Method and apparatus for detecting fibrillation using cardiac local impedance

Abstract

A cardiac rhythm management (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.

1 . A method for detecting tachyarrhythmia, the method comprising:
 sensing a cardiac local impedance signal indicative of a cardiac local wall motion using a first pair of impedance sensing electrodes at a distal end of a first implantable lead; and   detecting a predetermined-type tachyarrhythmia using the cardiac local impedance signal.   
     
     
         2 . The method of  claim 1 , further comprising sensing one or more electrograms, and wherein detecting the predetermined-type tachyarrhythmia comprises detecting the predetermined-type tachyarrhythmia using the cardiac local impedance signal and the one or more electrograms. 
     
     
         3 . The method of  claim 1 , wherein sensing the cardiac local impedance signal comprises:
 delivering current pulses through the first pair of impedance sensing electrodes at a frequency between approximately 3 and 500 Hz, the current pulses each have an amplitude between approximately 20 microamperes and 400 microamperes and a pulse width between approximately 10 microseconds and 100 microseconds;   sensing a voltage across the first pair of impedance sensing electrodes; and   producing the cardiac local impedance signal using the sensed voltage.   
     
     
         4 . The method of  claim 3 , wherein producing the cardiac local impedance signal comprises producing the cardiac local impedance signal as a ratio of the sensed voltage to the amplitude of the delivered current pulses. 
     
     
         5 . The method of  claim 1 , wherein detecting the predetermined-type tachyarrhythmia comprises:
 producing a ventricular fibrillation (VF) detection zone specified by one or more threshold amplitudes; and   indicating a VF detection when the cardiac local impedance signal falls into the VF detection zone.   
     
     
         6 . The method of  claim 5 , wherein detecting the predetermined-type tachyarrhythmia further comprises adjusting the VF detection zone based on a trend of the cardiac local impedance signal. 
     
     
         7 . The method of  claim 1 , further comprising producing a cardiac local impedance derivative signal indicative of a rate of change in the cardiac local impedance, and wherein detecting the predetermined-type tachyarrhythmia comprises detecting the predetermined-type tachyarrhythmia using the cardiac local impedance derivative signal. 
     
     
         8 . The method of  claim 7 , wherein producing the cardiac local impedance derivative signal comprises using a high-pass filter having a cutoff frequency between approximately 0.1 Hz and 1 Hz. 
     
     
         9 . The method of  claim 7 , wherein detecting the predetermined-type tachyarrhythmia comprises:
 detecting an impedance event by comparing the cardiac local impedance derivative signal to an event threshold, the impedance event representative of a cardiac local wall motion during a systolic phase of each cardiac cycle; and   adjusting the event threshold based on a trend of the cardiac local impedance derivative signal.   
     
     
         10 . The method of  claim 1 , wherein sensing the cardiac local impedance signal using the first pair of impedance sensing electrodes at the distal end of the first implantable lead comprises sensing a left ventricular (LV) local impedance signal (LVZ) indicative of an LV local impedance using a pair of LV impedance sensing electrodes at a distal end of an implantable LV lead. 
     
     
         11 . The method of  claim 10 , wherein the LV impedance sensing electrodes are spaced within approximately 40 millimeters. 
     
     
         12 . The method of  claim 10 , further comprising sensing a right ventricular (RV) local impedance signal (RVZ) indicative of an RV local impedance using a pair of RV impedance sensing electrodes at a distal end of an implantable RV lead. 
     
     
         13 . The method of  claim 12 , wherein the RV impedance sensing electrodes are spaced within approximately 20 millimeters. 
     
     
         14 . The method of  claim 12 , further comprising:
 producing an LV local impedance derivative signal (LV dZ/dT) using the LVZ, the LV dZ/dT indicative of a rate of change in the LVZ; and   producing an RV local impedance derivative signal (RV dZ/dT) using the RVZ, the RV dZ/dT indicative of a rate of change in the RVZ,   and wherein detecting the predetermined-type tachyarrhythmia comprises detecting ventricular fibrillation (VF) using the LV dZ/dT and the RV dZ/dT.   
     
     
         15 . The method of  claim 14 , wherein detecting VF comprises:
 detecting LV impedance events by comparing the LV local impedance derivative signal (LV dZ/dT) to an LV event threshold, the LV impedance events each representative of an LV local wall motion during a systolic phase of a cardiac cycle; and   detecting RV impedance events by comparing the RV local impedance derivative signal (RV dZ/dT) to an RV event threshold, the RV impedance events each representative of an RV local wall motion during the systolic phase of the cardiac cycle.   
     
     
         16 . The method of  claim 15 , wherein detecting VF further comprises determining whether a 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. 
     
     
         17 . The method of  claim 1 , wherein detecting the predetermined-type tachyarrhythmia comprises detecting ventricular fibrillation (VF). 
     
     
         18 . The method of  claim 17 , further comprising producing a cardiac local impedance derivative signal indicative of a rate of change in the cardiac local impedance, and wherein detecting the VF comprises detecting the VF using the cardiac local impedance derivative signal. 
     
     
         19 . The method of  claim 17 , further comprising delivering a defibrillation pulse in response to a detection of the VF. 
     
     
         20 . The method of  claim 19 , wherein sensing the cardiac local impedance signal comprises sensing the cardiac local impedance signal using two impedance sensing electrodes spaced at a distance between approximately 2 millimeters and 40 millimeters.