Abstract:
Methods, systems, and apparatus for recharging medical devices implanted within the body are disclosed. An illustrative method of recharging an implanted medical device includes delivering a charging device to a location adjacent to the implanted medical device, activating a charging element coupled to the charging device and transmitting charging energy to a receiver of the implanted medical device, and charging the implanted medical device using the transmitted charging energy from the charging device.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority under 35 U.S.C. § 119 to U.S. Provisional Application No. 61/108,635, filed on Oct. 27, 2008, entitled “Methods and Systems for Recharging Implantable Devices,” which is incorporated herein by reference in its entirety for all purposes. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to implantable medical devices including rechargeable power sources. More specifically, the present invention pertains to methods, systems, and apparatus for recharging medical devices implanted within the body. 
     BACKGROUND 
     Actively powered implantable medical devices sometimes require a power supply such as a battery or power capacitor to provide electrical power to the device, in some cases over an extended period of time. In cardiac rhythm management applications, for example, an implantable medical device such as a pressure sensor may require a power supply capable of operating the device over a period of several years. In some cases, the time required to power the device is beyond the capability of the power supply, requiring replacement of the power supply or the implantation of a new device within the body. 
     With advances in power management and battery technology, more recent trends have focused on the use of small rechargeable power sources for providing power to implantable devices. Current charging techniques often rely on the patient and/or a health-care provider to ensure that the battery is charged periodically. In some cases, the patient may be required to undergo recharging within a clinical environment, which can be burdensome to the patient and often adds to the overall costs associated with recharging. If recharging is to be performed in a clinic, for example, a special area may be required for the patient while the recharging is being performed, adding to the overall cost and time associated with the maintenance. 
     SUMMARY 
     The present invention pertains to methods, systems, and apparatus for recharging medical devices implanted within the body. An illustrative recharging system includes a device implanted within a body lumen having a rechargeable power source and a receiver, and a charging device adapted to provide charging energy to the implanted device from a location within the body adjacent to the device. A charging element coupled to the charging device is configured to transmit energy at a location within the body proximate to the receiver. In some embodiments, for example, the charging element includes a source transducer adapted to transmit an acoustic signal to a target transducer coupled to the implanted device for acoustically recharging the device. Alternatively, and in other embodiments, the charging element includes an electromagnetic transmitter adapted to transmit an electromagnetic signal to an antenna or coil coupled to the implanted device for recharging the device using RF or other forms of electromagnetic energy. Other energy transfer modes can also be employed for recharging the implanted device. 
     An illustrative method of recharging a medical device implanted within a body lumen of a patient&#39;s body includes delivering a distal section of the charging device to a location adjacent to the implanted device, activating a charging element operatively coupled to a power source and wirelessly transmitting energy to a receiver coupled to the implanted device, and converting the energy received by the receiver into electrical energy for charging the implanted device. The charging device can be positioned at a target location within the same body lumen as the implanted device, or alternatively, within a different body lumen. For recharging a pressure sensor implanted within a pulmonary artery, for example, the charging device can be delivered to a location within the pulmonary artery, an adjacent artery, or an adjacent lumen or cavity such as the aorta or esophagus. Once positioned adjacent to the implanted device, the charging element can be activated to transmit charging energy to the device from a position within the body. In some embodiments, the charging device can be used to perform other functions within the body such as calibrating the implanted device, confirming the proper operation of the charging device, and/or performing therapy within the body. 
     While multiple embodiments are disclosed, still other embodiments of the present invention will become apparent to those skilled in the art from the following detailed description, which shows and describes illustrative embodiments of the invention. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature and not restrictive. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of an illustrative system for recharging a medical device implanted within a body lumen; 
         FIG. 2  is a partial cross-sectional view showing the distal section of the charging device of  FIG. 1  inserted at a target location within the body adjacent to the implanted device; 
         FIG. 3  is a partial cross-sectional view showing the distal section of a charging device in accordance with another illustrative embodiment including a sensor; 
         FIG. 4  is a partial cross-sectional view showing the distal section of a charging device in accordance with another illustrative embodiment including a thermocouple wire; 
         FIG. 5  is a partial cross-sectional view showing the distal section of a charging device in accordance with another illustrative embodiment including a cooling lumen; 
         FIG. 6  is a partial cross-sectional view showing the distal section of a charging device in accordance with another illustrative embodiment including a cooling lumen; 
         FIG. 7  is a schematic view showing another illustrative system for recharging a medical device implanted within a body lumen; 
         FIG. 8  is a schematic view showing another illustrative system for recharging a medical device implanted within a body lumen; 
         FIG. 9  is a partial cross-sectional view showing the distal section of the charging device of  FIG. 8  inserted at a target location within the body adjacent to the implanted device; 
         FIG. 10  is a schematic view showing another illustrative system for recharging a medical device implanted within a body lumen; 
         FIG. 11  is another view of the charging device of  FIG. 10 ; and 
         FIG. 12  is a transverse view of a patient&#39;s thorax, showing the insertion of the charging device of  FIG. 10  in the esophagus adjacent to a medical device implanted within the right pulmonary artery. 
