Abstract
Introduction: Surgical intensive care unit (SICU) beds are a limited resource. We hypothesized that nighttime transfers from the SICU are associated with higher rates of SICU readmission. Methods: All patients who underwent thoracic surgery between 2022 and 2024 were included. Patients were grouped based on time of transfer. Nighttime transfers occurred between 5:30 PM and 5:30 AM. Risk-adjusted multivariable logistic regression was used to assess the association between transfer timing and SICU readmission, complications, and mortality. Results: A total of 216 patients were analyzed, including 169 Day Transfer patients and 47 Night Transfer patients. Baseline characteristics were similar between groups. The Day Transfer group experienced significantly longer transfer times compared with the Night Transfer group (564.1 minutes [IQR 474.3-653.9] vs. 324.1 minutes [IQR 203.4-444.7], p = 0.002). Complication rates were comparable between groups (12.8% Day vs. 14.9% Night, p = 0.71). However, Night Transfer patients had higher SICU readmission rates within 24 hours (10.6% vs. 1.2%, p = 0.001), within 7 days (19.2% vs. 5.2%, p = 0.002), and during the overall readmission rates (27.7% vs. 6.4%, p < 0.001). Nighttime transfer was independently associated with increased odds of SICU readmission at all measured time points. There were no significant differences in mortality or complication odds between groups. Conclusion Nighttime SICU transfers were associated with a significantly increased risk of readmission, indicating a potential opportunity to improve patient safety through optimized transfer timing and post-transfer care.
Introduction
Health care systems are increasingly focused on cost efficiency and patient-centered care, prompting efforts to optimize hospital length of stay, reduce readmissions, and ensure safe, timely discharges. Surgical intensive care units (SICUs) have drawn particular attention, as SICU readmissions are consistently associated with higher mortality, especially in North American and European cohorts.1,2 Unplanned SICU readmission within 48-72 hours of discharge is now recognized not only as a key quality metric but also as a marker of underlying patient instability and risk for adverse events3-6
Among surgical patients, unplanned SICU readmissions are most often driven by cardiac or respiratory deterioration. Despite this, the development of reliable risk-prediction tools remains challenging due to patient complexity, variability in laboratory and physiologic parameters, and limited external validation of existing scoring systems.7,8 Additionally, few studies have evaluated system-level contributors to readmission. Identifying the drivers of early SICU readmissions is essential for developing targeted interventions that can improve patient outcomes and optimize resource utilization. Prior studies have highlighted a range of potential contributors, but findings have been inconsistent and often limited by single-center designs or heterogeneous patient populations. As health care systems strive to balance SICU capacity with rising demand, understanding these risk factors becomes increasingly urgent. Lastly, prior studies also have not consistently isolated transfer timing as an independent system-level factor in surgical ICUs.
Given that surgical ICU beds are a finite, high-demand resource, patients are often transferred to the surgical ward as soon as they are deemed clinically stable from a critical care perspective. However, the timing of these transfers may have important implications for patient safety. Overnight transfers may coincide with reduced staffing, limited ancillary support, and delays in recognition of clinical deterioration on the receiving unit. We hypothesize that nighttime transfers from the surgical ICU are independently associated with an increased risk of unplanned readmission.
Methods
Study Design and Patient Population
This study included all patients who underwent thoracic surgical intervention and were admitted to the surgical ICU at a single center between 2022 and 2024. Patients were grouped by their time of transfer, defined by the time of arrival to the surgical floor. Nighttime transfers were defined as transfers from 5:30 PM to 5:30 AM, which coincides with the providers’ shifts. This study was approved by our institution. Patient type was defined based on the primary reason for hospital admission: “Thoracic” patients were admitted primarily for a thoracic surgical procedure, while “Medical,” “Cardiac,” “Transplant,” “Trauma,” or “Other” patients were admitted for these respective reasons and may have subsequently required thoracic interventions such as decortication.
Statistical Analysis
Categorical variables were compared using the chi-square test. Continuous variables were assessed for normality and homogeneity of variances; comparisons between groups were performed using Student’s t-test or Welch’s t-test, as appropriate. Univariate and multivariable logistic regression models were constructed to assess the association between timing of transfer from the surgical ICU (daytime vs nighttime) and outcomes of interest, including unplanned SICU readmission, post-transfer complications, and in-hospital mortality. Post-transfer complications were defined as the occurrence of pneumonia, sepsis, shock or cardiac arrest, pulmonary embolism or deep vein thrombosis (PE/DVT), or re-operation during the index hospitalization following SICU transfer.
Multivariable models were risk-adjusted for clinically relevant covariates selected a priori, including Charlson Comorbidity Index, race, gender, patient type, time of discharge from the SICU, duration of mechanical ventilation, and total SICU length of stay. Race and gender were included as covariates a priori based on their clinical relevance and established associations with health outcomes, regardless of statistical significance in univariable analyses. Collinearity among continuous predictors (ICU length of stay, duration of mechanical ventilation, and timing of discharge) was assessed using variance inflation factors, with all VIFs <2, indicating no evidence of problematic collinearity. Adjusted odds ratios (ORs) with 95% confidence intervals (CIs) were reported. Statistical significance was defined as a two-sided P-value <0.05. All analyses were performed using SAS software (SAS Institute Inc., Cary, NC).
