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Instructions:
- Page down through these ultrasound images and one at a time identify the structures you see in them.
- Visually identify the triangle containing the femoral nerves.
- Pass your mouse cursor over the picture to reveal a diagram to see if we agree.
All of the images are oriented as are chest x-rays, you are standing in front facing the patient, their right is your left.
Identify first the femoral artery, round and usually smaller than the vein which if visible lies below the artery and medial to it. In a live survey, the artery usually visibly pulsing remains round and full when you press hard enough to collapse the larger veins.
Depending on the position of your ultrasound probe relative to the inguinal ligament, you may see a second artery below the superficial one. This is the deep femoral (profunda) and simply means you are below the bifurcation. This second artery will merge with the superficial one when you move the probe cephalad. The bifurcation usually occurs a few centimeters below the emergence of the femoral artery from below the inguinal ligament, but sometimes the bifurcation occurs at or even above the level of the inguinal ligament.
The deep femoral is only important as an indication of how proximal or distal your survey is, and for pinpointing something you want to avoid. Only the common or superficial femoral artery is necessary as a landmark for locating the femoral nerve group.
Sometimes a brighter, density is noted beneath the femoral artery, this is a suspensory ligament that reaches up into the pelvis.
It is not necessary to identify the actual nerve bundles which comprise the components of the femoral nerve group. Although by adjusting the angle of ultrasound probe to the surface you will almost always be able to gain a view of the nerves.
The femoral nerve group gives off branches immediately as it emerges from the pelvis beneath the inguinal ligament. The branches that distribute first are smallest and lie lateral to the main nerve mass, therefore the nerve target that you generally will see will be densities in line abreast, thicker medially - thinning laterally.
For the block to be successful you must only identify the space containing the nerve group which lies lateral to the femoral artery. The target space will exist and appear as a triangle with a short side next to the artery and the other sides extending laterally.
When placing the needle for the block, it should be positioned at least twice. Once medially close to the artery (but not too close), and once more laterally toward the tip of the triangle. Local anesthetic solution injected at both of the points will produce a strong block.
All local anesthetic solution should be injected within the target triangle. Watch the ultrasound screen as the injection proceeds. It is common for the tip of a needle held rigidly to be displaced from a space which is deforming with the injected volume. The target triangle should be seen to expand as the solution is instilled. If the spaces above or below the triangle are seen to expand, the needle must be repositioned.
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