Abstract
Background
Induction of general anesthesia is associated with rapid-onset lung atelectasis, which increases intrapulmonary shunt and potentially impairs oxygenation. We aimed to evaluate whether applying positive end-expiratory pressure (PEEP) to facemask ventilation during induction of general anesthesia can reduce post-induction lung atelectasis.
Methods
In this single-center, three-arm randomized controlled trial, one hundred and twenty ASA I-II patients undergoing elective non-cardiothoracic cancer surgery under general anesthesia were randomly assigned into three groups. Facemask ventilation was initiated in volume-controlled mode with preassigned PEEP levels (0, 5, or 10 cmH2O) after anesthetic administration. Electrical impedance tomography (EIT) was used to continuously assess lung atelectasis during induction of general anesthesia, including baseline spontaneous breathing, facemask ventilation, and post-intubation mechanical ventilation. The primary outcome was the dorsal change in end-expiratory lung volume (△EELV) 2 min after endotracheal intubation. The secondary outcomes included driving pressure, EIT-derived ventilation homogeneity, systemic hemodynamics, and PaO2/FiO2 ratio.
Results
Atelectasis (defined as △EELV < 0) occurred in both the PEEP0 and PEEP5 groups during facemask ventilation [-58.8% (-110.4%, 4.0%) and − 34.2% (-200%, 19.2%), respectively] and after induction [-26.9% (-127.4%, 33.6%) and − 33.2% (-52.8%, 5.0%), respectively], but not in the PEEP10 group [25.3% (-12%, 104.7%)]. Facemask ventilation with 10 cmH2O PEEP demonstrated better dorsal lung recruitment than both lower PEEP levels (P < 0.05), reduced post-induction driving pressure (P < 0.05), and improved ventilation homogeneity (P < 0.05). Hemodynamics and PaO2/FiO2 ratio were comparable among the three groups.
Conclusions
Use of 10 cmH2O PEEP during induction of general anesthesia effectively mitigated dorsal lung collapse by optimizing respiratory mechanics without inducing overdistension and hemodynamic compromise. It is worth investigating whether this improvement can be extended to the postoperative period.
Trial registration number
NCT06900426 (registered at clinicaltrials, principal investigator: Jun Zhang, registration date: March 17, 2025).
Keywords: Induction of general anesthesia, Facemask ventilation, Lung atelectasis, Electrical impedance tomography, Positive end-expiratory pressure
Introduction
Induction of general anesthesia is consistently associated with rapid formation of pulmonary atelectasis, occurring in 85–90% of patients within minutes following anesthetic administration [1]. This pathophysiological change increases intrapulmonary shunt fraction and impairs oxygenation, even in patients with previously healthy lungs [2]. The clinical significance of anesthesia-induced atelectasis extends beyond transient hypoxemia. Emerging evidence indicates that alveolar collapse triggers localized biological responses characterized by inflammatory activation, immune dysregulation, and compromised alveolar-capillary barrier integrity [3]. These alterations may predispose patients to impaired pulmonary fluid clearance, increased protein permeability and risk of infection, potentially leading to postoperative pulmonary complications.
The severity of atelectasis is determined by the patient characteristics and perioperative management. Obesity and high inspired oxygen concentration (FiO2) have been identified as significant risk amplifiers [4, 5]. Importantly, effective atelectasis prevention during induction of general anesthesia may enhance patient safety through improved oxygenation reserves, particularly in challenging airway scenarios requiring prolonged intubation attempts.
The application of positive end-expiratory pressure (PEEP) to facemask ventilation during induction of general anesthesia has demonstrated potential to mitigate alveolar collapse, even when using high FiO2 [6]. Current guidelines recommend maintaining intraoperative PEEP at 5 cmH2O during mechanical ventilation [7], however, the optimal PEEP level used for facemask ventilation remains controversial. Low PEEP levels may inadequately prevent dorsal lung region collapse, whereas high PEEP strategies (≥ 10 cmH2O) may compromise hemodynamic stability [8], induce gastric insufflation [9] and alveolar overdistension [10] during facemask ventilation. This safety-efficacy dilemma creates critical knowledge gaps in respiratory management during induction of general anesthesia.
This study aimed to validate whether applying 10 cmH2O PEEP to facemask ventilation could reduce post-induction lung atelectasis without adverse effects compared to use of lower PEEP levels.
