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In this 147th episode I welcome back Drs. Segna and Rayaz to finish up our discussion of lower extremity nerve blocks. This time we cover the front of leg blocks (femoral, adductor canal, fascia iliaca and obturator).
CME: https://earnc.me/AyfE0B
I do have a question on LA-Dosing:
Does each maximum dose of an LA stand for itself or do I need to reduce the dosages in case of a combination of LAs. In the case of the latter, how would you recommend it?
Great question. Here is Dr. Rayaz’s response:
You calculate the number of milligrams for both local anesthetics and then use the max dose that is the lower of the two local anesthetics. For example, if you give 100mg plain lidocaine without epinephrine (max dose of 4.5-5mg/kg depending on the source) and then intend on giving bupivacaine (max dose of 2.5mg/kg) for a 100kg patient, then you would do the following calculation:
Use the max dose for bupivacaine since it is the lower max dose of the two local anesthetics being used.
2.5mg/kg x 100kg = 250mg
250mg – 100mg = 150mg
That leaves 150mg of bupivacaine that can be used or about 60cc of 0.25% bupivacaine (150mg/2.5mg/kg) left over for a tap block.
For a 70kg patient:
2.5mg/kg x 70kg = 175mg
175mg – 100mg = 75mg
That leaves 75mg of bupivacaine for a tap block or 30cc of 0.25% bupivacaine (75mg/2.5mg/kg) or 15cc of 0.5% bupivacaine (75mg/5mg/kg)
It’s a bit more simple in practice than the math problems make it seem. In general, combining local anesthetics should be done with caution. When used as a mixture for the same nerve block, combining local anesthetics can result in unpredictable onset and duration times. Combining local anesthetics can also limit your overall max dose as you default to the lower max dose of the two combined local anesthetics. I understand that people will say, “Wait a second! These molecules are not the same and you’re treating them like they’re the same in your equations!” and they’re absolutely right. Max doses and our local anesthetic calculations are estimates to keep us away from toxic plasma levels of local anesthetics. You can actually measure plasma levels of local anesthetics (and that’s one of the labs I get in the case of local anesthetic toxicity), but these are send out labs and take a while to result. I’ll get a local anesthetic plasma level during a case of local anesthetic toxicity to help confirm the diagnosis, not to guide therapy. Until we get more accurate and readily available local anesthetic plasma levels, we’ll have to resort to our estimates.
Great talk! I have heard widely differing opinion (and subsequently seen widely differing practice) among my colleagues in using femoral nerve and/or fascia iliaca blocks for total hip arthroplasty and/or femoral head pinning. Perhaps Dr. Segna or Dr. Rayaz can share their opinion on the utility and efficacy for these blocks for the particular procedures?
Great point!
Great question. Here are Dr. Rayaz’s thoughts:
The implicit question within the femoral vs fascia iliaca for hip fracture question is how do we best utilize regional anesthesia to help our hip fracture patients? Let’s peel back the layers:
1. Cover the fracture site with your block. It seems simple, but that’s the biggest factor in answering the femoral vs. fascia iliaca question. They both cover the femoral nerve distribution and thus will have an impact on a significant portion of the patient’s pain. How these blocks are used will impact which one is better for your institution.
2. Regional block is non-inferior and/or superior to IV opioids in treating hip fracture pain when timed properly. A nerve block will not help postoperative pain better than opioids when placed immediately prior to the hip arthroplasty (the exception to this may be the femoral nerve block catheter). When a block is placed in the ED or on the floor prior to surgery, that’s when we get better analgesia and decreased opioid consumption from our blocks (blocks provide better pain control with leg movement and non-inferior pain control at rest when compared to systemic opioids). Furthermore, a block done in the ED may actually help prevent postoperative delirium due to decreased opioid consumption in the geriatric population that often suffers from hip fractures.
3. Ultrasound seems to matter. Studies examining landmark techniques for these blocks are equivocal for improved analgesia. Studies examining ultrasound guided blocks, while fewer in number, show more promise.
In summary, if you have two anesthesiologists arguing over which block should I do for my patient immediately prior to surgery for postoperative pain control, I’d say they’re both barking up the wrong tree (unless, based on one European study, they use a femoral catheter). Either regional anesthesia technique will work as long as they cover the femoral distribution. The real discussion is how you develop the infrastructure to get people skilled in ultrasound guided blocks down to the ED or the floor to catch hip fracture patients early enough to make a significant difference.
The top end study I’d like to try in an ideal world is to see if we can do a femoral nerve catheter done in the ED and measure opioid consumption through the entire perioperative study. There are lots of logistical challenges in making that happen, but it could be a beautiful thing for our patients. Why not a fascia iliaca catheter? Less local anesthetic is necessary for a femoral nerve block to be effective and so that works better than a high volume fascia iliaca catheter where you may be at greater risk of hitting the toxic dose ceiling.
Those are my two cents. By no means is this definitive. Great question.