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The aim of this prospective cohort study was to determine the incidence of cerebral desaturation events (CDE) in patients undergoing shoulder arthroscopy in the beach chair position (BCP) compared to patients undergoing shoulder arthroscopy in the semi-upright sitting position (SSP).This prospective study included 220 patients, 150 men (68.18%) and 70 women (31.82%). The average age was 46 yr (24-68) years. Patients under 18 yr of age, those with uncontrolled diabetes, a previous cerebral stroke, a myocardial infarction, rheumatoid arthritis, congestive heart failure, severe cervical disc prolapsed, or severe obesity (body mass index >30) were excluded from the study. The patients were divided into two groups (110 patients in each group). Group A patients had shoulder arthroscopy done in the 60-degree beach chair position and Group B had shoulder arthroscopy done in a semi-upright sitting position at 90 degrees. Mean arterial blood pressure and cerebral oxygen saturation were measured at different stages: before induction of anesthesia (T0), after induction (T1 [baseline]), after positioning (T2), after induction of hypotension (T3), and 1 hr after final position (T4). Cerebral desaturation was defined as a reduction in regional cerebral oxygen saturation to less than 80% of baseline value for 15 sec or longer. Each stage was compared with others in the same group and with the other group.In both groups, the mean arterial blood pressure (MAP) values at T2 were significantly lower than at T1. The MAP values at T3 and T4 were significantly lower than those at T1 and T2. There was a significant decrease in regional cerebral oxygen saturation in stage T3 when compared to previous stages in the same group. Without any significant decrease in stage T4 compared to T3 in the same group. We found no significant difference in regional cerebral oxygen saturation when comparing each stage in both groups.No significant difference was noted in regional cerebral oxygen saturation between patients operated on in the beach chair position and those operated on in the semi-upright sitting position. A direct relation was noted between the decrease in mean arterial pressure and decrease in cerebral oxygen saturation.