Choosing the adequate size of endotracheal tube(ETT) in pediatric patients is important in all anesthesiologists, pediatricians, and critical care specialists. Inadequate ETT size will results in soft tissue injury to trachea by means of the pressure, insufficient ventilation, and aspiration, therefore choosing the adequate ETT size has very important clinical aspect.

Choosing the adequate size of endotracheal tube(ETT) in pediatric patients is important in all anesthesiologists, pediatricians, and critical care specialists. Inadequate ETT size will results in soft tissue injury to trachea by means of the pressure, insufficient ventilation, and aspiration, therefore choosing the adequate ETT size has very important clinical aspect.

Subglottis is the narrowest portion of upper airway. Thus, ETT size should be determined according to the subglottic diameter. Several conventional methods are available to determine ETT size. These methods are based on the demographic data of patient, such as age, weight, height, but could not be customized for each patient. Ultrasonography (US) is one of the most accurate method to measure the subglottic diameter. Also, US is quick, easy to perform, and non-invasive measure. The investigators tried to measure the exact subglottic diameter with US and analyze the correlation of the diameter and demographical data. Furthermore, the investigators will try to develop the equation that could determine the adequate ETT size based on demographical data, instead of actually measuring the subglottic diameter using US.

Inclusion Criteria:

• children undergoing elective urological surgeries

Exclusion Criteria:

• children with anatomical airway problem or prematurely delivered children

University Hospital (Severance Hospital, Seoul, S.Korea)