Persistent train-of-four fade in myasthenia gravis patients after sevoflurane anaesthesia
Seoul, South Korea
* E-mail: ylkwak{at}yumc.yonsei.ac.kr
Editor—We read with great interest the article by Nitahara and colleagues1 showing concentration-dependent inhibitory effects of sevoflurane on neuromuscular transmission in myasthenia gravis (MG) patients which was more prominent in patients with a pre-anaesthetic T4/T1<0.9. In their study, T4/T1 returned to baseline value in all patients at the end of anaesthesia. The importance of monitoring pre-anaesthetic T4/T1 ratio in MG patients has been previously mentioned in a study showing significantly lower atracurium requirements in patients with pre-anaesthetic T4/T1<0.9.2 We also investigated T4/T1 ratio as a predictor for early extubation in 11 MG patients using sevoflurane and remifentanil without concomitant use of neuromuscular blocking agents. Interestingly, five patients, of whom only one patient showed pre-anaesthetic T4/T1<0.9, had persistent T4/T1<0.9 at the end of anaesthesia, even when the end-tidal sevoflurane concentration was below 0.3%. Of the five patients, two had pre-anaesthetic T4/T1>0.9, had T4/T1<0.8 and did not meet the extubation criteria (inspiratory force >25 cm H2O and vital capacity of at least 10 ml kg–1), and could not be extubated in the operating room. These two patients had similar characteristics as the other nine patients, including serum anti-acetylcholine receptor antibody titre, except that they had bulbar symptoms and a more severe grade of MG.3 Sevoflurane may be a suitable anaesthetic agent for MG patients owing to the concentration-dependent inhibitory action on neuromuscular transmission with rapid reversibility after discontinuation.1 2 However, as we observed, persistent fade can occur at the end of sevoflurane anaesthesia, regardless of the presence of pre-anaesthetic fade, hindering early extubation even when no neuromuscular blocking agents were used. The association of MG severity and recovery of fade needs to be validated in a further study and we recommend monitoring post-recovery T4/T1 ratio as well, during sevoflurane anaesthesia without the use of neuromuscular blocking agents in MG patients to avoid unnecessary risk of respiratory compromise and delay in extubation.
Fukuoka, Japan
* E-mail: nitahara{at}fukuoka-u.ac.jp
Editor—We appreciate the interest of Dr Kwak and colleagues in our article.1 In our study, the train-of-four ratios (TOFRs) of some patients with fade or non-fade did not return to a value 
0.9. At the end of anaesthesia, although mean TOFR returned to values not significantly different from baseline in all three groups, TOFRs were <0.9 in three of 10 patients with baseline TOFR0.9 and in three of six patients with baseline TOFR<0.9. It is unclear whether the fade at the end of anaesthesia in MG patients was due to the residual neuromuscular effect of sevoflurane. We extubated all patients with fade since the inspiratory force and vital capacity met the extubation criteria. It has been reported that during partial neuromuscular block, although impairment of inspiratory flow and upper airway obstruction still occurs,4 forced vital capacity and inspiratory force recovery to an acceptable level at TOFR>0.6.4 5 Recently, TOFR0.9 has become a standard for safe recovery from the neuromuscular blocking drugs in patients without neuromuscular disease. However, if we set the extubation criteria at a TOFR above 0.9 in MG patients, we would keep many patients intubated after operation. In our institute, we extubate if the patient is fully awake and inspiratory force and vital capacity meet the criteria. Respiratory conditions are closely monitored after extubation. To our knowledge, no standard TOFR level has been established for safe recovery with regard to extubation in MG patients.
Postoperative respiratory insufficiency in MG patients may not only be caused by residual neuromuscular effect of inhalation anaesthetics, other intraoperative factors such as surgical invasion and the i.v. anaesthetics used, which cannot be measured by TOFR, can also affect postoperative respiratory function. In our study, anticholinesterase and steroids were continued until the morning of surgery. Preoperative control of MG symptoms may also influence the postoperative respiratory conditions. It is difficult to speculate what will be the postoperative respiratory conditions simply from the preoperative TOFR from single peripheral muscles.
References
1 Nitahara K, Sugi Y, Higa K, et al. Neuromuscular effects of sevoflurane in myasthenia gravis patients. Br J Anaesth (2007) 98:337–41.
2 Mann R, Blobner M, Jelen-Esselborn S, et al. Preanesthetic train-of- four fade predicts the atracurium requirement of myasthenia gravis patients. Anesthesiology (2000) 93:346–50.[CrossRef][Web of Science][Medline]
3 Jaretzki A III, Barohn RJ, Ernstoff RM, et al. Myasthenia gravis: recommendations for clinical research standards. Task Force of the Medical Scientific Advisory Board of the Myasthenia Gravis Foundation of America. Neurology (2000) 55:16–23.
4 Eikermann M, Groeben H, Husing J, Peters J. Accelerometry of adductor pollicis muscle predicts recovery of respiratory function from neuromuscular blockade. Anesthesiology (2003) 98:1333–7.[CrossRef][Web of Science][Medline]
5 Ali HH, Wilson RS, Savarese JJ, Kitz RJ. The effect of tubocurarine on indirectly elicited train-of-four muscle response and respiratory measurements in humans. Br J Anaesth (1975) 47:570–4.
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