programmer  18  to program therapy parameters for electrical stimulation. The therapy parameters may include an electrode combination for delivering stimulation signals, as well as an amplitude, which may be a current or voltage amplitude, and, if IMD  14  delivers electrical pulses, a pulse width, and a pulse rate for stimulation signals to be delivered to patient  12 . The electrode combination may include a selected subset of one or more electrodes located on implantable lead  16  coupled to IMD  14  and/or a housing of IMD  14 . The electrode combination may also refer to the polarities of the electrodes in the selected subset. By selecting particular electrode combinations, a clinician may target particular anatomic structures within patient  12 , such as the renal nerves. In addition, by selecting values for amplitude, pulse width, and pulse rate, the physician can attempt to generate an efficacious therapy for patient  12  that is delivered via the selected electrode subset. 
     As another example, the user may use programmer  18  to retrieve information from IMD  14  regarding the performance or integrity of IMD  14  or other components of therapy system  10 , such as lead  16  or a power source of IMD  14 . With the aid of programmer  18  or another computing device, a user may select values for therapy parameters for controlling therapy delivery by IMD  14 . The values for the therapy parameters may be organized into a group of parameter values referred to as a “therapy program” or “therapy parameter set.” “Therapy program” and “therapy parameter set” are used interchangeably herein. 
     Programmer  18  may communicate with IMD  14  via wireless communication using any techniques known in the art. Examples of communication techniques may include, for example, low frequency or radiofrequency (RF) telemetry, but other techniques are also contemplated. In some examples, programmer  18  may include a programming head that may be placed proximate to the patient&#39;s body near the IMD  14  implant site in order to improve the quality or security of communication between IMD  14  and programmer  18 . 
       FIG. 2  is a conceptual diagram illustrating IMD  14  and lead  16  of therapy system  10  in greater detail. In the example illustrated in  FIG. 2 , distal segment  16 A of lead  16  is implanted in left renal vein  26 A, and distal segment  16 B of lead  16  is implanted in right renal vein  26 B. In this manner, electrodes  38 A on distal segment  16 A may deliver stimulation to renal nerves  28 A, and electrodes  38 B on distal segment  16 B may deliver stimulation to renal nerves  28 B. 
     Renal nerves  28 A and  28 B (collectively “renal nerves  28 ”) illustrate the approximate location of the nerves that innervate kidneys  20 . For example, renal nerves  28  may exit spinal cord  22  approximately at the level of kidneys  20  and may approach kidneys  20  in a similar manner as renal arteries  32 A and  32 B (collectively “renal arteries  32 ”) and renal veins  26  (collectively “renal veins  26 ”). Renal nerves  28  may lie directly adjacent to renal arteries  32 . Renal nerves  28 , as described herein, may refer to the renal plexus as a whole, any individual nerve of the renal plexus, and/or any other nerve that innervates kidneys  20 . 
     To aid in positioning distal segments  16 A and  16 B proximate to renal nerves  28 A and  28 B, respectively, lead  16  may be inserted into inferior vena cava  30 . Once lead  16  is inserted into inferior vena cava  30 , distal segment  16 A may be guided into left renal vein  26 A using a first guidewire and distal segment  16 B may be guided into right renal vein  26 B using a second guidewire. Methods and systems for guiding distal segments of a bifurcated lead to different tissue sites are described in further detail in U.S. Pat. No. 7,142,919 to Hine at el., which issued on Nov. 28, 2006 and is entitled, “RECONFIGURABLE FAULT TOLERANT MULTIPLE-ELECTRODE CARDIAC LEAD SYSTEM,” and is incorporated herein by reference in its entirety. 
     Although distal segments  16 A and  16 B are implanted within renal veins  26  in the example of  FIG. 2 , in other examples distal segments  16 A and  16 B may be implanted at any other location proximate to renal nerves  28 . As one example, distal segments  16 A and  16 B may be implanted within renal arteries  32 A and  32 B, respectively. In some examples, lead  16  may be inserted through one of common iliac arteries  36  that branch off of abdominal aorta  34  to allow guidewires to direct distal segments  16 A and  16 B to renal arteries  32 A and  32 B. Since renal nerves  28  may lie adjacent to renal veins  26  and renal arteries  32 , implanting distal segments  16 A and  16 B within renal veins  26  and/or renal arteries  32  may allow IMD  14  to stimulate renal nerves  28 . 
     Renal veins  26  are larger than renal arteries  32  and may allow for easier intravascular lead implantation compared to renal arteries  32 . On the other hand, renal arteries  32  may be located closer than renal veins  26  to renal nerves  28 . Thus, stimulation from electrodes implanted within renal veins  26  may more easily capture renal nerves  28 . 
     In the example illustrated in  FIG. 2 , distal segment  16 A includes one or more electrodes  38 A and distal segment  16 B includes one or more electrodes  38 B. Electrodes  38 A may be ring electrodes that extend substantially completely around the circumference of distal segment  16 A or partial ring electrodes that extend partially around the circumference of the distal segment  16 A. Partial ring electrodes may be useful in directing electrical stimulation in a particular direction, e.g., toward renal nerves  28 A. Electrodes  38 B may also be ring or partial ring electrodes. The number, configuration, and type of electrodes  38 A and  38 B (collectively “electrodes  38 ”) illustrated in  FIG. 2  are merely exemplary. Other examples may include any configuration, number, or type of electrodes  38 . 
     IMD  14  may also include one or more housing electrodes, such as housing electrode  40 , which may be formed integrally with an outer surface of a hermetically-sealed housing of IMD  14  or otherwise coupled to the housing of IMD  14 . In some examples, housing electrode  40  is defined by an uninsulated portion of an outward facing portion of the housing of IMD  14 . In some examples, housing electrode  40  comprises substantially all of the IMD housing. Other divisions between insulated and uninsulated portions of the housing may be employed to define two or more housing electrodes. 
