Effect of ultrasound-guided individualized Positive end-expiratory pressure on the severity of postoperative atelectasis in elderly patients: a randomized controlled study

doi:10.21203/rs.3.rs-4697774/v1

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Effect of ultrasound-guided individualized Positive end-expiratory pressure on the severity of postoperative atelectasis in elderly patients: a randomized controlled study

https://doi.org/10.21203/rs.3.rs-4697774/v1

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Background

The mortality rate of postoperative pulmonary complications (PPCs) in general anesthesia patients can reach up to 20%. Our research hypothesis is that ultrasound-guided individualized PEEP titration can reduce the risk of PPCs.

Methods

This single-center randomized controlled trial. Eighty-nine patients scheduled for elective laparoscopic radical surgery for colorectal cancer were enrolled in our study. Patients were randomly assigned to two group: the study group (individualized PEEP groups, PEEP Ind group) and the control group (Fixed PEEP group, PEEP 5 group). All patients in these two groups received volume-controlled ventilation during general anesthesia. Patients in the study group were given ultrasound-guided PEEP, while those in the control group were given a fixed 5 cmH₂O PEEP. Bedside ultrasound assessed lung ventilation. The primary outcome was PPC incidence within seven days post-surgery.

Results

Lung ultrasound scores (LUSs) in the study group during postoperative seven days was significantly decreased compared with that in the control group (P < 0.05). The severity of postoperative atelectasis in the study group was significantly improved. The incidence of PPCs during postoperative seven days in the study group was significantly less than that in the control group (48.6% vs. 77.8%; RR = 0.625; CI = 0.430–0.909; P = 0.01).

Conclusions

Non-obese elderly patients undergoing laparoscopic radical resection by lung ultrasound-guided individualized PEEP can alleviate the severity of postoperative atelectasis.

Health sciences/Medical research

Health sciences/Medical research/Study design

Health sciences/Medical research/Study design/Randomized controlled trials

Atelectasis

Elderly

Positive end-expiratory pressure (PEEP)

Ultrasound-guided

Colorectal Neoplasms

The incidence of postoperative pulmonary complications (PPCs) in general anesthesia patients ranges from 6–80%.¹ As the underlying pathophysiology of PPCs, atelectasis can prolong hospitalization and increase postoperative mortality, with a 30-day mortality rate of up to 20% in PPCs patients. ^2–4 Studies have shown that factors such as advanced age, smoking, general anesthesia, pneumoperitoneum, prolonged surgical duration, and the Trendelenburg position increase the risk of perioperative atelectasis. ⁵ Especially in patients undergoing laparoscopic surgery under general anesthesia, pneumoperitoneum and the Trendelenburg position can exacerbate atelectasis.^6,7

Positive end-expiratory pressure (PEEP) is a key element of lung-protective ventilation strategies, as it, in combination with low tidal volume ventilation, can reduce the risk of atelectasis. ^8–11 However, there has been ongoing controversy regarding the optimal level of intraoperative PEEP. Higher levels of PEEP can reduce the risk of atelectasis, and enhance respiratory system compliance but may lead to hypotension. Conversely, lower levels of PEEP may decrease the risk of barotrauma but are less effective in preventing atelectasis. Considering patient-specific factors, surgical factors, and anesthesia-related considerations, the ideal PEEP level varies significantly among individual patients, thus emphasizing the importance of individualized settings.

Bedside lung ultrasound (LUS) has demonstrated significant value in the management of critically ill patients and anesthesia care for the critically ill. It enables the timely detection of critical conditions, such as pulmonary edema, pneumothorax, atelectasis, pleural effusion, even before clinical symptoms manifest. When it comes to diagnosing pulmonary pathologies like atelectasis and pleural effusion, LUS has shown superior accuracy compared to chest X-rays (CXR) and can be on par with Computed Tomography scans (CT).¹² In recent years, LUS has been utilized for optimizing PEEP settings and facilitating lung recruitment. ^13,14 However, it remains unclear whether LUS can guide individualized PEEP titration in elderly patients undergoing general anesthesia for laparoscopic procedures to reduce postoperative postoperative pulmonary complications (PPCs).

Therefore, we hypothesize that individualized PEEP titration guided by lung ultrasound, as opposed to a fixed PEEP of 5 cmH₂O, can ameliorate the severity of postoperative atelectasis in non-obese elderly patients undergoing laparoscopic colorectal cancer resection, thereby potentially reducing the incidence of PPCs.

