A catheter insertion assembly includes a coupler body (1) having a passageway therethrough communicating between a coupler adapted for attachment to an administration set tube at an inlet end of said passageway and at the outlet end of said passageway communicating with the operatively inlet end of a catheter (5) or communicating with a catheter and a cannula (9) carried by a base (11) and extending through a portion of said passageway co-axially into the catheter via an entrance separate from the coupler and laterally offset relative thereto with the cannula passing through a self-sealing grommet (10) or plug associated with the entrance, the coupler having an air receiving cavity (25) in communication with the passageway and the cavity having a gas permeable outlet.
