Digital Surgery & American Society of Anesthesiologists
An Interactive App Experience
Digital Surgery and the American Society of Anesthesiologists are excited to provide all ASA members with access to an interactive simulation module on the Touch Surgery app for learning about the performance of Train of Four for monitoring neuromuscular blockade and reversal.
If you have already created an account and been granted access using your ASA email, all you need to do is click on the below link from your mobile browser, download the Touch Surgery app (if you haven’t yet) and start practicing.
If you don’t have an account yet, please get in touch on firstname.lastname@example.org to get you started!
Neuromuscular blocking agents given during anesthesia and surgery facilitate endotracheal intubation, surgical relaxation and, in some cases, immobility at critical points during surgery. The key to safe management of these drugs is to avoid intraoperative over dosage, by monitoring the responses to incremental redosing, and to ensure that the administration of reversal agents at the end of surgery successfully accomplishes the goal of full reversal. Residual paralysis in the postoperative period can result in adverse clinical outcomes, such as; complete respiratory failure requiring emergent reintubation; partial airway obstruction and hypoxemia requiring a jaw thrust or oral or nasal airway insertion; recognized or unrecognized aspiration or microaspiration; patient discomfort; and post-PACU atelectasis and pneumonia. It is, therefore, imperative that anesthesia providers understand the need for close perioperative neuromuscular monitoring.
At the present time, most clinicians rely on the qualitative (subjective) assessment of the response to peripheral nerve stimulation – by observing the number of twitches seen after a ‘Train-of-Four’ (four electrical pulses delivered at a rate of 2 Hz to the ulnar or posterior tibial nerve), and occasionally examining the ‘post-tetanic count’ (PTC) if there is no response to a TOF, or by attempting to subjectively determine whether ‘fade’ is present in the TOF. While the use of a twitch count and carefully performed PTC may be sufficient to aid in the intraoperative dosing of neuromuscular blocking drugs, qualitative assessment of fade cannot rule out the presence of residual paralysis. Moreover, supplementing such qualitative assessments with clinical assessments of paralysis (e.g. grip strength, head lift, etc.) borders on worthless. Given the well-known limitations of such approaches, it should not be surprising that the incidence of residual paralysis in the PACU is 30-60%.
The only means by which such problems can be solved is via the continuous intraoperative use of quantitative neuromuscular blockade monitoring. Quantitative monitoring allows the provider to directly measure the magnitude of the response to peripheral nerve stimulation, for example, the ‘TOF ratio’ (the ratio of the amplitudes of the 4th twitch to the 1st twitch of a TOF). At present, three quantitative assessment systems are available: accelerometry, kinetomyography and electromyography. Each has its pros and cons – but all represent a major improvement over qualitative assessment.
Indications / Contraindications to Quantitative Monitoring
Monitoring is indicated for all patients receiving nondepolarizing neuromuscular blockade (NMB) for any purpose. Some clinicians also advocate the use of such monitoring in patients receiving depolarizing agents (succinylcholine).
There are no meaningful contraindications to monitoring – although there may be situations in which monitoring may be difficult or impossible. For example, extensive burns or skin injuries around the wrists or ankles may preclude electrode placement (although needle electrodes have been used). Tucking a patient’s arms may preclude the use of accelerometry or kinetomyography (but not electromyography) during the procedure – but does not prevent their use to assess reversal after the arms are untucked. TOF monitoring can be performed in awake patients (e.g. to assess reversal in the PACU) with few difficulties, given the patient is warned before activating the stimulator.
Note that this course focuses on monitoring in adults. While the material presented applies equally well to children, the status and role of quantitative NMB monitoring in neonates and infants is unclear.
This course is intended to demonstrate the correct steps and processes by which quantitative NMB monitoring is performed, covering the placment of stimulating electrodes and electrode sites, the monitoring of NMB onset with TOF, intraoperative monitoring and redosing, the correct use of PTC, and the assessment of reversal with both neostigmine and sugammadex.
The program is divided into four sections:
- Set-up and Block Onset
- Assessing Block and Redosing
- Reversal and the Assessment of Reversal
- Summary of Quantitative Monitoring
Set-up and Block Onset
Monitoring can be performed via either the ulnar nerve/thumb or the posterior tibial nerve/big toe. The authors do not recommend using facial nerve sites (e.g. facial nerve/orbicularis). Current quantitative devices cannot be used at these locations. The relaxant/dose-response characteristics of the facial nerve differ dramatically from the ulnar or posterior tibial sites, and invite overdosage.
Select the most appropriate site for peripheral nerve monitoring. The ulnar nerve is preferred, largely because responses at this site are well-correlated with postoperative problems and other measures of the adequacy of paralysis. The posterior tibial nerve is a useful alternate site when ulnar nerve stimulation is not possible.
After the induction of anesthesia, turn on the nerve stimulator and start TOF cycling (usually every 15-20 seconds). Some devices are capable of automatically determining a ‘supramaximal stimulus’ – if not, set the stimulus current at 40-60 MA. The TOF ratio is not particularly sensitive to stimulus current as long as there is a strong twitch in the pre-relaxant period.
