Effects of Permissive Hypercapnia During Lung Surgery on Inflammatory Biomarkers in Serum and Bronchoalveolar Lavage Fluid

Following admission to the preoperative area, demographic and clinical data were recorded, including age, sex, height, body weight, ideal body weight (IBW), comorbidities, smoking status, and ASA physical status classification. An arterial catheter was inserted before induction of anesthesia to allow continuous invasive blood pressure monitoring and arterial blood gas analysis.

Patients were pre-oxygenated with 100% oxygen for 3-5 minutes before induction of anesthesia. General anesthesia was induced and maintained using target-controlled infusion (TCI) of propofol with target effect-site concentration (Ceff) of 2.0-6.0 ng/ml (Schnider model) and remifentanil with a target-effect site concentration (Ceff) of 2.0-8.0 ng/ml (Minto model). Neuromuscular blockade was achieved with rocuronium bromide (0.6 mg/kg). The depth of anesthesia was monitored using the Bispectral Index (BIS) and were maintained between 40 and 60.

Following induction, all patients were intubated with a double-lumen endotracheal tube (Rüsch, Bronchopart, Teleflex, USA). Correct placement of the tube was confirmed by fiberoptic bronchoscopy immediately after intubation and reassessed following lateral positioning.

During two-lung ventilation, patients in the VC group received a tidal volume of 7 mL/kg IBW. During OLV, tidal volume was reduced to 5 mL/kg IBW. For patients ven-tilated using the PC mode, inspiratory pressure was adjusted to achieve equivalent tidal volumes. PEEP was initially set at 5 cm H₂O and remained unchanged unless hypoxemia occurred. The fraction of inspired oxygen (FiO₂) was initially set at 1.0 and reduced to 0.5 after initiation of OLV. The intended strategy was to minimize FiO2 at 0.5; however, FiO2 was increased when significant hypoxemia persisted for several minutes. Prior to and following the initiation of OLV, an alveolar recruitment maneuver was applied (manual inflations for 10-40 seconds to achieve a peak airway pressure of 30-40 cmH20). In cases of severe hypoxemia (<88%), the first intervention was an increase in the inspired oxygen fraction (FiO₂). If adequate oxygenation was not achieved, further ventilatory adjustments were implemented, including increases in PEEP, and inspiratory time, as well as application of the recruitment maneuver according to the clinical situation and ventilatory parameters.

Respiratory rate was adjusted to maintain the predefined PaCO₂ targets:

• Normocapnic group: 35-45 mmHg
• Hypercapnic group: 55-75 mmHg An inspiratory-to-expiratory (I:E) ratio ranging from 1:2 to 1:1 was maintained throughout the procedure. Throughout the surgical procedure, patients were monitored using a 5-lead electrocardiogram (ECG), invasive blood pressure measurement, heart rate, and depth of anesthesia using the Bispectral Index (BIS).

Ventilatory parameters, including tidal volume (TV), fraction of inspired oxygen (FiO2), respiratory rate (RR), SpO2, expired carbon dioxide (EtCO2), PEEP, peak (Ppeak), inspiratory pressure above PEEP (Pinsp), plateau airway pressure (Pplat), driving pres-sure (ΔP) - calculated as the difference between Pplat and PEEP), and the dynamic com-pliance, together with arterial blood gas parameters, were assessed at predefined time points.

In VC subgroup, Ppeak and Pplat were recorded. In PC subgroup, the set inspiratory pressure above PEEP (Pinsp) was recorded, as it represented the target inspiratory airway pressure. In PC mode, Pinsp was used as a surrogate for inspiratory airway pressure under controlled mechanical ventilation.

The predefined time points were: prior to anesthesia induction (Tbas), after induction, after intubation and initiation of two-lung ventilation (T1), after positioning in the lateral decubitus position (T2), 15 minutes from the onset of OLV (T3), 1 hour after the onset of OLV (T4), and after conversion back to two-lung ventilation in the supine position (T5).

2.7. Bronchoalveolar Lavage Fluid and Biomarker Analysis Prior to initiation of one-lung ventilation and following the return to two-lung ven-tilation and lung re-expansion, bronchoalveolar lavage (BAL) fluid was collected from two bronchopulumonary segments (the same segments before and after intervention) of both the collapsed and ventilated lungs. Samples from both lungs were pooled and analyzed together. BAL was performed using fiberoptic bronchoscope and 60 ml of sterile saline, with gentle aspiration under negative pressure of -50 cmH20. The recovered lavage fluid was collected, and 10-15 ml of BAL fluid was obtained for laboratory analysis. Due to variability in recovery volume, BAL samples were analyzed without normalization to the initial instilled lavage volume. BAL fluid was centrifuged at 1500 rpm for 15 minutes at room temperature. The supernatant was frozen and stored at -80 °C and interleukin levels were determined enzyme-linked immunosorbent assay (ELISA) with commercially available kits (FineTest,Wuhan China) according to the manufacturer's instructions. All samples were analyzed in the same assay run. The detection limits of the kits were as follows: TNF -α 3.8 pg/ml, IL-1β 1.56 pg/ml, IL-6 2.5 pg/ml, and IL-8 7.8 pg/ml.

Peripheral venous blood samples (10 ml) were obtained at the beginning and end of surgery for the measurement of serum leukocyte count (WBC), C-reactive protein (CRP) and IL-6 levels. At the end of the procedure, all patients were extubated in the operating room and transferred either to the intensive care unit (ICU) or to the surgical ward based on the clinician's judgment.

Perioperative blood samples and BAL fluid samples were analyzed at the Central Laboratory of the University Clinical Center Niš and the Laboratory for Functional Ge-nomics and Proteomics, Scientific Research Center for Biomedicine, Faculty of Medicine, University of Niš.

The duration of the surgery and anesthesia, duration of OLV, intraoperative fluid administration, urine output, and intraoperative blood loss were recorded. Potential adverse effects associated with hypercapnia were monitored, including postoperative visual disturbances, the occurrence of tachycardia and arrhythmias, postoperative emo-tional dysfunction, agitation, disorientation, panic symptoms, and convulsive seizures.

No protocol deviations were recorded during the study period.