An introducer assembly includes a catheter having a proximal end, a distal end extending to a distal tip of the introducer assembly, and an outer catheter wall. The catheter includes a medical device holding portion proximate the distal end, a guide wire lumen extending between the proximal and distal ends, and a side opening extending through the outer wall to the guide wire lumen. The side opening and the guide wire lumen are simultaneously open and the guide wire lumen and side opening are able to receive a guide wire therethrough. The catheter is flexible at least in the location of the side opening. The catheter also includes a plurality of one stiffening mandrel lumens extending from the proximal end and a plurality of stiffening mandrels.