This study was approved by the Medical Ethics Committee of Anhui Provincial Children’s Hospital, China, on March 23, 2022 (approval number: EYLL-2022-026) and was registered at http://www.chictr.org.cn/ (trialnumber: ChiCTR2200058653; April 13, 2022). For infants in this study, voluntary informed consent was obtained and signed by their parents or legal guardians. This single-center prospective randomized controlled trial was conducted from April 2022 to December 2023 at Anhui Provincial Children’s Hospital in China.

Infants aged 1–6 months, classified as American Society of Anesthesiologists (ASA) physical status I-II, and scheduled for laparoscopic abdominal surgery under general anesthesia (>2 h) between April 2022 and December 2023 at Anhui Provincial Children’s Hospital were recruited for this study.

The exclusion criteria were those who were preterm infants, had recent pulmonary inflammation (within one month), had preoperative conditions increasing the risks of severe infections, sepsis, or regurgitation and aspiration, or contraindications to radial artery cannulation, classified as ASA III or higher, lacked family consent, or were needed to taken to the postoperative intensive care unit (ICU) after surgery.

Patients were randomized via computer-generated sequence, and their group assignment was sealed in envelopes. Patients were divided into two groups: the lung protective ventilation group (LPV group: small tidal volume, positive end-expiratory pressure, and recruitment maneuvers) and the conventional ventilation group (control group: zero PEEP and no recruitment maneuvers) at a 1:1 ratio. A single investigator opened the envelopes and implemented the respective mechanical ventilation protocols. The assessor responsible for conducting the lung ultrasound evaluations was blinded to the group assignments.

An intravenous cannula was placed prior to transferring the infant to the operating room. Upon the patient’s arrival in the operating room, routine monitoring of blood pressure (BP), electrocardiogram (ECG), oxygen saturation (SpO₂), and body temperature was initiated. Following anesthesia induction, invasive arterial pressure monitoring was commenced. Anesthesia induction involved a sequential intravenous slow injection protocol comprising midazolam (0.05 mg/kg), sufentanil (0.3 μg/kg), propofol (3 mg/kg), and cisatracurium (0.1 mg/kg). Following this, a 3.5–4.5 uncuffed endotracheal tube (10) was inserted, and pressure-controlled ventilation was initiated after successful intubation. Prior to intubation, all patients underwent preoxygenation with 60% (11) oxygen. Anesthesia maintenance involved administering a single caudal block of 0.25% ropivacaine (0.6 mL/kg) combined with an intravenous infusion of remifentanil (0.1 to 0.3 μg/kg/min) and inhalation of sevoflurane (2 to 3%) during surgery. The pneumoperitoneum pressure was consistently maintained at 5–7 mmHg.

Throughout the surgery, haemodynamic stability was maintained, and vasopressors were used as needed. Lactated Ringer’s solution was administered at a rate of 10–15 mL/kg/h during surgery. Additionally, suspended red blood cells, plasma, and albumin were administered as needed. At the end of surgery, the administration of sevoflurane via inhalation and the infusion of remifentanil were ceased. In the LPV group, during surgery, the following ventilation settings were used: tidal volume (TV) of 6 mL/kg, respiratory rate (RR) of 24–28 breaths/min, inspiratory/expiratory (I: E) ratio of 1:1.5, PEEP of 5 cmH₂O, fractional inspired oxygen tension (FiO₂) of 60% (12) and a flow rate of 2 L/min, with a maximum pressure limit of 30 cmH₂O. Lung recruitment maneuvers were performed every 30 min (13). Blood pressure and respiratory parameters were assessed prior to lung recruitment maneuvers to ensure the child was ready for the procedure (14). Recruitment maneuvers were performed, in pressure-controlled mode, with a constant driving pressure of 15 cmH₂O. PEEP was increased in steps of 5 cmH₂O, from 5 to 15 cmH₂O, every three breaths. The target recruitment pressure of 30 cmH₂O was maintained for 10 breaths (15). For the control group, a tidal volume (TV) of 8–10 mL/kg, a respiratory rate (RR) of 24–28 breaths/min, an I:E ratio of 1:1.5, a PEEP of 0 cmH₂O (16), a FiO₂ of 60%, and a flow rate of 2 L/min were used, with a maximum pressure limit of 30 cmH₂O, and lung recruitment maneuvers were not performed. When mechanical ventilation was performed after induction in both groups, the presence or absence of autoPEEP (17) was observed and recorded. Postoperatively, upon awakening from anesthesia, 0.02 mg/kg atropine and 0.05 mg/kg neostigmine were administered after the infant recovered spontaneous respiration (18). The tracheal tube was removed upon observing indications like a conjugate gaze, purposeful movements, the eye opening, a tidal volume > 5 mL/kg, and facial ghosting (19). All infants were transferred to the postanesthesia care unit (PACU) and received supplemental oxygen at a rate of 2 L/min through a simple mask equipped with an oxygen reservoir. Gradually reduced to room air when the SpO₂ level exceeded 95% and remained stable. After meeting the criteria for discharge from the PACU (20), the infants were returned to their wards.

