iv lines get backed up with blood from the patient's vein when the pressure of the blood in the vein exceeds the pressure exerted by the iv fluid in the iv line. this is the reason that the iv bag is always kept hanging above the level of the patient's heart. an iv that is dripping by gravity (without a pump to assist it) will reach a point at which it no longer drips as you lower it closer and closer to the patient. lower it even further and you'll see blood start to back up into the tubing. patient movements can create enough pressure in peripheral veins to override the external pressure of the iv fluid and gravity, including the pressures exerted by iv pumps, enough to cause some blood to sometimes back up into the iv tubings and saline lock mini-tubing connectors. an example is if a patient makes a fist and holds it as tight as they can. blood will wash back into their iv tubing.
because the iv tubing and iv solution are sterile, any blood that might creep back into the tubing will also be sterile. if blood is backed up all the way to the iv pump, something is wrong. blood will also back up that high into tubings when the iv pump has been turned off and no iv fluid is infusing or somebody made a boo-boo and forgot to prime the iv tubing with iv solution and the tubing only has air in it. in either case, that's too much blood in the line for it to be cleared out and the tubing should be changed. it's not as much blood as you would think--only a couple mls. the problem with trying to flush that much blood out of a line is that you can never flush all the blood out completely. rbcs are left behind to lurk in the nooks and crannies of the tubing which are then a potential medium for bacterial growth at sometime in the future.
with heparin locks, if the dwelling solution is heparin, it is more than likely that any backed up blood is not clotted off because of the anticoagulation effect of the heparin. it is ok to push that blood back into the vein. it won't harm the patient, in that, the clotted blood isn't going to become an embolism that will lodge in a major organ and cause an infarction. however, you really want to be careful of pushing clotted blood through any iv cannula, whether the iv is being used for a continuous infusion or as a lock. more likely than not (1) the actual forceful pushing of the clot into the vein is going to hurt the patient as the wad of clotted blood is rammed into the vein and (2) result in a phlebitis soon to follow due to the physical irritation the clot created as it exploded onto the walls of the vein. that is traumatic. the better action upon discovering a saline lock with blood backed up into it, is to remove all the connecting caps and mini-tubings, connect a syringe directly to the hub of the cannula and make several attempts to aspirate the clot using a kind of piston motion--pull back on the plunger and then release it (don't push on it) and repeat this over and over. if, by some miracle, the clot does aspirate (suck) out and into the syringe, continue to draw out a little more blood to confirm the patency of the iv, remove the syringe, connect a new syringe already filled with saline and gently flush the iv. then, apply new caps and/or mini-connector tubings. our iv team always flushed any lines that were just freed of any blood with a good deal of saline just to wash out any stray rbcs that you can't see. in the more likely event that you can't aspirate the blood clot out--well, it's time to thrown in the towel, d/c the iv and restart it.
it is far more efficient to keep track of your iv devices and do regular maintenance to keep them patent so things like this don't happen. it's like having a car. some people only take it to a mechanic when it breaks down; others have it checked on a regular basis to prolong it's life of service. this often is a personal practice issue, a decision and behavior that you will have to make for yourself. saline locks that are not being used should be flushed on a regular basis. they will develop phlebitis as a natural body response to a foreign body by the very nature of their presence despite all the hype the manufactures tell you about the biofriendly materials these products are made with. when i was working as a staff nurse i made it a regular practice to flush all heparin and saline locks as part of my first rounds (didn't want any surprises later when i went to hang or give any iv medication). if a heparin or saline lock is no longer being used and there really is no reason for it, then the doctor should be asked if it can be d/c'd.