Apparatus for puncturing a fossa ovalis includes a catheter, which has a distal portion that is shaped so as to define a catheter distal end opening at a distal end of the catheter, and first and second lateral openings. The catheter is shaped so as to define a central catheter lumen open through the catheter distal end opening. A flexible longitudinal member passes from a proximal portion of the catheter to the distal portion of the catheter, out of the first lateral opening, and into the second lateral opening, so as to form a loop outside the catheter. A puncturing element is slidably disposed within the central catheter lumen, and is configured to puncture a hole through the fossa ovalis by being advanced out of the catheter distal end opening and through the fossa ovalis. Other embodiments are also described.