a catheter has an inflatable assembly at its distal portion , including a containment chamber , an axial core and a plurality of longitudinally oriented partitions extending from the axial core to the wall of the chamber to divide the containment chamber into at least four inflatable sectors . hydraulic valves are connected to respective sectors to enable selective inflation of the sectors by a fluid when the valves are connected to a source of the fluid , and at least one surface electrode is mounted on each of the sectors . when introduced into a heart chamber and diametrically opposed sectors are inflated the assembly is stably fixed against the walls of the heart chamber and readings can be obtained from the surface electrodes of the inflated sectors .