Patent Publication Number: US-2011077540-A1

Title: Method and apparatus for detecting fibrillation using cardiac local impedance

Description:
CLAIM OF PRIORITY 
     This application is a divisional of and claims the benefit of priority under 35 U.S.C. §120 to U.S. patent application Ser. No. 11/550,923, filed on Oct. 19, 2006, which is hereby incorporated by reference herein in its entirety. 
    
    
     TECHNICAL FIELD 
     This document relates generally to cardiac rhythm management (CRM) systems and particularly to an anti-tachyarrhythmia system that detects fibrillation using cardiac local impedance indicative of cardiac local wall motion. 
     BACKGROUND 
     Tachyarrhythmias are abnormal heart rhythms characterized by a rapid heart rate. Tachyarrhythmias generally include supraventricular tachyarrhythmia (SVT, including atrial tachyarrhythmia, AT) and ventricular tachyarrhythmia (VT). Fibrillation is a form of tachyarrhythmia further characterized by an irregular heart rhythm. In a normal heart, the sinoatrial node, the heart&#39;s predominant natural pacemaker, generates electrical impulses, called action potentials, that propagate through an electrical conduction system to the atria and then to the ventricles of the heart to excite the myocardial tissues. The atria and ventricles contract in the normal atrio-ventricular sequence and synchrony to result in efficient blood-pumping functions indicated by a normal hemodynamic performance. VT occurs when the electrical impulses propagate along a pathologically formed self-sustaining conductive loop within the ventricles or when a natural pacemaker in a ventricle usurps control of the heart rate from the sinoatrial node. When the atria and the ventricles become dissociated during VT, the ventricles may contract before they are properly filed with blood, resulting in diminished blood flow throughout the body. This condition becomes life-threatening when the brain is deprived of sufficient oxygen supply. Ventricular fibrillation (VF), in particular, stops blood flow within seconds and, if not timely and effectively treated, causes immediate death. In very few instances a heart recovers from VF without treatment. 
     Cardioversion and defibrillation are used to terminate most tachyarrhythmias, including AT, VT, and VF. An implantable cardioverter/defibrillator (ICD) is a CRM device that delivers an electric shock to terminate a detected tachyarrhythmia episode by depolarizing the entire myocardium simultaneously and rendering it refractory. Another type of electrical therapy for tachyarrhythmia is anti-tachyarrhythmia pacing (ATP). In ATP, the heart is competitively paced in an effort to interrupt the reentrant loop causing the tachyarrhythmia. An exemplary ICD includes ATP and defibrillation capabilities so that ATP is delivered to the heart when a non-fibrillation VT is detected, while a defibrillation shock is delivered when VF occurs. 
     The efficacy of cardioversion, defibrillation, and ATP in terminating tachyarrhythmia depends on the type and origin of the tachyarrhythmia. An unnecessary therapy delivered during a non-life-threatening tachyarrhythmia episode may cause substantial pain in the patient and reduces the longevity of the ICD while providing the patient with little or no benefit. On the other hand, a necessary therapy withheld during a life-threatening tachyarrhythmia episode may result in irreversible harm to the patient, including death. For these and other reasons, there is a need for accurate tachyarrhythmia detection that ensures patient safety while reducing unnecessary delivery of anti-tachyarrhythmia therapy. 
     SUMMARY 
     A CRM system detects tachyarrhythmia using cardiac local impedance indicative of cardiac local wall motion. A cardiac local impedance signal indicative of an impedance of a cardiac region is sensed by using a pair of bipolar electrodes placed in that cardiac region. Tachyarrhythmia such as VF is detected by analyzing one or more cardiac local impedance signals sensed in one or more cardiac regions. 
     In one embodiment, a CRM system includes an implantable lead and an implantable medical device. The implantable lead includes a proximal end, a distal end, and an elongate lead body coupled between the proximal end and the distal end. The proximal end is to be coupled to the implantable medical device. The distal end is to be placed in the heart and includes a pair of impedance sensing electrodes for sensing a cardiac local impedance signal. The implantable medical device includes an impedance sensing circuit and an impedance-based tachyarrhythmia detector. The impedance sensing circuit senses the cardiac local impedance signal using the pair of impedance sensing electrodes. The impedance-based tachyarrhythmia detector detects a predetermined-type tachyarrhythmia using the cardiac local impedance signal. 
     In one embodiment, a method for detecting tachyarrhythmia is provided. A cardiac local impedance signal is sensed using a pair of impedance sensing electrodes at a distal end of an implantable lead. A predetermined-type tachyarrhythmia is detected using the cardiac local impedance signal. 
     This Summary is an overview of some of the teachings of the present application and not intended to be an exclusive or exhaustive treatment of the present subject matter. Further details about the present subject matter are found in the detailed description and appended claims. Other aspects of the invention will be apparent to persons skilled in the art upon reading and understanding the following detailed description and viewing the drawings that form a part thereof, each of which are not to be taken in a limiting sense. The scope of the present invention is defined by the appended claims and their legal equivalents. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings, which are not necessarily drawn to scale, illustrate generally, by way of example, but not by way of limitation, various embodiments discussed in the present document. 
