This trial is for patients with a central venous catheter (a vascular access device) that is not functioning properly (unable to withdraw blood). The trial compares a new blood clot dissolving agent (alfimeprase) against the currently used treatment.
Subjects must: * have a dysfunctional Central Venous Access Device (CVAD) defined as unable to withdraw 3 mL blood * have had the device in place for \> 48 hours * be clinically stable * be 18 or older * give informed consent