To the Editor:
In a thought-provoking study, assessing the effects of muscle relaxants on facemask ventilation (FMV) in the anesthetized patients with normal upper airway anatomy, Ikeda et al.1 showed that rocuronium did not deteriorate FMV without airway interventions, and FMV was improved after succinylcholine administration in association with airway dilation during pharyngeal fasciculation. However, apart from the limitations described in the discussion, there are two aspects of this study that should be discussed.
First, it would be interesting to know why this study was performed in a neutral head and mandible position without airway interventions, which are not the practical airway management methods during anesthesia induction. Actually, upper airway obstruction is common during anesthesia induction due to loss of muscle tone present in the awake state.2 To obtain an adequate FMV and then make an easy laryngoscopy, a sniffing position is generally recommended in the clinical practice, especially for the patients with a difficult airway.2,3 A previous study from the authors’ team in patients with obstructive sleep apnea showed that compared with the neutral position, the sniffing position structurally improved maintenance of the passive pharyngeal airway at both retropalatal and retroglossal segments,4 which are the most common sites of upper airway obstruction.5 Moreover, the simple airway interventions, such as head tilt, jaw thrust, and open mouth (known as the triple airway maneuver), are the reliable methods frequently used to achieve upper airway patency in the anesthetized patients.2 The another study from the authors’ team showed that anesthesia induction and complete paralysis caused the upper airway obstruction in all patients with obstructive sleep apnea when the jaw thrust was not performed, while a combination of head tilt and jaw thrust restored airway patency and allowed adequate FMV ventilation.6 Considering the fact that the four patients in the rocuronium group were excluded from this study because of inadequate FMV, we would like to know whether the study design required a minor or moderate upper airway obstruction by a neutral head and mandible position without airway interventions. It is certain that the head and neck position of the anesthetized patients can affect the longitudinal tension on the upper airway and the manual airway interventions may change the caliber of the retrolingual and retropalatal airways, worsening or improving airway patency.2,5 Thus, we believed that if the patients are placed in a sniffing position with airway interventions in this study, as needed in the routine anesthesia induction, different result would have been obtained.
Second, in this study, the endoscopy at the isthmus of the fauces showed that the narrowed oral airway space abruptly and significantly dilated during oscillatory movements of the soft palate and the tongue base (pharyngeal fasciculation) after succinylcholine administration. Thus, the FMV improvement after succinylcholine administration is contributed to reopening of the pharyngeal airway by the pharyngeal muscle contraction. However, other than the soft tissue airway at the pharynx, the laryngeal aperture is another important site that may significantly affect gas flow of the upper airway.2,5 It has been shown that the vocal cord closure is a primary source of difficult or impossible FMV during anesthesia induction with sufentanil.7,8 After anesthesia induction, it is also possible for the epiglottis to overlie and obstruct the laryngeal aperture or to seal against the posterior pharyngeal wall, especially when the patients are placed in a neutral head and mandible position without any airway intervention.5 Because the authors did not observe changes of both position of the epiglottis in the pharynx and configuration of the laryngeal aperture during succinylcholine-induced upper airway muscle fasciculation, contribution of these factors to the FMV improvement by succinylcholine cannot be excluded.
