Saturday, October 26, 2013

The Science of Getting an IV & Collecting Blood

The case was a young trauma victim, he wasn't that sick, but sick enough to get admitted for observation. As part of the trauma protocol we collect blood for labs, type and screen and the usual stuff. Primary survey completed, we had a nice 16 g cannula in his left AC fossa but blood had not been collected yet. The lab tech arrived and was preparing to stick the patient in the R arm when I asked him. Why don't you take the blood from the cannula? - I'm not allowed to do that. He replied. I like to avoid unnecessary pain to my patients whenever possible so I reapplied a tourniquet to the L arm, pulled 3 cc of blood to discard, then collected the blood directly into the tubes. As I was doing that, the ED chief nurse saw me from across the room and almost had a stroke. She went into a rant about how this was a violation of the protocols and yara yara yara... To my amusement, she took the tubes I had just collected from that freshly-started IV and trash them into the sharps container. Then she order the lab tech to stick the patient again in the other arm for a "correct" blood sample. This was a senior nurse and I just couldn't believe she would do that. What is the difference from that blood to the blood in the other arm?!

So I did a lit search about this topic and found some interesting data. 

- The cannulas are made with soft plastic that remains open by the positive pressure exerted by the infusing fluid, the walls tend to collapse if too much negative pressure is exerted, this causes turbulence and higher rate of hemolysis. This hemolysis is more likely to occur using the larger vacutainer tubes of 10 cc because of higher negative pressure early in the draw or when using a syringe and pulling too hard on the plunger. So if you use a syringe, pull the plunger just a little bit until you see blood flowing into the barrel and then continue gently, and for the vacutainer system, use the 5 ml tubes to minimize hemolysis. 
- And what is the deal of tapping the site to "pop" the vein into view? That doesn't work, the skin gets red but the vein doesn't change in size and it also can cause hemolysis inside the vein, so let's not do that either. 
- Already inserted cannulas can be used to collect a blood sample even when IV fluids of medications have been infused through it. The only caveat is that for glucose containing fluids it is recommended to wait 3 minutes after stopping the infusion. When using the inserted IV, "washing" the cannula with 5 times its capacity and discarding that blood will eliminate any possibility of contamination. The volume capacity of a 22g cannula is about 0.05 ml and for a 14g  0.15 ml (yes, go ahead and measure it), therefore drawing 1 ml is technically enough, but get 2 ml and no one will argue.
- If you have a prolonged vein hunt for more than 2 minutes, go ahead and get the cannula in, remove the tourniquet, elevated the arm and then bring it down again to allow "fresh" blood into the extremity, reapply the tourniquet and then collect the samples. This is particularly important if you use venous lactate  in sepsis screening to avoid false positive results. 
- Follow this order when filling up the tubes: Blood cultures -> Lactate -> Coagulation -> Serum -> Heparin -> EDTA. This will minimize the chance of contamination, false positives, hemolysis and sample clotting. 
- Don't overfill, shake too hard or drop the tubes; and if you open the tubes transfer the blood from the syringe, do it without the needle.

And this is the summary of all of these articles. If you have a couple of hours and there is nothing on TV, go ahead and read...

And before I go, a final advice for my fellow physicians. If you think that starting IV's and collecting blood is the nurses' job, then you are missing the critical concept of team work. As leader of the team you should know how to do this and every other procedure in the ED; from starting lines, mixing drips, work up the pumps, etc. This will not only benefit the patient, it will also earn some golden points with the rest of the team. So pull up your sleeves and get busy.  


  1. Where's the "like" button for this post? As always it is great to hear from you, Dr Atriham :-)

  2. That is a good idea, we should have a "like" button. :)