Notes on Local anesthetics
/Selecting the right local anesthetic is a bit of art form. There are many different cocktails is use, each with their own ardent supporters. The science is hard to interpret given the many factors that go into creating a successful block. Putting it all together this is my ideal line up for local anesthetics in emergency practice:
The Ultra-short & ultra-safe procedural block:
3% 2-Chloroprocaine
3% 2-Chloroprocaine for ultra-short blocks in the 60-90 minute range. This is perfect for reductions or procedures where you would like a brief block. As an ester rapidly metabolized in the blood ( <60 second half-life), the risk of toxicity is very low. This low toxicity allows safe use of a high concentration ( 3%) that likely contributes to the very fast onset of the block. Example would be an intersalene block for shoulder reduction
Intermediate blocks. Colles' fracture reduction
1.5% Mepivacaine
Perfect for intermediate blocks when a several hour window of surgical level anesthesia is needed. Fast onset of 2-3 hours of dense surgical anesthesia. Consultants can be unexpectedly delayed, it is very very disappointing to have a block wear off just as the procedure is starting. Mepivacaine gives dense block and a nice window for this scenario. Mepivacaine has a strong following as a very consistent, low toxicity, amide local anesthetic. Mepivacaine does not have a strong vasodilatory affect and is typically used without epinephrine. Example would be an infraclavicular block for a distal radius fracture reduction and splinting.
The long block. Hip fractures etc...
0.5% Ropivacaine
Ropivacaine is structurally similar to bupivacaine with reduce cns and cardiovascular toxicity and some degree of preferential sensory blockade. It remains a powerful local anesthetic that should be used with caution, an understanding of the recognition and treatment of LAST with intralipid at hand. We use 40ml of 0.5% ropivacaine for hip fracture analgesia.
What about lido and bupiv?
1% lidocaine is commonly stocked in the ED. At 1% this is probably the weakest local anesthetic you could use with acceptable clinical result. 1.5%- 2% is the more common dosage for lidocaine. Lidocaine has strong vasodilatory properties and is commonly used with epinephrine.
Bupiv is the strongest, most potentially toxic local anesthetic in use. Full LAST precautions are required. Blocks can be quite dense and of long duration > 10 hrs.
What about epi?
There is no consensus at this point, but animal study and in vitro evidence of neurotoxicity has sufficiently raised concerns that most are moving away from epi when possible.
What about dexamethasone?
To early to say. Has the potential to dramatically lengthen block duration. Unclear if dexamethasone needs to be injected perineurally or can be given IV with the block.
KNOW HOW TO RECOGNIZE AND TREAT LAST
Further reading on the web about local anesthetics