Ultrasound for prediction of endotracheal tube size in the pediatric age group: Are we there yet?

Airway management remains the prime domain and expertise of anesthesiologists and difficult airway, a concern for them. This holds further significance in the pediatric population with evolving airway anatomy and physiology with growth. Ultrasound is being increasingly used for airway assessment and for selecting a proper-sized endotracheal tube (ETT). A meta-analysis by Gupta et al.[1] published in this issue concluded that ultrasound could accurately predict the ETT size required in the pediatric age group as opposed to normal age-based or height-based formulas. This is also associated with fewer reintubation attempts for changing an improper-sized ETT and can also mitigate the reintubation attempts with uncuffed ETT in the pediatric age group. Another study published recently also reported that airway ultrasound scans predicted the appropriate ETT size in 52 out of 53 participants compared to Cole’s age-based formula that accurately predicted appropriate ETT size in only 35 out of 53 participants (P < 0.001).[2]

Though the pooled evidence seems promising in favor of ultrasound assessment of tracheal diameter, we may still be far from the final conclusion. This is because of the heterogeneity of the studies involved in the analysis and a few other issues. The narrowest portion of the pediatric larynx is the glottis[3] or the subglottic region, and hence, it is the site of ultrasound measurement. Since the pliable and paralyzed glottic region offers minimal resistance when passing an ETT, the area of concern for damage during intubation is the unyielding portion at the cricoid cartilage (“functionally” narrowest portion of the pediatric upper airway). Most of the studies included in this meta-analysis have evaluated the subglottic transverse diameter or the minimal transverse diameter of the subglottic airway (MTDSA)[4,5] but not the diameter at the cricoid level. The measured airway dimensions could also be altered by tracheal stretching in sniffing position[6,7] compared to a neutral position[4,8] and correlation with ultrasound-measured diameters may be affected for cuffed versus an uncuffed ETT.[9] The manual ventilation with positive pressure can also be anticipated to lead to tracheal dilation affecting the baseline measurement. With respect to the age group, some of the recent evidences favor ultrasound assessment to be accurate in children aged <1 year[10] while others agree upon the older age group,[11] though both favor ultrasound to be a useful tool compared to age-based formula. There are still others who found that the ETT size by age-based formula strongly correlated with the size measured by ultrasound and thus could not justify the routine use of ultrasound for calculating ETT size for intubation in pediatric patients.[6] Similarly, equivocal results were reported by Makireddy et al.[12] who found no difference in the number of correct predictions of ETT size by US measurement, universal formula, and locally derived formula.

Another issue is the ETT size, which reflects the internal diameter of the tube, while the ultrasound measurements reflect the external diameter of the ETT that can be accommodated. Different manufacturers have variations in outer ETT diameter of up to 0.7 mm.[13,14] The formulas for determination of the size of the ETT required also differ among studies, using Cole’s age-based formula,[6] Penlington formula,[15] or the formulas proposed by Motoyama or Khine.[8]

The demographic parameters may also limit the result to a specific subset of the population. An additional point for research could be whether the two parameters be considered in isolation or as a mixed approach. A recent study found that both methods are highly sensitive and independent predictors and suggested the formula for the optimal ETT size prediction as ID = −0.091 + 0.814 (ID obtained by the US formula) +0.192 (ID obtained by Cole’s formula).[11]

The age-based formulas may still hold significance in resource-limited settings. Though the meta-analysis has brought evidence in favor of ultrasound assessment, considering the heterogenicity in studies—the majority being observational in nature—there is still a need for a well-designed multicentric randomized controlled trial (RCT) with a larger sample size. We may also extend the research domain to see whether the results hold true for anatomical variations as well, as in a study in pediatric patients with cervical lateral bending where the correlation was found to be moderate and an overestimation of ETT size by ultrasound.[16]