Abstract:
A cardiac pacing apparatus has a source of pacing pulses that are transmitted through the animal by a radio frequency signal. An electrode-stent is implanted in a blood vessel adjacent to the point at which stimulation is desired. The electrode-stent contains an electrical circuit that is tuned to the radio frequency signal and which responds to receipt of that signal by applying an electric current to tissue of the animal.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to implantable medical devices which deliver energy to cardiac tissue for the purpose of maintaining a regular heart rate. Such devices are commonly referred to as cardiac pacing devices. 
     A remedy for people with slowed or disrupted natural heart beating is to implant a cardiac pacing device into the patient. A cardiac pacing device is a small electronic apparatus that stimulates the heart to beat at regular rates. It consists of a pulse generator, implanted in the patient&#39;s chest, which produces electrical pulses to stimulate heart contractions. Electrical leads extend from the pulse generator to electrode placed adjacent to specific muscles of the heart, which when electrically stimulated produce contraction of the adjacent heart chambers. 
     Modern cardiac pacing devices adapt their pulse rate to adjust the heartbeat to the patient&#39;s level of activity, thereby mimicking the heart&#39;s natural beating. The pulse generator modifies that rate by tracking the activity at the sinus node of the heart or by responding to other sensors that monitor body motion and rate of breathing. 
     Different pacing needs are met by adjusting the programming of the pulse generator and by the location of the electrodes. It is quite common that the leads extend through veins which enter the heart so that the electrodes can be placed in the muscle of the heart chamber requiring stimulation. This requires that the leads extend for some distance through the veins and may also necessitate that the leads pass through one or two heart valves. In other patients, patch electrodes are placed on the exterior heart surface with wires extending through tissue to the pacing device. With either type of lead placement, it is important that the electrodes be attached to the proper positions on the heart to stimulate the muscles and produce contractions. Thus it is desirable to properly locate the electrodes for maximum heart stimulation with minimal adverse impact to other physiological functions, such as blood circulation. 
     SUMMARY OF THE INVENTION 
     An apparatus, for electrically stimulating tissue of an animal, comprises a generator which produces a stimulation signal having pulses occurring at a rate corresponding to a rate at which stimulation is desired. Where the stimulation controls the animal&#39;s heart rate, the stimulation signal pulses occur at the heart rate that is desired for the animal. The stimulation signal is fed to a transmitter which emits a radio frequency signal. 
     An electrode-stent is implanted into a blood vessel of the animal at a location where the stimulation is desired, such as a blood vessel in a muscle of the heart. Upon receipt of the radio frequency signal the electrode-stent applies an electric current through tissue of the animal. In a preferred embodiment, the electrode-stent includes an antenna for receiving the radio frequency signal and a detector tuned to the frequency of the radio frequency signal. When the radio frequency signal is received, the detector produces an electric current that is applied to electrodes which in turn are in contact with the tissue to be stimulated. 
     The use of a radio frequency signal eliminates the need for hard wire connection between the source of the pacing signal and the stimulation electrodes. Therefore, a wire does not have to be permanently inserted through the vascular system of the animal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic view of the present invention implanted in a patient; 
     FIG. 2 is a schematic block diagram of the pacing device in FIG. 1; 
     FIG. 3 is an isometric cut-away view of a cardiac vein with a vascular electrode-stent according to the present invention; and 
     FIG. 4 is a schematic block diagram of an electrical circuit on the vascular electrode-stent. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With initial reference to FIG. 1, an apparatus for applying electrical stimulation to pace a heart  10  comprises a pacing device  12  and one or more vascular electrode-stents located in veins  14  which supply blood to the heart muscles. As will be described in greater detail, the pacing device  12  emits a radio frequency signal  16  which produces an electric current in the implanted vascular electrode-stent thereby stimulating the heart muscle. 
     Referring to FIG. 2, the pacing device  12  comprises a conventional pacing signal generator  20  similar to that utilized in previous cardiac pacers that use electrodes connected to leads. The internal circuitry and operation of the pacing signal generator is similar to those 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  22 . Both the pacing signal generator  20  and the RF transmitter  22  are powered by a battery (not shown). In response to the stimulation signal (also known as a pacing signal) from the generator  20 , the radio frequency transmitter  22  generates a correspondingly long pulse of the radio frequency signal  16  that is transmitted throughout the chest cavity via an antenna  24 . Preferably the antenna  24  either is located relatively close to the heart or is of a type which focuses the radio frequency signal toward the heart. 
