Just found this article put out by CA pheb. school. (Glad I found it because I couldn't remember the rational exactly right)
Procedure Using a Butterfly
The Butterfly or "Winged Infusion Set" is sometimes used on difficult draws such as infants and patients with small veins or veins that may have a tendency to collapse. In recent years, this technique has been improperly used as well as used unnecessarily. Most phlebotomists believe that vacuum tubes can be used on a butterfly. This is incorrect. If the physics of the technique are taken into consideration, as well as to when its necessary to use the butterfly, the industry will see a rapid decline in using this method to draw blood, not to mention a decrease in hemolysis, microclotting and short draws..
The butterfly would ONLY be used under the following conditions:
*Neonate / Pediatric draws
*Small vein draws, such as the back of the hand or foot
Butterflies should NEVER be used for:
*Normal draws, using the standard veins in the arm
*With a vacuum tube
When suing the butterfly, you should never ever use a evacuated tube. In considering the physics of this devise, the evacuated tube works against you when drawing the blood sample. Also using the wrong size syringe, complicated by pulling too hard on the plunger, will also give basically the same result.
The butterfly has a 12" tube. At the other end it has a small bore connector attached to a multi-draw device on which an evacuated tube will be used. A 5 cc evacuated tube must use its vacuum to draw the blood through the tubing from a small vein - thus you have increased pressure on the vein by 40% As the pressure is exerted on the vein, it begins to collapse - perhaps not all the way, allowing some blood to be drawn out. As this pressure is exerting on the vein, it is also "forcing" the blood out, which elongates the cells. This causes microclotting and some hemolysis. Even before the blood enters the tube, this sample is useless. Now, from the point of the needle to the tube, there is a 12" tube that the blood must travel through. As its does, pressure in the tube decreases, drawing the blood slower and slower. By the time the blood reaches the tube, 1/3 of the vacuum has been used to draw the blood. Thus, if there is not enough pressure in the vein, and if the vein is not weak, the additional pressure to fill the tube must come from the patient.
If the blood sample must be drawn from a small vein, such as the hand, or from a Neonate, the following technique should be used:
23g needle used with a 5 cc syringe.
25g needle used with a 3 cc syringe.
Notice that as the needle reduces in size, so does the syringe. Never ever use a 10 cc syringe. This will double to triple the pressure on the vein as well as cause microclotting and hemolysis. Always match the syringe with the needle used. If your using a 22g needle, a 3 cc to 5 cc syringe can be used. Never pull faster than the blood will allow. When using a butterfly, you have chosen to draw only the amount of blood needed for the tests, and the size of the syringe your using will accommodate all the tubes, using microtainer if necessary. As you pull back on the plunger, so not pull faster than the blood will allow.
Do not pull on the syringe plunger faster then the blood will allow.
You are in complete control of the pressure on the vein and on the cells. Thus increasing your chance of a good sample
Following these major decision factors in determining when and how to use the butterfly, the usage's of this devise will decline. Its unknown why phlebotomists feel that the butterfly should be used for normal draws. Many say that its because they are drawing from a small vein that could collapse. This makes no sense because using the butterfly on a vein that may collapse using a vacuum tube, does not prevent this because the phlebotomist is not in total control of how much pressure is exerted on the vein.
Although the preferred method of obtaining a venipuncture specimen is using the evacuated multi-draw, the use of a syringe may become necessary in patients where your unsure of getting a vein, or veins that may collapse. When using an evacuated tube, you only know if your in the vein by site and sound. The site of seeing the needle go into the vein and the blood being drawn into the tube, and hearing the pop sound as the needle enters the vein. Therefore, if you can't hear the pop sound and if no blood enters the tube, then you have to dig around for it, or draw again.
The syringe can be used as a safety margin for those unsure of draws. Using the syringe, a blood spot will appear in the hub of the syringe when the needle has entered the vein. By pulling slowly at first and then increasing the tension to the median limit of the vein, a complete successful draw can be made. Using the same puncture site with an evacuated tube could have not been successful because the vein was too weak to accommodate the pressure of the tube. Again, as we stated describing the butterfly, you are in complete control when using the syringe.