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Most people are now using Ultrasound to aid in cannulation of the femoral and internal jugular veins, but if you find yourself without the ultrasound machine you can increase your chance of successful cannulation of the femoral vein by positioning the leg properly.
Werner et al looked at the common femoral veins of 25 healthy volunteers and noted that the femoral vein was accessable more often when the hip was abducted and external rotated. This simple position change increased the mean diameter of the vein, and prevented the vein from being directly posterior to the artery.
References
Werner SL, Jones RA, Emerman CL. Effect of hip abduction and external rotation on femoral vein exposure for possible cannulation. J Emerg Med. 2008 Jul;35(1):73-5.