         FIG. 1  is an illustration of an embodiment of a CRM system and portions of the environment in which the CRM system operates. 
         FIG. 2  is an illustration of an embodiment of cardiac local impedance sensing. 
         FIG. 3  is an illustration of examples of signals resulting from cardiac local impedance sensing. 
         FIG. 4  is a block diagram illustrating an embodiment of an implantable medical device of the CRM system. 
         FIG. 5  is a block diagram illustrating an embodiment of an impedance sensing circuit of the implantable medical device. 
         FIG. 6  is a block diagram illustrating another embodiment of the impedance sensing circuit. 
         FIG. 7  is a block diagram illustrating an embodiment of an arrhythmia detection circuit of the implantable medical device. 
         FIG. 8  is a block diagram illustrating an embodiment of an impedance-based VF detector of the arrhythmia detection circuit. 
         FIG. 9  is a block diagram illustrating another embodiment of the impedance-based VF detector. 
         FIG. 10  is a block diagram illustrating another embodiment of the impedance-based VF detector. 
         FIG. 11  is a flow chart illustrating an embodiment of a method for detecting tachyarrhythmia using cardiac local impedance. 
         FIG. 12  is a flow chart illustrating an embodiment of a method for detecting VF using cardiac local impedance. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that the embodiments may be combined, or that other embodiments may be utilized and that structural, logical and electrical changes may be made without departing from the scope of the present invention. The following detailed description provides examples, and the scope of the present invention is defined by the appended claims and their legal equivalents. 
     In this document, the terms “a” or “an” are used, as is common in patent documents, to include one or more than one. In this document, the term “or” is used to refer to a nonexclusive or, unless otherwise indicated. Furthermore, all publications, patents, and patent documents referred to in this document are incorporated by reference herein in their entirety, as though individually incorporated by reference. In the event of inconsistent usages between this documents and those documents so incorporated by reference, the usage in the incorporated reference(s) should be considered supplementary to that of this document; for irreconcilable inconsistencies, the usage in this document controls. 
     It should be noted that references to “an”, “one”, or “various” embodiments in this document are not necessarily to the same embodiment, and such references contemplate more than one embodiment. 
     This document discusses a CRM system that detects tachyarrhythmia episodes using cardiac local impedance indicative of cardiac local wall motion. A tachyarrhythmia episode is detected by detecting one or more abnormalities in the mechanical activities of the heart. A cardiac local impedance signal indicative of a cardiac local impedance of a cardiac region is sensed by bipolar electrodes, such as bipolar electrodes on a pacing or defibrillation lead, placed in that cardiac region. Tachyarrhythmia such as VF is detected by analyzing one or more cardiac local impedance signals sensed in one or more cardiac regions, or one or more cardiac local impedance derivative signals each indicative of the rate of change in one of the one or more cardiac local impedances. For example, VF is detected by analyzing a motion pattern of a cardiac region indicated by the cardiac local impedance signal sensed from that cardiac region, or by analyzing the synchrony of local wall motions in two cardiac regions indicated by the cardiac local impedance signals sensed from those two cardiac regions. 
     In this document, an “impedance signal” or “Z” includes a signal indicative of impedance. In one embodiment, the impedance signal is produced as a ratio of a sensed voltage to a current delivered for impedance sensing. In another embodiment, the impedance is a sensed voltage signal indicative of impedance, for example, when the current delivered for impedance sensing is from a constant-current source. An “impedance derivative signal” or “dZ/dT” indicates a rate of change in the impedance signal. For example, a “cardiac local impedance signal (Z)” includes a signal indicative of a cardiac local (regional) impedance, a “cardiac local impedance derivative signal (dZ/dT)” in indicates a rate of change in the cardiac local impedance, a “left ventricular (LV) local impedance signal (LVZ)” includes a signal indicative of an LV local (regional) impedance, an “LV local impedance derivative signal (dZ/dT)” in indicates a rate of change in the LV local impedance signal, a “right ventricular (RV) local impedance signal (RVZ)” includes a signal indicative of an RV local (regional) impedance, an “RV local impedance derivative signal (dZ/dT)” in indicates a rate of change in the RV local impedance. 
     As discussed in this document, the cardiac local impedance is indicative of cardiac local wall motion, which includes thickening of the cardiac wall due to systolic contraction and reorientation of impedance sensing electrodes relative to the contracting myocardium. The cardiac local impedance is also affected by displacement of blood in the myocardium due to its contraction. 
       FIG. 1  is an illustration a CRM system  100  and portions of an environment in which system  100  operates. CRM system  100  includes an implantable medical device  105  that is electrically coupled to a heart through implantable leads  110 ,  115 , and  125 . An external system  190  communicates with implantable medical device  105  via a telemetry link  185 . 