       While the invention is amenable to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and are described in detail below. The intention, however, is not to limit the invention to the particular embodiments described. On the contrary, the invention is intended to cover all modifications, equivalents, and alternatives falling within the scope of the invention as defined by the appended claims. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a schematic view of an illustrative system  10  for recharging a medical device implanted within a body lumen. The system  10 , illustratively a system for recharging a device  12  implanted within a pulmonary artery, includes a charging catheter  14  adapted for insertion at a target location within a patient&#39;s body such as in or near the heart  16 . The heart  16  includes a right atrium  18 , a right ventricle  20 , a left atrium  22 , and a left ventricle  24 . The right ventricle  20  leads to the main pulmonary artery  26 , which further branches to the right pulmonary artery  28  and the left pulmonary artery  30 , as shown. 
     The charging catheter  14  includes an elongate shaft  32  having a proximal section  34  located outside of the patient&#39;s body, and a distal section  36  insertable into the patient&#39;s body at a location adjacent to the implanted device  12 . In the illustrative embodiment of  FIG. 1 , the distal section  36  of the elongate shaft  32  is shown inserted into the main pulmonary artery  26  of the heart  16  at a location adjacent to the device  12 , which is shown secured within a portion of the left pulmonary artery  30  via a fixation element  38 . The distal section  36  of the charging catheter  14  can be positioned at other locations within the body, including the right pulmonary artery  28 , the left pulmonary artery  30 , or an adjacent vessel or body lumen such as the aorta  40 , as discussed further herein. The positioning of the charging catheter  14  within the body will typically depend on the implantation location and configuration of the implanted device  12  to be recharged, the anatomy surrounding the implanted device  12 , as well as other factors. 
     In some embodiments, the charging catheter  14  can be inserted into the main pulmonary artery  26  via an intravenous approach from a percutaneous access site such as a femoral artery or jugular vein. As shown in  FIG. 1 , for example, delivery of the charging catheter  14  to a target location within the body can occur intravenously through the superior vena cava  42 , the right atrium  18 , the right ventricle  20 , and the main pulmonary artery  26 . Other techniques for inserting the charging catheter  14  into the right pulmonary artery  26  are also possible. In some alternative embodiments, for example, the charging catheter  14  can be inserted into the main pulmonary artery  26  via an intra-arterial approach, percutaneously without the aid of vascular conduits, or via the esophagus, airway, or other conduit. 
     The implanted device  12  can be configured to perform one or more designated functions, including the sensing of physiological parameters within the body and/or providing therapy to the patient. Example physiological parameters that can be sensed using the implanted device  12  include, but are not limited to, blood pressure, blood or fluid flow, temperature, and strain. Various electrical, chemical, and/or magnetic properties may also be sensed within the body using the implanted device  12 . The specific configuration and function to be performed by the implanted device  12  will typically vary depending on the particular therapeutic needs of the patient. 
     In some embodiments, the implanted device  12  comprises a pressure sensor adapted to sense arterial blood pressure within a pulmonary artery. As shown in the illustrative system  10  of  FIG. 1 , for example, the implanted device  12  may comprise a pressure sensor implanted in the left pulmonary artery  30  for sensing arterial blood pressure. Alternatively, and in other embodiments, the device  12  may be implanted in the right pulmonary artery  28 , the main pulmonary artery  26 , or in another vessel leading into or from the heart  16 . The implanted device  12  can also be implanted at other locations within the heart  16  such as in the right atrium  18 , the right ventricle  20 , the left atrium  22 , or the left ventricle  24 . In some embodiments, the implanted device  12  can be placed at other locations in the body such as within an organ such as the liver or kidney, the vasculature, muscle tissue, or an airway within the body. 
     The implanted device  12  can be used as part of a cardiac rhythm management (CRM) system to predict decompensation of a heart failure patient, to optimize pacing and/or defibrillation therapy, as well as perform other designated functions within the body. In certain embodiments, for example, the implanted device  12  can be configured to transmit sensed physiological parameters to other CRM system components located within the body such as a pacemaker or defibrillator. In some embodiments, the implanted device  12  can be configured to transmit sensed physiological parameters to an external device such as a monitor or programmer for further monitoring and/or processing. Based on this information, an indication of any abnormalities within the heart  16  can be determined and an appropriate therapy provided to the patient, as necessary. 
