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Intravenous fluid administration is commonly used for critically ill and perioperative patients, but increasing attention is being given to its potential negative effects. In these cases, balancing fluid needs for hemodynamic stability with avoiding fluid overload and organ damage is crucial.

This thesis aimed to describe fluid administration practices in ICUs and postoperative care units in Sweden, focusing on how clinicians’ approach four key areas. It also sought to highlight the consequences of these practices and encourage clinicians to reconsider routine fluid prescriptions in both intensive care and perioperative settings.

In Paper I, a prospective multicentre interventional cross-over study, we investigated if limiting the availability of standard-sized fluid bags of Ringer’s acetate and replacing them with smaller-sized fluid bags would affect the total amount of fluid administered to ICU patients (n=437) and the impact on morbidity or mortality. However, data did not support our hypothesis.

Paper II is a retrospective, multicentre study examining fluid types given to ICU patients (n=241) in the post-resuscitation phase. We found that maintenance and drug fluids significantly exceeded resuscitative fluids, with patients receiving more maintenance fluids, drug diluents, and sodium than recommended.

Paper III is a post-hoc analysis of Paper II data. We found that plasma urea's share of estimated plasma osmolality increases during fluid volume reduction and with higher osmolality, independent of nitrogen administration and renal function. This shift from ionic osmolytes to urea resembles patterns seen in estivating animals.

Paper IV is a single-centre study examining the relationship between ADH levels, fluid administration, and urine output in postoperative major abdominal surgery patients (n=54). We found that elevated ADH levels did not predict urine output or oliguria, but hypotension did. Increased fluid administration did not normalize urine output after oliguria.