The Effects of Positive End-Expiratory Pressure on Cross-Sectional Area of Internal Jugular Vein in Obese Patients

72 obese adult patients (BMI>30 kg/m2) with ASA status II-III between 18 and 65 years age old who were scheduled for elective surgery under general anesthesia were enrolled in the study. In the operating room all patients recevied a balanced hydroxyethyl starch solution, (HES 130/0.4/6%; Voluven, Fresenius Kabi),6 ml/kg of the calculated ideal body weight (IBW), over 15 minute before induction of anesthesia. All anaesthetics administered were given as part of standard care. Anesthesia was induced with propofol 1.5-2.5 mg/kg of lean body weight(LBW) and fentanil 2-3 mcg/kg of LBW and rocuronium 0.6-1.2 mg/kg of IBW. Anesthesia was maintained with sevoflurane in air:oxygen mixture (50: 50) to keep a target MAC of 0.8 during the image recordings and data collection to reduce possible hypotension due to anesthesia without surgical stimulation. Lungs were ventilated in a volume-controlled mode with tidal volume of 7 ml/kg IBW. All of the measurements were performed in <20degree contralateral neck rotation. The cross-sectional area (CSA) of right internal jugular vein (IJV) measured at the level of the cricoid cartilage using a two-dimensional ultrasound machine with a 10-MHz linear probe (Mindray M5 'Registered Trademark, Shenzhen). Probe was placed perpendicular to the skin with minimal pressure to ensure that the examined vein was not compressed. While the probe was held in the proper position an investigator manipulated the ventilator 3 different conditions according to the order of the randomly assigned sequence: 1-a baseline condition without Positive end-expiratory pressure (PEEP) (P0), 2-a PEEP of 5 cm H2O(P5), 3-a PEEP of 12 cm H2O(P12). Randomization was achieved using the Orthoganol Latin Square Design method. Each PEEP condition is administered for one minutes and the ultrasound images were obtained at least 3 seconds after instituting each maneuver for 10 seconds. Images capture timing of the study was considered as a washout time. The same practitioner performed all ultrasound examinations. Following measurements were calculated at each condition using preloaded software installed in the ultrasound machine by an independent observer who was blinded to the study protocol; 1-CSA of the right IJV, 2-Transvers diamater(TD) and anteroposterior diameter(APD) of the IJV, 3-Margin of safety(MOS)(defined as the distance from the lateral most border of the IJV and the lateral-most border of the carotid artery(CA) at which the IJV could be punctured without touching the CA). 4-Depth of the IJV from the skin(measured by drawing a line between the skin and the closest margin of the vein to the skin’s surface), 5-Degree of overlap(categorized on the basis of the percentage of overlap; 1- 0%(no overlap); 2- IJV overlapped<25% of the diameter of the CA; 3- IJV overlapped 25–50% of the diameter of the CA; 4- IJV overlapped>50% of the diameter of the CA.