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Overtime work of anesthesiologists is associated with increased delirium in older patients admitted to intensive care unit after noncardiac surgery: a secondary analysis

BMC Anesthesiology volume 24, Article number: 465 (2024) Cite this article

Abstract

Background

Overtime work is common in anesthesiologists due to shortage of manpower. Herein, we analyzed if overtime work of anesthesiologists was associated with delirium development in older patients after surgery.

Methods

This was a secondary analysis of the database from a randomized trial. Seven hundred older patients (aged ≥ 65 years) who were admitted to the intensive care unit (ICU) after elective noncardiac surgery were enrolled in the underlying trial. Anesthesiologists who worked continuously for more than 8 h by the end of the surgery were marked as “work overtime”. Delirium was assessed with the Confusion Assessment Method for the ICU twice daily during the first 7 postoperative days. The association between overtime work of anesthesiologists and development of postoperative delirium was analyzed with multivariable logistic regression models.

Results

All 700 patients (mean age 74.3 years, 39.6% female) were included in this analysis. Anesthesiologists of 281 patients (40.1%) were marked as “work overtime” at the end of surgery. When compared with patients whose anesthesiologists didn’t work overtime, patients whose anesthesiologist worked overtime had a higher incidence of delirium within 7 days (20.3% [57/281] vs. 12.9% [54/419], P = 0.009). After correction for confounding factors, both overtime work (OR 1.87, 95% CI 1.19–2.94, P = 0.007) and prolonged continuous working hours of anesthesiologists (OR 1.08, 95% CI 1.01–1.15, P = 0.020) were associated with an increased risk of postoperative delirium.

Conclusions

Overtime work of anesthesiologists was associated with an increased risk of delirium development in older patients admitted to ICU after major noncardiac surgery.

Trial registration

The underlying trial was registered with Chinese Clinical Trial Registry (https://www.chictr.org.cn/showproj.html?proj=8734; ChiCTR-TRC-10000802; March 18, 2010).

Peer Review reports

Background

Delirium is an acutely occurred cerebral dysfunction syndrome characterized by fluctuating disturbances in attention, consciousness, and cognitive function [1]. It is a common complication in older patients after noncardiac surgery, with reported incidence ranges from 4 to 35% due to differences in patient populations and surgical types [2,3,4,5]. The incidence is especially high in patients who were admitted to the intensive care unit (ICU) [6]. The development of delirium is associated with worse outcomes, including more postoperative complications, prolonged hospital stay, and increased perioperative and long-term mortality [7,8,9,10,11,12,13]. Furthermore, postoperative delirium is considered a strong predictor of cognitive decline and decreased quality of life in early and long-term survivors [8, 10, 14, 15].

With aging population, the number of surgical cases is increasing rapidly [16]. The situation is especially true in China, as evidenced by long working hours and frequently occurring overtime work of anesthesiologists in large tertiary hospitals [17,18,19]. Anesthesia care requires close monitoring of vital signs and prompt management of physiological changes. Tiredness of anesthesiologists caused by working overtime may produce negative impact on the safety of surgical patients. Indeed, an early study showed that the risk of accidents increased exponentially with each additional hour after the ninth consecutive hour of work [20]. It is therefore suggested that a 20-min break should be considered for shifts of > 6 h, and found that short 5–15 min breaks every 1–2 h may improve performance [21].

We suppose that overtime work of anesthesiologists might increase the risk of postoperative delirium in high-risk patients. There is currently a lack of evidence exploring this topic. The purpose of this secondary analysis is to evaluate the association between the working duration of anesthesiologists and the risk of delirium in older patients who were admitted to ICU after noncardiac surgery.

Methods

Study design

This was a secondary analysis of the database from our previously published randomized trial [8]. The underlying trial was approved by the local Clinical Research Ethics Committee (2011–10), registered with the Chinese Clinical Trial Registry (ChiCTR-TRC-10000802; March 18, 2010), and conducted in the surgical ICUs of two tertiary hospitals in Beijing, China. Protocol for this secondary analysis was approved by the Biomedical Research Ethics Committee of Peking University First Hospital (2022–470; October 27, 2022). Since all data were obtained from the available database and the inpatient medical record system, and no patient or family member contact was needed, the Ethics Committee agreed to waive written consents from participants for this analysis. All personal data was kept strictly confidential.

