An implantable medical device, is designed to collect a signal representative of the electric activity of the heart and determine a cardiogenic impedance signal for at least a portion of the heart. An R-wave detector of the IMD detects the timing of an R-wave during a cardiac cycle based on the signal representative of the electric activity. A minimum detector detects the timing of a cardiogenic impedance minimum in the cardiogenic impedance signal and within a systolic time window of the cardiac cycle. A detected arrhythmia is then classified by the IMD based on the timing of the R-wave detected by the R-wave detector and the timing of the cardiogenic impedance minimum detected by the minimum detector.