       FIG. 2  is a partial cross-sectional view showing the distal section  36  of the charging catheter  14  of  FIG. 1  inserted at a target location within the body adjacent to the implanted device  12 . As can be further seen in  FIG. 2 , and in some embodiments, the distal section  36  of the charging catheter  14  includes a charging element  44  adapted to transmit energy to the implanted device  12  that can be used to charge a rechargeable battery  46  within the device  12 . In certain embodiments, for example, the charging element  44  includes an ultrasonic transducer  48  electrically coupled to an external power source  50  via a number of wires  52  extending through an interior lumen  54  of the charging catheter  14 . An example piezoelectric transducer that can be used for acoustically transmitting charging energy to the implanted device  12  is described, for example, in U.S. Pat. No. 6,140,740, entitled “Piezoelectric Transducer,” which is incorporated herein by reference in its entirety. Other types of acoustic transducers can also be utilized for providing charging energy to the implanted device  12 . In some embodiments, the ultrasonic transducer  48  includes an array of ultrasonic elements. 
     During recharging, the power source  50  can be configured to deliver a time-varying excitation current to the ultrasonic transducer  48 , causing the transducer  48  to generate an acoustic signal  56  within the body that is received by a receiver  58  coupled to the implanted device  12 . In some embodiments, for example, the receiver  58  comprises an ultrasonic transducer sensitive to the frequency of the acoustic signal  56  transmitted from the source ultrasonic transducer  48 . The acoustic signal  56  received by the target ultrasonic transducer  58  is then converted into electrical energy that can be used to recharge the battery  46 . 
     During delivery, the source ultrasonic transducer  48  on the charging catheter  14  can be positioned in close proximity to the target ultrasonic transducer  58  of the implanted device  12 . In certain embodiments, for example, the distal section  36  of the charging catheter  14  can be positioned such that the source ultrasonic transducer  48  is located a distance of between about 1 mm to about 10 mm apart from the target ultrasonic transducer  58 . The distance at which the two transducers  48 , 58  are spaced apart from each other may be greater or lesser, however, depending on the type of transducers  48 , 58  employed, the intensity and frequency characteristics of the acoustic signal  56  transmitted, the anatomy surrounding the transducers  48 , 58 , as well as other factors. In some embodiments, the positioning of the charging catheter  14  can be accomplished under the aid of fluoroscopy. A radiopaque marker band  60  placed at or near the distal end of the charging catheter  14  can be used in conjunction with a fluoroscope to visualize the location of the charging catheter  14  during delivery so as to minimize the distance between the transducers  48 , 58 . 
     Once the distal section  36  of the charging catheter  14  is positioned at a target location within the body adjacent to the implanted device  12 , the ultrasonic transducer  48  can be activated to transmit an acoustic signal  56  to the implanted device  12  for recharging the battery  46  in vivo. The time required to deliver a sufficient amount of charging energy to recharge the battery  46  may be affected by several factors, including the location of the device  12  within the body, the location of the charging catheter  14  within the body, the distance between the source and target ultrasonic transducers  48 , 58 , and the intensity and frequency of the acoustic signal  56 . Typically, the acoustic intensity of the acoustic signal  56  falls off inversely proportional to the square of the distance from the ultrasonic transducer  48 . Thus, for a given flux of energy, there is an initial rapid decrease in intensity in the near field followed by a more gradual decline further away from the transducer  48 . 
     By placing the source and target transducers  48 , 58  in close proximity to each other, the attenuation loss associated with the rapid fall off of acoustic energy in the near field is reduced, resulting in an increase in charge coupling efficiency. This increase in efficiency reduces the overall time required to recharge the battery  46 , and subjects the body to less energy than would otherwise be required to recharge the battery  46  via an external recharging approach with the source ultrasonic transducer transmitting the charging energy directly into the body. This results in a higher intensity field in the vicinity of the implanted device  12  while maintaining a lower overall energy flux transmitted into the body. In addition, because the source transducer  48  is located in close proximity to the target transducer  58 , a smaller portion of the transmitted acoustic energy is absorbed and/or scattered within the body, resulting in more efficient charging with reduced body tissue and fluid heating. 
     Although the illustrative charging catheter  14  of  FIG. 2  includes an ultrasonic transducer  48  for acoustically recharging the implanted device  12 , in other embodiments the charging element  44  can use other energy transfer modes for transmitting charging energy to the device  12 . Examples of other types of energy transfer modes can include, but are not limited to, inductive, electromagnetic, RF, optical (e.g., infrared light, visible light, ultraviolet light, and X-ray), vibration (e.g., transverse and longitudinal mechanical vibrations), radioactive energy, heat, and/or pressure. In one alternative embodiment, for example, the charging element  44  comprises a transmitter adapted to transmit electromagnetic energy to the implanted device  12 . The electromagnetic energy transmitted by the charging element  44  is received by an antenna or coil coupled to the implanted device  12 , which is then converted into electrical energy for recharging the battery  46 . As with an acoustic energy transfer mode, the transmitter can be positioned in close proximity to the antenna or coil in order to reduce attenuation and absorption of the transmitted electromagnetic energy. In certain embodiments, for example, the charging catheter  14  can be positioned such that the charging element  44  is located a distance of between about  1  mm to about  10  mm apart from the antenna or coil for the implanted device  12 . 