Participants

During the underlying trial, we enrolled patients aged ≥ 65 years who were admitted to the ICU after elective noncardiac surgery under general anesthesia. We excluded patients who had (1) Parkinson’s disease, schizophrenia, epilepsy, or myasthenia gravis before surgery; (2) no ability to communicate before surgery (severe dementia, coma or language barriers); (3) neurosurgery or brain injury; (4) sick sinus syndrome (heart rate < 50 beats/min), second-degree or higher atrioventricular block without pacemaker, or preoperative left ventricular ejection fraction (LVEF) < 30%; (5) preoperative history of severe renal dysfunction (undergoing renal replacement therapy) or severe liver dysfunction (Child–Pugh C grade); (6) an expected survival of ≤ 24 h [8]. All patients enrolled in the underlying trial were included in this analysis.

Anesthesia and perioperative care

Intraoperative monitoring generally included electrocardiogram, pulse oxygen saturation, non-invasive/invasive blood pressure, concentrations of inhalational anesthetics and expired carbon dioxide, bispectral index (BIS), and urine output.

General anesthesia with endotracheal intubation was performed for all patients. Anesthesia was induced with midazolam, sufentanil or fentanyl, and propofol and/or etomidate, and maintained with intravenous infusion of propofol and remifentanil, intravenous injection or infusion of fentanyl or sufentanil, with or without inhalation of sevoflurane and/or nitrous oxide. Muscle relaxation was induced and maintained with rocuronium or cisatracurium. Epidural block was performed in combination with general anesthesia, at the discretion of responsible anesthesiologists. The target was to maintain BIS between 40 and 60, blood pressure within 30% from baseline, and urine output ≥ 0.5 mL/kg/h. Fluid infusion and blood transfusion were performed as per clinical routine.

After surgery, all patients were admitted to the ICU, either with endotracheal intubation or after extubation. Patient-controlled epidural analgesia was provided for those who were given combined epidural-general anesthesia; otherwise, patient-controlled intravenous analgesia was provided. Low-dose dexmedetomidine infusion (0.1 μg/kg/h, from ICU admission on the day of surgery until 8 am on postoperative day 1) was provided according to randomization results of the underlying trial [8]. Supplemental sedatives (for patients with endotracheal intubation) and analgesics were provided when considered necessary. Other perioperative care was provided according to routine practice.

Overtime work of anesthesiologists

The normal working duration is 8 h a day in China. The same policy is also adopted by the many developed countries. However, due to shortage of staff, overtime working was common among anesthesiologists. During the period of our underlying trial, handover of anesthesia care was not a routine practice; responsible anesthesiologists usually worked from 8 am until the end of surgery in an operating room. “Overtime work” was therefore defined when anesthesiologists worked continuously for more than 8 h. We checked anesthesia records of all enrolled patients in the Anesthesia Information System of the participating hospitals. For each enrolled patient, the continuous working hours of responsible anesthesiologist were defined from the beginning of that working day until the end of anesthesia. Anesthesiologists who took care of patients continuously for more than 8 h by the end of the surgery were marked as “work overtime”.

Data collection and outcome assessment

Baseline and intraoperative data were collected during the underlying trial [8]. Preoperative comorbidity was evaluated with the Charlson comorbidity index [22]. The degree of operative stress was stratified into 5 physiologic categories, i.e., very low stress, low stress, moderate stress, high stress, and very high stress [23]. After surgery, delirium was assessed twice daily (8–10 am and 6–8 pm) during the first 7 days or until hospital discharge. Before each delirium assessment, the sedation/agitation level was evaluated with the Richmond Agitation Sedation Scale (RASS; the score ranges from − 5 [unarousable] to + 4 [combative], and 0 indicates alert and calm) [24]. Deeply sedated or unarousable (RASS − 4 to − 5) patients were recorded as comatose and not assessed for delirium. Patients with a RASS score from − 3 to + 4 were assessed for delirium with the Confusion Assessment Methods for the Intensive Care Unit (CAM-ICU) [25, 26] by investigators who had been trained by a psychiatrist [8]. The CAM-ICU detects four features of delirium including (1) acute onset of mental status changes or a fluctuating course, (2) inattention, (3) disorganized thinking, and (4) altered level of consciousness. Patients presented features (1) and (2), with either (3) or (4) were diagnosed as delirium.

