A catheter has a body including a proximal region, a neck region, and a distal region. A shaping wire is disposed within the distal region to predispose it into at least a partial loop, which may have a fixed or variable radius of curvature. The shaping wire includes a distal portion having a circular transverse cross-sectional shape and a proximal portion having a non-circular (e.g., rectangular) transverse cross-sectional shape. The proximal portion of the shaping wire can have a width-to-thickness ratio of at least about 4, such as about 4.67. A transition portion can promote a gradual transition from the circular to the non-circular transverse cross-sectional shape, for example by increasing a width of the shaping wire by about 0.001″ and/or by decreasing a thickness of the shaping wire by about 0.001″ for every about 0.004″ in length through the transition portion.
