A Formula for Estimating the Appropriate Tube Depth for Intubation

Abstract

An estimation of the appropriate tubing depth for fixation is helpful to prevent inadvertent endobronchial intubation and prolapse of cuff from the vocal cord. A feasible estimation formula should be established. We measured the anatomical length of the upper-airway tract through the oral and nasal pathways on cephalometric radiographs and tried to establish the estimation formula from the height of the patient. The oral upper-airway tract was measured from the tip of the incisor to the vocal cord. The nasal upper-airway tract was measured from the tip of the nostril to the vocal cord. The tracts were smoothly traced by using software. The length of the oral upper-airway tract was 13.2 ± 0.8 cm, and the nasal upper-airway tract was 16.1 ± 0.9 cm. We found no gender difference ( p > .05). The correlations between the patients' height and the length of the oral and nasal upper-airway tracts were 0.692 and 0.760, respectively. We found that the formulas (height/10) - 3 (in cm) for oral upper-airway and (height/10) + 1 (in cm) for nasal upper-airway tract are the simple fit estimation formulas. The average error and standard deviation of the estimated values from the measured values were 0.50 ± 0.66 cm for the oral tract and 0.39 ± 0.63 cm for the nasal tract. Thus, considering the length of the intubation marker of each product (DM), we would like to propose the length of tube fixation as (height/10) + 1 + DM for nasal intubation and (height/10) - 3 + DM for oral intubation. In conclusion, the estimation formulas of (height/10) - 3 + DM and (height/10) + 1 + DM for oral and nasal intubation, respectively, are within almost 1 cm error in most cases.

Keywords: Cephalogram; General anesthesia; Intubation tube; Larynx; Nasal intubation; Oral intubation; Tracheal.

Figure 1.

A cephalometric analysis of oral and nasal airway length. We employed 4 landmarks for oral airway and 5 landmarks for nasal airway. These landmarks were smoothly connected by using the DICOM viewer.

Figure 2.

Correlation between patient height and oral tract length. A strong correlation between patient height and the length of the oral tract was confirmed (r = 0.691; p > .01). No significant difference was seen between separate plots for men and women.

Figure 3.

Correlation between patient height and nasal tract length. A strong correlation between patient height and the length of the nasal tract was confirmed (r = 0.760; p > .01). No significant difference was seen between separate plots for men and women.

Figure 4.

Bland-Altman plots for the bias between the estimated and measured values of oral tract length. A significant fixed bias was observed (p < .05).

Figure 5.

Bland-Altman plots for the bias between the estimated and measured values of nasal tract length. No significant fixed bias or proportional bias was observed (p > .05).

Figure 6.

Intubation marker of the endotracheal tube. The distance to add to the airway length to help ensure appropriate tube depth (DM; distance between the distal edge of intubation guide mark and the tip).