The Volume-Outcome Relationship in Critically Ill Patients in Relation to the ICU-to-Hospital Bed Ratio

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.