Determine the correct dosing of blocking agent based on the patient’s weight, gender and age. Most agents are dosed on the basis of ideal body weight, with small doses required in women and in the elderly. The typical induction dose is twice the ED95 (the average dose required to produce a 95% reduction in single twitch height) – for rocuronium, this is 0.6 mg/kg, and for cisatracurium, this is 0.08-0.1 mg/kg. The authors recommend using this larger dose to speed the onset of complete paralysis. Full paralysis (a twitch count of 0-1) should be achieved in about 3 minutes.
Assessing Block and Redosing
Continue cycling the nerve stimulator at 15-20 second intervals. A target value twitch count of 1-2 is sufficient for most surgeries. In most patients, a return of the 1st or 2nd twitch is observed within 20-40 minutes with rocuronium – but it is important to be aware of the enormous variation of this time period, ranging from 15 to 180 minutes. The supplementary dose of NMB when the TOF count returns to 1-2 is quite small (5-10 mg of rocuronium). Larger doses (10-20 mg) can be used if the expected duration of the procedure from that point forward is long (>1 hr).
For some laparoscopic and robotic procedures, a deeper block is appropriate. This is where the monitoring of PTC comes into play. If the TOF count is 0, switch the monitor to PTC. This delivers a 5-second-long, 50 Hz tetanic stimulus. After a brief pause, the monitor then delivers single 1 Hz twitches (either 10 or 20). The goal is to avoid a PTC of 0, which represents a clear overdose, and to maintain a PTC of 2-5. Again, the NMB dose to maintain a PTC in this range is small. It is important to note that tetanic stimuli can only be given every 2-3 minutes – most devices have a ‘lockout’ feature that prevents more frequent attempts. If a PTC of 10-20 is seen, it is likely that a TOF count of at least 1 can also be seen. In this case, if the deep block is no longer needed, switch back to TOF mode.
Reversal and the Assessment of Reversal
If neostigmine is to be used for reversal, a TOF count of 4 is advised before administration; deeper block cannot be reliably reversed in any reasonable period of time with neostigmine. Even with a TOF count of 4, the time to full reversal (a TOF ratio of ≥0.9) is often longer than expected, one should expect to wait at least 10 full minutes. In some cases, even with a starting TOF count of 4, full reversal does not occur, but rather, the TOF ratio ‘stalls out’ at 0.5-0.8. If a full reversal dose is given (max 0.07 mg/kg, or 5 mg regardless of patient size), administering further neostigmine is pointless, as, once all acetylcholinesterase activity is blocked, more drug does nothing – all one can do is wait and support the patient.
Sugammadex binds rocuronium and is, hence, a direct and specific reversal agent, unlike neostigmine. If the starting TOF count is >1-2, the recommended reversal dose of 2 mg/kg generally provides complete reversal (TOF ratio ≥0.9), within 1-3 minutes. Deeper blocks can be reversed with larger doses (4 mg/kg), but there is less certainty of full reversal. Avoid the temptation to give doses >4 mg/kg for excessively deep blocks. First, the cost is exorbitant, and second, full reversal is questionable. Again, if full reversal is not successful with 4 mg/kg after a deep block, the authors would advise waiting and supporting the patient.
Summary of Quantitative Monitoring
Continuous quantitative monitoring, starting whenever possible prior to the administration of a NMB drug, is the single best way to avoid intraoperative overdosage, to ensure that a patient is, in fact, reversible at the end of the case, and to objectively verify that full reversal has, in fact, occurred (i.e. a TOF ratio ≥0.9).
It is also valuable to be able to convey to the PACU nursing team the information regarding reversal, particularly which drug has been used for reversal and when it was given. It is important not to equate the administration of the reversal agent with actual reversal. Simply giving neostigmine or sugammadex without continued monitoring does not guarantee true reversal. Also, note that assessing such clinical endpoints as head lift and grip strength provide little useful information on reversal status.
Authors and References
Michael Todd, MD.
Anesthesiologist, Professor and Vice-chair of Research in the Department of Anesthesiology, University of Minnesota, MN, USA.
Stephen Thilen, MD, MSMS.
Anesthesiologist and Assistant Professor of Anesthesiology and Pain Medicine, University of Washington School of Medicine, Seattle, WA.
Andrew Bowdle, MD, PhD, FASE.
Anesthesiologist, Professor of Anesthesiology and Pain Medicine and Director of Cardiothoracic Anesthesiology Simulation, University of Washington School of Medicine, Seattle, WA.
- Brull SJ, Murphy GS (2010). Residual neuromuscular block: lessons unlearned. Part II, methods to reduce the risk of residual weakness. 2010. 111: 129-140
- Brull SJ, Kopman AF (2017). Current status of neuromuscular reversal and monitoring: Challenges and opportunities. Anesthesiology 126: 173-190
- Murphy GS, Brull SJ (2010). Residual neuromuscular block: lessons unlearned. Part I. definitions, incidence and adverse physiologic effects of residual block. Anesth Analg. 111: 120-128.
- Murphy GS et al. (2015). Residual neuromuscular block in the elderly. Incidence and clinical implications. Anesthesiology 123: 1322-1336.
- Thilen S et al. (2018) Management of rocuronium neuromuscular blockade using a protocol for qualitative monitoring a reversal with neostigmine. Br. J. Anaesth. 121: 367-377.
- Todd, M. and Hindman, B. (2015). The Implementation of Quantitative Electromyographic Neuromuscular Monitoring in an Academic Anesthesia Department. Anesth Analg., 121: 836-838.