A guide catheter assembly for accessing lateral branch veins of the coronary sinus includes a guide catheter and a pacing lead. The guide catheter has a central lumen therethrough, a proximal end and a distal end pre-formed with a guide curve. The pacing lead is slidably receivable within the guide catheter lumen. The pacing lead has a proximal end and a distal end formed with a first curve and a second curve extending proximally from the first curve. The second curve is adapted to mate with the guide curve to direct the first curve into a selected branch vein of the coronary sinus. The second curve may be pre-formed in the pacing lead, or may be imparted to the pacing lead via a stylet or inner catheter.