The cricoid cartilage, as the narrowest part of the larynx in children, plays an important role in the selection of optimal ETT size for intubation[11, 12]. However, recently, Dalal et al found that the vocal cord and subvocal cord area was the narrowest portion in pediatric airways. We found that the vocal cord is narrower than the cricoid cartilage in ultrasound images. Compared with the vocal cords, the cricoid is a complete and relatively rigid cartilaginous ring, and also the most frequently damaged structure during endotracheal intubation. Therefore, theoretically, the cricoid cartilage is the limiting factor during intubation, and can be a predictive factor in the selection of optimal ETT size for intubation.
The leading edge of the cricoid cartilage and the air-column in the airways were identified on ultrasound images. The former is a round, hypoechoic structure with hyperechoic edges, whereas the latter is hyperechoic with a hypoechoic mucosal edge. The transverse air-column diameter can be measured by ultrasonography (Fig. 2). The transverse diameter of the cricoid cartilage is represented by the transverse air-column diameter at the cricoid cartilage level. In our study, the cricoid cartilage diameters were estimated from the measurements of the transverse air-column diameter. According to the measurements of the cricoid cartilage diameter by ultrasonography, the corresponding ETT size was selected.
Leak test was a classic experimental method and has been applied to determine the bestfit ETT size for many years. Therefore, in our study, bestfit ETT size was chosen according to the leak test. In leak test, the allowed leak pressure often was 15-30cm H2O for cuffed ETT[17,18,19]. Scoliosis can affect pulmonary function, and lung function abnormalities are mainly of the restrictive type. During the procedure, the children were positioned in prone recumbency, and the operation can bring pressure to bear on the chest. All these aspects can cause the elevated of the airway pressure.Therefor, we chose a higher leak pressure 25 cm H2 O to determine the bestfit ETT size.
The study results show a strong correlation and high agreement between the ETT size predicted by ultrasonography and the bestfit ETT size in pediatric patients with thoracic scoliosi (r = 0.93, p < 0.001) and lumbar scoliosis(r = 0.95, p < 0.001). The Bland–Altman analysis showed that there was no obvious bias between the ETT size predicted by ultrasonography and the bestfit ETT size in pediatric patients with thoracic scoliosis (bias = 0.02 mm) and lumbar scoliosis (bias = 0.09 mm). Our finding was consistent with that reported by Rahul Pillai6. In their study, the correlation was 0.98, p < 0.001, and the bias was 0.041 mm. Therefore, it is feasible to to predict ETT size through measuring thetransverse diameter of cricoid cartilage by ultrasonography in pediatric patients with thoracic scolios and lumbar scoliosis.
However, in pediatric patients with cervical lateral bending, Bland–Altman analysis showed that the ETT size was over estimated by ultrasound (bias = 0.73 mm).
MRI is considered the gold-standard method for evaluating the larynx. High-quality images of the cricoid cartilage can be acquired by MRI and the cricoid cartilage diameter can be accurately measured. Therefor, in order to ascertain the cause of this result, we reviewed the MRI of the patients with cervical lateral bending. We found that the cricoid cartilage of the pediatric patients with cervical lateral bending was rotary(Fig. 5). Previous research shown that the rotation of the centrum can produce displacement or rotation of the mainstem bronchi[9, 10]. Thefore, we speculate that the rotation of the cricoid cartilage results from the deviation or rotation of the cervical vertebra.
Under normal circumstances, the cricoid cartilage is elliptical, and the transverse diameter is smaller than the anteroposterior diameter. When measuring the transverse diameter of the cricoid cartilage by ultrasound, the probe was positioned on the anterior side of the neck, and the transverse air-column diameter measured by ultrasound was considered for estimating the cricoid cartilage diameter. The rotation of the cricoid cartilage can make the air-column measured by ultrasound broader, resulting in the measurement of cricoid cartilage diameter was larger, and the ETT size was over estimated(Fig. 6). In our study, we found that the ETT size predicted by ultrasonography was larger than the bestfit ETT size in pediatric patients with cervical lateral bending. Therefore, compared to what was predicted by ultrasonography, pediatric patients with cervical lateral bending need a smaller sized ETT.
The present study has some limitations. First, the sample size was not equally distributed among groups. Thoracic scoliosis has the highest proportion of children with scoliosis. Second, we did not include children with kyphosis, throat disorder and anticipated difficult airway. These children require additional airway assessment. Third, we did not investigate the incidence of respiratory complications such as post-extubation stridor and laryngospasm. All these aspects need further investigation.