     Implantable medical device  105  includes a hermetically sealed can housing an electronic circuit that senses physiological signals and delivers therapeutic electrical pulses. The hermetically sealed can also functions as an electrode for sensing and/or pulse delivery purposes. In one embodiment, implantable medical device  105  includes an arrhythmia detection circuit that detects tachyarrhythmias and determines whether a therapy is to be delivered from implantable medical device  105 . For example, if VF is detected, implantable medical device  105  delivers a defibrillation therapy. In one embodiment, implantable medical device  105  is an ICD with cardiac pacing capabilities. In another embodiment, in addition to a pacemaker and a cardioverter/defibrillator, implantable medical device  105  further includes one or more of other monitoring and/or therapeutic devices such as a neural stimulator, a drug delivery device, and a biological therapy device. 
     Lead  110  is a right atrial (RA) pacing lead that includes an elongate lead body having a proximal end  111  and a distal end  113 . Proximal end  111  is coupled to a connector for connecting to implantable medical device  105 . Distal end  113  is configured for placement in the RA in or near the atrial septum. Lead  110  includes an RA tip electrode  114 A, and an RA ring electrode  114 B. RA electrodes  114 A and  114 B are incorporated into the lead body at distal end  113  for placement in or near the atrial septum, and are each electrically coupled to implantable medical device  105  through a conductor extending within the lead body. RA tip electrode  114 A, RA ring electrode  114 B, and/or the can of implantable medical device  105  allow for sensing an RA electrogram indicative of RA depolarizations and delivering RA pacing pulses. In one embodiment, RA electrodes  114 A and  114 B function as a pair of RA impedance sensing electrodes for sensing an RA local impedance signal. The distance between RA tip electrode  114 A and RA ring electrode  114 B is in a range of approximately 2 millimeters to 20 millimeters, with approximately 5 millimeters being a specific example. 
     Lead  115  is a right ventricular (RV) pacing-defibrillation lead that includes an elongate lead body having a proximal end  117  and a distal end  119 . Proximal end  117  is coupled to a connector for connecting to implantable medical device  105 . Distal end  119  is configured for placement in the RV. Lead  115  includes a proximal defibrillation electrode  116 , a distal defibrillation electrode  118 , an RV tip electrode  120 A, and an RV ring electrode  120 B. Defibrillation electrode  116  is incorporated into the lead body in a location suitable for supraventricular placement in the RA and/or the superior vena cava. Defibrillation electrode  118  is incorporated into the lead body near distal end  119  for placement in the RV. RV electrodes  120 A and  120 B are incorporated into the lead body at distal end  119 . Electrodes  116 ,  118 ,  120 A, and  120 B are each electrically coupled to implantable medical device  105  through a conductor extending within the lead body. Proximal defibrillation electrode  116 , distal defibrillation electrode  118 , and/or the can of implantable medical device  105  allow for delivery of cardioversion/defibrillation pulses to the heart. RV tip electrode  120 A, RV ring electrode  120 B, and/or the can of implantable medical device  105  allow for sensing an RV electrogram indicative of RV depolarizations and delivering RV pacing pulses. In one embodiment, RV electrodes  120 A and  120 B function as a pair of RV impedance sensing electrodes for sensing an RV local impedance signal. The distance between RV tip electrode  120 A and RV ring electrode  120 B is in a range of approximately 2 millimeters to 20 millimeters, with approximately 8 millimeters being a specific example. 
     Lead  125  is a left ventricular (LV) coronary pacing lead that includes an elongate lead body having a proximal end  121  and a distal end  123 . Proximal end  121  is coupled to a connector for connecting to implantable medical device  105 . Distal end  123  is configured for placement in the coronary vein. Lead  125  includes an LV tip electrode  128 A and an LV ring electrode  128 B. The distal portion of lead  125  is configured for placement in the coronary sinus and coronary vein such that LV electrodes  128 A and  128 B are placed in the coronary vein. LV electrodes  128 A and  128 B are incorporated into the lead body at distal end  123  and each electrically coupled to implantable medical device  105  through a conductor extending within the lead body. LV tip electrode  128 A, LV ring electrode  128 B, and/or the can of implantable medical device  105  allow for sensing an LV electrogram indicative of LV depolarizations and delivering LV pacing pulses. In one embodiment, LV electrodes  128 A and  128 B function as a pair of LV impedance sensing electrodes for sensing an LV local impedance signal. The distance between LV tip electrode  128 A and LV ring electrode  128 B is in a range of approximately 2 millimeters to 40 millimeters, with approximately 11 millimeters being a specific example. 
