Sometimes the blood doesn't come out of Jemma's catheter and that's a big problem. Her catheter is used to give medicine and take blood. Basically it's a long-term iv that is inserted directly into her heart. The catheter must be cleaned internally at least once per week. This is done by the nurses squirting a saltwater type of fluid into the tube.
Sometimes catheters are positioned inside the vein in such a way that the end opening is blocked by the vein wall. This could create a blood extraction issue. Sometimes platelets grow over the end of the tube and act as a one way valve. In that case fluid could go into the catheter, but would be difficult to withdraw.
In the past when blood hasn't come out of the catheter Jemma tries to change her body position while the nurses are trying to take blood. Sometimes this works and sometimes it doesn't. Then other nurses and doctors give it a try. Again sometimes this works and sometimes it doesn't. Then they inject a medicine in the catheter to break-up the blockage. The medicine can stay inside the catheter between one and twenty-four hours. In the past they have left it in for one hour and then tried to take blood again. Each time this has been successful.
Unfortunately the medicine isn't especially good for the body, so using it each time blood is required is undesirable. Since Jemma had the medicine last week, the doctors decided to investigate further this week when blood did not come out. She had an ultrasound of the heart and an x-ray. The conclusion was the catheter was positioned correctly and there were no platelets blocking the entrance. Perhaps the platelets were too minimal to be seen on the ultrasound.