Muscle Relaxants And Neurophysiological Monitoring

So you want to tell anesthesia they can’t use muscle relaxant on this case? You’re bound to get a wide ranges of responses, from acknowledgement, dirty looks and maybe even a full blown temper tantrum. It’s helpful to go in with a little understanding of how muscle relaxants affect our monitoring to clearly state your reasoning.

The drug of choice around my area is rocuronium. It’s a non-depolarizing muscle relaxant that doesn’t take all day to wear off, and doesn’t take too long to take effect. Intubation is smooth, patient positioning is smooth, they can keep the patient from moving during surgery, they’re good at timing their wake up to give a little neuro exam shortly after and the patient doesn’t wake up with muscle soreness (or run  risk of malignant hyperthermia or hyperkalemia like what can be seen with succinylcholine). Thumbs up all around.

Muscle relaxant side effects

But there are some cases where you’ll need full twitches (ok, some will argue 2 or 3 twitches should suffice) right away. This might be in a tcMEP case, thyroidectomy with NIMS tube neuromonitoring, minimally invasive, peripheral nerve cases, etc. Our options then become a small dose of non-depolarizing, depolarizing (succinylcholine) or nothing at all.

Planning The Use Of Pre-induction Muscle Relaxant With Anesthesia

First off, we need to be realistic as to when we actually need to test what we’re testing. If we’re taking tcMEP baselines in a stabile patient (not worried about positional problems), we might be able to go 30 minutes before we need to set our baselines (with all the setup and prep being done). If we’re doing prepositional tcMEP for a trauma case going prone, then we’re going to need full reversal about 10-15 minutes after induction (depending on how many channels you’re running and how much you can get done before the nurse tries to put in the foley). If you’re monitoring with a NIMS tube, then you’ll want to test muscle activity in less than 5 minutes of intubation.

Next, we need to compare those realistic times with the duration of medications. Rocuronium (also called zemuron) has a significant does response effect on recovery time. A recent paper (A comparison between succinylcholine and rocuronium on the recovery profile of the laryngeal muscles during intraoperative neuromonitoring of the recurrent laryngeal nerve: A prospective porcine model) showed that a low dose (0.3 mg/kg) of rocuronium recovered 80% in 15 minutes, with full recovery in 30 minutes of the laryngeal muscles. A 0.6 mg/kg dose of rocuronium reached 80% recovery around 30 minutes, but took greater than 1 hour to recover 100%.

What’s surprising to me was their findings comparing low dose rocuronium and 1.0 mg/kg of succinylcholine. The low dose rocuronium was around 5 minutes faster to reach 80%recover, and was at 100% recovery, it was about the same time (30 minutes). I typically associate succinylcholine as the necessary evil to get up-front baselines ASAP.

Low dose roc was faster to 80% baseline, same at 100% as succ

Wouldn’t it be nice to not have to restrict anesthesia to using succinylcholine for anything? But I’m not sure that’s how things are. A low dose is fine for baseline tcMEP before incision, probably not a great idea if you’re getting pre-positional baselines (unless everyone is aware it might take a little longer to wear off and are willing to wait it out), and most likely not a good option during thyroidectomies using a NIMS tube (since you want to get impedances checked and muscle activity recorded before they tape the tube). But at least now you have some numbers to give them as to what qualifies as a “small dose of roc up front.”

If they absolutely can’t intubate without muscle relaxants or using succinylcholine, then maybe the tube shouldn’t be used, but rather needle insertion into laryngeal muscles.

Please Note

*** The article used for the clinical numbers should not be taken as gospel. This was done in a porcine model, and was monitoring muscles that are well known as some of the first to recover from muscle relaxants. Neurophysiological monitoring of transcranial motor evoked potentials in an adult abductor hallucis brevis may not necessarily correlate with a pig’s voice box.***

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