Construction and validation of a nomogram for predicting prolonged air leak after minimally invasive pulmonary resection
World Journal of Surgical Oncology volume 20, Article number: 249 (2022)
Prolonged air leak (PAL) remains one of the most frequent postoperative complications after pulmonary resection. This study aimed to develop a predictive nomogram to estimate the risk of PAL for individual patients after minimally invasive pulmonary resection.
Patients who underwent minimally invasive pulmonary resection for either benign or malignant lung tumors between January 2020 and December 2021 were included. All eligible patients were randomly assigned to the training cohort or validation cohort at a 3:1 ratio. Univariate and multivariate logistic regression were performed to identify independent risk factors. All independent risk factors were incorporated to establish a predictive model and nomogram, and a web-based dynamic nomogram was then built based on the logistic regression model. Nomogram discrimination was assessed using the receiver operating characteristic (ROC) curve. The calibration power was evaluated using the Hosmer-Lemeshow test and calibration curves. The nomogram was also evaluated for clinical utility by the decision curve analysis (DCA).
A total of 2213 patients were finally enrolled in this study, among whom, 341 cases (15.4%) were confirmed to have PAL. The following eight independent risk factors were identified through logistic regression: age, body mass index (BMI), smoking history, percentage of the predicted value for forced expiratory volume in 1 second (FEV1% predicted), surgical procedure, surgical range, operation side, operation duration. The area under the ROC curve (AUC) was 0.7315 [95% confidence interval (CI): 0.6979–0.7651] for the training cohort and 0.7325 (95% CI: 0.6743–0.7906) for the validation cohort. The P values of the Hosmer-Lemeshow test were 0.388 and 0.577 for the training and validation cohorts, respectively, with well-fitted calibration curves. The DCA demonstrated that the nomogram was clinically useful. An operation interface on a web page (https://lirongyangql.shinyapps.io/PAL_DynNom/) was built to improve the clinical utility of the nomogram.
The nomogram achieved good predictive performance for PAL after minimally invasive pulmonary resection. Patients at high risk of PAL could be identified using this nomogram, and thus some preventive measures could be adopted in advance.
With the popularization of low-dose computed tomography screening, small pulmonary nodules have been increasingly detected in recent years . Minimally invasive pulmonary resection, including robotic-assisted thoracic surgery (RATS) and video-assisted thoracic surgery (VATS), has replaced conventional thoracotomy as the dominant surgical treatment for pulmonary nodules with satisfactory outcomes [2, 3]. Prolonged air leak (PAL) remains one of the most frequent postoperative complications after pulmonary resection, with a reported incidence of up to 30% [4,5,6]. PAL is closely associated with increased pain from longer duration of chest tubes, prolonged hospital length of stay, higher financial cost, increased risk of some complications, and poor postoperative quality of life [7,8,9]. Although many methods for PAL management, such as fibrin sealants, pleural tents, pleurodesis, digital chest drainage system, and endobronchial valves, have been proven effective, PAL remains a great challenge for thoracic surgeons [4, 10]. Therefore, identification of patients at high risk for PAL could enable the doctors to take additional effective procedures to prevent its occurrence in advance.
At present, many risk factors have been identified to increase the incidence of postoperative PAL, including male sex, low body mass index (BMI), smoking history, reduced percentage of the predicted value for forced expiratory volume in 1 second (FEV1% predicted), previous chemoradiotherapy, presence of diabetes and lobectomy [5, 6, 11]. Although a few predictive models for stratifying patients with high risk for PAL have been developed in recent years [12,13,14,15,16,17,18], there is still no standard model to estimate the incidence of PAL. The majority of the previously reported models were based on the data from European and the USA, which might make it not applicable for Asian populations [12,13,14, 16,17,18]. In addition, the differences in PAL definitions and patient selection criteria made the results of these studies less credible [12,13,14,15, 18]. Moreover, in the era of minimally invasive pulmonary resection as the dominant surgical procedure, traditional thoracotomy is rarely performed, and thus the clinical utility of previous models incorporating thoracotomy as a risk factor would be greatly reduced.
