 This study will test the effectiveness of low-dose recombinant tissue plasminogen activator rtPA, or alteplase in dissolving blood clots in major arm or neck veins. rtPA is given to patients with heart attacks to dissolve blood clots in blocked coronary arteries. Blood clots that develop in major arm or neck veins usually develop after a venous access device VAD or catheter has been placed in the vein. The clot often causes arm, shoulder or neck swelling and pressure or discomfort. Current treatments include removing the VAD, using blood thinners such as heparin and warfarin, or using rtPA to dissolve the clot. All these options have disadvantages, however, including the risk of abnormal bleeding. This study will evaluate whether lower doses of rtPA can effectively dissolve clots without requiring an extended hospital stay, as is needed with the current higher-dose regimen. 