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

Summary

Aim

Regional cerebral oxygen saturation (rSO₂) is currently the most used measure in clinical practice to monitor cerebral ischemia in patients undergoing carotid endarterectomy (CEA). Although end-tidal carbon dioxide (P_ETCO₂) is known as a factor that influences rSO₂, the relationship between P_ETCO₂ and rSO₂ has not been quantitatively evaluated in patients with severe arteriosclerosis. This study aimed to evaluate the effect of P_ETCO₂ on rSO₂ in patients undergoing CEA under general anaesthesia.

Methods

The intervention to change P_ETCO₂ was conducted between skin incision and clamping of the carotid artery. The rSO₂ values were observed by changing P_ETCO₂ in the range of 25-45 mmHg. The P_ETCO₂–rSO₂ relationship was characterized by population analysis using a turnover model.

Results

In total, 1,651 rSO₂ 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_ETCO₂ on rSO₂ 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 hypertension attenuates the response of rSO₂ after a change in P_ETCO₂. When cerebral blood flow is in a state of decline caused by a decrease in SBP to less than 90 mmHg, the response of rSO₂ to P_ETCO₂ is increased. It is advisable to maintain SBP above 90 mmHg in patients with hypertension during CEA.

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