Individualized PEEP titration guided by static lung compliance reduces postoperative atelectasis in elderly patients during laparoscopic Trendelenburg surgery: a randomized controlled trial

Abstract

Objective

To investigate the lung protective effects of individualized positive end-expiratory pressure (PEEP) titration guided by static lung compliance (Cstat) in elderly patients undergoing laparoscopic surgery in the Trendelenburg position.

Methods

Seventy elderly patients aged 65–75 years scheduled for elective laparoscopic rectal cancer surgery with anticipated duration > 2 h were enrolled. Patients had ASA physical status II-III and BMI 19–28 kg/m². Patients were equally divided into two groups: individualized PEEP guided by static lung compliance (CV group) and fixed PEEP group (DV group). Lung ultrasound scores (LUS) were performed after invasive arterial monitoring establishment (T1), immediately post-surgery (T4), before discharge from PACU (T5). Central venous blood samples were collected at T1 and T5 for ELISA determination of IL-6, CC16, and SP-A. Peak airway pressure, plateau pressure, alveolar dead space fraction (VD/VT), PaO₂/FiO₂ ratio, and invasive arterial pressure were recorded.

Results

Compared with the DV group, the CV group demonstrated significantly lower lung ultrasound scores at T4 (mean difference: -4.0, 95% CI: -7.0 to -2.0) and T5 (mean difference: -4.06, 95% CI: -6.3 to -1.7), significantly higher SP-A (median difference: 33.39, 95% CI:15.90 to 47.44) at T5, and increased dynamic and static compliance with decreased driving pressure at T3 and T4 (all p < 0.05). No significant differences were observed in VD/VT, PaO₂/FiO₂ ratio, or invasive arterial pressure.

Conclusion

For elderly patients undergoing laparoscopic rectal cancer surgery in the Trendelenburg position, individualized PEEP titration based on static lung compliance is associated with lower postoperative lung ultrasound scores (primarily located in the lateral and posterior zones) and may potentially alleviate atelectasis. This strategy is associated with improved intraoperative lung compliance, decreased driving pressure, and increased postoperative serum SP-A concentration, potentially providing intraoperative lung protection. Future studies should evaluate hard clinical outcomes, such as postoperative pulmonary complications (PPCs), as primary endpoints.

Trial registration

China Clinical Trial Registry, 7/26/2024, ChiCTR2400087409.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13741-026-00708-y.

Introduction

The number of elderly patients undergoing surgical procedures under general anesthesia is steadily increasing. Postoperative atelectasis is a common complication following general anesthesia, with severe cases requiring reintubation and potentially leading to respiratory failure and other serious postoperative complications [1]. Among patients without preoperative cardiopulmonary dysfunction, approximately 90% develop varying degrees of atelectasis following general anesthesia [2]. Laparoscopic pneumoperitoneum causes cephalad displacement of the diaphragm, affecting chest-lung compliance and increasing the incidence of atelectasis in patients under general anesthesia [3–5]. This effect becomes more pronounced in the Trendelenburg position [6, 7]. Lung protective ventilation strategies can reduce atelectasis occurrence and minimize postoperative pulmonary complications [8, 9]. However, optimal PEEP settings remain challenging. Recent years have seen proposals for individualized PEEP settings, though research conclusions remain inconsistent [10, 11]. Elderly patients have reduced pulmonary functional reserve [12]. Determining optimal PEEP to minimize atelectasis during laparoscopic surgery in Trendelenburg position is a critical clinical focus. Static lung compliance (Cstat) reflects lung tissue elasticity without airway resistance influence. This study aimed to evaluate the effects of PEEP titration based on static lung compliance on postoperative atelectasis in elderly patients undergoing laparoscopic surgery in Trendelenburg position.

Methods

Study design and participants

This randomized controlled clinical trial was approved by the institutional ethics committee in Shandong Cancer Hospital and Institute (approval number: SDTHEC2024007022) and registered with the China Clinical Trial Registry (registration number: ChiCTR2400087409) before patient enrollment. Written informed consent was obtained from all participants. The principles of Declaration of Helsinki were followed for this study. This manuscript adheres to the applicable CONSORT guidelines. Seventy elderly patients scheduled for elective laparoscopic rectal tumor surgery at Shandong Cancer Hospital and Institute (Shandong First Medical University) from July 2024 to May 2025 were enrolled. Inclusion criteria: Anticipated surgical duration > 2 h; age ≥ 65 years; BMI 19–28 kg/m²; ASA physical status I-III; intermediate ARISCAT risk score. Exclusion criteria: Patient or family refusal to participate; severe respiratory diseases (COPD, asthma, bronchiectasis, pneumonia, OSAHS, pulmonary interstitial fibrosis); history of thoracic surgery; severe cardiovascular diseases (heart failure, arrhythmias, coronary artery disease, cardiomyopathies, congenital heart disease, valvular heart disease). Withdrawal criteria: Intraoperative severe hypotension (systolic BP < 90 mmHg) or hypoxemia (SpO₂ < 90%) due to ventilation strategy; conversion to open surgery; incomplete data collection.

Randomization and blinding

A total of 70 recruited patients were allocated 1:1 to either the individualized PEEP group guided by static lung compliance (CV group) or fixed PEEP group (DV group) (Fig. 1). Random sequence generation was performed by an independent statistician using computer-generated block randomization (block sizes of 2, 4, and 6). To ensure strict allocation concealment, the randomization sequence was concealed in sequentially numbered, opaque, and sealed envelopes. These envelopes were kept by an independent researcher and were only opened by the attending anesthesiologist immediately before anesthesia induction. Random number generation, concealment, and allocation were performed by three different researchers (Jinze Li, Xuepeng Cao and Liquan Huang respectively). The study was patient- and family-blinded. Due to the nature of the mechanical ventilation intervention, the attending anesthesiologists could not be blinded to the group allocation. However, strict blinding was maintained for the patients, their families, and all other personnel involved in the study. Specifically, the outcome assessors (ultrasonographers, performing LUS evaluations), the laboratory technicians analyzing blood samples, and the statisticians performing data analysis were completely blinded to the group allocation.

Fig. 1.

CONSORT (Consolidated Standards of Reporting Trials) flow diagram showing the progress of participants during the trial

Anesthesia management and perioperative ventilation

Following admission to the operating room, peripheral venous access was established and standard monitoring was initiated, including blood pressure, electrocardiography, heart rate, pulse oximetry, and bispectral index monitoring. Left radial artery catheterization was performed under local anesthesia for invasive arterial pressure monitoring. Pre-induction preoxygenation was administered via pressure-support face mask. Drug dosing and respiratory parameter adjustments were calculated based on corrected body weight using the formula: 50 + 0.91 × (height in cm—152.4).

