Anesthesia-induced atelectasis is a well-known entity observed in approximately 68-100% of pediatric patients undergoing general anesthesia. The collapse of dependent lung zones starts with anesthesia induction but can persist for hours or even days after surgery.

Lung collapse is a pressure-dependent phenomenon. Each acinus has a critical closing pressure, i.e., the minimum transpulmonary pressure (Ptp) below that the acinus begins to collapse. While airway pressure is homogeneously distributed within all lung units, Pleural pressure increases along the vertical gravitational vector because of the lung's weight. As a consequence, the decreased Ptp in the dependent zones promotes collapse. This means that patients in the supine position suffer from increasing closing pressures in the ventral to dorsal direction.

Alveolar recruitment maneuvers recruit collapsed alveoli, increase gas exchange, and improve arterial oxygenation.

The investigators hypothesized that in children with anesthesia-induced atelectasis, postural changes have recruiting effects and improve lung aeration assessed by lung ultrasound.

Compare lung aeration between two different lung recruitment strategies (recruitment maneuvers in supine position and recruitment maneuvers with postural changes of lateral decubitus with 10 cmH2O level of positive-end expiratory pressure (PEEP) during 180 seconds; in pediatric patients scheduled for surgery under general anesthesia using ultrasound imaging and a four-point-aeration score to assess the lung aeration.

Inclusion Criteria:

• Written informed consent by parents.
• Patients aged 6 months to 5 years old
• Scheduled for surgery under general anesthesia with tracheal intubation with atelectasis assessed by lung ultrasound after anesthesia induction.
• American Society of Anesthesiologists classification: physical status I-II

Exclusion Criteria:

• Acute airway infection
• Cardiovascular or pulmonary disease
• Previous thoracic procedure