     In some embodiments, the charging catheter  14  further includes a focusing or collimating element adapted to direct and focus the charging energy transmitted to the implanted device  12 . In those embodiments in which the charging element  44  includes an ultrasonic transducer  48 , for example, the charging element  44  may further include an acoustic baffle or lens for focusing the acoustic signal  56  in the direction of the target transducer  58 . In some embodiments, focusing of the acoustic signal  56  may occur by selectively activating one or more ultrasonic transducer elements within a transducer array, by adjusting the timing or phase of one or more ultrasonic transducer elements, and/or by adjusting the intensity levels of one or more ultrasonic transducer elements. Other techniques for focusing the transmitted acoustic signal  56  are also possible. 
     In certain embodiments, the charging element  44  is configured to provide charging energy to the implanted device  12  by directly contacting a surface on the device  12 . In one such embodiment, for example, the charging element  44  includes an electrode adapted to electrically contact a corresponding electrode on the implanted device  12 . During delivery, the distal section  36  of the charging catheter  14  can be positioned within the body such that the two electrodes make electrical contact with each other. Once positioned, the electrode on the charging catheter  14  can be energized, causing current to flow to the electrode on the implanted device  12 . As with other energy transmission modes discussed herein, the charging energy received by the implanted device  12  can then be used to recharge the battery  46 . 
       FIG. 3  is a partial cross-sectional view showing the distal section  62  of a charging catheter  64  in accordance with another illustrative embodiment including a sensor. As shown in  FIG. 3 , the distal section  62  of the charging catheter  64  includes a charging element  68  adapted to transmit energy to the implanted device  12 , which can be used to charge a rechargeable battery  46  within the device  12 . In certain embodiments, for example, the charging element  68  includes an ultrasonic transducer  70 , or alternatively, multiple ultrasonic transducers  70  each electrically coupled to an external power source via a number of wires  72  extending through an interior lumen  74  of the charging catheter  64 . In use, the ultrasonic transducer(s)  70  can be activated to transmit an acoustic signal  76  to the implanted device  12  for recharging the battery  46  in vivo in a manner similar to that described above with respect to the charging catheter  14  of  FIG. 2 . 
     In some embodiments, and as further shown in  FIG. 3 , the distal section  62  of the charging catheter  64  further includes at least one sensor  78  electrically coupled to the power source  50  via a wire  80  and adapted to monitor one or more parameters associated with the operation of the charging catheter  64  and/or the surrounding environment. Example parameters related to the operation of the charging catheter  64  that can be monitored can include, but are not limited to, parameters related to the energy transmitted by the ultrasonic transducer(s)  70  such as peak power, average power, or power gradient. Example parameters related to the surrounding environment that can be monitored can include, but are not limited to, temperature, electrical impedance, electrical potential, dielectric coefficient, and/or changes in one or more of these parameters. The sensor  78  can also be configured to sensor other parameters associated with the operation of the charging catheter, the implanted device  12 , and/or the surrounding environment. 
     In certain embodiments, the sensor  78  is a temperature sensor  78  adapted to measure the temperature of body tissue and/or the local blood temperature at or near the location of the implanted device  12 . In some embodiments, for example, the temperature sensor  78  can be configured to sense the local blood temperature of blood in the path of the acoustic signal  76 , which can be used to estimate the temperature of the body tissue adjacent to the implanted device  12 . The charging catheter  64  can be positioned within the vessel such that the temperature sensor  78  contacts the body tissue within the vessel, allowing the sensor  78  to directly sense the body tissue temperature adjacent to the implanted device  12 . Based on the monitored temperature, the system can then either reduce the power of the acoustic energy transmitted by the ultrasonic transducer(s)  70 , or alternatively, disable one or more of the transducers  70  in the event the temperature exceeds a maximum temperature threshold value. The monitored temperature can also be provided as feedback to notify a clinician of a potentially hazardous condition related to the operation of the charging catheter  64 . The temperature sensor  78  can also be utilized to perform other tasks such as calibrating the charging element  68 . 