Our primary endpoint was the occurrence of delirium during the first 7 postoperative days. Secondary endpoints include occurrence of major complications within 30 days, lengths of stay in ICU and hospital, and all-cause 30-day mortality after surgery. Major complications were defined as new-onset events other than delirium that were deemed harmful to patients’ recovery and required therapeutic intervention, that was grade II or higher on the Clavien-Dindo classification [27].

Statistical analysis

Patients were divided into two groups according to the presence of working overtime or not of their anesthesiologists. Continuous variables were analyzed with independent samples t-test or Mann–Whitney u test. Categorical variables were analyzed with Chi square test or Fisher’s exact test. Time-to-event variables were analyzed with Kaplan–Meier survival analysis and log-rank test.

Univariate logistic regression analysis was used to screen factors that might be related to the development of postoperative delirium, and those with P < 0.20 or were considered clinically important were included in the multivariate logistic regression model using a backward stepwise procedure to evaluate the association between overtime working of anesthesiologists and the development of postoperative delirium. As exploratory analyses, we also analyzed the associations between overtime working of anesthesiologists and the development of major complications and the length of hospital stay after surgery using multivariable logistic regression or Cox proportional hazards models as above, with overtime work or continuous working hours of anesthesiologists entered compulsorily.

SPSS 26.0 software package was used for statistical analysis. A P-value of less than 0.05 was considered statistically significant.

Results

From August 17, 2011, to November 20, 2013, 700 patients were enrolled in the underlying trial and completed the study. All these patients (mean age 74.3 years, 39.6% female sex) were included in this analysis (Fig. 1).

Fig. 1
figure 1

Flowchart of the study

Full size image

Anesthesiologists of 281 patients (40.1%) were marked as “work overtime” at the end of surgery. When compared with patients whose anesthesiologists didn’t work overtime, those whose anesthesiologists worked overtime were younger and less often female; they had lower proportions with hypertension and albumin < 30 g/L, but higher proportions with smoking and alcoholism (Table 1).

Table 1 Baseline variables
Full size table

During the perioperative period, patients whose anesthesiologists worked overtime received more benzodiazepines, endured longer anesthesia and surgery, underwent more intra-thoracic (less superficial and transurethral) surgery and more high and very high stress (less low and moderate stress) surgery, lost more blood, and received more blood transfusion and fluid infusion during surgery; they were more often admitted to the ICU with endotracheal intubation, received less prophylactic dexmedetomidine but more PCEA and propofol sedation (Table 2).

Table 2 Perioperative variables
Full size table

Postoperative outcomes

When compared with patients whose anesthesiologists didn’t work overtime, patients whose anesthesiologist worked overtime had a higher incidence of delirium within 7 days (20.3% [57/281] vs. 12.9% [54/419], P = 0.009; Fig. 2); they also had a higher incidence of major complications within 30 days (22.8% [64/281] vs. 14.6% [61/419], P = 0.005; Supplement Fig. S1) and stayed longer in hospital after surgery (13.0 days [IQR 9 to 11] vs. 10.0 days [IQR 12 to 14]; P < 0.001; Supplement Fig. S2; Table 3).

