Comparison of Age-Based Formula and Ultrasound-Guided Subglottic Diameter Measurement for Endotracheal Tube Sizing

To compare the accuracy of ultrasound-guided subglottic diameter measurement versus age-based formulas in determining optimal ETT size in pediatric patients with single ventricle physiology to avoid post-extubation complications. All legal guardians will provide written informed consent after a full explanation of the study. During the pre-anesthetic evaluation, demographic data, ASA physical status, relevant comorbidities, and a recent echocardiogram confirming single ventricle physiology will be documented. In the operating room, patients will be monitored using standard ASA monitors including ECG, non-invasive blood pressure, pulse oximetry, and capnography, followed by preoxygenation with 100% oxygen. If intravenous access is available, anesthesia will be induced with ketamine, fentanyl, and rocuronium. If IV access is not available, inhalational induction with sevoflurane will be used until IV access is secured, after which fentanyl and rocuronium will be administered. Following induction, ventilation will continue with FiO₂ 100% and sevoflurane 1.5% for 3 minutes. Patients will be divided into two groups: Group A (Age-Based Formula): In neonates and infants under 1 year, a weight-based sizing is used: * Infants \<1 kg (Preterm): 2.5 mm ETT * Infants 1-2 kg: 3 mm ETT * Infants 2-3 kg (Full Term): 3-3.5 mm ETT * Infants \>3 kg (3 months to 1 year): 3.5-4 mm ETT In Infants \> 1 year, ETT size will be selected based on the standard formula: Cole's Formula for uncuffed ETT. ETT ID (mm) = (Age/4) + 4. Group B (Ultrasound Group): During mask ventilation, a trained anesthesiologist will perform transverse subglottic diameter measurement using a high-frequency linear ultrasound probe (7-15 MHz). The patient will be positioned supine with mild neck extension. The probe will be placed transversely just below the thyroid cartilage to identify the subglottic airway at the level of the cricoid cartilage. The measured diameter (in mm) will be recorded and used to determine ETT size from the equation: ETT ID (mm) = Subglottic diameter (mm) × 0.8 To confirm appropriate ETT sizing, all patients will be placed on pressure-controlled mechanical ventilation and a leak test will be performed in both groups. Start with pressure 20-25 cm H₂O, if there is resistance to ETT passage into the trachea or no audible leak or ventilator-detected leak when the lungs are inflated, the tube will be exchanged for one that is 0.5 mm smaller. If a leak is detected, decrease the pressure to 10 cm H₂O. If there's still a detected leak at 10 cm H₂O, the ETT will be exchanged for one that is 0.5 mm larger. If there's no leak at 10 cm H₂O then the ETT is optimal. Tube size will be considered optimal when a tracheal leak is not detected at an inflation pressure between 10-20 cm H₂O with uncuffed tubes.