A sheath assembly for the insertion of a percutaneous pump includes a tubular sheath body dimensioned for insertion into a blood vessel through a vessel aperture. The tubular sheath body includes a wall having a proximal end portion, a distal end portion, a longitudinal axis, an outer surface, an inner surface defining a first lumen substantially parallel to the longitudinal axis, and a second lumen disposed within the wall between the inner surface and the outer surface and extending from the proximal end portion to the distal end portion. The first lumen is dimensioned to allow passage of a portion of the percutaneous pump, and the second lumen is dimensioned for passage of a guidewire. A stylet is removably positioned to substantially occlude the second lumen. The stylet has a proximal end releasably secured to the sheath assembly.