After doing routine pre-anaesthetic evaluation ,Paediatric patients undergoing surgery under general anaesthesia with ET Tube intubation will be enrolled. Patient will be kept NPO for solids 8hrs, liquids 2hrs prior to surgery. General anaesthesia will be initiated by the attending anaesthesiologist. After loss of  consciousness, bag and mask ventilation will be confirmed and weight based dose of long acting muscle relaxant will be given. The patient will be ventilated for 4minutes while performing the sonography.GE LOGIQ P7 machine with high resolution linear probe (4.5-13 MHz) in B mode will be used for scanning.  With head in extension and neck flexed (sniffing position) initial scanning will be done in sagittal section in the midline immediately above supra-sternal notch upwards. This is to identify the tracheal rings, and cricoid cartilage. At the level of cricoid cartilage, the probe will be rotated transversely with marker pointing towards right side. The  vocal cords are identified in transverse section  by moving the probe upwards. The probe is then moved again downwards to identify the sub glottis immediately below the vocal cords.  Transverse air column diameter will be measured at the cephalic half of cricoid cartilage taken as subglottic diameter. This is the outer diameter of the ETT that can be used for the patient.

Cuffed ETT OD = Subglottic diameter – 1mm

Uncuffed ETT OD = Subglottic diameter

The ETT will be chosen by matching the measured OD with manufacturers OD. Time taken to decide the size of the ETT from the time of scanning till matching the OD will be noted. Successful first pass of the ETT will be observed. After intubation, cuff pressure will be inflated to 15-20 cmH2O and audible air leak with stethoscope will be noted. Leak occurring at <10cmH2O ETT will be exchanged with 0.5mm smaller size ETT, Leak occurring at >20cm H20 will be exchanged with 0.5mm larger size ETT.

 Similarly, circuit pressure equilibration will be checked. Closed circuit will be connected to the ETT and flow rate will be  set to 6L/min with a closed adjustable pressure limiting valve closed to 30cmH2O allowing the circuit to fill up and pressurize to a circuit pressure of 30cm H2O. Then the flow meter will be closed fully and fall in pressure will be noted for 10 seconds , when the pressure equilibrates to a new value at the end of 10 seconds .If it equilibrates to 10 to 20 cm H2O after 10secs ETT will be considered appropriate. ETT will be considered large if the circuit pressure equilibrates at >20 cm H20 and ETT size will be considered small if the circuit pressure equilibrates to <10 cm H20 after 10 secs and will be changed accordingly by 0.5mm smaller or larger respectively. Adequate ventilation will be confirmed. The final size of the ETT will be compared with age based formula.