Study design

This study is a single-center randomized controlled trial (No. ChiCTR2200062979, https://www.chictr.org.cn, 26/8/2022. Main researcher: Junyang Ma).

Ethics

Ethical approval

for this study was approved by the Ethics committee of General hospital of Ningxia medicine university on 11 April 2022 (reviewer: Chen Juan. No. KYLL-2022-0411). Written informed consent was obtained from all eligible participants. The study adheres to relevant Consolidated Standards of Reporting Trials (CONSORT) guidelines.

Inclusion criteria

Patients scheduled for elective laparoscopic radical resection for colorectal cancer, between August 2022 and May 2023, aged ≥ 60, with a BMI of 18.5 to 27.9 kg/m², ASA (American Society of Anesthesiologists) physical status classification II-III, and an ARISCAT (Assess Respiratory Risk in Surgical Patients in Catalonia) score of ≥ 26 points.

Exclusion criteria

Patients undergoing elective right hemicolectomy, individuals with a medical history marked by substantial cardiopulmonary ailments, such as chronic obstructive pulmonary disease (COPD), prior pulmonary surgeries, bronchial asthma, the presence of pulmonary bullae, reliance on home oxygen therapy, preoperative oxygen saturation levels falling below 90%, decompensated heart disease (NYHA class 3 or 4), as well as patients slated for a change in their surgical procedure. Additionally, individuals who experience intraoperative blood loss exceeding 400 mL, choose to voluntarily withdraw from the trial postoperatively, or exhibit missing clinical data will be subject to exclusion.

Measurements

The ventilation parameters during the operation were collected. The results of perioperative arterial blood gas analysis were recorded using a portable blood gas analyzer (RADIOMETER ABL90 FLEX model). The bedside lung ultrasound scores (LUSs) with a mobile ultrasound device (China Wisonic Navi P) were comprehensively documented. These assessments were conducted at distinct time points: before induction of anesthesia (T0), after application of mechanical ventilation (T1), after application of pneumoperitoneum and Trendelenburg position (T2), 10 min after application of PEEP (T3), 1 hour after insufflation (T4), 10 min after the end of pneumoperitoneum and Trendelenburg position (T5), 10 min before extubation (T6), 10 min after entering the PACU (T7), first day postoperatively (D1), third day postoperatively (D3), and seventh day postoperatively (D7).

Anesthesia and Intraoperative ventilation

Details of anesthesia induction and maintenance, as well as the data of mechanical ventilation using standard methods, can be found in the supplementary materials.

Randomization and Blinding

The randomization process was based on a computer-generated allocation sequence. Patients were blinded. During the perioperative period, lung ultrasonography is performed by the same anesthesiologist who is blinded to the grouping. The researcher responsible for evaluating participants' group information regarding the primary outcomes remained blinded. Data were collected intraoperatively and in the post-anesthesia care unit (PACU), allowing the attending anesthesiologists involved in patient care and implementing the PEEP titration protocol to be aware of the group allocations.

Lung Ultrasonography

All patients were performed lung ultrasound scanning by the same anesthesiologist at six time points: T0, T2, T3, T7, D1, and D3. Patients were scanned in a supine position (Fig. 1). with each hemithorax divided into six regions. Twelve lung zones were scanned sequentially from left to right, cranially to caudally, and anterior to posterior. Using two-dimensional views, the probe was oriented parallel to the ribs. Transverse scans were performed in at least two intercostal spaces in each zone, and representative images or clips were recorded for postoperative data analysis. Lung ultrasound scores (LUSs), as modified by Mongodi et al.¹⁵ (see supplementary table 2 and video 1), were assigned to each patient based on their lung ultrasonography findings. In lung ultrasonography, the observation of no more than two B-lines indicates normal ventilation and is scored as 0. If the cumulative percentage of B-lines affecting the pleura does not exceed 50%, it indicates moderate ventilation loss and is scored as 1. If the cumulative percentage of B-lines affecting the pleura exceeds 50%, it indicates severe ventilation loss and is scored as 2. The presence of lung consolidation indicates complete ventilation loss and is scored as 3. A total score, ranging from 0 to 36, was calculated for each patient. Higher scores indicated poorer ventilation. If atelectasis were found in more than three regions with a score of 1 or higher (approximately 25% of the total lung surface), anesthesia-induced atelectasis can be diagnosed.¹⁶

