Comparative study on prediction of paediatric endotracheal tube size by ultrasonography little finger breadth and age based formula: A prospective observational study

Background

Ultrasound is said to be a reliable, safe and non-invasive modality for evaluation of upper airways and a useful tool for estimating the proper size of the ET tube. Transverse subglottic diameter measured by ultrasound may be used to choose the correct size of tracheal tube by predicting the outer diameter of ET tube. There are studies comparing ultrasonography with age-based formulas and ultrasonography with little finger breadth for ET tube estimation. But there are no studies comparing all three techniques. In this study we wanted to find the utility of ultrasound to predict ET tube size in paediatric population.

Methods

This was a prospective observational study involving 30 paediatric patients aged 6 months to 6 years undergoing elective surgery. Preoperatively, the breadth of the distal phalanx of little finger at level of distal inter-phalangeal groove of all patients was measured with the help of a calliper. Measurement of transverse subglottic diameter at cricoid level was performed using a portable ultrasound machine. An uncuffed ET tube with the OD close to the subglottic diameter measured by ultrasound was used to intubate the patient.

Results

Mean values for age and weight were 2.25 ± 1.87 and 11.10 ± 3.65, respectively. Mean values for USG guided MTDSA (Minimal Transverse Diameter of Subglottic Airway) diameter vs. O.D (Outer diameter) of ET tube were 6.51 ± 0.91 vs. 6.33 ± 0.88, respectively. Mean values for age-based formula size vs. I.D (Inner diameter) of ET tube were 4.35 ± 0.56 vs. 4.62 ± 0.64, respectively. Mean values for little finger breadth vs. O.D. of ET tube were 6.52 ± 1.04 vs. 6.33 ± 0.88, respectively. The scatter plot showed a correlation (r) between USG-guided MTDSA measurement and the OD of the ET tube used (r value = 0.95) and Bland-Altman analysis (mean bias of 0.16). The correlation between little finger breadth and OD of the ET tube showed a correlation (r) of 0.83 in the scatter plot. The mean bias shown by Bland-Altman analysis using the little finger breadth method was 0.17. The scatter plot between age-based formula and ID of ET tube showed a correlation (r) of 0.92, and Bland-Altman analysis showed a mean bias of 0.27. The highest level of correlation was observed between ultrasound measurement and OD of the ET tube used (r = 0.95). Least bias was found between ultrasound measurement and OD of ET tube used (−0.16).

Conclusions

Ultrasound predicted the correct tube size in 80 % of the population. Ultrasound is a better tool compared to conventional age-based formulas and the little finger breadth method in predicting the appropriate size of the ET tube in paediatric population.