Inability to maintain patient’s airway associate with hypoventilation, hypoxemia, hypoxic brain injury or even death [18–20], so the management of the difficult airway is at high-risk. Awake intubation via FOB is still the recommended method for dealing with difficult airway [3–6] and a clear airway that allows the passage of the FOB is critical to awake intubation via FOB.
In this study, we compared the effect and safety issues of three head positions (neutral position, sniffing position and extension position) during AFOI. The results showed us that the time for viewing the vocal cords was significantly shorter and the POGO scores were significantly higher in the extension position group compared with the other two groups, in addition at before intubation, the SpO2 was higher in the EP group compared with neutral position group and at immediate after intubation the SpO2 was also higher in the EP group compared with the sniffing position group. The results suggested that the extension position facilitated the FOB passing through the base of the tongue and easy to view the whole glottis. The results of SpO2 also confirmed that the extension position was better than sniffing position or neutral position to keep the airway open in sedated patients.
Tracheal intubation via laryngoscope requires the operator to view the glottis directly and clearly, therefore, how to reduce the angle of airway axes (oral, pharyngeal and laryngeal) is the key to successful intubation. Magill first reported that the sniffing position was better than the neutral position during tracheal intubation via laryngoscopy . This recommendation is based on the so-called three-axes alignment theory that the sniffing position make three axes closer alignment [22, 23]. Although the clinical benefit of sniffing position is controversial [24, 25], sniffing position is still recommended for tracheal intubation using laryngoscopy [26, 27].
It seems to be different of tracheal intubation using FOB for the anterior of FOB can move freely, so the FOB can cross the angle among the three axes, however the FOB can not provide enough support to the laryngeal tissue, resulting in the base of tongue, soft palate and epiglottis were closer to the posterior pharyngeal wall , lead to the operater unable to find the glottis easily in sedated patients using FOB. Therefore, sufficient space of pharyngeal cavity and laryngeal cavity is necessary for FOB to successfully pass through the base of the tongue and quickly view the glottis during AFOI. So the space of pharyngeal cavity and laryngeal cavity is the key to successful AFOI. Several studies found that the manoeuvre of jaw-thrust elevated the epiglottis and tongue base away from the wall of posterior pharyngeal, provided more space in the pharyngeal cavity and laryngeal cavity [9–14]. So the manoeuvre of jaw thrust facilitated viewing the vocal cords and increased the success rate of oral FOB intubation . Durga et al demonstrated that used jaw thrust and lingual traction alone unable to clear the airway completely, whereas the combining used of jaw thrust and lingual traction cleared the airway more effectively . Stacey et al found a clear airway at the level of the palate and larynx with a combined of FOB and laryngoscopy technique . The problem with these methods is that one or two trained assistants are required. Therefore, in some emergency situations, such as unpredictable difficult airway the success rate of orotracheal intubation via FOB may reduce for without the aid of assistants. In addition, during awake orotracheal intubation the patients only sedated and without the use of muscle relaxants, so the manoeuvre of jaw thrust, lingual traction or assistance by laryngoscope may increase the discomfort of patients and cause the resistance of patients. In present study, all patients in the SP group with head and cervical extension by placing a 7-cm firm pillow under the shoulders of patients, so this positions tightened the muscles and tissues in the front of neck then moved the oropharyngeal structures anteriorly and emptied the oropharyngeal airspace. In addition, with head extension position, the operator made the bite block as a fulcrum and lifted the mandible upward more easily than other two positions during intubation, which may achieve similar effects as jaw thrust that lifting the base of tongue enabling the FOB easily to pass through the base of the tongue. Most important one was the whole process of intubation with no need for the assistance of others.
Successful inserting a FOB into the trachea does not guarantee a successful AFOI, because the anterior of the tracheal tube may impinge on the laryngeal structures when advance the tracheal tube into trachea over the FOB [30, 31], which may lead to serious injury of laryngeal  or be catastrophic in some cases . This difficulty can be reduced by reducing the gap between the tracheal tube and FOB . In this study, the outside diameter of FOB was 5.2 mm and the inner diameter of tracheal tube was 6.5 mm for female and 7.0 mm for male, so the gap between the tracheal tube and FOB was very small, furthermore we used silicone flexible tracheal tube and the inside and outside of the tracheal tube was lubricated with dyclonine hydrochloride mucilage, so the advancement of a tracheal tube over the FOB very smoothly in all the patients.
In our study, we found no statistical differences in the hemodynamic changes and coughing scores among groups, the reasons may be that topical anaesthesia of the trachea and combined use of dexmedetomidine and remifentanil not only preserved the patients' spontaneous breathing but also achieve adequate sedation depth during intubation .
The patients were interviewed by an independent anesthesiologist one day after operation and the results showed us that all the patients were unable to recall the procedure of topical anaesthesia and intubation, the reasons may be that the application of low dose midazolam and the combined use of dexmedetomidine and remifentanil elimination of the patients' memory of these procedure. In addition, the small size of the tracheal tube was selected and the tracheal tube was adequately lubricated with dyclonine hydrochloride mucilage which may reduced the incidence of adverse complications and improved the comfort of patients.
There were also some limitations in our study. First, this method is not suitable for patients with cervical vertebra sickness; Second, this method is not suitable for patients with incisors loose or missing; Third, this method may not be applicable to the patient with oversize-chin or the operator with little experience in the management of FOB; Fourth, it was not possible to blind investigator to the technique, consequently we cannot rule out the possibility of biases by investigator in this study.