     In various embodiments, one or more pairs of impedance sensing electrodes are used, with each pair configured to sense a cardiac local impedance signal. The impedance sensing electrodes of each pair are spaced to sense an impedance that is indicative of local wall motion in a cardiac region. Each impedance sensing electrode may also be used for sensing an electrogram and/or delivering pacing or defibrillation pulses. The lead configuration including RA lead  110 , RV lead  115 , and LV lead  125  is illustrated in  FIG. 1  as an example. Other lead configurations may be used, depending on monitoring and therapeutic requirements. For example, additional leads may be used to provide access to additional cardiac regions, and leads  110 ,  115 , and  125  may each include more or fewer electrodes along the lead body at, near, and/or distant from the distal end, depending on specified monitoring and therapeutic needs. 
     External system  190  allows for programming of implantable medical device  105  and receives signals acquired by implantable medical device  105 . In one embodiment, telemetry link  185  is an inductive telemetry link. In an alternative embodiment, telemetry link  185  is a far-field radio-frequency telemetry link. Telemetry link  185  provides for data transmission from implantable medical device  105  to external system  190 . This may include, for example, transmitting real-time physiological data acquired by implantable medical device  105 , extracting physiological data acquired by and stored in implantable medical device  105 , extracting therapy history data stored in implantable medical device  105 , and extracting data indicating an operational status of implantable medical device  105  (e.g., battery status and lead impedance). Telemetry link  185  also provides for data transmission from external system  190  to implantable medical device  105 . This may include, for example, programming implantable medical device  105  to acquire physiological data, programming implantable medical device  105  to perform at least one self-diagnostic test (such as for a device operational status), programming implantable medical device  105  to run a signal analysis algorithm (such as an algorithm implementing the tachyarrhythmia detection method discussed in this document), and programming implantable medical device  105  to deliver pacing and/or cardioversion/defibrillation therapies. 
     The circuit of CRM system  100  may be implemented using a combination of hardware and software. In various embodiments, each element of implantable medical device  105  as illustrated in  FIGS. 2-8 , including its specific embodiments, may be implemented using an application-specific circuit constructed to perform one or more particular functions or a general-purpose circuit programmed to perform such function(s). Such a general-purpose circuit includes, but is not limited to, a microprocessor or portions thereof, a microcontroller or portions thereof, and a programmable logic circuit or portions thereof. For example, a “comparator” includes, among other things, an electronic circuit comparator constructed to perform the only function of comparing two or more signals or a portion of a general-purpose circuit driven by a code instructing that portion of the general-purpose circuit to perform the comparing. 
       FIG. 2  is an illustration of an embodiment of cardiac local impedance sensing. A lead  215  represents portions of lead  115  including RV electrodes  120 A and  120 B function as a pair of RV impedance sensing electrodes. A lead  225  represents portions of lead  125  including LV electrodes  128 A and  128 B function as a pair of LV impedance sensing electrodes. RV electrodes  120 A and  120 B are used for injecting a current  221  and sensing the resulting voltage indicative of the RV local impedance. LV electrodes  128 A and  128 B are used for injecting a current  229  and sensing the resulting voltage indicative of the LV local impedance. The cardiac local impedance is sensed using two closely spaced impedance sensing electrodes (e.g., within 20 millimeters for the RA or RV, or within 40 millimeters for the LV) placed over or near the myocardium. In one embodiment, the distance between the two impedance sensing electrodes is within approximately 20 millimeters. The sensed cardiac local impedance signal is indicative of local motion and/or geometrical changes of the myocardial region in the vicinity of the impedance sensing electrodes. 
     In this document, an signal sensed or event detected using an RV lead such as lead  115  or  215  is referred to as an “RV” signal or an “RV” event, and an signal sensed or event detected using an LV lead such as lead  125  or  225  is referred to as an “LV” signal or an “LV” event. For example, when electrode  120 A and  120 B are used to deliver pacing pulse to the RV-LV septum to control LV activation, the cardiac local impedance sensed using these two electrodes are still referred to as an RV local impedance indicative of RV local motion. An “interventricular delay” between an RV event and an LV event includes a delay between an event detected using an RV lead such as lead  115  or  215  and an event detected using an LV lead such as lead  125  or  225 . 
       FIG. 3  is an illustration of examples of signals resulting from cardiac local impedance sensing. The illustrated signals include an LV impedance signal (LVZ), an LV impedance derivative signal (LV dZ/dT), an RV impedance signal (RVZ), and an RV impedance derivative signal (RV dZ/dT), sensed during a regular cardiac rhythm. 
     The LVZ represents an impedance signal sensed using LV electrodes  128 A and  128 B. The LV dZ/dT represents the rate of change in the LVZ. The RVZ represents an impedance signal sensed using RV electrodes  120 A and  120 B. The RV dZ/dT represents the rate of change in the RVZ. These signals and their uses are further discussed below. The LV dZ/dT includes LV impedance events  326 . The RV dZ/dT includes RV impedance events  327 . In one embodiment, such impedance events are each representative of a cardiac local wall motion during the systolic phase of each cardiac cycle. The LV impedance events each represent the LV local wall motion during the systolic phase of each cardiac cycle. The RV impedance events each represent the RV local wall motion during the systolic phase of each cardiac cycle. During a normal sinus rhythm or a tachycardia with a regular rhythm, such illustrated in  FIG. 3 , the LV and RV contract in synchrony, and the LV and RV impedance events during each cardiac cycle occur approximately simultaneously or within a limited interventricular delay. 