Results
A total of 216 thoracic surgery SICU transfers were analyzed, including 169 daytime transfers and 47 nighttime transfers (Table 1). Baseline demographics were similar between groups, with no significant differences in age, sex, race, patient type, or overall complication rates. Although patients transferred at night tended to have a higher Charlson comorbidity burden, this difference did not reach statistical significance (P = 0.06). Nighttime transfers were associated with a significantly shorter interval from SICU admission to transfer compared with daytime transfers (median 324 vs 564 minutes, P = 0.002).
Total SICU length of stay demonstrated a right-skewed distribution in both groups. Median SICU length of stay was similar between daytime and nighttime transfers (10 [IQR 6-17] vs 11 [IQR 6-28], respectively), and no statistically significant difference in the overall distribution was observed (P = 0.29), despite greater variability among nighttime transfers.
Unplanned SICU readmissions occurred more frequently among patients transferred at night. Nighttime transfer was associated with significantly higher rates of SICU readmission within 24 hours (10.6% vs 1.2%, P = 0.001), within 7 days (19.2% vs 5.2%, P = 0.002), and over the entire hospital stay (27.7% vs 6.4%, P < 0.001). Rates and types of post-transfer complications did not differ significantly between groups. Cardiopulmonary etiologies were the most common causes of SICU readmission in both daytime and nighttime transfer groups, and the distribution of readmission diagnoses did not differ significantly between groups (P = 0.71). These included atrial fibrillation with rapid ventricular response, cardiac arrest, shock, and hypoxia with or without associated altered mental status and hypotension.
Discharge disposition did not differ significantly between daytime and nighttime transfer groups (P = 0.44). Among patients transferred during the day, most were discharged home with or without services (118, 69.8%), followed by discharge to a post-acute care facility (36, 21.3%), hospice (3, 1.8%), inpatient transfer (1, 0.6%), death during hospitalization (11, 6.5%), or other dispositions (1, 0.6%). Similarly, among nighttime transfers, 24 patients (51.1%) were discharged home, 17 (36.2%) to a post-acute care facility, 1 (2.1%) to hospice, and 5 (10.6%) died during hospitalization; no nighttime transfers resulted in inpatient transfer or other disposition.
On multivariable logistic regression, nighttime transfer was not independently associated with overall complications (OR 1.15, 95% CI 0.42-3.12; P = 0.80) (Table 2). However, nighttime transfer remained a strong independent predictor of SICU readmission within 24 hours (OR 10.40, 95% CI 1.62-66.81; P = 0.01), within 7 days (OR 3.75, 95% CI 1.34-10.55; P = 0.01), and during the remainder of the hospital stay (OR 4.71, 95% CI 1.86-11.89; P = 0.001).
Discussion
In this study, nighttime transfers from the surgical ICU were strongly associated with increased rates of unplanned readmission, despite similar rates of post-transfer complications. Specifically, patients transferred at night experienced higher readmission within 24 hours, within 7 days, and across the entire hospital stay. These findings highlight the critical role of transfer timing as a system-level factor influencing patient outcomes.
Although thoracic SICU patients represent a heterogeneous group with varying procedural risk profiles, our cohort was categorized by patient type—Cardiac, Medical, Thoracic, Transplant, Trauma, and Other—with the majority being thoracic (69.2% of day transfers, 57.5% of night transfers; P = 0.21). The thoracic group primarily included patients admitted for a thoracic surgical procedure, whereas other patients were admitted for medical, cardiac, or transplant reasons and later required thoracic interventions such as decortication. Within the thoracic group, procedures included lobectomy, pneumonectomy, wedge resections, and esophagectomies. Importantly, the distribution of patient types did not differ significantly between daytime and nighttime transfers, suggesting that cohort composition is unlikely to account for the observed higher rates of nighttime SICU readmissions. Therefore, these findings support the conclusion that transfer timing, rather than underlying patient type, is the primary system-level factor associated with increased readmission risk.
