Association Between Hour‐by‐Hour ICU Admission Timing and 24‐h Mortality: A Single‐Center Retrospective Observational Study

Abstract

ABSTRACT Aim Prior studies of ICU admission timing often use broad categories (day vs. night), potentially masking hour‐level effects. We examined hour‐by‐hour ICU admission timing and mortality, with attention to nighttime periods and routine handovers. Methods We conducted a single‐center retrospective observational study in a closed ICU with 24/7 on‐site intensivist coverage, using prospectively registered data from the Japanese Intensive Care Patient Database. Adults (≥ 18 years) admitted to the E‐ICU between April 2017 and July 2025 were included. Admission hour (0–23) was modeled as a continuous exposure using restricted cubic splines (reference 14:00). The primary outcome was 24‐h mortality and the secondary outcome was ICU mortality. Multivariable logistic regression was adjusted for demographics, APACHE II, invasive mechanical ventilation at admission, comorbidities, diagnostic categories, admission source, and post‐cardiac arrest status. Bootstrap resampling was used to estimate 95% confidence intervals. Additional analyses examined handover, nighttime, and 24‐h periodic patterns. Results Among 5724 patients, 24‐h and ICU mortality were 3.4% and 10.6%, respectively. Compared with 14:00, adjusted odds of 24‐h mortality were higher in the early morning. The early‐evening estimate was imprecise, with the 95% CI including unity. In contrast, ICU mortality showed no clear temporal pattern. Handover comparisons showed no clear mortality association; nighttime admission was not associated with higher mortality, and cosinor analysis was nonsignificant. Conclusion Hour‐by‐hour analysis showed higher adjusted odds of 24‐h mortality in the early morning, whereas evidence of an increase in the early evening was inconclusive. Direct comparisons of handover periods showed no clear association with mortality.