In cases where there is no ventilation-perfusion problem, the end-tidal carbon dioxide (ETCO2) value is closely associated with partial arterial carbon dioxide pressure (PaCO2); therefore, the PaCO2 value can be estimated using ETCO2 measurements in patients without significant cardiopulmonary disorders. The aim of the investigator's study is to evaluate the reliability of pulmonary ventilation monitoring with ETCO2 value and to investigate at what tidal volume values ETCO2 monitoring provides reliable information.

The patients were grouped as follows: Group 1, if the tidal volume is 4 ml/kg; Group 2, if TV=6 ml/kg; Group 3, if TV=8 ml/kg; and Group 4, if TV=10 ml/kg. It was determined how many breaths per minute (minute ventilation) would be given with tidal volume x respiratory rate=6 L. The patients were ventilated with the same ventilation parameters for 30 minutes and then arterial blood gas analysis was performed. Arterial blood gas was collected at t1 for group 1, t2 for group 2, t3 for group 3, and t4 for group 4. The ETCO2 value, Peak pressure (Ppeak), Plateau pressure (Pplato), mean airway pressure (Pmean) values were recorded at the time of arterial blood gas collection. Systolic blood pressure (SBP), diastolic blood pressure (DBP), mean arterial pressure (MAP), peak heart rate (PHR) values were evaluated. The patients with a mean arterial pressure of less than 65 mmHg were excluded from the study. Partial arterial oxygen pressure (PaO2), partial arterial carbon dioxide pressure (PaCO2), oxygen saturation (SO2) values were recorded from the arterial blood gas analysis. P (a-ET) CO2 gradient value was calculated.

The groups were compared, in each patient, under the same conditions, when the different tidal volume was applied, how the ETCO2 monitorization, PaO2 values and hence the P (a-ET) CO2 gradient changed.

Inclusion Criteria:

American Society of Anesthesiologists (ASA) classification 1, 2 risk group

Head and neck surgery under general anesthesia lasting more than 120 minutes and who progressed normotensive.

Exclusion Criteria:

Cardiac disease

Pulmonary disease

Obese patients

Double