The purpose of this study is to develop a clinical prediction model and nomogram to estimate the risk of PAL after minimally invasive pulmonary resection for both benign and malignant lung tumors using preoperative and intraoperative characteristics in a large cohort from a single center in China. The predictive nomogram could stratify the patients into different risk categories and assist thoracic surgeons in making clinical decision.
Patients and methods
This retrospective study was approved by the Ethics Committee of the Qilu Hospital of Shandong University (registration number: KYLL-202008-023-1) and all patients provided informed consent for the use of their clinical information.
A prospectively maintained departmental database of the Qilu Hospital of Shandong University was retrieved for patients who underwent minimally invasive pulmonary resection from January 2020 to December 2021. The exclusion criteria were: (I) patients aged < 18 years old; (II) lung volume reduction surgery and bulla resection; (III) pneumonectomy; (IV) pulmonary resection with mediastinal mass resection; (V) thoracotomy; and (VI) incomplete perioperative data. All enrolled patients were randomly assigned to the training cohort or validation cohort at a 3:1 ratio using a random split-sample method. The training cohort was used to develop the predictive nomogram, while the validation cohort was used to verify the performance of the nomogram.
Data collection and variable definitions
The following data of eligible patients were collected from the database of Qilu Hospital: (I) demographic data: age, gender, BMI, smoking history, preoperative comorbidities [hypertension, diabetes mellitus, and chronic obstructive pulmonary diseases (COPD)], neoadjuvant therapy, and history of thoracic surgery; (II) preoperative evaluation data: FEV1% predicted, percentage of the predicted value of maximal voluntary ventilation (MVV% predicted), American Society of Anesthesiologists (ASA) score, ABO blood type, peripheral blood lymphocyte count, albumin, and prognostic nutritional index (PNI); (III) surgical data: surgical technique (RATS or VATS), surgical procedure (sublobar resection or lobectomy), surgical range (mono-lobe or multi-lobe), operation side, operation duration, total number of dissected lymph nodes (LNs), and tumor size.
PAL was defined as air leakage more than 5 consecutive days after surgery. A conventional drainage system was used for the majority of the cases, and a digital continuous negative pressure drainage device with a negative pressure range of 6-10 cm water column was applied for patients with significant PAL. The chest tube could be removed if there was no pneumonia, subcutaneous emphysema or pneumothorax with a daily drainage of less than 200ml. PNI was calculated using the following formula: PNI (%) = albumin (g/L) + 5 × lymphocyte (109/L). Tumor size was defined as the maximum tumor diameter.
Construction of the PAL nomogram
Univariate logistic regression analysis was performed to identify potential risk factors for PAL. All factors with a P value less than 0.20 in univariate analysis were included in further multivariate logistic regression analysis. Independent risk factors (P <0.05 in multivariate logistic regression) were finally used to develop the predictive model. A nomogram was then constructed based on the results of a multivariate logistic regression model by using the “rms” and “DynNom” packages in R project software (version 4.1.1; http://www.r-project.org/). A score of each variable was calculated using the regression model, and the predicted probability of PAL could be derived by summing the scores of the individual variables.
The performance of the predictive nomogram was assessed by discrimination, calibration, and clinical utility. Discrimination is the capacity that a model can correctly distinguish between events and non-events. We used the receiver operating characteristic (ROC) curve to evaluate the discrimination efficiency of the predictive nomogram . Calibration measures how closely the predicted probabilities are consistent with the actual outcomes. The Hosmer-Lemeshow test was used to evaluate the calibration power, and a P value larger than 0.05 indicated satisfactory calibration . A nomogram calibration plot was then formed to further evaluate calibration. Internal validation was performed by using the bootstrapping method with 1000 repetitions . Decision curve analysis (DCA) was performed to evaluate the clinical utility of the predictive nomogram based on net benefits at different threshold probabilities . The optimal cutoff value was determined based on the ROC curve analysis of the training cohort when the Youden index (sensitivity + specificity − 1) reached the maximum value.
Categorical variables were compared using the Pearson Chi-squared test or Fisher exact test. Normally distributed continuous variable was presented as mean ± standard deviation (SD), and the Student t-test was used for comparison. For continuous variables that are not normally distributed, the data was presented as median (interquartile range [IQR]) and compared by the Mann–Whitney U test between the groups. A two-sided P value < 0.05 was considered statistically significant. R Project software (v4.1.1; http://www.R-project.org) and SPSS software (v25.0; SPSS Inc., Chicago, IL, USA) were used for data analysis.
