Explore what the ASA guidelines actually say about neuromuscular monitoring—and whether your practice is aligned with today’s standards of care.
THE PATIENT IS WAKING UP? (NOT)
Every anesthesia professional has heard this remark from the surgical team on more than one occasion. While light anesthesia and the potential for the patient to experience awareness is always a concern, most often, the patient is not actually “waking up” or experiencing awareness of the surgical procedure. The observation that prompts this comment is typically patient movement in response to a change in the degree of painful stimulation. Patients are usually receiving a more than adequate dose of an intravenous or inhaled hypnotic to prevent awareness. So why do they move? There are a couple of explanations.
One explanation is an inadequate dose of analgesic medications. The dose of hypnotic can be reduced substantially if analgesic medications are given as well without increasing the risk of movement. Analgesic medications can be local anesthetics used to block the sensation from the surgical site, potent narcotics like fentanyl or other classes of medications like ketamine. Virtually all general anesthetics administered during painful procedures include a combination of hypnotic and analgesic medications.
Bolus Dosing of Relaxants Can Result in Wide Variations of Relaxant Effect
Another explanation for patient movement is the management of muscle relaxant dosing, especially by a bolus technique. Muscle relaxants are used during general anesthesia to facilitate tracheal intubation (placing a breathing tube into the trachea after the patient is asleep), to keep the patient immobile during the procedure without needing excessively deep anesthesia and to make it easier to do the surgery by eliminating muscle tone. The bolus technique is most commonly used, consisting of an initial dose given prior to intubation and repeated doses to maintain muscle relaxation, ideally guided by nerve stimulation to assess the degree of relaxation and need for repeated dosing. Not uncommonly, especially if there is a prolonged period of time between induction of anesthesia and the start of surgery, the relaxant effect will diminish enough to allow the patient to move in response to skin incision. Intermittent qualitative nerve monitoring requires an active intervention by the anesthesia professional to assess the degree of blockade and decide on further relaxant administration. In the busy period between induction and start of surgery, it is easy to get distracted and fail to assess the need for additional muscle relaxant before incision. This sets up the potential for patient movement when surgical stimulation starts if the relaxant effect has diminished.
This bolus dose scenario also sets the stage for overdosage of muscle relaxant since the knee jerk response to patient movement can be to quickly administer additional muscle relaxant. In the haste to eliminate the patient movement, too much relaxant can be administered leading to a prolonged effect. If the procedure ends too soon after the additional dose was given, reversal of the muscle relaxant effect has traditionally been a problem. With the introduction of sugammadex, reversal can be accomplished for most patients from any level of block if rocuronium or vecuronium are being used as muscle relaxants. If Atracurium is being used, reversal remains a problem.
Another challenge to providing optimal muscle relaxant dosing is the variability in response between patients to a given dose. All medications have guidelines for dosing based upon studies of populations of patients. Dosage recommendations are typically based upon the ED 50 or the dose required to obtain a desired effect in half the patients studied. In the case of muscle relaxants, the ED95 is a term used to describe the dose required to achieve a 95% reduction in twitch height for half the patients. It is a bit confusing but basically the ED95 dose recommendation for a relaxant is the dose at which half of the population studied (ED50) will have a 95% reduction in twitch height. Recommended induction doses to facilitate intubation are typically multiples of the ED95 to ensure that most, not just half of the patients will have the desired effect with a rapid onset. (Table 1). 
From; Nash M, Strupp K. Neuromuscular Blockade: Miscellaneous Topics. Open Anesthesia. Last Updated. 11/10/2023
Finally, the effect of a muscle relaxant on a given patient is influenced by other factors other than the dose administered. The age of the patient, other medications being used like inhalation anesthetics and underlying diseases will all influence the result obtained by a given dose both in terms of degree of relaxation achieved and the duration of effect.
Throughout the procedure, the anesthesia professional needs to make repeated assessments of muscle response to nerve stimulation to maintain a stable degree of muscle relaxation consistent with the needs of the procedure. It is therefore quite possible for the anesthesia professional to miss a timely assessment of muscle function and fail to appreciate the need for additional muscle relaxant. As a result, variation in the degree of muscle relaxation is common.
The bottom line is that bolus dosing of muscle relaxants can be associated with inadequate muscle relaxation manifesting as undesired patient movement or excessive relaxation leading to prolonged effect. Careful attention to intermittent monitoring of neuromuscular function can minimize the swings in degree of relaxation at the cost of additional attention by the anesthesia professional.
The Importance of Neuromuscular Blockade Monitoring
Due to the variability of depth and duration of muscle relaxant effect after a bolus dose, some means to assess neuromuscular function is essential to be able to assess the effect on an individual patient after a bolus dosage. Intermittent qualitative monitoring using peripheral nerve stimulation is a useful tool but requires attention by the anesthesia professional to perform an assessment and make a subjective judgement of the degree of relaxation. With careful attention, monitoring can provide insight into the effect of a given dosage on an individual patient and help to guide subsequent management. As we have noted previously, the attention required to make this intermittent assessment can lead to gaps in relaxant administration. In addition, the subjective assessment of relaxant effect has been shown to be both variable and inaccurate, and can lead to residual paralysis and post-operative complications if the patient is inadequately reversed.
