To the Editor:—
In the September issue of Anesthesiology, Murphy et al.1
demonstrated that residual neuromuscular blockade may produce adverse respiratory outcomes. We believe that the methodology used in their study is significantly flawed, and that their conclusions comparing qualitative (conventional nerve stimulators) and quantitative (acceleromyography) monitoring are not supported by their results. In our opinion, their study failed to demonstrate that quantitative monitoring is superior to qualitative monitoring in reducing the incidence of adverse respiratory events in the perioperative period.
Naguib et al.2
have written: “(In neuromuscular blockade studies,) nuances in protocol and apparently ‘minor' variations in methodology may markedly affect outcome.” We find three types of faults with the present methodology. First, they used visual rather than tactile evaluation of train-of-four (TOF) responses. The present study, involving 20 faculty and 50 residents and nurse anesthetists, and similar papers from the same institution3,4
indicate that they routinely produce a level of blockade which results in two to three visual TOF responses. This practice is based upon an early study comparing electromyogram findings with a single surgeon's subjective evaluation of abdominal relaxation.3,5
We believe that evaluating visual as compared to tactile TOF responses tends to underestimate the level of blockade and overestimate the amount of recovery. We have observed patients with 4/4 visual and simultaneous 0/4 tactile TOF responses. Others6,7
have noted that visual TOF tends to overestimate the return of neuromuscular function, as compared with tactile monitoring. Furthermore, we are not aware of any studies that support the use of visual TOF monitoring or maintaining two to three visual TOF responses to attain satisfactory surgical relaxation. There are numerous studies8–10
that rely upon tactile TOF monitoring, a clinical simplification of the original investigations of TOF using mechanomyography.
Secondly, we do not believe that the present study truly compared one group of patients who were managed with conventional neuromuscular monitoring with another group who were managed with quantitative monitoring. Both groups of patients received conventional TOF monitoring to maintain two to three visual TOF responses and guide the administration of additional doses of rocuronium. Acceleromyography was not used in the quantitative group until after administration of the last dose of rocuronium to assure that a TOF ratio of > 0.8 was reached before tracheal extubation. The conventional group underwent tracheal extubation after a conventional nerve stimulator demonstrated the loss of visual TOF fade, an inaccurate indicator of TOF ratio.7
Thirdly, the authors did not follow common practices of neuromuscular monitoring and management of extubation when using a conventional monitor. It has been demonstrated that using intermediate neuromuscular blockers and waiting until the appearance of 2 or 3-4 tactile TOF responses before the administration of neostigmine markedly increases the likelihood of adequate (TOF ratio of > 0.8) recovery within 20–30 min, at which time the trachea can be safely extubated.9,10
It is predictable from the present study design that patients in the quantitative group who are extubated with a TOF ratio of > 0.8 will not have residual blockade in the postanesthesia care unit, while those in the conventional group whose tracheas are extubated on the basis of loss of visual TOF fade will have a deeper level of blockade and more likely demonstrate residual blockade.
While Murphy et al.
have alerted us to the relationship between residual neuromuscular blockade in the setting and practices described in the article, they have failed to support the editorial opinion that acceleromyography should be available in every operating room where neuromuscular blockers are administered.11,12
Instead, they have demonstrated that acceleromyography reduces the risk of residual neuromuscular blockade in a setting where evidence based standards for conventional monitoring are not routinely followed. We agree with Naguib et al.2
: “What makes the difference in the incidence of (residual blockade) is not the monitor but the anesthetist.” To reduce the incidence of residual blockade and adverse respiratory events in Evanston, we recommend that these authors and practitioners more carefully evaluate the degree of neuromuscular blockade required for their surgical patients, and either follow the recommendations above for using conventional nerve stimulators or use acceleromyography to guide the timing of tracheal extubation.
Peter E. Horowitz, M.D.,*
William M. Gild, M.B., Ch.B., J.D.
*National Institutes of Health Clinical Center, Bethesda, Maryland. firstname.lastname@example.org
1. Murphy GS, Szokol JW, Marymont JH, Greenberg SB, Avram MJ, Vender JS, Nisman M: Intraoperative acceleromyographic monitoring reduces the risk of residual neuromuscular blockade and adverse respiratory events in the postanesthesia care unit. Anesthesiology 2008; 109:389–98
2. Naguib M, Kopman AF, Ensor JE: Neuromuscular monitoring and postoperative residual curarisation (in reply). Br J Anaesth 2007; 99:297–9
3. Murphy GS, Szokol JW: Monitoring neuromuscular blockade. Int Anesthesiol Clin 2004; 42:25–40
4. Murphy GS, Szokol JW, Franklin M, Marymont JH, Avram MJ, Vender JS: Postanesthesia care unit recovery times and neuromuscular blocking drugs: A prospective study of orthopedic surgical patients randomized to receive pancuronium or rocuronium. Anesth Analg 2004; 98:193–200
5. De Jong RH: Controlled relaxation 1. Quantitation of electromyogram with abdominal relaxation. JAMA 1966; 187:393–7
6. Tammisto T, Wirtavuori K, Linko K: Assessment of neuromuscular block: Comparison of three clinical methods and evoked electromyography. Eur J Anaesthesiol 1988; 5:1–8
7. Brull SJ, Silverman DG: Visual and tactile assessment of neuromuscular fade. Anesth Analg 1993; 77:352–5
8. Kopman AF: Tactile evaluation of train-of-four count as an indicator of reliability of antagonism of vecuronium- or atracurium-induced neuromuscular blockade. Anesthesiology 1991; 75:588–93
9. Kopman AF, Zank LM, Ng J, Neuman GG: Antagonism of cisatracurium and rocuronium block at a tactile train-of-four count of 2: Should quantitative assessment of neuromuscular function be mandatory? Anesth Analg 2004; 98:102–6
10. Kirkegaard H, Heier T, Caldwell JE: Efficacy of tactile-guided reversal from cisatracurium-induced neuromuscular block. Anesthesiology 2002; 96:45–50
11. Eriksson LI: Evidence-based practice and neuromuscular monitoring: It's time for routine quantitative assessment. Anesthesiology 2003; 98:1037–9
12. Kopman AF: Undetected residual neuromuscular block has consequences. Anesthesiology 2008; 109:363–4
© 2009 American Society of Anesthesiologists, Inc.