Patients were placed in the supine position and scanned using ultrasound (SONIMAGE HS2-KONICA MINOLTA, Shanghai, China) according to the lung ultrasound score examination method described by Acosta et al. (9, 21). Each half of the chest was divided into three regions (anterior, posterior, and lateral) by the anterior and posterior axillary lines and further divided into six regions by an axial line 1 cm above the nipples. The 12 regions in the lung were scanned sequentially from right to left, from cranial to caudal and anterior to posterior ends. Lung ultrasound scores were assessed using ultrasound at T1 (5 min post-intubation), T2 (5 min post-pneumoperitoneum), T3 (at the end of surgery), T4 (post-extubation), and T5 (prior to discharge from the PACU). The 12 quadrants were assigned a score of 0 to 3 based on the following scoring system: 0, 0 to 2 B lines; 1, at least three B lines or one or multiple small subpleural consolidations separated by a normal pleural line; 2, multiple coalescent B lines or multiple small subpleural consolidations separated by a thickened or irregular pleural line; and 3, consolidation or small subpleural consolidation of more than 1 cm × 2 cm (Figure 1). The consolidation scores were recorded at each time point. Significant atelectasis was determined if any region had a consolidation score of 2 (22). The lung ultrasound score (0–36) was then calculated by adding up the 12 individual quadrant scores, with higher scores indicating more severe aeration loss.

Lung ultrasound scores and significant atelectasis incidence rates were assessed at T1 (5 min post-intubation), T2 (5 min post-pneumoperitoneum), T3 (at the end of surgery), T4 (post-extubation), and T5 (prior to discharge from the PACU).

Measurements of PaO₂ (partial pressure of arterial oxygen), PaCO₂ (partial pressure of arterial carbon dioxide), the PaO₂/FiO₂ ratio, HR (heart rate), MAP (mean arterial pressure), and SpO₂ (oxygen saturation) were recorded at T1, T2, T3 and T4. Peak airway pressure (Ppeak) and dynamic lung compliance (Cdyn) were directly measured on the anesthesia machine at T1, T2 and T3. The presence or absence of automatic PEEP and its value were recorded at T1. The time from the recovery of spontaneous breathing to extubation, and the total duration of their stay in the PACU, were both recorded. An independent investigator, blinded to the study details, evaluated the incidence of pulmonary complications within 7 days after surgery. These complications were scored based on the operational definitions of postoperative pulmonary complications provided by Hulzebos et al. (23) (Figure 2).

The study evaluated the likelihood of potential side effects, such as transient hypotension (defined as less than 80% of baseline blood pressure) and oxygen desaturation (SpO₂ below 95%), following regular alveolar recruitment maneuvers.

