Abstract:
An implantable device electrically stimulates an organ of an animal in response to a trigger event. Between trigger event that device receives a wireless signal, such as a radio frequency signal, and stores energy from the signal in a plurality of capacitors. The capacitors are located within a electrical lead that extends to a stimulation electrode attached to the organ. That electrical lead has a hollow outer insulating tube with a pair of conductors extending longitudinally therein. The plurality of capacitors are connected between the pair of conductors. Thus the electrical lead serves as both a conductor of a stimulation current to the electrode and a housing for the plurality of capacitors.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   Not Applicable 
   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not Applicable 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to implantable medical devices which produce electrical pulses that to stimulate organs of an animal, and more particularly to the storage of energy in such medical devices. 
   2. Description of the Related Art 
   A remedy for people with slowed or disrupted natural heart activity involves implanting a cardiac pacing device which is a small electronic apparatus that stimulates the heart to beat at regular rates. 
   Typically the pacing device is implanted in the patient&#39;s chest and has sensor electrodes that detect electrical impulses associated with in the heart contractions. These sensed impulses are analyzed to determine when abnormal cardiac activity occurs, in which event a pulse generator is triggered to produce electrical pulses. Wires carry these pulses to electrodes placed adjacent specific cardiac muscles, which when electrically stimulated contract the heart chambers. It is important that the stimulation electrodes be properly located to produce contraction of the heart chambers. 
   Modern cardiac pacing devices vary the stimulation to adapt the heart rate to the patient&#39;s level of activity, thereby mimicking the heart&#39;s natural action. The pulse generator modifies that rate by tracking the activity of the sinus node of the heart or by responding to other sensor signals that indicate body motion or respiration rate. 
   U.S. Pat. No. 6,445,953 describes a cardiac pacemaker that has a pacing device, which can be located outside the patient, to detect abnormal electrical cardiac activity. In that event, the pacing device emits a radio frequency signal, that is received by a stimulator implanted in a vein or artery of the patient&#39;s heart. Specifically, the radio frequency signal induces a voltage pulse in an antenna on the stimulator and that pulse is applied across a pair of electrodes, thereby stimulating adjacent muscles and contracting the heart. 
   Although this cardiac pacing apparatus offered several advantages over conventional pacemakers, it required that sufficient energy be derived from the received radio frequency signal to power the implanted circuit and to stimulate the adjacent organ. The amount of energy required may be relatively great, especially when the apparatus is an implantable defibrillator. 
   Therefore, it is desirable to provide an energy storage mechanism in the implanted apparatus which will accumulate energy from a radio frequency signal and provide that accumulated energy when needed for organ stimulation. 
   SUMMARY OF THE INVENTION 
   An apparatus is provided to artificially stimulate an organ of an animal. That apparatus includes a first electrode and a second electrode for implantation into the animal. An electrical lead has a first conductor and a second conductor with a plurality of capacitors connected to the first and second conductors. The plurality of capacitors may be connected in parallel or in series between the first and second conductors. 
   An element is provided for receiving wireless signals, which may be in the radio frequency spectrum, for example. A stimulation circuit is connected to the first electrode, the first conductor, the second conductor and the element. The stimulation circuit uses energy from a received first wireless signal to charge the plurality of capacitors with electrical energy. In response to a trigger event, the stimulation circuit applies the electrical energy from plurality of capacitors to the first and second electrodes to electrically stimulate the organ of the animal. 
