Effect of driving pressure on early postoperative lung gas distribution in supratentorial craniotomy: a randomized controlled trial

Study design

This was a single-center, randomized, parallel group, patient and outcome assessor-blinded trial exploring a ventilation strategy targeting DP during supratentorial craniotomy conducted between June 21, 2020, and July 1, 2021, at Beijing Tiantan Hospital, Capital Medical University. The study adhered to the Consolidated Standards of Reporting Trials (CONSORT) guidelines. The study was approved by the Ethics Committee of China (ChiECRCT20200137) on June 12, 2020, and registered at ClinicalTrials.gov (NCT04421976). Participants were included after obtaining written informed consent.

Study population

Participants were recruited if they met the following criteria: Glasgow Coma Scale score of more than 8 points, age between 18 and 70 years, American Society of Anesthesiologists (ASA) level ≥ II, mechanical ventilation duration ≥ 2 h, and elective supratentorial craniotomy. Patients were excluded if they met at least one of the following criteria: preexisting severe respiratory disease (chronic lung disease, pneumonia, acute lung injury or acute respiratory distress syndrome) or heart disease, dysphagia resulting from preoperative cranial nerve damage, body mass index (BMI) ≥ 35 kg ⋅ m− 2, mechanical ventilation > 1 h within 2 weeks before the operation, progressive neuromuscular disease, pregnancy and any contraindication to EIT or LUS scan.

Randomization and blinding

Randomization was conducted using computer-generated random numbers sealed in opaque envelopes. Patients were randomly allocated into two groups by the corresponding envelope. Knowing the group task, the anesthesiologist was responsible for the intervention, and the other researchers, blinded to the random allocation, participated in the follow-up visit and data analysis. Chest EIT and lung ultrasonography were performed by the relevant technicians and analyzed by a researcher.

Anesthesia

Smokers had quit smoking more than four weeks before surgery. For intravenous induction, sufentanil (0.2–0.3 µg ⋅ kg− 1), propofol (2–2.5 mg ⋅ kg− 1), and rocuronium (0.6 mg ⋅ kg− 1) were used. Anesthesia was maintained with sevoflurane (0.4–0.5 MAC), remifentanil (0.05–0.2 µg ⋅ kg− 1⋅ min− 1), propofol (3–4 mg ⋅ kg− 1⋅ h− 1) and rocuronium (0.2 mg ⋅ kg− 1, per 50–60 min). Routine perioperative monitoring included invasive blood pressure and the bispectral index (BIS). Mean arterial pressure (MAP) was maintained at > 70 mmHg. Neuromuscular blockade was reversed with neostigmine 0.04 mg·kg-1 and atropine 0.02 mg·kg− 1 when the train-of-four (TOF) ratio was below 0.9. Extubation was performed when patients recovered consciousness and demonstrated sufficient spontaneous breathing. All patients were transferred to the postanesthesia care unit (PACU) after successful extubation and monitored for at least 1 h in the PACU. Supplemental oxygen was administered at 3 L·min− 1 via a face mask. Postoperative pain was assessed at 20 min postoperatively by using the visual analog scale (VAS).

Ventilation protocol

Volume-controlled mechanical ventilation was provided (Datex Ohmeda S/5 Advance, General Electric Healthcare, Helsinki, Finland). All patients were preoxygenated with a 0.8 FiO2 before tracheal intubation for 3 min. After intubation, the initial settings were TV 8 ml ⋅ kg-1 of predicted body weight (PBW), fresh gas 2 L·min-1, FiO2 0.4 or higher if the SpO2 < 92%, inspiratory to expiratory ratio 1:2, and a respiratory rate (RR) adjusted according to normocapnia (PaCO2 between 35 and 45 mmHg). Mechanical ventilation parameters, such as plateau pressure (Pplat), peak pressure (Ppeak) and compliance, were acquired from the anesthetic machine. In the control group, PEEP was set at 5 cmH2O until respiratory restoration [15]. In our study, the ventilation strategy with a fixed PEEP of 5 cmH2O was applied safely to patients with brain tumors [15]. The retrospective study showed that the application of PEEP had no effect on either ICP or cerebral perfusion pressure (CPP) for those without severe lung injury. We adopted the titration of PEEP strategy similar to the literature [16]. In the titration group, the PEEP value was increased stepwise by 1 cmH2O from 2 cmH2O to 10 cmH2O after the patient was placed in position, with TV and RR unchanged. Each level was maintained for 10 breathing cycles, with DP in the last cycle recorded. Then, the level producing the lowest DP was identified as “optimal”, and the individual PEEP was maintained throughout mechanical ventilation. The Pplat should not exceed 30 cmH2O in each group; otherwise, the titration would be terminated in advance.

