Association between serum osmolarity and mortality in patients who are critically ill: a retrospective cohort study

Objectives This research aims to explore the association between serum osmolarity and mortality in patients who are critically ill with specific categories of disease. Design A retrospective cohort study. Setting and participants Data were extracted from an online database named ‘Multiparameter Intelligent Monitoring in Intensive Care II’. 16 598 patients were included. Methods Patients were divided into six disease subgroups based on the diagnosis at admission: cardiac, cerebral, vascular, gastrointestinal, respiratory and non-respiratory. The association between maximum osmolarity (osmolarity max ) and hospital mortality in each subgroup was evaluated using osmolarity max as a design variable (six levels). Results Analysis of the 16 598 patients revealed a ‘U’-shaped relationship between osmolarity and mortality with a threshold of 300 mmoL/L. For patients with non-respiratory disease, both hypo-osmolarity and hyperosmolarity max were associated with increased mortality, with the OR increasing from osmolarity max level 3 (OR: 1.98, 95% CI 1.69 to 2.33, p<0.001) to level 6 (OR: 4.45, 95% CI 3.58 to 5.53, p<0.001), using level 2 (290–309 mmoL/L) as the reference group. For patients with respiratory disease, however, neither hypo-osmolarity nor hyperosmolarity max was significantly associated with mortality (levels 1 to 5) except for extreme hyperosmolarity max (≥340 mmoL/L, OR: 2.03, 95% CI 1.20 to 3.42, p=0.007). ORs of mortality in the other four subgroups (cardiac, cerebral, vascular, gastrointestinal) were similar, with OR progressively increasing from level 3 to 6. In all six subgroups, vasopressin use was consistently associated with increased mortality. Conclusions Hyperosmolarity is associated with increased mortality in patients who are critically ill with cardiac, cerebral, vascular and gastrointestinal admission diagnoses, with thresholds at 300 mmoL/L. For patients with respiratory disease, however, no significant association was detected.