Abstract:
An apparatus and method of automatically measuring the lead impedance of a high energy shock lead before delivery of high energy therapy used to treat heart arrhythmia. In one example, an impedance measurement circuit measures the impedance between electrodes in a plurality of pairs of electrodes. The measured lead electrode impedance is compared to a predetermined value to detect if the lead is shorted to another lead. If a high-energy shock electrode is shorted to another lead, a shorted lead indicator is set to a fault state. Based on the state of the shorted lead indicator, a processor prevents or allows the delivery of high energy therapy. By checking for a lead short before delivery of the therapy, all of the energy of the therapy is delivered to the patient rather than being bypassed by a shorted lead connection.

Description:
TECHNICAL FIELD 
   This document relates to pacemakers, defibrillators, and any other devices that are capable of diagnosing and treating cardiac arrhythmia, and in particular, to an apparatus and method for ensuring effective delivery of shock therapy by automatic measurement of shock lead impedance. 
   BACKGROUND 
   Pacemakers deliver timed sequences of low energy electrical stimuli, called pace pulses, to the heart, such as via an intravascular lead (hereinafter referred to as a “lead”). By properly timing the delivery of pace pulses, the heart can be induced to contract in proper rhythm, greatly improving its pumping efficiency. 
   Defibrillators are devices capable of delivering higher energy electrical stimuli to the heart. A defibrillator is capable of delivering a high energy electrical stimulus that is sometimes referred to as a defibrillation countershock. The countershock interrupts a fibrillation, allowing the heart to reestablish a normal rhythm for efficient pumping of blood. 
   One problem that may arise is if a shock lead dislodges and the shock electrode shorts to either a pacing lead or another shock lead. The short may cause all of the energy from the countershock to be delivered internal to the device itself instead of to the heart which may damage the device. There is a need in the art for detection of shorted leads. 
   SUMMARY 
   This document discusses an apparatus and method of automatically measuring the lead impedance of a high energy shock lead before delivery of high energy therapy used to treat heart arrhythmia. In one example, an impedance measurement circuit measures the impedance between different pairs of electrodes. The measured lead electrode impedance is compared to a predetermined value to detect if the lead is shorted to another lead. If a high-energy shock electrode is shorted to another lead, a shorted lead indicator is set to a fault state. Based on the state of the shorted lead indicator, a processor prevents or allows the delivery of high energy therapy. By checking for a lead short before delivery of the therapy, all of the energy of the therapy is delivered to the patient rather than being bypassed by a shorted lead connection. 
   In one example, the lead impedance is measured after the defibrillator or defibrillator/pacemaker device has charged in preparation for a countershock. If the shorted lead is not set to a fault state and the lead impedance is greater than a predetermined value, the delivery of shock therapy is continued. If the shorted lead indicator is set to a fault state or the measured lead impedance is less than a predetermined value, shock therapy is aborted. 
   In another example, the lead impedance is measured while the device is charging in preparation for a countershock. If the charging is complete, the shorted lead indicator is not set to a fault state, and if the lead impedance is greater than a predetermined value, the delivery of shock therapy is continued. The delivery is also continued if the charging completed before the impedance measurement completed and the shorted lead indicator is not set to a fault state. The delivery of the shock therapy is aborted if the charging completed and either the shorted lead indicator was set or the lead impedance is less than or equal to a predetermined value. 
   This summary is intended to provide an overview of the subject matter of the present patent application. It is not intended to provide an exclusive or exhaustive explanation of the invention. The detailed description is included to provide further information about the subject matter of the present patent application. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a general illustration of one embodiment of portions of a system to treat cardiac arrhythmia and an environment in which it is used. 
       FIG. 2  is a block diagram of portions of a device for treating cardiac arrhythmia coupled to a heart. 
       FIG. 3  is a flowchart showing one embodiment of a method of delivering shock therapy based on the result of a lead impedance measurement. 
       FIG. 4  is a flowchart showing another embodiment of a method of delivering shock therapy based on the result of a lead impedance measurement. 
       FIG. 5  is a flowchart showing another embodiment of a method of delivering shock therapy based on the result of a lead impedance measurement. 
   

   DETAILED DESCRIPTION 
   In the following detailed description, reference is made to the accompanying drawings which form a part thereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. Other embodiments may be used and structural changes may be made without departing from the scope of the present invention. 
   The various embodiments will generally be discussed in the context of cardiac therapy given by delivering shock therapy to the coronary sinus region, having electrodes coupled to the coronary sinus region and the right atrial and ventricular regions. However, the methods described herein can be adapted to treat other forms cardiac arrhythmia by disposing leads in other selected cardiac regions. Furthermore, the methods described herein can also be adapted to unichamber therapies, having multiple lead sites within a single chamber. 