     IMD  14  may deliver electrical stimulation to renal nerves  28  via any combination of electrodes  38  and housing electrode  40 , e.g., any unipolar or multipolar electrode configuration, to decrease renal sympathetic activity. Each of electrodes  38  may be individually activated by IMD  14  to deliver stimulation using a variety of electrode configurations. In some examples, IMD  14  may deliver a stimulation signal between one of electrodes  38  and housing electrode  40 , i.e., in a unipolar configuration. As another example, IMD  14  may deliver a stimulation signal between a plurality of electrodes  38 , e.g., in a multipolar configuration. IMD  14  may deliver the same or different stimulation signal to both sets of electrodes  38 A and  38 B, i.e., to deliver bilateral stimulation. As another example, IMD  14  may deliver a stimulation signal to one set of electrode  38 A and  38 B, i.e., to deliver unilateral stimulation. 
     Distal segments  16 A and/or  16 B may include one or more fixation elements to prevent migration of distal segments  16 A and/or  16 B. For example, distal segment  16 A may include an expandable fixation element, e.g., an expandable stent or cage. The expandable fixation element may be inserted into inferior vena cava  30  in an unexpanded configuration and expanded to engage an inner surface of renal vein  26 A once distal segment  16 A is properly placed within renal vein  26 A. The fixation element may fixate distal segment  16 A within renal vein  26 A without impeding blood flow within renal vein  26 A. 
     In some examples, the fixation element may be conductive and IMD  14  may use the fixation element as an electrode to stimulate renal nerve  28 A. In other examples, the fixation element may include a plurality of electrically isolated conductive portions such that IMD  14  may independently activate the various conductive portions as electrodes for sensing and/or stimulation. Distal segment  16 B may also include a fixation element to fixate distal segment  16 B within renal vein  26 B. Although expandable fixation elements are described for purposes of example, distal segments  16 A and  16 B may include any appropriate type of fixation element. Additionally, the fixation elements may be sized and configured to fixate distal segments  16 A and  16 B in other vessels of patient  12 , e.g., renal arteries  32 . 
     In other examples, distal segments  16 A and/or  16 B may be positioned extravascularly. For example, electrodes  3   8 A and  3   8 B of distal segments  16 A and  16 B may be included on cuff electrode assemblies that wrap at least partially around renal nerves  28 A and  28 B, respectively. Since renal nerves  28  may include fibers that run in close proximity to renal veins  26  and renal arteries  32 , the cuff electrode assemblies may be implanted around renal veins  26  and/or renal arteries  32  instead of directly around renal nerves  28 . A cuff electrode assembly may include a U-shaped cross section configured to fit about a selected portion of the circumference of a nerve, e.g., one of renal nerves  28 , or vessel, e.g., one of renal veins  26  or renal arteries  32 . A cuff electrode assembly may also include one or more conductive portions that serve as electrodes  38 . Examples of cuff electrode assemblies are described in U.S. Pat. No. 5,344,438 to Testerman et al., which issued on Sep. 4, 1994 and is entitled, “Cuff Electrode,” and is incorporated herein by reference in its entirety. 
     In yet other examples, distal segments  16 A and/or  16 B may be transvascularly positioned renal nerves  28 A and  28 B. For example, electrodes  38 A and  38 B of distal segments  16 A and  16 B may be positioned extravascularly although other portions of distal segments  16 A and  16 B may be implanted intravascularly. As one example, lead  16  may be inserted into superior vena cava  30 , distal segment  16 A may be guided into right renal vein  26 A, and distal segment  16 B may be guided into left renal vein  26 B. The portions of distal segments  16 A and  16 B carrying electrodes  38 A and  38 B may be guided through walls of respective veins  26  such that electrodes  38 A and  38 B are positioned extravascularly proximate to renal nerves  28 A and  28 B, respectively. Transvascular implantation of electrodes is described in further detail in U.S. patent application Ser. No. 10/411,891 by Lamson et al., which was filed on Apr. 11, 2003, is entitled, “Devices and Methods for Transluminal or Transthoracic Interstitial Electrode Placement,” and is incorporated herein by reference in its entirety. 
     IMD  14  may sense electrical signals attendant to the sympathetic activity of renal nerves  28  that innervate kidneys  20  via electrodes  38  and/or housing electrode  40 . For example, IMD  14  may monitor electrogram (EGM) signals of renal nerves  28  to determine the level of renal sympathetic activity within patient  12 . In some examples, IMD  14  may monitor separate signals from one or more of electrodes  38 A on the right side of patient  12  and one or more of electrodes  38 B on the left side of patient  12  to evaluate renal sympathetic activity associated with kidneys  20 A and  20 B individually. The configurations of electrodes used by IMD  14  for sensing may be unipolar or bipolar. 
     Additionally or alternatively, lead  16  may include other sensors to monitor physiological parameters indicative of sympathetic tone. In the example illustrated in  FIG. 2 , distal segment  16 A of lead  16  includes sensor  39 A proximate to kidney  20 A, and distal segment  16 B of lead  16  includes sensor  39 B proximate to kidney  20 B. Other examples any include any number or configuration of sensors  39 . For example, lead  16  may include one or more chemical sensors  39  to monitor blood chemistry, e.g., to monitor norepinephrine levels and/or plasma renin levels, within patient  12 . Lead  16  may also include one or more sensors  39  to detect blood pressure, blood flow, and/or assess vascular tone within one or more vessels, e.g., inferior vena cava  30 , renal veins  26 , renal arteries  32 , and/or common iliac arteries  36 , of patient  12 . In some examples, sensors  39  are positioned within renal vessels, e.g., renal veins  26  and/or renal arteries  32 , or otherwise proximate to kidneys  20  to monitor renal sympathetic activity. Monitoring renal sympathetic tone may provide more specific feedback to IMD  14  than monitoring systemic sympathetic tone. In some examples, therapy system  10  may include sensors  39  that are intravascularly implanted within patient  12  but not carried by lead  16 . Such sensors may be in wired and/or wireless communication with IMD  14 . 