     In some embodiments, the sensor  78  can be configured to monitor for the presence of any electrical leakage from the charging element  68 . For example, the sensor  78  can comprise a sensor adapted to detect the presence of any current leakage from the ultrasonic transducer(s)  70  into the surround anatomy. The monitoring of electrical leakage from the ultrasonic transducers  70  can be accomplished, for example, by measuring the current into and out of the transducers  70  using a differential current transformer, a bridge circuit, or the like. If an electrical leakage is detected, and depending on its magnitude, the system can then either adjust the operating power provided to one or more of the ultrasonic transducers  70  or disable the transducers  70  in order to reduce or eliminate the electrical leakage. This may be useful, for example, in acoustic charging systems that deliver relatively high voltages to the transducer elements. The charging system can also be configured to notify the clinician if a fault condition has occurred in the charging catheter  64 . 
     The implanted device  12  can be further configured to monitor a number of parameters associated with the acoustic signal  76  received from the charging catheter  64 . For example, in those embodiments in which the implanted device  12  includes an energy exchanger (e.g., an ultrasonic transducer), the implanted device  12  can be configured to monitor the power or intensity of the acoustic signal  76  transmitted by the charging catheter  64  to determine whether the signal  76  is within an acceptable range. If the received acoustic signal  76  exceeds a maximum power or intensity value, for example, the implanted device  12  can be configured to communicate a signal back to the charging catheter  64 , which can be used by the catheter  64  as feedback to adjust the intensity or power of the signal  76 . In some embodiments, the feedback signal can also be used by the clinician to aid in repositioning the charging catheter  64  within the vessel to maximize the charge coupling efficiency between the catheter  64  and the implanted device  12 . In one embodiment, for example, the feedback signal can be used to adjust the placement location of charging catheter  64 , and in particular the location of the charging element  68  within the vessel, in order to optimize the charging energy received by the implanted device  12 . 
       FIG. 4  is a partial cross-sectional view showing the distal section  82  of charging catheter  84  in accordance with another illustrative embodiment including a thermocouple wire for sensing temperature. As shown in  FIG. 4 , the distal section  82  of the charging catheter  84  includes a charging element  86  adapted to transmit a signal  88  to the implanted device  12 . In certain embodiments, the charging element  86  includes an ultrasonic transducer  90 , or alternatively, multiple ultrasonic transducers  90  each electrically coupled to an external power source via a number of wires  92  extending through an interior lumen  94  of the charging catheter  84 . 
     In some embodiments, and as further shown in  FIG. 4 , the charging catheter  84  includes a thermocouple wire  96  adapted to sense the temperature of the body tissue and/or local blood temperature within the vessel at or near the location of the implanted device  12 . The thermocouple wire  96  can be embedded within the charging catheter  84  at a location at or near the charging element  86 , and can be electrically coupled to sensing electronics within the external power source  50  via a wire  98 . The thermocouple wire  96  can be fabricated from a relatively thin gauge metal capable of sensing relatively small changes in temperature. In some embodiments, the sensing electronics can comprise a differential amplifier adapted to convert sensed thermal potential differences into an electrical potential difference indicative of a change in temperature due to the transmitted charging energy. 
     In use, the thermocouple wire  94  can be configured to sense temperature at the distal section  82  of the charging catheter  84 , which can then be used to estimate the temperature of the body tissue and/or blood in the path of the acoustic signal  88 . In some embodiments, an exposed portion  100  of the thermocouple wire  96  may permit the wire  96  to sense the local temperature within the blood vessel or, if placed into contact with the vessel wall, the body tissue temperature. The exposed portion  100  of the thermocouple wire  96  can also be used to sense other parameters within the vessel. In certain embodiments, for example, the exposed portion  100  of the thermocouple wire  96  may also function as a voltimeter probe to detect the presence of any electrical leakage from the charging element  86  by measuring electrical potentials within the vessel. 
     In another embodiment, the thermocouple wire  96  can be coupled directly to the charging element  86  for monitoring the temperature of the element  86  itself. For an acoustic recharging system including an ultrasonic transducer  90 , for example, the thermocouple wire  96  can be attached to a portion of the transducer  90  to monitor the temperature of the transducer  90  during recharging. Since heating in the vessel is due in part to heat conduction from the ultrasonic transducer  90 , the temperature within the vessel can be monitored indirectly using the thermocouple wire  96 . The sensed temperature on the ultrasonic transducer  70  can then be used as feedback for regulating the operating power provided to the transducer  70 . 
       FIG. 5  is a partial cross-sectional view showing the distal section  102  of a charging catheter  104  in accordance with another illustrative embodiment including a cooling lumen. As shown in  FIG. 5 , the charging catheter  104  includes an elongate shaft  106  having an internal lumen  108  in fluid communication with a cooling medium  110  that can be used for cooling a charging element  112  and/or the body tissue and fluids surrounding the distal section  102  of the catheter  104 . The cooling medium  110  may comprise a liquid or gas delivered to the distal section  102  of the charging catheter  104  from a source operatively coupled to the proximal section of the catheter  104 . In some embodiments, the cooling medium  110  can be configured to change its aggregation state from a liquid to gas when heated. In certain embodiments, for example, the cooling medium  110  comprises a pressurized source of liquid nitrogen operatively coupled to the charging catheter  104  at a location external to the body. Upon heating from the charging element  112 , the liquid nitrogen can be configured to change from an initial liquid state to a gaseous state, absorbing heat produced by activation of the element  112  during recharging. Examples of other suitable cooling mediums  110  can include, but are not limited to, air, carbon dioxide, helium, neon, argon, saline, water, or a Freon-based solution. 
