The primary objective of this study is to evaluate and compare the incidence and severity of intraoperative pulmonary atelectasis, measured by Lung Ultrasound Score (LUS) and EIT (Electrical Impedance Tomography), between two different modes of mechanical ventilation: Volume Control Ventilation (VCV) and Pressure Regulated Volume Control (PRVC/PCV-VG)

Background Mechanical ventilation is often required in pediatric anesthesia. However, the optimal intraoperative ventilatory strategy for pediatric patients remains a matter of debate, with no definitive consensus to date. Among conventional modes, Pressure-Controlled Ventilation (PCV) is time-triggered, pressure-limited, time cycled; set parameters include peak inspiratory pressure (PIP), inspiratory time (Ti) and respiratory rate (RR). Tidal Volume (TV) is a dependent variable. With the constant pressure throughout inspiration, PCV is thought to provide the maximum inspiratory pressure for the entire inspiratory-time favoring lung recruitment1.

In contrast, Volume-Controlled ventilation (VCV) delivers the preset TV, and the PIP is dependent on the respiratory mechanics of the patient. VCV has the advantage of certain TV delivery, while PCV has the advantage of not exceeding the set PIP1.

Both have limitations and to overcome the inherent compromises of traditional modes, modern ventilation modes such as Pressure Regulated Volume Control (PRVC) have been developed. These modes are designed to combine the benefits of VCV and PCV. They use a pressure-controlled flow and ventilation application approach, integrated with digital feedback mechanisms that continuously monitor the applied tidal volume, targeting a fixed tidal volume. PRVC allows the ventilator to measure the patient's lung compliance on a breath-to-breath basis and determine the pressure required to be given for the set Ti to achieve the set tidal volume. As a result, the ventilator can deliver a square wave pressure waveform like PCV but also ensure that a constant tidal volume is delivered to the patient like VCV.

Regardless of the ventilation strategy, general anesthesia inevitably induces atelectasis2, due to different mechanisms such as: shift of the diaphragm towards the thorax thus causing compressions of some lung areas, surfactant alteration due to inhalational anaesthetics, high inspired fractions of oxygen (FiO2) which are reabsorbed from the alveoli into the bloodstream causing reduction of the alveolar size3. These effects are demonstrable both immediately after induction and at the endo of surgery.

Children are particularly vulnerable to anaesthesia-induced atelectasis, due to the relatively higher compliant chest wall and the presence of diaphragmatic compression by large abdominal organs4. The clinical consequences of atelectasis are significant and include increased intrapulmonary shunt (blood passing through the lungs without being oxygenated), perioperative desaturation, higher risk of pneumonia, and a broader spectrum of postoperative pulmonary complications (PPCs)5. Given the peculiarities that predispose paediatric patients to atelectasis, they cannot be considered simply as miniature adults; their unique physiology requires tailored protective ventilation strategies. This highlights the urgency of identifying optimal ventilatory modes capable of minimizing atelectasis in this vulnerable population Since the best ventilatory strategy to avoid atelectasis is not defined currently, with the present randomized clinical trial the investigators aim to compare the effects of two ventilatory modes (VCV and PRVC) at same settings in terms of tidal volume, PEEP, FiO2, on atelectasis as described by lung ultrasound and EIT. The investigators hypnotized that PRVC (combining pressure control with guaranteed tidal volume) reduces atelectasis severity compared to VCV, as measured by LUS and EIT.

Inclusion Criteria:

• Patients requiring intraoperative mechanical ventilation via endotracheal intubation
• Written informed consent obtained from parents or legal guardians. For children capable of understanding, assent will also be obtained based on age and cognitive ability.
• ASA (American Society of Anesthesiologists) physical status I-II

Exclusion Criteria:

• Patients with significant pre-existing lung disease (e.g., cystic fibrosis, severe bronchopulmonary dysplasia, severe uncontrolled asthma, neuromuscular disease with respiratory compromise).
• Patients with complex congenital heart disease or significant hemodynamic instability.
• Patients undergoing thoracic surgery or procedures that could significantly alter lung mechanics (e.g., pre-existing pneumothorax).
• Refusal of parents/guardians or patient to participate in the study.
• History of previous intrathoracic procedure.

Patients will not be aware of the randomization arm