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GENERAL ARTICLES

Lengthening of the Trachea During Neck Extension: Which Part of the Trachea Is Stretched?

David T. Wong, MD^*, Hao Weng, MD, Eunice Lam, Hai-Bao Song, MD^£, and Jin Liu, MD^£

From the *Department of Anesthesiology, University of Toronto, Canada; Department of Anesthesiology, Dongfeng General Hospital, Yunyang Medical College, No.10 Da-Ling-Lu street, Shiyan, Hubei, 442008, People’s Republic of China; and Student, Health Science Program, McMaster University, Hamilton, Canada; £Department of Anesthesiology, West China Hospital, Sichuan University, Chengdu, Sichuan, People’s Republic of China.

Address correspondence and reprint requests to David T. Wong, MD, Department of Anesthesiology, Toronto Western Hospital, University of Toronto, MC2-405 Toronto, Ontario M5T 258, Canada. Address e-mail to david.wong{at}uhn.on.ca.

	Abstract

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BACKGROUND: We sought to determine the distances of the three segments of the airway from upper incisors to carina in intubated patients in three different neck positions.

METHODS: Twenty patients undergoing elective surgery were studied. The airway was divided into three segments: upper incisor to vocal cords (UI-VC), vocal cords to sternal notch (VC-SN), and sternal notch to the carina (SN-CA). After general anesthesia and tracheal tube placement, the circuit was connected and the lungs ventilated. A bronchoscope was inserted through a ported elbow adapter until the tip just contacted carina. A marker tape was placed on the bronchoscope immediately above the adapter port. As the bronchoscope was withdrawn to the sternal notch (by transillumination), vocal cords and upper incisor (endoscopic visualization), three corresponding markers were placed along the bronchoscope. The three segments of the airway were obtained by measuring the distances between the four markers. Measurements were taken with the patient’s neck in flexion (10 cm pillow), neutral (5 cm pillow), and extension (no pillow) positions. Repeated measure analysis of variance and paired t-tests were used for analysis of the data.

RESULTS: The UI-VC, VC-SN, and SN-CA distances were 12.01 ± 1.49, 5.37 ± 0.95, 8.24 ± 1.16 cm. From neck flexion to extension, UI-VC and VC-SN increased by 0.36 ± 0.68 cm (P = 0.027) and 1.74 ± 0.48 cm (P < 0.001) respectively; SN-CA decreased by 0.12 ± 0.70 cm (NS). Overall, UI-CA increased by 1.99 ± 0.70 cm (P < 0.001). SN-CA represented 64%, 61%, 56% of the VC-CA distance with the neck in flexion, neutral, and extension respectively. SN-CA did not change significantly among the head positions (NS).

CONCLUSIONS: From neck flexion to extension, the UI-CA distance increased by 1.99 cm. The major contribution to this lengthening was an increase of the VC-SN distance by 1.74 cm; UI-VC increased by 0.36 cm whereas SN-CA did not change significantly. Averaging the three neck positions, SN-CA represented 60% of the VC-CA distance. Our findings may explain why tracheal tubes fixed at the mouth ascend in the trachea with neck extension.

	Introduction

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Studies have shown that in intubated patients, neck flexion results in movement of the endotracheal tube towards the carina, and extension displaces the endotracheal tube cephalad towards the vocal cords.1–5 Endotracheal tube movement and tracheal length alteration during neck adjustments have been demonstrated through chest radiography1,3,4 and fiberoptic bronchoscopy.2,5 Toung et al. and Kim et al. reported that the distance from the vocal cords to carina increases with neck extension.1,5 However, it is still unclear which segment of the airway is lengthened or shortened. To elucidate the mechanism of endotracheal tube displacement with neck movement, it is important to measure the distances of different segments of the airway while the neck position is altered.

