For this month of December we will continue with our theme of micro-anatomy of nerves but emphasize the very related topic of why continuous peripheral nerve blocks (CPNB) fail. Secondary block failure is a very prevalent issue and, IMHO, one of the main reasons why CPNB’s have been decreasing in popularity. The secondary block is when the primary block, where a larger volume of a high concentration local anesthetic (LA), had worn off and the low volume of a low concentration LA for the block takes over. This is where the Fick Equation kicks in as the drug can only reach the axons if the concentration gradient is high enough and the barriers between the drug and the axons are penetrable enough. The low concentration and volume of the LA for the secondary block – typically ropivacaine 0.2% at 3 – 5 mL/h (why are we still using a long-acting drug for CPNB’s and not lidocaine, by the way??) is too low, the drug is continually removed by the blood and lymphatic flow, and there are too many barriers between the drug and the axons the day after the CPNB had been placed, and the block fails.
In institutions where they do not measure the 2º block failure this failure of course “never happens”. Where it is honestly measured, however, there is a 40 – 60% incidence of 2º block failure. The reason is simply that the catheter is placed in the sub-epimyseal space and not in the sub-circumneural space. (The members who followed us last month will know exactly what this means, but it is not too late for the others. All the information is available on the website). Unfortunately we can only see the circumneurium with high-definition ultrasound and not with the fanciest of our garden variety regular ultrasound machines – and most, if not all of us, do not have access to high-definition ultrasound.
The Trending Presentation this month is the brilliant talk by Associate Editor-in-Chief Mike Kent on “Why Continuous Nerve Blocks fail”. I highly encourage everyone to listen to this fascinating talk and I encourage you to participate in the discussions. If you follow me (#aboezaart1) or any of the other regular Twitters on the Regional Anesthesiology sites on Twitter, you will be able to participate in this and many other discussions. You can of course also comment of this site here and I will post your comments on the Twitter group.
Click HERE to access Mikes talk. Next month we will focus on why (and how) we do thoracis epidural blocks. If you’d like to understand why up to 24% of thoracic epidurals fail, please study the Space of Okada in the meantime. We will feature it specifically early next year.
André P Boezaart MD, PhD Editor-in-Chief