Rationale: The aging population may strain intensive care unit (ICU) capacity and adversely affect patient outcomes. Existing fluctuations in demand for ICU care offer an opportunity to explore such relationships. Objectives: To determine whether transient increases in ICU strain influence patient mortality, and to identify characteristics of ICUs that are resilient to surges in capacity strain. physician staffing models (interaction P value ¼ 0.02). The relationship between census and mortality was stronger when the census was composed of higher acuity patients (interaction P value , 0.01). Averaging strain over the first 3 days of patients' ICU stays yielded similar results except that the proportion of new admissions was now also associated with mortality (OR, 1.04 for each 10% increase; 95% CI: 1.02, 1.06). Conclusions: Several sources of ICU strain are associated with small but potentially important increases in patient mortality, particularly in ICUs employing closed staffing models. Although closed ICUs may promote favorable outcomes under static conditions, they are susceptible to being overwhelmed by patient influxes.Keywords: critical care; resource allocation; intensive care unit; physician staffing; regionalization Intensive care units (ICUs) in the United States will encounter increased demand for critical care in the next two decades because of the aging of the American population (1). This sustained increase in baseline demand will compound the effects of routine fluctuations in demand for critical care, such as from influenza epidemics and mass casualties. Concomitant increases in critical care supply are unlikely, due to projected staffing shortages and fiscal constraints (1-6). Thus, ICUs will be increasingly challenged to deliver high-quality care under conditions of increased capacity strain (7).We reported that several measures of ICU capacity strain near the time that patients are discharged from ICUs may enhance the efficiency of critical care delivery without harming patients (8). However, mixed evidence exists regarding the effects of capacity strain closer to the time of ICU admission, when patients' trajectories are often determined by the care they receive. European studies have found associations between increased ICU workload and decreased patient safety (9, 10). By contrast, a large U.S. study found that one metric of strain, ICU census on the day of patients' admissions, was not associated with mortality (11).We therefore sought, in the largest study of capacity strain to date, to determine whether several metrics of strain are associated with in-hospital mortality. We also sought to determine whether certain types of ICUs are more "elastic"-that is, better able to accommodate increases in strain without experiencing worse patient outcomes.
METHODS Study DesignWe performed a retrospective cohort study of patients admitted to U.S. ICUs included in the Project IMPACT database (Cerner Corporation, Author Contributions: All authors were involved in the study design, data analysis and/...