Vented piston and push-rod subassembly for use in a syringe barrel

Abstract

This invention relates to an improved vented syringe of a type ideally suited to the taking of blood samples for blood gas analysis and, more particularly, to a novel piston and push-rod subassembly for use therewith wherein the rubber piston on the tip of a rigid push-rod cooperate with one another to vent air out the rear end of the syringe barrel while trapping that contaminated portion of the blood sample that came in contact with the vented air inside the piston's hollow interior. Specifically, the piston is encircled on both ends by annular ribs sized for sliding contact with the interior of the syringe barrel, the upstream rib of the two having slits therein to pass both air and blood to a medial portion of reduced cross section while the downstream one maintains a continuous annular fluid and air-tight seal. A slit in the medial section admits the blood and air passing the upstream rib to the hollow interior of the piston when, with the piston subassembly in a passive state, a fluid-tight seal effective to prevent the passage of blood rearwardly out of the hollow interior of the piston is established that will freely pass air. These same elements of the piston subassembly cooperate when the latter is forcefully urged in either direction to establish an annular seal between opposed surfaces of the piston and push-rod tip that is both liquid and air-tight.