Fig. 2
figure 2

Cumulative incidence (A) and daily prevalence of delirium (B) up to 7 days after surgery in patients whose anesthesiologists worked overtime or not

Full size image
Table 3 Postoperative outcomes
Full size table

Association between overtime work of anesthesiologists and postoperative outcomes

Univariable analyses identified 23 factors with P < 0.20 in association with delirium development after surgery, including overtime work and continuous working hours of anesthesiologists (Supplement Table S1). Among these, we excluded intraoperative propofol use (correlated with intraoperative etomidate), postoperative propofol use within 7 days (correlated with ICU admission with endotracheal intubation), and estimated blood loss (correlated with blood transfusion during surgery) from multivariable analysis. According to previous studies, we included chronic use of analgesics or benzodiazepines and perioperative benzodiazepines [28,29,30,31,32,33,34,35], education level [36,37,38,39], renal injury [40,41,42], hematocrit < 30% [43,44,45,46,47,48,49], and Operative Stress Score [23, 50, 51] in the multivariable model. Overtime work and continuous working hours of anesthesiologists were included separately in the multivariable models.

After correction for confounding factors, both overtime work (OR 1.87, 95% CI 1.19–2.94, P = 0.007) and prolonged continuous working hours of anesthesiologists (OR 1.08, 95% CI 1.01–1.15, P = 0.020) were associated with an increased risk of postoperative delirium. Among other factors, older age, female sex, previous stroke, surgeries in other than intra-abdominal or intra-thoracic sites, and ICU admission with intubation were associated with an increased risk, whereas higher body mass index and prophylactic dexmedetomidine use were associated with a decreased risk of postoperative delirium (Table 4).

Table 4 Variables in association with postoperative delirium within 7 days
Full size table

After correction for confounding factors, we did not find significant associations between overtime work or continuous working hours of anesthesiologists and major complications within 30 days or length of hospital stay after surgery (Supplement Tables S2 and S3).

Discussion

Results of this analysis showed that in older patients admitted to ICU after major noncardiac surgery under general anesthesia, overtime work of anesthesiologists was associated with an increased risk of postoperative delirium after correction for confounding factors.

The development of postoperative delirium is a result of complex interaction of multiple risk factors [5, 33, 52, 53]. Generally accepted predisposing factors include old age [3, 35, 42, 48,49,50], low body mass index [50, 51], cognitive impairment [42], comorbidity (like cerebrovascular disease [41, 48, 49], heart failure [42], hypertension [39, 42], and kidney disease [41, 42]), high ASA grade [3, 41], and malnutrition (low albumin [47, 49] and low hematocrit [47,48,49]). And generally recognized precipitating factors include perioperative medication (benzodiazepines [34, 35] and opioid use [42, 49]), surgical strategies [48], long-duration surgery [47, 49,50,51], blood loss/transfusion [3, 47, 49, 50], and mechanical ventilation during ICU stay [35]. Delirium prevention usually requires multicomponent interventions [54]. Indeed, measures included in Enhanced Recovery After Surgery (ERAS) were found to be effective in preventing delirium in geriatric patients [55, 56]. Our study provides clew of an additional precipitating factor and thus a potential new target of intervention. Avoiding overtime work or relieving workload (such as via artificial intelligence) of anesthesiologists may help reduce delirium in these high-risk patients but requires further confirmation [57].

Aging population is increasing in China. It is estimated that more than half of old people will receive at least one surgery during their remaining life span [58]. The development of economy and medical science also makes it possible that more people to receive surgical treatment. These explain why the number and complexity of surgeries are increasing rapidly [16, 59]. Alone with this, overtime work and tiredness of anesthesiologists is unavoidable due to shortage of manpower [60]. Indeed, the number of anesthesiologists per 10,000 people is only 0.4 in China, compared with 3 in the United States and 2.8 in the UK [61].

In a study of German working population, overtime work was associated with an increased risk of occupational accidents [20]. Similar association seems to exist among anesthesiologists. For example, investigations showed that up to 50% of anesthesiologists believed that they experienced medical errors when they were tired [62,63,64]. In a survey of New Zealand anaesthetists, 86% of respondents reported fatigue-related errors which were more likely to occur when the working hours exceeded their safety limits [65]. These medical errors might have increased the risk of postoperative delirium.

In addition, surgeries that required overtime work of anesthesiologists are often complex and last longer, and usually lead to more severe pain and sleep disturbances as well as inflammation. These might also have contributed to the risk of postoperative delirium [66, 67]. As can be expected, our results also showed that patients whose anesthesiologists worked overtime developed more major complications and stayed longer in hospital after surgery, although we did not find independent associations between anesthesiologists’ workload and these outcome measures.