Measurements

The dynamic changes in aeration distribution can be visualized and evaluated by EIT [17]. The global inhomogeneity index (GI) is a measure that describes the regional ventilation distribution and homogeneity [18]. A lower GI implies more ventilation homogeneity in the lung [18, 19]. In the present study, EIT measurements were performed at three specific time points (PulmoVista 500, Draeger Medical, Lübeck, Germany): preinduction, immediately after extubation and 1 h after extubation. An oblique belt with 16 electrodes was placed between the 5th and 6th intercostal spaces, and the data were recorded after the main cable was linked. Data for 5 min at each time point were recorded, and the belt position on the patient’s skin was marked. GI [20] was calculated offline using customized software to evaluate the distribution of ventilation.

LUS can be used as a fast and easily available bedside test to evaluate lung areation. A-lines are a single line or multiple lines parallel to the pleural line and occur in normal lungs [14, 21]. B-lines are defined as hyperechoic vertical artifacts that originate from the pleural line, reach the bottom of the screen without fading and move synchronously with lung sliding [22]. Sets of at least three hyperechoic B-lines arising from the pleural line in one intercostal space are indicative of interstitial lung syndrome [23]. Each hemithorax was divided into six quadrants using two longitudinal lines (anterior and posterior axillary) and one axial line at the level of the nipples. Each quadrant was assigned a score of 0 to 3 according to a modified grading system: 0, normal lung with sliding pleura and A-lines; 1, three or more scattered B-lines; 2, coalescent B-lines; and 3, consolidated lung. The LUSs (0–36) were then calculated by summing the 12 individual quadrant scores, with higher scores indicating more severe aeration loss [24, 25]. A complete ultrasound examination using an ultrasound machine (Sonosite M-Turbo, Sonosite, USA) and a 6–13 MHz linear transducer array (L25) required an average duration of 10 min. The lung ultrasonic measurement time was consistent with the EIT. LUS was performed by trained anesthesiologists (Fei L and Wei Z, with 1 year and 3 years of experience in LUS, respectively).

To evaluate gas exchange, arterial blood gas was tested (ABL 800, Radiometer, Copenhagen, Denmark), and pH and PaO2 were recorded.

Brain relaxation was evaluated by a neurosurgeon after craniotomy and before cutting the dura mater [26]. It was assigned a score from 1 to 4 points: scores of 1 and 2 points were considered soft/adequate/no swelling/moderate swelling and good, and scores of 3 and 4 points were considered tight and pronounced swelling and bad.

Pulmonary complications within 3 days after the operation were evaluated by the Melbourne Group Scale Version 2 (MGS-2) [27]. PPCs were diagnosed if four or more factors were present.

Outcomes

The primary outcome was the GI value immediately after extubation. The secondary outcomes were LUSs, respiratory system compliance, PaO2/FiO2 ratio and PPCs within 3 days postoperatively.

Sample size calculation

The sample size was estimated for a previous study. A difference of 0.1 in GI between groups according to a previous study was detected [20], with an alpha level of 0.05 and an SD of 10% using the independent t test at 90% power. Considering a dropout rate of 5%, 24 participants were needed per group.

Statistical analysis

Categorical variables are reported as the number (proportion) of patients, normally distributed data are presented as the mean and standard deviation (SD), and nonnormally distributed data are presented as the median (IQR [range]). The Kolmogorov–Smirnov test was used to assess the normality of the distribution. For baseline characteristics between groups, the standardized mean difference(SMD) with 95% confidence intervals (95% CIs) was calculated. Two-tailed unpaired Student’s t test was conducted to compare continuous variables between two groups. The Mann–Whitney U test was conducted to assess the differences between groups for nonnormally distributed data (age, BMI, ventilation duration, intraoperative fluid input, intraoperative bleeding and intraoperative urine). The chi-square test or Fisher’s exact test was used to compare two or more proportions (PPC with 3 days). Two-way ANOVA was conducted to evaluate the effects of group, time, and the interaction on GI, LUSs, respiratory mechanics, PaO2/FiO2 ratio and hemodynamic variables. Because of the repeated measurement, the Holm–Bonferroni method was used to adjust the P value for outcomes. Data analysis was performed using GraphPad Prism 8.0 (GraphPad software, USA). P less than 0.05 was considered to indicate a significant difference.

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