Et-C02 is used to predict PaCO2 during surgery. There are controversies regarding the value of ETCO2 to predict PaCO2. The aim of this study was to investigate the relation between Et-CO₂ and PaCO₂ measurements in patients undergoing laparoscopic colorectal surgery. Patient and surgical characteristics, mechanical ventilation adjustments, and intraabdominal pressure values and the corelation between these parameters and EtCO₂ - PaCO₂ difference were also investigated.

Patient records from patients underwent colorectal surgery between 2010-2011 will be studied. It was planned to include patients undergoing laparoscopic colorectal surgery with a predetermined anaesthesia protocol.

Patients will receive a standardized general anesthetic and thoracal epidural anaesthesia. Ventilatory parameters will be adjusted to maintain ETCO2 35 mmHg, Paw 25 cmH2O.Patient age, gen-der, BMI, ASA classification, the duration of surgery, anesthesia, Trendelenburg position and pneumoperitoneum. Hemodynamic parameters, blood gas analyzes, ventilation parameters and EtCO₂ values will be recorded. We determined 6 timelines to compare the difference between PaCO₂ and EtCO₂. These timelines were the times where we could collect the most data regarding blood gas analyzes. Preoperative period: to, 10th minute after the first insuflation and trendelendburg position: t₁,the first hour after the first insuflation and trendelendburg position: t₂, 10th minute after desuflation and supine position: t₃, 10th minute after the second insuflation and trendelendburg position: t₄, and 10th minute after the second desuflation and supine position:t₅. Primary outcome measure is the difference between PaCO₂ and EtCO₂ a difference greater than 5 mmHg was assumed to be important.

Inclusion Criteria:

• ASA I-III patients underwent laparoscopic colorectal resection

Exclusion Criteria:

• convertion to the open surgery

Patients scheduled whom planned for elective laparoscopic colorectal resection.