Fig 1.
Measurement of the distance between the carina and vocal cords.
The measurement was performed using a fiberoptic bronchoscope. (A) The fiberoptic bronchoscope was inserted through the nose and towards the carina. (B) The fiberoptic bronchoscope was withdrawn until the vocal cords were just visualized.
Fig 2.
Insertion depth of the nasotracheal tube’s cuff and tip.
The maximum allowable proximal-cuff-to-tip distance was determined as 50 mm less than the calculated distance between the vocal cords and carina; 2 cm between the vocal cords and the tube’s proximal cuff and 3 cm between the tube’s tip and carina.
Fig 3.
Flow chart of patient enrolment.
Among the 306 patients scheduled for elective surgery under general anesthesia with nasotracheal intubation between September 2017 and December 2018 at our institution, 64 patients did not meet the inclusion criteria and 96 declined to participate. Among the remaining 146 patients, three withdrew consent, two cancelled the operation, and four had failed measurements. Finally, 137 patients were included.
Table 1.
Patient characteristics and airway lengths.
Fig 4.
Difference in the safety margin between the maximum allowable proximal-cuff-to-tip distance and actual proximal-cuff-to-tip distance.
The selection of the tube diameter was based on nostril size or sex. A negative value indicated that the length from the tube’s proximal cuff to its tip was too long. This means that the actual proximal-cuff-to-tip distance of the selected tube exceeded the maximum allowable proximal-cuff-to-tip distance; the tip was too close to the carina for maintaining 2 cm safety margin for the distance between the vocal cords and the tube’s proximal cuff.
Fig 5.
Nostril-size-based vs. sex-based selection.
The number of patients expected to have the appropriate tip location when the tube sizes were selected according to the nostril size or sex, with the tube’s proximal cuff located 2 cm below the vocal cords. A red area means that the tube’s tip was too close to the carina causing a risk of endobronchial intubation in various neck positions.