The aim of this study will be to explore the clinical value of ultrasonic monitoring in the diagnosis of anesthesia-induced atelectasis, the assessment of the effects of lung recruitment, the best positive end-expiratory pressure (PEEP) after RM and in the detection of the point of lung re-collapse after RM in pediatric patients undergoing lengthy microsurgery operations using two levels of intraoperative FIO2 (0.5 VERSUS 0.3).

• To maximize the benefits, minimize the drawbacks and assess the adequacy of the recruitment maneuver; adequate monitoring at the bedside is essential. Several methods have been proposed, including measuring end-expiratory lung volume or pulmonary compliance, volumetric capnography, oxygenation indices, electrical impedance tomography, computerized tomography and lung ultrasound.
• For lung CT examination patients must be transported out to the radiation unit, which carries risk of transfer, high cost, and radiation exposure. The oxygenation method which is the most commonly used, but it is necessary to repeat arterial blood collection many times, which is cumbersome and expensive.
• Using lung ultrasound (LUS) as real-time guidance during the recruitment maneuver has gained popularity recently owing to its' easy applicability and non-invasive nature. Sonography is a radiation-free methodology which plays an important role in diagnosing pulmonary diseases in children, including obstructive and compressive atelectasis of different origins. Furthermore, lung ultrasound has shown reliable sensitivity and specificity for the diagnosis of anaesthesia-induced atelectasis in children.
• LUS can identify children needing a recruitment maneuver to re-expand their lungs and help optimize ventilator treatment during anesthesia.

Inclusion Criteria:

• Age: 2-12 years old.
• ASA physical status classification I-II.
• Undergoing lengthy microsurgery operations
• and requiring endotracheal intubation and mechanical ventilation for more than 4 hours.-

Exclusion Criteria:

• ASA Physical status classification >II.

  • Thoracic or abdominal surgery.
  • Preexisting lung disease.
  • Pre-operative chest infection or abnormal chest US findings.
  • Cardiac patients.-

Ventilator adjustments, lung recruitment, PEEP titration and ultrasonographic assessments will be done by well-trained investigators not involved in data collection. The surgeon, the parents or legal guardians and the data collection personell will be blinded to the patients' group assignment.