     FIG. 3 illustrates an electrode-stent  30  that is placed in the vein  14  of the heart  10 . The body  33  of the electrode-stent  30  has a design similar to well-known expandable vascular stents that are employed to enlarge a restricted vein or artery. Such vascular stents have a generally tubular design that initially is collapsed to a relatively small diameter enabling them to pass freely through an artery or vein of a patient. 
     The procedure for implanting the electrode-stent  30  is similar to that used for conventional vascular stents. For example, the balloon at the end of a standard catheter is inserted into the electrode-stent  30  in a collapsed, or reduced diameter, configuration. That assembly then is inserted through an incision in a vein or artery near the skin of a patient and threaded through the vascular system to the appropriate location adjacent the heart  10 . Specifically, the electrode-stent  30  ultimately is positioned in a cardiac vein  14  adjacent to a section of the heart muscle where stimulation should be applied. The balloon of the catheter then is inflated to expand the vascular electrode-stent  30  which expansion also slightly enlarges the vein  14 , as seen in FIG. 3 which embeds the electrode-stent  30  in the wall of the vein. This slight enlargement of the vein and the tubular design of the electrode-stent allows blood to flow relatively unimpeded through the device. The balloon is deflated, the catheter is removed from the patient, and the incision is closed. The electrode-stent  30  remains in the vein without any wire connecting an electrode to pacing device  12 . 
     With reference to FIGS. 3 and 4, the vascular electrode-stent  30  has a body  33  on which is mounted a signal receiving circuit  32 . The signal receiving circuit  32  includes an antenna  34 , a radio frequency signal detector  36 , and a stimulator, that is formed by first and second electrodes  38  and  40 , for example. The antenna  34  is connected to an input of the radio frequency signal detector  36 . That detector is tuned to the frequency of the RF signal  16  that is emitted by the pacing device  12 . Upon detecting the radio frequency signal  16 , the detector  36  converts the energy of that signal into an electric current that is applied to the first and second electrodes  38  and  40 . Those electrodes form an electric circuit path with the patient&#39;s heart tissue allowing for stimulation of that tissue. Thus, each time the pacing device  12  emits a radio frequency signal  16 , a pulse of electrical current is produced in the vicinity of the electrode-stent  30 , thereby stimulating the heart muscle adjacent to that electrode. 
     Therefore, instead of coupling the pacing device to the electrodes by wires extending through the vascular system and even the heart itself, the present invention employs radio frequency signals to provide that coupling. This eliminates the need for electrical leads that extend through the veins which can break thus disabling the cardiac pacing. Furthermore, the present electrode-stents  30  and  31  can be located in the cardiac veins  14  at points that are directly associated with the specific muscles requiring stimulation. 
     With reference to FIG. 1, a plurality of vascular electrode-stents  30  and  31  which are tuned to the same radio frequency can be positioned in cardiac veins at different locations in the heart to provide simultaneous stimulation of the adjacent tissue regions. 
     Alternatively, the plurality of electrode-stents  30  and  31 , implanted in various veins of the heart muscle, can be tuned to different radio frequencies. In this embodiment, the radio frequency transmitter  22  also is tunable to produce output signals at several different radio frequencies, in response to an electrical control signal from the pacing signal generator  20 . The pacing signal generator  20  now specifies the duration and the frequency of the RF signal  16  in order to select a specific electrode-stent to stimulate the heart muscle at a particular location. As a consequence, different portions of the heart muscle can be stimulated independently and sequentially by varying the radio frequency of the emitted signal  16  to correspond to the frequency to which the electrode-stent  30  in a given location is tuned. Furthermore, the plurality of electrode-stents  30  can be activated in a given sequence by producing a series of pacer signals at different radio frequencies. This enables the pacing device  12  to produce a sequential contraction of the heart chambers to increase cardiac efficiency. 
     The foregoing description was primarily directed to a preferred embodiment 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. For example, although the invention has been described in the context of a cardiac pacing device, the inventive concept may be applied to devices for electrically stimulating other organs of the body, such as the brain for seizure control. Accordingly, the scope of the invention should be determined from the following claims and not limited by the above disclosure.