Patent Publication Number: US-7711434-B2

Title: Wireless intravascular medical device with a double helical antenna assembly

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a divisional of U.S. patent application Ser. No. 11/166,889 filed on Jun. 24, 2005, now U.S. Pat. No. 7,295,879. 

   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 are controlled by a wireless signal that is received by the device, and more particularly to cardiac stimulation 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 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 activity. 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. 
   The stimulator in that wireless system is powered by the energy of the received signal thus requiring that the pacing device transmit a relatively strong radio frequency signal in order to provide adequate energy to the stimulator implanted deep in the patient&#39;s chest. It is desirable to place the stimulator in a blood vessel located closer to the skin of the patient with stimulation electrodes implanted in one or more cardiac blood vessels and connected to the stimulator by wires extending through the electronic circuit circulatory system. This would enable more of the energy from the frequency signal to reach the stimulator, however, the blood vessels close to the skin are not sufficiently large to accommodate the size of the stimulator. 
   SUMMARY OF THE INVENTION 
   A medical device, such as a cardiac pacing device or an implanted defibrillator for example, includes an antenna assembly with an intravascular coil for engaging a wall of a first blood vessel to receive a radio frequency signal. The coil has a first end and a second end along a longitudinal axis. A first winding of the coil is wound helically in a rotational direction along a longitudinal axis from a first terminus at the first end to the second end, and a second winding connected to the a first winding at the second end and wound helically in that same rotational direction along a longitudinal axis from the second end to a second terminus at the first end. 
   The medical device also has an electronic circuit implanted in the patient and connected to receive an electrical signal from the receiver antenna assembly. In the case of a cardiac pacing device, the electronic circuit determines when stimulation of the heart is required and applies a voltage pulse to tissue of the heart. 
   The preferred embodiment of the medical device also includes a transmitter antenna outside the patient and a transmitter that generates a radio frequency signal which is applied to the transmitter antenna. The antenna assembly includes a detector that rectifies the radio frequency signal received from the transmitter antenna to produce a direct current. A storage device in the electronic circuit is connected to the detector for storing electrical energy derived from the radio frequency signal to provide electricity for powering other components of the electronic circuit. 
   Another aspect of the present invention is to implant the antenna assembly in a blood vessel of a limb or the neck of the patient and place the transmitter antenna so that it is positioned around the limb or neck. Ideally the longitudinal axis of the windings of the antenna assembly are substantially parallel with the axis of the transmitter antenna or its generated field to optimize signal coupling there between. A conductor extends from the antenna assembly through the patient&#39;s vascular system to the stimulator. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a representation of a cardiac pacing system attached to a medical patient; 
       FIG. 2  is an isometric cut-away view of the patient&#39;s blood vessels in which an receiver antenna, a stimulator and a stimulation electrode have been implanted at different locations; 
       FIG. 3  is a schematic diagram of a power source that transmits a radio frequency signal to the implanted components; 
       FIG. 4  is a schematic diagram of the implanted circuitry of the cardiac pacing apparatus; 
       FIG. 5  depicts the receiver antenna in a configuration during implantation; and 
       FIG. 6  illustrates the receiver antenna in a deployed configuration. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   With initial reference to  FIG. 1 , a cardiac pacing system  10  for electrically stimulating a heart  12  to contract comprises an external power source  14  and a pacing apparatus  15  implanted in the circulatory system of a human medical patient. The pacing apparatus  15  receives a radio frequency (RF) signal from the power source  14  worn outside the patient and the implanted electrical circuitry is electrically powered from the energy of that signal. 
   With additional reference to  FIG. 3 , the power source  14  comprises a radio frequency transmitter  30  that is powered by a battery  32 . The transmitter  30  periodically emits a signal at a predefined radio frequency that is applied to a transmitter antenna  34 . The transmitter antenna  34  is a coil of wire within a band  36  that is placed around the patient&#39;s upper arm  28 . Thus the antenna coil is wound around the patient&#39;s arm. Alternatively another limb or area of the body, such as the neck, with an adequately sized blood vessel close to the skin surface of the human medical patient can be used. In a basic version of the cardiac pacing system  10 , the radio frequency signal merely is used to convey energy for powering the pacing apparatus  15  implanted in the patient. In a more sophisticated version of the cardiac pacing system  10 , the transmitter  30  modulates the radio frequency signal with commands received from optional control circuits  35  that configure or control the operation of the pacing apparatus  15 . 
   Referring to  FIGS. 1 and 2 , the implanted pacing apparatus  15  includes an intravascular stimulator  16  located a vein or artery  18  in close proximity to the heart. Because of its electrical circuitry, the stimulator  16  is relatively large requiring a blood vessel that is larger than the arm vein, e.g. the basilic vein which is approximately five millimeters in diameter. As a result, the stimulator  16  may be embedded in the superior or inferior vena cava. Electrical wires lead from the stimulator  16  through the cardiac vascular system to one or more locations in smaller blood vessels, e.g. the coronary sinus vein, at which stimulation of the heart is desired. At such locations, the electrical wires are connected to electrodes  20  and  21  implanted into the blood vessel walls. 
   Because the stimulator  16  of the pacing apparatus  15  is near the heart and relatively deep in the chest of the human medical patient, a receiver antenna  24  is implanted in a vein or artery  26  of the patient&#39;s upper right arm  28  at a location surrounded by the transmitter antenna  34  with the arm band  36 . That arm vein or artery  26  is significantly closer to the skin and thus receiver antenna  24  picks up a greater amount of the energy of the radio frequency signal emitted by the power source  14 , than if the receiver antenna was located on the stimulator  16 . 