       FIG. 4  is a block diagram illustrating an embodiment of an implantable medical device  405 . Implantable medical device  405  is a specific embodiment of an implantable medical device  105  and includes a cardiac sensing circuit  430 , a pacing circuit  432 , a defibrillation circuit  434 , an impedance sensing circuit  436 , an arrhythmia detection circuit  438 , an implant controller  440 , and an implant telemetry circuit  442 . Cardiac sensing circuit  430  senses one or more electrograms from the heart through electrodes such as those selected from RA electrodes  114 A and  114 B, RV electrodes  120 A and  120 B, LV electrodes  128 A and  128 B, and the can of implantable medical device  405 . Pacing circuit  432  delivers pacing pulses to the heart through electrodes such as those selected from RA electrodes  114 A and  114 B, RV electrodes  120 A and  120 B, LV electrodes  128 A and  128 B, and the can of implantable medical device  405 . Defibrillation circuit  434  delivers cardioversion/defibrillation pulses through electrodes such as those selected from defibrillation electrodes  116  and  118  and the can of implantable medical device  405 . Impedance sensing circuit  436  produces one or more cardiac local impedance signals each by sensing a voltage across a pair of impedance sensing electrodes placed in a cardiac region. In one embodiment, impedance sensing circuit  436  produces each cardiac local impedance signal as the ratio of the sensed voltage to a current delivered for the impedance sensing. In another embodiment, impedance sensing circuit  436  produces each cardiac local impedance signal by isolating the signal component indicative of the cardiac local impedance from the sensed voltage, when the current delivered for the impedance sensing is from a constant-current source. Examples of the pair of impedance sensing electrodes include the pair of RA impedance sensing electrodes  114 A and  114 B, the pair of LV impedance sensing electrodes  128 A and  128 B, and the pair of RV impedance sensing electrodes  120 A and  120 B. Arrhythmia detection circuit  438  detects tachyarrhythmias using at least the sensed one or more cardiac local impedance signals. Implant controller  440  controls the operation of implantable medical device  405 , including delivery of an anti-tachyarrhythmia therapy in response to the detection of tachyarrhythmia, such as the delivery of a ventricular defibrillation therapy in response to a detection of VF. Implant telemetry circuit  442  receives signals from, and transmits signals to, external system  190  via telemetry link  185 . 
       FIG. 5  is a block diagram illustrating an embodiment of an impedance sensing circuit  536 . Impedance sensing circuit  536  is a specific embodiment of impedance sensing circuit  436  and includes a current source circuit  546 , a voltage sensing circuit  548 , an impedance detector  552 , and a differentiator  554 . 
     Current source circuit  546  includes delivers a current through a pair of impedance sensing electrodes. In one embodiment, current source circuit  546  delivers constant current pulses at a frequency between approximately 3 Hz and 500 Hz, with approximately 20 Hz as a specific example. The constant current pulses each have an amplitude between approximately 20 microamperes and 400 microamperes, with approximately 80 microamperes as a specific example, and a pulse width between approximately 10 microseconds and 100 microseconds, with approximately 40 microseconds as a specific example. Voltage sensing circuit  548  senses a voltage across the pair of impedance sensing electrodes and produces a sensed voltage. Impedance detector  552  produces a cardiac local impedance signal (Z) using the sensed voltage. In one embodiment, impedance detector  552  produces the cardiac local impedance signal (Z) as a ratio of the voltage sensed by voltage sensing circuit  548  to the current delivered from current source circuit  546 . In another embodiment, impedance detector  552  produces the cardiac local impedance signal (Z) by isolating the signal component indicative of the cardiac local impedance from the voltage sensed by voltage sensing circuit  548 , when the current delivered from current source circuit  546  is in the form of constant current pulses. Differentiator  554  produces a cardiac local impedance derivative signal (dZ/dT) that indicates the rate of change in the cardiac local impedance. In one embodiment, differentiator  554  includes a high-pass filter having a cutoff frequency between approximately 0.1 Hz and 1 Hz, with approximately 0.5 Hz being a specific example. 
       FIG. 6  is a block diagram illustrating an embodiment of an impedance sensing circuit  636 . Impedance sensing circuit  636  is a specific embodiment of impedance sensing circuit  536  that allows for sensing of multiple cardiac local impedance signals. In the illustrated embodiment, impedance sensing circuit  636  includes two impedance sensing sub-circuits: an LV impedance sensing circuit  636 A and an RV impedance sensing circuit  636 B sensing. LV impedance sensing circuit  636 A produces an LV local impedance signal indicative of an LV local wall motion. RV impedance sensing circuit  636 B produces an RV local impedance signal indicative of an RV local wall motion. In other embodiments, impedance sensing circuit  636  includes two or more impedance sensing sub-circuits each sensing a local impedance signal in a cardiac region. 
