A model-based validation study of postoperative complications with considerations on operative timing
Original Article

A model-based validation study of postoperative complications with considerations on operative timing

Kun Qian1, Simeng Wu1, Weishan Lee2, Shiwen Liu2, Ailun Li2, Jing Cang2, Fang Fang2

1Department of Information and Intelligence Development, Zhongshan Hospital, Fudan University, Shanghai, China; 2Department of Anesthesia, Zhongshan Hospital, Fudan University, Shanghai, China

Contributions: (I) Conception and design: J Cang, F Fang; (II) Administrative support: None; (III) Provision of study materials or patients: None; (IV) Collection and assembly of data: K Qian, S Wu, W Lee, S Liu, A Li; (V) Data analysis and interpretation: K Qian, S Wu, J Cang, F Fang; (VI) Manuscript writing: All authors; (VII) Final approval of manuscript: All authors.

Correspondence to: Jing Cang; Fang Fang. Department of Anesthesia, Zhongshan Hospital, Fudan University, Shanghai, China. Email: cangjing_zs@sina.com; fang.fang@zs-hospital.sh.cn.

Background: Surgery is a highly technical procedure relying on high mental acuity and manual dexterity. The possibility that surgical outcomes and post-operative complications could be subject to influence by fatigue and/or circadian rhythms in surgeons has been investigated with inconsistent results.

Methods: We conducted a retrospective study to assess the significance of operative timing on classifying surgical complications using an interpretable machine learning approach. We trained various linear, generative as well as tree models on the surgical record data collected from a university-affiliated, tertiary teaching hospital in China by performing parameter tuning using grid search cross-validation for optimizing the F1 score.

Results: The results indicated that XGBoost was the best-performing model overall and its feature importance was shown to provide insight into possible timing-related associations with postoperative complications. We observed that the duration of surgery acted as the strongest indicator, and while surgery initiated at night (between 9 pm and 7 am) also ranked higher on the feature importance scale, it bore less significance than other factors such as the patient’s age, gender, and type of surgery performed.

Conclusions: We showed that surgical records could be used to demonstrate that operative timing might affect the occurrence of postoperative complications, but only in a relatively mild way while potentially entangling with multiple factors.

Keywords: Postoperative complication; circadian rhythm; machine learning; cross validation; XGBoost; feature importance

Submitted Dec 24, 2020. Accepted for publication Mar 30, 2021.

doi: 10.21037/atm-21-669


Sleep deprivation (SD) is prevalent in the general population and may affect human attention and working memory (1). During prolonged wakefulness, the homeostatic drive for sleep competes with an effort to remain awake, resulting in impaired cognitive functions (2), errors, and accidents, especially during the circadian night (3,4). Circadian variation in performance is most evident when SD is present and chronic insufficient sleep (fewer than 5.6 hours of sleep per day) could negatively impact neurobehavioural performance, self-assessment, and alertness, even without extended wakefulness (5).

In medical circumstances, acute and/or chronic SD were found to affect the performance, learning, and personal well-being of medical trainees (6-8). A nationwide survey of 2,737 American residents in different specialties showed that extended working hours raised the risk of self-reported medical errors, patient fatalities, and attention failures (9). Deterioration of fine motor skills with SD was also shown in surgical residences (10,11), while anesthesia residents demonstrated sleepiness like narcolepsies even with no call duty over the preceding 48 hours (12). Since surgery is a highly technical procedure and is potentially vulnerable to fatigue and circadian rhythms, it is of great importance to investigate the relationship between operative start time as well as the duration and outcomes of surgery, for the purpose of improving healthcare.

In this study, we took postoperative complications from hospital stay as the outcome of interest, as previous research on general and vascular surgical procedures indicated that morbidity, instead of mortality could be strongly associated with the start time of surgery after appropriate adjustment (13). However, the results of possible associations between the two were found to be inconsistent and dependent on surgical disciplines. For example, operative timing in emergency general surgery was associated with increased postoperative complications (14) and the operative start time of cardiac surgery affected intraoperative transfusion rates (15). However, postoperative complications and overall survival rates did not vary with start time in the performance of radical gastrectomy (16), hip fracture fixation (17), renal transplantation (18), and liver resection (19). The outcome of minimal invasive endometrial cancer surgery and laparoscopic sacrocolpopexy (20) also had no association with operative start time (21) and although afternoon surgeries were found to be associated with increased levels of cortisol and inflammatory cytokines [interleukin (IL)-6, IL-8] (22), no conclusion of correlation between operation time and postoperative infectious complications (23).