     In some examples, therapy system  10  includes one or more sensors  39  implanted in extravascular spaces, such as the intraperitoneal space within the abdominal cavity, of patient  12 . Sensors  39  implanted in extravascular spaces may permit monitoring of blood parameters without requiring intravascular implantation. Such sensors  39  may be in wired and/or wireless communication with IMD  14 . In examples in which lead  16  is implanted extravascularly, these sensors  39  may be carried by lead  16 . 
     IMD  14  may use the physiological signals sensed by electrodes  38  and/or other intravascular and/or extravascular sensors  39  to control stimulation delivery to renal nerves  28 . For example, IMD  14  may initiate, modify, or cease stimulation delivery based on one or more sensed physiological parameters. As one example, IMD  14  may identify an increase in sympathetic activity based on one or more sensed physiological parameters and deliver a stimulation signal to renal nerves  28  in response to the increase in sympathetic activity. For example, IMD  14  may initiate stimulation delivery or modify the stimulation parameters in response to the detected increase in sympathetic activity. IMD  14  may modify one or more stimulation parameters, e.g., electrode configuration, amplitude, pulse width, and/or pulse rate, to increase the intensity of the stimulation signal in response to the detected increase in sympathetic activity. 
     IMD  14  may identify a level of sympathetic activity based on one or more physiological signals from one or more sensors, e.g., electrodes  38  or sensors  39 . For example, IMD  14  may identify an increase in sympathetic activity by detecting an increase in plasma renin levels, e.g., within a renal vessel, and deliver a stimulation signal in response to the detection. IMD  14  may monitor the magnitude and time-course of changes in one or more physiological signals, such as plasma renin levels, renal blood flow, or other biomarkers, to identify changes in sympathetic activity of patient  12 . IMD  14  may monitor the magnitude and time-course of changes in one or more physiological signals while IMD  14  delivers stimulation to decrease renal sympathetic activity to investigate the effectiveness of stimulation delivery and/or the effectiveness of the physiological signals in measuring the effectiveness of stimulation delivery. 
     In some examples, IMD  14  may use two or more physiological signals to monitor the sympathetic activity of patient  12 . As one example, IMD  14  may monitor a norepinephrine level with the patient&#39;s blood, e.g., within a renal vessel. Elevated norepinephrine levels may indicate elevated sympathetic activity. If the norepinephrine level rises above a threshold, IMD  14  may monitor renal blood flow and/or renal blood pressure, e.g., within renal arteries  32 . Since sympathetic efferent activation causes renal vasoconstriction and a reduction in renal blood flow, blood flow and/or blood pressure in a renal vessel may indicate the level of renal sympathetic activity. If blood flow to kidneys  20  is decreased and/or renal blood pressure is increased, IMD  14  may identify an increase in sympathetic activity and deliver a stimulation signal to renal nerves  28 . Once the blood flow and/or blood pressure return to normal, IMD  14  may switch back to monitoring norepinephrine levels. The methods of sensing physiological parameters and identifying increases in sympathetic activity described herein are merely examples. 
     In other examples, IMD  14  may utilize a plurality of sensors, e.g., electrodes  38  and/or sensors  39 , in complimentary and/or orthogonal manners to detect changes in sympathetic activity and regulate stimulation delivery. For example, IMD  14  may sense a first physiological parameter. When the first physiological parameter indicates increased sympathetic activity, IMD  14  may sense a second physiological parameter. The second physiological parameter may be used to confirm the increase in sympathetic activity. For example, IMD  14  may only identify an increase in sympathetic activity when both the first and second physiological parameters indicate increased sympathetic activity. 
     In general, IMD  14  may identify changes in the sympathetic activity level of patient  12  based on one or more sensed physiological parameters and control stimulation delivery to renal nerves  28  in response to the identified changes. In some examples, the sensed physiological parameters indicate renal sympathetic activity, and IMD  14  identifies changes in renal sympathetic activity. In some examples, IMD  14  may maintain sympathetic activity below a threshold level by adjusting stimulation delivery based on the sensed sympathetic physiological parameters. IMD  14  may use the sensed physiological parameters to determine when patient  12  requires stimulation and the minimum level of stimulation required to maintain renal sympathetic activity below a desired level. IMD  14  may sense physiological parameters on the right and left sides of patient  12 , e.g., proximate to kidneys  20 A and  20 B, and/or control stimulation delivery to the right and left sides of patient  12 , e.g., renal nerves  28 A and  28 B, individually. 
     Additionally or alternatively, IMD  14  may control stimulation delivery based on parameters other than sensed physiological parameters. For example, IMD  14  may deliver stimulation during specific portions of the day, e.g., according to a schedule. The intensity of the stimulation may be preprogrammed, e.g., via programmer  18 , or responsive to sensed physiological parameters. As one example, a schedule may specify therapy intensities and/or therapy parameters for certain portions of the day instead of or in addition to specifying which portions of the day IMD  14  delivers stimulation. Alternatively, patient  12  may activate IMD  14 , e.g., via programmer  18 , to deliver stimulation when needed. Again, the intensity of the stimulation may be preprogrammed, e.g., via programmer  18 , or responsive to sensed physiological parameters. 