Anesthesia induction consisted of intravenous administration of midazolam 0.2 mg/kg, etomidate 0.3 mg/kg, cisatracurium besylate 0.2 mg/kg, and sufentanil 0.4 μg/kg. After 120 s, endotracheal intubation was performed using video laryngoscopy, followed by connection to the Avance CS2 anesthesia system. Ventilation parameters were established as follows: volume-controlled ventilation mode with an oxygen-air mixture at 1:1 ratio, gas flow rate of 2 L/min, tidal volume of 7 mL/kg corrected body weight, inspiratory-to-expiratory ratio of 1:2, and respiratory rate of 10–12 breaths per minute to maintain end-tidal carbon dioxide between 35–45 mmHg (1 mmHg = 0.133 kPa). All patients received manual lung recruitment maneuvers immediately after intubation, applying 30 cmH₂O pressure for 30 s, with the procedure repeated once before tracheal extubation. Following intubation, right subclavian central venous catheterization was performed.

International consensus-based recommendations currently advise PEEP levels of 4 to 6 cmH₂O during perioperative mechanical ventilation [9]. So, PEEP was maintained at a fixed level of 4 cmH₂O in DV group throughout the procedure. In the CV group, individualized PEEP titration commenced 5 min after establishment of pneumoperitoneum and Trendelenburg positioning. The titration protocol began at 1 cmH₂O PEEP, increasing in 2 cmH₂O increments, with each level maintained for 10 respiratory cycles. Maximum PEEP was limited to 15 cmH₂O. Static lung compliance was calculated at each PEEP level using the formula: Cstat = VT/(P_plat—PEEP) = VT/ΔP. The plateau pressure (P_plat) could be obtained from the anesthesia machine. Titration continued until the calculated Cstat showed no further increase or began to decrease, at which point titration was terminated and the PEEP corresponding to maximum Cstat was selected as the optimal PEEP for maintenance throughout surgery. The Trendelenburg position was standardized at 15 degrees head-down tilt, with pneumoperitoneum pressure maintained at 12 mmHg.

Anesthesia maintenance was identical for both groups, consisting of a continuous infusion of propofol at 0.08 mg/kg/min and remifentanil at 0.06 μg/kg/min, supplemented with 1–2% sevoflurane inhalation as a combined intravenous-inhalational anesthesia strategy to maintain hemodynamic stability and bispectral index values between 40–60. Blood pressure was maintained within ± 15% of baseline values using ephedrine or phenylephrine as needed. Fluid management was tailored to perioperative requirements, primarily using Ringer's solution with colloid supplementation when necessary. Adequate muscle relaxation and hemodynamic stability were maintained during PEEP titration. Acid–base balance and electrolyte disturbances were corrected based on intraoperative arterial blood gas analysis.

Postoperatively, tracheal extubation was performed after confirmation of consciousness recovery, adequate muscle strength restoration, normal tidal volume and minute ventilation, and absence of factors predisposing to airway obstruction. Patients were then transferred to the post-anesthesia care unit and returned to the ward once their Steward score exceeded 4 points.

Data collection and outcome measures

The primary observation time points were designated as follows: T1 represented the baseline measurement following establishment of invasive arterial monitoring upon operating room admission; T2 was recorded 5 min after endotracheal intubation; T3 was measured either 5 min after successful completion of the first PEEP titration in the CV group or 10 min after establishment of pneumoperitoneum and Trendelenburg position in the DV group; T4 was assessed immediately upon surgery completion; and T5 was evaluated before discharge from PACU.

Heart rate and invasive arterial blood pressure were recorded at all five time points from T1 through T5. Lung ultrasound scoring was performed at three specific time points: T1 for baseline assessment, T4 for immediate postoperative evaluation, and T5 for post-extubation assessment. Respiratory mechanics parameters were measured and documented at T2, T3, and T4, including peak airway pressure, plateau pressure, mean airway pressure, alveolar dead space fraction calculated using the formula VD/VT = (PaCO₂—PeCO₂)/PaCO₂, and the ratio of arterial oxygen partial pressure to fraction of inspired oxygen (PaO₂/FiO₂).

For biomarker analysis, central venous blood samples of 5 ml were collected at T1 and T5. These samples underwent centrifugation at 3000 revolutions per minute for 10 min to separate the serum, which was subsequently stored at minus 80 degrees Celsius in an ultra-low temperature freezer. Serum concentrations of Clara cell secretory protein (CC16), interleukin-6 (IL-6), and surfactant protein-A (SP-A) were determined using enzyme-linked immunosorbent assay according to the manufacturer's protocol. Absorbance values were measured and concentrations were calculated using standard curve methodology. The single primary outcome measure was the lung ultrasound score (LUS) evaluated immediately upon surgery completion (T4). Secondary outcome measures encompassed the changes in serum cytokine concentrations (IL-6, SP-A, and CC16) from baseline to post-surgery, the LUS at the remaining observation time points (T1 and T5), intraoperative vital signs, and respiratory mechanics parameters throughout the surgical procedure.

Study protocol

In both group, lung ultrasonography was performed on immediately before induction(T1), immediately after surgery(T4), before discharge from PACU (T5). RMs were carried out after intubation(T1) and the procedure repeated once before tracheal extubation in both groups. After the establishment of pneumoperitoneum and Trendelenburg position, PPEP titration was started in CV group. Initial PEEP was set to 1 cmH₂O. PEEP was increased at steps of 2 cmH₂O, and each PEEP level is sustained for 10 respiratory cycles, during which Cstat was calculated (in accordance with the formula: Cstat = VT/Plat-PEEP). We ended the titration process the moment the calculated Cstat displays a downward trend and choose its previous PEEP (the PEEP that maximizes Cstat) as the optimal PEEP for this patient. The upper limit of PEEP was set to 15 cmH₂O. The subclavian venous blood of the two groups was collected at T1 and T5. After centrifugation, serum samples were taken to measure the concentrations of interleukin (IL) 6 (IL-6), CC-16 and SP-A via enzyme-linked immunosorbent assay (ELISA). Details of the study protocol were provided in Fig. 2.

Fig. 2.

Schematic of study protocol and intervention for both CV and DV groups

Statistical analysis

The sample size calculation was determined strictly based on the single primary outcome, which is the lung ultrasound score (LUS) at T4. Based on our preliminary results, the mean LUS at T4 was 7.83 ± 1.47 in the CV group and 10.6 ± 2.41 in the DV group. To detect this difference using a two-independent samples t-test with a two-sided α level of 0.05 and a power (1-β) of 80%, a minimum of 30 patients per group was required. Accounting for a potential dropout rate of 10–20%, 35 patients per group were enrolled, yielding a total sample size of 70 patients.

