The non-pulsating blood flow caused by the cardiopulmonary bypass device damages multiple organs in the body, especially the brain. Non-physiological blood flow alters blood supply to the brain tissues, and damage to the transmission of arterial pulsation to the “pulse brain” tissue will alter the brain structure and function [
9,
10]. Brain injury after cardiovascular surgery will increase the degree of pain in patients and reduce their quality of life [
2]. Therefore, it is crucial to implement intraoperative brain monitoring and timely intervention to reduce the incidence of postoperative brain injury. In this study, TCCD was used to monitor the changes in cerebral blood flow during cardiac surgery, and rSO
2 was used to monitor the changes in cerebral oxygen metabolism. Taken together, TCCD and rSO
2 could effectively predict early postoperative brain injury, and this study also identified the risk factors of early brain injury after cardiovascular surgery.
NSE is considered a specific neuro-biochemical marker of neuronal injury [
11]. The level of serum NSE was significantly increased after brain injury and was significantly correlated with the severity of brain injury after ischemia [
12]. In this study, the patients were divided into the early postoperative brain injury and normal groups according to the measured NSE concentration 24 h post-surgery. We observed significant differences in CPB time, decreased rSO
2 level, decreased cerebral blood flow level, aortic occlusion time, and NSE concentration between the two groups. Further analysis indicated that decreased rSO
2 level was strongly correlated with NSE concentration. In the model without decreased rSO
2, decreased cerebral blood flow level, aortic occlusion time, and cerebral infarction history were correlated with NSE. Therefore, decreased rSO
2 was considered the most important factor affecting NSE concentration. Patients with a history of cerebral infarction were more likely to be affected by ischemia, and the degree and time of low cerebral blood flow that could be tolerated were also reduced. Hence, patients with a history of cerebral infarction in this trial were more likely to develop postoperative NSE abnormalities.
The parameter rSO
2 has good stability and is not affected by arterial pulse. Additionally, relatively stable rSO
2 values could be observed in patients with hemodynamic instability or even cardiac arrest [
13]. In this study, decreased rSO
2 level was identified as an independent risk factor for postoperative brain injury with high diagnostic value. If decreased rSO
2 level was more than 21.4%, postoperative brain injury was more likely to occur. Similarly, Klinkova et al. observed that 19% of 200 patients with a 20% or greater decrease in rSO
2 during CPB had a higher risk of neurological complications [
14]. It was also reported that maintaining intraoperative rSO
2 levels above 80% of the baseline value during cardiac surgery could reduce the incidence of cognitive impairment seven days post-surgery [
15]. In a coronary artery bypass grafting study, patients with rSO
2 levels maintained above 75% of baseline value spent significantly less time in the intensive care unit and hospital stay after surgery than patients in the no-intervention group [
16]. These studies demonstrated that monitoring rSO
2 is important to improve cardiac surgery. However, rSO
2 levels may be affected inevitably. rSO
2 monitoring is affected by the position of its electrodes. Usually, the electrodes are selected to be placed on the forehead. Although global cerebral oxygen sufficiency can be evaluated, it may not be possible to detect changes in areas far from the monitored site. Another factor is its signal may be disturbed by external information, such as skull thickness, skin pigment, etc., and we may eliminate some of the effects by maintaining the same position during the operation. Moreover, rSO
2 monitoring is also susceptible to hemoglobin concentration, especially during cardiac surgery, when blood is diluted to a certain extent, such that decreased hemoglobin levels can also lead to decreased rSO
2 levels [
17]. Therefore, it is inadequate to evaluate cerebral blood flow with only rSO
2 levels during surgery. Likewise, this study further evaluated TCCD to measure cerebral hemodynamics, obtain cerebral blood flow, observe vascular misalignment, and determine the presence of plaque [
18]. Our findings revealed that, in the absence of decreased rSO
2 levels, decreased cerebral blood flow (measured by TCCD) was identified as an independent risk factor for early postoperative brain injury, and the decrease of more than 37.4% could predict early postoperative brain injury. Six patients in the brain injury group had no significant decrease in rSO
2, while the decrease in cerebral blood flow measured by TCCD was much greater than 37.4%. Hence, TCCD could be used to supplement rSO
2 monitoring for more accurate and comprehensive results. According to Catena et al. [
19], during aortic arch surgery, TCCD could monitor cerebral blood flow, optimize the rate of anterograde and retrograde cerebral perfusion, and avoid hyper-perfusion or hypo-perfusion during cardiac arrest. Additionally, during cardiopulmonary bypass, the cerebral blood flow spectrum was altered with a jagged waveform, which was consistent with our observation (Fig.
4). This change is related to the use of non-pulsating pumps during cardiopulmonary bypass. In addition, TCCD can also detect the occurrence of embolic events [
20]. However, its limitations can lead to difficulties. TCCD operation is dependent on the operator’s proficiency. Therefore, it is not easy for any one person to obtain it, provided that it is systematically trained.
In cardiovascular surgery, it is often necessary to block the aorta to facilitate the operation. However, the results of this study suggested that the duration of aortic occlusion over 86 min may indicate early postoperative brain injury. Long-term aortic occlusion may not only affect cerebral perfusion but also damage the vascular endothelium and induce coagulation disorders. In addition, a history of atrial fibrillation was a risk factor for early postoperative brain injury. Atrial fibrillation causes cerebral blood flow and fluctuations in blood perfusion to the brain. Studies have found that atrial fibrillation can damage the blood-brain barrier, causing patients to be more susceptible to cognitive impairments later in life [
21]. In this study, the duration of hospital stay in the brain injury group was longer than that in the normal group, but there was no statistical difference. However, in some studies, longer hospital stays after surgery were associated with lower rSO
2 levels [
4,
22], which could be due to other clinical factors.
There were some limitations in this study, particularly the inclusion of more than one type of cardiovascular surgery, which might have affected the results. Likewise, further research is required to investigate specific interventions to reduce early postoperative brain injury.