     In some examples, IMD  14  may modify one or more stimulation parameters over time to prevent or minimize accommodation. For example, IMD  14  may deliver stimulation signals using different electrode combinations, waveforms, amplitudes, or frequencies to prevent or minimize accommodation. In examples in which IMD  14  delivers electrical pulses, IMD  14  may also deliver signals with different pulse widths and/or pulse rates to prevent or minimize accommodation. 
     In some examples, IMD  14  delivers a high frequency, biphasic stimulation signal to renal nerves  28  to decrease renal sympathetic activity. For example, IMD  14  may generate a stimulation signal with a frequency of approximately 100 hertz to approximately 20 kilohertz and deliver the stimulation signal to renal nerves  28  of patient  12 . In some examples, the stimulation signal may have a frequency of approximately 100 hertz to approximately 10 kilohertz. In other examples, IMD  14  may generate a stimulation signal with a frequency of approximately 2 kilohertz or higher. The stimulation signal may have a voltage amplitude of approximately 0.5 volts to approximately 20 volts and, in some examples, a voltage amplitude of approximately 0.5 volts to approximately 10 volts. Alternatively, the stimulation signal may have a current amplitude of approximately 1 to approximately 12 milliamperes. A biphasic stimulation signal has portions with opposite polarities, e.g., positive and negative portions. 
     High-frequency biphasic electrical stimulation may create a reversible functional conduction block in the efferent and afferent nerve fibers that innervate kidneys  20 , e.g., renal nerves  28 . High-frequency biphasic electrical stimulation may be effective in producing reversible nerve conduction block in unmyelinated nerve fibers, such as the unmyelinated post-ganglionic nerve fibers of the renal sympathetic nerves. Biphasic electrical stimulation may also be charge-balanced, and thereby prevent and/or reduce corrosion of electrodes  38 . 
     IMD  14  may use alternating current (AC) to deliver stimulation signals to reduce renal sympathetic activity. High-frequency AC stimulation has been shown to produce block of nerve conduction in motor nerves and may also be effective at producing conduction block in renal sympathetic nerves, e.g., renal nerves  28 . IMD  14  may also use monopolar and/or multipolar electrode configurations to achieve at least partial conduction block in renal nerves  28 . Example stimulation waveforms that IMD  14  may utilize to achieve at least partial renal nerve blockage include sinusoidal waveforms, square waveforms, and other continuous time signals. As an alternative, IMD  14  may deliver stimulation in the form of pulses. 
     In some examples, IMD  14  delivers high voltage stimulation in addition to or as an alternative to high frequency stimulation. High voltage stimulation may use voltages significantly higher than the physiological voltages renal nerves  28  use to conduct neural signals. For example, IMD  14  may deliver high voltage stimulation at approximately  15  volts or higher. High voltage stimulation may stun renal nerves  28  and at least partially prevent renal nerves  28  from conducting neural signals. High voltage stimulation may utilize direct current (DC) signals and may be configured to minimize damage to renal nerves  28 . 
     As another example, IMD  14  may deliver stimulation to create a unidirectional or collision block. In this manner, IMD  14  may deliver stimulation signals that propagate in a direction that opposes the efferent neural signals traveling toward kidneys  20 . The stimulation signals delivered by IMD  14  may collide with the neural signals traveling from the central nervous system of patient  12  to kidneys  20  and at least partially prevent conduction of the efferent neural signals. IMD  14  may configure at least some of electrodes  38  as anodes and cathodes to achieve collision block in renal nerves  28 . In other examples, IMD  14  may deliver stimulation signal that propagate in a direction that opposes the afferent neural signals traveling from kidneys  20  to the central nervous system to at least partially block the afferent neural signals from reaching the central nervous system of patient  12 . 
       FIG. 3  is a functional block diagram illustrating various components of IMD  14  according to one example. In the example of  FIG. 3 , IMD  14  includes processor  50 , memory  52 , signal generator module  54 , sensing module  56 , telemetry module  58 , and power source  60 . Telemetry module  58  may permit communication with programmer  18  to receive, for example, new therapy programs or adjustments to therapy programs. Telemetry module  58  may also permit communication with programmer  18  to transfer, for example, sensed physiological parameters to programmer  18 . 
     Memory  52  includes computer-readable instructions that, when executed by processor  50 , cause IMD  14  and processor  50  to perform various functions attributed to IMD  14  and processor  50  herein. Memory  52  may include any volatile, non-volatile, magnetic, optical, or electrical media, such as a random access memory (RAM), read-only memory (ROM), non-volatile RAM (NVRAM), electrically-erasable programmable ROM (EEPROM), flash memory, or any other digital media. As described in further detail below, memory  52  may store, for example, diagnostic information  61  regarding sensed physiological parameters, therapy programs  62  defining therapy parameters for stimulation delivery, sensor functions  63  including instructions for sensing sympathetic activity of patient  12 , and/or schedules  64  that define when to deliver stimulation therapy. 
     Processor  50  may include any one or more of a microprocessor, a controller, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or equivalent discrete or integrated logic circuitry. In some examples, processor  50  may include multiple components, such as any combination of one or more microprocessors, one or more controllers, one or more DSPs, one or more ASICs, or one or more FPGAs, as well as other discrete or integrated logic circuitry. The functions attributed to processor  50  herein may be embodied as software, firmware, hardware or any combination thereof. 
