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.