Materials and methods
Patient population
This single-center, randomized clinical trial was performed at the Fudan University Shanghai Cancer Center (FUSCC). Ethical approval for this study (No. IRB 2503-Exp148) was provided by the Ethical Committee of FUSCC, Shanghai, Republic of China (Chairperson Prof. Zhen Chen) on 10 March 2025. Written informed consent was obtained from all participating patients before enrollment. This study was registered in ClinicalTrials. gov (registry number: NCT06900426) on March 17, 2025. The subjects were recruited from 28 March, 2025 to 28 April, 2025.
Inclusion criteria were as follows: (1) adult patients (18–80 years); (2) ASA physical status I-II; (3) BMI 18–30 kg/m²; (4) elective non-cardiothoracic surgery; (5) general anesthesia. Exclusion criteria: (1) preexisting respiratory disorders (e.g., chronic obstructive pulmonary disease or asthma), (2) history of thoracic surgery or chest wall deformities, (3) conditions contraindicating standard airway management (e.g., high aspiration risk, anticipated difficult airway), (4) implanted electronic devices (e.g., pacemakers), and (5) pregnancy. A blinded investigator performed eligibility assessments 24 h preoperatively.
Anesthesia protocol and ventilation strategy
The study design is shown in Fig. 1. The study procedure commenced with preoxygenation using 100% oxygen delivered through a circle system (Flow-I, Maquet Inc., Heidelberg, Germany) and a face-fitted mask (PAIM, Congren Medical Device Co., Ltd, Xiamen, China) until the fraction of expired oxygen reached 0.8 or higher. Induction of general anesthesia was conducted with propofol target-controlled infusion (TCI, Marsh mode) at a plasma concentration of 4 µg ml−1, along with sufentanil 0.3 µg kg−1 and rocuronium 0.6 mg kg−1. General anesthesia was maintained with sevoflurane or desflurane inhalation and remifentanil TCI (Minto mode, plasma concentration 1–2 ng ml−1), supplemented with intermittent rocuronium and sufentanil, if necessary.
Fig. 1.
Diagram of study design. EIT = electrical impedance tomography. PEEP = positive end expiratory pressure. TCI = target-controlled infusion. VCV = volume-controlled ventilation. TV = tidal volume. EOS = end of the study. SOS = start of the study
Once apnea was confirmed by the absence of airflow and respiratory effort, face mask ventilation was initiated. This was conducted with volume-controlled ventilation on an anesthetic ventilator set to 12 breaths per minute, with an inspiratory to expiratory ratio of 1:2, and tidal volumes of 6–8 ml kg−1 to keep the partial pressure of end-tidal CO2 (PetCO2) between 35 and 45 mmHg. The patients were randomly assigned to three groups: PEEP = 0 cmH2O (PEEP0 group), PEEP = 5 cmH2O (PEEP5 group), or PEEP = 10 cmH2O (PEEP10 group). The facemask was held in place using a two-handed technique and the jaw was positioned to optimize airway patency.
Following endotracheal intubation, all of three groups received continuous volume-controlled mechanical ventilation (12 breaths per minute, with an inspiratory to expiratory ratio of 1:2, and tidal volumes of 6–8 ml kg−1 to keep PetCO2 between 35 and 45 mmHg), with a consistent PEEP of 5 cmH₂O.
If, at any point, pulse oxygen saturation (SpO2) fell below 90% or PetCO2 more than 50 mmHg), the study was terminated and routine airway management was resumed.
The entire procedure of induction of general anesthesia was documented using a video recorder, which captured the dynamic parameters displayed on the anesthetic monitor and machine. The recorded data were then subjected to subsequent review and analysis. The parameters revisited included heart rate and blood pressure at (1) spontaneous breathing, (2) facemask ventilation (immediately before intubation), and (3) mechanical ventilation 2 min after endotracheal intubation. Arterial blood gas analysis was conducted at the same three time points using a GEM3500 analyzer (Instrumentation Laboratory, USA). Additionally, the post-induction driving pressure was also recorded.