Additionally, to verify whether this sample size would provide adequate statistical power for exploring our key secondary outcomes (serum cytokines), we performed a supplementary power estimation. Using IL-6 as a reference biomarker based on previous research [13] (estimated means: 2.3 ± 1.85 vs. 4.0 ± 2.67), calculations indicated that 30 patients per group would concurrently yield approximately 80% power at an α level of 0.05. Thus, the sample size primarily driven by the clinical LUS outcome is also considered statistically sufficient for evaluating secondary inflammatory responses.

Statistical analysis was performed using SPSS version 22.0. The normality of continuous variables was assessed using the Shapiro–Wilk test. Normally distributed data were expressed as mean ± SD. For continuous variables measured repeatedly at multiple time points (e.g., LUS across T1, T4, T5; respiratory mechanics and hemodynamics), a Repeated Measures Analysis of Variance (RM-ANOVA) was employed, incorporating 'group', 'time', and the 'group-by-time interaction' as factors. To strictly control the Type I error rate, the Bonferroni correction was applied for all post-hoc multiple comparisons between different time points within the RM-ANOVA model. Non-normally distributed data were expressed as median (interquartile range) and analyzed using the Mann–Whitney U test for intergroup comparisons and the Wilcoxon signed-rank test for intragroup comparisons. Categorical data were expressed as n (%) and analyzed using the Chi-square or Fisher's exact test. A two-sided P-value of < 0.05 was considered statistically significant.

Results

Study population

All 70 patients received intended interventions according to protocol. No study-related adverse events occurred. Three patients in the CV group and two in the DV group converted to open surgery; Data collection was incomplete for one patient in the DV group. Final analysis included 32 patients per group (Fig. 1).

Patient demographics

No significant differences were observed between groups in baseline characteristics or surgery-related parameters (P > 0.05) (Table 1).

Table 1.

Baseline characteristics and perioperative data of elderly patients undergoing laparoscopic rectal cancer surgery

Variables	CV Group(n = 32)	DV Group(n = 32)	Z, χ2 or t	P value
Ages(y)	67.9 ± 6.9	69.0 ± 5.5	−0.564	0.575
BMI	23.4 ± 3.3	24.0 ± 3.3	−0.463	0.643
Sex(n)			0.065	0.797
Male	20	19
Female	12	13
Hypertension (n)	22	24	0.309	0.578
Diabetes (n)	10	11	0.071	0.790
ASA II/III	20/12	19/13	0.066	0.797
Preoperative radiotherapy (n)	5	5	0.000	1.000
Preoperative chemotherapy (n)	3	4	0.160	0.689
ARISCAT score	29 ± 5	28 ± 6	0.890	0.350
Intraoperative fluid volume (mL)	2000(1500,2000)	2000(2000,2000)	−1.410	0.159
Surgical duration (min)	162 ± 38.06	157 ± 33.7	0.900	0.372
Trendelenburg position duration (min)	138 ± 33.75	130 ± 31.2	−1.150	0.250
Anesthesia duration (min)	173 ± 34.03	165 ± 36.8	−0.686	0.493
Sufentanil dose (μg)	45.8 ± 5.2	47.34 ± 5.68	−1.126	−0.264
Remifentanil dose (μg)	466 ± 71.5	478 ± 71.3	−0.700	−0.487
Preoperative albumin (g/L)	40.9 ± 3.4	39.6 ± 4.9	1.143	0.257
Preoperative WBC (10⁹/L)	6.1 ± 2.1	7.7 ± 3.6	−2.137	0.370
Preoperative Hb (g/L)	129 ± 20	127 ± 19	0.503	0.617

Data expressed as mean ± SD, Median (Q1, Q3) or n

CV group individualized PEEP guided by static lung compliance, DV group fixed PEEP group, BMI body mass index, ASA American Society of Anesthesiologists, ARISCAT score Assess Respiratory Risk in Surgical Patients in Catalonia score, WBC white blood cell, Hb hemoglobin

Primary and secondary outcomes

Comparison of lung ultrasound scores at T1, T4, and T5 between groups

Between-group comparisons revealed no significant difference in lung ultrasound scores at baseline (T1). The CV group demonstrated significantly lower lung ultrasound scores compared to the DV group at T4, with values of 7.5 ± 4.2 versus 11.7 ± 4.9 respectively (mean difference: −4.0, 95% CI: −7.0 to −2.0, p < 0.01). At T5, the CV group continued to show significantly lower scores than the DV group, with values of 10.3 ± 4.6 versus 14.4 ± 4.7 (mean difference: −4.06, 95% CI: −6.3 to −1.7, p < 0.01).

Within-group analysis of the CV group showed significant elevation of lung ultrasound scores post-extubation, with T5 values exceeding both T1 and T4 measurements (p < 0.01), while no significant difference was observed between T1 and T4 scores (8.3 ± 4.3 versus 7.5 ± 4.2, P > 0.05). In the DV group, lung ultrasound scores at T4 were significantly higher than at T1 (11.7 ± 4.9 versus 7.8 ± 4.8, p < 0.01), with further elevation at T5, resulting in T5 scores exceeding both T4 and T1 values (p < 0.01). These findings are presented in Table 2 and Fig. 3.

Table 2.

LUS scores across time points

Group	T1	T4	T5
CV (n = 32)	8.3 ± 4.3	7.5 ± 4.2a	10.3 ± 4.6abc
DV (n = 32)	7.8 ± 4.8	11.7 ± 4.9b	14.4 ± 4.7bc
Mean Difference (95% CI)	1.0(−1.0 to 3.0)	−4.0 (−7.0 to −2.0)	−4.06 (−6.3 to −1.7)
t value	0.354	−3.632	−3.485
P value	0.724	0.001	0.001

Data expressed as mean±SD

Compared with DV group: ᵃP<0.01; Within-group comparison, compared with T1: ᵇP<0.01; compared with T4: ᶜP<0.01. CV group: individualized PEEP guided by static lung compliance, DV group: fixed PEEP group

Fig. 3.

LUS images across time points

LUS 12-zone were compared across time points. At T4, compared to the DV group, there were 7 regions (L2, L3, L4, L6, R2, R4, R6) with a statistically significant decrease in LUS (p < 0.05). At T5, compared to the DV group, there were 9 regions (L3, L4, L5, L6, R1, R3, R4, R5, R6) with a statistically significant reduction in LUS (p < 0.05). Meanwhile, at T4, compared to T1, no region in the CV group showed a statistically significant increase in LUS, whereas the DV group had 6 regions (L3, L4, L5, L6, R4, R5) with a statistically significant increase in LUS (p < 0.05). At T5, compared to T4, the CV group had 4 regions (L4, L5, L6, R6) with a statistically significant increase in LUS (p < 0.05), while the DV group had 5 regions (L6, R3, R4, R5, R6) with a statistically significant increase in LUS (p < 0.05). These findings are presented in Table 3 and Fig. 4.

