Individualized vs Standard PEEP and Oxygen Reserve Index in Laparoscopic Hernia Surgery

General anesthesia combined with pneumoperitoneum and Trendelenburg positioning significantly alters respiratory mechanics. These changes can reduce functional residual capacity (FRC), promote atelectasis, and impair gas exchange - particularly in laparoscopic surgery.

Individualized PEEP, determined through a recruitment maneuver followed by PEEP titration, has been shown to reduce driving pressure and improve dynamic lung compliance compared to fixed PEEP strategies.

The Oxygen Reserve Index (ORI) is a novel non-invasive monitoring parameter that continuously tracks partial oxygen pressure changes in the 100-200 mmHg range, providing early warning of desaturation before SpO2 drops.

In this prospective observational study, 60 adult patients (ASA I-III, aged 18-75) scheduled for elective laparoscopic unilateral inguinal hernia repair under general anesthesia will be enrolled. Patients will receive either standard PEEP (5 cmH₂O) or individualized PEEP based on the attending anesthesiologist's routine clinical preference.

Hemodynamic and respiratory parameters including ORI, SpO2, airway pressures, tidal volume, dynamic compliance, and BIS will be recorded at six time points: before anesthesia (T0), 5 minutes post-intubation (T1), 10 minutes post-Trendelenburg (T2), 20 minutes post-Trendelenburg (T3), after desufflation (T4), and at postoperative 20 minutes (T5).

Lung ultrasound scoring (0-3 per zone, 12-zone protocol) will be performed preoperatively and postoperatively to assess atelectasis. Arterial blood gas analysis will provide additional oxygenation data.

The primary outcome is the effect of individualized versus standard PEEP on perioperative oxygenation as measured by ORI. The secondary outcome is the comparison of atelectasis prevention efficacy between the two PEEP strategies assessed by lung ultrasound score.

• Standard PEEP Ventilation (Procedure): Mechanical ventilation with a fixed positive end-expiratory pressure (PEEP) of 5 cmH₂O applied throughout the surgical procedure. Volume-controlled ventilation mode with tidal volume 6-8 ml/kg ideal body weight, inspiratory-to-expiratory ratio 1:2, and fraction of inspired oxygen (FiO₂) 0.40. Respiratory rate adjusted to maintain end-tidal CO₂ (EtCO₂) between 35-40 mmHg. No recruitment maneuver is performed.
• Individualized PEEP Ventilation (Procedure): Individualized PEEP determined by a stepwise recruitment maneuver followed by PEEP titration, applied at 10 minutes after Trendelenburg positioning. The optimal PEEP level is identified based on best dynamic lung compliance and maintained for the remainder of surgery. All other ventilation parameters are identical: volume-controlled mode, tidal volume 6-8 ml/kg, I/E ratio 1:2, FiO₂ 0.40, EtCO₂ 35-40 mmHg.

Primary outcomes

• Perioperative Oxygenation Assessed by Oxygen Reserve Index (ORI) [From anesthesia induction to postoperative 20 minutes (approximately 3 hours)]: Continuous non-invasive monitoring of the Oxygen Reserve Index (ORI) to assess perioperative oxygenation. ORI values range from 0 to 1 and reflect partial oxygen pressure changes in the 100-200 mmHg range. ORI is recorded at six time points: before anesthesia induction (T0), 5 minutes after intubation (T1), 10 minutes after Trendelenburg positioning (T2), 20 minutes after Trendelenburg positioning (T3), after desufflation (T4), and at postoperative 20 minutes (T5). ORI values will be compared between the standard PEEP and individualized PEEP groups.

Secondary outcomes

• Perioperative Atelectasis Assessed by Lung Ultrasound Score [Preoperatively and at postoperative 20 minutes]: Lung aeration assessed using a 12-zone lung ultrasound protocol in supine position. Each zone is scored 0-3: 0 = normal aeration (A-lines, fewer than 3 B-lines), 1 = mild interstitial syndrome (3 or more well-defined B-lines), 2 = moderate interstitial syndrome (confluent B-lines), 3 = severe interstitial syndrome or lung consolidation. Total score ranges from 0 to 36. Scores will be compared between standard PEEP and individualized PEEP groups.
• Dynamic Lung Compliance (Cdyn) [From anesthesia induction to 20 minutes after admission to the post-anesthesia care unit]: Dynamic lung compliance (Cdyn), expressed in mL/cmH₂O, recorded from the anesthesia ventilator at six predefined perioperative time points. All six observations represent repeated measurements of the same outcome using the same unit of measure
• Arterial Partial Pressure of Oxygen (PaO₂) [From anesthesia induction to 20 minutes after admission to the post-anesthesia care unit]: Arterial partial pressure of oxygen (PaO₂), measured in mmHg in blood samples obtained through the radial arterial cannula at the predefined time points.
• Postoperative Pulmonary Complications [Up to 7 days after surgery]: Incidence of postoperative pulmonary complications including desaturation, dyspnea, pneumonia, or respiratory-related hospital readmission assessed at postoperative day 7 via telephone follow-up.
• Arterial Partial Pressure of Carbon Dioxide (PaCO₂) [From anesthesia induction to 20 minutes after admission to the post-anesthesia care unit]: Arterial partial pressure of carbon dioxide (PaCO₂), measured in mmHg in blood samples obtained through the radial arterial cannula at the predefined time points.
• Arterial Blood pH [From anesthesia induction to 20 minutes after admission to the post-anesthesia care unit]: Arterial blood pH, reported as a unitless numerical value in blood samples obtained through the radial arterial cannula at the predefined time points.

Sex: All, ages: 18 Years to 75 Years

Inclusion Criteria:

* Aged 18-75 years
* ASA physical status I-III
* Scheduled for elective laparoscopic unilateral inguinal hernia repair under general anesthesia
* Body mass index (BMI) < 35 kg/m²
* Written informed consent obtained

Exclusion Criteria:

* Age below 18 or above 75 years
* ASA physical status IV-V
* Emergency surgical procedure
* Severe chronic or restrictive pulmonary disease
* Pulmonary hypertension
* Obstructive sleep apnea syndrome
* Preoperative peripheral oxygen saturation < 94% on room air
* Respiratory tract infection within the last two weeks
* Severe cardiac failure (NYHA III-IV)
* Pregnancy
* Communication difficulties preventing NRS assessment
* Surgical complications during the procedure
* Operation duration exceeding 3 hours
* Refusal to provide informed consent

• öznur demiroluk, MD, Study chair, ISTANBUL PROVINCIAL HEALTH DIRECTORATE FATIH SULTAN MEHMET TRAINING AND RESEARCH HOSPITAL