     Processor  50  controls operation of IMD  14 , e.g., controls signal generator module  54  to deliver stimulation therapy according to a selected one or more therapy programs  62 , which may be stored in memory  52 . For example, processor  50  may control signal generator module  54  to deliver electrical signals with current or voltage amplitudes, pulse widths (if applicable), and rates specified by one or more stimulation programs  62 . Processor  50  may also control signal generator module  54  to deliver the stimulation signals via subsets of the electrodes  38  and  40  with polarities, the subsets and polarities specified as electrode combinations or configurations by one or more therapy programs  62 . In some examples, signal generator module  54  includes two stimulation generators  54 A and  54 B to deliver separate stimulation signals to renal nerves  28 A and renal nerves  28 B, respectively. Processor  50  may control signal generators  54 A and  54 B to deliver the same or different stimulation signals to renal nerves  28 A proximate to kidney  28 A on the right side of patient  12  and renal nerves  28 B proximate to kidney  28 B on the left side of patient  12 . Processor  50  may also include separate circuitry to separately control stimulation delivery to renal nerves  28 A and  28 B. 
     In some examples, processor  50  may control signal generator module  54  to deliver stimulation signals to patient  12  based on physiological parameter values sensed by sensing module  56 , which may indicate a level of sympathetic activity that patient  12  is experiencing. In other examples, processor  50  may control signal generator module  54  to deliver stimulation signals to patient  12  according to one or more predetermined schedules  64  that are independent of physiological parameter values sensed by sensing module  56 . The schedules  64  may be determined by a clinician and stored in memory  52 . The schedules  64  may indicate times that IMD  14  should initiate, increase, decrease, and/or cease stimulation delivery. 
     Sensing module  56  may monitor signals from at least two of electrodes  38  and  40  to monitor electrical activity of renal nerves  28 , via electrogram (EGM) signals. Sensing module  96  may also include a switch module to select the available electrodes  38  and  40  that are used to sense the electrical activity of renal nerves  28 . In some examples, processor  50  may select the electrodes  38  and  40  that function as sense electrodes via the switch module within sensing module  56 , e.g., by providing signals via a data/address bus. For example, processor  50  may access sensing functions  63  stored in memory  52  and select a plurality of electrodes  38  and  40  to function as sense electrodes based on sensing functions  63 . In some examples, sensing module  56  includes one or more sensing channels, each of which may comprise an amplifier. In response to the signals from processor  50 , the switch module within sensing module  56  may couple the outputs from the selected electrodes  38  and  40  to one of the sensing channels. 
     Sensing module  56  may also receive signals from other sensors  39  in wired communication with IMD  14 , and telemetry module  58  may receive signals from sensors in wireless communication with IMD  14 . For example, sensing module  56  may receive signals from non-electrode sensors  39 , e.g., chemical, pressure, and/or flow sensors, coupled to lead  16 . Telemetry module  58  may receive signals from any sensors in wireless communication with IMD  14  and may provide the received data to processor  50  and/or sensing module  56 . Processor  50  may control sensing module  56  and/or telemetry module  58  to retrieve sensed physiological signals based on sensing functions  63  stored in memory  52 . Sensing functions  63  may define which sensors are activated to identify changes in the sympathetic activity level of patient  12  and/or the values of sensed physiological parameters that indicate elevated sympathetic activity. 
     Telemetry module  58  may also permit IMD  14  to transmit information regarding the physiological parameters, e.g., sensed physiological parameters, information regarding renal function, and/or heart failure status, to an external device such as programmer  18  for view by a clinician, patient  12 , and/or another user. In some examples, memory  52  stores diagnostic information  61 , e.g., information regarding the physiological parameters, and telemetry module  58  may retrieve diagnostic information  61  from memory  52  for transmission to an external device such as programmer  18 . 
     Telemetry module  58  includes any suitable hardware, firmware, software or any combination thereof for communicating with another device, such as programmer  18  ( FIG. 1 ) or sensors. Under the control of processor  50 , telemetry module  58  may receive downlink telemetry from and send uplink telemetry to programmer  18  with the aid of an antenna, which may be internal and/or external. Processor  50  may provide the data to be uplinked to programmer  18  and the control signals for the telemetry circuit within telemetry module  58 , e.g., via an address/data bus. In some examples, telemetry module  58  may provide received data to processor  50  via a multiplexer. 
     The various components of IMD  14  are coupled to power source  60 , which may include a rechargeable or non-rechargeable battery or a supercapacitor. A non-rechargeable battery may be selected to last for several years, while a rechargeable battery may be inductively charged from an external device, e.g., on a daily or weekly basis. In some examples, power source  60  recharge via induction or ultrasonic energy transmission, and include an appropriate circuit for recovering transcutaneously received energy. For example, power source  60  may be coupled to a secondary coil and a rectifier circuit for inductive energy transfer. 
     As described in further detail with respect to  FIG. 8 , in some examples data generated by sensing module  56  and stored in memory  52  may be uploaded to a remote server, from which a clinician, patient or another user may access the data to, for example, evaluate the progression or heart failure, renal failure, or hypertension. An example of a remote server includes the CareLink Network, available from Medtronic, Inc. of Minneapolis, Minn. An example system may include an external device, such as a server, and one or more computing devices that are coupled to IMD  14  and programmer  18  via a network. 
       FIG. 4  is a block diagram of an example medical device programmer  18 . As shown in  FIG. 4 , programmer  18  includes processor  70 , memory  72 , user interface  74 , telemetry module  76 , and power source  78 . Programmer  18  may be a dedicated hardware device with dedicated software for programming of IMD  14 . Alternatively, programmer  18  may be an off-the-shelf computing device running an application that enables programmer  18  to program IMD  14 . 
     A user, e.g., clinician, may use programmer  18  to select therapy programs (e.g., sets of stimulation parameters), generate new therapy programs, modify therapy programs through individual or global adjustments or transmit the new programs to IMD  14  ( FIG. 1 ). The user may program, modify or control any aspect of the operation of IMD  14  via programmer  18 . For example, the user may modify therapy programs  62 , sensor functions  63 , or schedules  64 . In this manner, the user may modify the manner in which stimulation is delivered to the renal nerves, including the timing and intensity of such stimulation, the manner in which physiological parameters indicative of sympathetic activity are sensed, and the manner in which the stimulation is delivered based on the sensed parameters. The user may interact with programmer  18  via user interface  74 , which may include a display to present a graphical user interface to a user, and a keypad or another mechanism for receiving input from a user. 
