" Mediastinal mass excision is typically performed via video-assisted thoracoscopic surgery (VATS). To secure a clear surgical field, the ipsilateral lung must be deflated, achieved through one-lung ventilation (OLV) and intrathoracic CO₂ insufflation. However, OLV increases intrapulmonary shunt due to continued perfusion of the non-ventilated lung, potentially leading to hypoxemia and hypercapnia.

When performed in the supine position, gas exchange becomes more challenging compared to lateral decubitus due to limited gravitational redistribution of blood flow. Although CO₂ insufflation aids surgical exposure through passive lung deflation, it may also increase CVP and PCWP, reduce cardiac output, and raise PaCO₂, contributing to respiratory acidosis.

End-inspiratory pause (EIP), a ventilatory setting that pauses airflow at end-inspiration, prolongs alveolar gas exchange and improves ventilation-perfusion matching. Prior studies show EIP can enhance gas exchange, reduce microatelectasis, and improve CO₂ clearance in patients with acute lung injury. We therefore aimed to assess the effect of EIP application during VATS mediastinal mass excicion.

Inclusion Criteria:

• Adult patients aged 20 to 80 years scheduled for video-assisted thoracoscopic mediastinal tumor resection.
• American Society of Anesthesiologists (ASA) physical status classification of II (patients with mild systemic disease) or III (patients with severe systemic disease limiting activity).

Exclusion Criteria:

• Moderate to severe chronic obstructive pulmonary disease (COPD) according to GOLD criteria: defined as FEV₁/FVC < 0.7 and FEV₁ ≤ 80% of predicted on pulmonary function testing.
• Diffusion capacity of the lung for carbon monoxide (DLCO) < 80% of predicted.
• History of pneumothorax or presence of bullae or blebs on preoperative imaging.
• Inability to read or understand the subject information sheet and consent form.