My goal is to insert catheter full of saline in femoral vein of rats and then I can intravenously inject pentobarbital through linking catheter to a pump. However, I face a problem. When I succeed to insert it, I always find that there is some saline leaking. I guess that I must break the vein when I insert it. But there is no blood when I separate the vein and the artery. Sometimes it is fine at first, and then the vein starts leaking out saline so I fail to link the catheter to the pump.

Does anyone face the same problem like I do?

How to solve it?