A formula to predict the correct size of the endotracheal tube (ETT) to be used in the paediatric patient was first proposed by Cole in 1957 and this is the most commonly used formula even today. Due to the wide variations in the size of children and the frequent inaccuracy of the calculated ETT size, other complex formulae using multiple variables such as age, height and weight were proposed, but did not gain wide use in clinical practice. Intubation of paediatric patients with an ETT that is too small may result in insufficient ventilation, poor reliability of end-tidal gas monitoring, leakage of anaesthetic gases into the operating room environment, and an enhanced risk of aspiration. In contrast, an ETT that is too large can cause upper airway damage (e.g., laryngeal oedema, stridor, local ischaemia, ulceration, granulation, scar formation) and potential for subsequent subglottic stenosis. Repeated laryngoscopies and reintubations may be needed to identify the appropriately sized tube. Some recent reports suggest that the diameter of the subglottic upper airway can be determined by ultrasound in young adults and paediatric patients. ETT size in paediatric population is traditionally determined by age related formulae. Altogether 22 formulae to estimate appropriate endotracheal tube size for paediatric patients (age 0-12 years) were identified: 12 age-based formulas for tubes without a cuff, 4 height-based formulas for tubes without a cuff, 2 weight-based formulas for tubes without a cuff and one multivariate formula for tubes without a cuff as well as 3 age-based formulas for cuffed endotracheal tubes.

Ultrasound has been used in adults to verify accurate placement of ETT but minimally used in paediatric population. Ultrasound can reliably measure the sub glottic diameter which corresponds to maximum outer diameter of ET tube that can be secured. This will also help in reducing wastage of ET tubes, eventually helping with reducing the economic burden and being environmentally friendly.