A method includes receiving a bipolar signal sensed by a pair of electrodes at a location in a heart of a patient. A unipolar signal is received, which is sensed by an electrode at the location in the heart. A derivative of the received bipolar signal is computed. A derivative of the received unipolar signal is computed. A ratio is evaluated, between the derivative of the bipolar signal and the derivative of the unipolar signal at local minima of the derivative of the unipolar signal. When the ratio is smaller than a preset threshold ratio value, an occurrence of ventricular activity is indicated.