The study will be designed as a prospective clinical trial. Patients scheduled for lower limb arterial bypass surgery will be randomly allocated for treatment with spinal anesthesia associated with spontaneous ventilation (nasal cannula with supplemental oxygen - Group 1) or treatment with general anesthesia under controlled mechanical ventilation (tidal volume 6 to 8 ml / kg of the predicted body weight and PEEP of 5 cmH2O - Group 2).

In this prospective and randomized study, only adult ASA II to IV patients, aged over 18 years, scheduled to undergo lower limb arterial revascularization surgery at the Hospital de Clínicas in Porto Alegre and at the Hospital Nossa Senhora da Conceição will be included.

The study will be designed as a prospective clinical trial. Patients will be randomly allocated for treatment with spinal neuraxial anesthesia associated with spontaneous ventilation (nasal glasses with supplemental oxygen - Group 1) or treatment with general anesthesia under controlled mechanical ventilation (tidal volume from 6 to 8 ml / kg of the assumed weight and PEEP of 5 cmH2O - Group 2).

Sampling will be carried out for convenience, including patients routinely seen in the operating room of the hospitals involved and who meet the inclusion and exclusion criteria of this study. The allocation of patients will be performed through a table of random numbers generated prior to the beginning of the protocol. The process of randomization of patients in the groups will be carried out using random blocks. The method for blinding the study will be carried out using sealed envelopes and will include the blinding of the researchers, evaluators and those responsible for statistical analysis, but will not include the blinding of the attending physician and the patients.

The data will be stored in Excel and analyzed statistically using STATA 12.0 software. The statistical approach will be carried out through the "intention-to-treat" model. The assessment of ventilatory and hemodynamic parameters between the groups will be performed using ANOVA associated with Bonferroni for parametric data or the Kruskal-Wallis test associated with the Mann-Whitney test for nonparametric data. For categorical variables, we will use Fisher's exact test or Chi-square test. Repeated measures ANOVA will be used to assess outcomes between treatment groups over the follow-up period. In order to control important variables, such as surgical time, surgical complications, age, body mass index, and ASA classification (comorbidities), a statistical approach to assess confounding factors (linear regression model) will be performed after the period data collection. Results will be considered statistically significant when type I (P) error is less than 0.05.

Inclusion Criteria:

• Adult patients ASA II to IV; age greater than 18 years; scheduled for arterial revascularization surgery on lower limbs.

Exclusion Criteria:

• Patients with a body mass index above 40 kg / m2 who underwent emergency surgery, with a history of pulmonary surgery, persistent hemodynamic instability in the preoperative period, history of asthma or chronic use of corticotherapy, or patients with history neuromuscular disorder. Patients with a history of use of anticoagulants or antiplatelet agents in the preoperative period who contraindicate the performance of spinal anesthesia will also be excluded from the study. In this study, only patients with peripheral vascular disease with critical and symptomatic ischemia in the lower limbs and scheduled to undergo elective arterial revascularization of the lower limbs will be included, and all patients with acute vascular obstruction or other associated vascular complications will be excluded.