An electrophysiology catheter introduced through the groin and inferior vena cava into the right side of the heart comprises an elongate flexible shaft having a steerable distal section and a prolapsing section located proximally of the distal section. The distal section is inserted into the coronary sinus and a back-steering force is applied to the catheter to anchor the distal section therein, after which the catheter is further advanced to prolapse the prolapsing section against the high right atrium. Electrical pathways in both the coronary sinus and the high right atrium are mapped using respective electrode pairs carried on the distal and prolapsing sections of the catheter.