Adaptive electrode for bi-polar ablation

An ablation apparatus, comprising:

• a first flexible probe adapted for insertion into a heart of a living subject and having a distal segment and a first ablation electrode disposed at the distal segment to be brought to a target tissue at a first side of a wall of the heart;
• a second flexible probe adapted for insertion into the living subject and having a distal end and a second ablation electrode disposed at the distal end, the second ablation electrode to be brought to an opposing second side of the wall to oppose the first ablation electrode, the second ablation electrode comprising a plurality of segments, the segments being arranged in a spiral and electrically insulated from one another;
• and a power generator connectable to the first ablation electrode and switchably connectable to selected ones of the segments of the second ablation electrode for passing electric current from the first ablation electrode and through the target tissue and for passing electric current from the selected segments of the second ablation electrode and through the tissue, the first ablation electrode having a fixed current transmission area and the second ablation electrode having a variable current transmission area, the variable current transmission area being selected to form a transmural lesion between the first ablation electrode and the second ablation electrode.