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

Abstract

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 (EtCO 2 ) 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 EtCO 2 and cerebral oxygen saturation (rSO 2 ) 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 EtCO 2 was steadily increased and then decreased by adjusting the ventilator settings every 3 min. so that time lag of rSO 2 response to EtCO 2 changes could be delineated. Near-infrared spectroscopy was used to monitor rSO 2 response. An indirect response model was used to examine the relationship between EtCO 2 and rSO 2 . To determine the relevant covariates, a stepwise approach was used. There was a linear relationship between rSO 2 and EtCO 2 with a slight delay in the peak of rSO 2 relative to EtCO 2 . Increase in end-tidal desflurane concentration led to a slower response of rSO 2 to the changes of EtCO 2 (p = 0.0002). The presence of diabetes mellitus reduced the reactivity of rSO 2 to EtCO 2 changes (p < 0.0001). This model-based approach revealed that diabetes mellitus attenuates the response of rSO 2 to changes in EtCO 2 . 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.The beach chair position (BCP) under general anaesthesia with induced hypotension is commonly used in arthroscopic shoulder surgery [1]. Unlike the BCP in an awake state in which the compensatory mechanism to increase blood pressure occurs, the BCP under general anaesthesia is associated with significant hypotension and cerebral desaturation [2][3][4]. Further hypotension induced for better visualization of the surgical field might aggravate cerebral desaturation [4,5]. As cerebral desaturation during surgery is associated with postoperative cognitive dysfunction and stroke [6,7], prevention and treatment of cerebral desaturation is necessary.As cerebral oxygenation is dependent on cerebral perfusion and oxygen transport, interventions for the prevention and treatment of cerebral desaturation during shoulder surgery include increasing systemic arterial pressure with vasopressors, increasing end-tidal carbon dioxide (EtCO 2 ) with ventilation adjustment and increasing fraction of inspired oxygen (FiO 2 ) [3,4,8,9]. Although increases in systemic arterial pressure may cause rapid increases in cerebral blood flow (CBF), CBF returns to its baseline value within a few seconds by the mechanism of cerebral autoregulation, which aims to maintain a stable CBF over a wide range of mean arterial pressures (MAP) of 70-150 mmHg [10][11][12]. Moreover, increases in arterial pressure may worsen visualization of the sur...

Quantitative analysis of the effect of end‐tidal carbon dioxide on regional cerebral oxygen saturation in patients undergoing carotid endarterectomy under general anaesthesia

“…In an earlier study, cerebrovascular carbon dioxide reactivity was greatly reduced in patients with severe cerebral vascular disease . The slope factor in the stimulatory effect of EtCO 2 on rSO 2 was about 2–3 times smaller in patients undergoing carotid endarterectomy than in patients from previous studies without cerebrovascular disease . In a previous study, DM was a significant covariate on the slope factor, but not in this study.…”

“…In a previous study, DM was a significant covariate on the slope factor, but not in this study. Although DM was a significant covariate in the previous study, the slope factor was similar regardless of DM (presence of DM: slope factor = 0.011; absence of DM: slope factor = 0.015) . However, the slope factor of patients without HTN was about twice as large as that of patients with HTN in this study, which may suggest that HTN is a more significant covariate than is DM.…”

“…In a previous study, a time delay was observed between P ET CO 2 and rSO 2 . The turnover model could appropriately describe it between drug concentration and effect .…”

“…H(c) is a stimulation function to explain the relationship between P ET CO 2 and rSO 2 . Within the P ET CO 2 range used in this study, the relationship is known to be linear . H(c) can be expressed as follows : where slope is a slope factor in the stimulatory effect of P ET CO 2 on the production of the response (rSO 2 ).…”

“…Moreover, the model is best conceptualized when the effect is an endogenous biomarker . Considering rSO 2 as an endogenous substance in the human body, the turnover model can be used to characterize the effect of P ET CO 2 on rSO 2 .…”