this invention is a single - lumen balloon catheter and assembly having a variable stiffness shaft that allows for greater guidewire trackability and proximal pushability than previous designs . the catheter has an elongate tubular body that is made of inner and outer coaxial tubes , the inner tube being made of proximal and distal sections , the proximal section being stiffer than the distal section and the distal section being stiffer than the outer tube .