The procedure of identification and selection of the eligible patients is illustrated in Fig. 1. Finally, a total of 2213 eligible patients were included in our study, among whom, the incidence of PAL was 15.4% (341/2213). The enrolled patients were then randomly assigned to the training cohort (n = 1660) or validation cohort (n = 553) at a 3:1 ratio, and there were no significant differences in all variables between the two cohorts (Table 1). According to the presence or absence of PAL, patients were divided into PAL and non-PAL groups. The characteristics of both groups in the training and validation cohorts are presented in Table 2.
Identification of risk factors for PAL
Univariate and multivariate logistic regression analyses were performed in the training cohort to explore independent risk factors for postoperative PAL (Table 3). Univariate analysis indicated that age, gender, BMI, smoking history, presence of COPD, FEV1% predicted, MVV% predicted, ASA score, surgical procedure, surgical range, operation duration, number of LNs dissected, and tumor size were potential risk factors for PAL (P < 0.05). Further multivariate logistic regression revealed that age [odds ratio (OR) = 1.020; 95% confidence interval (CI): 1.004–1.037; P = 0.013], BMI (OR = 0.884; 95% CI: 0.842–0.928; P < 0.001), Smoking history (OR = 1.628; 95% CI: 1.048–2.531; P = 0.030), FEV1% predicted (OR = 0.991; 95% CI: 0.981–1.000; P = 0.046), surgical procedure (lobectomy vs. sublobar resection; OR = 2.100; 95% CI: 1.424–3.096; P < 0.001), surgical range (multi-lobe vs. mono-lobe; OR = 1.820; 95% CI: 1.251–2.648; P = 0.002), operation side (right vs. left; OR = 0.730; 95% CI: 0.545–0.977; P = 0.035), and operation duration (OR = 1.009; 95% CI: 1.005–1.012; P < 0.001) were independently associated with the occurrence of PAL.
All eight independent risk factors for PAL were included to build a logistic regression model. Details of the predictive model are presented in Table 4. Based on the coefficients of the multivariate logistic regression model, a predictive nomogram for PAL was drawn by using the “rms” package of R statistical software (Fig. 2). As shown in this nomogram, there were 11 axes in total, and the axes 2–9 represented the eight variables in the predictive model. The estimated score of each risk factor could be calculated by drawing a perpendicular line to the top points axis, and a further summation could be made to obtain a total point. The total points axis was then used to predict the probability of PAL after minimally invasive pulmonary resection. In addition, to facilitate the wide use of our predictive nomogram for thoracic surgeons, we used the “Dynnom” package to build an operation interface on a web page (https://lirongyangql.shinyapps.io/PAL_DynNom/) to calculate the probability of PAL. By entering or choosing a patient’s preoperative and intraoperative characteristics, the user can obtain the predictive probability of PAL after minimally invasive pulmonary resection.
Predictive performance and validation of the PAL nomogram
The discrimination capacity of the predictive model and nomogram was evaluated by the ROC curve (Fig. 3). The area under the ROC curve (AUC) was 0.7315 (95% CI: 0.6979–0.7651) for the training cohort and 0.7325 (95% CI: 0.6743–0.7906) for the validation cohort, indicating a relatively good prediction accuracy of the nomogram. The optimal cutoff value of the estimated probability of PAL was approximately 16%, and the sensitivity and specificity were 0.649 and 0.708 respectively (see Additional file 1). The Hosmer-Lemeshow test and calibration plot were used to assess the calibration power. The P value of the Hosmer-Lemeshow test was 0.388 for the training cohort and 0.577 for the validation cohort, which suggested an insignificant difference between the predicted probabilities and actual observed probabilities. The calibration plots for both the training (Fig. 4A) and the validation cohorts (Fig. 4B) also demonstrated a good calibration of the predictive nomogram. In addition, the predicted PAL probabilities of different risk categories based on the nomogram were closely consistent with the actual observed PAL rates in the validation cohort, as shown in Table 5.