The sternal notch has been considered to be located approximately at mid-trachea, halfway between the vocal cords and carina.6,7 It has been used as an anatomical landmark to determine endotracheal tube placement through suprasternal notch palpation, a technique used primarily in emergency airway management, and neonatal care.6,8,9 Furthermore, the sternal notch is also used as a landmark in transillumination intubation techniques.7 Hung and Stewart recommended that the endotracheal tube tip be placed by transillumination techniques just below the sternal notch, which was considered to be near mid-trachea. However, there are little data to show that the sternal notch is a reliable marker of the mid-trachea.

The purposes of the study were (a) to determine the distance of the three segments of the airway through bronchoscopy, from upper incisors to carina, in intubated patients with the neck in flexed, neutral, and extended positions, and (b) to determine if the sternal notch was a reliable surface landmark as the midpoint of the trachea.

	METHODS

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This study was approved by the IRB, and written, informed consent was obtained from each patient. Twenty consecutive ASA I-II adult patients undergoing elective surgery that required general anesthesia and tracheal intubation were entered into the study. The patients with anatomical abnormalities of the face, neck, upper airway, cervical spine, Body Mass Index >30, or coagulopathy were excluded from the study. Patients’ demographic, medical, and surgical data were recorded preoperatively. All patients were premedicated with IM injection of atropine 0.5 mg and phenobarbital 0.1 g 30 min prior to surgery.

Patient monitoring included electrocardiography, noninvasive arterial blood pressure and pulse oximetry. Anesthesia was induced with midazolam 0.05–0.1 mg/kg, fentanyl 3–5 µg/kg, propofol 1–2 mg/kg and vecuronium 0.1 mg/kg, and was maintained with fentanyl, vecuronium, and isoflurane. Using a MAC 3 or 4 laryngoscope blade, tracheal tubes of ID 8.0 mm were inserted for male patients, whereas ID 7.5 mm tracheal tubes were used for female patients. After intubation, the tracheal tube was fixed firmly with tape at the patient’s mouth at the level of the upper incisors.

The airway was divided into three segments: upper incisors to vocal cords (UI-VC), vocal cords to sternal notch (VC-SN), and sternal notch to carina (SN-CA). Sternal notch was defined as the midpoint between the superior borders of the clavicles. The distances of the three airway segments were determined by placing markings on the fiberoptic bronchoscope (Olympus BF-P 40, outer diameter 4.9 mm; Olympus Optical Co, Tokyo, Japan) as described below.

The patient’s neck was kept in an anatomically neutral position with the head resting on a head ring with a height of 5 cm. The head rings were custom-made in the hospital. An elongated sponge sheet was rolled into a straight tube-shaped structure, which was then bent to form a donut-shaped ring of 15 cm in outer diameter and a height of 5 cm. A thin layer of flannel 5 cm in diameter was circumferentially wrapped around the sponge donut to form the head ring. The midpoint between the superior borders of the clavicles was marked using a pen at the sternal notch. With the room lights dimmed, the fiberoptic bronchoscope was inserted through a modified right angle elbow ported connector (Opti-Port^TM, Mallinckrodt, Dublin, Ireland), which allowed mechanical ventilation to continue while bronchoscopy was performed. The bronchoscope was inserted until the distal end was seen to just contact the carina (Fig. 1, left). A 1/4 in. marking tape was placed on the bronchoscope immediately above the bronchoscope port of the elbow connector. The operating room lights were shut off while one radiograph box light was left on. The bronchoscope was then withdrawn until the leading edge of its transillumination was externally visualized at the sternal notch mark; the second marking tape was placed on the bronchoscope. The bronchoscope was then further withdrawn until the vocal cords were just visible via the endoscope through the walls of the endotracheal tube (Fig. 2); the third marking tape was placed. Finally, the bronchoscope was withdrawn until the upper incisors (or gingival margin if the patient was edentulous) were just visible via the endoscope through the walls of the endotracheal tube; the fourth marking tape was placed (Fig. 1, right). The distances between the four markings along the bronchoscope (UI-VC, VC-SN, SN-CA) were measured using a ruler.