Study intervention

Patients were randomly assigned to two group: the study group (individualized PEEP groups, PEEP Ind group) and the control group (Fixed PEEP group, PEEP 5 group). Patients in PEEP 5 group were fixed at 5 cmH₂O after tracheal intubation during the surgery. Patients in PEEP Ind group were set to 5 cmH₂O after endotracheal intubation, and PEEP titration by using lung ultrasound was performed immediately after establishing pneumoperitoneum and Trendelenburg position (T2), starting with an initial value of 0 cmH₂O. PEEP was adjusted incrementally using a stepwise approach, increasing by 2 cmH₂O¹⁷ if LUSs was ≥ 2.^18,19 Lung ultrasound was repeated 1 minute after each PEEP change until lung aeration (LUSs < 2)¹⁷ was achieved, with a maximum PEEP of 15 cmH₂O.²⁰ After releasing pneumoperitoneum, PEEP was gradually decreased in increments of 2 cmH₂O until no atelectasis was detected (LUSs < 2). If there were intraoperative changes in patient position, ultrasound examination and PEEP titration were performed. To minimize hemodynamic interference, during PEEP titration, if PEEP reached 15 cmH₂O without improvement in atelectasis in zones 5 and 6 (LUS ≥ 2), a minimum effective PEEP was set to optimize aeration in zones 1, 2, 3, and 4. (see Fig. 2)

Outcomes

Primary outcome was the severity of atelectasis within the first 7 postoperative days. The scores of all 12 regions were summed, and the number of regions with scores ≥ 2 was recorded. Higher scores indicate more severe atelectasis.

Secondary outcomes were postoperative hypoxemia, postoperative pulmonary embolism, duration of hospital stay and PPCs within 7 days postoperatively. Peak airway pressure (P_peak), plateau pressure (P_plat), driving pressure (ΔP), static respiratory system compliance (C_rs), and mechanical power (MP) were recorded during the procedure. Plasma concentrations of pulmonary injury biomarkers including IL-1, IL-6, IL-10, CC16, and FGF21 were tested during the perioperative period.

Statistical analyses

Sample size calculation was based on our pilot study. There were 10 patients in each group to measure perioperative LUSs. The mean and standard deviation of LUSs in the PEEP Ind group were 5.1 ± 3.3, and those in the PEEP 5 group were 8.2 ± 3.4. Assuming an α of 0.05, a β of 0.1, and accounting for a 20% attrition rate, each group requires at least 32 patients.

For continuous variables that adhere to a normal distribution (validated by the Shapiro-Wilk test and Q-Q plot), data are presented as mean ± standard deviation (SD). For variables that do not conform to a normal distribution, data are described using the median and interquartile range (IQR). Categorical variables are presented as counts and percentages.

The impact of the intervention on the outcome is quantified in terms of both quantity and percentage. The relative risk (RR) with a 95% confidence interval (CI) is calculated to estimate the effect. Hypothesis testing is conducted using the Wald likelihood ratio test and two-tailed χ2 test. Continuous variables are analyzed with independent sample t-tests or Mann-Whitney U tests, if necessary, while categorical variables are assessed using χ2 tests or Fisher’s exact tests. The Wilcoxon signed-rank test is employed to compare intragroup LUSs.

To examine intergroup differences at various time points, repeated-measures analysis of variance (ANOVA) with Bonferroni correction is applied. To analyze intragroup variations at different time points, a one-way repeated-measures ANOVA is used. Results are deemed statistically significant when p-values are less than 0.05.

All statistical analyses are conducted using IBM SPSS software (version 21.0).

Study Population

The CONSORT flowchart for patient selection is shown in Fig. 3, and the baseline characteristics of the two groups are in Table 1.