     FIG. 1  shows one embodiment of portions of a system for treating cardiac arrhythmia  100 . System  100  includes an implantable pulse generator (PG)  105  that is coupled by a first cardiac lead  110  and a second cardiac lead  115 , or one or more additional leads, to a heart  120  of a patient  125 . Implantable PG  105  can take the form of a pacemaker, a defibrillator, or a defibrillator that includes pacing capability. System  100  also includes an external programmer  140  that provides for wireless communication with the implantable PG  105  using telemetry device  145 . The first cardiac lead  110  and the second cardiac lead  115  each include a proximal end and a distal end, where the distal end of the leads  110  and  115  are implanted in, or on, the heart  120  at a first cardiac region and a second cardiac region, respectively. Each lead includes one or more electrodes that allow for combinations of either unipolar and/or bipolar sensing and delivery of energy to the heart  120  for pacing, and/or defibrillation. In some embodiments, the one or more electrodes include electrodes such as sensing, pacing, and shock electrodes. 
     FIG. 2  is a schematic diagram of one embodiment of portions of control circuitry  200  of an implantable PG  105  coupled to the heart  120 . The implantable PG  105 , as shown in  FIG. 2 , includes a sensing circuit  205  and a therapy circuit  220  coupled to shock leads  110  and  115 . The implantable PG  105  further includes a shock lead impedance measurement device  260 , a power source  270 , and a control circuit/processor  225 . In the embodiment shown, the control circuit/processor  225  incorporates a cardiac signal analyzer  230 , a comparator  240 , and a memory  250  to control implantable PG  105 . In one embodiment, the functions of the analyzer  230  and the comparator  240  are implemented in software within the control circuit/processor  225 . 
   Sensing circuit  205  is coupled to implantable leads  110  and  115 . In some embodiments, sensing circuit  205  is coupled to multiple leads. Each of the leads includes one or more shock/pacing electrodes to deliver low/high energy therapy to the heart  120 . The electrodes are disposed in multiple selected cardiac regions of the heart  120 , such as the coronary sinus region, the ventricular region, and the superior vena cava region. The electrodes coupled to leads  110  and  115  can include sensing, pacing, and/or shock electrodes. Sensing circuit  205  receives cardiac signals from the sensing electrodes and amplifies the received cardiac signals. 
   Shock lead impedance measurement device  260  is coupled to the electrodes and measures shock lead electrode impedances by measuring impedance between each possible pair of electrodes that includes at least one shock electrode from all of the disposed electrodes. One example of a method for measuring defibrillation or shock lead impedance is to measure the voltage difference between the lead electrode and another electrode resulting from a test current sent through the lead to the other electrode. The impedance is then determined by dividing the measured voltage by the test current. This method is discussed in Linder et al. U.S. Pat. No. 6,317,628, entitled “Cardiac Rhythm Management System with Painless Lead Impedance Measurement System” and is incorporated by reference herein in its entirety, including its discussion of a lead impedance measurement of a defibrillation lead. Another example of a method for measuring defibrillation lead impedance is to calculate the impedance value from the voltage droop of a capacitively coupled output voltage pulse over a fixed period of time. This method is discussed in Citak U.S. Registered Invention No. H1,929, entitled “Cardiac Rhythm Management System with Lead Impedance Measurement” and is incorporated by reference herein in its entirety. 
   Each possible pair of electrodes can include two or more shock electrodes, a shock electrode and a pacing electrode, a shock electrode and a sensing electrode, a shock electrode and two or more pacing/sensing electrodes, and a shock electrode and a conductive housing that covers part of the implantable PG  105 . 
   Comparator  240  which is coupled to the shock lead impedance measurement device  260 , then compares each of the measured shock lead electrode impedances to a predetermined acceptable shock lead electrode impedance value. In some embodiments, the predetermined acceptable lead electrode impedance value is about 20 ohms. 
   If the lead electrode impedance measurement is greater than a predetermined value, analyzer circuit  230  which is coupled to comparator  240  allows shock therapy to be delivered through the lead. If the lead electrode impedance is less than or equal to the predetermined value, the lead is presumed to be in an electrically shorted condition and analyzer circuit  230  prevents delivery of shock therapy using that lead. 
   An electrically shorted shock electrode condition can occur when one or more dislodged shock electrodes can come in contact with one or more disposed sensing/pacing electrodes, a dislodged shock electrode coming in contact with one or more other disposed shock electrodes, and a shock electrode having exposed coils. 
   In some embodiments, analyzer circuit  230  sets a shorted lead indication corresponding to a shock electrode based on the outcome of the lead impedance measurement. In some embodiments, analyzer circuit  230  sets one or more shorted lead indications to each of the shock electrodes whose measured shock lead electrode impedances are below the predetermined acceptable shock lead acceptance value. In some embodiments, setting shorted lead indications comprises setting shorted lead flags. Setting shorted lead indications can also include writing to one or more locations in a memory  250 . In some embodiments the shorted lead indications are cleared if the measured impedance value of the corresponding lead is greater than the predetermined value. 