Electrical impedance tomography (EIT) monitoring and analysis
EIT was used to continuously monitor and record global and regional ventilation (PulmoVista500; Draeger Medical, Luebeck, Germany) during the study period. Briefly, an EIT electrode belt containing 16 electrodes, each 40 mm wide, was positioned around the thorax at the fifth intercostal space with a reference electrode placed on the abdomen. Customized software, programmed with Matlat 2023a (MathWorks, MA, US), was utilized for quantitative offline data analysis (Fig. 2). During facemask ventilation or post-induction mechanical ventilation, results were obtained by averaging all breaths occurring in the last minute of each time point. Tidal variation (TVEIT) was computed by subtracting the end-expiration image from the end-inspiration image to reflect changes during tidal breathing. The TVEIT image was divided into quadrants (four regions of interest (ROIs): ROI1 denotes the top-left side of the image, which corresponds to the right-ventral lung regions). Based on the TVEIT the following variables were calculated:
Fig. 2.
The represented EIT tidal images and the analysis of impedance images. A: impedance changes across the peri-induction period including spontaneous breathing, facemask ventilation and post-induction ventilation. B: EIT image at spontaneous breathing. C: EIT image at facemask ventilation with ZEEP. D: EIT image at post-induction mechanical ventilation with 5 cmH2O PEEP. E: EIT image at facemask ventilation with 10 cmH2O PEEP (The impedance changes are not displayed in Fig. 2A). F: The calculation formula of ΔEELI. EELI(n) denotes the end-expiratory lung impedance at breath n. EELIbaseline and TVbaseline represent the EELI and tidal variation (TVEIT) at the baseline (stable breathing cycles before induction of general anesthesia)
The global inhomogeneity (GI) index was calculated from tidal EIT images to summarize ventilation heterogeneity [11].
 |
Where TVl denotes the value of pixel l in the tidal images, and pixel l is considered as the lung region if TVl >20% × max (TVEIT). TVlung pixels represent the lung area. A high GI index indicates a high degree of ventilation heterogeneity.
Centre of ventilation (CoV) depicts ventilation distribution in the anteroposterior co-ordinate) [12]:
 |
Where TVi is the impedance change in the TVEIT image for pixel i, yi is the pixel height, and of pixel i scaled such that the bottom of the image (dorsal) is 100% and the top (ventral) is 0%.
The change of end-expiratory lung volume (△EELV) was significantly correlated with the change of end expiratory lung impedance (△EELI) [13].
 |
 |
We use the normalized version of △EELI to represent the △EELV in the present study.
Where EELI(n) denotes the end-expiratory lung impedance at breath n. EELIbaseline and TVbaseline represent the EELI and TVEIT at the baseline (stable breathing cycles before induction of general anesthesia).
Lung atelectasis was defined as △EELV < 0.
Study outcomes
The primary outcome was dorsal △EELV at 2 min after endotracheal intubation. The secondary outcomes included driving pressure, GI, CoV, ventral EELI, systemic hemodynamics, and PaO2/FiO2 ratio.
Statistical analyses
According to our pilot study, a sample size of 120 patients was required (40 in each group), with a calculated effect size of 0.57 to achieve an α error probability of 0.05 and power of 0.8. Considering a dropout rate of 20%, 150 patients were enrolled in this study.
Randomization was conducted using the MinimPy2 software (version 2.0, OSDN, Columbus, OH, USA). After the patients entered the operating room, randomization was performed by a study team member blinded to the study protocol. Continuous variables are presented as mean ± standard deviation (SD) or median with interquartile range (IQR) depending on the normality of the data distribution. Differences between continuous variables were tested using analysis of variance (ANOVA) or the Kruskal–Wallis test. Categorical variables were presented as numbers and proportions. The chi-square test or Fisher’s exact test was used. Differences in repeated measurements between groups were tested using repeated measurement analysis of variance (ANOVA), followed by post hoc Bonferroni analysis. Statistical analyses were performed using SPSS 19.0 (SPSS, Inc., Chicago, IL, USA). Statistical significance was defined as P < 0.05, and all P values were two-sided.
Results
Participant flow and baseline characteristics
During the study period, 141 consecutive patients underwent eligibility screening, of whom 21 were excluded before randomization (Fig. 3). The remaining 120 participants were equally allocated to the three PEEP groups (PEEP0, PEEP5 and PEEP10 groups). All of the patients achieved successful first-attempt endotracheal intubation. Careful monitoring revealed no clinical or radiological evidence of pulmonary aspiration in any patient. The demographic and clinical characteristics were comparable among the groups (Table 1).
Fig. 3.