Table 1

Patients’ Baseline Characteristics and Surgical Information
	Individualized PEEP group (n = 37)	Fixed PEEP group (n = 36)	P Value
Age, y	67.1 ± 5.3	69.0 ± 5.7	0.146
Male, NO. (%)	20(54.1)	23(63.9)	0.393
Height, cm	164.7 ± 6.9	165.3 ± 8.1	0.769
Weight, kg	64.7 ± 7.9	65.0 ± 7.8	0.890
BMI, kg/m2	23.8 ± 2.3	23.7 ± 2.4	0.911
ASA classification NO. (%)			0.733
II	21(56.8)	19(52.8)	-
III	16(43.2)	17(47.2)	-
Coronary heart disease NO. (%)	7(18.9)	6(16.7)	0.901
Sleep apnoea NO. (%)	1(3)	0(0)	-
Hypertension NO. (%)	17(45.9)	13(36.1)	0.393
Diabetes NO. (%)	5(13.5)	6(16.7)	0.707
ARISCAT score			0.416
26≤	0(0)	0(0)	-
27–44	13(35.1)	16(44.4)	-
≥ 45	24(64.9)	20(55.6)	-
Smoking status, NO. (%)
Never	26(70.3)	26(72.2)	0.854
Former	2	2	-
Current	9	8	-
Respiratory infection within the last month, No. (%)	3	2	>0.999
Types of surgery			0.258
LAP rectectomy, NO. (%)	28(75.7)	31(86.1)	-
LAP colectomy, NO. (%)	9(24.3)	5(13.9)	-
Preoperative hemoglobin < 10 g/dL	3(8.0)	4(11.1)	0.970
SpO₂, %	94.8 ± 1.8	95.0 ± 2	0.638
≥ 96%, No. (%)	15(40.5)	19(52.8)	-
91%-95%, No. (%)	21(56.8)	17(47.2)	-
≤ 90%, No. (%)	1(2.7)	0	-
Duration of surgery, min	213.3 ± 47.5	203.8 ± 55.9	0.436
Duration of anesthesia, min	248.8 ± 51.1	238.3 ± 55.6	0.402
Fluid volume
Crystalloids, ml	2200(1850–2500)	2200(1700–2200)	0.634
Phenylephrine needed, NO. (%)	19(51.4)	17(47.2)	0.724
Data are shown as mean ± SD, or NO. (%), differences were considered to be statistically significant if P < 0.05.

The primary outcomes

LUSs in the PEEP Ind group were significantly lower than those in the PEEP 5 group at the T3 (4[3–5] vs. 7[4–8]; P = 0.005), T7 (5[3–7] vs. 8[6–10]; P = 0.001), D1 (4[1–5] vs. 6[4–10]; P = 0.001), and D3 (2[0–4] vs. 4[1–5]; P = 0.028) (supplementary Table 4).

Compared to the PEEP 5 group, there was a significant decrease in the proportion of patients with LUSs ≥ 2 in the PEEP Ind group after surgery (see Fig. 4), reflecting fewer regions with scores ≥ 2 among the 12 chest regions assessed. The proportion of patients with at least one chest region scoring ≥ 2 was significantly lower in the PEEP Ind group postoperatively, compared to the PEEP 5 group: at the T7 (43.2% vs. 88.9%; RR = 0.486, CI = 0.330–0.716, P < 0.001), D1 (32.4% vs. 75%; RR = 0.468, CI = 0.291–0.755, P < 0.001), D3 (16.2% vs. 38.9%; RR = 0.417, CI = 0.180–0.965, P = 0.03).

The secondary outcomes

The incidence of hypoxemia within seven days postoperatively was 63.9% in the PEEP 5 group and 45.9% in the PEEP Ind group (RR = 0.719, CI = 0.469–1.102; P = 0.124). The incidence of PPCs within seven days postoperatively was significantly lower in the PEEP Ind group compared to the PEEP 5 group (48.6% vs. 77.8%; RR = 0.625; CI = 0.430–0.909; P = 0.01). Among all patients, postoperative atelectasis primarily occurred in zones 3, 4, 5, and 6, with zones 5 and 6 exhibiting more severe cases (see supplementary Video 3,4,5,6). The incidence of atelectasis within seven days postoperatively was significantly lower in the PEEP Ind group compared to the PEEP 5 group (40.5% vs. 75%; RR = 0.541; CI = 0.350–0.834; P = 0.003). There were no significant differences in the incidence of other PPCs between the two groups (see supplementary table 3). Two patients in the PEEP 5 group experienced pleural effusion (> 500 mL, see supplementary Video1,2), while no patients in the PEEP Ind group developed pleural effusion.

When examining the incidence of pulmonary embolism within seven days postoperatively, there was no significant differences between the two groups (supplementary table 3).

