This study investigates the effects of three extrinsic PEEP settings-5 cmH2O, 0 cmH2O, and an individualized PEEP (70% of the measured intrinsic PEEP)-on intrinsic PEEP and hemodynamic stability in patients with intrinsic PEEP undergoing lung resection surgery, using a randomized, crossover design.

Intrinsic positive end-expiratory pressure (iPEEP) frequently occurs during one-lung ventilation (OLV), which is essential for lung resection. This occurrence is driven by three primary factors. First, a significant portion of patients undergoing lung surgery have concomitant chronic obstructive pulmonary disease (COPD) and thus possess a baseline intrinsic PEEP. Second, high-frequency ventilation applied to prevent carbon dioxide accumulation during OLV shortens the expiratory time, thereby generating intrinsic PEEP. Third, the use of a narrow-bore double-lumen endobronchial tube for OLV increases airway resistance, accelerating the development of intrinsic PEEP. Intrinsic PEEP causes lung hyperinflation and elevates intrathoracic pressure, which can subsequently decrease venous return and lead to abrupt hypotension. However, research remains insufficient regarding the optimal level of extrinsic PEEP for patients who develop intrinsic PEEP during OLV. While an extrinsic PEEP of around 5 cmH2O is generally applied during OLV, some guidelines recommend completely removing extrinsic PEEP (0 cmH2O) during OLV in COPD patients. Nevertheless, this recommendation lacks robust scientific evidence. Furthermore, whether the strategy of setting extrinsic PEEP at approximately 70% of intrinsic PEEP-as suggested by studies in critically ill patients-is equally valid in the intraoperative OLV environment remains unverified. Theoretically, if extrinsic PEEP exceeds intrinsic PEEP, it may exacerbate lung hyperinflation by increasing expiratory resistance. Conversely, an extrinsic PEEP that is too low may cause airway collapse during expiration, paradoxically worsening gas trapping. Therefore, this study aims to evaluate the effects of three extrinsic PEEP settings-5 cmH2O, 0 cmH2O, and an individualized PEEP (70% of the measured intrinsic PEEP)-on intrinsic PEEP and hemodynamic stability using a randomized, crossover design.

Inclusion Criteria:

• American Society of Anesthesiologists Physical Status I - III
• The Eastern Cooperative Oncology Group Performance Status Grade 0 - 2
• Lung resection surgery requiring one-lung ventilation for over 60 minutes
• Indication of double-lumen endobronchial tube (female 35 Fr, male 37 Fr)
• Patient who is diagnosed with intrinsic PEEP during one-lung ventilation

Exclusion Criteria:

• Large bullae
• Emergency surgery
• Mechanical ventilation before surgery
• Hemodynamic instability before surgery
• Surgery requiring cardiopulmonary bypass
• Pregnancy, breastfeeding patient
• Patient's refusal to participate
• No intrinsic PEEP during one-lung ventilation

Although all patients will receive three interventions, the order of intervention will be randomly determined. The order of interventions will be blinded to participants, the investigators, and the outcome assessor.