In pediatric anesthesia, unexpected displacement of the uncuffed endotracheal tube (ETT) caused by extension and flexion of the neck can result in serious complications, such as accidental extubation and endobronchial intubation (1,2). Accurate placement of the ETT to an ideal depth is thus of prime importance, and it is reasonable that the ideal depth of the ETT should be one that allows its displacement by movements of the head and neck while still avoiding extubation or endobronchial intubation.
The ETT displacement with extension or flexion of the neck has been described as an increase or decrease in the distance between ETT tip and carina in neonates, infants, and young children (2–5). The ETT tip has been known to move caudally with neck flexion and cephalad with neck extension. With the uncuffed ETT fixed at the lip, the distance between ETT tip and vocal cords will decrease as much as the increase in the distance that occurs between ETT tip and carina during neck extension, unless the tracheal length changes (Fig. 1). However, some previous studies demonstrated that the trachea might be lengthened during neck extension (6–8). Therefore, the changes in the distance between ETT tip and carina may not be equal to the changes in the distance between ETT tip and vocal cords during neck extension. It would be reasonable that the changes in the distance between ETT tip and vocal cords during neck extension should be considered to assess the risk of the extubation in children, as are the changes in the distance between ETT tip and carina to assess the risk of endobronchial intubation during neck flexion. Despite this, there has been no evaluation of the effect of neck extension on the tracheal length and the distance between ETT tip and vocal cords in children.
The purpose of this investigation was to evaluate the degree of tracheal elongation and to verify the changes in the distance between ETT tip and vocal cords during extension using fiberoptic bronchoscopy in children.
The study protocol was approved by the IRB of Seoul City Boramae Hospital. Twenty-five children aged from 2 to 8 yr were enrolled in the study after providing informed parental consent. All patients were scheduled for elective surgery under general anesthesia. Patients with pulmonary diseases such as bronchial asthma, abnormal breath sounds, and malformations of trachea or bronchus on chest radiograph were excluded from the study. Patients having difficulties in neck flexion or extension were also excluded. Anesthesia was induced with oxygen, nitrous oxide, and sevoflurane. Orotracheal intubation was performed after IV administration of vecuronium 0.1 mg/kg. The ETT (Contour ™, Mallinckrodt, Dublin, Ireland) was positioned by deliberately intubating the patient’s right main bronchus and then withdrawing the ETT 3 cm into the trachea (9). After ETT positioning, the ETT length was measured at the upper incisor teeth and secured to the upper lip.
The patients were kept in an anatomically neutral position during bronchoscopic evaluation. Heart rate, respiration, arterial blood pressure, and pulse oximetry were monitored. A fiberoptic bronchoscope (Olympus LF-P, outer diameter 2.2 mm; Olympus Optical Co, Tokyo, Japan) was inserted through a modified right angle connector (Opti-Port ™, Mallinckrodt), which allowed the procedure during mechanical ventilation. When the tip of the fiberoptic bronchoscope touched the carina, the first mark was made on the bronchoscope corresponding to the port of the connector. Then the bronchoscope was withdrawn until the ETT tip was visualized, and the second mark was made on the bronchoscope. The distance between these two marks on the bronchoscope corresponded to the distance between ETT tip and carina (10). The distance between ETT tip and carina was also measured after full extension of the neck.
Finally, with the head and neck in the neutral position, the tracheal length was measured using the method of Hartrey and Kestin (11). This procedure was performed by withdrawing the bronchoscope and ETT as one unit from the trachea, starting with the bronchoscope at the carina and stopping when the vocal cords were just visible through the bronchoscope. At this point, a mark was made on the ETT at its point of exit from the upper lip, and the amount of tube removed represented the tracheal length. Then, the tracheal length was measured again with the neck in full extension to measure the changes in the tracheal length after full extension. Each procedure lasted no more than 30 s. The distance between ETT tip and vocal cords was calculated as the tracheal length minus the distance between ETT tip and carina (Fig. 1).
The number of patients required to demonstrate an elongation of the tracheas in children was calculated with the assumption that a 10% elongation would be clinically relevant. Based on a statistical power of 0.8, α level of 0.05, and β of 0.2, 21 patients were suggested. Twenty-five patients were studied to account for methodological difficulties that could have led to exclusion from the study. Statistical analysis was performed with SPSS 10.0 for Windows (SPSS, Chicago, IL). Data were analyzed by paired Student’s t-test. Statistical significance was established at P < 0.05. Data are represented as mean ± sd.
