Apparatus, including a tube having a proximal end, and a distal end for insertion into a body cavity including anatomical structures, and a blade mounted at the distal end. The apparatus also includes a handle at the proximal end and including a device which generates sensible outputs upon receiving activation signals, and a position sensor fixed in a predefined disposition relative to the tube. The apparatus additionally includes a processor configured to acquire an image of the cavity, to determine, in the image, locations for each of the structures, to receive, from the sensor, a position signal indicative of a blade location of the blade within the cavity, to determine, based on the blade location and the respective structure locations, a proximity of the blade to a given structure, and to convey, to the warning device, a given activation signal in response to the proximity to the given structure.
