An apparatus includes a shaft and an end effector at a distal end of the shaft. The end effector is sized to fit in an anatomical passageway within a subject's cardiovascular system. The end effector includes at least one electrode pair that is configured to contact cardiovascular tissue and thereby pick up electrocardiogram signals. Each electrode pair includes first and second electrodes spaced apart along a longitudinal axis from each other by a gap area located between the electrodes, the gap area having a gap length with respect to the longitudinal axis such that a length of one of the electrodes along the longitudinal axis is equal to or greater than the gap length; and a ratio of an area defined by the gap area to one electrode area is equal to or less than one.