The demographic data of the 25 patients included in this study are shown in Table 1. After full extension of the neck (49 ± 5 degrees), the distance between ETT tip and carina increased by a distance of 2.02 ± 0.58 cm (Table 2).
The tracheal length was 7.97 ± 0.85 cm in the neutral position. After full extension, the tracheal length was 8.92 ± 0.90 cm, which was significantly longer than that in the neutral position (P < 0.05). The trachea was elongated by 0.95 ± 0.43 cm, 12.1% of the tracheal length in the neutral position (Table 2).
The change in the distance between ETT tip and vocal cords after full extension was −1.08 ± 0.47 cm. It was equivalent to 52.9% of the change in the distance between ETT tip and carina (Table 2).
The reported lengths of the trachea in the pediatric age group are highly variable. The tracheal length in children four to six years of age has been reported as 5.6, 6.4, and 7.2 cm, respectively (7,12,13). In the present study, the tracheal length of this age group was 7.97 cm, which is longer than the results of the previous studies. Our results were obtained from the actual displacement of the ETT from the carina to vocal cords, which was guided by fiberoptic bronchoscope. This measurement can avoid some potential problems associated with radiologic assessments of the trachea, such as radiation exposure, high costs, inaccurate image, or magnification of the field (10,14). The reason why the results are different is not clear. However, there may be a gross discrepancy between measurements at autopsy and those in living children. In addition, the children were mechanically ventilated after muscle relaxation in the present study, whereas some children breathed spontaneously only with sedation in the previous studies. These facts might also contribute to the discrepancy. Interestingly, Fearon and Whalen (15) reported that the tracheal length of children between the ages of 12 and 18 months was 8.1 cm. This result is similar to the tracheal lengths we found in children two to eight years of age. The similar tracheal lengths observed in Fearon and Whalen’s younger patients may have been secondary to tracheal stretching caused by hyper-extension of the neck during the rigid bronchoscopic examination. In addition to our study, previous studies have also suggested the possibility of tracheal lengthening during neck extension (6–8). Toung et al. (6) demonstrated evidence of increase in the tracheal length in an adult using computed tomography scans. However, the data have never been reported as to the degree of tracheal lengthening during neck extension in children. In the present study, the fiberoptic bronchoscopy revealed that the trachea was elongated by 0.95 cm (12.1% of the tracheal length) during full extension of the neck in older children.
ETT can be displaced by neck extension, flexion, and head rotation causing accidental endobronchial intubation or tracheal extubation (1,2,5,11,14). Sugiyama and Yokoyama (2) reported in children 16–19 months of age that full flexion displaced the ETT tip 0.9 cm toward the carina, whereas full extension displaced the ETT tip 1.7 cm toward the vocal cords. The results after full extension, however, represent displacements of the ETT tip from the carina, not actual displacements of the ETT tip to the vocal cords, because they were obtained only by measuring the changes in the distance between ETT tip and carina after neck extension. Unlike the previous studies, the present study considered the tracheal elongation to assess actual displacements of the ETT tip to the vocal cords after extension. The results revealed that the distance between ETT tip and vocal cords decreased by 1.08 ± 0.47 cm after full extension and that the actual displacements of the ETT tip to the vocal cords was 52.1% of the displacement of the ETT tip from the carina (2.02 ± 0.58 cm). The reduction of the ETT displacement to the vocal cords was caused by the tracheal elongation during neck extension. Of note, our results revealed that the ETT tip actually moved to the vocal cords despite the tracheal elongation during neck extension. Consistent with the previous report (6), these results also clarify that neck extension may risk accidental extubation when the ETT is not in the proper position. In general, the more the ETT tip is out, the easier it is to extubate. However, the result of neck extension depends on multiple factors such as an initial placement of ETT tip, increase in the distance from incisors to vocal cords, and whether the tracheal elongation is even or uneven along the tracheal length. Therefore, these would be areas of future study to assess the risk of extubation of the ETT during neck extension in children.
In conclusion, tracheal length was increased by approximately 1 cm during full extension of the neck in older children. Despite the tracheal elongation, neck extension actually displaced the ETT tip to the vocal cords, increasing the risk of extubation.
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