Table 3.

LUS 12-zone across time points

Time	Group	L1	L2	L3	L4	L5	L6
T1	CV	0.09 ± 0.29	0.19 ± 0.39	0.34 ± 0.49	0.66 ± 0.55	1.28 ± 0.63	1.28 ± 0.63
	DV	0.16 ± 0.37	0.19 ± 0.40	0.56 ± 0.56	0.78 ± 0.71	1.09 ± 0.69	1.33 ± 0.65
	t	0.74	0.00	1.66	0.79	1.33	0.39
	P	0.45	0.99	0.10	0.43	0.26	0.70
T4	CV	0.06 ± 0.25	0.06 ± 0.25a	0.28 ± 0.45a	0.63 ± 0.66a	1.22 ± 0.61	1.31 ± 0.54a
	DV	0.06 ± 0.25	0.25 ± 0.44	0.88 ± 0.56b	1.22 ± 0.79b	1.47 ± 0.67b	1.84 ± 0.72b
	t	0.00	2.10	4.68	3.25	1.56	3.34
	P	0.99	0.04	0.01	0.01	0.12	0.01
T5	CV	0.12 ± 0.33	0.22 ± 0.42	0.47 ± 0.57a	0.97 ± 0.65abc	1.47 ± 0.57abc	1.69 ± 0.59abc
	DV	0.12 ± 0.33	0.44 ± 0.50	1.00 ± 0.57b	1.46 ± 0.67b	1.84 ± 0.52bc	2.03 ± 0.65b
	t	0.00	1.89	3.74	2.67	2.77	2.21
	P	0.99	0.06	0.01	0.01	0.01	0.03
Time	Group	R1	R2	R3	R4	R5	R6
T1	CV	0.16 ± 0.37	0.22 ± 0.42	0.63 ± 0.61	0.69 ± 0.59	1.25 ± 0.83	1.43 ± 0.62
	DV	0.13 ± 0.33	0.22 ± 0.50	0.59 ± 0.61	0.63 ± 0.70	1.06 ± 0.67	1.15 ± 0.75
	t	0.35	0.00	0.20	0.38	1.01	1.82
	P	0.72	0.99	0.83	0.70	0.31	0.07
T4	CV	0.09 ± 0.30	0.16 ± 0.37a	0.50 ± 0.62	0.59 ± 0.67a	1.19 ± 0.74	1.41 ± 0.56a
	DV	0.25 ± 0.44	0.44 ± 0.50	0.78 ± 0.61	1.03 ± 0.78b	1.50 ± 0.67b	1.86 ± 0.61
	t	1.67	2.55	1.83	2.41	1.77	3.20
	P	0.10	0.01	0.07	0.02	0.08	0.01
T5	CV	0.16 ± 0.37a	0.31 ± 0.54	0.69 ± 0.59a	0.88 ± 0.70ab	1.56 ± 0.50ab	1.81 ± 0.54abc
	DV	0.41 ± 0.50b	0.56 ± 0.50b	1.09 ± 0.64bc	1.44 ± 0.56BC	1.91 ± 0.53bc	2.09 ± 0.45bc
	t	2.27	1.92	2.63	3.51	2.66	2.24
	P	0.02	0.06	0.01	0.01	0.01	0.03

Data expressed as mean ± SD

Compared with DV group: ᵃP < 0.01; compared with T1: ᵇP < 0.01; compared with T4: ᶜP < 0.01. CV group: individualized PEEP guided by static lung compliance, DV group: fixed PEEP group

Fig. 4.

Schematic diagram of LUS 12-zone at different time points. The blue areas represent regions where the LUS increased at T4 compared to T1. These were not observed in the CV group. In the DV group, there were 6 such regions (L3, L4, L5, L6, R4, R5), accounting for 50% of the total lung regions assessed. The green areas represent regions where the LUS increased at T5 compared to T4. In the CV group, there were 4 such regions (L4, L5, L6, R6), accounting for 33% of the total lung regions assessed. In the DV group, there were 5 such regions (L6, R3, R4, R5, R6), accounting for 42% of the total lung regions assessed. The red areas represent regions where the LUS increased at T5 compared to T1. In the CV group, there were 6 such regions (L4, L5, L6, R4, R5, R6), accounting for 50% of the total lung regions assessed. In the DV group, there were 10 such regions (L3, L4, L5, L6, R1, R2, R3, R4, R5, R6), accounting for 83% of the total lung regions assessed

Comparison of preoperative and postoperative serum IL-6, CC16, and SP-A levels

Compared to preoperative baseline values at T1, both groups demonstrated significant increases in IL-6, CC16, and SP-A levels at T5 (p < 0.01). When comparing between groups, the CV group exhibited significantly higher SP-A levels at T5 compared to the DV group, with median values of 147 (119,165) versus 112 (103,132) ng/L (median difference: 33.39, 95% CI:15.90 to 47.44, p < 0.01). These results are illustrated in Figure S1 and Table 4.

Table 4.

Levels of IL-6, CC16, and SP-A

Time	Biomarker	CV (n = 32)	DV (n = 32)	Z value	Median Difference (95% CI)	P value
	IL-6 (ng/L)	13.1(12.3,14.4)	12.3(12.0,14.0)	1.202	0.58(−0.47 to 1.31)	0.229
T1	CC16 (ng/L)	43.9(39.3,50.3)	43.3(40.5,47.2)	0.228	0.35(−2.33 to 3.33)	0.819
	SP-A (ng/L)	127(100,135)	108(103,125)	0. 942	7.26(−5.85 to 20.01)	0.346
	IL-6 (ng/L)	14.5(13.6,15.2)b	14.0(13.5,15.8)b	0.537	0.26(−0.68 to 1.18)	0.591
T5	CC16 (ng/L)	47.1(43.1,55.2)b	47.5(44.6,51.2)b	0.295	−0.52(−3.59 to 3.00)	0.768
	SP-A (ng/L)	147(119,165)ab	112(103,132)b	3.582	33.39(15.90 to 47.44)	0.001

Data expressed as M (Q1, Q3)

Compared with DV group: ᵃP < 0.01; Compared with T1: ᵇP < 0.01. CV group: individualized PEEP guided by static lung compliance, DV group: fixed PEEP group

Comparison of intraoperative respiratory mechanics parameters

No statistically significant differences were observed between groups for peak airway pressure, plateau pressure, or tidal volume at any observation time point. At T2, no significant differences existed between groups for dynamic compliance, static compliance, or driving pressure. The PEEP value titrated in the CV group was 7.1cmH₂O, which showed a statistically significant difference compared to the fixed PEEP value of 4cmH₂O in the DV group(p = 0.001).