Clinical utility of the predictive nomogram
DCA was applied to evaluate the clinical utility of the predictive nomogram (Fig. 5). The results showed that the nomogram provided a greater net benefit with a wider range of threshold probabilities for predicting the risk of PAL in both the training and the validation cohorts, suggesting that the nomogram was clinically useful and could enable surgeons to make better clinical decisions.
PAL remains one of the most frequent complications after lung surgery, with a reported incidence of approximately 5% to 30% [4,5,6]. Although minimally invasive surgical techniques have been relatively developed, PAL is still common, leading to a longer hospital stay, increased financial cost, and higher risk of postoperative infection [6, 8, 9]. In this study, we developed a clinical prediction model and devised a nomogram with good predictive performance for PAL after minimally invasive pulmonary resection. The probability of PAL for individual patients could be estimated using this predictive nomogram, and preventive measures could be adopted in advance for high-risk patients.
Prolonged parenchymal air leakage is often caused by impaired healing of damaged alveoli, which is usually associated with poor lung alignment with the parietal pleura . In recent years, several effective innovations in techniques to deal with PAL intraoperatively and postoperatively have emerged, including fibrin sealants, pleural tents, water seal or suction, doxycycline pleurodesis, digital chest drainage system, and endobronchial valves [4, 11, 24,25,26,27,28]. Although these procedures can reduce the incidence of postoperative PAL, the routine use of these adjuncts might result in an increase of unnecessary cost and the waste of medical resources . Therefore, a clinical prediction tool that can identify those patients with a high risk of PAL might guide the doctors to selectively use these adjuncts.
A large number of risk factors for PAL after lung surgery have been identified by prior studies [5,6,7, 30]. However, no studies have focused on the risk factors for PAL after minimally invasive pulmonary resection. Consistent with previous risk factor studies, we identified five independent risk factors for PAL: advanced age, low BMI, low FEV1% predicted, lobectomy, and smoking history. Age is an important factor affecting physical condition of the patients. Elderly patients usually have poor wound healing ability, and thus they are more prone to PAL [14, 31]. Overweight and obese patients are less likely to develop PAL due to their better nutritional status and intrathoracic milieu that favors sealing of lung parenchymal defects [12, 32]. Low FEV1% predicted and smoking history are closely associated with poor lung function, including reduced lung compliance and increased airway resistance, which could impair the recovering from lung injury [14, 33]. It is worth noting that we found that patients who underwent multi-lobar pulmonary resection were more likely to develop PAL. The lower level of pleural pressure caused by less resection of lung parenchyma during sublobar or mono-lobar resection might lead to a reduced risk of PAL . The pleura is usually supported by the chest wall and by adhesions to the chest wall, thereby relieving pleural stress . There will be less support and adhesion of the chest wall to the pleura if more lung parenchyma is removed, and the pleural pressure will increase correspondingly, thereby increasing the risk of PAL. Unlike the previous study performed by Attaar et al, we found that patients undergoing left-sided surgery were at higher risk of PAL compared with right-sided surgery. This might be due to the longer and hypoplastic oblique fissure of the left lung, resulting in greater damage to the pulmonary parenchyma during surgery [36, 37]. Interestingly, we first found that prolonged operation duration was an independent risk factor for PAL. Longer operation duration often means greater difficulty in surgery and more damage to lung tissue, which might increase the risk of PAL. In addition, the duration of lobectomy and multi-lobar pulmonary resection tends to be longer.
