Nasotracheal (NT) intubation is commonly performed in patients undergoing head and neck surgeries, such as intraoral, dental, or microlaryngeal procedures or mandibulectomies. Malpositioned tube can cause endobronchial intubation if too deep or accidental extubation, vocal cord damage if too shallow.^1,2 Previous studies have suggested that the proximal cuffs of NT tubes should be placed >2 cm^3,4 or at 3 cm⁵ below the vocal cords to prevent complications associated with too shallow intubation. An optimal endotracheal tube (ETT) placement should also ensure sufficient distance (2–5 cm) between the tip of the ETT and the carina.³  The increase in ETT tip-carina distance in adults with head and neck extension has been reported to range from a mean of 0.6 cm^6,7 to a mean of 2.7 cm.⁸ This increase is attributed to lengthening of trachea and proximal migration of the ETT due to head and neck extension.⁹ Most ETTs for the adults have two black insertion guide marks at 2 and 4 cm above the cuff or one mark at 2–3 cm above the cuff.⁵ Alignment of the marks with the vocal cords helps to place the ETT at the correct depth.⁶ However, this technique relies on visualization of the vocal cords with a laryngoscope which is difficult in patients with large tongue, prominent teeth, a short neck. Besides, in cases with limited mouth opening where only nasotracheal route of intubation is possible, the depth markers cannot be observed. 

Although some studies have used fiberoptic bronchoscope (FOB) to make sure that the ETT cuff was 2–3 cm below the vocal cords,^17,18 it was found difficult to identify the cuff and the vocal cords via FOB when the ETT was already in place, especially in flexometallic tubes and silicon Ring-Adair-Elwyn (RAE) tubes as this tube is opaque. Fiberoptic examination through the tracheal tube will indicate the position of the tip relative to the carina, but gives little information about the proximity of the cuff to the vocal cords.⁵ In patients with craniofacial anomalies or any abnormalities of the upper airway such as tumor and temporomandibular joint disorders, or post radiotherapy for head and neck cancers have potential difficulty with airway management and it is important in these cases to ensure appropriate positioning of the endotracheal tube. In various studies measurements were made with the head in neutral position. If the head position changes during surgery, the tube depth should be adjusted accordingly. In addition to routinely measuring ETT tip to carina distance, the mark at vocal cords can be visualized using FOB through opposite nostril, a safe distance between the vocal cords and the ETT cuff can potentially be guaranteed by this method.

We hypothesize that FOB can be used to reliably estimate the distance between the upper edge of the ETT cuff and the vocal cords in adults so that the depth of the ETT can be adjusted accordingly after positioning the head and neck during surgery.