Choosing the right anesthetic for a procedure is critical to performing a successful nerve block. In this video from our Ultrasound-Guided Nerve Block Masterclass, you'll learn how to select an anesthetic based on the procedure you're about to do, the critical factors affecting dosage selection, and why you only really need to know about two anesthetics to successfully perform blocks.
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[00:00:00] What do I need to know about the wide, wide world of local anesthetics? There are countless types. I would suggest asking your pharmacy which two or three are most available in your clinical practice. Become familiar with these, forget all the rest. The most common anesthetics that we use are
[00:00:30] lidocaine, ropivacaine, and bupivacaine. Ropivacaine likely is the safest in regards to cardiotoxicity, however, the other two are plenty safe as well, especially when using ultrasound to confirm anesthetic delivery to the correct location and not intravascularly. As for dosages, these are weight based and unique to each anesthetic. For instance, the maximum dose of bupivacaine, which is longer acting and readily
[00:01:00] available for me has a dosing limit of 2 mg / kg. This amount is far, far more than you will need for the majority of nerve blocks. These dosages are easy to look up once you determine which anesthetics are readily available at your institution. Singular isolated nerve blocks require less anesthetic. Usually 5 to 10 cc or mL of fluid. I like to think of the length of the nerve
[00:01:30] you plan to block as a surrogate for how much anesthetic you need. For instance, if you are blocking the nerve in the arm, you won't need as much anesthetic as you would if you were blocking a nerve in the groin and you expect it to have to travel all the way down the leg. Compartment or plane blocks require more. Usually around the order of 20 to 300 cc or mL of fluid. It is with these larger volume blocks where you should be conscious of the
[00:02:00] maximum dosing for the particular agent you choose. For these blocks, you will rely upon the diffusion of the agent along the fascial planes, not hydrodissection. I will often dilute these agents with 10 cc or mL of saline to promote this diffusion. You want adequate diffusion so that the anesthetic spreads all along the fascial plane and captures the entirety of the compartment you wish to block. The onset of time of
[00:02:30] action also varies between agents. Lidocaine is the shortest onset usually in the order of minutes. Bupivacaine and ropivacaine take a little bit longer to take effect. Adding adrenaline or epinephrine to your agent will hasten the onset of a nerve block. Some data suggest that it may prolong the duration of the block as well. It reduces local blood flow and thereby enhances nerve uptake and slows the clearance of lidocaine from the nerve.
[00:03:00] Like onset of action, duration of action varies with the agent. Anesthetics can be either short, in order of 1 to 3 hours or longer-acting, 4 to even 18 hours. Lidocaine is a short-acting agent, and ropivacaine and bupivacaine are long-acting agents. The major complication with anesthetic choice, which we've already briefly eluded to you, is to understand at which point you may be reaching toxic levels. For instance,
[00:03:30] bupivacaine provides the longest duration of the commonly used local anesthetics but also has the worst cardiotoxic profile. However, if a reduction in the dose of bupivacaine is used, which is seen more frequently now with ultrasound guidance at our disposal, in equivalent dose or even a greater safety profile is achieved when compared to a larger dose of the safer ropivacaine, the drug to which it is often compared. The advent of ultrasonography
[00:04:00] has allowed for a dramatic reduction in the volumes of local anesthetics necessary to achieve many common nerve blocks. Every procedure worth its salt [00:04:09] or every procedure that has the ability to provide substantial patient benefit has risks tied to them. Nerve blocks are not without risk. Having a basic understanding of the anesthetic you chose will help you to avoid using too much or using the wrong one for a particular procedure.
[00:04:30] I recommend only knowing two agents, one short-acting and one long-acting. This will be much easier for you and safer for your patient.