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