     Furthermore, the user may view information via user interface  74 . For example, a user may view diagnostic information  61  collected by IMD  18 , or other sensors. As indicated in  FIG. 4 , memory  72  of programmer  18  may store diagnostic information  72 . Based on such information, a user may evaluate the progression or status of a patient condition, such as heart failure, renal failure, or hypertension. 
     Processor  70  can take the form one or more microprocessors, DSPs, ASICs, FPGAs, programmable logic circuitry, or the like, and the functions attributed to processor  70  herein may be embodied as hardware, firmware, software or any combination thereof. Memory  72  may store instructions that cause processor  70  to provide the functionality ascribed to programmer  18  herein, and information used by processor  70  to provide the functionality ascribed to programmer  18  herein. Memory  72  may include any fixed or removable magnetic, optical, or electrical media, such as RAM, ROM, CD-ROM, hard or floppy magnetic disks, EEPROM, or the like. Memory  72  may also include a removable memory portion that may be used to provide memory updates or increases in memory capacities. A removable memory may also allow patient data to be easily transferred to another computing device, or to be removed before programmer  18  is used to program therapy for another patient. Memory  72  may also store information that controls therapy delivery by IMD  14 , such as stimulation parameter values. For example, memory  72  may store therapy programs  62 , which telemetry module  76  may access and transmit to IMD  14 . 
     Programmer  18  may communicate wirelessly with IMD  14 , such as using RF communication or proximal inductive interaction. This wireless communication is possible through the use of telemetry module  76 , which may be coupled to an internal antenna or an external antenna. An external antenna that is coupled to programmer  18  may correspond to the programming head that may be placed proximate to the patient&#39;s body near the IMD  14  implant site. Telemetry module  76  may be similar to telemetry module  58  of IMD  14  ( FIG. 3 ). 
     Telemetry module  76  may also be configured to communicate with another computing device via wireless communication techniques, or direct communication through a wired connection. Examples of local wireless communication techniques that may be employed to facilitate communication between programmer  18  and another computing device include RF communication according to the 802.11 or Bluetooth specification sets, infrared communication, e.g., according to the IrDA standard, or other standard or proprietary telemetry protocols. In this manner, other external devices may be capable of communicating with programmer  18  without needing to establish a secure wireless connection. 
     Power source  78  delivers operating power to the components of programmer  18 . Power source  78  may include a battery and a power generation circuit to produce the operating power. In some examples, the battery may be rechargeable to allow extended operation. Recharging may be accomplished by electrically coupling power source  78  to a cradle or plug that is connected to an alternating current (AC) outlet. In addition or alternatively, recharging may be accomplished through proximal inductive interaction between an external charger and an inductive charging coil within programmer  18 . In other examples, traditional batteries (e.g., nickel cadmium or lithium ion batteries) may be used. In addition, programmer  18  may be directly coupled to an alternating current outlet to power programmer  18 . Power source  78  may include circuitry to monitor power remaining within a battery. In this manner, user interface  74  may provide a current battery level indicator or low battery level indicator when the battery needs to be replaced or recharged. In some cases, power source  78  may be capable of estimating the remaining time of operation using the current battery. 
       FIG. 5  is a flow diagram illustrating an example technique for delivering electrical stimulation to a patient to decrease renal sympathetic activity. Sensing module  56  of IMD  14  may sense a physiological parameter of patient  12  ( 80 ). For example, processor  50  may control IMD  14  to sense a physiological parameter of patient  12  via any combination of electrodes  38  and  40 . As another example, sensing module  56  may sense one or more physiological parameters via non-electrode sensors  39 , e.g., chemical or mechanical sensors, coupled to IMD  14  via lead  16 . Additionally or alternatively, telemetry module  58  of IMD  14  may receive one or more physiological signals from sensors in wireless communication with IMD  14 . 
     Signal generator  54  of IMD  14  may generate a stimulation signal based on the sensed physiological parameter ( 82 ). For example, the physiological parameter may be indicative of sympathetic activity, e.g., systemic or renal sympathetic activity, within patient  12 . Processor  50  within IMD  14  may identify an increase in sympathetic activity based on the physiological parameter and generate the stimulation signal in response to the increase in sympathetic activity. 
     Alternatively, telemetry module  58  of IMD  14  may transmit information regarding the sensed physiological parameter to an external device outside of patient  12 , e.g., programmer  18 . Processor  70  of programmer  18  may analyze the information regarding the physiological parameter to identify changes in sympathetic activity within patient  12 . In response to an increase in sympathetic activity, programmer  18  may direct signal generator  54  of IMD  14  to generate a stimulation signal ( 82 ). Processor  70  of programmer  18  may identify increases in sympathetic activity in examples in which IMD  14  does not include circuitry to perform the analysis of the physiological parameter, e.g., due to size constraints of IMD  14 . In other examples in which processor  50  of IMD  14  identifies increases in sympathetic activity, telemetry module  58  may transmit information regarding the physiological parameter to programmer  18  or another external device for viewing by a user, e.g., to supplement the automatic identification of increased sympathetic activity performed by processor  50  of IMD  14 . 