     LV impedance sensing module  636 A includes an LV current source circuit  646 A, an LV voltage sensing circuit  648 A, an LV impedance detector  652 A, and an LV differentiator  654 A. LV current source circuit  646 A delivers an LV current through a pair of LV impedance sensing electrodes, such as LV electrodes  128 A and  128 B. LV voltage sensing circuit  648 A senses an LV voltage across the pair of LV impedance sensing electrodes. LV impedance detector  652 A produces an LV local impedance signal (LVZ). In one embodiment, LV impedance detector  652 A produces the LV local impedance signal (LVZ) as a ratio of the LV voltage to the LV current. In another embodiment, LV impedance detector  652 A produces the LV local impedance signal (LVZ) by isolating the signal component indicative of the LV local impedance from the LV voltage, when the LV current is delivered as constant-current pulses. LV differentiator  654 A produces an LV local impedance derivative signal (LV dZ/dT), which indicates the rate of change in the LV local impedance. 
     RV impedance sensing module  636 B includes an RV current source circuit  646 B, an RV voltage sensing circuit  648 B, an RV impedance detector  652 B, and an RV differentiator  654 B. RV current source circuit  646 B delivers an RV current through a pair of RV impedance sensing electrodes, such as RV electrodes  120 A and  120 B. RV voltage sensing circuit  648 B senses an RV voltage across the pair of RV impedance sensing electrodes. RV impedance detector  652 B produces an RV local impedance signal (RVZ). In one embodiment, RV impedance detector  652 B produces the RV local impedance signal (RVZ) as a ratio of the RV voltage to the RV current. In another embodiment, RV impedance detector  652 B produces the RV local impedance signal (RVZ) by isolating the signal component indicative of the RV local impedance from the LV voltage, when the RV current is delivered as constant-current pulses. RV differentiator  654 B produces an RV local impedance derivative signal (RV dZ/dT), which indicates the rate of change in the RV local impedance. 
       FIG. 7  is a block diagram illustrating an embodiment of an arrhythmia detection circuit  738 , which is a specific embodiment of arrhythmia detection circuit  438 . In the illustrated embodiment, arrhythmia detection circuit  738  includes an impedance-based tachyarrhythmia detector  758  and an electrogram-based tachyarrhythmia detector  760 . Impedance-based tachyarrhythmia detector  758  detects tachyarrhythmia using one or more cardiac local impedance signals. Electrogram-based tachyarrhythmia detector  760  detects tachyarrhythmia using one or more electrograms. In one embodiment, arrhythmia detection circuit  738  detects a predetermined-type tachyarrhythmia using the one or more cardiac local impedance signals and the one or more electrograms. In one embodiment, a detection of the predetermined-type tachyarrhythmia is indicated when impedance-based tachyarrhythmia detector  758  and electrogram-based tachyarrhythmia detector  760  both indicate a detection of the tachyarrhythmia. In another embodiment, a detection of the predetermined-type tachyarrhythmia is indicated using weighted outputs of impedance-based tachyarrhythmia detector  758  and electrogram-based tachyarrhythmia detector  760 . In one embodiment, impedance-based tachyarrhythmia detector  758  and electrogram-based tachyarrhythmia detector  760  supplement each other in tachyarrhythmia detection. For example, electrogram-based tachyarrhythmia detector  760  detects a fast heart rate, and impedance-based tachyarrhythmia detector  758  is activated in response to a detection of the fast heart rate. In one embodiment, arrhythmia detection circuit  738  includes only impedance-based tachyarrhythmia detector  758 . 
       FIG. 8  is a block diagram illustrating an embodiment of an impedance-based VF detector  858 . Impedance-based VF detector  858  is a specific embodiment of impedance-based tachyarrhythmia detector  758  and includes a comparator  862  and a detection zone generator  864 . 
     In one embodiment, impedance-based VF detector  858  detects VF using a cardiac local impedance signal (Z). Detection zone generator  864  produces a VF detection zone specified by one or more threshold amplitudes. Comparator  862  has a signal input that receives the cardiac local impedance signal (Z), one or more threshold inputs that receives the one or more threshold amplitudes, and an output that indicates a VF detection when the amplitude of the cardiac local impedance signal (Z) falls into the VF detection zone. In one embodiment, detection zone generator  864  adjusts the VF detection zone based on a trend of the cardiac local impedance signal (Z). 