This inconsistency in the results of studies investigating the association between operative timing and postoperative complications led us to consider the following alternative but closely related question: How does operative start time and duration rank in importance with other factors signifying a case of complication? To answer this question, we used an interpretable machine learning approach (24) by training various interpretable models and performing cross-validation on the training data, which in this case were retrospective surgical records collected, and chose the best-performing model for the classification task. We checked whether different time-domain features were important to classifying a case of complication from this model via the feature importance scores. In our study, we provide details of the dataset for this study and the pipeline of modeling and obtaining feature importance. We also discussed the factors that might lead to our observations and the implications of the results. We present the following article in accordance with the TRIPOD reporting checklist (available at http://dx.doi.org/10.21037/atm-21-669).


Study design

This single-center, retrospective validation study was approved by the Review Board of the Ethics Committee of Zhongshan Hospital affiliated with Fudan University. All procedures performed in this study involving human participants were in accordance with the Declaration of Helsinki (as revised in 2013). Individual consent for this retrospective analysis was waived. Zhongshan is a 2,005-bed major tertiary teaching hospital serving over 100,000 inpatients and 4,000,000 outpatients as well as emergencies annually. The records of 167,001 patients were collected from the hospital database, which involved all surgical procedures performed between January 1, 2018, and November 2, 2020. The following attributes were extracted: date and time the surgery was commenced and completed; duration of surgery; length of stay; surgical discipline; patient age and gender; admission and discharge consultation summaries; preoperative comorbidity (if any); and postoperative complications (if any). We excluded surgeries/patients under specific conditions, such as interventional procedures without anesthesia or sedation, anesthesiology surgeries, and transplants, resulting in 107,481 records being obtained for the study. Details of exclusion are shown in Figure 1.

Figure 1 Pipeline of surgical record exclusion to generate final records for this study.

We explored the associations of attributes (or features) with postoperative complications to achieve a better understanding of what features had the strongest correlation with the presence of complications. We also observed the hourly rate of complications trending with operative start times which we reason could result from multiple factors. The machine learning models used for generating cross-validation scores for the classification task included Logistic Regression (LR), Naive Bayes, CART, Random Forest (RF) (25), GBDT (26), AdaBoost (27), and XGBoost (28), LightGBM (LGB) (29), and CatBoost (30), which extracted information hidden in the surgical record data and detected those features having the strongest correlation with the binary target.

Statistical analysis

Prior to modeling, categorical features were one-hot encoded and all feature values were standardized. This resulted in 70% of the (shuffled) records used to train the models, leaving 30% as the validation set for model evaluation. To address the issue of class imbalance, we used the SMOTE oversampling algorithm (31) which generates synthetic data of the minority class to balance the classes and Recursive Feature Elimination (RFE) feature selection for extracting relevant features. Parameter tuning was performed via grid search 5-fold cross-validation for optimizing the F1 score [2 * precision * recall /(precision + recall)].

For model evaluation, accuracy, precision, recall, and the F1 score were calculated. We generated a Receiver Operating Characteristic (ROC) curve for each model and calculated the area under the curve (AUC). The F1 score and AUC were used as the main performance metrics for model comparison.

We also performed a time series analysis of the monthly rate towards complications, where the series was decomposed into additive components of trend, (periodic) seasonality, and residual. The trend component indicates a general tendency of complications that occurred during the past 3 years, which could be used for forecast and further planning.


Table 1 summarizes the basic statistics of several attributes of the 107,481 surgical records for this study, among which 7,187 (6.69%) postoperative complications were detected. Table 2 describes complications categorized by the surgical discipline and grouped start time of the surgery {morning: [7 am, 12 pm); afternoon: [12 pm, 5 pm); evening: [5 pm, 7 am)}. The results show that the percentage of complications is highest in cardiac surgeries (26.32%), followed by neurological (10.73%) and plastic surgeries (7.82%). The percentage of complications is highest in surgeries which commence in the morning (7.73%) while lower in the afternoon group (6.18%) and lowest in the evening group (4.44%).

