An apparatus configured to assess a lesion formed to prevent electrical signals from travelling between a heart and blood vessel emanating from the heart, the apparatus comprising: a first medical probe comprising a first electrode adapted to contact tissue at a stimulus location in the heart; a second medical probe comprising multiple second electrodes adapted to contact tissue at respective sensing locations in the blood vessel; a display; and a processor configured: to apply to or to sense from the first electrode in contact with the tissue at the stimulus location in the heart, a first activation signal having a first activation peak at a first time, to receive, from the second electrodes in contact with the tissue at the respective sensing locations in the blood vessel, respective second activation signals (144A-D) having respective second activation peaks (148A-D) sensed by the electrodes following the first activation signal, to identify one of the multiple electrodes proximal to a gap in the lesion by finding a given electrode among the multiple electrodes that has a respective second activation peak closest in time after the first activation peak among the second activation signals, and to present, on the display, a map of the body cavity with the identified electrode marked on the map by presenting, on the map, an icon representing a distal end of the second medical probe, and highlighting the found electrode on the map. a first medical probe comprising a first electrode adapted to contact tissue at a stimulus location in the heart; a second medical probe comprising multiple second electrodes adapted to contact tissue at respective sensing locations in the blood vessel; a display; and a processor configured: to apply to or to sense from the first electrode in contact with the tissue at the stimulus location in the heart, a first activation signal having a first activation peak at a first time, to receive, from the second electrodes in contact with the tissue at the respective sensing locations in the blood vessel, respective second activation signals (144A-D) having respective second activation peaks (148A-D) sensed by the electrodes following the first activation signal, to identify one of the multiple electrodes proximal to a gap in the lesion by finding a given electrode among the multiple electrodes that has a respective second activation peak closest in time after the first activation peak among the second activation signals, and to present, on the display, a map of the body cavity with the identified electrode marked on the map by presenting, on the map, an icon representing a distal end of the second medical probe, and highlighting the found electrode on the map. to apply to or to sense from the first electrode in contact with the tissue at the stimulus location in the heart, a first activation signal having a first activation peak at a first time, to receive, from the second electrodes in contact with the tissue at the respective sensing locations in the blood vessel, respective second activation signals (144A-D) having respective second activation peaks (148A-D) sensed by the electrodes following the first activation signal, to identify one of the multiple electrodes proximal to a gap in the lesion by finding a given electrode among the multiple electrodes that has a respective second activation peak closest in time after the first activation peak among the second activation signals, and to present, on the display, a map of the body cavity with the identified electrode marked on the map by presenting, on the map, an icon representing a distal end of the second medical probe, and highlighting the found electrode on the map.