     In some embodiments, an additional lumen  114  can be used as a return line to return the cooling medium  110  back to the proximal section of the charging catheter  104  once heated. During recharging, the cooling medium  110  can be circulated through the interior of the distal section  102  to dissipate the heat generated by the charging element  112  and to reduce heating of the body tissue and fluids surrounding the catheter  104 . In some embodiments, the temperature of the cooling medium  110  can be reduced to a temperature below room temperature to further aid in dissipating heat generated by the charging element  112 . 
     During recharging, the presence of the cooling medium  110  within the lumens  108 , 114  facilitates operation of the charging element  112  at higher intensity levels without causing significant heating in the surrounding body tissue and fluids. When the charging catheter  104  is implanted in a pulmonary artery, for example, the presence of the cooling medium  110  facilitates operation of the charging element  112  at greater intensity levels without heating the blood within the artery. The ability to operate at higher intensity levels without heating may reduce the overall time required to recharge the battery within the implanted device. 
       FIG. 6  is a partial cross-sectional view showing the distal section  116  of a charging catheter  118  in accordance with another illustrative embodiment including a cooling lumen. Similar to the embodiment of  FIG. 5 , the charging catheter  118  includes an elongate shaft  120  having one or more internal lumens  122 , 124  in fluid communication with a cooling medium  110  that can be used for cooling a charging element  126  and/or body tissue and fluids surrounding the distal section  116  of the catheter  118 . In the embodiment of  FIG. 6 , the lumens  122 , 124  terminate distally at an exit port  126  disposed at or near the distal end  128  of the charging catheter  118 . In some embodiments, multiple exit ports may be provided at or near the distal end  128  of the elongate shaft  120  and/or may be provided at various other locations along the length of the shaft  120 . 
     During recharging, a pressurized cooling medium  110  (e.g., saline) contained within the lumens  122 , 124  is ejected through the port  126  and into the surrounding anatomy. In recharging applications where the catheter  118  is positioned in a pulmonary artery adjacent to an implanted pressure sensor, for example, the cooling medium  110  may be ejected through the exit port  126  and into the artery for cooling the blood within the artery as well as the pressure sensor. As with the embodiment of  FIG. 5 , the passage of the cooling medium  110  through the lumens  122 , 124  further dissipates heat generated by the charging element  126 . 
       FIG. 7  is a schematic view of another illustrative system  130  for recharging a medical device  12  implanted within a body lumen. In the illustrative embodiment of  FIG. 7 , a charging catheter  132  including an elongate shaft  134  having a proximal section (not shown) and a distal section  136  is inserted into a different body vessel in close proximity to the vessel containing the implanted device  12 . In an implantable pressure sensor disposed within a pulmonary artery, for example, the distal section  136  of the charging catheter  132  may be positioned within an adjacent body vessel such as the aorta  138 . Delivery of the charging catheter  132  to the aorta  138  can be accomplished, for example, via a catheterization approach through a coronary artery. Other delivery techniques, however, are possible. 
     By positioning the charging catheter  132  into a different vessel than the implanted device  12 , access to a target site for recharging may be easier and/or may be less invasive than inserting the catheter  132  directly into the same vessel as the device  12 . In some cases, for example, the implanted device  12  may be implanted within a body lumen that is difficult to access. In such case, delivery of the charging catheter  132  to a different vessel within the body (e.g., the aorta, the right pulmonary artery, the esophagus, etc.) may reduce the overall time and difficulty associated with the recharging process. 
     Once the charging catheter  132  is positioned at a target location within an adjacent body lumen (e.g., the aorta  138 ), a charging element  140  coupled to the catheter  132  can be activated to transmit charging energy into the adjacent vessel (e.g., the left pulmonary artery  30 ) for recharging the implanted device  12 . In those embodiments in which the charging element  140  includes an ultrasonic transducer  142 , for example, the transducer  142  can be configured to transmit an acoustic signal  144  that can be received by the implanted device  12  and converted into electrical energy for recharging the device  12 . 