Table 1
Table 1 Summary statistics of several attributes of the 107,481 surgical records
Full table
Table 2
Table 2 Postoperative complications (total: 7,187) in the 107,481 surgical records, categorized by surgical disciplines and grouped start time of the surgery {morning: [7 am, 12 pm); afternoon: [12 pm, 5 pm); evening: [5 pm, 7 am)}
Full table

Figure 2 shows the hourly rate of complications against the histogram of surgeries, revealing that surgeries starting between 8–9 am lead to the highest percentage of complications (1,431/10,405, 13.75%), and that a linear trend drawn on the hourly rate of complications leans slightly upwards as the day progresses. Surgeries that start after 9 pm were observed to result in local peaks in complication rates, including 10–11 pm (23/320, 7.19%), 1–2 am (5/50, 10%), 4–5 am (4/12, 33.33%), and 6–7 am (10/72, 13.89%).

Figure 2 Postoperative complications (percentage: blue line; 95% CI: black dotted lines) in terms of the start time (hour) of surgery, with general linear trend (red line) and 95% CI (red shade), against the histogram of surgeries.

We performed RFE feature selection and then applied different machine learning models with cross validation, and the resulting performance metrics (accuracy, precision, recall, F1 score, and AUC) are listed in Table 3. It is clearly seen that XGBoost (F1: 0.95, AUC: 0.98) achieves the best evaluation performance among these models in all categories of performance metrics. Figure 3 compares the ROC curves of these models and shows XGBoost has the largest AUC and best optimal cutoff point. We calculated the relative feature importance for the XGBoost model and Figure 4 displays the most important eight features with duration of surgery, patient age, and cardiac surgical discipline as the three most important features for classifying a case of complication. Surgical disciplines such as plastic, liver, and general are also relatively important for classification, while patient sex—female [0] or male [1], ranks after these surgical disciplines in feature importance, as there is a higher proportion of male patient complication cases (4,520/55,515, 8.14%) than female cases (2,667/51,966, 5.13%) in the relatively balanced patient samples (male/female sample size ratio: 1.0682). The manually constructed feature ’Night’ (1: start time of surgery between 9 pm–7 am; 0 otherwise) also appears to be important, indicating it could be useful in classifying a case of complication.

Table 3
Table 3 Results of model evaluations for postoperative complication classification
Full table
Figure 3 ROC curves of machine learning models for postoperative complication classification.
Figure 4 Feature importance of XGBoost for postoperative complication classification.

The monthly variation of complication rates is shown in Figure 5. For the past 3 years, the month of February was associated with the highest percentage of complications (405/4,416, 9.17%) while the month of July with the lowest (664/11,371, 5.84%), with a general trend decreasing during the year. The traditional Spring Festival is the biggest family holiday period in China normally beginning in February, which could explain why the number of surgeries is the lowest in that month while the percentage of complications tends to be the highest as only patients with severe conditions might undergo surgery during that period. Figure 6 shows the monthly variation of complication rates over the 3-year span. A general decreasing trend in the percentage of complications can be seen across each year (generally between 7.5% and 10.0% for 2018; between 5.0% and 7.5% for 2019; and approximately 5.0% for 2020). An additive decomposition of the monthly complication rates into trend, seasonal, and residual components is shown in Figure 7 and shows a decreasing trend over the past 3 years, as well as a roughly 12-month seasonal periodicity with highest and lowest peaks occurring in the months of February and July, respectively.

Figure 5 Postoperative complications (percentage of each month for 2018–2020 total: red dots connected by blue line, 95% CI: black dotted lines) in terms of the month of surgery, with general linear trend (red line) and 95% CI (red shade), against the histogram of surgeries.
Figure 6 Postoperative complications (percentage of each month for 2018–2020 separately: blue line, 95% CI: black dotted lines) in terms of the month of surgery, and the histogram of surgeries.
Figure 7 Decomposition of monthly postoperative complication rates into trend, seasonal, and residual components.


We performed cross validation on the collected records of patients undergoing surgery at the Zhongshan Hospital between January 1, 2018 and November 2, 2020. The resulting best-performing model was XGboost, for which the feature importance scale indicated that duration of surgery most strongly signified postoperative complications. In addition, operative timing features showed less correlation with complications, with only one feature of operative start time, i.e., surgeries that started during the night (between 9 pm and 7 am), ranking 10th in feature importance. We observed that the rate of complications had some peaks during the night (see Figure 2), although there was insufficient evidence that surgeries started during that time period were correlated with a more frequent occurrence of complications. Some of these surgeries might be emergencies and with limited samples, there is not enough evidence supporting a relatively worse performance during the night in terms of the frequency of complications, and more data needs to be collected for analyzing surgeries commencing after midnight that led to postoperative complications. On the other hand, patient age and sex, and specific surgical disciplines, were more strongly correlated with classifying a case of complication. More data needs to be collected to strengthen the support of these observed characteristics, especially regarding operative timing during the night.

