The difference in survival outcomes between closed and open intensive care unit (ICU) designs with respect to trained intensivist coverage remains unknown. We aimed to investigate whether trained intensivist coverage is associated with mortality in critically ill patients admitted to the ICU in South Korea.

This population-based cohort study used nationwide registration data from South Korea. This study enrolled all adult patients admitted to the ICU between January 1, 2016, and December 31, 2019. Patients, who were admitted ICU in a hospital that employed trained intensivists, were designated as the intensivist group.

This study included 1,147,493 critically ill patients admitted to the ICU. The intensivist and non-intensivist groups consisted of 484,004 (42.2%) and 663,489 (57.8%) patients, respectively. Mixed effect logistic regression revealed a 22% lower in-hospital mortality rate (odds ratio: 0.78. 95% confidence interval: 0.74, 0.81; P   <  0.001) than that in the non-intensivist group. Mixed effect Cox regression revealed a 15% lower 1-year mortality rate (hazard ratio: 0.85. 95% confidence interval: 0.83, 0.89; P   <  0.001) in the intensivist group than that in the non-intensivist group. Moreover, the in-hospital mortality was significantly lower in the intensivist group than that in the non-intensivist group, irrespective of age, Charlson comorbidity index, surgery or non-surgery associated admission, and invasive treatment during ICU stay.

A closed ICU design with trained intensivist coverage was associated with lower in-hospital and 1-year mortality rates. Our results suggest that hospitals should employ trained intensivists to improve both short-term and long-term survival outcomes of critically ill patients.