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

Abstract

Aims: Regional cerebral oxygen saturation (rSO 2 ) is currently the most used measure in clinical practice to monitor cerebral ischaemia in patients undergoing carotid endarterectomy (CEA). Although end-tidal carbon dioxide (P ET CO 2 ) is known as a factor that influences rSO 2 , the relationship between P ET CO 2 and rSO 2 has not been quantitatively evaluated in patients with severe arteriosclerosis. This study aimed to evaluate the effect of P ET CO 2 on rSO 2 in patients undergoing CEA under general anaesthesia. Methods: The intervention to change P ET CO 2 was conducted between skin incision and clamping of the carotid artery. The rSO 2 values were observed by changing P ET CO 2 in the range of 25–45 mmHg. The P ET CO 2 –rSO 2 relationship was characterized by population analysis using a turnover model. Results: In total, 1651 rSO 2 data points from 30 patients were used to determine the pharmacodynamic characteristics. Hypertension (HTN) and systolic blood pressure (SBP) were significant covariates on the slope factor in the stimulatory effect of P ET CO 2 on rSO 2 and fractional turnover rate constant (k out ), respectively. The estimates of the parameters were k out (min −1 ): 3.59 for SBP <90 mmHg and 0.491 for SBP ≥90 mmHg, slope: 0.00321 for patients with HTN and 0.00664 for patients without HTN. Conclusion: The presence of HTNattenuates the response of rSO 2 after a change in P ET CO 2 . When cerebral blood flow is in a state of decline caused by a decrease in SBP to <90 mmHg, the response of rSO 2 to P ET CO 2 is increased. It is advisable to maintain SBP >90 mmHg in patients with HTNduring CEA.