There are some limitations. First, like all observational studies, we cannot establish a causal relationship between overtime work of anesthesiologists and development of postoperative delirium according to results of this secondary analysis. Second, our underlying trial only recruited patients who were admitted to ICU after elective major noncardiac surgery, and excluded those with schizophrenia, dementia, and severe organ dysfunction and thus at high-risk of postoperative delirium. This limited the generalizability of our results. Third, although multiple regression models were used to adjust for confounding factors in statistical analysis, we cannot rule out the interference of other factors such as differences in anesthesiologists’ experience and patients’ lifestyles. Lastly, as a secondary analysis, the limited sample size diminished the study power (power = 0.72). Nevertheless, this study remains unique in the context of existing literature and provides clues for further investigations.

Conclusions

Among older patients undergoing major noncardiac surgery with scheduled ICU admission, overtime work of anesthesiologists might increase the risk of postoperative delirium. Further studies are required to confirm if limiting working hours of anesthesiologists could reduce delirium and improve outcomes in this patient population after surgery.

Data availability

Anonymized individual patient data collected for this study can be made available upon publication to researchers who provide a sound proposal and ethics approval, considering possible legal restrictions of China. Proposals should be submitted to corresponding author (dxwang65@bjmu.edu.cn or wangdongxin@hotmail.com).

Abbreviations

ICU:

The intensive care unit

RASS:

Richmond Agitation Sedation Scale

CAM-ICU:

Confusion Assessment Methods for the Intensive Care Unit

OSS:

Operative Stress Score

BMI:

Body Mass Index

COPD:

Chronic Obstructive Pulmonary Disease

ASA:

American Society of Anesthesiologists

PCIA:

Patient-Controlled Intravenous Analgesia

PCEA:

Patient-Controlled Epidural Analgesia

NSAIDs:

Non-Steroid Anti-Inflammatory Drugs

OR:

Odds ratio

HR:

Hazard ratio

IQR:

Interquartile range

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Acknowledgements

The authors gratefully acknowledge Drs. Zhao-Ting Meng and Fan Cui (Department of Anesthesiology, Peking University First Hospital, Beijing, China) for their help in data collection.

Funding

This study was supported by National Natural Science Foundation of China (Major Program No. 82293644) and National High Level Hospital Clinical Research Funding (High Quality Clinical Research Project of Peking University First Hospital No. 2022CR78).

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Authors and Affiliations

  1. Department of Anesthesiology, Peking University First Hospital, Beijing, 100034, China

    Yi Zhao, Na-Ping Chen, Xian Su, Jia-Hui Ma & Dong-Xin Wang

  2. Outcomes Research Consortium, Houston, TX, USA

    Dong-Xin Wang

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  1. Yi Zhao
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  3. Xian Su
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Contributions

YZ, XS and DXW designed this study. YZ collected the data. YZ, NPC and JHM analyzed the data. YZ drafted the manuscript. DXW critically revised the manuscript. All authors read and approved the final manuscript.

Corresponding author

Correspondence to Dong-Xin Wang.

Ethics declarations

Ethics approval and consent to participate

This study was approved by the Biomedical Research Ethics Committee of Peking University First Hospital on October 27, 2022 (2022[470]). Ethical approval for the original study was provided by the Clinical Research Ethics Committee of Peking University First Hospital (Chairperson Prof Guo Xiaohui) and registered with the Chinese Clinical Trial Registry (ChiCTR-TRC-10000802). Participants or their legal representatives were provided informed consent prior to enrollment into the original study.

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Not applicable.

Competing interests

The authors declare no competing interests.

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Zhao, Y., Chen, NP., Su, X. et al. Overtime work of anesthesiologists is associated with increased delirium in older patients admitted to intensive care unit after noncardiac surgery: a secondary analysis. BMC Anesthesiol 24, 465 (2024). https://doiorg.publicaciones.saludcastillayleon.es/10.1186/s12871-024-02825-x

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Keywords

BMC Anesthesiology

ISSN: 1471-2253

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