Unless otherwise specified, all the data are presented as the mean ± standard deviation or median (interquartile range). The Shapiro–Wilk test was used to assess the normality of the distribution. Outcomes were assessed using independent t tests, Mann–Whitney U tests and chi–square tests. A two-tailed p value <0.05 was considered to indicate statistical significance. Repeated-measures ANOVA was used to detect differences in the measured parameters between treatment groups and over time via a mixed-model procedure, with Bonferroni’s test for multiple comparisions. p values <0.05 were considered to indicate statistical significance. Data analysis was performed using SPSS software (version 25.0; SPSS, Inc., Chicago, Illinois, USA) and data visualization were carried out with GraphPad Prims 9 (GraphPad Software, SanDiego, CA, USA).

We used PASS 15.0 to calculate the sample size, basing our calculations on data from prior research. One study reported that lung ultrasound had an accuracy of 88% for detecting pulmonary atelectasis (9). According to our pilot study, the incidence of ultrasound-detected pulmonary atelectasis was 70%. We assumed this incidence would be halved by lung recruitment maneuvers and PEEP (15). With an alpha error of 0.05 and a power of 80%, the required sample size was calculated to be 29 patients per group. Accounting for a 20% dropout rate, we determined that a total of 73 patients would be needed, and thus we planned to enroll 80 patients in the study.

There were statistically significant differences between the LPV group and the control group at T2 in terms of ultrasound scores (6.35 ± 2.66 vs. 10.95 ± 4.07, 95% CI (−6.13 to −3.07), p < 0.001) and the incidence of atelectasis (62.5% vs. 87.5%, p = 0.010); at T3 in terms of ultrasound scores (6.65 ± 3.15 vs. 12.53 ± 4.29, 95% CI (−7.55 to −4.20), p < 0.001) and the incidence of atelectasis (55% vs. 90%, p = 0.0005); and at T4 in terms of ultrasound scores (5.50 ± 2.75 vs. 8.00 ± 2.92, 95% CI (−3.76 to −1.24), p < 0.001) and the incidence of atelectasis (35% vs. 72.5%, p = 0.001). At T1, there was no statistically significant difference in lung ultrasound score (7.35 ± 5.06 vs. 5.85 ± 4.14, 95% CI (−0.56 to 3.56), p = 0.151), the incidence of atelectasis (80% vs. 77.5%, p = 0.785). Moreover, there were no statistically significant differences between the LPV group and the control group at T5 in terms of ultrasound score (4.30 ± 1.87 vs. 5.00 ± 2.43, 95% CI (−1.67 to 0.27), p = 0.153) or incidence of atelectasis (32.5% vs. 47.5%, p = 0.171) (Table 2). Ultrasound scores of both groups at various time points are presented in Figure 4.

At T1, there was no statistically significant difference in PaO₂ (260.64 ± 23.43 vs. 262.39 ± 25.89, 95% CI (−12.74 to 9.25), p = 0.752), PaCO₂ (38.32 ± 3.15 vs. 37.94 ± 4.16, 95% CI (−1.27 to 2.02), p = 0.653), or the PaO₂/FiO₂ ratio (434.40 ± 39.05 vs. 437.31 ± 43.16, 95% CI (−21.23 to 15.41), p = 0.752). There were statistically significant differences between the LPV group and the control group at T2 and T3 in terms of the PaO₂, PaCO₂, PaO₂/FiO₂ or Cdyn; at T4, in terms of the PaO₂ or PaO₂/FiO₂. The LPV group demonstrated shorter extubation times and reduced lengths of stay in the PACUcompared to the control group (Table 2). There was no significant difference in autoPEEP between the two groups at T1. The two groups showed no significant difference in HR, MAP or SpO₂ at T1, T2, T3, or T4 (Table 3). No significant difference was observed in the incidence of pulmonary complications within 7 days after surgery (p = 0.516) (Table 4). Alveolar recruitment maneuvers did not show transient hypotension and oxygen desaturation.