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Haven’t We Talked Enough About the Interscalene Block?
Short answer, No! Now is the time to consider a big change in the technique and use ultrasound to its greater potential.
I have been trying to think of an apt analogy for the concept I am trying to explain in this page, many have come to mind and I will probably flip back and forth between them as I go. Essentially I want to describe the interscalene environs of the neck in terms of its connections and barriers, its compartments and connections, its rooms and hallways. Hence the working title of this page; Floorplan.
In nerve blocks, it is sometimes hard to think in 3 dimensions about the space your working in and seeing on a 2-dimensional ultrasound screen, but it is necessary, at least in the planning stages of a block. Most of the images and illustrations on this site depict the area containing the brachial plexus (and most other nerve targets) as flat, 2-dimensional work areas, this is a convenient simplification for me and for you, but mostly for me.
Here I must add a dimension in order to clarify.
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In other words, we can create brachial plexus nerve blocks of consistently high quality and overall fewer side effects, with much less volume using ultrasound guidance.
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It seems to me we’ve been missing at least one of the big advantages of ultrasound guidance. Since in some cases we can identify structures surrounding target nerves, we can use a 3-dimensional reconstruction of the environment to take advantage of naturally occurring dividers and connectors to understand, contain and direct the distribution of the volume of local anesthetic you inject. In other words, we can create brachial plexus nerve blocks of consistently high quality and overall fewer side effects, with much less volume using ultrasound guidance.
Follow along as I explain my thoughts on getting the most block out the least volume and keeping the side effects at a minimum.
On another page of Neuraxiom wee have introduced the “Phrenic Sparing Interscalene Block” PSIB Project and the open study which is intended to produce the best possible technique of the interscalene block. Many hours of discussion and planning about PSIB have served to focus attention on the factors creating some variability in the standarrd technique. Namely the sheath surrounding the brachial plexus and whether your injecting the local anesthetic within the sheath or around it.
If this seems like a small difference, then welcome to where I was a couple months ago. I knew there was a difference in appearance on ultrasound but I didn’t understand the difference in the outcome of the injection and I had even less of clue as to how to take advantage of it.
What Sheath? -- If it walks like a Duct, and acts like a Duct, ...
I know a recent article in the journal Anesthesiology called into question the accepted existence of an anatomical sheath surrounding the brachial plexus in the axillary space. I’ve heard of this article but have read only its abstract, I’m not sure whether the author questions the existence of the sheath in the supraclavicular space and northwards, but it matters little to this particular line of thought. If there is no demonstrable anatomic sheath surrounding brachial plexus in the interscalene through supraclavicular space, then the confluence of adjacent structures creates a space that acts exactly like a sheath. (I don’t really believe this, but I thought I’d give everybody an out). Meaning if you inject volume into the distal end of this “sheath”, effects occur which can only be explained if the local spread directly retrograde to the nerve roots. So no more talk about the “sheath”, onward.
Ground Rules of Nerve Blocks within Compartments (Rules are Made to be Broken)
We must have some guiding principles. (Most of these are obvious)
- If you inject within a compartment, the volume cannot cross to another compartment without a damn good reason.
- Movement of the volume within the compartment will occur from an areas of higher pressure (the point(s) of injection) to areas of lower pressure, along pre-existing planes wherever possible.
- Injecting into connecting structure (like a sheath) the volume will travel along the connecting structure to an extent which is in direct relation to the compliance of the structure and to pressure exerted on it from structures which abut it.
- Movement of the volume along a connecting structure occurs in both directions (or more if branches exists) favored by “least resistance” principle and only slightly by gravity
- Distribution of the volume occurs over time starting a the time injection starts, and continuing until the pressures and concentrations are equalized.
- Since this happens there is no “reservoir of local anesthetic” laying around waiting to prolong the block.
- The speed with which local molecules diffuse into and bind with sodium channels within nervous tissues is most directly related by pKa (available non-ionized molecules) and by the number of local molecules (solution concentration) initially produced.
- Volume of local anesthetic may increase the surface area of the nerve-anesthetic interface but this does not necessarily prolong the length of time a block is intact. (All segments of blocked nerve would presumably unblock at the same rate, so unless there was a significant difference in the time of initial contact they would end at about the same time.)
- Any volume that diffuses away from the nerve is not available to participate in the block.
- Local anesthetic molecules diffuse and become bound in every direction while they are available.
- Once the concentration of the local molecules in the nerve tissue becomes the same across its diameter and at least equals its surrounding environment the block is set and will begin slowly wearing off.
- Any local anesthetic molecules which are bound in non-nervous tissues are not available for the block.
- Any techniques or structures that hold the local volume in close contact with the nerves for some time after injection and prevent diffusion of the volume away from the nerves would advantage the onset of the block.
- Nerve blocks wear off at a rate based on the size of nerve bundle, and the gradient of local anesthetic molecule concentration between the nerve and its surrounding environment (epinephrine and clonidine slow the development of large gradients thereby prolonging the duration of the block.
I said all of that so that I could say this. While local anesthetic volume injection around the brachial plexus bundle at both the interscalene and supraclavicular positions produce wonderfully reliable blocks, local volume directly injected into the sheath containing the nerve plexus bundle will create a equally solid reliable block, but with a fraction of the volume needed outside of the sheath, and with a much lower chance of concurrent phrenic block.*
How much lower volume? Well a well placed 10 - 20 mls of local outside the sheath will produce a good block with a relatively low chance of phrenic involvement. Placing 3-5mls of the local inside the sheath should produce the same block. The difference between these 2 techniques is, the intra-sheath injection will almost surely produce at least a partial cervical sympathetic block (Horner’s) because even 3mls is enough to travel up the sheath to the nerve roots and catch the sympathetic nerve as they exit the foramen.
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