Patent Document

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 deliver energy to heart tissue to stimulate cardiac contractions, and more particularly to such cardiac pacing devices that are implantable in a vein or artery. 
   2. Description of the Related Art 
   A remedy for people with slowed or disrupted natural heart activity is to implant 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 irregular cardiac activity occurs, in which event a pulse generator is triggered to produce electrical pulses. Wires carry these pulses to patch-type stimulation 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 activity. The pulse generator modifies that rate by tracking the activity at 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 irregular or weak cardiac activity. In that event, the pacing device emits a radio frequency signal, that is received by a circuit mounted on a stent 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 stent and that pulse is applied across a pair of electrodes on the stent, thereby stimulating adjacent muscles and contracting the heart. Although this cardiac pacing apparatus offered several advantages over other types of pacemakers, it required placement of sensing electrodes on the patient&#39;s chest in order for the external pacing device to detect when the heart requires stimulation. 
   SUMMARY OF THE INVENTION 
   A cardiac pacing apparatus is provided to artificially stimulate contractions of a heart in an animal. That apparatus includes a power transmitter which periodically transmits a pulse of a radio frequency signal to a vascular electrode-stent that is implanted preferably in a vein or artery the animal. 
   The vascular electrode-stent comprises an pickup device, such as a coil of wire for example, for receiving the radio frequency signal and a cardiac signal emitted from the sinus node of the heart. A pacing signal circuit is connected to the pickup device and a pair of electrodes that are in contact with tissue of the animal. The pacing signal circuit has an electrical storage device that is charged by electrical energy from the radio frequency signal. In response to detecting the cardiac signal, the pacing signal circuit applies a stimulation voltage pulse across the pair of electrodes to cause a contraction of the heart. 
   In a preferred embodiment of the vascular electrode-stent, the pacing signal circuit includes a discriminator and a pulse circuit. The discriminator is connected to the pickup device and controls charging of the electrical storage device in response to detecting a pulse of the radio frequency signal. When the discriminator detects the cardiac signal, a trigger signal is produced, which causes the pulse circuit to apply the stimulation voltage pulse across the pair of electrodes. 