When comparing T3 with T2, both groups demonstrated significant reductions in dynamic and static compliance with concurrent elevation in driving pressure (p < 0.05). Compared to T3, at T4 both groups showed improved dynamic and static compliance with significantly decreased driving pressure (p < 0.05). When comparing T4 to T2, no statistically significant differences were observed (P > 0.05).

The CV group demonstrated significantly higher dynamic and static compliance compared to the DV group at T3, with dynamic compliance values of 28.4 ± 7.6 versus 23.3 ± 6.5 mL/cmH₂O (P = 0.006) and static compliance values of 29.3 ± 8.1 versus 24.1 ± 8.1 mL/cmH₂O (P = 0.011). At T4, the CV group continued to show significantly higher dynamic and static compliance than the DV group, with dynamic compliance values of 42.1 ± 10.4 versus 33.6 ± 9.5 mL/cmH₂O (P = 0.001) and static compliance values of 43.6 ± 25.3 versus 35.5 ± 11.4 mL/cmH₂O (P = 0.010).

Driving pressure in the CV group was significantly lower than in the DV group at both T3 and T4 time points. At T3, driving pressure values were 13.28 ± 3.57 versus 15.41 ± 3.67 cmH₂O, respectively (P = 0.022), and at T4, values were 8.78 ± 1.95 versus 10.78 ± 2.72 cmH₂O, respectively (P = 0.001). Detailed results are presented in Table 5 and Figure S2.

Table 5.

Comparison of intraoperative respiratory mechanics parameters

Parameter	Group (n = 32)	T2	T3	T4
PIP (cmH₂O)	CV	14.0 ± 4.0	19.7 ± 3.7	15.4 ± 1.8
	DV	14.4 ± 4.0	20.3 ± 4.1	15.3 ± 2.5
Mean Difference (95% CI)		0.00(−2.00 to 1.00)	0.00(−2.00 to 2.00)	0.00(−1.00 to 1.00)
t value		−0.375	−0.610	0.344
P value		0.709	0.544	0.732
Pplat (cmH₂O)	CV	13.5 ± 4.0	19.3 ± 3.6	14.9 ± 2.0
	DV	13.6 ± 3.8	25.7 ± 36.0	14.8 ± 2.7
Mean Difference (95% CI)		0.00(1.00 to 1.00)	−0.09(1.89 to 1.70)	0.09(1.08 to 1.27)
t value		−0.097	−0.992	0.159
P value		0.923	0.325	0.874
PEEP(cmH₂O)	CV	3.6 ± 1.1	7.1 ± 0.9ᵃ	7.1 ± 0.9ᵃ
	DV	3.69 ± 0.7	4.0 ± 0	4.0 ± 0
Mean Difference (95% CI)		0.00(−1.00 to 1.00)	2.00(2.00 to 2.00)	2.00(2.00 to 2.00)
t value		0.39	13.89	13.89
P value		0.69	0.001	0.001
Vt (ml)	CV	357 ± 84	368 ± 67	381 ± 72
	DV	345 ± 59	359 ± 52	361 ± 68
Mean Difference (95% CI)		23.00 (−15.00 to 54.00)	8.78 (−21.06 to 38.62)	20.44 (−14.51 to 55.38)
t value		0.655	0.588	1.169
P value		0.515	0.559	0.247
Cdyn	CV	37.5 ± 13.5	28.4 ± 7.6ab	42.1 ± 10.4ac
	DV	35.2 ± 10.4	23.3 ± 6.5b	33.6 ± 9.5c
Mean Difference (95% CI)		2.31(−3.71 to 8.33)	5.08(1.53 to 8.63)	8.52(3.54 to 13.48)
t value		0.766	2.866	3.427
P value		0.446	0.006	0.001
Cstat	CV	39.4 ± 14.3	29.3 ± 8.1ab	43.6 ± 25.3ac
	DV	38.1 ± 11.4	24.1 ± 8.1b	35.5 ± 11.4c
Mean Difference (95% CI)		1.35(−5.10 to 7.80)	5.52(1.22 to 9.28)	9.79(3.77 to 16.27)
t value		0.418	2.606	2.674
P value		0.677	0.011	0.010
ΔP	CV	9.91 ± 3.31	13.28 ± 3.57ab	8.78 ± 1.95ac
	DV	9.91 ± 3.61	15.41 ± 3.67b	10.78 ± 2.72c
Mean Difference (95% CI)		0.00(−1.00 to 2.00)	−2.21(−3.93 to −0.31)	−2.00(−3.18 to −0.81)
t value		0.000	−2.348	−3.382
P value		1.000	0.022	0.001

Data expressed as mean ± SD. Compared with DV group: ᵃP < 0.01; Compared with T2: ᵇP < 0.05; Compared with T3: ᶜP < 0.05; PIP Peak inspiratory pressure, Pplat Plateau pressure, PEEP Positive end-expiratory pressure, Vt Tidal volume, Cdyn Dynamic compliance, Cstat Static compliance, ΔP P_plat – PEEP. CV group: individualized PEEP guided by static lung compliance, DV group: fixed PEEP group

Intraoperative gas exchange parameters

Between-group comparisons revealed no statistically significant differences in PaO₂, PaCO₂, alveolar dead space fraction, or PaO₂/FiO₂ ratio at T2 (P > 0.05 for all parameters). At T3 and T4, both groups demonstrated reduced PaO₂ values compared to T2 baseline measurements (p < 0.01). Similarly, both groups showed decreased PaO₂/FiO₂ ratios at T3 and T4 compared to T2 values (p < 0.01). Complete data are presented in Table 6.

Table 6.

