The purpose of this pilot study is to identify the optimal way to ventilate patients during abdominal surgery in order to reduce the amount of post-operative pulmonary complications in patients at moderate and high-risk for them.

The investigators plan to prospectively compare two methods to individualize Positive End Expiratory Pressure (PEEP) settings in the operating room during abdominal surgery: (1) Maximization of respiratory compliance during a decremental PEEP titration, and (2) Prevention of negative end-expiratory transpulmonary pressures.

The investigators will exploit the usual intraoperative requirement for a naso/orogastric tube to assess transpulmonary pressures,and respiratory mechanics measurements from anesthesia machines to titrate PEEP.

The investigators will measure biomarkers of lung injury and lung function to compare those methods between themselves and to the control group. In the process, the investigators will assess the ease and reliability of anesthesia teams in implementing the methods. These data will allow us to determine the PEEP strategy best suited for the full-scale trial, and to estimate the degree of separation the experimental lung protective approach will have from the protocolized usual care control settings.

Inclusion Criteria:

• Adults ( 18 years) scheduled for elective surgery expected to last 2 h,
• elective intraperitoneal abdominal or pelvic surgery including: gastric; biliary; pancreatic; hepatic; major bowel, ovarian, renal tract, bladder, and prostatic; radical hysterectomy; and pelvic exenteration;
• at least intermediate risk of PPCs defined by a risk score 26

Exclusion Criteria:

• Inability or refusal to provide consent
• Refusal of clinicians caring for patient to follow the protocol
• Participation in interventional investigation within 30 days of the time of the study
• Pregnancy
• Emergency surgery
• Severe obesity (above Class I, BMI 35)
• Significant lung disease: any diagnosed or treated respiratory condition that (a) requires home oxygen therapy or non-invasive ventilation, (b) severely limits exercise tolerance to <4 METs (e.g. patients unable to do light housework, walk flat at 4 miles/h or climb one flight of stairs), or (c) required previous lung surgery80
• Significant heart disease: cardiac conditions that limit exercise tolerance to <4 METs
• Renal failure: peritoneal or hemodialysis requirement or preoperative creatinine 2 mg/dL;
• Neuromuscular disease that impairs ability to ventilate without assistance
• Severe chronic liver disease (Child-Pugh Score of 10 -15)
• Sepsis
• Malignancy or other irreversible condition for which 6-month mortality is estimated 50%
• Bone marrow transplant.