Flowchart of subjects’ enrollment. EIT = electrical impedance tomography. PEEP = positive end expiratory pressure
Table 1.
The demographics and clinical characteristics of patients in three groups
| PEEP0 group (n = 40) |
PEEP5 group (n = 40) |
PEEP10 group (n = 40) | |
|---|---|---|---|
| Age (<65/≥65) | 25/15 | 23/17 | 24/16 |
| Gender (male/female) | 19/21 | 18/22 | 20/20 |
| ASA statues (Ⅰ/Ⅱ/Ⅲ) | 7/28/5 | 8/29/3 | 7/28/5 |
| Weight (kg) | 65.3 ± 6.8 | 64.9 ± 5.9 | 64.4 ± 6.0 |
| Height (cm) | 165.3 ± 7.8 | 166.7 ± 6.9 | 166.1 ± 7.5 |
| BMI (kg.m-2) | 23.8 ± 2.1 | 23.4 ± 3.0 | 23.9 ± 1.9 |
| ARISCAT scores | 19.2 ± 3.5 | 16.5 ± 4.8 | 16.7 ± 5.1 |
| Non-smoking / Smoking | 26/14 | 24/16 | 22/18 |
| Hypertension (%) | 14 (35%) | 15 (37.5%) | 14 (35%) |
| Diabetes (%) | 9 (22.5%) | 10 (25%) | 8 (20%) |
Data are presented as the mean ± SD unless otherwise noted
ASA American Society of Anesthesiologists, BMI Body mass index, ARISCAT Assess respiratory risk in surgical patients with catalonia
Atelectasis incidence and lung volume changes
Atelectasis (defined as △EELV < 0) was found in both the PEEP0 and PEEP5 groups but not in the PEEP10 group. The dorsal △EELV during facemask ventilation was − 58.8% (−110.4%, 4.0%) in the PEEP0 group, −34.2% (−200%, 19.2%) in the PEEP5 group and 81.0% (19.2%, 153.0%) in the PEEP10 group. The dorsal △EELV in post-induction ventilation was − 26.9% (−127.4%, 33.6%) in PEEP0 group, −33.2% (−52.8%, 5.0%) in PEEP5 group and 25.3% (−12%, 104.7%) in PEEP10 group, respectively.
There were significant differences in dorsal △EELV among the three groups, both in facemask ventilation and post-induction ventilation, as well as in the regional lung ventilation parameters GI, CoV, and ventral and dorsal EELI (all P < 0.05, Table 2). Compared with 0 cmH2O PEEP, 5 cmH2O PEEP did not increase the dorsal △EELV in either facemask ventilation or post-induction ventilation (both P > 0.05, Table 2). In contrast, 10 cmH2O PEEP increased the dorsal △EELV in both facemask ventilation and post-induction ventilation compared with 0 cmH2O PEEP or 5 cmH2O PEEP (all P < 0.05, Table 2).
Table 2.
EIT parameters during peri-induction period
| PEEP0 group (n = 40) |
PEEP5 group (n = 40) |
PEEP10 group (n = 40) |
P value | |
|---|---|---|---|---|
| Spontaneous breathing | ||||
| GI | 0.51 ± 0.23 | 0.51 ± 0.11 | 0.51 ± 0.14 | 0.233 |
| CoV (%) | 49.5 ± 5.5 | 49.5 ± 6.0 | 49.1 ± 5.4 | 0.162 |
| Ventral EELI | 898.7 ± 542.4 | 1068.4 ± 654.2 | 1176.8 ± 1092.7 | 0.631 |
| Dorsal EELI | 1050.8 ± 531.9 | 1169.4 ± 596.8 | 1231.5 ± 910.3 | 0.773 |
| Facemask ventilation | ||||
| GI | 0.55 ± 0.16 | 0.45 ± 0.15 | 0.38 ± 0.06* | 0.003 |
| CoV (%) | 41.1 ± 6.3 | 42.3 ± 5.4 | 45.9 ± 4.3* | 0.005 |
| Ventral EELI | 1490.3 ± 813.7 | 2346.1 ± 1468.0 | 5428.6 ± 2357.4*# | 0.001 |
| Dorsal EELI | 654.6 ± 481.1 | 1112.9 ± 700.7 | 2969.3 ± 1160.2*# | 0.001 |
| Dorsal △EELV (%) | −58.8 (−110.4, 4.0) | −34.2 (−200, 19.2) | 81.0 (19.2, 153.0) *# | 0.001 |
| Post-induction ventilation | ||||
| GI | 0.50 ± 0.10 | 0.50 ± 0.10 | 0.40 ± 0.04* | 0.010 |
| CoV (%) | 40.6 ± 6.0 | 41.2 ± 4.5 | 43.9 ± 5.0* | 0.012 |