Comparison of Intraoperative Gas Exchange Parameters

Parameter	Group(n = 32)	T2	T3	T4
PaO2(mmHg)	CV	241 (219,280)	209 (156,226)a	202 (148,225)a
	DV	233 (189,277)	198 (160,222)a	183 (151,214)a
	Median Difference (95% CI)	17.83(−18.00 to 52.00)	4.00(−16.00 to 28.00)	9.00(−16.00 to 28.00)
	Z value	−0.369	−0.692	−0.927
	P value	0.712	0.489	0.354
PaCO₂ (mmHg)	CV	39.6 (34.4,42.2)	39.3(36.4,46.4)	39.8 (36.5,43.3)
	DV	38.0(33.2,42.1)	41.0 (37.3,44.2)	43.1(38.8,44.9)
	Median Difference (95% CI)	0.00(−1.56 to 3.44)	−0.1(−3.00 to 3.00)	−1.7(−4.50 to 0.80)
	Z value	−0.128	−0.987	−1.457
	P value	0.898	0.324	0.145
VD/VT	CV	0.098(0.0,0.1)	0.056(0.0,0.1)	0.064(0.0,0.1)
	DV	0.087(0.0,0.1)	0.070(0.0,0.1)	0.084(0.0,0.1)
	Median Difference (95% CI)	0.01(−0.03 to 0.05)	−0.03(−0.07 to 0.02)	−0.02(−0.06 to 0.01)
	Z value	−0.571	−1.255	−1.255
	P value	0.568	0.209	0.209
PF ratio	CV	397.5 (362.4,463.6)	344.6 (257.4,373.6)a	333.9 (244.6,371.9)a
	DV	384.3 (312.8,457.0)	327.3 (267.5,366.1)a	301.7 (248.8,353.7)a
	Median Difference (95% CI)	33.6(−29.75 to 87.60)	6.61(−26.44 to 46.28)	14.88(−23.14 to54.54)
	Z value	−0.927	−0.443	−0.692
	P value	0.354	0.658	0.489

Data expressed as M (Q1, Q3). Compared with T2: ᵃP < 0.01. PaO₂: arterial partial pressure of oxygen, PaCO₂: arterial partial pressure of carbon dioxide, VD: physiological dead space volume, VT: tidal volume, PF ratio: PaO₂/FiO₂, FiO₂: fraction of inspired oxygen. CV group: individualized PEEP guided by static lung compliance, DV group: fixed PEEP group

Comparison of hemodynamic parameters

Between-group comparisons revealed no significant differences in invasive radial arterial systolic pressure, diastolic pressure, or mean arterial pressure at any of the five time points from T1 through T5. No other significant differences in hemodynamic parameters were observed between time points. Results are detailed in Table 7 and Figure S3.

Table 7.

Hemodynamic Parameters Comparison in Two Patient Groups (n = 32)

Time	Parameter	CV Group	DV Group	t value	Mean Difference (95% CI)	P value
T1	SBP (mmHg)	133 ± 15.5	139 ± 19.5	−1.279	−5.62(−14.42 to 3.17)	0.206
	DBP (mmHg)	75.1 ± 8.9	80.0 ± 11.5	−1.906	−4.91(−10.05 to 0.24)	0.062
	MAP (mmHg)	91.4 ± 10.9	95.9 ± 13.6	−1.439	−4.44(−10.60 to 1.73)	0.155
T2	SBP (mmHg)	128 ± 19.9	133 ± 20.4	−0.937	−4.72(−14.78 to 5.35)	0.352
	DBP (mmHg)	73.3 ± 10.4	73.0 ± 13.2	0.094	−1.00(−7.00 to 5.00)	0.925
	MAP (mmHg)	90.4 ± 14.4	92.5 ± 15.1	−0.568	−3.00(−11.00 to 5.00)	0.572
T3	SBP (mmHg)	126 ± 16.0	130 ± 18.9	−0.979	−4.28(−13.02 to 4.46)	0.331
	DBP (mmHg)	69.1 ± 10.6	70.3 ± 11.3	−0.433	−1.36(−7.00 to 4.00)	0.666
	MAP (mmHg)	88.1 ± 12.1	89.0 ± 13.1	−0.287	−1.00(−7.00 to 5.00)	0.775
T4	SBP (mmHg)	121 ± 14.8	131 ± 17.6	−2.626	−10.00(−20.00 to −3.00)	0.051
	DBP (mmHg)	70.4 ± 11.0	71.1 ± 12.4	−0.235	−0.69(−6.54 to 5.16)	0.815
	MAP (mmHg)	83.8 ± 12.4	89.1 ± 13.8	−1.6	−4.00(−10.00 to 2.00)	0.115
T5	SBP (mmHg)	137 ± 18.8	140 ± 14.6	−0.708	−4.00(−10.00 to 3.00)	0.482
	DBP (mmHg)	78.8 ± 10.9	78.9 ± 11.6	−0.033	0.00(−5.00 to 6.00)	0.974
	MAP (mmHg)	92.3 ± 15.7	94.8 ± 12.7	−0.717	−1.00(−9.00 to 5.00)	0.476

Data expressed as mean ± SD. All P > 0.05 between groups. SBP Systolic blood pressure, DBP Diastolic blood pressure, MAP Mean arterial pressure. CV group: individualized PEEP guided by static lung compliance, DV group: fixed PEEP group

At T1, both CV and DV groups showed smooth normal pleural lines (0 points). At T4 and T5, the CV group still showed normal pleural lines (0 points), while the DV group showed subpleural small consolidations separated by thickened pleura with confluent B lines (2 points). At T5, the DV group continued to show irregular pleural lines and confluent B lines (2 points)

Discussion

Our study investigated individualized PEEP titration guided by static lung compliance in elderly patients with rectal cancer undergoing laparoscopic surgery in the Trendelenburg position. The results indicated that intraoperative application of static lung compliance-guided individualized PEEP was associated with reduced lung ultrasound scores immediately after surgery and before PACU discharge, increased postoperative serum SP-A concentration, decreased driving pressure, and mitigated intraoperative lung compliance loss from pneumoperitoneum, suggesting a possible reduction in atelectasis.

While laparoscopic techniques have become increasingly widespread, they simultaneously demand more refined anesthetic management [14]. Laparoscopic pneumoperitoneum exerts pressure on the thoracic cavity, and rectal surgery requires the additional establishment of Trendelenburg positioning to ensure adequate surgical visualization. Due to gravitational effects, abdominal organs further compress the thoracic cavity. Given current population aging trends, the proportion of elderly patients (≥ 65 years) increases annually. The probability of postoperative atelectasis following general anesthesia increases with age [15]. Additionally, elderly patients have higher baseline prevalence of pulmonary diseases such as chronic bronchitis and COPD, increasing the likelihood of postoperative pulmonary complications. These factors collectively underscore the particular importance of effective lung protective measures for elderly patients undergoing laparoscopic surgery in the Trendelenburg position.

Individualized PEEP can be determined through various methods including lung compliance, driving pressure, electrical impedance tomography, pressure–volume curves, and esophageal pressure monitoring [16–18]. While driving pressure-guided PEEP titration offers simplicity [19], controversy exists regarding its effectiveness in reducing postoperative pulmonary complications [16, 20]. During our preliminary experiments, we observed that multiple PEEP values could correspond to identical driving pressures during titration. Consequently, we incorporated tidal volume into our approach, utilizing static lung compliance for individualized PEEP titration.

