Objective To evaluate the effect of therapeutic hypercapnia on the balance between cerebral O2 supply and demand in the patients undergoing arthroscopic shoulder surgery in the beach chair position(BCP). Methods Forty-eight patients of both sexes, aged 21-64 yr, weighing 45-80 kg, of American Society of Anesthesiologists physical status Ⅰ or Ⅱ, scheduled for elective arthroscopic shoulder surgery in the beach chair position, were divided into 2 groups(n=24 each)using a random number table: control group(group C, end-tidal pressure of carbon dioxide 35-40 mmHg)and therapeutic hypercapnia group(group H, end-tidal pressure of carbon dioxide 45-50 mmHg). The regional cerebral oxygen saturation(rSO2)was recorded after induction and before BCP, immediately after BCP, at 3, 6, 9, 12, 15, 18, 21, 24, 27 and 30 min after BCP and at the end of surgery(T0-12). The occurrence of cerebral desaturation events, extubation time, duration of stay in postanesthesia care unit, development of nausea and vomiting and requirement for vasoactive drugs were recorded during surgery. Results Compared with the baseline at T0, the rSO2 was significantly decreased at T1-12 in group C and at T2-6 in group H(P 0.05). Conclusion Therapeutic hypercapnia can improve the balance between cerebral O2 supply and demand in the patients undergoing arthroscopic shoulder surgery in the BCP. Key words: Hypercapnia; Posture; Brain; Shoulder joint; Oxygen consumption