     In some embodiments, the charging catheter  132  can be used to perform other functions within the body and/or to provide therapy to the patient. In certain embodiments, for example, the charging catheter  132  may be used during a diagnostic or therapeutic coronary artery catheterization (e.g., a right heart catheterization) for treating coronary artery disease within the body. In one such embodiment, the charging element  140  may be provided as part of a coronary balloon catheter for performing an angioplasty procedure on the patient. In such case, recharging of the implanted device  12  may be performed in conjunction with the therapy using the same catheterization. 
       FIG. 8  is a schematic view showing another illustrative system  146  for recharging a medical device  12  implanted within a body lumen. The system  146 , illustratively a system for recharging a pressure sensor implanted within a pulmonary artery, includes a charging catheter  148  having a distal section  150  inserted at a target location within the patient&#39;s body, an external charging element  152  coupled to a proximal section  154  of the charging catheter  148 , and a power source  156 . 
     The charging catheter  148  includes an elongate shaft  158  having an interior lumen  160  adapted to transmit charging energy generated by the charging element  152  from a location outside of the patient to the distal section  150  of the charging catheter  148 . In some embodiments, for example, the charging element  152  comprises an external ultrasonic transducer that, when energized by the power source  156 , generates an acoustic signal  162  that is transmitted through the interior lumen  160  to the distal section  150  of the catheter  148 . In some embodiments, the ultrasonic transducer  152  comprises an array of ultrasonic transducer elements each of which can be selectively actuated to generate the acoustic signal  162 . During recharging, the interior lumen  160  acts as an acoustic waveguide for the acoustic signal  162 , reducing attenuation and scattering that would normally occur during transmission of the signal  162  directly through the body. Because the charging element  152  is located outside of the patient&#39;s body, the transducer  152  can be of any size and power without significantly impacting the acoustic energy transmitted into the body. 
       FIG. 9  is a partial cross-sectional view showing the distal section  150  of the charging catheter  148  of  FIG. 8  inserted at a target location within the body adjacent to an implanted device  12 . As can be further seen in  FIG. 9 , the distal section  150  of the charging catheter  148  includes a port  164  adapted to direct the acoustic signal  162  transmitted through the interior lumen  118  in a direction towards the implanted device  12 . In some embodiments, the interior lumen  160  may contain a liquid or solid material (e.g., saline), which acts as an interface to facilitate transmission of the acoustic energy through the interior lumen  160 . In certain embodiments, the distal section  150  of the charging catheter  148  may further include a focusing or collimating element  166  such as an acoustic baffle or lens to further focus the acoustic signal  162  in a direction towards the implanted device  12 . 
     In some embodiments, the charging catheter  148  may include other components for use in focusing the charging energy generated by the charging element  152 , either passively or actively. In the embodiment of  FIG. 9 , for example, the charging catheter  148  includes a sensor  168  adapted to sense various parameters of the acoustic signal  162  as it is transmitted from the port  164  towards the implanted device  12 . In one embodiment, the sensor  168  is an ultrasonic pressure sensor adapted to sense the intensity and/or phase of the acoustic signal  162  as it exits the port  164 . The sensor  168  can be configured to relay pressure sensor readings to an external controller  170  via a wired or wireless communications link. Based on the sensor readings, the external controller  170  can be configured to run an adaptive algorithm or routine that is used to optimize the direction, focusing, phase, intensity, timing, and/or bandwidth of the acoustic signal  162  generated by the charging element  152 . In some embodiments, for example, the sensor readings can be used to analyze the intensity and phase of the acoustic signal  162  exiting the port  164 , and responsive to these parameters, adjust the intensity and timing of the electrical signal  172  provided to one or more transducer elements of the charging element  152 , either simultaneously or sequentially. 
     Alternatively, and in other embodiments, the distal section  150  of the charging catheter  148  may include a passive element such as a reflector or an active element such as a repeater adapted to generate a signal  172  that is received by an array of transducer elements. In certain embodiments, for example, the reflected or repeated signal may serve as a reference signal for a time-reversal acoustic algorithm that can be used to generate time reversals on one or more of the transducer elements in order to focus the acoustic signal  162  towards the implanted device  12 . The sensor  168  on the charging catheter  148  can be configured to sense an acoustic signal transmitted by the implanted device  12 . The sensed acoustic signal can then be transmitted to the external controller  170  for computing phase delays for each of the transducer elements. The external controller  170  can then adjust one or more parameters associated with the ultrasonic elements to focus or steer the acoustic signal  162  towards the implanted device  12 . Example parameters that can be adjusted include, but are not limited to, direction, focusing, phase, intensity, timing, and/or bandwidth. 
     The sensor  168  can be used to perform other functions within the body such as calibrating the implanted device  12 . In those embodiments in which the implanted device  12  comprises a pressure sensor, for example, the sensor  168  may be used as a reference pressure sensor to calibrate the device  12 . In one embodiment, the reference pressure sensor and charging element can be combined into a single catheter. The ability to calibrate the implanted device  12  without subjecting the patient to an additional catheterization process may reduce the time and complexity associated with servicing the implanted device  12 . 
