Prognostic value of the early lung ultrasound B-line score for postoperative pulmonary insufficiency in patients undergoing thoracic surgery: an observational study

Patients and study design

This study was approved by the Ethics Committee of The Affiliated Changzhou No. 2 People’s Hospital of Nanjing Medical University ([2020]KY134-0l) and registered with the Chinese Clinical Trials Registry (ChiCTR2000040374). Written informed consent was obtained from all patients. Patients described as ASA classification 1 to 3 who were undergoing elective lung surgery under general anesthesia at The Affiliated Changzhou No. 2 The People’s Hospital of Nanjing Medical University between December 15, 2020, and March 1, 2021, was considered for inclusion in this prospective cohort study. Patients were excluded from the study if they had pre-existing pulmonary fibrosis, if they were undergoing several surgeries during hospitalization, if they were undergoing cardiac surgery, if they had a thoracic deformity (leading to difficult image acquisition), or if they had obvious postoperative stress factors (e.g., bleeding, severe pain, severe arrhythmia). Eligible study patients were then divided into two groups based on the ratio of partial pressure of arterial oxygen to fraction of inspired oxygen (PaO2/FiO2) 3 days after surgery [3, 18,19,20]: the normal group (PaO2/FiO2 ≥ 300) and the pulmonary insufficiency (PPI) group (PaO2/FiO2 < 300) [21] (Fig. 1).

Fig. 1figure 1

Flowchart demonstrating patient selection

Anesthesia

For induction of anesthesia, patients were administered sufentanil 1 μg/kg, propofol 2 mg/kg, and rocuronium 0.9 mg/kg. Midazolam 0.1 mg/kg was also administered. For maintenance of anesthesia, patients were given 1% sevoflurane by continuous inhalation, remifentanil 0.2 ug/kg/min, propofol 5 mg/kg/h, and cisatracurium 1 mg/kg/h. The dose rate of all medications was dynamically adjusted to maintain the bispectral index at 40 to 60. A double-lumen tube was used to achieve single-lung ventilation. A pressure controlled-volume guaranteed ventilation mode was used, with tidal volume set at 4 to 6 mL/kg and airway peak pressure < 30 mbar. The respiratory rate was adjusted to maintain an end-tidal carbon dioxide level between 35 and 45 mmHg. Epidural analgesia was administered to all patients.

Lung ultrasound B-line score

The B-line is described as a comet-tailed artifact originating from the pleural line, which extends vertically from the pleura to the bottom of the screen without attenuation, moving with sliding of the lung. Multiple B-line fusions present as diffuse subpleural hyperechoic shadows [16]. A portable ultrasound convex array probe (probe Model C5-1, Phillips-CX50, Andover, MA, USA) was used for lung ultrasound, with the patient placed in the supine position. Lung ultrasound was performed by the same sonographer for every case using the classic 8-zone method. In each area, the probe was first placed perpendicular to the ribs, and the B-line count was conducted once the bat sign was identified. The probe was then placed parallel to the intercostal space, and the maximum number of B-lines was counted twice. In particular, the probe was adjusted three times in each region to obtain the maximum value (Fig. 2). B-line scoring rules are as follows: A line or no B-line, 0 points; each B-line, 1 point; fusion B-line accounting for 50% of the screen area, 5 points; an area of 75%, 8 points; full screen B-line, 10 points; total score, 0–80 points (Fig. 3).

Fig. 2figure 2

Zones followed for lung ultrasound examination

Fig. 3figure 3

Example of lung ultrasound B-line score. a, b, c, and d correspond to scores of 0, 3, 4, and 5, respectively

Fig. 4figure 4

Change in PaO2/FiO2 from 30 min after extubation to postoperative day 3. The blue and red lines indicate PaO2/FiO2 < 300 and ≥ 300 on the third postoperative day, respectively

Data collection

The B-line score was recorded 30 min after extubation, and the PaO2/FiO2 values were recorded 30 min after extubation and on the 3rd day after surgery. Data regarding patient demographics (age, sex, body mass index, ASA classification, New York Heart Association [NYHA] classification at admission, liver and kidney function indices, hemoglobin level, history of smoking) and surgical factors (operative method, procedure duration, infusion volume) were collected, as well as information about each patient’s postoperative hospital stay.

Statistical analysis

Based on a previously reported incidence of PPI in thoracic surgery of 20% [22] and the anticipated sensitivity and specificity of lung ultrasound of 95% with a 10% margin of error, the estimated sample size was 91. Considering 5% attrition during the study, a total of 95 patients were enrolled. Data processing was performed using IBM SPSS Statistics 20 (IBM Corporation, Armonk, NY, USA) and GraphPad Prism Software version 8.0.2 (GraphPad Software, La Jolla, CA, USA). The data were checked for normality using the Shapiro Wilks test, and the continuous variables were expressed as the mean and standard deviation or the median and interquartile range accordingly. Student’s t test and Mann‒Whitney U rank-sum tests were used for comparison of normal and nonnormally distributed data, respectively. Categorical variables were expressed as numbers and percentages, and data differences between the two groups were compared using a chi-square test. The difference in PaO2/FiO2 at 30 min after extubation and on postoperative Day 3 was compared using a paired t test. Linear regression models were used to assess the interdependence of baseline parameters, B-line scores and PaO2/FiO2 on postoperative Day 3. Variance inflation factors (VIFs) were calculated to assess continuous variable covariance. A stepwise forward logistic regression model (p for entry < 0.1, for exit > 0.1) was constructed to identify independent risk factors for postoperative pulmonary insufficiency. Odds ratio values were calculated, and the best predictive values for continuous variables were calculated based on recipient operating characteristic curves and the Youden index. The model fit was assessed using the Hosmer and Lemeshow statistic. A P-value < 0.05 was considered statistically significant (see Additional file 1).

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