How to take blood with an intravenous cannula - a novel technique for the smallest veins!
Please only try this technique with appropriate supervision! I came up with this technique as a junior doctor many years ago when I just could not take blood from a patient. the veins were too small, ultrasound wasn't available, and I wasn't able to do a central line or femoral access. I remember thinking, the only reason I couldn't use the small veins was that a needle or cannula was obstructing their inflow. So why not do the cannula in reverse! This was a tricky technique to start with but once I got used to the different angle and positioning it wasn't that difficult, and much easier than a femoral stab or central line! A cannula is also less likely to rupture a vein than a needle. So in the smallest veins, where even the slightest tremor could cause the needle to contact the vein wall causing rupture, a plastic cannula sitting comfortably in a vein is far easier in my hands. There are times I use this technique in long theatre cases. Usually I would use an arterial line to sample repeat arterial blood gases, but often the arterial line doesn't withdraw blood easily. Instead of trying to insert another arterial line, I will insert a reverse cannula. This allows me to sample venous gases when I need to, and still get accurate readings for the Hb and electrolytes. AND the PaCO2/pH readings correlate well PvCO2 is approximately 6mmHg higher than PaCO2 and Venous pH is approximately 0.03 lower arterial pH [Peer-Reviewed, Web Publication] Ogele E, Lang S (2018, October 15). VBG vs ABG in the ED. [NUEM Blog. Expert Commentary by Walter J]. Retrieved from http://www.nuemblog.com/blog/vbg-abg
