Some say Stimulating Catheters are obsolete: Hang on, not so fast!
By: Cameron R. Smith MD. PhD
The advent of ultrasound has been a huge boon for medicine in general, allowing for everything from bedside evaluation of cardiac function to pre-natal fetal evaluation, and simplifying procedures from intravenous access to prostate biopsy. Arguably one of the most dramatically impacted fields within medicine has been acute pain medicine where the introduction of ultrasound has shed new light on anatomical variation, improved safety and simplicity of many classical nerve blocks, and allowed for the development of a host of new fascial plane blocks (for better or worse…). All joking aside, ultrasound is an invaluable tool. I trained in the age of ultrasound, so I’m very comfortable with ultrasound as a tool in my toolbox, but I also trained in an environment where anatomic landmark- and nerve stimulation- based blocks were (and are) commonplace, so even if the ultrasound isn’t available, there are very few blocks that will be delayed.
Since ultrasound-guided regional anesthesia is now becoming the de facto standard of care in the developed world, why would anyone at all choose to use an outdated technology and mindset such as nerve stimulation to guide the placement of something as mission-critical as a continuous peripheral nerve catheter?
Give me a minute to put on my curmudgeon clothes and grey wig…
While ultrasound is an incredible tool that I use every day, it, like every tool, has its limitations. I think most practitioners in the acute pain/regional anesthesia world would agree that anatomy is key to all our interventional procedures. So then how come ultrasound, which provides a real-time view of the individual anatomy of the patient currently being treated, hasn’t completely replaced archaic tools like nerve stimulators? Have you ever stopped to consider what that beautiful ultrasound image doesn’t show you? Are there important structures relevant to nerve block placement that even the newest, most coveted ultrasound devices from the likes of Phillips and SonoSite aren’t able to reveal? Let me be unequivocal here – the answer is a resounding YES. Not only is gross anatomy (like being able to locate the femoral nerve adjacent to the femoral artery at the level of the inguinal crease) critical to ultrasound-guided regional anesthesia, but so is microanatomy.
Nerves are not single tubes, akin to a drinking straw. There are multiple layers of tissue, from the innermost individual axons to the outer epimysium which defines the borders of adjacent muscle tissue, and thus the spaces which nerves and other extramuscular tissues can occupy, all of which are critical to multiple aspects of placement and ultimately the function of peripheral nerve blocks.
For continuous peripheral nerve blocks to function well, the catheter needs to remain in close contact with the target nerve – it needs to track with the nerve through any twists and turns the nerve may make. In order to do that, the catheter must sit within a space which also tracks with the nerve through it’s twists and turns. The ideal space for this is the subcircumneural space – a space which is deep to the circumneurium (sometimes called the perineurium), one of the outer tissue layers which is actually a part of the nerve itself rather than surrounding tissue, so it tracks with the nerve.
Why does this matter – who cares about tissue planes and spaces too small to see? If you were to place a needle next to a nerve as best as can be done under ultrasound guidance and then deposit some fluid to ‘open a pocket’ to thread a catheter into, generating a beautiful ‘donut sign’, you still don’t really know if that pocket of fluid and catheter are deep to the circumneurium and tracking with the nerve, or outside the nerve in the subepimyseal space. In the short term you won’t be able to tell, either. If you bolus some local anesthetic through this catheter then a relatively large dose (10mL or more) of relatively concentrated local anesthetic (0.5% ropivacaine, for example) will generate a beautiful nerve block no matter which of these spaces the catheter sits in. The proof of the pudding will emerge late that night or early the next morning when the relatively low-volume (even as much as 5-10mL per hour) of relatively low concentration local anesthetic (0.2% ropivacaine for example) will work beautifully IF THE CATHETER IS IN THE SUBCIRCUMNEURAL SPACE, but will provide almost no analgesia IF THE CATHETER IS IN THE SUBEPIMYSEAL SPACE. The extra layer of tissue is enough to prevent enough local anesthetic from diffusing all the way to the axons where it needs to be do to its job. This phenomenon – a catheter that seems to work great with an initial bolus, but later fails to provide adequate analgesia, is what is termed ‘secondary block failure’ and is entirely a product of the catheter failing to end up in the correct microanatomic tissue plane.
And now we circle back to that archaic nerve stimulator… By stimulating the catheter as it is advanced into position and maintaining an appropriate muscle twitch until the catheter is in its final position, we suddenly are able to confirm that the catheter has been successfully placed into the correct tissue plane – the subcircumneural space. If it were outside this tissue space, it would not track with the nerve and the twitch would not be maintained – the business end of the catheter would track wherever the sub-epimyseal space goes, usually away from the nerve. This is the key to eliminating secondary block failure and the ire of surgeons frustrated by peripheral nerve catheters of unpredictable utility; continuous blocks that work great on the day of surgery but fail the next day and days.
I know I can’t force or convince anyone reading this piece to start using nerve stimulation to guide the placement of all their future peripheral nerve catheters, but I can promise that if you do, 2 things will happen: 1) you will experience some wonderful frustration early on trying to figure out what sort of voodoo you need to channel to get the stupid catheter to tread and maintain the twitch (but this will get better with time and experience), and 2) it won’t be long before people are asking why your continuous nerve blocks almost never fail and work just great for days and even weeks after placement. This is a small investment in skill you will need to make to achieve great results.
Until the Ultrasound Technology has developed far enough so we can see all the perineural membranes and spaces, especially the circumneurium, I am happy to stick with this archaic technology that WORKS and GIVES ME CONSISTENT RELIABLE GREAT RESULTS.
Please visit the Must-know Anatomy section under ANATOMY and have a look at the Micro-anatomy of peripheral nerves.
This is also very well addressed under the High-yield Block Section.
To perhaps understand this better, it may be advisable to study this drawing by Mary K. Bryson:
Schematic drawing of the sciatic nerve. The axons (1) are myelinated or unmyelinated and in the endoneurium (2). The axons and endoneurium are surrounded by the perineurium (3) to form a fascicle (4). A bundle of fascicles is, in turn, surrounded by an epineurium (6), which forms a nerve – the tibial nerve (7) and the common peroneal nerve (8) in this case. Both of these nerves are bundled together by a circumneurium (10) (formerly called the paraneurium) to form the sciatic nerve (11) approaching the popliteal fossa. The fascia in which the nerves, arteries, and muscles are housed is the epimysium (13). Each of these layers has a compartment deep to it: the subepimyseal (12), subcircumneural (9), subepineural (5), and subperineural (2)compartments. The latter is referred to as intrafascicular, while the subepineural space is interfascicular (or intraneural). All the compartments except the intrafascicular (sub-perineural) contain adipose tissue. (With permission from Mary K. Bryson).