   
     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 electrode-stent and a second electrode have been implanted; 
       FIG. 4  is a block diagram of an electrical circuit on the vascular electrode-stent shown in  FIG. 2 ; and 
       FIGS. 5A , B, and C are waveform diagrams of three electrical signals in the cardiac pacing apparatus. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   With initial reference to  FIG. 1 , a pacing apparatus  10  for electrically stimulating a heart  12  to contract comprises a power transmitter  14  and a vascular electrode-stent  20 . The power transmitter  14  preferably is worn outside the patient&#39;s body adjacent the chest and emits a radio frequency signal  16  which is received by the vascular electrode-stent  20 . Alternatively, the power transmitter  14  may be implanted in the patient. As will be described in greater detail, receipt of radio frequency signal  16  provides electrical power for circuitry on the electrode-stent. The vascular electrode-stent  20  is placed in an artery or vein  18  which carries blood through the heart in close proximity to the sinus node. For example the vascular electrode-stent  20  may be positioned in the ______ artery. 
   Referring to  FIG. 2 , the power transmitter  14  comprises a radio frequency (RF) transmitter  22  connected to a timing circuit  24  and to an antenna  26 . Both the RF transmitter  22  and the timing circuit  24  are powered by a battery  28 . The timing circuit  24  controls the RF transmitter  22  to emit periodic pulses of the radio frequency signal  16 . For example, the pulses have relatively slow rising and falling edges, as shown in  FIG. 4A , so that the signal level gradually increases and decreases. 
   As illustrated in  FIG. 3 , the electrode-stent  20  includes a body  30  similar to well-known expandable vascular stents that are employed to enlarge a restricted vein or artery. Such vascular stents have a generally tubular shape that initially is collapsed to a relatively small diameter enabling them to pass freely through blood vessels of a patient. The procedure for implanting the electrode-stent  20  is similar to that used for conventional vascular stents. For example, a balloon at the end of a standard catheter is inserted into the vascular electrode-stent  20  in a collapsed configuration. That assembly is inserted through an incision in a vein or artery near the skin of a patient and pushed through the vascular system to the appropriate location proximate to the sinus node of the heart  12 . The balloon of the catheter then is inflated to expand the vascular electrode-stent  20 , thereby slightly enlarging the blood vessel  18  which embeds the electrode-stent 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 stent may be utilized as the body  30 . The slight enlargement of the blood vessel  18  and the tubular design of the stent&#39;s body  30  allows blood to flow relatively unimpeded through the vascular electrode-stent  20 . 
   With reference to  FIGS. 3 and 4 , the vascular electrode-stent  20  has a pacing signal circuit  32  and a pickup device  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 pacing signal circuit  32  includes a pulse discriminator  38  connected to the pickup device  34 . As will be described, the pulse discriminator  38  distinguishes between electrical pulses induced in the pickup device by electrical events at the sinus node of the heart and by the RF signal  16  from the power transmitter  14 . That distinguishing is based on the shape of the respective signal waveform and the pulses of those waveforms as illustrated in  FIG. 4A  for the RF signal  16  and in  FIG. 4B  for the cardiac signal from the sinus node. The RF signal has relatively long duration pulses with gradually rising and falling edges. In contrast, the electrical pulses of the cardiac signal are very short duration and rise and fall quickly. The pulse discriminator  38  also is able to detect when both types of pulses coincide in time. 
   Whenever an RF signal pulse is detected, the pulse discriminator  38  uses the energy of that signal to charge a storage capacitor  40  which supplies electrical power to the circuitry on the vascular electrode-stent  20 . Other types of electrical storage devices may be employed. The radio frequency signal supplies power to the vascular electrode-stent, and unlike prior wireless pacemakers does not trigger cardiac stimulation. 
   The sinus node of the heart  12  emits an electrical cardiac signal which causes contraction of the heart chambers. The cardiac signal travels from cell to cell in paths through the heart to muscles which contract the atria. This signal also propagates along another path until reaching the atrioventricular (AV) node, which is a cluster of cells situated in the center of the heart between the atria and ventricles. The atrioventricular node serves as a gate that slows the electrical current before the cardiac signal is permitted to pass to the ventricles. This delay ensures that the atria have a chance to fully contract before the ventricles are stimulated. 
   Due to the placement of the vascular electrode-stent  20  in proximity to the sinus node, emission of the cardiac signal also induces an electric current pulse in the pickup device, or coil,  34  of the vascular electrode-stent  20 , as depicted in  FIG. 5B . The pulse discriminator  38  recognizes the rapid rise time of this pulse as being produced by the cardiac signal, as compared to a RF signal pulse shown in  FIG. 5A . When a cardiac signal pulse is detected, the pulse discriminator  38  issues a trigger signal to a pulse circuit  42 . The pulse circuit  42  is similar to circuits used in previous cardiac pacing devices which generate voltage pulses for stimulating a contraction of the heart, as shown in  FIG. 5C . Specifically, upon being triggered the pulse circuit  42  uses the charge on the capacitor  40  to produce a voltage pulse that is applied between the first electrode  36 , that extends around the stent body, and a second electrode  44 , which is remote from the vascular electrode-stent  20 . 
   As shown in  FIG. 3 , the second electrode  44  is secured to the wall of a blood vessel  46  in another section of the heart and is connected to the pulse circuit  42  by a thin insulated wire  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 electrode-stent  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 . 
   Depending upon whether the second electrode  44  is placed to stimulate contraction of an atrium or a ventricle, the pulse circuit  42  delays a predefined amount of time after receiving the trigger signal from the pulse discriminator  38  before applying the voltage pulse to the first and second electrodes. Therefore, timing of muscle stimulation corresponds to that which occurs with respect to naturally induced contraction of the atrium or ventricle. The duration of that delay is programmed into the pulse circuit  42  by the surgeon upon implantation and is a function of the location of the second electrode. 
   In another version of the vascular electrode-stent  20 , one or more additional electrodes, such as a third electrode  50 , can be implanted in other cardiac blood vessels  52  to stimulate further sections of the heart. In this case, individual voltage pulses can be applied between the first electrode  36  and each of the additional electrodes  44  and  50  to separately stimulate contraction of those other sections of the heart. A stimulation pulse also may be applied between the second and third electrodes  44  and  50 , without using the first electrode  36 . 
   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.

Technology Category: 1