     IMD  14  may deliver the stimulation signal to one or more of renal nerves  28  ( 84 ). For example, IMD  14  may deliver the stimulation signal to one or more of renal nerves  28  in response to the increase in sympathetic activity within patient  12 . The stimulation signal may be configured to decrease renal sympathetic activity within patient  12 . In one example, IMD  14  delivers the stimulation signal to renal nerves  28 A via electrodes  38 A carried by distal segment  16 A of lead  16 . Additionally or alternatively, IMD  14  may deliver the stimulation signal to renal nerves  28 B via electrodes  38 B carried by distal segment  16 B of lead  16 . IMD  14  may deliver unilateral stimulation, e.g., to either renal nerves  28 A on the right side of patient  12  or renal nerves  28 B on the left side of patient  12 , or bilateral stimulation, e.g., to both renal nerves  28 A on the right side of patient  12  and renal nerves  28 B on the left side of patient  12 . In examples in which IMD  14  delivers bilateral stimulation, IMD  14  may deliver the same or different stimulation signals to renal nerves  28 A and  28 B. 
     In some examples, the stimulation signal may be a high frequency, biphasic stimulation signal. High-frequency biphasic electrical stimulation may create a reversible functional conduction in renal nerves  28 . Biphasic electrical stimulation may also prevent and/or reduce corrosion of electrodes  38 . As one example, the stimulation signal may be a biphasic stimulation signal with a frequency of approximately 100 hertz to approximately 20 kilohertz. 
     As previously described, telemetry module  58  of IMD  14  may transmit information regarding the physiological parameter to programmer  18  or another external device ( 86 ), and the external device may present the information to a user, e.g., patient  12  or a clinician, for viewing ( 88 ). For example, telemetry module  58  may transmit information regarding the physiological parameter itself, e.g., blood pressure and plasma renin level. As another example, telemetry module  58  may transmit other diagnostic information based on the sensed physiological parameter, such as renal function and heart failure status. The user may interpret the information and provide programming instructions to IMD  14 , e.g., to improve therapy effectiveness based on the information. 
     In some examples, IMD  14  does not necessarily sense a physiological parameter of patient  12 . Instead, IMD  14  may control stimulation delivery based on parameters other than sensed physiological parameters. For example, IMD  14  may deliver stimulation during specific portions of the day, e.g., according to a schedule, or in response to patient activation, e.g., received via programmer  18 . 
       FIG. 6  is a flow diagram illustrating an example technique for modifying stimulation delivery based on a sensed physiological parameter. As described with respect to  FIG. 5 , IMD  14  may deliver a stimulation signal to one or more of renal nerves  28  ( 84 ), and sensing module  56  of IMD  14  may sense a physiological parameter of patient  12  ( 80 ). If IMD  14  identifies an increase in sympathetic activity within patient  12  based on the sensed physiological parameter ( 90 ), IMD  14  may generate a modified stimulation signal ( 92 ). For example, IMD  14  may modify one or more stimulation parameters, e.g., electrode configuration, amplitude, pulse width, and/or pulse rate, to increase the intensity of the stimulation signal in response to the detected increase in sympathetic activity. IMD  14  may deliver the modified stimulation signal to one or more of renal nerves  28  in response to the increase in sympathetic activity ( 94 ). 
       FIG. 7  is a flow diagram illustrating an example technique for sensing physiological parameters. IMD  14  may monitor a norepinephrine level with the patient&#39;s blood, e.g., within a renal vessel ( 100 ). Elevated norepinephrine levels may indicate elevated sympathetic activity. If the norepinephrine level rises above a threshold ( 102 ), IMD  14  may monitor renal blood flow, e.g., within renal arteries  32  ( 104 ). Since sympathetic efferent activation causes renal vasoconstriction and a reduction in renal blood flow, blood flow in a renal vessel may indicate the level of renal sympathetic activity. If blood flow to kidneys  20  is decreased below a threshold ( 106 ), IMD  14  may identify an increase in sympathetic activity within patient  12  ( 108 ). The sensed blood flow may confirm an increase in sympathetic activity detected by the norepinephrine level. For example, IMD  14  may only identify an increase in sympathetic activity when both norepinephrine and blood flow indicate increased sympathetic activity. 
     IMD  14  may deliver a stimulation signal to renal nerves  28  in response to the increase in sympathetic activity ( 110 ). For example, if IMD  14  was not previously delivering stimulation, IMD  14  may initiate stimulation delivery. If IMD  14  was already delivering stimulation therapy, IMD  14  may modify the stimulation signal, as described with respect to  FIG. 6 . 
     If blood flow to kidneys  20  is not decreased below a threshold ( 106 ), IMD  14  may determine if blood flow to kidneys  20  is normal, e.g., within a specified range ( 112 ). If blood flow is outside of the acceptable range, e.g., below the acceptable range but not below the threshold, IMD  14  may continue to monitor blood flow ( 104 ). Once the sensed blood flow returns to normal, e.g., is within a specified range, IMD  14  may switch back to monitoring norepinephrine levels ( 100 ). 
     The technique illustrated in  FIG. 7  is merely an example. In general, IMD  14  may identify changes in the sympathetic activity level of patient  12  based on one or more sensed physiological parameters and control stimulation delivery to renal nerves  28  in response to the identified changes. In some examples, the sensed physiological parameters indicate renal sympathetic activity, and IMD  14  identifies changes in renal sympathetic activity. In some examples, IMD  14  may maintain sympathetic activity below a threshold level by adjusting stimulation delivery based on the sensed sympathetic physiological parameters. IMD  14  may use the sensed physiological parameters to determine when patient  12  requires stimulation and the minimum level of stimulation required to maintain renal sympathetic activity below a desired level. IMD  14  may sense physiological parameters on the right and left sides of patient  12 , e.g., proximate to kidneys  20 A and  20 B, and/or control stimulation delivery to the right and left sides of patient  12 , e.g., renal nerves  28 A and  28 B, individually. 