| Ventral EELI | 1695.0 ± 963.6 | 2028.3 ± 1305.1 | 3287.5 ± 1679.4*# | 0.006 |
| Dorsal EELI | 825.7 ± 520.4 | 850.3 ± 507.9 | 1714.0 ± 695.4*# | 0.001 |
| Dorsal △EELV (%) | −26.9 (−127.4, 33.6) | −33.2 (−52.8, 5.0) | 25.3 (−12, 104.7) *# | 0.010 |
Data are presented as the mean ± SD or median (IQR) unless otherwise noted
GI Global inhomogeneity index, CoV Center of ventilation (100% = entirely dorsal), EELI End-expiratory lung impedance, EELV End-expiratory lung volume
*P < 0.05 (PEEP0 group vs. PEEP10 group); #P < 0.05 (PEEP5 group vs. PEEP10 group). Two-way ANOVA was used for multiple comparisons
Respiratory mechanics, oxygenation and hemodynamics
The PEEP10 group demonstrated a significantly lower post-induction driving pressure than the PEEP0 and PEEP5 groups (both P < 0.05, Table 2). However, there were no significant differences in PaO2/FiO2 ratio and systemic hemodynamics among the three groups (all P > 0.05, Table 3).
Table 3.
Driving pressure, hemodynamics and PaO2/FiO2 ratio during peri-induction period
| PEEP0 group (n = 40) |
PEEP5 group (n = 40) |
PEEP10 group (n = 40) |
P value | |
|---|---|---|---|---|
| Spontaneous breathing | ||||
| HR (bpm) | 74.9 ± 16.4 | 76.7 ± 10.5 | 81.4 ± 14.9 | 0.443 |
| SBP (mmHg) | 153.7 ± 20.7 | 155.8 ± 23.9 | 145.9 ± 17.5 | 0.412 |
| DBP (mmHg) | 75.1 ± 12.4 | 82.0 ± 15.2 | 72.3 ± 11.3 | 0.121 |
| MAP (mmHg) | 103.5 ± 11.0 | 109.8 ± 17.1 | 101.1 ± 13.5 | 0.233 |
| PaO2/FiO2 ratio | 422.7 ± 59.9 | 413.4 ± 52.8 | 443.2 ± 56.1 | 0.513 |
| Facemask ventilation | ||||
| HR (bpm) | 68.0 ± 15.3 | 69.1 ± 8.4 | 75.8 ± 15.6 | 0.242 |
| SBP (mmHg) | 116.2 ± 18.7 | 117.0 ± 19.1 | 107.5 ± 15.5 | 0.281 |
| DBP (mmHg) | 61.6 ± 10.4 | 68.3 ± 12.0 | 63.6 ± 7.4 | 0.192 |
| MAP (mmHg) | 80.5 ± 10.3 | 82.1 ± 13.1 | 78.4 ± 8.8 | 0.663 |
| PaO2/FiO2 ratio | 381.7 ± 84.6 | 392.0 ± 56.5 | 435.0 ± 59.0 | 0.221 |
| Post-induction ventilation | ||||
| Driving pressure (cmH2O) | 11.6 ± 4.1 | 10.9 ± 2.6 | 9.1 ± 1.8 | 0.032 |
| HR (bpm) | 72.1 ± 14.0 | 75.2 ± 11.9 | 75.3 ± 13.5 | 0.751 |
| SBP (mmHg) | 118.0 ± 15.8 | 126.5 ± 30.9 | 108.7 ± 17.1 | 0.113 |
| DBP (mmHg) | 64.0 ± 10.2 | 70.8 ± 13.5 | 61.9 ± 9.6 | 0.113 |
| MAP (mmHg) | 86.1 ± 13.7 | 91.3 ± 20.1 | 78.5 ± 11.4 | 0.091 |
| PaO2/FiO2 ratio | 382.9 ± 96.0 | 385.9 ± 84.5 | 446.8 ± 54.2 | 0.133 |
Data are presented as the mean ± SD unless otherwise noted
HR Heart rate, SBP Systolic blood pressure, DBP Diastolic blood pressure, MAP Mean arterial blood pressure
Discussion
This study demonstrated that the application of 10 cmH2O PEEP rather than lower PEEP levels to facemask ventilation after apnoea effectively prevented post-induction atelectasis formation in lung-healthy surgical patients, as evidenced by the EIT-monitored dorsal ΔEELV improvements, lower GI index, and post-induction drive pressure. Notably, this effect was achieved without inducing significant alveolar overdistension or hemodynamic compromise, which is a critical balance in perioperative respiratory management. Our findings provide insights into PEEP use during induction of general anesthesia for prevention of post-induction atelectasis in adult surgical patients.
