This prospective observational study will include critically ill patients with hemodynamic compromise. It will compare passive leg raising and a mini volume challenge (MVC) carried out on every patient sequentially. The endogenous release of stress hormones will also be assayed in order to investigate their modifying effect on the hemodynamic reaction of the participants. The effect of fluid resuscitation on renal perfusion will also be assessed and the data compared with the release of cystatin C.

Fluids are administered either before starting with or during ongoing vasopressor treatment in critically ill patients with hemodynamic instability. The endogenous release of stress hormones, namely renin, vasopressin, norepinephrine and cortisol, may be variable in critically ill patients, which may have a modifying effect on the hemodynamic response. Passive leg raising (PLR), a validated, simple and dynamic test, is currently recommended for testing fluid responsiveness. The test might possibly be false negative under extreme dehydration or increased intra-abdominal pressure. This prospective observational study aims to compare PLR and MVC with 300 ml Ringer acetate solution administered as a bolus within 5 minutes in critically ill medical patients with a hemodynamic instability. MVC will be carried out in every participant following PLR. Blood samples will be collected immediately before PLR for measurement of renin, copeptin A, cortisol and norepinephrine in those patients not yet receiving a vasopressor. These biomarkers will be compared with the Magnitude of fluid responsiveness. Additionally, renal resistive index will be assessed with ultrasound immediately before PLR and 1 and 24 hours after fluid challenge and the data compared with Initial serum cystatin C in those patients who have not yet developed acute renal failure. Blood samples for measurement of angiopoietin-2, a marker of capillary leak, will be drawn immediately before PLR and the data will be correlated with the amount of fluid required for Initial resuscitation.

Inclusion Criteria:

• mean arterial pressure <65 mm Hg and/or
• blood lactate >2 mmol/l and/or
• mottling or capillary refill >3 seconds and/or
• oliguria and
• critical care decision for testing fluid responsiveness

Exclusion Criteria:

• cardiopulmonary resuscitation
• uncontrolled bleeding
• irreversible brain damage
• pregnancy and lactation
• Age under 18 years

critically ill adult medical patients with signs of hemodynamic instability