A catheter assembly employs an outer catheter with a pre-formed distal end and an open lumen. An inner catheter having an open lumen and a pre-formed distal end is movably disposed within the outer catheter. Relative rotation and extension of the inner and outer catheters provides the distal end of the catheter assembly with an adjustable range of two- and three-dimensional shapes. The inner catheter can include sections of varying stiffness, such that extension of the inner catheter within the outer catheter modifies the shape of the outer catheters pre-formed distal end. One or both of the outer and inner catheters includes an occlusion balloon attached to the distal end thereof. The adjustable shaping of the catheter assemblys distal tip provides an improved system for locating and cannulating cardiac venous structures, particularly the coronary sinus via the right atrium.