Continuous quantitative neuromuscular blockade monitoring is an advance that substantially reduces, but does not eliminate, the burden on the clinician of intermittent monitoring. Devices that automate the process of nerve stimulation and response measurement provide convenient regular updates on the degree of muscle relaxation. Importantly, it is still incumbent on the anesthesia professional to pay attention to the measurement, assess the measurement relative to the time and dosage of relaxant that has been given and make a decision on whether or not administering additional relaxant is indicated.
Is there a role for Relaxant Infusions?
The goal of administering muscle relaxants by continuous infusion is to reduce variability in the degree of relaxation. A bolus or loading dose is given to achieve the desired effect rapidly, followed by an infusion to maintain a stable degree of relaxation. (Table 2) 
Table 2: Typical dosage schemes for starting and maintaining muscle relaxation with different agents. Note the recommended frequency of dosing for maintenance of neuromuscular blockade. In clinical use, the exact dosages should be adjusted based upon neuromuscular function monitoring to achieve an optimal effect for an individual patient.
From: Clar DT, Liu M. Nondepolarizing Neuromuscular Blockers. [Updated 2024 Oct 18]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK534828/
A typical approach to a relaxant infusion is to give the loading dose and monitor neuromuscular function until a nerve stimulation elicits a response, at which time the infusion is started. Depending upon the desired degree of relaxation, the desired response could be just a post-tetanic response to assure deep relaxation, or a single twitch after train-of-four stimulation if less relaxation is needed. Infusion rates needed to maintain stable relaxation would be similar to the recommended maintenance dosing for each drug given in Table 2. One would expect to adjust the infusion rate as guided by nerve stimulation, especially early after starting the infusion, until a constant stable rate of drug administration and relaxation is achieved. While the infusion technique is fairly commonly used in the operating room, it is not routine, nor is there a lot of recent literature on continuous infusions of relaxants. A quote from a 1987 paper on relaxant infusions describes the potential advantages:
A variable rate continuous infusion of short-acting competitive neuromuscular blocking agents with monitoring of the neuromuscular blockade is the most flexible and accurate method of maintaining a precise degree of neuromuscular blockade during prolonged surgery. -GH Beemer. Continuous Infusions of Muscle Relaxants – Why and How. Anaesth Intensiv Care. 1987;15:83.
If we define optimal muscle relaxation as a relatively rapid onset of deep relaxation for intubation followed by stable degree of relaxation during the procedure with readiness for reversal at the proper time, infusions become a convenient clinical tool.
So why is it that relaxant infusions are not used more commonly? Many procedures only require a single dose of muscle relaxant either because relaxation is not required beyond intubation, or because the duration of effect after a single dose is sufficient for the procedure. In longer procedures where repeated dosing of relaxants is required, infusions are arguably superior to bolus dosing. Infusions do require an infusion pump and disposables to connect the syringe of relaxant to the patient. The equipment and supplies add cost and an environmental impact. Attention to monitoring neuromuscular function is still required, with manual adjustment of the infusion based upon the patient’s response. Nevertheless, with practice, infusions become a convenient tool to maintain stable, optimal relaxation over a long period of time.
There are some patient safety considerations whenever a muscle relaxant is administered. Unintended administration of relaxant is a known medication error and programming errors for infusion pumps are unfortunately not uncommon. Safety precautions are needed to avoid unintended patient harm from muscle relaxant drugs especially once infusions are prepared at the bedside.
Is there a Future for Muscle Relaxant Infusions?
Given the frequent need for long periods of muscle relaxation during surgical procedures, there is an opportunity to improve patient care by using infusions more commonly than they are used today. Continuous automated quantitative neuromuscular blockade monitoring is an enabling technology given the frequency, convenience and quality of information about the degree of relaxation that is provided. In the current state, the bedside clinician still has a burden to observe the monitor display, connect the trend of relaxation to the dosages given, either by bolus or infusion, and make manual adjustments to the pump. However, using muscle relaxant infusions combined with continuous automated quantitative neuromuscular blockade monitoring, undesired fluctuations in relaxant effect are less likely to occur since both neuromuscular blocking and reversal agents can be more easily titrated to individual patient needs.
CASE STUDY OF RELAXANT INFUSION TECHNIQUE
The patient is a 55 yo 75 kg male scheduled to undergo a craniotomy for clipping of an intracranial aneurysm. The expected duration of the procedure is a minimum of four hours. The patient will be positioned supine with fixation of the skull to the OR table using a Mayfield device. Intravenous induction of general anesthesia is planned using Propofol and Fentanyl and vecuronium for muscle relaxation. Given the importance of stable muscle relaxation to ensure complete immobility, a vecuronium infusion is planned with train of four monitoring.
Infusion Dosage Scheme – Vecuronium
Maintenance Goal: TOF Count = 1
Bolus Dosage: 7.5 mgms (0.1 mg/kg)
Initial Infusion Rate: 0.02 mg/kg/hr when TOF Count = 1
Adjust infusion as needed to maintain TOF count = 1
Turn off infusion when Mayfield fixation is removed.
Reversal with Sugammadex 3-4 mg/kg
Confirm reversal and readiness for extubation when TOF Ratio > 90%
Learn how quantitative EMG monitoring outperforms subjective twitch assessments in reducing residual paralysis and improving patient outcomes.
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