Anesthesia-induced atelectasis formation and ventilation redistribution to non-dependent lung regions impair gas exchange by increasing the ventilation–perfusion mismatch and pulmonary shunt fraction. Brismar et al.. demonstrated that the development of atelectasis was associated with increased pulmonary densities on computed tomography scans [14]. In our study, the EIT method confirmed lung atelectasis during and after induction of general anesthesia, and the change in pulmonary densities could be reflected partly by ΔEELI.
Several factors are responsible for lung atelectasis during induction of general anesthesia. First, induction of general anesthesia leads to cranial displacement of the abdominal organs and diaphragm due to loss of muscle tone, resulting in compression of basal lung segments [15]. Secondly, the high oxygen concentration used for preoxygenation contributes to the formation of resorption atelectasis. Furthermore, both muscle relaxation and ventilation patterns were found to significantly influence the development of atelectasis [16–18]. Atelectasis formation varies widely among surgical patients, with up to 90% affected. Notably, up to 20% of the lung tissue in the basal sections can collapse following induction of general anesthesia [15]. In this study, we chose the dorsal △EELV as a substitute for atelectasis. It has been reported that △EELI produced by PEEP strongly correlates with △EELV measured with the nitrogen washout technique [13, 19]. Post-induction atelectasis formation and airway closure are also associated with an increase in ventilation inhomogeneity [20]. Actually, we found higher GI indices in the two lower PEEP groups.
PEEP is recommended to prevent intraoperative development of atelectasis by maintaining positive transpulmonary pressure when alveolar pressure reaches its lowest end-expiratory level [3]. Further, PEEP improves intraoperative gas exchange and respiratory mechanics [17, 18], and prevents the mechanical and biological injuries associated with atelectasis, which consequently improves postoperative pulmonary outcomes [19, 21]. It is suggested that pre-oxygenation with positive pressure (PEEP with or without pressure support) during induction of general anesthesia is a simple and effect approach to improve oxygenation and limit atelectasis formation by increasing expiration lung volume [22]. Therefore, a PEEP could be applied preferably immediately after the onset of apnoea, especially in clinical situations with increased risk of airway closure and atelectasis. However, the impact of PEEP application on facemask ventilation in adults is unknown. Our results showed that 10 cmH2O PEEP was associated with less post-induction atelectasis and a driving pressure than 5 cmH2O PEEP. Higher post-induction PaO2/FiO2 ratio in the PEEP10 group than in the other two groups, although there were no significant differences among the three groups, suggesting a potentially better oxygenation reserve with 10 cmH2O PEEP during facemask ventilation. As individualized PEEP settings based on body mass index and surgical conditions are needed [23], whether it is optimal for obese patients with 10 cmH2O PEEP during induction of general anesthesia requires further study. Interestingly, in this study 5 cmH2O PEEP did not improve lung atelectasis when compared with 0 cmH2O PEEP. It is reported that the mean closing pressure is estimated as 6 cmH2O in a small number of anesthetized patients with mechanical ventilation [24]. Five cmH2O PEEP may not be sufficient to keep alveolar pressures above critical closing pressures, thus fail to open collapsed lung units, at end-exhalation, especially in the dorsal lung.
