Cannula sizes

Hi, littlehobo! I worked for some time on IV Teams. I would think that most A&E's would not like to use small cannulas. The reason is because if the patient needs a bolus or an IV push of some medication immediately, those blue cannulas (22g in the states) just don't get the medication in fast enough and there is a risk of blowing the vein if the fluid is pushed too hard. Smallar cannulas and smallar volume syringes = high pressures in the vein. Blood can be drawn through those blue cannulas for blood sampling, but if too much pressure is used to aspirate the blood it will hemolyze and the specimen will be worthless to the lab. A larger cannula is always better for blood drawing. When I worked on IV Teams I always drew a patients labwork (if there was any ordered) before hooking up a new IV line just after I had started a new IV site. Actually, IV cannulas can and are used for blood drawing. Some facilities just don't want nurses to do it. The less an IV site is tampered with, the less likely it is to get a phlebitis. The other bigger here is that every time you open an IV site, you potentially expose the patient to bacterial invasion. However, we placed IV cannulas specifically for blood drawing in young child diabetics who need frequent blood sampling to save them from the trauma of being stuck repeatedly. We also placed them in patients just before receiving one of the clot buster drugs because they were going to be so anticoagulated that they couldn't be stuck or they'd hemorrhage. We, upon a doctor's order, also placed a cannula for blood drawing in some of our long term dialysis patients.

To draw blood from an IV cannula you need several syringes with leur-lok connections, tubes and a tourniquet. Place a tourniquet 8 or 12 inches above the IV cannula--not tight, just slightly snug to help you in withdrawing blood. Remove all tubes and caps from the IV needle hub. Attach a syringe and gently aspirate. If you are successful in getting blood, remove at least 5 cc. This blood will be discarded. Attach another syringe and withdraw the blood you need for the blood drawing. Withdraw gently so you don't hemolyze the blood. Most tubes take anywhere from 5 to 7 to 10cc of blood. Depending on the amount of blood you need to fill the test tubes you will probably need a 30cc or larger syringe with a leur-lok hub. Flush the IV cannula with at least 5 (10 is better) cc's of saline. Cap the hub or reattach the IV lines. Attach a needle to the syringe with the blood sample and let the negative pressure of each test tube take the blood it needs from the syringe. Do this ASAP before the blood clots in the syringe. It is not a good idea to draw blood out of an IV that is being used to give IV fluids. The reason is because no matter how much blood you withdraw and discard before taking a blood sample, it is always possible that there will be IV fluid lurking around in some of the tributaries of the vein that will contaminate and dilute your blood sample. So, it is best to only draw blood from a heparin or saline lock.

The way you get around the hospital not having a policy to do this is to get an order from the doctor that it is OK to draw blood samples from the existing saline lock.

Hope that answers your questions and gives you some information about this.