Guide extension catheters and related methods are disclosed. A guide extension catheter can comprise, among other things, an elongate tube member and a push member. The push member can be operably coupled to the tube member at an eccentric coupling position and can extend proximally therefrom for slidably positioning the elongate tube member over and partially beyond a distal end of the guide catheter. The inner diameter of the elongate tube member may be adjustable, such that it constricts when not bound along its inner surface by the guide catheter. The elongate tube member can, for example, include a longitudinal slit configured to expand and contract to accommodate insertion and removal of the guide catheter, respectively.