       FIG. 10  is a schematic view of another illustrative system  174  for recharging a medical device  12  implanted within a body lumen using a transesophageal approach. In the embodiment of  FIG. 10 , a charging device  176  is inserted transesophageally into the esophagus  178  of the patient. The esophagus  178  is located posterior to the heart  16  and the airway  180 , and extends downwardly to the stomach  182  at a location adjacent to the aorta  40  and the posterior wall  184  of the heart  16 , as shown. 
     As can be further understood in conjunction with  FIG. 11 , the charging device  176  includes an elongate shaft  186  having a proximal section  188  and a distal section  190 . The distal section  190  of the charging device  176  includes a cylindrically-shaped charging element  192  adapted to transmit energy from a position within the esophagus  178  to acoustically recharge an implanted device  12  located in an adjacent body lumen such as a pulmonary artery. In some embodiments, the charging element  192  comprises an array of ultrasonic transducers  194  which, when energized via an external controller  196 , generate a radial, omnidirectional acoustic signal that is transmitted through the esophageal wall and into the adjacent pulmonary artery for recharging the implanted device  12 . In some embodiments, the charging element  192  can include a polymeric coating or layer that matches the acoustic impedance of the charging element  192  with the surrounding fluid and tissue in the esophagus  178 . In one embodiment, the charging element  192  and external controller  196  may be provided as part of a transesophageal echocardiogram (TEE) device. 
     The external controller  196  can include a signal generator  198  and a tuning circuit  200  that can be used to tune the frequency of the acoustic signal generated by the ultrasonic transducers  194  to a particular frequency or range of frequencies based on the resonance characteristics of the ultrasonic transducer elements used to transmit and receive the acoustic charging energy. In certain embodiments, for example, the signal generator  198  and tuning circuit  200  can be used to tune the ultrasonic transducer elements to a frequency of about 40 kHz, which can correspond to a resonance frequency of the ultrasonic transducer on the implanted device  12 . In some embodiments, the signal generator  198  and tuning circuit  200  can be used to tune the ultrasonic transducer elements to operate over a desired range of frequencies (e.g., between about 10 kHz to 200 kHz). Other operating frequencies and frequency ranges are possible, however. 
     To recharge an implanted device  12  positioned in or near the heart  16 , the distal section  190  of the charging device  176  can be inserted into the patient&#39;s esophagus  178  and advanced to a position within the esophagus  178  adjacent to the implantation location of the device  12 . In those embodiments in which the implanted device  12  is located within a pulmonary artery  30 , for example, the distal section  190  of the charging device  176  can be inserted into the esophagus  178  and positioned such that the charging element  192  is located in the mediastinum immediately posterior to the artery  30 , as shown, for example, in  FIG. 10 . In some embodiments, the distal section  190  of the charging catheter  176  can be configured to substantially fill the lumen of the esophagus  178  such that a portion of the charging element  192  contacts the esophageal wall. In this position, the charging element  192  is located a short distance (e.g. 1 cm to 2 cm) from the adjacent artery  30 , and provides a direct acoustic path between the charging element  192  and the implanted device  12 . The esophagus  178  comprises primarily water and soft tissue, and is therefore acoustically matched with the impedance of the transducer elements, which helps to reduce losses in acoustic energy due to impedance mismatches. 
     Once the charging device  176  is positioned within the esophagus  178  adjacent to the body vessel or lumen containing the implanted device  12 , the charging element  192  can be activated to generate an acoustic signal that travels through the esophageal wall. As can be further seen in a transverse view of the patient&#39;s thorax in  FIG. 12 , the charging element  192  can be positioned within the esophagus  178  at a location immediately adjacent to a device  12  implanted within the right pulmonary artery  28 . In this position, the charging element  192  can be activated to generate an acoustic signal  204  that can be received by the implanted device  12  and converted into electrical energy for recharging the device  12 . During recharging, the direct acoustic pathway and relatively short distance between the esophagus  178  and the artery  28  results in an increase in charge coupling efficiency between the charging element  192  and the implanted device  12 . As with other embodiments discussed herein, this increase in efficiency reduces the overall time required to recharge the battery within the implanted device  12 , and subjects the body to less energy than would otherwise be required to recharge the device  12  via an external recharging approach. 
     Various modifications and additions can be made to the exemplary embodiments discussed without departing from the scope of the present invention. For example, while the embodiments described above refer to particular features, the scope of this invention also includes embodiments having different combinations of features and embodiments that do not include all of the described features. Accordingly, the scope of the present invention is intended to embrace all such alternatives, modifications, and variations as fall within the scope of the claims, together with all equivalents thereof.