Medical apparatus includes an insertion tube configured for insertion into a body cavity of a patient and a distal assembly, including a plurality of spines having respective proximal ends that are connected distally to the insertion tube. Each spine includes a rib extending along a length of the spine, a flexible polymer sleeve disposed over the rib and defining a lumen running parallel to the rib along the spine, and one or more electrodes disposed on the sleeve and configured to contact tissue within the body cavity.