     In one embodiment, impedance-based VF detector  858  detects VF using a cardiac local impedance derivative signal (dZ/dT). Detection zone generator  864  produces a VF detection zone specified by one or more threshold amplitudes. Comparator  862  has a signal input that receives the cardiac local impedance derivative signal (dZ/dT), one or more threshold inputs that receives the one or more threshold amplitudes, and an output that indicates a VF detection when the amplitude of the cardiac local impedance derivative signal (dZ/dT) falls into the VF detection zone. In one embodiment, detection zone generator  864  adjusts the VF detection zone based on a trend of the cardiac local impedance derivative signal (dZ/dT). 
       FIG. 9  is a block diagram illustrating an embodiment of an impedance-based VF detector  958 . Impedance-based VF detector  958  is another specific embodiment of impedance-based tachyarrhythmia detector  758  and includes a motion event detector  966  and an event threshold generator  968  to detect VF. 
     Motion event detector  966  detects an impedance event from a cardiac local impedance derivative signal (dZ/dT). In one embodiment, the impedance event is representative of a cardiac local wall motion during the systolic phase of each cardiac cycle. Motion event detector  966  indicates a detection of the impedance event when the cardiac local impedance derivative signal (dZ/dT) exceeds an event threshold. Event threshold generator  968  adjusts the event threshold based on a trend of the cardiac local impedance derivative signal (dZ/dT). Impedance-based VF detector  958  detects VF using a pattern of the impedance events (i.e., a pattern of cardiac local wall motion). In one embodiment, impedance-based VF detector  958  indicates a VF detection when the pattern of the impedance events becomes irregular while the heart rate falls into a predetermined VF detection zone. 
       FIG. 10  is a block diagram illustrating an embodiment of an impedance-based VF detector  1058 . Impedance-based VF detector  1058  is another specific embodiment of impedance-based tachyarrhythmia detector  758  and includes a ventricular motion event detector  1066 , an event threshold generator  1068 , and an interventricular synchrony analyzer  1070  to detect VF based on whether the LV and RV contract in synchrony. During a normal sinus rhythm or a tachycardia with a regular rhythm, the LV and RV contract in synchrony, and the LV and RV impedance events during each cardiac cycle occur approximately simultaneously, such as shown in  FIG. 3 . Cardiac disorders such as heart failure may cause a certain degree of dyssynchrony in the LV and RV local wall motions, but during a normal or fast but regular rhythm, the LV and RV contractions generally have a one-to-one relationship and occur within a limited interventricular delay during each cardiac cycle. 
     Ventricular motion event detector  1066  detects an LV impedance event by comparing the LV local impedance derivative signal (LV dZ/dT) to an LV event threshold, and detects an RV impedance event by comparing the RV local impedance derivative signal (RV dZ/dT) to an RV event threshold. Event threshold generator  1068  adjusts the LV event threshold based on a trend of the LV local impedance derivative signal (LV dZ/dT), and adjusts the RV event threshold based on a trend of the RV local impedance derivative signal (RV dZ/dT). Impedance-based VF detector  1058  detects VF using a pattern of the LV impedance events and the RV impedance events. In the illustrated embodiment, interventricular synchrony analyzer  1070  detects VF by determining whether the pattern of the LV impedance events and the RV impedance events indicates a degree of dyssynchrony between the LV and RV local wall motions that exceeds a predetermined threshold degree. In one embodiment, interventricular synchrony analyzer  1070  indicates a VF detection when the degree of dyssynchrony between the LV and RV local wall motions falls below the predetermined threshold degree while the heart rate falls into a predetermined VF detection zone. In one embodiment, the degree of dyssynchrony between the LV and RV local wall motions is measured by the interventricular delay between the LV and RV local wall motions. Interventricular synchrony analyzer  1070  detects an interventricular delay between the LV impedance event and the RV impedance event during each cardiac cycle and detects VF by comparing the interventricular delay to a predetermined threshold delay. Interventricular synchrony analyzer  1070  indicates a VF detection when the interventricular delay exceeds the predetermined threshold delay while the heart rate falls into a predetermined VF detection zone. 
       FIG. 11  is a flow chart illustrating an embodiment of a method  1100  for detecting a tachyarrhythmia using cardiac local impedance. In one embodiment, method  1100  is performed by system  100 . 
     A cardiac local impedance signal is sensed at  1110 . The cardiac local impedance signal is sensed using a pair of impedance sensing electrodes placed to sense cardiac local wall motion. In one embodiment, the pair of impedance sensing electrodes includes a pair of bipolar pacing-sensing electrodes at a distal end of an implantable pacing or pacing-defibrillation lead. To sense the cardiac local impedance signal, current pulses are delivered through the pair of impedance sensing electrodes at a frequency between approximately 3 Hz and 500 Hz, with approximately 20 Hz as a specific example. The current pulses each have an amplitude between approximately 20 microamperes and 400 microamperes, with approximately 80 microamperes as a specific example, and a pulse width between approximately 10 microseconds and 100 microseconds, with approximately 40 microseconds as a specific example. The voltage across the pair of impedance sensing electrodes is sensed. In one embodiment, the cardiac local impedance signal (Z) is produced as a ratio of the sensed voltage to the delivered current. In another embodiment, the cardiac local impedance signal (Z) is produced by isolating the signal component indicative of the cardiac local impedance from the sensed voltage, when the delivered current is in the form of constant-current pulses. 