View larger version (14K): [in this window] [in a new window]   Figure 1. Left: The bronchoscope was inserted until the distal end just contacted the carina; a tape (marked CA) was placed on the bronchoscope immediately above the bronchoscope port of the elbow connector. Right: The bronchoscope was withdrawn until the upper incisors were just seen via the endoscope; the fourth marking tape (UI) was placed. UI = upper incisor, VC = vocal cords, SN = sternal notch, CA = carina.

View larger version (172K): [in this window] [in a new window]   Figure 2. The visualization of the vocal cords through the wall of the endotracheal tube. A = the radio-opaque marker of the endotracheal tube, B = vocal cords, C = the anterior commissure of the vocal cords.

The above procedure was repeated with the neck in flexion, in which the head rested on a custom-made head ring with a height of 10 cm; and in extension, in which the head ring was removed and the head extended. Thus, we were able to measure the distances of UI-VC, VC-SN, and SN-CA in the three neck positions. The changes in the segmental distances among the three positions were calculated, as was the SN-CA to VC-CA ratio, which provided an indication if the sternal notch was located at mid-trachea. All measurements were performed by one anesthesiologist with experience in bronchoscopy.

Statistical analysis was performed with SPSS 10.0 for Windows (SPSS, Chicago, IL). Repeated-measures analysis of variance and paired t-test were performed among all three neck positions and among flexion and extension respectively for each of the following segmental distances: UI-VC, VC-SN, SN-CA, VC-CA, and UI-CA. A P value of <0.05 was considered statistically significant. To determine whether the sternal notch was located at mid-trachea, SN-CA was divided by VC-CA in each of the three neck positions.

	RESULTS

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Fourteen man and six women patients of Chinese origin were studied. Patients’ age was 43 ± 16 yr, height 163.25 ± 8.45 cm, and weight 54.40 ± 10.43 kg. The distances of UI-VC, VC-SN, and SN-CA measured in the three neck positions are shown in Table 1. The mean ± sd. UI-VC, VC-SN, and SN-CA distances of the three neck positions were 12.01 ± 1.49, 5.37 ± 0.95, 8.24 ± 1.16 cm respectively (Table 1). The changes in segmental distances when the neck position was changed from flexion to extension are shown in Table 1. UI-VC and VC-SN increased significantly by 0.36 ± 0.68 cm (P = 0.027) and 1.74 ± 0.48 cm (P < 0.001) respectively, and SN-CA decreased by 0.12 ± 0.70 cm (P = 0.453). Overall, the VC-CA and UI-CA distances increased significantly by 1.62 ± 0.69 and 1.99 ± 0.70 cm respectively from neck flexion to extension (P < 0.001). The change of VC-SN accounted for 87.44% of the lengthening of the airway (UI-CA) (Fig. 3).

View larger version (41K): [in this window] [in a new window]   Figure 3. The vocal cords to sternal notch distance varied the most among the three parts of the airway when the position of the neck was changed. UI-VC = distance between upper incisor teeth and vocal cords; VC-SN = distance between vocal cords and the midpoint of the superior borders of both clavicles; SN-CA = distance between the midpoint of the superior borders of both clavicles and carina. *P < 0.05 comparing flexion to extension.

SN-CA represented 64%, 61%, 56% of the VC-CA distance in neck flexion, neutral, and extension positions respectively. The mean SN-CA distance of the three positions was 8.24 ± 1.16 cm. SN-CA did not change significantly among the three positions of the neck (P = 0.936).

	DISCUSSION

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Our study shows that the length of the airway from upper incisors to carina in intubated patients increased by 1.99 ± 0.70 cm from neck flexion to extension. Most of the airway length alteration was accounted for by the increase of VC-SN distance. SN-CA distance represented, on average, 60% of the trachea distance in the three neck positions.

