Selection and insertion of an endotracheal tube (ETT) of appropriate size for airway management during general anesthesia in pediatric patients is an essential, albeit challenging task. The outer diameter of the ETT is very important to control leakage and protect from aspiration, while the lumen of the ETT is important for ventilation pressure. The use of the inner diameter to estimate the optimal ETT size has been established clinically. Although traditional age- or height-based formulas have been used to determine the ETT size [1], the initial size selected may not be appropriate, and replacement may be required. If a very small ETT is inserted, there is a risk of inadequate ventilation and decreased reliability of end tidal carbon dioxide monitoring. There is also a risk of air leakage, anesthetic gas leakage, aspiration, and high airway resistance [2–5]. Similarly, a very large ETT may cause serious complications including airway damage, such as ulceration, local ischemia, scar formation, post-intubation croup, and, in severe cases, subglottic stenosis [6,7]. Therefore, this review article aims to discuss the advantages and disadvantages of conventional and novel methods that are used to select an appropriate ETT size in pediatric patients.

The larynx is mainly funnel-shaped during infancy, with the narrowest part being the nondistensible cricoid cartilage. According to Eckenhoff [8], the larynx is located between the pharynx and trachea, and extends from the base of the tongue to the cricoid cartilage. It consists of the thyroid cartilage, cricoid cartilage, paired arytenoids, epiglottis, and corniculate and cuneiform cartilages located around the epiglottis. These pieces of cartilage are connected with ligaments and affect laryngeal muscle movement. Among them, the cricoid cartilage is located below the largest thyroid cartilage at the C4 level at birth, before settling at the C5 level by 6 years of age [9]. Owing to the small size of the cricoid cartilage in children, mucosal edema may cause severe airway obstruction, and prolonged endotracheal intubation increases the risk of subglottic stenosis [10]. The epiglottis is wide and runs parallel to the trachea in adults, but is narrower, softer, and relatively horizontal in children [11].

Recently, in vivo investigations and review articles have examined whether the larynx is funnel-shaped in children. Holzki et al. [12] reported that when the aryepiglottic, vestibular, and vocal folds, which constitute the structure of the laryngeal upper airway, are maximally abducted, they have a larger opening in the glottis than the outlet of the cricoid ring. Moreover, although the pediatric larynx is conical, the narrowest part, the rima glottidis, is cylindrical in the anteroposterior dimension regardless of development, and the cricoid ring is slightly elliptical [13,14]. However, based on the active contraction of the laryngeal muscles, Litman et al. [15] suggested that the narrowest part of the larynx is the transverse dimension at the level of the vocal cords, while the cricoid ring is functionally the narrowest part.

Uncuffed ETTs and cuffed ETTs have advantages and disadvantages (Table 1) [13]. Traditionally, uncuffed ETTs have been used in children below 8 years old, because the cricoid cartilage is the narrowest part in this age range [16]. Litman et al. [15] and Dalal et al. [17] found that, as the cricoid cartilage is nearly ellipse-shaped, circular uncuffed ETTs may cause leakage in the empty anterior and posterior spaces, thus possibly putting excessive pressure to the sides [14]. Therefore, cuffed ETTs can reduce the number of endotracheal intubation attempts, and if cuff pressure can be maintained within a safe range, the risk of airway damage may not exceed that of an uncuffed ETT [18–20]. In addition, the use of cuffed ETTs reduces the risk of leakage during positive pressure ventilation, ensuring patient safety through adequate ventilation and lack of aspiration, and reduces contamination of the surrounding air by inhaled anesthetics (Fig. 1) [21–23]. Therefore, the use of cuffed ETTs in children is increasing. However, excessive pressure in the cuff can inhibit perfusion to the tracheal mucosa and cause postoperative sore throat, subglottic edema, or stenosis [24–26]. Pediatric ETT cuffs must be very thin to meet the high-volume low-pressure standards, and the cuff must be short enough, so that its upper boundary is stably positioned in the subglottic region below the cricoid cartilage (Fig. 2) [14]. In addition, although the tip of the ETT should have a sufficient safety margin over the tracheal bifurcation, selection of an appropriate ETT is challenging, because the outer diameter, size, and location of the cuff vary between manufacturers. The cuff may not meet the criteria for high volume and low pressure, and the depth may not be marked [27].

Microcuff ETTs are a new type of ETTs, which are of polyurethane. The cuff is located more distally on the ETT shaft, and there is no Murphy’s eye (Fig. 3). Therefore, the cuff may be positioned below the non-distensible cricoid ring, where a 20 cmH₂O inflation pressure maintains a cross-sectional area of approximately 150% of the maximal internal tracheal cross-sectional area. In the deflated state, the outer diameter of the microcuff ETT is minimally increased; therefore, it can be safely used in children [28,29]. However, this type of cuff has not been introduced in Korea yet. With the development of newer polyvinyl chloride high-volume low-pressure cuffed ETTs in the late 1990s and ultrathin polyurethane microcuff ETTs in the early 2000s, the use of cuffed ETTs has increased in infants and children weighing ≥ 3 kg. While most studies on cuffed ETTs in infants only included infants ≥ 3 kg [20,21,30–34], Thomas et al. [33] reported that microcuff ETTs can be safely used in neonates weighing < 3 kg.

The age-based formula presented by Cole [ID (mm) = (age/4) + 4.0] has long been used to select the appropriate ETT size in children. This formula is suitable for uncuffed ETTs; however, recent changes in the understanding of pediatric airway anatomy, redefined the use of cuffed ETTs. Cuffed ETTs can reduce the number of endotracheal intubation attempts, and if cuff pressure can be maintained within a safe range, the risk of airway damage may not increase, compared to an endotracheal tube without a cuff. When estimating cuffed ETT size using an age-based formula, Duracher’s formula [ID (mm) = (age/4) + 3.5] is more accurate. However, a tube 0.5 mm larger or smaller than the calculated size should always be possible to use. Because age-based formulas in children are not always accurate, various methods for estimating ETT size, such as chest radiography, ultrasound, and three-dimensional airway models, have been investigated. However, these methods have several limitations, and further studies are warranted to ensure proper use in clinical situations.