Evaluation of the Effects of Intraoperative Ventilation Modes on Perioperative Atelectasis

Postoperative pulmonary complications (such as atelectasis, pneumonia, pulmonary embolism, pleural effusion, pulmonary edema, and pneumothorax) are clinically significant because hospital stay is prolonged and morbidity and mortality are increased. Atelectasis is one of the most common respiratory complications in the perioperative period and occurs in 80-100% of patients undergoing general anesthesia. Anesthesia may lead to alveolar inhomogeneity, and the use of positive pressure ventilation may cause lung injury through mechanisms such as atelectotrauma, barotrauma, and biotrauma. Therefore, identifying the relationship between routine perioperative practices and the development of atelectasis may help guide the development of preventive strategies. Ventilator-induced lung injury (VILI) arises from the interaction between the energy delivered by the ventilator to the lung tissue and the tissue's response to this energy. Ventilator-associated lung injury occurring in patients receiving mechanical ventilation significantly increases morbidity and mortality. In recent years, numerous studies have focused on improving standard ventilation strategies-such as reducing tidal volume and adjusting positive end-expiratory pressure (PEEP)-to reduce the risk of VILI. However, VILI rates remain high and continue to contribute to postoperative pulmonary complications as well as complications in intensive care patients. One of the principal challenges of mechanical ventilation is the inability to accurately assess patient-specific lung mechanics using routinely monitored parameters. The lung characteristics predisposing to VILI are particularly dependent on the degree of pulmonary edema. Edema promotes atelectasis, increases inhomogeneity, elevates mechanical stress, and leads to cyclic opening and closing of alveoli. Ventilator-induced lung injury results from the interaction between mechanical power and the ventilated lung parenchyma. Tidal volume, ΔPaw (difference in plateau airway pressure), peak airway pressure, respiratory rate (RR), flow rate, and positive end-expiratory pressure (PEEP) are components of this interaction, each contributing to mechanical power in different ways. Factors affecting the development of atelectasis include the type, location, and duration of surgery, as well as patient positioning. Hysterectomy procedures are considered a high-risk group for postoperative atelectasis because they involve abdominal surgery, prolonged operative times, and the use of the Trendelenburg position. Anesthesia-related atelectasis is often too small to be detected on conventional chest radiographs. Comparative studies using computed tomography (CT) or magnetic resonance imaging (MRI) have demonstrated that lung ultrasonography can reliably detect anesthesia-related atelectasis with approximately 90% sensitivity, specificity, and diagnostic accuracy. Lung ultrasonography is therefore recommended for the diagnosis of anesthesia-related atelectasis and for monitoring respiratory complications. Flow-controlled ventilation (FCV) is a ventilation mode in which a constant low flow is applied during both inspiration and expiration. In this technique, airway pressure increases linearly during inspiration and decreases linearly during expiration. The applied flow is adjusted to maintain normocapnia (normal arterial carbon dioxide levels), and the inspiration-to-expiration (I:E) ratio is set at 1:1. This technique represents an innovative approach in mechanical ventilation. With constant flow and direct measurement of tracheal pressure, individualized lung mechanics can be assessed, which is not possible with conventional ventilation modes in which flow is variable and tracheal pressure cannot be directly monitored. Flow-controlled ventilation allows accurate calculation of dynamic compliance, enabling precise adjustment of positive end-expiratory pressure and peak inspiratory pressure (Ppeak). Thus, ventilation can be delivered within the lower and upper inflection points of the pressure-volume curve, tailored to individual lung mechanics. In pressure-controlled ventilation (PCV), peak airway pressure is controlled, and mandatory respiratory rate and inspiratory time are set. Once the preset pressure is reached, gas flow ceases; however, expiration does not begin until the preset inspiratory time has elapsed. In volume-controlled ventilation (VCV), a fixed tidal volume is delivered using a user-defined flow rate and inspiratory time. Airway pressure may vary, and excessive pressure may lead to barotrauma. In this study, the investigators aimed to evaluate the effects of flow-controlled ventilation, volume-controlled ventilation, and pressure-controlled ventilation modes used in the operating room on the development of postoperative atelectasis using lung ultrasonography. Following approval numbered 2024/241 by the Scientific Research Ethics Committee of Sancaktepe Şehit Prof. Dr. İlhan Varank Training and Research Hospital, patients scheduled for elective hysterectomy who met the inclusion criteria were screened. Patients who provided written informed consent after receiving detailed information about the study were included. Age, American Society of Anesthesiologists Physical Status Classification (ASA score), body mass index (BMI), and duration of surgery were recorded. Due to the observational nature of the study, lung ultrasonographic evaluation-considered a routine component of the pre-anesthesia assessment in the clinic-was performed, and lung ultrasound scores were recorded. After each participant was positioned on the operating table, standard anesthesia monitoring was applied, including heart rate (HR), electrocardiography (ECG), noninvasive arterial blood pressure monitoring, mean arterial pressure (MAP), peripheral oxygen saturation (SpO₂), and processed electroencephalography measured by the bispectral index (BIS). These parameters were recorded. Adequate surgical depth of anesthesia was achieved using propofol-remifentanil infusion guided by processed electroencephalography monitoring. After induction and adjustment of the mechanical ventilator by the responsible anesthesiologist, the observer recorded vital signs (SpO₂, HR, arterial pressure, processed electroencephalography), ventilation mode, peak airway pressure (Ppeak), positive end-expiratory pressure, plateau pressure, tidal volume, respiratory rate (f), end-tidal carbon dioxide (EtCO₂), fraction of inspired oxygen (FiO₂), flow rate, and inspiration-to-expiration ratio. Measurements were recorded at the following time points: preoperatively, after induction, before and after pneumoperitoneum, before and after positioning, and at 30-minute intervals thereafter. Arterial blood gas analysis was performed preoperatively and in the post-anesthesia care unit (PACU). Using vital signs, blood gas analysis, and mechanical ventilator parameters, heart rate, mean arterial pressure, peripheral oxygen saturation (SpO₂), mechanical power, lung compliance, end-tidal carbon dioxide (EtCO₂), partial pressure of carbon dioxide (pCO₂), partial pressure of oxygen (pO₂), the ratio of arterial partial pressure of oxygen to fraction of inspired oxygen (PaO₂/FiO₂ ratio), bispectral index (BIS), and intra-abdominal pressure were evaluated. For postoperative analgesia, techniques and medications selected by the responsible anesthesiologist, including intravenous drugs and neuraxial techniques, were recorded. In accordance with routine clinical practice, patients were transferred from the post-anesthesia care unit to the ward when the Visual Analog Scale (VAS) score was ≤3 and the Modified Aldrete Score was ≥9. Lung ultrasonographic evaluations were performed at the time of transfer and at postoperative 2 and 24 hours. Changes in lung ultrasound scores relative to preoperative values were assessed. Pain scores were recorded using the Visual Analog Scale at postoperative 2, 6, 12, and 24 hours. The statistical power of the study was expressed as 1-β (β = type II error probability), and a power of 80% was considered adequate. To achieve 80% power at a significance level of α = 0.05, a minimum of 22 patients per group (66 patients in total) was required. Considering the possibility of data loss, the study was planned to include 78 patients.