 The hypothesis is that the enoxaparin treatment group will have a median adverse catheter event free survival time of 12 weeks versus 4 weeks for the control group with a hazard ration of 0.4. A total sample size of 50 25 in each arm will detect such a difference with 90 power at an α0.05. If there is a drop out rate of 10 in each arm, a difference can still be detected with 80 power. 