The intensive care unit volume–mortality relationship, is bigger better? An integrative literature review

To explore the association between patient volume in intensive care units (ICUs) and risk-adjusted mortality. Large multi-speciality ICUs are emerging in response to increasing demand for critical care. Consolidation of resources through regionalisation of services aims to contain costs and optimise demand management and operational synergies. Higher patient volumes in ICU have been associated with improved outcomes. Limited evidence exists, however, to suggest an optimal volume of patients in terms of risk-adjusted mortality. Retrospective integrative literature review. EMBASE, PubMed and Cumulative Index to Nursing and Allied Health Literature electronic databases. Primary studies of risk adjusted mortality in adult ICU patients published between 1995 and 2012. Studies of admissions following elective procedures. Twenty quantitative observational studies were included in this review. Studies were primarily retrospective with three conducted prospectively. Nine studied mechanically ventilated patients, six included all admissions to ICU, three reported on patients with sepsis and one study each on patients post cardiac arrest and those receiving renal replacement therapy. A significant association was evident in sixteen studies suggesting a lower risk of adjusted mortality in higher-volume units. The association was not consistent across all diagnosis. A non-linear relationship observed in two studies noted no mortality benefit occurring above a volume threshold of 450 cases annually per diagnostic category and above 711 cases not specific to a diagnostic group. Patient mortality may be improved in large capacity ICUs. However, the association is not consistent across all diagnostic groups. Risk adjusted mortality is increased in low volume ICUs. There appears to be a high volume threshold at which point the risk adjusted mortality benefit is also lost suggesting a window of optimal ICU organisational performance exists between low and high volumes. Further prospective research is recommended into clinical outcomes in high volume ICUs to explore association between organisational efficiency and quality of care.