An introducer having an elongate tubular member and a device connector releasably attached to a proximal end of the tubular member. The introducer allows exchange of medical instruments, such as a blood filter and cardioplegia catheter, through a single lumen. An obturator having a retractable blade for making incision on a tissue is insertable through the lumen of the introducer. Methods of using the obturator and the introducer for introducing medical device(s) into body cavity, such as a vessel or cardiac tissue, are also disclosed.