Objective To evaluate the effect of therapeutic hypercapnia on cerebral oxygen metabo?lism in the patients undergoing thoracoscopic surgery in beach chair position ( BCP ) . Methods Sixty pa?tients of both sexes, aged 18-32 yr, with body mass index of 19-24 kg∕m2 , of American Society of Anes?thesiologists physical statusⅠorⅡ, scheduled for elective bilateral thoracic sympathectomy performed via a thoracoscope, were divided into control group ( group C ) and hypercapnia group ( group H ) , with 30 patients in each group using a random number table. After induction of anesthesia, all the patients in both groups were tracheally intubated and mechanically ventilated using the ventilation regimen low tidal vol?ume intermittent positive pressure ventilation combined with low level of positive end?expiratory pressure ( 5 cmH2 O) , maintaining arterial carbon dioxide partial pressure ( PaCO2 ) at 35-45 mmHg. PaCO2 was maintained at 45-55 mmHg by adjusting the respiratory rate after the patients were placed in BCP in group H. Anesthesia was maintained with target?controlled infusion of propofol and intermittent intravenous boluses of rocuronium and sufentanil. Bispectral index value was maintained at 45-55. Before anesthesia induction ( baseline) , at 5 min after intubation, and at 5, 10, 15 and 20 min after the patients were placed in BCP, blood samples were taken from the radial artery and jugular bulb for blood gas analysis, jugular ve?nous bulb oxygen saturation was measured, and arteriovenous blood O2 content difference, cerebral O2 ex?traction rate, and venous to arterial blood lactate concentration difference were calculated. Results Com?pared with group C, PaCO2 and jugular venous bulb oxygen saturation were significantly increased, and ar?teriovenous blood O2 content difference and cerebral O2 extraction rate were significantly decreased at at 5, 10, 15 and 20 min after the patients were placed in BCP in group H ( P<0?05) , and there was no signifi?cant change in venous to arterial blood lactate concentration difference at each time point between the two groups ( P>0?05) . Conclusion Therapeutic hypercapnia can improve the cerebral oxygen metabolism in the patients undergoing thoracoscopic surgery in BCP .