   The trigger event may be the receipt of a second wireless signal that has a unique format which indicates that stimulation should occur. In this case, the second wireless signal may emanate from a pacing circuit that detects when artificial pacing of a heart if required. In another case, the wireless signal may emanate from a circuit that detects when cardiac defibrillation is required. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a representation of a cardiac pacing apparatus attached to a medical patient; 
       FIG. 2  is a circuit diagram of a power transmitter for the cardiac pacing apparatus; 
       FIG. 3  is an isometric cut-away view of cardiac blood vessels in which a vascular stimulator and a second electrode have been implanted; 
       FIG. 4  is a block diagram of an electrical circuit on the vascular stimulator shown in  FIG. 2 ; 
       FIG. 5  is a longitudinal cross sectional view through an electrical conductor which contains energy storage capacitors according to the present invention; 
       FIG. 6  is a longitudinal cross sectional view through a second electrical conductor which contains energy storage capacitors connected in series; and 
       FIG. 7  is a longitudinal cross sectional view through another electrical conductor which contains energy storage capacitors connected in parallel in groups that are then connected in series. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   With initial reference to  FIG. 1 , a pacing apparatus  10  electrically stimulates a medical patient&#39;s heart  12  to contract or to convert from fibrillation to a normal rhythm. That apparatus comprises an external pacing device  14  and a stimulator  20  that is implanted in a blood vessel  18  of a muscle in the heart. As will be described in greater detail, the pacing device  14  transmits a radio frequency signal  16  which causes the stimulator  20  to emit an electric current that stimulates the heart muscle. 
   Referring to  FIG. 2 , the pacing device  14 , that preferably is worn outside the patient&#39;s body adjacent the chest, comprises a conventional pacing signal generator  22  connected to input electrodes  23  attached to the patient&#39;s body. Alternatively, the pacing device  14  may be implanted in the patient. The internal circuitry and operation of the pacing signal generator  22  are similar to prior cardiac pacers. However, instead of the output stimulation signals being applied to the electrodes via leads, the pacing signals are applied to an input of a radio frequency (RF) transmitter  24 . In response to the stimulation signal (also known as a pacing signal) from the generator  22 , the radio frequency transmitter  24  generates a pulse of the radio frequency signal  16  that is transmitted throughout the chest cavity via an antenna  26 . Preferably the antenna  26  either is located relatively close to the heart  12  or is of a type which focuses the radio frequency signal toward the heart. Both the pacing signal generator  22  and the RF transmitter  24  are powered by a battery  28 . 
   As illustrated in  FIG. 3 , the stimulator  20  is placed in an artery or vein  18  in close proximity to the atria or ventricles of the heart  12 . For example the vascular stimulator  20  may be positioned in the coronary sinus vein. The stimulator  20  includes a body  30  similar to well-known expandable vascular stents that are employed to enlarge a restricted vein or artery. The body  30  has a generally tubular shape that initially is collapsed to a relatively small diameter enabling the stimulator to pass freely through blood vessels of a patient. The procedure for implanting the stimulator  20  is similar to that used for vascular stents. For example, a balloon at the end of a standard catheter is inserted into the stimulator  20  in a collapsed configuration. That assembly is inserted through an incision in a vein or artery near the skin of a patient and passed through the vascular system to the appropriate location proximate to the atria or ventricles of the heart  12 . The balloon of the catheter then is inflated to expand the stimulator  20 , thereby slightly enlarging the blood vessel  18  which embeds the stimulator in the wall of the vein or artery. The balloon is deflated, the catheter is removed from the patient, and the incision is closed. Alternatively, a self-expanding body  30  may be utilized. The slight enlargement of the blood vessel  18  and the tubular design of the body  30  allows blood to flow relatively unimpeded through the vascular stimulator  20 . 
   With additional reference to  FIG. 4 , the stimulator  20  has a stimulation circuit  32  and a receive antenna  34  in the form of a wire coil wound circumferentially around the body  30 . A first electrode  36  in the form of a ring encircles the body. The stimulation circuit  32  includes an RF signal detector  38  having an input connected to the receive antenna  34  and tuned to the frequency of the RF signal  16  that is emitted by the pacing device  14 . The RF signal detector  38  converts the energy of that RF signal into an electric voltage that charges a storage capacitor  40  which supplies electrical power to the circuitry on the vascular stimulator  20 . The periodic pulses of the RF signal charge the storage capacitor  40  so that it will have sufficient stored energy when stimulation of the heart is required. 
