This study will compare the effectiveness of two blood thinners, heparin and lepirudin, in preventing withdrawal occlusion (blockage) in a venous access device (VAD). A VAD is a catheter (plastic tube) placed in a vein beneath the collarbone to deliver medication and withdraw blood samples during treatment. The device may become clogged, possibly by formation of a clot around the tip, blocking its opening and making it difficult or impossible to use. The clot can be dissolved by a medication called tPA. The blood thinner heparin has been used for many years to try to prevent the blockage from occurring, but it is still a problem in as many as 25 percent of VADs. This study will test whether a new blood thinner called lepirudin is more effective than heparin in preventing withdrawal occlusion caused by a small clot.