Respiratory Mechanics and Hemodynamics

Respiratory and circulatory system data can be found in Fig. 5, supplementary Table 5, and supplementary Table 6. In comparison to the PEEP 5 group, patients in the PEEP Ind group exhibited higher values for intraoperative PEEP [9 (7–11)], P_peak, and P_plat, while displaying lower ΔP. Following pneumoperitoneum, patients in the PEEP Ind group demonstrated improved lung compliance and oxygenation. During pneumoperitoneum and Trendelenburg positioning, patients the PEEP Ind group experienced higher MP when compared to the PEEP 5 group. Throughout the entire surgical procedure, there were no significant differences between the two groups regarding MAP and the number of patients requiring vasoactive medications (see Table 1).

Plasma Lung Injury Biomarkers

The baseline values of plasma levels for lung injury biomarkers showed no significant differences between the two groups (refer to supplementary Fig. 6 and supplementary Table 7). When comparing the data with baseline values, it was evident that the plasma CC16 levels in the PEEP Ind group had a lower increase at T7 compared to that in the PEEP 5 group (as shown in supplementary Fig. 6). Similarly, concerning the comparison with baseline values, the plasma IL-6 levels in the PEEP Ind group at D3 exhibited a lower increase in contrast to that in the PEEP 5 group.

This study demonstrates that in elderly patients undergoing laparoscopic rectal cancer surgery, individualized PEEP guided by LUS can significantly reduce the severity of atelectasis within postoperative seven days. It also led to a substantial 34.5% decrease in postoperative atelectasis incidence and an 18% reduction in postoperative hypoxemia. These improvements can be attributed to the optimization of PEEP settings, guided by intraoperative LUS, which effectively prevents atelectasis during general anesthesia mechanical ventilation, lowers ΔP, enhances oxygenation, improves lung ventilation, and mitigates pulmonary inflammatory responses. Previous studies have indicated that the incidence of postoperative pulmonary complications (PPCs) ranges from 6–80%^1,21–23 primarily depending on factors, such as surgical type, definition, and severity of PPCs. It is worth noting that in our study, the incidence of PPCs was significantly lower in the PEEP Ind group compared to the PEEP 5 group (48.6% vs. 77.8%). The significant reduction in PPCs is evidently attributed to the decreased occurrence of atelectasis in the study group. However, in both groups, the incidence of postoperative atelectasis was high. This elevated occurrence could potentially be attributed to a higher prevalence of male smokers among patients in China, a greater number of underlying pulmonary diseases in the elderly population, and a correlation with infections related to COVID-19.²⁴

Many previous studies^25–28 have utilized a decremental PEEP titration approach and reported that high PEEP levels have a significant impact on hemodynamics. Our research also observed that during pneumoperitoneum and Trendelenburg positioning, MAP in the study group was lower than that in the control group, but patients in the study group did not experience severe hemodynamic disturbances (Fig. 5, h). This may be attributed, in part, to our incremental PEEP titration strategy, which produced favorable outcomes in mitigating the adverse effects of elevated PEEP on hemodynamics and preventing pulmonary overdistension. Additionally, during laparoscopic surgery, the Trendelenburg position and pneumoperitoneum can lead to a significant 35% increase in mean arterial pressure (MAP), this increase returns to baseline levels after surgery.²⁹ This observation provides a reasonable explanation for why our study did not detect severe hemodynamic fluctuations when employing higher individualized PEEP settings during pneumoperitoneum and the Trendelenburg position. Throughout the entire surgical procedure, the PEEP values in PEEP Ind group were dynamically adjusted. For instance, during pneumoperitoneum and Trendelenburg position, PEEP reached its maximum, and following the over of pneumoperitoneum and Trendelenburg position, we would lower PEEP to ensure hemodynamic stability. Furthermore, our research highlights the importance of individualized PEEP settings during laparoscopic rectal cancer surgery, as evidenced by PEEP levels in the PEEP Ind group ranging from 3 to 14 cmH₂O, with a median of 9 cmH₂O [7–11]. This finding emphasizes the importance of individualized PEEP settings throughout the entire surgical process.

