The purpose of the study is to confirm the hypothesis that a dysfunction of the autonomic nervous system could complicate the postoperative course in the patient undergoing surgery. Measurement of heart rate variability (HRV) is a non-invasive method for evaluation of the autonomic nervous system.

The autonomic nervous system (ANS) plays an important role in the maintenance of systemic homeostasis. The homeostasis is modified continually by internal and external stimuli to which the autonomic nervous system modulates the response of organism by tight control on essential functions such as circulation, respiration, hormonal secretion and thermoregulation. Dysfunction of the autonomic nervous system could complicate the perioperative course in the patient undergoing surgery; therefore it should be considered as another risk factor during preoperative evaluation. Nowadays the evaluation of the autonomic nervous system function like a tool for prediction postoperative outcome is not considered in daily anaesthesiology practice. Measurement of heart rate variability (HRV) is a method for evaluation the autonomic nervous system function. It is a non-invasive method that provides information about the autonomic regulation of organism and thus it identifies patients with the autonomic nervous system dysfunction.

Inclusion Criteria:

• 21 years of age and above
• anticipated duration of surgery more than 120 minutes
• estimated blood loss exceeding more than 15% of blood volume
• American Society of Anesthesiology (ASA) classification 2 or 3

Exclusion Criteria:

• pregnant or lactating women
• age below 21 years of age
• emergency surgery
• American Society of Anesthesiology (ASA) classification 1
• sepsis and septic shock
• severe cardiac arrhythmias

Patients undergoing elective major abdominal surgery.