Patients over the age of 18 who underwent a surgical procedure in an operating room and required general anesthesia and endotracheal intubation were included in the study. In the analysis we only considered patients who were extubated at the end of the procedure. Exclusion criteria were cardiac and thoracic procedures (since the opening of the chest cavity and single lung ventilation present unique perturbations to pulmonary physiology that we believe should be evaluated separately), surgical procedure within four weeks before the index operation, height less than 119 cm (calculations of predicted body weight become inaccurate at the extremes of size),16 (link) and patients with missing data elements.