Many studies have demonstrated the ascending and descending movement of the endotracheal tube in intubated patients during neck extension and flexion respectively.1–5 The ascending tube movement from a neutral to extended neck position ranged from 0.9 to 2.0 cm,2–4 whereas the descending tube movement from a neutral to flexed neck position ranged from 1.5 to 2.8 cm.2–5 However, the mechanism of endotracheal tube displacement with neck movement is not clear. Our results showed that from neck flexion to extension, the UI-VC distance, the trachea (VC-CA), and the overall airway (UI-CA) lengthened by 0.36 ± 0.68, 1.62 ± 0.69, and 1.99 ± 0.70 cm respectively. Moving from a neutral to extended neck position, we found tracheal elongation of 1.01 ± 0.57 cm and airway elongation of 0.65 ± 0.60 cm. Only two studies have assessed the alterations of airway length with neck movements.1,5 Toung et al. studied the alteration of the airway during neck adjustments using computed tomography scans in a single intubated adult.1 They showed on sagittal computed tomography lengthening in two segments of the airway moving from neck flexion to extension: lips to vocal cords and vocal cords to carina. However, the exact change in tracheal length during neck alteration was not measured. Toung et al. reported an increase of 2.2 cm in the lips to vocal cords segment, while we found a smaller increase of the UI-VC distance (0.36 cm). The different methods used to determine the changes in airway length might have contributed to these differences. Computed tomography was used in Toung et al.’s study, and fiberoptic bronchoscopy was used in our study. Furthermore, the lips to vocal cords distance measured on the computed tomography scan was determined by summing two components: distance from lips to oropharynx (measured in a straight line parallel to the upper teeth), and oropharynx to vocal cords (measured in a straight line). In our study, the UI-VC distance measured by bronchoscopy followed the curved contour of the endotracheal tube. In addition, different degrees of flexion and extension might have been used. Toung et al. did not specify the degree of neck angulations, whereas we defined the flexion position as the head resting on a 10 cm high head ring, and extension position with the head ring removed. Lastly, measurements in Toung et al. were taken from a single patient only.

Kim et al. measured the change in tracheal length from neutral to full neck extension in children scheduled for elective surgery under general anesthesia.5 Bronchoscopy was used to assess the tracheal length alteration. However, the UI-VC distance was not measured. Kim et al. found the tracheal length increased by 0.95 ± 0.43 cm moving from neutral to extended neck position as compared to 1.01 ± 0.57 cm from our study. We had anticipated a larger difference in our results because our study population comprised adults as opposed to children 2–8 yr in Kim et al.’s study. Different degrees of angulations used might have contributed to this discrepancy; Kim et al.’s full neck extension position was at 49 ± 5 degrees, whereas our extension position was defined as full extension with the head ring removed. Different measurement techniques were also used. Kim et al. used the method of Hartrey and Kestin to determine the distance of the trachea, in which the fiberoptic bronchoscope and endotracheal tube were withdrawn from the trachea as one unit.2

The change in the length of the airway seems to be a logical explanation for the reported endotracheal tube movement with various neck positions. With the proximal end of the tracheal tube anchored at the mouth level and the airway lengthening during neck extension, it is a logical consequence for the tracheal tube to ascend and move away from the carina. Similarly, the endotracheal tube will descend towards the carina during neck flexion due to the shortening of the airway. During neck extension, as the endotracheal tube moves away from the carina, the tracheal tube cuff may press against the vocal cords and potentially cause damage.10 If further movement occurs, accidental extubation may result.10 During neck flexion, with the endotracheal tube moving towards the carina, accidental bronchial intubation may occur, leading to hypoxemia.10

Although previous studies have suggested that the sternal notch is located at mid-trachea,6,7 our results suggested that it was not halfway between the vocal cords and carina in any of the three neck positions. We found the mean SN-CA distance to be 8.24 ± 1.16 cm, representing 60% of the trachea distance when averaging the three head positions. Hung and Stewart 7 and Stewart et al.6 indicated that using lighted stylet intubation, the endotracheal tube was optimally placed when the tranillumination which matched the position of the tracheal tube tip was seen at the sternal notch. Stewart et al. suggested using the sternal notch as a landmark in order to position the endotracheal tube tip at the recommended guideline of 5 ± 2 cm above the carina with the neck in a neutral position.11 Our findings suggest that the sternal notch may not be a reliable anatomical point of reference and that positioning the tracheal tube tip at the sternal notch by radiograph or tranillumination method may, in fact, be too high in the trachea.

