 Even though mild hypercapnia in the beach chair position BCP under general anesthesia was effective in reducing cerebral desaturation, there is no comprehensive data with regard to the dose-effect relationship of EtCO2 on rSO2 in the BCP under general anesthesia. Pharmacodynamic modeling can be useful in describing this relationship in which concurrent mean arterial pressure changes and disease state can be considered as covariates. Therefore, we investigated the relationship between the EtCO2 and rSO2 and identified the covariates in the pharmacodynamic relationship between EtCO2 and rSO2. 