Usefulness of ultrasound for selecting a correctly sized uncuffed tracheal tube for paediatric patients


Jin-Tae Kim


The purpose of this study was to assess whether ultrasonography is useful for determining uncuffed tracheal tube sizes for paediatric patients. The equation for selecting the correctly sized tracheal tube was developed using data on the subglottic diameter measured by ultrasonography and air leak test. The efficacy of the new equation was evaluated by comparing it with the conventional age-based formula (4 + age/4) in another 100 patients. Tracheal tube sizes were selected using two methods, and air leakage pressure was measured after each intubation. The ultrasonographic method allowed the correct tube size to be selected in 60% of cases, whereas the age-based method enabled this in 31% of cases (p < 0.001). Ultrasound can offer a useful means of selecting correct tracheal tube size compared with the age-based formula in paediatric patients. However, even using ultrasound, the success rate of correct tube size selection is still not very high.

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Choosing the correct tracheal tube size is important in paediatric patients because an inappropriately large tracheal tube may cause damage to the airway. If the tracheal tube is too small, this may lead to difficulties in ventilation or the need to re-intubate with a different size of tracheal tube. Various methods (based on age, weight, height and finger size) have been suggested for the selection of tracheal tube size [1–3]. However, these methods are not always suitable because the size of the airway varies considerably between patients. Clinically, when air leakage around the tracheal tube occurs at 10–30 cmH2O, the size of the tracheal tube is considered to be appropriate [2, 4].

Ultrasonography has been suggested to be a useful tool for airway management in children [5, 6], and may be used to measure the transverse tracheal diameter of the neck in adults and children [7, 8]. Furthermore, a strong correlation exists between ultrasound and magnetic resonance imaging measurements of transverse tracheal diameter at the level of the cricoid cartilage [9]. Recently, Shibasaki et al. showed that the rate of agreement between the predicted tracheal tube size based on ultrasonic measurement and the final tracheal tube size selected clinically was 98% for cuffed tracheal tubes and 96% for uncuffed tracheal tubes [10].

As the transverse diameter of the trachea is smaller than the anteroposterior diameter at the cricoid cartilage level [11], it may be hypothesised that the transverse diameter measured by ultrasound may be used to choose the correct size of tracheal tube. However, there are reasons that ultrasonography may not useful for selecting the correct tracheal tube size: ultrasound may measure only transverse diameter, and thus it is not able to provide information on anteroposterior size discrepancies. The level of sono-probe application may not be at the smallest tracheal lumen diameter, in which case air will not leak within the targeted pressure range.

The purpose of this study was to evaluate the usefulness of ultrasonography for selecting the correct size of uncuffed tracheal tubes in paediatric patients, and to compare these findings with a conventional age-based method.


This study was conducted with local ethics committee approval. Informed written consent was obtained from the parents or patients, if the patient was considered to be old enough to give consent. After applying the exclusion criteria, 141 children < 8 years of age scheduled for elective surgery were enrolled. Exclusion criteria included an anticipated difficult airway, the presence of a neck mass and an unstable cardiopulmonary condition.

Anaesthesia was induced with intravenous thiopental sodium 5−1 according to the institutional standard. Rocuronium 0.6−1 was administered to facilitate airway manipulation and surgery. The lungs were ventilated with 4–8% sevoflurane in 100% oxygen, via a facemask before intubation. All tracheas were intubated with an uncuffed Mallinckrodt tracheal tube with a Murphy’s eye. The tip of the tracheal was placed 2–3 cm above the carina by withdrawing the tube, after confirming by auscultation.

The ‘correct’ tracheal tube size was defined as that size which allowed an audible air leak around the tube at an inspiratory airway pressure of 15–30 cmH2O, with the head and neck in a neutral position. The presence of an air leak was assessed by closing off the pop-off valve and allowing pressure to rise slowly until an audible leak was heard using a stethoscope. When the airway pressure reached 35 cmH2O, no further measurements at higher pressures were taken to avoid barotrauma, and the trachea was re-intubated with a smaller tracheal tube.

