Background: Volume replacement strategies and type of fluid used in patients undergoing cardiac surgery have changed during the last years. Currently used crystalloid solutes have a variable composition and a major impact on organ function and outcome. Additionally critically ill patients are prone to fluid overload, which is despite common perception, not a benign occurrence as it is associated with prolonged ICU- and hospital length of stay and increased mortality rates. Fluid resuscitation using bolus or continuous infusion of hypertonic saline was used for more than thirty years. Only a few studies have been conducted so far, but they showed that infusion of hypertonic saline results in less volume administered, increased renal function less weight gain in critically ill patients when compared to other crystalloids.

Aim: This preliminary randomized controlled double-blind study aims to identify whether fluid resuscitation using hypertonic saline (HS) when used in addition to lactated Ringers solution results in less total fluid amount administered in patients following cardiac surgery. Additionally we want to evaluate whether the use of hypertonic saline results less need for pharmacological cardiocirculatory support, increased renal function, less postoperative volume overload shortened post-cardiac bypass immune suppression and increased postoperative outcomes.

Study intervention: At admission to the ICU patients will receive 5ml/kg body weight of 7.3% NaCl or 0.9% NaCl by infusion pump over 60 minutes. If necessary, fluid resuscitation will thereafter be performed with Ringer's lactate to normalize peripheral perfusion and to allow weaning of vasopressors.

Inclusion Criteria:

• Adult patients undergoing cardiac surgery for ischemic or valvular heart disease

Exclusion Criteria:

• Patients unable to give informed consent
• Patients with age <18 years
• Pregnancy or breastfeeding
• Left-ventricular ejection fraction (LVEF) < 30% preoperatively
• Preexisting renal insufficiency with an eGFR <30 ml/min/1.73m2
• Patients with postoperative circulatory support devices such as LVAD, IABP, Impella, ECMO
• Preexisting serum sodium of >145mmol/l or <135 mmol/L
• Preexisting serum chloremia >107mmol/l or < 98 mmol/L
• Systemic steroid therapy (at any dose at time of inclusion)
• Chronic liver disease (bilirubin >3 mg.dl)
• Any signs of infection or sepsis defined as clear clinical evidence for active infection or current antibiotic therapy

double-blind