A splined multilumen body for use in elongated medical devices is provided for carrying multiple conductors, wires or cables to multiple device components along the device body. Open lumens may be provided through which medical devices or therapies may be delivered. The multilumen body is constructed from a generally tubular outer member having inward-radiating splines that mate with outward-radiating splines on a generally tubular inner member. Lumens formed between sets of mated splines isolate conductors carried therein. Interaction of mated splines provides good torque transfer between outer and inner members. Materials for fabricating outer and inner members may be selected to achieve desired torque transfer properties, flexibility, and surface friction.