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Teaching points: Defasciculating Doses

Discussion in 'Anesthesiology' started by LostTommyGuns, Sep 18, 2006.

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  1. LostTommyGuns

    LostTommyGuns Saucy Gas Monkey

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    A friend mentioned the use of a defasciculating dose of a nondepolarizing muscle relaxant to eliminate the fasciculations that come with using Succinylcholine (in this case they wanted no fascic. to prevent transient rises in ICP for a neuro case). After discussing it I decided that I didn't understand the mechanism for how this would work.

    I can see how depolarizers work: mimic ACh to cause the endplate to generate an action potential, but whereas ACh releases the receptor, Sux. holds on and maintains end-plate depolarization. Which means the muscle contracts then becomes flaccid... So... if you give a "defasciculating dose" of Roc or Vec or whatever, it will competitively inhibit sux from binding, which means that you'll have to an increased dose of Sux... Which seems to leave you in the same situation - overriding ACh (and now Roc as well) in order to create an endplate depolarization... and fasciculations?

    Clearly I'm missing something. The only thing I can think of is that by using a nondepolarizing muscle relaxant first you paralyze some muscles fibers through nondepolarization, then others through depolarization. Since you give a larger dose of Sux, it ultimately outcompetes the Roc, but over a longer time which creates smaller/less fasciculations? :confused:
  2. Induc(junc)tion

    Induc(junc)tion Member

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    Just a fourth year medical student here with a couple of rotations under my belt, but I'll chime in. Anyways, I had an attending explain it to me like this. For a defasciculating dose, you would give about 1/10 of the intubating dose of a nondepolarizer. So say for vec, instead of 0.1 mg/kg, you would give 0.01 mg/kg right before the sux. The sux dose would remain the same. The vec would block enough receptors at the motor endplate to prevent fasciculations, but not enough to prevent the sux from doing its job.
  3. Amik25

    Amik25 Junior Member

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    According to most anesthesia textbooks, you do not give the same dose of Sux if you pre-treat with a nondepolarizer. You need to increase the dose of Sux by about 50% (In Miller, although I think in Baby Miller it says 70%).
  4. TIVA

    TIVA Member

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    Myths, my friends, they're all myths.

    Succinycholine does not increase ICP. It's purely theoretical. Studies in primates and in humans show no clinically significant increase in ICP from use of sux or the fasciculations from sux.

    What increases ICP? Laryngoscopy.

    So if you want to intubate a patient with head injury or with increased ICP and you don't want to raise they're ICP during intubation, they have to be adequately (read: deeply) anesthetized. For this, narcotics are the best, particularly the short-acting synthetic variants like fentanyl or remifentanil.

    Now, sux does cause a transient rise in intra-ocular pressure, but is it clinically significant? That's a different question. Whatever rise there is in IOP is extremely short-lived (normal IOP is about 5-8 mmHg, and sux will increase it for a second or two to about 12-15 mmHg). Thus, the quandary with full stomach/open eye.

    Similarly, sux also causes a transient rise in gastric pressure, which could theoretically increase the risk of aspiration. But will it lead directly to aspiration? Doubtful. I'd like to say no but I'm sure someone out there will swear they saw a case of aspiration secondary to sux, even in the face of inadequate anesthetic depth prior to intubation, trying to intubate before complete relaxtion, or someone who would've regurgitated anyway (i.e. foreign body esophagus, upper GI dysmotility, etc.)

    So where did this concept of a defasciculating dose arise from? It rose from wanting to prevent the post-operative myalgias which sux can induce. And these sux-related post-operative myalgias are in fact very real and very bothersome. In fact, for some patients, they can last a good several weeks if not months. Often they will report that the worst thing about they're perioperative experience was the long persistence of muscle aches/pains. One anesthesia attending here was so debilitated by it that she had to take time off to recover; thus she almost never uses sux now.

    This leads us to the next question? Does the defasciculating dose work? Does it prevent sux-induced muscle pain? Studies indicate that there is no statistical or clinical benefit. But then again, most people who are giving sux are probably giving it for purposes of rapid sequence intubation. And thus, these same people (myself included cuz I know I'm guilty of it as well), are probably not giving the "defasciculating dose" a full 3 minutes to work. Instead, we give our 1 mg of vec, wait 20-30 seconds, and then follow it with propofol and sux. And then we wonder why we still see fasciculations.

    So if I could pass along just one take home point:

    Sux doesn't increase ICP, laryngoscopy does. Therefore, make sure your patient is nicely anesthetized before manipulating that airway.
  5. The_Sensei

    The_Sensei Removed

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    No. The only thing proven to decrease the incidence of sux induced post operative myalgias is lidocaine 1mg/kg IV prior to induction. Has to do with blocking the Na channels.
  6. pgg

    pgg Laugh at me, will they? Moderator

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    Once upon a time a big fat dude stepped on my arm and gave me an awful comminuted fracture.

    The whole-body post-op myalgia I endured for the first 48 hours s/p my ORIF were worse than the pain of feeling my radius and ulna crunched. I had the surgery a week after the injury - a week during which I went to class, and functioned more or less normally. The week after my surgery I was utterly incapacitated.

    This was in my undergrad days, long before I'd ever heard of succinylcholine, so I thought maybe I'd arrested on the table and they'd done CPR by banging on my chest with a sledgehammer, with an ex-NFL-field-goal-kicker kicking my arms and legs for good measure. At my first postop appointment, the orthopod told me the myalgias were most likely from the muscle relaxant. Didn't really clue in on how that explained my pain until I saw a patient fasciculating for the first time.

    Interesting ... and surprising. This forum teaches me something every day.
  7. johankriek

    johankriek Removed

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    and its ladies deconditioned who get it mostly.... not young muscular guys so thats an argument against using succinylcholine... i would suppose.....
  8. Laryngospasm

    Laryngospasm Trench Dog

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    Waste O' time.:)
  9. bubalus

    bubalus Member

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    1: Anesthesiology. 2005 Oct;103(4):877-84. Links

    Prevention of succinylcholine-induced fasciculation and myalgia: a meta-analysis of randomized trials.Schreiber JU, Lysakowski C, Fuchs-Buder T, Tramer MR.
    Department of Anesthesiology and Critical Care Medicine, University Hospital of the Saarland, Homburg, Germany. [email protected]

    Fifty-two randomized trials (5,318 patients) were included in this meta-analysis. In controls, the incidence of fasciculation was 95%, and the incidence of myalgia at 24 h was 50%. Nondepolarizing muscle relaxants, lidocaine, or magnesium prevented fasciculation (number needed to treat, 1.2-2.5). Best prevention of myalgia was with nonsteroidal antiinflammatory drugs (number needed to treat, 2.5) and with rocuronium or lidocaine (number needed to treat, 3). There was a dose-dependent risk of blurred vision, diplopia, voice disorders, and difficulty in breathing and swallowing (number needed to harm, < 3.5) with muscle relaxants. There was evidence of less myalgia with 1.5 mg/kg succinylcholine (compared with 1 mg/kg). Opioids had no impact. Succinylcholine-induced fasciculation may best be prevented with muscle relaxants, lidocaine, or magnesium. Myalgia may best be prevented with muscle relaxants, lidocaine, or nonsteroidal antiinflammatory drugs. The risk of potentially serious adverse events with muscle relaxants is not negligible. Data that allow for a risk-benefit assessment are lacking for other drugs.

    It was on the written boards this year.

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