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Official Satellite Symposium of the 12th World Congress of Pain

“Ultrasound Guidance for Diagnosis and Interventional Management of Pain”

 August 15-16, 2008  Glasgow, Scotland

International Experts, Didactic Lectures, Cadaver Workshops, Interventional Procedures, Musculoskeletal Ultrasound

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Large-Nerve Blocks

The Puzzle

• Inject an equal, generous quantity of the same local anesthetic around 2 different nerves (presumably with the needle in the same proximity to the nerve).
   

• Assess the motor and sensory blocks of the 2 innervated areas 40 minutes later- Area 1 has a profound sensory and motor block, the other area (Area 2) is just starting to show signs of a sensory block.

• Check Area 2 again after 2 hours have elapsed and you find a fair sensory block (still not fully numb), and the motor block is only partially present.

• About 2 hours later (4 hours elapsed) and Area 2 has reached a full sensory and motor block. Area 1 blocks are beginning to recede.

Why do the 2 blocks differ so greatly in effects and what can we possibly infer from this situation?

Area 1 is inhabited by "small" caliber nerve bundles and Area 2 contains "large" caliber nerves. The blocks will likely recede at the same pace as they onset.
If the block of Area 2 took 4 hours for full onset then it will probably take another 4 hours to dissipate once the highest concentration of drug is reached in the center of the nerve mass.

The Logic

If the effectiveness of the block depends on the presence of the local anesthetic in contact with the nerve substance;

• and the transport of the drug relies largely on passive mechanisms, i.e.- diffusion,

• and even if the rate of diffusion remains constant for a given solution across a type of tissue, the time it takes to completely diffuse into a tissue mass must in part dependent upon the size of that tissue mass,

• and since if you increase the diameter of the nerve, you increase the mass within it disproportionately,

• therefore, the time it takes to block "Large Nerves", in a peripheral technique, <b>must</b> be greater than the onset time of a block of "Smaller Nerves", and in proportion to the difference in the size of the 2 nerve sets,

dissipation of the block by diffusion out of the nerve mass follows largely the same principles, diffusion of the drug toward areas of lower concentration must take place over an amount of time proportional to the size of the mass involved. Therefore, blocks take longer to wear off of larger nerves than smaller nerves.
 

The Extension

The use of local anesthetics associated with relatively short blocks in smaller peripheral nerves will exhibit longer effects when injected in sufficient quantity in close proximity to larger nerve. The prolongation of the effect will be proportional to the increased size of the nerve.
 

Trying to Find the Sciatic Nerve Using Ultrasound? Try This!

One of the limitations of having only a 5-10 mHz general-purpose ultrasound probe is that it’s inherent strength involves visualizing fairly superficial structures (say less than 3 cms down).  When you’re using this probe on humans over about 12 y/o to locate the sciatic you are, as they say, “out of your depth”. The average depth of the adult sciatic seems to be about 5 cms. 

Looking for the sciatic with the ultrasound probe in a transverse orientation to the nerve is difficult at best because in cross-section there is not much echo-reflective surface on, even a nerve as large as the sciatic. 

Looking for the sciatic with a longitudinal scan increases your chances for success by maximizing the possible amount of reflective surface that can be encountered by the ultrasound beam.  However unless your scan clearly shows the lower boundary of the gluteus maximus muscle, the longitudinal striations of the sciatic can appear to be a continuation of the striations of the gluteal muscle, obscuring it’s position.

So then by splitting the difference and holding the transducer in an orientation between transverse and longitudinal, with respect to the suspected path of the sciatic, we may gain access to a larger reflective surface while making the reflected object easily distinguishable from the gluteal muscle.

Try to identify the lower edge of gluteus maximus muscle and look for the sciatic density at that level.

Below is a diagram showing an illustration of this method.