According to previous research, several factors might lead to a higher incidence of postoperative complications after night surgeries. The homeostatic drive for sleep competes with efforts to remain awake after a long wakefulness, resulting in worse performance in professional activities including complicated surgical procedures (32-34), but not simple ones (35). SD and circadian hormonal rhythm disturbance could also impair patient recovery from surgery (36-39). The influence of circadian rhythms on both medical care providers and patients might simultaneously contribute to a higher rate of complications after surgeries commencing late in the night.

Long-lasting work leads to fatigue which is associated with decline in technical skills (11,40), slower reaction, and inappropriate decision making (41,42). Studies have shown that the number of hours already worked has a significant impact on increasing the risk of certain adverse outcomes during unscheduled deliveries (43). In the present study we found that surgeries starting during the night (between 9 pm and 7 am) were more likely to be associated with complications, although the increase was only small. In our hospital, most midnight surgeries are emergencies which are performed by chief residents during 24-hour shifts. Fatigue is a prominent issue for these practitioners and may contribute to this observation.

We also observed a higher rate of complications for surgeries starting between 8–9 am, which could be due to the hospital arranging a large portion of major and prolonged surgeries during that time period. Figure 8 shows the box plot of duration of surgery in terms of its start time. Surgeries commencing between 8–9 am have the largest medium duration is 255.0 (IQR, 135.0–345.0) min during the day, and variation of the mediums for duration of surgery closely match that of complication rates, which is another way to appreciate the strong correlation between these two variables.

Figure 8 Box plots of duration of surgery (min) in terms of the start time (hour) of surgery.

In the time series analysis, the monthly rate of complications demonstrated periodic seasonality with a general decreasing trend during the year as well as across the 3-year span. The relatively high rate of complications in the month of February could be explained by the timing of the traditional Spring Festival which is the most important holiday of the year in China. During that long vacation, elective surgeries are rare, and most surgeries are emergencies. It was curious that the month of July saw the least frequent occurrence of complications. If a seasonal rhythm exists, this occurrence pattern could be partly explained by previous research on mortality which showed a peak occurrence of postoperative complications in winter (44). The general decreasing trend of postoperative complications in the 3-year span indicates a constantly improving medical performance and working system in our hospital.

There are limitations of our study associated with data collection. Firstly, our data were collected from a single-center thus might not simply generalize to other settings. It is known that there are variations in working systems as well as patient characteristics between hospitals and locations. Secondly, the electronic medical records did not reveal detailed information about postoperative complications, such as their specific type and severity, and both surgical and non-surgical complications were listed together in the data. The extraction of such details would benefit future studies. Thirdly, the sample size of surgeries commencing during the night (9 pm to 7 am) was around 1,500, which may be too small to rule out false positives. Finally, we excluded transplant surgeries during data preparation because these patients are likely to be more complicated thus subject to an inherently higher rate of complication than in other surgeries. In addition, transplants are mostly scheduled to commence at night due to donor-related factors, which would further affect the complication rate for night surgeries.

The results of this retrospective study suggest operative timing could influence postoperative complications. However, more research is required to determine higher-order correlations.


Funding: This work was supported by WITMED Project of Zhongshan Hospital Affiliated to Fudan University (2020ZHZS23).


Reporting Checklist: The authors have completed the TRIPOD reporting checklist. Available at http://dx.doi.org/10.21037/atm-21-669

Data Sharing Statement: Available at http://dx.doi.org/10.21037/atm-21-669

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at http://dx.doi.org/10.21037/atm-21-669). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. All procedures performed in this study involving human participants were in accordance with the Declaration of Helsinki (as revised in 2013). This single-center, retrospective validation study was approved by the Review Board of the Ethics Committee of Zhongshan Hospital affiliated with Fudan University. Individual consent for this retrospective analysis was waived.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.


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Cite this article as: Qian K, Wu S, Lee W, Liu S, Li A, Cang J, Fang F. A model-based validation study of postoperative complications with considerations on operative timing. Ann Transl Med 2021;9(8):708. doi: 10.21037/atm-21-669