Two groups of patients are going to have abdominal surgery with low-flow general anaesthesia.

Group A (sevoflurane group, experimental group) Patients will receive midazolam for premedication 45 minutes before surgery. Dose is 0,08 mg/kg i.m. Patients will be preoxygenated with 100 % oxygen with oxygen flow of 7,5 L/min. When peripheral blood saturation reach 100 %, the induction of general anaesthesia will start. For induction patients will get anaesthetic thiopental 4,5 mg/kg i.v., analgetic fentanyl 3 mcg/kg i.v. and muscle relaxant vecuronium 0,1 mg/kg i.v. After 90 seconds patients will be intubated. After intubation anaesthetic gases will be switched on. Oxygen and nitrous oxide (50:50 %) and volatile anaesthetic sevoflurane 1 MAC (2,0 vol%). Patients will be ventilated by anaesthetic device. Intermittent positive pressure ventilation (IPPV) tidal volume 8 mL/kg, respiratory rate 12 times per minute, flow will be 1 L/min. First 20 minutes of anaesthesia will be period of saturation of the body with anaesthetic sevoflurane. After 20 minutes the depth of anaesthesia will be titrated with level of anaesthetic sevoflurane. Change will be +10 % of sevoflurane MAC (0,2 vol%) for every change of signs that indicate shallow anaesthesia. Change will be -10 % of sevoflurane MAC (0,2 vol%) for every change of signs that indicate too deep anaesthesia. Signs that show the depth of anaesthesia are: arterial blood pressure (systolic, diastolic and mean), heart rate and bispectral index (BIS). Measuring points will be every 5 minutes during surgery. The goal is to provide the values of this vital signs within normal physiology ranges. Analgetic fentanyl will be repeated every 45 minutes in the dose of 0,8 mcg/kg i.v. Muscle relaxant vecuronium will be repeated in the dose of 0,5 mg/kg i.v. when train of four (TOF) shows score 30 % or higher. It will also be measured (every 5 minutes): inspiratory and expiratory concentration of sevoflurane, saturation of the blood with oxygen and level of end-tidal carbon dioxide. At the end of surgery, begins awakening from general anaesthesia. All anaesthetic gases will be switched off. Patients will be ventilated manually with 100 % oxygen with oxygen flow of 7,5 L/min. Every patient will get atropin 1 mg + neostigmine 2,5 mg i.v. for decurarization. Measuring points will be: start of awakening, extubation and discharge from operating theatre. It will be measured: arterial blood pressure (systolic, diastolic and mean), heart rate, inspiratory and expiratory concentration of sevoflurane, saturation of the blood with oxygen, level of end-tidal carbon dioxide, BIS and TOF. During surgery each patient will get i.v. infusion 2000 mL of sodium chloride 0.9 %.

Group B (isoflurane group, control group) Patients will receive midazolam for premedication 45 minutes before surgery. Dose is 0,08 mg/kg i.m. Patients will be preoxygenated with 100 % oxygen with oxygen flow of 7,5 L/min. When peripheral blood saturation reach 100 %, the induction of general anaesthesia will start. For induction patients will get anaesthetic thiopental 4,5 mg/kg i.v., analgetic fentanyl 3 mcg/kg i.v. and muscle relaxant vecuronium 0,1 mg/kg i.v. After 90 seconds patients will be intubated. After intubation anaesthetic gases will be switched on. Oxygen and nitrous oxide (50:50 %) and volatile anaesthetic isoflurane 1 MAC (1,2 vol%). Patients will be ventilated by anaesthetic device. Intermittent positive pressure ventilation (IPPV) tidal volume 8 mL/kg, respiratory rate 12 times per minute, flow will be 1 L/min. First 20 minutes of anaesthesia will be period of saturation of the body with anaesthetic isoflurane. After 20 minutes the depth of anaesthesia will be titrated with level of anaesthetic isoflurane. Change will be +10 % of isoflurane MAC (0,12 vol%) for every change of signs that indicate shallow anaesthesia. Change will be -10 % of isoflurane MAC (0,12 vol%) for every change of signs that indicate too deep anaesthesia. Signs that show the depth of anaesthesia are: arterial blood pressure (systolic, diastolic and mean), heart rate and bispectral index (BIS). Measuring points will be every 5 minutes during surgery. The goal is to provide the values of this vital signs within normal physiology ranges. Analgetic fentanyl will be repeated every 45 minutes in the dose of 0,8 mcg/kg i.v. Muscle relaxant vecuronium will be repeated in the dose of 0,5 mg/kg i.v. when train of four (TOF) shows score 30 % or higher. It will also be measured (every 5 minutes): inspiratory and expiratory concentration of isoflurane, saturation of the blood with oxygen and level of end-tidal carbon dioxide. At the end of surgery, begins awakening from general anaesthesia. All anaesthetic gases will be switched off. Patients will be ventilated manually with 100 % oxygen with oxygen flow of 7,5 L/min. Every patient will get atropin 1 mg + neostigmine 2,5 mg i.v. for decurarization. Measuring points will be: start of awakening, extubation and discharge from operating theatre. It will be measured: arterial blood pressure (systolic, diastolic and mean), heart rate, inspiratory and expiratory concentration of isoflurane, saturation of the blood with oxygen, level of end-tidal carbon dioxide, BIS and TOF. During surgery each patient will get i.v. infusion 2000 mL of sodium chloride 0.9 %.

Duration of this randomised controled trial will be approximately 2 months. Estimated sample size will be 82 persons (41 in sevoflurane group and 41 in isoflurane group).