A guide catheter employs a retractable inner sheath movably disposed within an open lumen of an outer sheath. The inner sheath includes a pre-formed distal end and an open lumen adapted to receive a payload. The inner sheath is translatable and rotatable relative to the outer sheath. The inner sheath conforms to the outer sheath shape until it is distally extended beyond the outer sheath. The outer sheath contains a steering tendon that can be used to adjustably deflect the outer sheath. The guide catheter further includes a proximally attached guide handle. A steering handle can be pivotably connected to the guide handle. The steering handle is connected to the steering tendon and can apply a tensile force to the steering tendon for adjustably deflecting the outer sheath. A catheter according to the present invention provides an improved system for locating and cannulating cardiac venous structures, particularly the coronary sinus via the right atrium.