Ultrasound measurement of subglottic diameter and an empirical formula for proper endotracheal tube fitting in children.

BACKGROUND: The appropriate endotracheal tube (ETT) size is commonly determined using age-based formula; ETT size determination based on ultrasound (US) measurement of subglottic diameter (SD), the narrowest portion of the paediatric upper airway, may provide a better method for accurate fit. We aimed to validate ETT size determination using US measures of SD before intubation to establish an empirical formula for ETT fitting based on SD and biographic parameters. METHODS: We included 215 children aged 1-72 months undergoing general anaesthesia. US was performed on the anterior neck to measure SD during mask ventilation under anaesthesia. Endotracheal intubation was performed with a cuffed ETT selected by age-based recommendation; the transverse outer diameter (OD) of the ETT within the trachea at the subglottis level (OD-ETT at SD) was measured. RESULTS: The OD-ETT at SD was correlated with the actual OD-ETT outside the trachea (R(2) = 0.635), showing the validity of ultrasonographic measurement; moreover, the US-measured SD revealed strong correlation with the actual OD-ETT (R(2) = 0.834). US-measured SD and biographic data (age, height and weight) showed little correlation in children less than 12 months but good correlation (age, height) in children older than 12 months (P < 0.01). CONCLUSIONS: US-measured OD-ETT at SD was in good agreement with the actual OD-ETT, suggesting that US-measured SD helps in choosing the appropriate ETT diameter for children. In children older than 12 months, the equation 'OD (mm) = 0.01 × age (months) + 0.02 × height (cm) + 3.3' may help select the appropriate ETT.