Although several models for predicting PAL have been reported previously [12,13,14,15,16,17,18], no model has been focused on predicting the risk of PAL after minimally invasive pulmonary resection. Compared with previously published predictive models for PAL, ours has the following advantages. First, we visualized this predictive model as a nomogram and then built an operation interface for our nomogram on the web page (https://lirongyangql.shinyapps.io/PAL_DynNom/), which greatly optimized the calculation process and improved the clinical usability of this model . Second, we did not include patients undergoing thoracotomy and constructed a risk prediction model for predicting PAL after minimally invasive pulmonary resection for the first time. In an era of minimally invasive pulmonary resection as the mainstream surgical procedure, the clinical utility of our model might be better. Third, we developed a clinical model to predict the risk of PAL using preoperative and intraoperative characteristics without taking the patient's histopathological information into account. Although the pathological type and stage might be associated with the incidence of postoperative PAL , the histopathological results are usually not available prior to surgery and PAL often already occurs by the time we obtain the pathological results after surgery. With this model, we could identify high-risk patients at the end of surgery and intervene as early as possible to avoid PAL. Fourth, we investigated some intraoperative characteristics that have rarely been explored in previous studies but might be closely related to PAL (such as operation duration, number of LN dissected, and tumor size), and incorporated operation duration into the final model. In addition, unlike previous studies, we did not convert continuous variables (such as age, BMI, FEV1% predicted, and operation duration) to categorical variables, which provided our model with much greater discrimination accuracy between patients. Finally, we performed DCA to evaluate the clinical utility of the predictive nomogram . The decision curve demonstrated an obvious net benefit of the nomogram model within the threshold range of 5–30% (incidence of PAL reported in prior literature), indicating a great clinical utility of this nomogram.
The clinical prediction nomogram constructed in this study could assist thoracic surgeons in assessing the risk of PAL in patients after minimally invasive pulmonary resection using preoperative and intraoperative characteristics and then make better clinical decisions. Preventive interventions could be applied to high-risk patients to reduce the incidence of PAL, while the potential harm and increased costs caused by overtreatment could be avoided for low-risk patients. For example, if the individual is identified as a high-risk patient at the end of the surgery, fibrin sealants or pleural tents could be immediately used to reduce the incidence of PAL. And digital chest drainage system and endobronchial valves could also be applied as early as possible to high-risk patients after surgery to avoid PAL. In our institution, we usually use the digital chest drainage system for high-risk patients or patients with suspected PAL to avoid the occurrence of PAL, which has achieved satisfactory outcomes. In addition, better preoperative patient counseling could be also achieved by informing high-risk patients of the possibility of stricter chest tube management, longer hospital length of stay, and higher hospitalization costs.
This study has several limitations that should be considered. First, the single-center retrospective nature of this study might limit the generalization ability of our predictive nomogram, and some uncontrolled confounders might also arise. In addition, the predictive model was only internally validated, thus the selection biases present in the training cohort might also be present in the validation cohort. Further external validation in a multicenter setting is required to determine whether this nomogram could be widely used in other populations. Finally, some factors that may be associated with PAL were not considered, such as pleural adhesion, diffusing capacity of carbon monoxide, surgeons’ experience and radiographic imaging characteristics, which were not available in our database and could be investigated in future research. Despite the above limitations, the independent risk factors for PAL identified in this study and the predictive nomogram constructed in this study could provide the reference for thoracic surgeons' clinical decision-making and pave the way for future research in this field.
We developed a clinical nomogram for the prediction of PAL after minimally invasive pulmonary resection based on preoperative and intraoperative characteristics, and the nomogram achieved good predictive performance for PAL. The risk of PAL for individual patients can be estimated by using this nomogram, and preventive measures may be adopted in advance for those high-risk patients.
Availability of data and materials
The data that support the findings of this study are available on request from the corresponding author.
Prolonged air leak
Robotic-assisted thoracic surgery
Video-assisted thoracic surgery
Body mass index
- FEV1% predicted:
Percentage of predicted value for forced expiratory volume in 1 second
- MVV% predicted:
Percentage of predicted value of maximal voluntary ventilation
Chronic obstructive pulmonary diseases
American Society of Anesthesiologists
Prognostic nutritional index
Receiver operating characteristic
Area under ROC curve
Decision curve analysis
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This work was funded by the National Key Research and Development Program (2021YFC2500904, and 2021YFC2500905), Natural Science Foundation of Shandong Province (ZR2021LSW006), and the Taishan Scholar Program of Shandong Province (ts201712087).
Ethics approval and consent to participate
This retrospective study was approved by the Ethics Committee of the Qilu Hospital of Shandong University (registration number: KYLL-202008-023-1) and all patients provided informed consent for the use of their clinical information.
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The authors declare that they have no competing interests.
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Li, R., Xue, M., Ma, Z. et al. Construction and validation of a nomogram for predicting prolonged air leak after minimally invasive pulmonary resection. World J Surg Onc 20, 249 (2022). https://doi.org/10.1186/s12957-022-02716-w