# Effect of Carotid Stenosis Severity on Patterns of Brain Activity in Patients after Cardiac Surgery
## 1 Introduction
[25] studied the discriminatory characteristics of patients with vascular CI and healthy individuals using non-linear EEG analysis methods. It was found that the degree of EEG irregularity and complexity was significantly lower in patients with vascular CI compared to control subjects. We previously showed that a theta activity increase in the frontal and occipital sites, as well as high theta/alpha ratios, may be considered as the earliest EEG markers of vascular cognitive disorders [8]. [PERSON] et al. proposed several promising EEG markers that could be important in the differential diagnosis of vascular and neurodegenerative CI. The alpha3/alpha2 and theta/gamma indices showed prognostic significance for the progression of the neurodegenerative type of CI [26; 27; 28]. The changes in the electrical activity of neurons in the post-stroke period proved to be promising in the search for prognostic markers of clinical recovery in patients with ischemic brain damage. A study by [PERSON] et al. found that a bilateral increase in low-frequency activity and a decrease in hemispheric asymmetry in the acute phase of a unilateral stroke in the middle cerebral artery basin predicts a worse functional outcome in the future [29]. However, there is still insufficient information on the modification of the brain electrical activity in cardiac surgery patients. Cardiac surgery has been shown to be associated with local or diffuse brain damage [21; 30; 31; 32]. It is assumed that chronic cerebral ischemia in patients with cardiovascular diseases, as well as episodes of acute ischemia that occur during on-pump cardiac surgery, can contribute to specific changes in the brain's electrical activity. Our previous studies have shown that EEG patterns associated with coronary artery bypass grafting (CABG) have specific features, depending on the presence of pre-operative CI or cognitive decline in the early postoperative period [21; 33]. We found that the presence of early POCD was accompanied by negative postoperative dynamics of EEG parameters with the increase in low-frequency activity. [PERSON] et al. found that the occurrence of nonconvexity status epilepticts after open cardiac surgery is associated with mitigating secondary brain injury [34]. Thus, recent studies have shown that the patterns of brain activity are associated with perioperative brain damage in cardiac surgery patients. However, the role of the severity of carotid stenosis in the development of the postoperative changes in brain activity and cognitive functions is uncertain. It has previously been shown that hemodynamically significant stenoses of CA (70-99%) can be a risk factor for brain damage during cardiac surgery [31; 35]. However, little is known about the effects of small stenoses of CA (<50%) on the state of the brain in cardiac surgery patients. There have been several research studies into the negative effects of asymptomatic stenosis of CA on the state of the brain after cardiac surgery [33; 35]. This has resulted in the perception that CA stenoses of less than 50% are hemodynamically insignificant. Therefore, this has led to insufficient attention being paid to preoperative management and intraoperative brain protection in patients with CA stenoses of less than 50%. There are some data in the literature about the serious neurological complications (stroke, postoperative delirium, etc.) that occur in the group of patients with hemodynamically significant stenoses [36; 37]. Research studies about the brain activity changes associated with postoperative cognitive decline in patients with stenoses of the coronary and carotid arteries are rare, especially after simultaneous cardiac surgery. It is important to note that the intraoperative episodes of brain ischemia during combined coronary and carotid revascularization does not necessarily lead to brain damage such as stroke. Meanwhile, less pronounced, diffuse ischemic brain damage may have a significantly higher frequency.