Effect of Crystalloids With Different SID on pH, and Urinary Electrolytes During General Anesthesia

Inclusion Criteria: * Adult patients (between 18 and 75 years old); * ASA (American Society of Anesthesiologists) physical status classification system 1 or 2; * Patients undergoing spinal surgery under general anesthesia, intubated and mechanically ventilated; * Normal preoperative albumin; * Surgery with an estimated duration of at least 3 hours. Exclusion Criteria: * Patients who are pregnant; * Patients with obesity (BMI \> 35); * Patients with a chronic obstructive pulmonary disease (COPD); * Patients with obstructive sleep apnea (OSA) treated with CPAP; * Patients with chronic heart failure (CHF) with a NYHA (New York Heart Association) class ≥2; * Patients receiving diuretics in the pre-operative period; * Patients with a chronic kidney disease (CKD), defined as a GFR \< 60 ml/min/1.73 m2; * Patients with diabetes mellitus treated with insulin; * Patients with myopathies; Patients undergoing a surgery with likely fluid losses or with unexpected bleeding during surgery.