A system and method for treating a blood vessel that is at least partially obstructed by an occlusion which divides the lumen into a proximal lumen segment and a distal lumen segment. The system includes an orienting catheter having an orientation element positionable in an intrawall space of the vessel and an occlusion catheter having an occlusion balloon inflatable in the proximal lumen segment so as to isolate a target volume including the intrawall space. The pressure inside the target volume is reduced to a pressure below the pressure of the distal lumen segment so that the intima presses against the orienting element of the orienting catheter. A distal end of a reentry device may be advanced from the orienting catheter through the intima and into the distal lumen segment.