Our results are consistent with prior literature demonstrating that early SICU readmissions are clinically meaningful events, though their impact on long-term outcomes appears to be variable across populations. Patients with unplanned SICU admissions have been shown to experience higher mortality, longer hospital length of stay, and fewer discharges home, with comorbidities such as cirrhosis, cancer, and CHF contributing to risk. 9 Early SICU readmissions are also correlated with higher mortality in trauma patients, reinforcing the clinical impact of timing on patient trajectories. 10 Conversely, some studies suggest that unplanned SICU readmissions do not necessarily affect overall mortality in trauma populations, indicating that readmission may serve more as a marker of instability rather than a direct cause of adverse outcomes. 11 A large proportion of unplanned SICU transfers occur within 48 hours of admission or prior SICU discharge, most commonly due to respiratory distress, highlighting under-triage or insufficient monitoring as key drivers of early “bounce-backs.” 12
Nighttime transfers in our cohort occurred significantly sooner after SICU admission than daytime transfers, likely reflecting pressures to free SICU beds and differences in overnight workflows. Although post-transfer complication rates were similar between groups, patients transferred at night experienced higher SICU readmission rates. This study does not evaluate causation, however potential contributing factors of nighttime transfer can include reduced staffing, limited ancillary services, and delayed recognition of clinical deterioration on the receiving floor which would require further study. The nighttime environment increases the likelihood of SICU return without necessarily translating into overt complications. These observations are consistent with systematic review data demonstrating that after-hours or nighttime SICU transfers are associated with increased SICU readmission and mortality. 13 However, our findings diverge from prior literature in that increased readmission rates were not accompanied by higher complication rates, mortality, or non-home discharge.14-16 One possible explanation is the widespread adoption of Rapid Response Teams, which have been shown to improve early detection of clinical deterioration and reduce failure-to-rescue events, potentially enabling earlier SICU re-escalation without progression to adverse outcomes. 17
Patient safety during the transition from the surgical ICU to general wards represents a critical period of vulnerability, particularly when transfers occur after hours. During these periods, staffing ratios, specialist support, and immediate access to experienced surgical decision-makers may be reduced. Consistent with guidance from the Joint Commission and the Institute for Healthcare Improvement (IHI), after-hours ICU discharge is increasingly recognized as a system-level vulnerability rather than an individual provider failure. Several studies have reported increased rates of in-hospital complications associated with after-hours ICU discharge even when controlling for illness severity, further emphasizing the systemic nature of this risk. In response, many academic surgical centers have implemented proactive vigilance mechanisms to address this high-risk transition period.
Emerging evidence supports targeted post-ICU discharge monitoring—particularly for patients transferred after evening hours—through standardized ICU-to-ward handoffs, risk stratification using validated early warning scores, and proactive follow-up by rapid response or critical care outreach teams. When embedded within a nonpunitive, high-reliability framework, these approaches promote earlier recognition of postoperative deterioration, more reliable escalation of care, and fewer unplanned ICU readmissions, thereby helping mitigate known hazards during transitions of care.
These findings have several implications for SICU discharge planning and hospital operations. Clear communication between SICU and ward teams may be particularly important during nighttime transitions, when ancillary staffing and support services are often limited. Institutions may benefit from implementing structured handoff protocols and dedicated nighttime transfer checklists that account for overnight workflow constraints, including confirmation of monitoring plans and clearly defined escalation pathways. Additionally, proactive monitoring strategies—such as early reassessment or targeted rapid response team rounding for patients transferred after evening hours—may help mitigate the risk of delayed recognition of clinical deterioration during the immediate post-transfer period.
Although patients met established stability criteria at the time the transfer decision was made, the shorter interval from SICU admission to transfer observed in the nighttime group may narrow the physiologic margin for error if subtle clinical changes emerge during or shortly after transition to the floor. Recognizing this potential vulnerability reinforces the importance of heightened vigilance, clear communication, and early reassessment for patients undergoing evening or nighttime SICU transfers.
Limitations
This study has several limitations. Its single-center, retrospective design limits causal inference and may reflect institution-specific SICU workflows, staffing patterns, and discharge practices that are not generalizable to other settings. Although standardized SICU discharge criteria exist, transfer timing—particularly overnight—remains influenced by clinical judgment and operational pressures, introducing potential selection bias.
The relatively small number of nighttime transfers may have limited power to detect differences in less frequent outcomes such as mortality or major complications and may explain why higher readmission rates were not accompanied by worse downstream outcomes. Despite risk adjustment, residual confounding is possible, as factors such as unit-level staffing, provider experience, hospital census, and availability of ancillary services were not directly measured.
Additionally, SICU readmission is a heterogeneous outcome and may reflect appropriate early escalation of care rather than failure of discharge, particularly in institutions with active rapid response systems. Finally, the focus on a thoracic surgical SICU population may limit generalizability, though the underlying system-level factors related to transfer timing are likely relevant across surgical SICUs.
Conclusion
Patient Demographics and Clinical Characteristics
*Cardiopulmonary includes atrial fibrillation with rapid ventricular rhythm, cardiac arrest, shock, and hypoxia with +/− altered mental status with hypotension. Neurologic included altered mental status. Renal/Metabolic includes severe hyponatremia and renal failure requiring dialysis. Infectious includes septic shock. Surgical/other includes re-operation.
**Other includes leaving against medical advice.
Logistic Regression Analysis
Footnotes
Funding
The authors received no financial support for the research, authorship, and/or publication of this article.
Declaration of Conflicting Interests
The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.