The recruitment maneuver is believed to prevent atelectasis during mechanical ventilation, which involves transiently elevating the airway pressure (e.g., 30–40 cmH2O for 30–40 s) to re-open the collapsed alveoli. One could argue that recruitment maneuvers after induction could be an alternative strategy to largely reverse post-intubation atelectasis. Nevertheless, the efficacy of recruitment maneuvers depends significantly on subsequent ventilation strategies. It has been reported that when pure oxygen is used for ventilation following a successful recruitment maneuver, rapid re-collapse of alveoli occurs [15]. In contrast, using 40% FiO2 in nitrogen results in more gradual atelectasis formation. Early atelectasis formation in the dorsal lung regions after induction of general anesthesia leads to a significant reduction in aerated lung volume; thus, the subsequent set tidal volume is distributed over less lung tissue in dependent areas but may lead to relevant overinflation in non-dependent areas. Furthermore, better post-induction oxygenation may increase the non-hypoxic apnea time in cases of difficult airways. Therefore, we emphasize that 10 cmH₂O PEEP during facemask ventilation provides superior physiological outcomes versus post-intubation RM through: (1) proactive prevention of dorsal atelectasis (vs. reactive reversal by RM); (2) sustained alveolar stability even with high FiO₂ (vs. rapid re-collapse post-RM); (3) preserved ventilation homogeneity in dependent lung regions; (4) critical safety extension for difficult airway scenarios. Nevertheless, it should be noted that both should be followed by mechanical ventilation with moderate FiO2 and individualized PEEP levels to maintain the alveolar opening [15].
The application of high PEEP may induce overdistension in non-dependent lung regions and impair systemic hemodynamics. EIT can identify overdistension during PEEP changes [10, 25]. When overdistension is present, the GI index would become greater [11]. One may argue that lung recruitment in dorsal regions might counteract the effects of overdistension on GI and CoV indices. With the improvement in ventilation homogeneity (decrease in GI index) and ventilation distributed towards the dorsal region, but still mainly in the ventral regions (CoV < 50%), overdistension is minimal, even if presented. A PEEP of 10 cmH2O also lowered the MAP and increased the heart rate, but the changes were minimal and statistically insignificant.
This study has several limitations. First, the application of PEEP to facemask ventilation carries the theoretical risk of gastric insufflation, and ultrasonography monitoring of gastric content volume can verify this risk. Second, we recruited only patients with normal pulmonary function and weight. Patients with pre-existing respiratory comorbidities (e.g., COPD or obesity-related pulmonary dysfunction) may require individualized PEEP strategies during induction of general anesthesia, as their distinct pathophysiological mechanisms necessitate tailored approaches to alveolar recruitment. Finally, while our EIT-based approach provided regional ventilation insights, future investigations should integrate other biomechanical analyses, such as transpulmonary pressure measurements and stress-strain curve assessments, to optimize patient-specific PEEP titration strategies.
Conclusion
In lung-healthy and normal-weight surgical patients, 10 cmH2O PEEP during induction of general anesthesia significantly reduced the formation of dorsal lung atelectasis while maintaining favorable respiratory mechanics and hemodynamic stability. These findings may provide a novel ventilation strategy during induction of general anesthesia for selected populations, such as obese patients, to reduce early atelectasis development and increase safe apnoea time after induction of general anesthesia. Further research is needed to evaluate the effects of this strategy on postoperative pulmonary outcomes.
Acknowledgements
The authors would like to thank the surgeon colleagues for their assistance in carrying out the study.
Authors’ contributions
Y. L., W. L., and Y. J. performed the trial procedures, analyzed the data, and drafted the manuscript. Z.Z. provided statistical advice and significantly revised the manuscript. Z. J. conceived the study, participated in its design and coordination, and helped revise the manuscript. All authors have read and approved the final manuscript.
Funding
Natural Science Foundation of China (No. 82171261, to Jun Zhang; No.82301443), and the Shanghai Science and Technology Commission (No. 22Y11904200, to Jun Zhang).
Data availability
The datasets used and/or analyzed during the current study are available from the corresponding author upon reasonable request.
Declarations
Ethics approval and consent to participate
This trial was approved by the Ethics Committee of Fudan University Shanghai Cancer Center (No. IRB 2503-Exp148), and written informed consent was obtained from all the participants before enrollment. This study adhered to the Declaration of Helsinki. Our manuscript adhered to CONSORT guidelines. All methods were performed in accordance with relevant guidelines and regulations.
Consent for publication
Not applicable.
Competing interests
The authors declare no competing interests.
Footnotes
Publisher’s Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Li Yang MD and Lei Wu MD are contributed equally to this work.
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Data Availability Statement
The datasets used and/or analyzed during the current study are available from the corresponding author upon reasonable request.