Preoxygenation Before General Anesthesia

Rationale of the study: we aim to clarify the question (related to still unclear and not univocal response) about the protective or unnecessary role of preoxygenation in non-critically ill patients (otherwise with no high risk of desaturation) undergoing general anesthesia before elective surgery. It will be also necessary differentiate the development of postoperative complications (pulmonary, cardiovascular, neurological, surgical) due to preoxygenation from the ones related with patient comorbidity, intraoperative and surgical causes, tube disconnection. Procedure: patient's informed consent signature for adhesion at the study will be initially requested. With their acceptance, parameters will be recorded anonymously in the Case Report Form, identified by their initials and an alphanumeric code, until hospital discharge. The parameters analyzed will be related to: * preoperative evaluation; about anamnesis, health general conditions, blood oxygen saturation (Sat02), Metabolic Equivalent of Task (METs) * intraoperative evaluation; about oxygenations values, recorded before/during induction and maintenance of general anesthesia * postoperative evaluation; about postoperative complications, pulmonary primarily, and secondary cardiovascular, neurological and surgical, based on the medical record. The data wil be transferred on Excel worksheet, utilized for descriptive analysis related at every variable. By multivariate logistic regression will be evaluated the major factors influencing postoperative pulmonary complications (PPCs) onset in patients undergoing preoxygenation for elective surgery