An atherectomy device comprising a rotatably motor-driven flexible hollow shaft that is coaxial with a longitudinal axis of the guidewire; an annular distal end of the hollow shaft having a diameter that is only slightly larger than the diameter of the guidewire and configured with cutting surfaces for piercing hardened atherosclerotic plaque! a tubular sleeve secured to a first peripheral surface of the hollow shaft and configured with cutting surfaces for enlarging an opening formed in the plaque; and an asymmetric cutting unit extending between the sleeve and a second peripheral surface of the hollow shaft, to enable eccentric rotation of the hollow shaft for cutting and removing additional occlusive material from the blood vessel to open a chronic total occlusion. In one embodiment, the cutting unit is a helical strand unit having a polygonal cross section with a first side that atraumatically contacts a blood vessel wall.