# Effect of Carotid Stenosis Severity on Patterns of Brain Activity in Patients after Cardiac Surgery
## 1 Introduction  
According to the World Health Organization, cardiovascular diseases (CVD), mainly associated with atherosclerosis, are the leading causes of death worldwide, including in Russia [1]. The Siberian region shows less favorable CVD epidemiology. Various climatic and ecological conditions of the region contribute to the high prevalence of this pathology [2]. Atherosclerosis often affects multiple vascular basins simultaneously. Significant atherosclerotic lesions of several vascular basins determine the severity of the disease, making it difficult to choose the optimal treatment strategy and calling into question the positivity of the prognosis, in particular, coronary artery disease. The global population of elderly people has been increasing every year, and the ageing of the population has posed new and complex challenges for health professionals not only to increase life expectancy but also to maintain its quality. A high standard of quality of life cannot be reached without preserving a person's intellectual functions. It is known that with age, cognitive functions diminish, and cognitive impairment (CI) develops in the form of memory loss, attention and executive impairment, etc. [3, 4, 5]. Cognitive disorders associated with cerebral and coronary atherosclerosis (vascular CI) are widespread among older persons and are more severe than age-related cognitive changes [6, 7, 8]. Previous studies have revealed significant interactions between cognitive disorders developing in the elderly and senile age, atherosclerotic changes in cerebralvessels, and accompanying disorders of cerebral blood flow [9; 10]. There is evidence that age-related structural and functional changes in arteries, arterioles and capillaries lead to dysregulation of cerebral blood flow and ischemia, leading to disruption of the blood-brain barrier. Additionally, metabolic disorders are developed with reduced delivery of energy substrates to neurons and excretion of by-products of the protein breakdown, increasing neuroinflammation and paracrine regulation dysfunction [11; 12]. It is suggested that the atherosclerotic remodeling of the brain vessels can lead to an accelerated progression of brain dysfunction [11]. In this case, carotid artery (CA) stenosis is one of the factors affecting self-regulation of brain perfusion [13]. It has been found that patients with vascular CI often show a decrease in blood flow velocity in the cerebral cortex, especially in the frontal and parietal regions [14; 15]. These brain regions are known to be the watersheds of the blood supply, at the boundaries between the vascular pools [16; 17; 18]. These zones are more disadvantaged than any other brain region in the case of systolic and/or diastolic dysfunction of the left ventricle, valvular pathology and atrial fibrillation accompanying cardiovascular pathology, as well as during cardiac surgery [11; 19]. There is a wide variety of epidemiological and clinical data on vascular CI, but only a few studies have examined changes in the neurophysiological parameters of cardiac surgery patients [20; 21]. At the same time, early manifestations of vascular and postoperative CI are subclinical and are detected only using an extended neurophysiological examination. In this regard, careful attention should be paid to the identification of objective and sensitive criteria for early diagnosis of CI in cardiac surgery patients. It is generally accepted that the electroencephalogram (EEG) rhythms reflect the activity of the neural network to be placed under recording electrode [22; 23]. As a consequence, the changes in EEG rhythms may be early indicators of structural and functional abnormalities in neural networks associated with vascular and postoperative CI. Previous studies have shown that the frequency-spatial pattern of brain electrical activity in patients with vascular CI has specific features [8; 24; 25]. The association between poststroke alpha slowing and CI, which may be mediated by attentional dysfunction, was revealed [24]. [PERSON] et al.