   In one example embodiment, a first shock lead is coupled to at least one shock electrode which is adapted to be disposed around a coronary sinus regions of a heart  120 . A second shock lead is coupled to multiple sensing/pacing/shock electrodes adapted to be disposed around the right atrium of the heart  120 . A third shock lead is coupled to at least one pacing/shock electrode adapted to be disposed around the superior vena cava region of the heart  120 . Therapy circuit  220  is coupled to the first electrode to deliver a high energy shock therapy to the coronary sinus region. Further, the therapy circuit  220  is coupled to the second and third electrodes to deliver low/high energy therapy to the right atrium and superior vena cava regions of the heart  120 , respectively. 
   Therapy circuit  220  delivers high energy therapy to the coronary sinus region using the first lead if the lead electrode impedance is greater than the predetermined value. If the disposed shock lead in the coronary sinus region becomes dislodged and comes in contact with at least one of the other disposed electrodes in the right atrium or superior vena cava regions, the measured lead electrode impedance will be less than or equal to a predetermined value and delivery of shock therapy will be prevented. Delivery is prevented to maintain integrity of the therapy circuit  220  and to further ensure patient safety. Also in this embodiment, therapy circuit  220  delivers low/high energy therapy to the right atrial and superior vena cava regions based on the outcome of the impedance measurements. It can also be envisioned that the shock leads can be disposed in the ventricular region of the heart  120 . 
     FIG. 3  is a flowchart illustrating one embodiment of a method  300  of delivering shock therapy based on the result of a lead electrode impedance measurement. At step  310 , sensing leads disposed in the atrium detect that shock therapy is to be delivered. At step  320 , shock lead electrode impedance is measured between each of all possible pairs of electrodes that include the shock electrode that is to be used to deliver the shock therapy. In some embodiments, shock leads includes electrodes such as pacing and sensing electrodes. In some embodiments, electrodes are disposed around multiple selected cardiac regions that include the coronary sinus region, ventricular region, the superior vena cava region, and the conductive housing covering a part of the implantable PG  105 . 
   At step  330 , therapy circuit  220  is charged. When the charging is completed, if a shorted lead fault  340  is not indicated and a lead electrode impedance measurement was made  350 , at step  360  each of the measured shock lead electrode impedances is compared to a predetermined shock lead electrode impedance value. If the lead electrode impedance is greater than the predetermined value, at step  370  the shock therapy continues. In one embodiment, the predetermined shock lead electrode impedance value is approximately 20 ohms. 
   If the charging  330  is completed, a shorted lead fault  340  is not indicated, and a lead electrode impedance measurement was not made  350  or not completed, at step  370  the shock therapy continues. 
   If the charging  330  is completed, and either a shorted lead fault  340  is indicated, or a lead electrode impedance measurement was made  350  and the measured lead electrode impedance is less than or equal to a predetermined shock lead electrode impedance value  360 , then a shorted lead fault is set to a fault state at step  375 , the delivery of shock therapy is aborted at step  380 , but the arrhythmia therapy is continued as if the shock therapy was delivered  385 . In some embodiments the event of an aborted shock therapy delivery is logged as having occurred. After a predetermined number of logged events, no further deliveries of shock therapy are allowed. In one embodiment, the number of logged events is 6. 
   In some embodiments, shorted lead flags are cleared when the shorted leads are corrected. Generally, flags are cleared by a physician or a trained health care professional. 
     FIG. 4  is an alternate embodiment of the method shown in  FIG. 3 . In this embodiment the lead impedance is measured at step  420  and the shorted lead fault is checked at step  440  without charging the therapy circuit  220 . 
     FIG. 5  is a flowchart illustrating another embodiment of a method  500  of delivering shock therapy based on the result of a lead electrode impedance measurement. At step  510 , it is determined that shock therapy is to be delivered. At step  520 , shock lead electrode impedance is measured between each of all possible pairs of electrodes that include the shock electrode that is going to be used to deliver the shock therapy while the therapy circuit  220  is charging. If charging is complete  530 , a shorted lead fault is not indicated  540 , the lead electrode impedance measurement was completed  550 , and the lead electrode impedance value is greater than a predetermined impedance value  555 , at step  565  the shock therapy continues. In one embodiment, the predetermined shock lead electrode impedance value is approximately 20 ohms. 
   If charging is complete  530 , a shorted lead fault is not indicated  540 , and the lead electrode impedance measurement is not complete  550 , then at step  560  the lead electrode impedance measurement is aborted and at step  565  the delivery of shock therapy is continued. 
   If charging is complete  530 , and either a shorted lead fault is indicated  540  or the lead electrode impedance measurement completed and the lead electrode impedance was not greater than the predetermined value, then the delivery of shock therapy is aborted at step  570 , the shorted lead fault is set to a fault state at step  575 , but the arrhythmia therapy is continued as if the shock therapy was delivered  580 . In some embodiments continuing as if the shock therapy was delivered includes logging the event of an aborted shock therapy delivery as having occurred. After a predetermined number of logged events, no further deliveries of shock therapy are allowed. 
   Although specific examples have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any other embodiment that exists that is calculated to achieve the same purpose may be substituted for the specific example shown. This application is intended to cover any adaptations or variations of the present invention. Therefore, it is intended that this invention be limited only by the claims and their equivalents.