The findings from this study highlight that ΔP in the PEEP Ind group demonstrated markedly lower when compared to the PEEP 5 group (Fig. 5, d). ΔP plays a crucial role in reducing PPCs. Existing research has consistently identified a direct correlation between elevated ΔP and the occurrence of PPCs during anesthesia.³⁰ The observed differences in driving pressure may help explain the partial effectiveness of the study intervention.³⁰ To mitigate this risk, previous studies have often adopted individualized PEEP titration methods, focusing on minimizing driving pressure.^22,31 In our study, we utilized bedside, user-friendly, and visual LUS guidance for individualized PEEP titration, employing LUS assessment to assist in diagnosing PPCs. While our study did not directly measure the influence on ΔP, it was evident that ΔP in the PEEP Ind group exhibited significantly lower compared to the PEEP 5 group. This observation strongly suggests that LUS-guided individualized PEEP titration serve to optimize intraoperative respiratory mechanics, achieve a delicate balance between averting pulmonary overdistension and atelectasis, ultimately reducing the risk of lung injury.

Mechanical power (MP) is a novel concept in mechanical ventilation and is associated with postoperative lung injury.^32,33 We observed that MP in the individualized PEEP group (10.1 ± 2.5 J/min) exhibited slightly higher than that in the fixed PEEP group (8.9 ± 1.5 J/min) (see Fig. 5, i). A study by Zhang et al.²² suggests that MP below the threshold for lung injury (12 J/min) ³⁴ do not appear to exacerbate PPCs. This explains why, in our study, the increase in mechanical power in the individualized PEEP group did not worsen PPCs, even though it was slightly elevated.

In the present study, the incidence of postoperative atelectasis is up to 75% in elderly patients undergoing laparoscopic rectal cancer surgery. This atelectasis is particularly common in gravity-dependent lung regions, consistent with prior research results.^22,35–41 Furthermore, the combined application of pneumoperitoneum and the Trendelenburg position during general anesthesia exacerbates atelectasis, aligning with findings from other studies.^5,8,17,27,35 A study indicates that postoperative atelectasis following general anesthesia can persist for several days, potentially leading to severe complications such as hypoxemia and pulmonary infections.⁴² This also explains the higher incidence of PPCs in the PEEP 5 group. There is still controversy regarding the persistence of atelectasis during the emergence from general anesthesia. In our study, all patients in both groups continued to experience ventilation impairment after the return of spontaneous ventilation, contrary to previous research.⁴⁰ Additionally, differences in lung ventilation and oxygenation were observed between the two groups, suggesting that low PEEP mechanical ventilation may not effectively prevent the occurrence of atelectasis in such surgeries (Fig. 5, a). The lung-protective ventilation strategy employed in this study can offer sustained benefits post-extubation. This may be particularly pronounced in older patients undergoing laparoscopic procedures, where the incidence of atelectasis is higher,⁴³ and intraoperative lung injury increases with age, especially in patients aged 65 and above,⁴⁴ Therefore, the benefits of individualized PEEP might be more useful in the elderly patient population.

The preceding text highlights that during mechanical ventilation, atelectasis predominantly occurs in gravity-dependent areas (zones 5 and 6). The optimal PEEP represents a compromise between lung recruitment and lung over-distension. The ideal PEEP may not align with the hemodynamic optimum and could potentially elevate pulmonary vascular pressures, impede venous return, decrease preload, and reduce cardiac output. Therefore, PEEP selection necessitates balancing its potential to prevent atelectasis while considering its impact on cardiac preload. The optimal PEEP should aim to prevent both lung over-distension and atelectasis. In our pilot study, we observed that some patients in the study group, even with PEEP increased to 15 cmH₂O, there was no significant improvement in atelectasis in zones 5 and 6. At the same time, there were significant hemodynamic fluctuations, with airway pressures reaching up to 30 cmH₂O. To avoid lung over-distension, for such patients, we opted to decrease the PEEP value to optimize ventilation in zones 1–4.

In this study, we examined plasma biomarkers related to lung injury and found that, in comparison to baseline values, CC16 plasma levels in the PEEP Ind group increased less than that in the PEEP 5 group after the patients entered the PACU. A previous study indicated that elderly patients with reduced lung function have lower circulating CC16 concentrations.⁴⁵ This may potentially explain the lower baseline levels of CC16 observed in our study before surgery. In the context of ventilator-induced lung injury, FGF21 is involved in lung injury repair by inhibiting the NLRP3/Caspase-1/GSDMD pyroptosis pathway.^46,47 In our study, both groups of patients showed an increase in FGF21 postoperatively, suggesting that mild lung injury may occur in both groups following general anesthesia mechanical ventilation. However, due to the limited sample size in our study, it may have been challenging to detect subtle differences in FGF21 between the two groups. The plasma concentration of IL-6 in both groups increased significantly on one day postoperatively, possibly related to surgical trauma and the inflammatory response to lung injury. However, by the third day postoperatively, the increased level of IL-6 in the PEEP 5 group exceeded that in the PEEP Ind group, indicating that the lung-protective effect of individualized PEEP titration guided by LUS can persist for up to three days postoperatively.

