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Estimation of the Length of the Nares-Vocal Cord

Han, Dong Woo MD*†; Shim, Yon Hee MD*†; Shin, Cheung Soo MD*†; Lee, Youn-Woo MD*†; Lee, Jong Seok MD*†; Ahn, So Woon MD*

doi: 10.1213/01.ANE.0000149900.68354.33
General Articles: Research Report

The nasal route is preferred for fiberoptic intubation. Placing a lubricated endotracheal tube through the nostril can guide the fiberoptic scope towards the larynx. It would be helpful for optimal visualization of the vocal cord when the scope is passed through the endotracheal tube if the length of nares-vocal cord (NV length) could be predicted and the tip of the endotracheal tube could be placed close to the vocal cord. In this study we measured the NV length and examined the relationship between the NV length and various external measurements. Using a fiberoptic scope, the NV lengths were measured in 50 male and 45 female patients scheduled to undergo elective surgery under general anesthesia. In addition, the distances from the lateral border of the nares to tragus of the ear (NE distance) and to the angle of the mandible (NM distance) were measured. The age, height, and weight of all the patients were recorded. The NV length of the males was 18.3 ± 0.8 cm, and that of the females was 16.3 ± 0.7 cm. The relationship between the NV length and body height (P < 0.001, r = 0.755) and the NE distance (P < 0.001, r = 0.636) showed a significant correlation but NM distance did not (P = 0.075). The length of the NV cord can be predicted using the body height or the NE distance.

IMPLICATIONS: The length of the nares-vocal cord can be predicted using the body height or the distance from the lateral border of the nares to the tragus of the ear.

Department of *Anesthesiology and Pain Medicine and †Anesthesia and Pain Research Institute, Yonsei University College of Medicine, Seoul, Korea

Accepted for publication October 21, 2004.

Address correspondence and reprint requests to Jong Seok Lee, MD, Department of Anesthesiology and Pain Medicine and Anesthesia and Pain Research Institute, Yonsei University College of Medicine, CPO Box 8044, Seoul 120–752, Korea. Address e-mail to jonglee@yumc.yonsei.ac.kr.

The nasal route is preferred during fiberoptic tracheal intubation; it provides an easier view of the laryngeal opening as a result of decreased interference from the tongue. In addition, the gag reflex is less pronounced with nasal intubation than with oral intubation (1–4).

Placing the lubricated endotracheal tube through the nostril can guide the fiberoptic scope towards the larynx (1). When the endotracheal tube is inserted blindly through the nostril, the glottis can be more easily visualized as the tip of the endotracheal tube becomes closer to the glottis. Knowing the length of the nares-vocal cord (NV length) is helpful for optimal visualization of the vocal cord when the scope is passed through the endotracheal tube. Moreover, it can be applied for either a blind nasotracheal intubation or proper positioning of the tip of the endotracheal tube. However, there are few studies that have measured the NV length using a fiberoptic scope (5,6).

The aim of this study was to measure the NV length and examine the relationship between the NV length and various external body measurements.

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Methods

Ninety-five ASA physical status I–II patients (50 male and 45 female) aged 20–65 yr who required endotracheal intubation for general anesthesia were enrolled in this study. This study was performed after obtaining approval from our IRB and informed consent from each patient. Patients with rhinitis, bleeding diatheses, or anatomical defects of the face, neck, and upper airway were excluded from this study. The age, gender, height, and weight of all patients were recorded.

Glycopyrrolate 0.2 mg and midazolam 2.0 mg were administered IV before anesthetic induction. The patients were placed on the operating table with the head and neck in the neutral position. We examined the distances from the lateral border of the nares to the tragus of the ear (NE distance) and the angle of the mandible (NM distance) using a tape measure. After induction with thiopental and rocuronium, the patients were ventilated for 5 min using 2%–2.5% sevoflurane in 100% oxygen. A well lubricated fiberoptic bronchoscope (Olympus LF-P; Olympus Optical Company, Tokyo, Japan) was inserted into the most suitable nostril and advanced down to the vocal cords. The fiberoptic tube was marked with tape at the nares when the tip of the fiberoptic bronchoscope was positioned precisely between the vocal cords. After the bronchoscope was withdrawn, a ruler was used to measure the length between the mark and the tip of the bronchoscope and the length from the nares to the vocal cord was obtained. Orotracheal intubation was performed using laryngoscopy, and anesthesia was continued with 50% N2O/oxygen and sevoflurane.

The data according to gender were analyzed and compared using unpaired Student’s t-test. Linear regression analysis of the measured data was performed and the relationships between the NV length and height, NE distance, or NM distance were analyzed. A P value < 0.05 was considered significant.

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Results

The weight, height, NE distance, and NV length were greater in males than females. The age and NM distance of the males and females were similar. The mean NV length of the males was 18.3 ± 0.8 cm and that of females was 16.3 ± 0.7 cm (Table 1).

