Introduction: This research work aims to stablish the relationship between the cerebral saturation value measured by NIRS (near infrared spectroscopy), as an indicator of cerebral blood flow, and the CO2 levels (etCO2), the mean arterial pressure (PAM) and the inspired oxygen fraction (FiO2). Methods: Longitudinal observational study of 64 patients undergoing to elective surgery under general anesthesia in the Fundación Cardioinfantil. Before and during surgery, the values of PAM, FiO2, EtCO2 and NIRS (bilateral somanetics invosoximeter equipment) were recorded. The NIRS and PAM levels were analyzed by linear regression models and the changes in the EtCO2 and the FiO2 and their relationship with the NIRS changes were analyzed with mixed linear regression models, using the identification of the patients as random effects term. All models were adjusted by age, sex and ASA.Results: The mean baseline NIRS was higher among men than in women (p <0.05) and slightly higher in ASA I patients’ and in the younger participants. Baseline NIR levels were not associated with PAM (βNIRS-Right: 0.08, IC-95%: -0.11; 0.27). The changes in NIRS levels was positively associated with the changes in EtcO2 levels (βNIRS-Right: 0.25, IC-95%: 0.21-0.29) and FiO2 levels (βNIRS-Right: 0 15; IC-95%: 0.12; 0.17). Discussion: The levels of NIRs are linearly associated with EtCO2 and FiO2 levels. The use of trans-operative levels close to: etCO2 of 40mmHg and FiO2 of 80%, could facilitate the patient to present adequate NIRS values during surgery.