In the present study, the POGO score indicating the non-obstructing area of glottic opening was significantly higher in the neutral position than in the sniffing position. This difference could be attributed to the anterior aspect of the vocal cord, which is covered by the epiglottis (Fig. 3). In the sniffing position, because the epiglottis is closer to the posterior pharyngeal wall  due to the stretched and collapsed retropharyngeal muscles at the neck flexion point, the area of vocal cord obscured by the epiglottis is wider than in the neutral position. In sedated patients, this tendency is stronger because of the reduced muscle tone of the retropharyngeal structure and soft palate [11, 23].
The sniffing position, achieved using a pillow, was recommended by Magill in 1936 as being better than the neutral position during endotracheal intubation using laryngoscopy . The average head height needed to obtain an optimal laryngeal view was 55 mm (range: 31–71 mm) . The anatomical basis of this recommendation is the three axes (oral, pharyngeal and laryngeal) alignment theory. Bannister and Macbeth suggested that the alignment of these three axes provides a better view of the vocal cords for endotracheal intubation using a laryngoscope .
However, the oral axes or line of view can be ignored during endotracheal intubation using FOB, because the view inside the oral cavity can be secured and the fiberoptic endoscope passed through the patient’s mouth. Thus, only the angle between the pharyngeal and laryngeal axes is important during intubation using FOB. According to a previous MRI-based study, this angle is wider in the sniffing position as the pharyngeal axes lean posteriorly with atlanto-occipital extension , and poorer visualization of the vocal cord during fiberoptic endotracheal intubation is assumed.
As glottic view is one of determining factor for Fremantle score using videolaryngoscopic intubation which supplies the direct glottic view , the better glottic view can lead the successful intubation using FOB. In the previous studies about fiberoptic intubation, there was no consistent evaluating tool for assessment of glottis view [11, 16]. Therefore, to assess the visualization of vocal cord we used the POGO score which is invented to estimate the glottic inlet in conventional laryngoscopic intubation. A validated scoring system of the percentage of glottic opening (POGO) score is used to investigate about a degree of glottic airway narrowing , and has high accuracy and inter-rater reliability during videolaryngoscopic intubation . For applying the POGO score to the fiberoptic view which has wide angle view, we paid attention to maintain the consistency along the pictures using ovassapian airway. Also, to compensate the inconsistency of the location taking pictures between two head positions, we selected the images viewing the widest glottic opening for scoring the POGO.
In the present study, despite the apparent difference in vocal cord view between the two positions, there were no differences in the ease of intubation and the time for the endotracheal tube insertion. These observations suggested that two different positions do not affect time or ease of orotracheal intubation using FOB. Also, while visualization of the vocal cord affects the difficulty of laryngoscopic endotracheal intubation , the effects of vocal cord view on duration or ease of fiberoptic intubation are unclear. The time for tube insertion and the difficulty of tube insertion showed no relationship with better vocal cord view. These observations suggested that visualization of the vocal cord does not affect time or ease of orotracheal intubation using FOB.
The present study has several limitations. First, the intubation time was not measured during whole procedure but only measured from acquiring images to completion of tube insertion. There could be the difference in the time getting the best view of glottis opening between two head position. Measurement of whole intubation time may reveal the relationship between head position and intubation time. Second, we included patients with low TAS score for reducing the airway variation of subjects, excluding patients expected difficult intubation. In patients with difficult direct laryngoscopic intubation (Cormack & Lehane grade III), elevation of the head and neck beyond the sniffing position improved visualization of glottic structures . In addition, sniffing position relieves obstruction of upper airway in patients with obstructive sleep apnea indicating difficult laryngoscopic intubation . These results limit the application of this study’s result to various clinical situations. To validate the superiority of different head positions during fiberoptic intubation, further studies are required with larger patient populations including various airway difficulty including measurement of the entire procedure time.