The objective of the study is to evaluate influence of perioperative fluid therapy on Methemoglobin levels, and to changes in hemoglobin. Further, changes in Methemoglobin will be related to metabolic signs of oxidative stress (changes in blood lactate). Patients undergoing major abdominal surgery will be included into the study.

Increasing plasma volume by intravenous administration of crystalloid and/or colloid solutions may cause a relative reduction in hemoglobin (Hb) concentration, a situation that is termed 'dilutional anemia'. Such hemodilution may lead to an iatrogenic reduction in oxygen-carrying capacity and the development of organ dysfunction. Paradoxically, large amounts of fluids that are administered with the aim of increasing oxygen delivery (DO2), as is frequently done as part of perioperative goal-directed therapy; this may lead to an actual decrease in the DO2 due to a decrease in Hb concentration.

Another potential parameter that may reflect the development of dilutional anemia is methemoglobin (MetHb), a form of Hb with reduced ability for oxygen binding. Experimental studies showed that dilutional anemia may lead to up-regulation of perivascular nitric oxide synthase (NOS) and increase NOS-derived nitric oxide (NO) leading to local vasodilation and oxidization of Hb to MetHb. MetHb may potentially serve as a biomarker of 'anemic stress' associated with reduced tissue perfusion during acute hemodilution.

Inclusion Criteria:

• major abdominal surgery
• informed consent

Exclusion Criteria:

• violation of study protocol

Patients undergoing major abdominal surgery