In a pacing system, the pacing controller induces the leads to deliver an impedance measurement pulse to the right ventricle of the heart at at least a 15 ms interval during a window of time following detection of the R-wave or the delivery of a pacing pulse. The window of time corresponds to...http://www.google.com.hk/patents/US5800467?utm_source=gb-gplus-share專利 US5800467 - Cardio-synchronous impedance measurement system for an implantable stimulation device 