Our study has several limitations. First, the number of subjects in the study is relatively small and type II errors are possible. However, the P values for the VC-SN and the UI-CA in the three positions of the head and neck were highly significant, indicating that the sample size was likely adequate. Second, the population in the study was relatively thin in physical stature and quite homogeneous in their racial background. The result may not be generalizable to a more diverse North American population. Third, although transillumination of the sternal notch using bronchoscopy was feasible in all subjects in the study, it may not be usable in obese subjects or patients with thickened neck soft tissue.

In conclusion, the airway lengthened by 1.99 ± 0.70 cm and VC-SN distance lengthened by 1.74 ± 0.48 cm from neck flexion to extension. To our knowledge, this is the first study to show that the main contribution to airway lengthening was VC-SN elongation. In contrast, SN-CA distance did not change significantly among the three neck positions. Our findings may explain why tracheal tubes, affixed at the mouth, ascend in the trachea that was lengthened with neck extension. SN-CA distance represented 60% of the tracheal length, suggesting that it may not be located at the midpoint of the trachea.

	Footnotes

 
Accepted for publication April 24, 2008.

Supported by Department of Anesthesiology, West China Hospital Sichuan University, People’s Republic of China.

David T. Wong is currently at Department of Anesthesiology, University of Toronto, Toronto, Canada.

	REFERENCES

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1. Toung TJ, Grayson R, Saklad J, Wang H. Movement of the distal end of the endotracheal tube during flexion and extension of the neck. Anesth Analg 1985;64:1030–2[Free Full Text]
2. Hartrey R, Kestin IG. Movement of oral and nasal tracheal tubes as a result of changes in head and neck position. Anaesthesia 1995;50:682–7[Web of Science][Medline]
3. Conrardy PA, Goodman LR, Lainge F, Singer MM. Alteration of endotracheal tube position: flexion and extension of the neck. Crit Care Med 1976;4:7–12[Medline]
4. Saito S, Dohi S, Naito H. Alteration of double-lumen endobronchial tube position by flexion and extension of the neck. Anesthesiology 1985;62:696–7[Web of Science][Medline]
5. Kim JH, Ro YJ, Min SW, Kim CS, Kim SD, Lee JH, Bahk JH. Elongation of the trachea during neck extension in children: implications of the safety of endotracheal tubes. Anesth Analg 2005;101:974–7[Abstract/Free Full Text]
6. Stewart RD, LaRosee A, Kaplan RM, Ilkhanipour K. Correct positioning of an endotracheal tube using a lighted stylet. Crit Care Med 1990;18:97–9[Web of Science][Medline]
7. Hung OR, Stewart RD. Lightwand intubation: I-A new lightwand device. Can J Anaesth 1995;42:820–5[Web of Science][Medline]
8. Pollard RJ, Lobato EB. Endotracheal tube location verified reliably by cuff palpation. Anesth Analg 1995;81:135–8[Abstract]
9. Jain A, Finer NN, Hilton S, Rich W. A randomized trial of suprasternal palpation to determine endotracheal tube position in neonates. Resuscitation 2004;60:297–302[Web of Science][Medline]
10. Cavo JW Jr. True vocal cord paralysis following intubation. Laryngoscope 1985;95:1352–9[Web of Science][Medline]
11. Goodman LR, Conrardy PA, Laing F, Singer MM. Radiographic evaluation of endotracheal tube position. AJR Am J Roentgenol 1976;127:433–4[Abstract]

This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a colleague Similar articles in this journal Similar articles in Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Permissions Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Wong, D. T. Articles by Liu, J. Search for Related Content PubMed PubMed Citation Articles by Wong, D. T. Articles by Liu, J. Related Collections Airway General