Ultrasonographic measurements of transverse diameter at the cricoid cartilage level were performed with a linear probe lightly placed on the middle of the anterior region with the neck extended (LOGIQ e; GE Healthcare System, Milwaukee, WI, USA). A continuous positive airway pressure of 10 cmH2O was maintained during measurements. All measurements were performed by the same practitioner (JTK). After finding the vocal cords, the 8–13 MHz linear probe (12L-RS; GE Healthcare System) was moved caudally to show the cricoid arch [2, 9]. The transverse air-column diameter, defined as the subglottic diameter, was measured at the level of the cricoid cartilage (Fig. 1).

Figure 1.

 The transverse air-column diameter measurement. Ultrasonographic measurement was made at the level of the cricoid arch. The air-column appeared hyperechoic (between two crosses) and created a posterior acoustic shadow.

The correct size of tracheal tube was identified with an air leak test in the first 41 patients whose transverse air-column diameter was measured using ultrasound. From this data, the linear regression equation predicting the correct tracheal tube size was made, based on the measured subglottic diameter.

In another 100 patients, we evaluated the efficacy of the method using ultrasonograpy by comparing the age-based method. Tracheal tube size was selected using the patient ages or the new equation based on ultrasonography. Tracheal tube size selection by age was performed as shown in Table 1 [2]. After determining tracheal tube sizes, the trachea was intubated with the two tracheal tubes in a computer generated- randomised order; leak pressures were also measured. An air leak test was performed by another investigator, who was blinded to the selection method. When selected tracheal tube sizes were identical, intubation was performed once.

Table 1.   Recommended internal diameter of tracheal tube sizes for infants and children. Values are in mm.
Neonate to 3 months3.0
3–9 months3.5
9–21 months4.0
> 21 monthsAge (years)/4 + 4

The equation for determining the tracheal tube size using subglottic diameter was made by linear regression analysis. The correct selection rates of the two methods were compared using McNemar’s test, and the tracheal tube sizes selected using the two methods were compared with Wilcoxon’s signed rank test. Sample size for McNemar’s test was calculated with an alpha error of 0.05 and a beta error of 0.2 based on data obtained from our pilot study from another 15 patients. In the pilot study, ultrasonographic findings selected the correct tracheal tube size in nine of the 15 patients (60%), whereas the age-based method selected the correct tracheal tube size in six patients (40%). The null hypothesis was that the rate of selecting the proper size tracheal tube by ultrasonography alone is identical to that selected using the age-based formula. A 20% difference in the rate of selecting the optimal size between the two methods was considered clinically significant. Assuming, a dropout rate of 5%, the required sample size was approximately 100 patients. Statistical significance was accepted for p values of < 0.05.


The mean (SD) age, weight and height of the first 41 patients were 44 (31) months, 16.2 (8.2) kg and 98 (22) cm, respectively. The correct size of tracheal tube was highly correlated with the subglottic diameter (internal diameter of tracheal tube = 0.705 × subglottic diameter –0.091, r2 = 0.925, p < 0.001) (Fig. 2).

Figure 2.

 The relationship between the correctly sized tracheal tube inner diameter and the subglottic diameter.

The mean (SD) age, weight and height of the second 100 patients were 39 (28) months, 14.8 (6.9) kg and 95 (21) cm, respectively. Ultrasonographic findings selected the correct tracheal tube size in 60 of the 100 patients (60%), whereas the age-based method selected the correct tracheal tube size in 31 patients (31%) (p 0.001). Ultrasonographic method selected improper tracheal tube size in 40 patients. Air leakage < 15 cmH2O occurred in 31 patients, and air leakage > 30 cmH2O occurred in nine patients. The age-based formula selected incorrect tracheal tube in 69 patients, of which 63 were too small and six were too large.