       FIG. 8  is a block diagram illustrating a system  120  that includes an external device  122 , such as a server, and one or more computing devices  124 A- 124 N that are coupled to the IMD  14  and programmer  18  via a network  126 , according to one example. In this example, IMD  14  uses telemetry module  58  ( FIG. 3 ) to communicate with programmer  18  via a first wireless connection, and to communicate with an access point  128  via a second wireless connection. In the example of  FIG. 8 , access point  128 , programmer  18 , external device  122 , and computing devices  124 A- 124 N are interconnected, and able to communicate with each other, through network  126 . 
     In some cases, one or more of access point  128 , programmer  18 , external device  122 , and computing devices  124 A- 124 N may be coupled to network  126  through one or more wireless connections. IMD  14 , programmer  18 , external device  122 , and computing devices  124 A- 124 N may each comprise one or more processors, such as one or more microprocessors, DSPs, ASICs, FPGAs, programmable logic circuitry, or the like, that may perform various functions and operations, such as those described herein. 
     Access point  128  may comprise a device that connects to network  126  via any of a variety of connections, such as telephone dial-up, digital subscriber line (DSL), or cable modem connections. In other examples, access point  128  may be coupled to network  126  through different forms of connections, including wired or wireless connections. In some examples, access point  128  may communicate with programmer  18  and/or IMD  14 . Access point  128  may be co-located with patient  12  (e.g., within the same room or within the same site as patient  12 ) or may be remotely located from patient  12 . For example, access point  128  may be a home monitor that is located in the patient&#39;s home or is portable for carrying with patient  12 . 
     During operation, IMD  14  may collect, measure, and store various forms of diagnostic data. For example, as described previously, IMD  14  may collect information regarding physiological parameters sensed via electrode  38 ,  40  and/or sensors  39 . In certain cases, IMD  14  may directly analyze collected diagnostic data and generate any corresponding reports or alerts. In some cases, however, IMD  14  may send diagnostic data to programmer  18 , access point  128 , and/or external device  122 , either wirelessly or via access point  128  and network  126 , for remote processing and analysis. 
     For example, IMD  14  may send programmer  18  collected physiological parameter values indicative of sympathetic activity, which is then analyzed by programmer  18 . Programmer  18  may generate reports or alerts after analyzing physiological parameter values and determine whether the values indicate that patient  12  requires medical attention, e.g., based on the physiological parameter values exceeding a threshold value. In some cases, IMD  14  and/or programmer  18  may combine all of the diagnostic data into a single displayable sympathetic activity report, which may be displayed on programmer  18 . The sympathetic activity report may contain information concerning the physiological parameter measurements, the time of day at which the measurements were taken, and identify any patterns in the physiological parameter measurements. A clinician or other trained professional may review and/or annotate the sympathetic activity report, and possibly identify any patient conditions (e.g., heart disease). 
     In another example, IMD  14  may provide external device  122  with collected physiological parameter data via access point  128  and network  126 . External device  122  includes one or more processors  130 . In some cases, external device  122  may request collected physiological parameter data, and in some cases, IMD  14  may automatically or periodically provide such data to external device  122 . Upon receipt of the physiological parameter data via input/output device  132 , external device  122  is capable of analyzing the data and generating reports or alerts upon determination that the physiological parameter data indicates a patient condition may exist. 
     In one example, external device  122  may combine the diagnostic data into a physiological parameter report. One or more of computing devices  124 A- 124 N may access the report through network  126  and display the report to users of computing devices  124 A- 124 N. In some cases, external device  122  may automatically send the report via input/output device  132  to one or more of computing devices  124 A- 124 N as an alert, such as an audio or visual alert. In some cases, external device  122  may send the report to another device, such as programmer  18 , either automatically or upon request. In some cases, external device  122  may display the report to a user via input/output device  132 . 
     In one example, external device  122  may comprise a secure storage site for diagnostic information that has been collected from IMD  14  and/or programmer  18 . In this example, network  126  may comprise an Internet network, and trained professionals, such as clinicians, may use computing devices  124 A- 124 N to securely access stored diagnostic data on external device  122 . For example, the trained professionals may need to enter usernames and passwords to access the stored information on external device  122 . In one example, external device  122  may be a CareLink server provided by Medtronic, Inc., of Minneapolis, Minn. 
     The techniques described in this disclosure, including those attributed to IMD  14 , programmer  18 , or various constituent components, may be implemented, at least in part, in hardware, software, firmware or any combination thereof. For example, various aspects of the techniques may be implemented within one or more processors, including one or more microprocessors, DSPs, ASICs, FPGAs, or any other equivalent integrated or discrete logic circuitry, as well as any combinations of such components, embodied in programmers, such as physician or patient programmers, stimulators, image processing devices or other devices. The term “processor” or “processing circuitry” may generally refer to any of the foregoing logic circuitry, alone or in combination with other logic circuitry, or any other equivalent circuitry. 
     Such hardware, software, firmware may be implemented within the same device or within separate devices to support the various operations and functions described in this disclosure. In addition, any of the described units, modules or components may be implemented together or separately as discrete but interoperable logic devices. Depiction of different features as modules or units is intended to highlight different functional aspects and does not necessarily imply that such modules or units must be realized by separate hardware or software components. Rather, functionality associated with one or more modules or units may be performed by separate hardware or software components, or integrated within common or separate hardware or software components. 
     When implemented in software, the functionality ascribed to the systems, devices and techniques described in this disclosure may be embodied as instructions on a computer-readable medium such as RAM, ROM, NVRAM, EEPROM, FLASH memory, magnetic data storage media, optical data storage media, or the like. The instructions may be executed to support one or more aspects of the functionality described in this disclosure. 
     Various examples have been described. Although described primarily in the context of bilateral leads and stimulation, some examples include a single lead that provides unilateral stimulation of renal nerves proximate to one of the kidneys. These and other examples are within the scope of the following claims.