   A first electrode  36  in the form of a ring encircles the body is connected to one terminal of the storage capacitor  40 . A second electrode  44  is adjacent to the wall of a blood vessel  46  in another section of the heart, as seen in  FIG. 3 , and is coupled to the stimulation circuit  32  by an insulated electrical lead  48  extending through the blood vessels. The relatively small size of the second electrode  44  allows it to be placed into a significantly smaller blood vessel  46  than the vascular stimulator  20 . As a result, the second electrode  44  can be placed is a greater variety of locations in the cardiac vascular system and in close proximity to the muscles that contract the desired portion of the heart  12 . 
   In order to provide a enough electrical energy for stimulation, especially for defibrillation, as relatively large storage capacitance is required. Instead of placing that capacitance on the body  30  of the stimulator  20 , a plurality of capacitors are placed along the length of the electrical lead  48  that connects the second electrode  44  to the stimulation circuit  32 . With reference to  FIG. 5 , the electrical lead  48  has a tubular shell  50  of insulated material with two conductors  51  and  52  extending longitudinally along the central opening. A plurality of disk-shaped capacitors  54  also are spaced along that central opening. These capacitors  54  may be conventional surface mount type devices, such as model PCC2232CT which is 4.7 μf, 16 WVDC capacitor available from Panasonic Corporation of North America in Secaucus, N.J. 07094. The terminal on one side of each capacitor  54  is connected to the first electrical conductor  51  and the other terminal is connected to the second conductor  52 . Therefore, the plurality of capacitors  54  are connected in parallel so that the individual capacitances are summed to form a large equivalent storage capacitor  40 . 
     FIG. 6  illustrates an alternative structure for the electrical lead  48  which has an tubular insulated shell  60  with first and second conductors  61  and  62  extending longitudinally along the central opening. A plurality of disk-shaped capacitors  64  are connected in series between the first and second conductors  61  and  62 , in contrast with electrical lead  48  in which the capacitors  54  are connected in parallel. 
   Referring again to  FIG. 4 , the pacing device  14  periodically transmits the radio frequency signal  16  to the stimulator  20 . The RF signal detector  38  derives electrical voltage from the energy of that RF signal and applies that voltage across conductors  51  and  52  to charge the plurality of capacitors  64  that form the storage capacitor  40 . Thus a sufficient charge is maintained on the storage capacitor  40  for when cardiac stimulation is needed. 
   The pacing device  14  also responds to the electrical signals from the heart, that are detected by the input electrodes  23 , by determining in a conventional manner when cardiac stimulation is to required. When stimulation is to occur, the RF transmitter sends a uniquely shaped RF signal pulse sequence. The RF signal detector  38  in  FIG. 4  responds to that RF signal pulse sequence by activating a pulse circuit  42  that closes a switch  45  connected to a second electrode. That action completes a circuit thereby dumping the voltage stored on the capacitor across the first and second electrodes  36  and  44  which stimulates the heart tissue between those electrodes. 
     FIG. 7  illustrates a third structure for the electrical lead  48  which has an tubular insulated shell  70  with first and second conductors  71  and  72  extending longitudinally along the central opening. A plurality of disk-shaped capacitors  74 - 80  are located within central opening. The capacitors are connected in groups with the devices in each group being coupled in series with the groups connected in parallel to the first and second conductors  71  and  72 . Specifically, one group comprises capacitors  74 ,  75  and  76  that are connected in series with the first capacitor  74  in that group having a terminal that is coupled to the first conductor  71  and the last capacitor  76  in that group having a terminal that is coupled to the second conductor  72 . Another group comprises capacitors  77 ,  78  and  79  connected in series with the first capacitor  74  coupled to the first conductor  71  and the last capacitor  76  coupled to the second conductor  72 . The number of capacitors in each group and the number of groups are chosen to provide the desired cumulative capacitance and working voltage for the storage capacitor  40  of the stimulator  20 . 
   The foregoing description was primarily directed to preferred embodiments of the invention. Even though some attention was given to various alternatives within the scope of the invention, it is anticipated that one skilled in the art will likely realize additional alternatives that are now apparent from disclosure of embodiments of the invention. Accordingly, the scope of the invention should be determined from the following claims and not limited by the above disclosure.