Table 1

Table 1

The correlations between the NV length and height (Fig. 1) or NE distance (Fig. 2) were significant (P < 0.001) and the regression lines were derived, but there was no significant correlation between the NV length and NM distance (P = 0.075). The correlation coefficient was higher in the NV length versus height (r = 0.755) than the length versus NE distance (r = 0.636). The formulae for these regression lines were the NV length (cm) = 0.113 × height (cm) − 1.245 or NV length (cm) = 1.158 × NE distance (cm) + 1.498.

Figure 1

Figure 1

Figure 2

Figure 2

None of the patients showed deoxygenation or epistaxis while we were examining the NV length. None of the patients developed a laryngospasm or bronchospasm.

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Discussion

According to this study, the relationship of airway lengths (the NV) to the body height or the NE distances are significantly correlated. The correlation coefficient for the NV length to the body height was higher than the NV length to NE distance. This suggests that it may be more accurate to estimate the length of the nasopharyngeal airway by the body height. As the body height may be obtained more easily than the NE distance, it might be used more routinely in clinical practice as an independent variable to estimate the nasopharyngeal airway length.

The proper length of the artificial nasal airway can be estimated as the distance from the nares to the meatus of the ear and that of the artificial oral airway from the lip to the angle of the mandible (7,8). Therefore, in this study we assessed the NE or NM distances as possible predictors of the nasopharyngeal length. Stoneham (5) evaluated the correlation between the nares-epiglottis distance and the distance from the tip of nose to the mandible angle or to the ear tragus, but there were no significant correlations. In this study, the lateral border of the nares was accepted as a measuring point instead of the tip of the nose, and the NV length could be predicted using the NE distance. The height of the nose may be individualized or show ethnic variations. All patients enrolled in our study were Oriental, whose nose usually has a bulbous nasal tip with lack of nasal tip projection compared to that of a Caucasian (9). Therefore, the lateral border of the nares is believed to be a more optimal point than the tip of nose.

There are two ways to enter the nasopharynx during fiberoptic intubation. A clinician can either place the lubricated endotracheal tube through the nostril to serve as a guide for the scope or place the scope in first to serve as a passageway for the endotracheal tube (10). We usually use the former method because sometimes the endotracheal tube has not advanced through the nostril after the fiberoptic scope had been passed through the trachea. In that case, the scope and endotracheal tube should be withdrawn, and it is advisable that the other nostril or a smaller endotracheal tube be used. A narrowing of the nasopharynx, which would not allow the passage of the endotracheal tube, could be discovered before investing the time and trouble to locate the trachea with the fiberoptic scope (6). It is often assumed that when the endotracheal tube is blindly inserted through the nostril, the closer the tip of endotracheal tube is to the vocal cord, the more easily the vocal cord is visualized; however, sometimes positioning the tip of the endotracheal tube too close to the glottis may prohibit the anesthesiologist from manipulating the tip of the scope and thereby make it more difficult to enter the tracheal lumen.

In addition, knowing the NV length can be useful for a blind nasotracheal intubation. Blind conscious nasal intubation is useful for an urgent intubation outside the operating room when mouth opening or neck movement is limited or prohibited (11). When a blind nasotracheal intubation is chosen, the tube is inserted until the maximum breath sounds can be heard, suggesting that the tube tip is just above the glottis (12–14). Anesthetized blind nasal intubation may also be attempted in apneic, paralyzed patients, but in this case there are no spontaneous breath sounds to aid placement. Estimating the length of the NV can be useful for predicting the optimal position.

Based on these results, the proper positioning of the tip of the endotracheal tube in patients for nasotracheal intubation is possible. The nasotracheal tube should be advanced until the external markings are 26 cm for men and 24 cm for women (3 cm more than for oral tubes) in adults (15), and the lengths of the vocal cord to the carina are 13 cm for men and 11 cm for women (16). The length from the carina to the tip of the endotracheal tube was calculated to be 5.3 cm for males and 3.3 cm for females because the average NV lengths of these results were 18.3 cm and 16.3 cm, respectively. However, it should be remembered that the range of the NV lengths of the 95 patients ranged from 14.7 cm to 20.0 cm. The recommended average positioning of the endotracheal tube for nasotracheal intubation, regardless of personal differences, may cause either endobronchial intubation or accidental extubation, particularly during neck flexion or extension. The formulae proposed in this study are useful for determining the optimal positioning of the endotracheal tube in adult patients who require nasotracheal intubation.

The fiberoptic bronchoscopic method in a clinical assessment of the airway length is simple and more accurate than that reported in other studies that used radiography. The position of the vocal cords can be easily and directly identified using a bronchoscope, which avoids the magnification effect of radiography (16,17). In this study, a fiberoptic scope was used to measure the NV length.

In conclusion, there were significant correlations between the NV and height or the NE distance.

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