Strengths: Firstly, our study progressively increased PEEP levels in real-time, facilitating gradual lung re-expansion without causing lung overdistension guided by LUS. We kept the maximum PEEP pressure below 15 cmH₂O and stopped increasing PEEP when LUSs was ≤ 1. This strategy ensured stable lung recruitment while effectively avoiding hemodynamic issues. Secondly, our study employed an individualized PEEP adjustment strategy. We dynamically adjusted PEEP levels based on changes in the Trendelenburg position and pneumoperitoneum during surgery, maintaining stable alveolar recruitment, minimizing shear injury, preventing intraoperative atelectasis, and consequently improving postoperative lung function. Thirdly, we utilized LUS technology for convenient and accurate assessment of perioperative pulmonary complications in patients, including lung infections, atelectasis, pleural effusions, and more.

Limitation: Firstly, surgical positioning can restrict the range of lung ultrasonography, particularly in scanning zones 5 and 6, which may not be fully examined. We used grooves to position the patient's arm to ensure as comprehensive a scan as possible of the patient's chest sides and back. Secondly, this study is based on a small single-center sample, and further validation with larger multi-center samples is needed. Third, the specificity of the study population limits the generalizability and applicability of the conclusions.

Despite these limitations, our study suggests that individualized PEEP guided by LUS, can alleviate the severity of postoperative atelectasis within 7 days postoperatively by optimizing intraoperative respiratory mechanics, improving oxygenation and lung ventilation, reducing lung injury in elderly patients undergoing laparoscopic colorectal cancer surgery.

Acknowledgments: Many thanks to Professor Deng Liqin for her expert guidance on this research and manuscript revisions. We also express our gratitude to all the doctors and nurses in the Colorectal Surgery Department at the General Hospital of Ningxia Medical University for their strong support of this study. The Health Commission of Ningxia Hui Autonomous Region (Effect of ultrasound-guided individualized PEEP on postoperative pulmonary complications in laparoscopic colorectal surgery: a randomized controlled study, Project: 2022-NWKY-067).

Authors’ contributions：

Conception of study design: Junyang Ma, Liqin Deng

Study design: Junyang Ma, Liqin Deng

Patient recruitment: Junyang Ma, Liqin Deng, Meiqi Sun

Data collection: Junyang Ma, Liqin Deng, Meiqi Sun, Fengxiang Song, Aiqi wang, Xiaoxia Tian, Yanan Wu, Lu Wang, Qian Zhao, Bin Liu, Shengfu Wang, Yuxue Qiu, Haitao Hou

Data analysis: Junyang Ma, Liqin Deng, Meiqi Sun, Fengxiang Song

Drafting the manuscript: Junyang Ma, Liqin Deng

Critical revision for important intellectual content: all authors.

Data availability statement：The raw data has been uploaded to the Chinese ResMan Research Manager（ResMan, http://www.medresman.org.cn/login.aspx）.

Conflicts of interest: The authors declare no competing interests.

Financial disclosure(s): The Health Commission of Ningxia Hui Autonomous Region (Effect of ultrasound-guided individualized PEEP on postoperative pulmonary complications in laparoscopic colorectal surgery: a randomized controlled study, Project: 2022-NWKY-067).

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No competing interests reported.

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Effect of ultrasound-guided individualized Positive end-expiratory pressure on the severity of postoperative atelectasis in elderly patients: a randomized controlled study

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Background

Methods

Study design

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Inclusion criteria

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Measurements

Anesthesia and Intraoperative ventilation

Randomization and Blinding

Lung Ultrasonography

Study intervention

Outcomes

Statistical analyses

Results

The primary outcomes

The secondary outcomes

Respiratory Mechanics and Hemodynamics

Plasma Lung Injury Biomarkers

Discussion

Conclusion

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