   As illustrated in  FIG. 2 , the intravascular stimulator  16  has a body  40  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 a blood vessel of a patient. The stimulator body  40  and the other components of the pacing apparatus  15  are implanted in the patient&#39;s circulatory system using a catheter and techniques similar to those employed to implant vascular stents. In an additional embodiment, the stimulator  40  is encapsulated in a biocompatible waterproof capsule floating in the bloodstream of the vessel which has significantly larger diameter. From the capsule multiple micro-coaxial cables are connected to a plurality of bipolar electrodes in small cardiac blood vessels. 
   With reference to  FIGS. 2 and 4 , the stimulator body  40  has a pacing circuit  42  mounted thereon. Depending upon its proximity to the heart  12 , the body  40  may have a stimulation electrode  46  in the form of a ring encircles the body. Alternatively, when the stimulator is relatively remote from the heart  12  the stimulation electrode  46  is replaced by a second stimulation electrode  21  ( FIG. 1 ) located in a small cardiac blood vessel. The pacing circuit  42  includes a power supply  48  to which a micro-coaxial cable  49  from the receiver antenna  24  is connected. The power supply  48  utilizes electricity from that antenna to charge a storage capacitor  50  that provides electrical power to the other components of the pacing circuit  42 . A stimulation control circuit  52 , of a conventional design, detects the electrical activity of the heart by means of two or more sensing electrodes  54  located either on the stimulator body  40  or implanted in blood vessels of the heart  12 . In response to that cardiac electrical activity, the stimulation control circuit  52  determines when electrical pulses need to be applied to the heart to stimulate cardiac contractions to provide a proper heart rate. When such stimulation is desired, the stimulation control circuit  52  sends a control signal to a pulse circuit  56  that applies electrical voltage from the storage capacitor  50  across the stimulation electrodes  20  and  21  or  46 . Alternatively when bipolar electrodes are employed as devices  20  and  21 , the electrical voltage is applied across the tissue contacts of each electrode. 
   The pacing apparatus  15  utilizes a unique receiver antenna  24 . With reference to  FIG. 5 , the receiver antenna  24  comprises a coil  60  formed by an electrical conductor wound in a double helix. The coil  60  has a first terminus  61  at a first end  62  and a first helical winding  64  is wound in one rotational direction (e.g. clockwise) from that first terminus along a longitudinal axis  63  to an opposite second end  65  of the antenna coil. At the second end  65 , the conductor loops into a second helical winding  66  that is wound in the same rotational direction going from the second end  65  back to the first end  62  where the second helical winding ends at a second terminus  69 . Thus the conductor of the coil  60  is wound in the same direction when forming the double helix. However, viewed from either end of the coil  60 , the first helical winding  64  extends from that end in one rotational direction and the second helical winding  66  extends from that same end in the opposite rotational direction so that convolutions of the helical windings cross each other. In the preferred embodiment illustrated in  FIG. 5 , the first and second helical windings  64  and  66  have the same number of turns which results in every convolution of each helical winding crossing the other helical winding at two locations  67 . Although the size of the coil  60  and the number of turns may differ depending upon the particular application in which the antenna is being utilized, one application for an implantable pacing device employs a coil  60  that has a diameter of five to six millimeters, a length of two inches when deployed, and twelve turns in each helical winding  64  and  66 . 
   The cross section of the wire used to wind the double helical coil  60  is selected to provide the desired spring coefficient. A coil made from round, or circular, wire has a uniform spring coefficient whereas a ribbon (wire with a rectangular cross section) exhibits different resistances to axial versus radial deformation. Various other cross sectional shapes can be used. 
   To implant the antenna  24  in a vein or artery of a patient, the coil  60  is stretched longitudinal which reduces its diameter, as depicted in  FIG. 6 . In this state the coil is releasably attached to a catheter that is used to guide and place the antenna  24  at the desired location within the patient&#39;s vascular system. When the antenna has been properly located, the catheter is operated to release the coil which, due to its resiliency, contracts longitudinally which increases its diameter, thereby springing into a shape illustrated in  FIG. 5 . When the coil  60  is in the deployed or contracted state the spacing between corresponding points  68  on adjacent convolutions is at least five times the width of the coil&#39;s conductor. In this expanded, or deployed, state the windings  64  and  66  are embedded into the wall of the blood vessel  26 , as seen in  FIG. 2 , thereby securing the antenna  24  at that location. The antenna  24  is preferably embedded in an artery or vein in the upper arm of the patient wherein the longitudinal axis  63  of the receiver coil  60  is substantially parallel to the longitudinal axis or the field of the coil of the transmitter antenna  34  in the band  36  placed around the arm in  FIG. 1 . That alignment maximizes the signal coupling between the two antenna coils. 
   With reference to  FIG. 4 , the termini  61  and  69  of the antenna coil  60  are connected to the inputs of a rectifier detector  70  which converts the radio frequency signal received by the coil  60  into a DC voltage at the output terminals  72  and  74 . Preferably the rectifier detector  70  is co-located with the antenna  24  in the arm  28  of the patient. The output terminals  72  and  74  are connected to a micro-coaxial cable  49  that extends through the patient&#39;s circulatory system to the power supply  48  in the circuit  42  of the intravascular stimulator  16 . As previously described, the voltage received from the antenna  24  electrically powers the stimulator circuitry. By converting the radio frequency signal to a direct current at the remotely located antenna  24 , the significant losses associated with sending a radio frequency signal in a wire extending through the vascular system are avoided. 
   The foregoing description was primarily directed to a 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.