Modelling of the Effect of End-Tidal Carbon Dioxide on Cerebral Oxygen Saturation in Beach Chair Position under General Anaesthesia.

Patients undergoing shoulder surgery in the beach chair position (BCP) under general anaesthesia may be at risk of cerebral desaturation. Increasing end-tidal carbon dioxide (EtCO2 ) is the most convenient and powerful method for the management of cerebral desaturation. The purpose of this study was to investigate the quantitative relationship between EtCO2 and cerebral oxygen saturation (rSO2 ) and to identify the associated influencing factors. Fifty-one patients undergoing arthroscopic shoulder surgery in the BCP under general anaesthesia completed this study. Desflurane and remifentanil were used, and EtCO2 was steadily increased and then decreased by adjusting the ventilator settings every 3 min. so that time lag of rSO2 response to EtCO2 changes could be delineated. Near-infrared spectroscopy was used to monitor rSO2 response. An indirect response model was used to examine the relationship between EtCO2 and rSO2 . To determine the relevant covariates, a stepwise approach was used. There was a linear relationship between rSO2 and EtCO2 with a slight delay in the peak of rSO2 relative to EtCO2 . Increase in end-tidal desflurane concentration led to a slower response of rSO2 to the changes of EtCO2 (p = 0.0002). The presence of diabetes mellitus reduced the reactivity of rSO2 to EtCO2 changes (p < 0.0001). This model-based approach revealed that diabetes mellitus attenuates the response of rSO2 to changes in EtCO2 . The management of cerebral desaturation by hypercapnia in patients with diabetes may be less effective than in non-diabetic patients under general anaesthesia with BCP.