Failed phlebotomy? Think William HarveyBMJ 2014; 349 doi: https://doi.org/10.1136/bmj.g5232 (Published 04 September 2014) Cite this as: BMJ 2014;349:g5232
- Keith L Dorrington, associate professor of physiology, Department of Physiology, Anatomy and Genetics, University of Oxford, Oxford OX1 3PT, United Kingdom,
- Jeffrey K Aronson, reader in clinical pharmacology, Nuffield Department of Primary Care Health Sciences, University of Oxford, Oxford OX2 6GG, United Kingdom
- Correspondence to:
Why do health professionals take blood in the wrong direction? Shortly before his death in 1657, William Harvey told the young Robert Boyle that the main thing that convinced him of the circulation of the blood was that the valves in arm veins prevent flow down the arm.1 This is a truth universally acknowledged, but ignored, given that it is the retrograde direction in which all hopeful phlebotomists expect blood to flow into their needles. We sometimes get away with it, when the blood, travelling headwards up a vein, struggles past the occluding needle, does an about turn, and ends up in the right place. Sometimes. The headwards approach will generally work in large veins. But the smaller the vein the more guaranteed we are of a mere dribble, loss of self-esteem, and another bruise in a disheartened patient.
When the needle is in a large vein in the arm, flow may be improved by invoking the phenomenon of vis a tergo. This Latin term (literally “force from behind”) entered English at the start of the 19th century and generally refers to the force exerted by the heart in pushing the blood around the circulation. Contracting the nearby muscles increases this, for example by asking the patient to clench and unclench the fist, but when a sample is sought from smaller vessels, such as those in the hand, this manoeuvre is less helpful. Other ways of increasing flow or inducing distension in a vein include tapping it, lowering the limb, and using a tourniquet.
Another solution is to remember Harvey. When asked to see a patient from whom others have failed to obtain blood, find that tiny vein in the arm or hand and gently insert a needle or cannula facing down the arm. It may be helpful to locate a nearby valve, using Harvey’s method, and then insert the needle just below it. Harvey’s method is to empty a segment of vein by stroking along it with a finger, moving away from the heart, and noting the point beyond which a valve prevents the retrograde refilling of the vein from the proximal end of the segment, the distal segment still being occluded by the finger. A cannula may be useful if repeated samples are needed; it can sit there for a while without spearing the wall of the vein and springing a leak that heralds another failure. However, there is evidence that using a cannula may be associated with a higher risk of haemolysis than using a butterfly needle. Even if the flow is sluggish, a needle or cannula facing fingerwards in a small vein will always serve up the required sample (see figure⇓). In this case, don’t use an aggressive vacuumed sample tube—it may be too impatient to cope.
In 125 pages of WHO guidelines on best practices in phlebotomy there is no concession to Harvey.2 Needles are shown dutifully pointing headwards. This is perhaps because we are used to delivering fluids and medications in the direction of flow. But a little physiology shows how to solve a very common problem in difficult cases.
Cite this as: BMJ 2014;349:g5232
Competing interests: We have read and understood the BMJ policy on declaration of interests and have no relevant interests to declare.