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Puncture Techniques

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A striking difference here from conventional, ‚landmark‘-based puncture techniques is that the anaesthetist in charge has a much greater range of „freedom“ in selecting the access paths to the target structures, i.e. the nerves. However, this does not relieve him of responsibility for carrying out an intensive investigation of the respective (cross-sectional) anatomy of the region in question. Only under this premise and in combination with corresponding coordinative and manual dexterity can ultrasound contribute to minimum-risk puncture with little duress for the patient. Attention can then be directed flexibly to the individual anatomic features rather than to external landmarks.

Basically, ultrasound-guided blocks can be divided into those with puncture more or less perpendicular to the ultrasonographic plane (out-of-plane, short axis) and techniques within in the ultrasonographic plane (in-plane, long axis).

Out-of-plane, the tip of the needle shows up as a small, hyperechogenic (white) dot in the image. Here it is to be noted, however, that the entire shaft of the needle is displayed as such a dot regardless of whether the ultrasound plane transects the needle tip – as is required – or the shaft.

out-of-plane

 

With the in-plane technique, the needle ideally ends up lying at full length in the ultrasound plane as a hyperechogenic (white) line, so that it can be advanced accurately to the target, i.e. to the nerve, which usually appears in cross-section. This procedure too is not always easy, even for the experienced user, since even a slight tilt of the ultrasound plane may impair the visibility of the needle in its entirety. Our basic preference in most blocks is the in-plane-technique, since it permits a more comprehensive overview of the path taken by the needle on its way to the nerve.

Our specific block techniques and procedures are described in the respective sections.

 

in-plane

 

The Nerve in Ultrasound

In most blocks, the corresponding nerves or parts of the plexus show up in cross-section, and the image is governed primarily by the histologic composition of the respective structures. Thus the display of nerves with little connective tissue (e.g. the interscalene brachial plexus) is hypoechogenic, that is, nearly „black“, in contrast to a so-called hyperechogenic („white“) image such as that typical of the sciatic nerve with its abundance of connective tissue. Between these extremes fall numerous intermediate types which are explained by the varying proportions of nerve tissue and connective tissue in the nerve as a whole.


The Procedure in Practice

Bringing about any ultrasonographically guided regional anaesthesia requires an inner-departmental procedure which is standardized as far as possible. The following proposals have proven helpful in clinical routine at our institution. Attention must first be given to adequate positioning of the patient and to a good ergonomic setup of the ultrasound device. In the ideal case, the person performing the block should have a synchronous view of the puncture site and the screen. Initially, non-sterile imaging for orientation, with identification of the anatomic landmarks, is to be recommended. This also makes it possible to recognize any potential obstacles as well as anatomic variation. After the puncture location and needle path have been selected, the region around the point of insertion is carefully disinfected (remember to let it take effect!!) and covered with a sterile cloth. With the help of the assisting nurse, the probe is now packed in a sterile wrapper. The choice of best available option (film, bag, glove, etc.) should be made within the department after testing. Then local infiltration of the puncture site and the intended path of the needle are performed under ultrasound imaging. If the decision is made not to use a nerve stimulator, the application may extend into the vicinity of the neural structures. Then the puncture needle is advanced into the perineural target area. A few millilitres of dextrose 5% solution can be used to check for proper dispersion of the injected fluid. Dextrose 5% does not impair nerve stimulation. After this the intended dose of local anaesthetic is administered around the nerve. If its distribution is inadequate, the position of the needle may have to be corrected. Finally, a catheter can be inserted, if desired, and attached with a sterile dressing.