A catheter assembly or catheter attachment for use in treatment of coronary artery disease resulting from atheromatous lesions whereby stenotic lesions can be dilated without interruption to the blood supply distal to the stenotic lesion being treated, the catheter further allowing blood pressure distal to the site to be measured throughout the treatment procedure. In one form of the invention, the catheter assembly comprises first and second coaxial telescoping tubes, the outer tube having a pattern of perforations through its side wall proximate the distal end thereof and an inelastic expander member sealed to its outer surface at spaced apart locations so as to span the pattern of perforations. The patient's own blood may be perfused through the lumen of the outer tube between the outer surface of the inner tube and the inner surface of the outer tube and through the pattern of perforations to uniformly expand the inelastic expander member. The inner and outer tubes are tapered at their distal ends so as to facilitate insertion of the catheter assembly through a blood vessel and, by relative longitudinal displacement of the inner tube with respect to the outer tube, the pressure and rate of flow of blood from the open distal end of the catheter can be controlled from its proximal end. In an alternative arrangement, the inner tube is replaced with a cylindrical wire of a predetermined diameter less than the inner diameter of said outer tube. Further, the distal end portion of the outer tube may be made distensible so that it can be made to expand by applying a suitable pressure at the proximal end of the catheter, and the catheter may be provided with a highly flexible guidance pigtail for facilitating the insertions of the instrument into a blood vessel, without interfering with the expanding and perfusing functions of the instrument.