A small tube connected to a catheter in a vein in the arm for easy access. It is an alternative in some cases to using an IV. Its called heplock because of the order of medicating using it which is saline, medication, saline then heparin. The final heparin dose being introduced to prevent blood clots in the line should more medication be needed later. Generally a saline lock is used for lines in the arm but it is sometimes refered to as a heplock for historical reasons.
Heparin is used to prevent blood clots either before surgury or if there are idications that the patient has blood clots forming in the veins arteries or lungs. Hep-Lock is a registered trademark for a brand of porcine heparin solution produced by Baxter. Porcine (from pigs) heparine can also be purchased without the brand name.