Our results demonstrated that compared to the DV group, the CV group showed increased dynamic compliance at T3 and T4 (P = 0.006 and P = 0.001, respectively), enhanced static compliance (P = 0.011 and P = 0.010, respectively), and reduced driving pressure (P = 0.022 and P = 0.001, respectively). These findings indicate that individualized PEEP is associated with the mitigation of intraoperative lung compliance loss from pneumoperitoneum, and better improves intraoperative lung compliance, reduces driving pressure, achieving optimal lung compliance with minimal driving pressure. We also observed that both groups experienced significantly decreased compliance at T3 due to pneumoperitoneum and Trendelenburg positioning effects, with values lower than those at T2 (post-intubation), and correspondingly, driving pressure at T3 was higher than at T2. Upon surgery completion, after pneumoperitoneum release and return to supine position, both compliance and driving pressure returned to post-intubation T2 baseline levels. These findings suggests that pneumoperitoneum and Trendelenburg position have a significant impact on chest-lung compliance in elderly patients. Static compliance-guided PEEP titration was associated with improved intraoperative compliance and reduced driving pressure, although the clinical relevance of these changes requires further evaluation.

Our study revealed that following establishment of pneumoperitoneum and Trendelenburg position, both groups showed decreased PaO2 and PF ratios at T3 and T4 compared to T2, with significant differences but no between-group variations. This resulted from our protocol of administering pure oxygen during induction followed by a 1:1 air-oxygen mixture after intubation, leading to the observed decreases in PaO2 and PF ratio.

Considering age-related physiological changes in our elderly study population, the control group received a fixed PEEP of 4 cmH₂O. The observation group's mean PEEP after static compliance titration was 7 cmH₂O. Shono et al. [6] compared 5 cmH₂O and 15 cmH₂O PEEP in robotic prostate surgery, similarly finding that higher PEEP increased intraoperative dynamic lung compliance and decreased driving pressure, with between-group differences disappearing post-surgery, consistent with our findings. De Meyer et al. [21] also concluded that higher PEEP during laparoscopic Trendelenburg positioning increases dynamic lung compliance and decreases driving pressure. Our study incorporated static lung compliance assessment, providing greater precision without airway resistance influence. Our PEEP titration protocol, starting at 1 cmH₂O and gradually increasing to identify the PEEP corresponding to maximum static compliance for intraoperative maintenance, aligns with the methodology of Wang et al. [22] who found static compliance-guided PEEP titration comparable to electrical impedance tomography-guided approaches. Our results confirm the advantages of static compliance-guided individualized PEEP. Recent meta-analyses and randomized controlled trials have increasingly highlighted the necessity of optimizing PEEP during pneumoperitoneum. Studies have consistently shown that while standard low PEEP strategies are insufficient to prevent atelectasis during the severe physiological derangements caused by capnoperitoneum and the Trendelenburg position, high fixed PEEP levels may compromise hemodynamics. Consequently, individualized PEEP titration—often combined with recruitment maneuvers—has emerged as a superior strategy in various abdominal surgeries to maximize lung protection and improve respiratory mechanics [23–25].

Lung ultrasound examination offers unique advantages in diagnosing and evaluating pulmonary diseases, being simple, non-invasive, and bedside-applicable [26–28]. The 12-zone scoring method is widely accepted for assessing perioperative atelectasis, quantifying ventilation loss through ultrasound-detected artifacts. Higher scores indicate greater aeration loss. Our results showed that the CV group had significantly lower lung ultrasound scores than the DV group at both immediate post-surgery and 30 min post-extubation (p < 0.001). Within-group analysis of the DV group revealed scores higher than baseline at both post-surgery and post-extubation time points (p < 0.01), with further elevation post-extubation (p < 0.01). The CV group showed elevated post-extubation scores compared to baseline and immediate post-surgery (p < 0.01), but no difference between immediate post-surgery and baseline (P > 0.05). This indicates that individualized PEEP titration based on pulmonary static compliance has a positive effect on intraoperative lung protection. It is important to note that while the reduction in LUS in the CV group compared to the DV group was statistically significant (7.5 vs. 11.7, p = 0.001), the direct clinical relevance of this finding should be interpreted with caution. Whether this approximately 4-point statistical improvement in ultrasound aeration scores translates directly into a clinically meaningful reduction in major postoperative pulmonary complications (e.g., postoperative pneumonia or respiratory failure) cannot be definitively concluded from our current data, and warrants future large-scale trials powered for these hard clinical endpoints. Specifically, regarding the zonal analysis, for the lung regions most susceptible to the effects of pneumoperitoneum and prolonged Trendelenburg positioning—namely the lower and dorsal lung segments—the CV group demonstrated superior protective effects. Through appropriately tailored PEEP, sustained alveolar inflation was better maintained, leading to a reduction in LUS scores. In contrast, the upper lung regions, being less influenced by body position and gravity, could maintain alveolar expansion even with fixed PEEP; thus, individualized PEEP may not show significant benefits in these areas. Meanwhile, during the surgery, individualized PEEP was associated with better maintenance of alveolar expansion compared to the DV group. From anesthesia induction to the end of the surgery, no significant increase in LUS was observed in any region in the CV group, whereas in the DV group, six regions already showed notable LUS elevation during the procedure. However, during the period from the end of surgery to extubation, the CV group exhibited LUS elevation in four regions, showing only a marginal advantage over the DV group, which had five. Overall, from patient admission to post-extubation, the CV group presented LUS elevation in a total of six regions, while the DV group showed elevation in ten regions. Overall, however, compared with fixed PEEP, individualized PEEP is associated with a more positive effect in reducing intraoperative atelectasis. These findings indicate that static compliance-based individualized PEEP titration was associated with lower ventilation loss and atelectasis in elderly patients undergoing laparoscopic surgery in Trendelenburg position, providing positive intraoperative lung protection. Our study showed that the reduction in lung ultrasound scores was primarily located in the lateral and posterior zones.

De Meyer et al. [21] similarly observed that higher PEEP produced lower lung ultrasound scores. Park et al. [29] advised against routine use of driving pressure-guided ventilation strategies in lung resection surgery. Despite not employing lung ultrasound scoring, their driving pressure-guided PEEP titration failed to reduce postoperative pulmonary complications, and bronchoscopic examination revealed no statistical difference in atelectasis incidence compared to fixed PEEP groups. Chen et al. [30] affirmed the advantages of static compliance-guided PEEP titration in elderly patients undergoing robot-assisted radical prostatectomy. Furthermore, we also observed elevated lung ultrasound scores in both groups following tracheal extubation. Généreux et al. [31] reported similar findings, showing ventilation loss manifested as increased lung ultrasound scores in both groups post-extubation when comparing 7 cmH₂O PEEP with recruitment maneuvers against zero PEEP without recruitment. We hypothesize this may relate to the need for additional ventilation support strategies during the emergence-to-extubation period, or that surgical anesthesia's impact on pulmonary ventilation cannot rapidly return to baseline immediately post-extubation regardless of intraoperative ventilation strategy. The specific mechanisms warrant further investigation.