     In one embodiment, a cardiac local impedance derivative signal (dZ/dT) is produced, for example, by high-pass filtering the cardiac local impedance signal (Z) using a cutoff frequency between approximately 0.1 Hz and 1 Hz, with approximately 0.5 Hz as a specific example. 
     Tachyarrhythmia is detected using the cardiac local impedance signal at  1120 . In one embodiment, tachyarrhythmia is detected using the cardiac local impedance derivative signal. In one embodiment, one or more electrograms are also sensed, and tachyarrhythmia is detected using the cardiac local impedance signal and the one or more electrograms. In one embodiment, a VF detection zone specified by one or more threshold amplitudes is produced, and a VF detection is indicated when the cardiac local impedance signal (Z) or the cardiac local impedance derivative signal (dZ/dT) falls into the VF detection zone. In a specific embodiment, the VF detection zone is adjusted using a trend of the cardiac local impedance signal (Z) or the cardiac local impedance derivative signal (dZ/dT). In one embodiment, an impedance event is detected by comparing the cardiac local impedance derivative signal (dZ/dT) to an event threshold. The impedance event represents a cardiac local wall motion during the systolic phase of each cardiac cycle. The event threshold is adjusted based on a trend of the cardiac local impedance derivative signal (dZ/dT). VF is detected using the pattern of the detected impedance events (i.e., pattern of cardiac local wall motion). 
     Delivery of an anti-tachyarrhythmia therapy is controlled at  1130 . In one embodiment, if VF is detected at  1120 , a defibrillation pulse is delivered at  1130 . 
       FIG. 12  is a flow chart illustrating an embodiment of a method  1200  for detecting VF using LV and RV local impedance signal. In one embodiment, the method is performed by system  100 . 
     The LV local impedance signal (LVZ) is sensed at  1210 . To sense the LV local impedance signal (LVZ), an LV current is delivered through a pair of LV impedance sensing electrodes, and an LV voltage across the pair of LV impedance sensing electrodes is sensed. In one embodiment, the LV local impedance signal (LVZ) is produced as the ratio of the sensed LV voltage to the delivered LV current. In another embodiment, the LV local impedance signal (LVZ) is produced by isolating the signal component indicative of the LV local impedance from the sensed LV voltage, when the delivered LV current is in the form of constant-current pulses. An LV local impedance derivative signal (LV dZ/dT) is produced at  1215 . LV impedance events are detected at  1220  by comparing the LV local impedance derivative signal (LV dZ/dT) to an LV event threshold. In one embodiment, the LV impedance events are each representative of an LV local wall motion during the systolic phase of a cardiac cycle. In one embodiment, the LV event threshold is adjusted based on a trend of the LV local impedance derivative signal (LV dZ/dT). 
     The RV local impedance signal (RVZ) is sensed at  1230 . To sense the RV local impedance signal (RVZ), an RV current is delivered through a pair of RV impedance sensing electrodes, and an RV voltage across the pair of RV impedance sensing electrodes is sensed. In one embodiment, the RV local impedance signal (RVZ) is produced as the ratio of the sensed RV voltage to the delivered RV current. In another embodiment, the RV local impedance signal (RVZ) is produced by isolating the signal component indicative of the RV local impedance from the sensed RV voltage, when the delivered RV current is in the form of constant-current pulses. An RV local impedance derivative signal (RV dZ/dT) is produced at  1235 . RV impedance events are detected at  1240  by comparing the RV local impedance derivative signal (RV dZ/dT) to an RV event threshold. In one embodiment, the RV impedance events are each representative of an RV local wall motion during the systolic phase of a cardiac cycle. In one embodiment, the RV event threshold is adjusted based on a trend of the RV local impedance derivative signal (RV dZ/dT). 
     The pattern of the detected LV and RV impedance events is analyzed at  1250 . A degree of dyssynchrony between the LV and RV local wall motions is produced based on the pattern. If the degree of dyssynchrony between the LV and RV local wall motions exceeds a predetermined threshold degree (i.e., the LV and RV do not contract in synchrony) at  1255 , a VF detection is indicated at  1260 . In one embodiment, an interventricular delay between the LV impedance event and the RV impedance event during each cardiac cycle is detected as a measure of the degree of dyssynchrony between the LV and RV local wall motions. The LV and RV contract in synchrony when the LV impedance events and the RV impedance events have approximately a one-to-one relationship and the interventricular delay is within a predetermined limit. 
     It is to be understood that the above detailed description is intended to be illustrative, and not restrictive. Other embodiments will be apparent to those of skill in the art upon reading and understanding the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.