Factors Misleading the Estimation of the Tracheal Tube Size for Children

In order to evaluate the validity of formulas for estimating tracheal tube size, we compared actual tube size with the calculated size from a formula based on age in 623 children over 1 year (tube size (mmID)=age(y)/4+4) and a formula based on height in 877 infants and children (tube size (mmID)=height(cm)/30+1 for infants under 1 year, tube size (mmID)=height/20 for children 1 year and older). In 2 cases in which the actual tube size deviated by 1 mm or more from the estimated tube size, the possible causes were investigated. Estimates from the height-based formula tended to be smaller than the actual size, while those from the age-based formula tended to be larger. Tubes at least 1 mm smaller than estimated by the height-based formula were used in 2 cases with micrognathia and repaired cleft palate. Similar deviations from the age-based formula were noted in 6 children with short stature and low weight. In one child with repaired esophageal atresia, short stature and low weight, the actual size was 1 mm smaller than both calculations. In 2 children in whom the actual tube sizes were at least 1 mm larger than the tube sizes estimated from the height-based formula, no obvious causes were found.