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Objective: Postoperative delirium (POD) is common after cardiac surgery. We have previously identified plasma sodium concentration and the volume of infused fluids during surgery as possible risk factors. Both are linked to the selection and composition of the pump prime used for cardiopulmonary bypass (CPB). Present study aims to examine whether hyperosmolality increases the risk for POD.

Design: Patients ≥65 years (n = 195) scheduled for cardiac surgery were prospectively enrolled into this double blinded randomised clinical trial. Study group received a pump prime containing mannitol and ringer-acetate (966 mOsmol) (n = 98) vs. ringer-acetate (388 mOsmol) (n = 97) in the control group. Postoperative delirium was defined according to DSM-5 criteria based on a test-battery pre- and postoperatively (days 1–3). Plasma osmolality was measured on five occasions and coordinated with the POD assessments. The primary outcome was the POD incidence related to hyperosmolality as the secondary outcome.

Results: The incidence of POD was 36% in the study group and 34% in the control group, without intergroup difference (p=.59). The plasma osmolality was significantly higher in the study group, both on days 1 and 3 and after CPB (p<.001). Post hoc analysis indicated that high osmolality levels increased the risk for delirium on day 1 by 9% (odds ratio (OR) 1.09, 95% CI 1.03–1.15) and by 10% on day 3 (OR 1.10, 95% CI 1.04–1.16).

Conclusions: Use of a prime solution with high osmolality did not increase the incidence of POD. However, the influence of hyperosmolality as a risk factor for POD warrants further investigation.