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Objectives A volume-outcome relationship in ICU patients has been suggested in recent studies. However, it is unclear whether the ICU-to-hospital bed ratio affects the volume-outcome relationship. The aim of this study is to investigate the relationship between hospital volume and in-hospital mortality of adult ICU patients in relation to the ratio of ICU beds to regular hospital beds. Design Retrospective cohort study. Setting Four hundred seventy-seven Japanese hospitals from 2007 to 2012 in the Japanese Diagnosis Procedure Combination database. Patients A total of 596,143 patients discharged from acute care hospitals. Interventions None. Measurements and main results We analyzed data from 596,143 ICU patients from 2007 through 2012 using a nationwide administrative database. Patients were categorized into nine subgroups (the tertiles of hospital volume of ICU patients combined with the tertiles of ICU-to-hospital bed ratio). Multivariable logistic regression analyses were performed to examine the concurrent effects of hospital volume of ICU patients and ICU-to-hospital bed ratio on in-hospital mortality, with adjustment for patient and hospital characteristics. Higher hospital volume of ICU patients and a higher ICU-to-hospital bed ratio were independently associated with lower mortality. When patients were stratified by ICU-to-hospital bed ratio categories, in-hospital mortality was significantly lower in the high-volume subgroup (odds ratio, 0.74; 95% CI, 0.58-0.93) compared with the low-volume subgroup in hospitals with a high ICU-to-hospital bed ratio. However, these relationships were not significant in hospitals with low ICU-to-hospital bed ratios (odds ratio, 0.94; 95% CI, 0.59-1.50) or in hospitals with intermediate ICU-to-hospital bed ratios (odds ratio, 0.80; 95% CI, 0.71-1.08). Conclusions An inverse relationship between hospital volume of ICU patients and mortality was seen only when the ICU-to-hospital bed ratio was sufficiently high. Regionalization and increasing the number of ICU beds in referral centers may improve patient outcomes.

Previous research assessing the impact of between-hospital trauma volume (high volume centers vs. low volume centers) and outcomes has been inconsistent. Furthermore, previous research has not considered temporal variations in within-hospital volume (a center having higher than average volume vs. lower than average volume) as a covariate. The objective of this study was to determine the relationship of between-hospital and within-hospital trauma volume and two measures of hospital quality of care.Multivariable, hierarchical, mixed effects, logistic regression analyses of a population-based nonconcurrent cohort from 1995 to 1999.Thirty-nine nonfederal California hospitals included in the California Patient Discharge Data Set designated by local Emergency Medical Services authorities as adult trauma centers.All nonelderly adult trauma patients, 16-64 yrs (n = 54,352), and elderly adult trauma patients,65 yrs (n = 47,656), admitted with an Injury Severity Score9.None.Severity adjusted in-hospital mortality rate and 30-day trauma-related readmissions were analyzed. Among nonelderly adult patients, higher annual between-hospital trauma volume was not associated with mortality rate (odds ratio, 1.02 for each 100 admissions; 95% confidence interval, 0.99, 1.06) but was associated with higher risk of readmission (odds ratio, 1.19 for each 100 admissions; 95% confidence interval, 1.13, 1.26). Among elderly adult patients, higher annual between-hospital trauma volume was associated with lower mortality (odds ratio, 0.79 for each 100 admissions; 95% confidence interval, 0.71, 0.87) but was not associated with risk of readmission (odds ratio, 0.96 for each 100 admissions; 95% confidence interval, 0.90, 1.04). Higher than average monthly within-hospital trauma volume was associated with higher odds of readmission (odds ratio, 1.11 for a volume deviation of ten patients per month; 95% confidence interval, 1.01, 1.21) among elderly adult patients.The findings of this study in the context of previous research suggest that relationships between trauma volume and outcomes exist but depend on which patient populations are studied and how the data are analyzed. Furthermore, trauma centers may be subject to the detrimental effects of high temporal volume overextending existing services and capacity. Since this study found that both between-hospital volume and within-hospital volume measures are associated with outcomes, we recommend that both measures be included in future volume-outcome investigations.