Abstract: A method is provided, including transvascularly inserting a guidewire into a right atrium of a subject. Apparatus is provided that includes (a) a catheter, and (b) a hollow needle disposed within a lumen of the catheter, the hollow needle including a proximal portion and a distal portion that is more flexible than the proximal portion. The apparatus is passed into the right atrium by passing the needle over the guidewire. Following the passing of the apparatus into the right atrium, the guidewire is partially withdrawn into the needle, while keeping a distal end of the guidewire within the needle and in a body of the subject. Thereafter, a hole is formed through an interatrial septum of the subject with the needle. Other embodiments are also described.