IL-6 activates neutrophils, triggering inflammatory cascades that cause alveolar damage and pulmonary disease [32]. Surfactant protein-A (SP-A), a major protein component of pulmonary surfactant, reduces surface tension and regulates surfactant phospholipid synthesis, secretion, and recycling while counteracting plasma protein inhibition of surfactant function during lung injury [33]. As a soluble innate immune molecule, SP-A maintains pulmonary homeostasis through anti-infective and immunomodulatory dual actions. Serum CC16 concentration serves as a reliable biomarker for early detection of acute airway injury and assessment of epithelial integrity, accurately detecting minor epithelial barrier permeability defects. CC16, secreted almost exclusively by Clara cells in terminal bronchial epithelium, represents a peripheral lung injury marker associated with epithelial cell integrity, with elevated concentrations linked to pneumonia development [34].

Both groups showed significant postoperative increases in IL-6, CC16, and SP-A compared to baseline. Between-group comparison revealed significantly higher SP-A in the CV group at T5 compared to the DV group (p < 0.01). Unlike Chen et al. [30], we found no between-group difference in postoperative IL-6, possibly due to our shorter mean surgical and Trendelenburg duration. Additionally, our study population of rectal cancer patients included some who had undergone preoperative radiotherapy or chemotherapy, potentially affecting cytokine values. Animal studies demonstrate that SP-A reduces plasma protein inhibition of pulmonary surfactant, thereby decreasing surfactant destruction, potentially benefiting surfactant deficiency-related atelectasis. Increased SP-A coordinates innate and adaptive immunity to limit inflammation, potentially representing a molecular mechanism of static compliance-guided individualized PEEP lung protection. Static compliance-guided PEEP titration, compared to fixed PEEP, accommodates individual variations, preventing both lung collapse and alveolar overdistension, thereby reducing lung injury. Various causes of lung injury allow blood components to enter alveolar spaces, where plasma proteins inhibit surfactant activity both in vivo and in vitro. The lack of significant between-group difference in postoperative CC16 suggests that static compliance-based PEEP titration may not significantly reduce epithelial lung injury.

Hemodynamically, no significant differences were observed between groups across all time points from T1 to T5, indicating the safety of static compliance-guided PEEP titration. Excessive PEEP is known to not only fail to provide lung protection but also induce lung injury and hemodynamic instability [35]. In our study, with the CV group's mean titrated PEEP of 7 cmH₂O, static compliance-guided PEEP titration did not affect hemodynamics.

Overall, in elderly patients undergoing laparoscopic rectal cancer surgery in the Trendelenburg position, individualized PEEP titration based on static lung compliance was associated with lower postoperative lung ultrasound scores (primarily in the lateral and posterior zones), suggesting a possible reduction in atelectasis. This strategy was also associated with improved intraoperative lung compliance, decreased driving pressure, and increased postoperative serum SP-A concentration. These findings indicate a potential intraoperative lung-protective effect without clinically significant adverse hemodynamic effects. However, no statistically significant improvement in gas exchange parameters was observed. The clinical importance of the observed changes in lung ultrasound scores and physiological parameters remains to be determined, and whether these benefits translate into reduced postoperative pulmonary complications requires further investigation.

Limitations

Our study has several limitations. First, we did not analyze postoperative pulmonary complications, which lack unified definition and involve multiple factors [36]. Since postoperative atelectasis directly relates to general anesthesia and surgery, this study focused solely on atelectasis. Second, we only observed operating room outcomes without longer-term follow-up of lung ultrasound scores and cytokine levels at 1–7 days postoperatively, which warrants future investigation. Third, as a single-center trial with strict inclusion criteria (e.g., a restricted BMI range of 19–28 kg/m², and the exclusion of patients with severe preoperative respiratory or cardiovascular diseases), the external validity of our findings may be limited. The generalizability and applicability of this strategy to obese patients or other surgical populations (e.g., patients with severe baseline pulmonary dysfunction or those undergoing surgeries in different positions) remains uncertain and requires validation in future multicenter studies.

Conclusion

For elderly patients undergoing laparoscopic rectal cancer surgery in the Trendelenburg position, static lung compliance-guided individualized PEEP titration is associated with lower postoperative lung ultrasound scores (primarily in the lateral and posterior zones) and may potentially alleviate atelectasis. This strategy is also associated with improved intraoperative lung compliance and increases postoperative serum SP-A concentration, which might contribute to limiting pulmonary inflammation. Overall, these findings indicate a potential lung protection without significant adverse hemodynamic effects, although it does not significantly improve gas exchange parameters. The clinical importance of the observed changes in lung ultrasound scores and physiological parameters remains to be determined, and whether these benefits translate into reduced PPCs remains uncertain; therefore, future studies should evaluate PPCs as primary endpoints.

Authors’ contributions

Jinze Li: Writing – original draft, Software, Project administration, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Xuepeng Cao: Visualization, Validation, Supervision, Software, Methodology, Investigation. Liquan Huang: Resources, Project administration, Methodology, Investigation, Formal analysis, Data curation. Guojie Han: Writing – review & editing, Supervision, Formal analysis, Conceptualization. Xiuqin Wang: Writing – review & editing, Project administration, Supervision, Methodology, Investigation, Formal analysis, Conceptualization.

Funding

This study did not receive any internal or external funding.

Data availability

The datasets used and analysed during the current study are not publicly available due to patient privacy but are available from the corresponding author on reasonable request.

Declarations

Ethics approval and consent to participate

This randomized controlled clinical trial was approved by the institutional ethics committee in Shandong Cancer Hospital and Institute (approval number: SDTHEC2024007022) and registered with the China Clinical Trial Registry (registration number: ChiCTR2400087409) before patient enrollment. Written informed consent was obtained from all participants. The principles of Declaration of Helsinki were followed for this study. This manuscript adheres to the applicable CONSORT guidelines.

Consent for publication

The publication was approval by all the authors. We confirm that all authors have read and approved the final version of the manuscript and consent to its publication. All data and materials presented in this manuscript are original, created by the authors, and do not require separate consent for publication from any third party.

Competing interests

The authors declare no competing interests.