Ultrasonography and the age-based method resulted in the selection of the same tracheal tube size in 37 patients (37%), and the correct size was chosen in 21 of these patients. In 63 patients (63%), the two methods resulted in different sizes. Of these patients, ultrasonography selected the proper size in 39 patients. Using the age-based method, selection of the correct tracheal tube size occurred in 10 patients, and both methods chose the incorrect size of the tracheal tube in 19 patients (p < 0.001) (Table 2).

Table 2.   Comparison between ultrasonography and age for the selection of correct size of the tracheal tube. Values are numbers.
 Age-based formulaTotal
Total 3169100

Mean (SD) transverse diameter of the cricoid arch in the cohort was 7.0 (1.3) mm. The median (range) internal diameter of tubes selected by ultrasonography (5 [3.0–6.5] mm) differed from that of tubes selected by age (4.5 [3.0–6.0] mm) (p < 0.001). The mean (SD) leakage pressure of tracheal tubes chosen by ultrasound was 18 (10) cmH2O, whereas that of tracheal tubes chosen by the age-slected method was 11 (10) cmH2O (p < 0.001).


The present study shows that ultrasonography offers a more accurate means of selecting a correctly sized uncuffed tracheal tube in children than the age-based formula. This concurs with the findings of previous studies, in which it was suggested that ultrasonography is useful for assessing subglottic diameter in the clinical setting [9, 12]. Our findings indicate that direct measurements of subglottic tracheal diameter by ultrasound may reduce unnecessary tube changes caused by an incorrect tracheal tube size. However, it should be noticed that even using ultrasonogaphy, selection of the correct tracheal tube size occurred in only 60% of cases.

The limitation of ultrasonography should be considered. Ultrasonography measures only the transverse diameter of the trachea at one level. Furthermore, the external diameter of tracheal tubes varies according to the manufacturer, and thus the tracheal tube size used must be assessed on an individual basis. Finally, diameter measurements are subject to variation and are also time consuming. These shortcomings can explain the inappropriate tube selections made by ultrasonography in 40% of cases.

In adults, pressures above 30 cmH2O compromise tracheal mucosal perfusion [13], and thus, it appears reasonable to set an upper pressure limit for children, especially for those likely to require intubation for some time. Air leak testing has been recommended as a means of assessing the appropriateness of tracheal tube size [14]. In fact, the absence of air leak at higher pressures has been associated with increased rates of adverse respiratory events or re-intubation after extubation in children in the intensive care unit [15, 16]. In addition, children undergoing general anaesthesia experienced significantly more adverse events after tracheal tube removal, when there was no air leak at 25 cmH2O [17]. A leak pressure of 30 cm H2O may be allowable in cases requiring a short intubation period. Therefore, we defined the correct size of tracheal tube as that allowing an air leak at an inspiratory airway pressure of 15–30 cmH2O with the head and neck in a neutral position.

Although the appropriateness of tracheal tube size was evaluated using the air leak test, this test has its shortcomings in terms of the determination of optimal tube size; for example, the air leak test is not able to predict an increased risk of adverse events after extubation or the risk of re-intubation in children [18]. Furthermore, the incidence of inter-observer variations for the assessment of leak pressures is high [19], and leak pressure depends on head position and the degree of neuromuscular blockade [20]. However, there are no other feasible practical methods for confirming the correct tracheal tube size after intubation. Therefore, in the present study, to reduce inter-observer error, the same anaesthetist blinded to the selection method performed the air leak test in all children. A continuous positive airway pressure of 10 cmH2O was maintained during ultrasonographic measurements to eliminate ventilation-induced diameter changes and simulate the condition of an air leak test.

Unlike adults, laryngeal calcification is not encountered in children [21]. Therefore, laryngeal calcification, one of the limitations of performing ultrasonographic measurements of the larynx, does not influence ultrasonographic findings in paediatric patients. Thus, ultrasonography may be more useful for the selection of tracheal tube size in children.

In conclusion, our findings show that ultrasonography offers a better alternative than the frequently used age-based tube selection method. However, even ultrasound is not a reliable method in terms of choosing the correct tracheal tube size in children.

Competing interests

No external funding and no competing interests declared.