A catheter assembly for cannulating an ostium of a coronary sinus of a heart comprises an outer catheter member and an inner catheter member. The outer catheter member has a distal end portion including a deflectable distal end and a lumen extending through the deflectable distal end, wherein the distal end portion includes regions of varying stiffness and has a pre-formed curvature. The inner catheter member is slidably and rotatably disposed within the lumen of the outer catheter member, and has a distal end portion including a distal end, and a lumen extending through the distal end. The distal end portion of the inner catheter member has regions of varying stiffness and a pre-formed curvature. The outer and inner catheter members are configured such that relative translation and rotation of the outer and inner catheter members causes the distal end of the outer catheter member to assume a plurality of two and three dimensional shapes.
