The use of near-infrared spectroscopy (NIRS) technology to monitor intraoperative local cerebral oxygen saturation (rSO2) in patients is becoming increasingly popular in clinical practice. Previous studies have focused more on the dynamic changes in rSO2 to reflect the real-time supply and demand status of cerebral oxygen. Recent studies have found that the baseline value of rSO2 (B-rSO2) is often taken as the average rSO2 when the patient inhales air while awake (B-rSO2). This not only defines the threshold for cerebral oxygen desaturation, but also reflects the preoperative cardiopulmonary function status of the patient [1], and may become a powerful predictor of poor prognosis [2]. This article will review the clinical applications and influencing factors of B-rSO2, providing reference for clinical practice.
*Note: The brain tissue oxygen saturation monitoring parameter "BL" of Bolian Zhongke MOC series is equivalent to B-rso2 in the article.NIRS monitoring rSO2The basic principle of
NIRS is a non-invasive real-time monitoring method for tissue hemoglobin oxygenation, widely used to evaluate regional brain oxygenation. This technology is based on near-infrared light waves with wavelengths ranging from 650 to 1100nm, which have good penetration into human tissues, while near-infrared light waves with wavelengths ranging from 700 to 850mm can greatly distinguish between oxygenated hemoglobin and deoxygenated hemoglobin. By emitting near-infrared light through the frontal lobe tissue of the brain, the probe detects the absorption attenuation of near-infrared light in the corresponding spectrum, and then uses the improved Beer Lambert law and diffuse reflectance correction to calculate the relative content of tissue oxygenated hemoglobin and deoxygenated hemoglobin, and then calculates rSO2 [3]. RSO2 is the mixed oxygen saturation of local brain tissue, reflecting the balance of oxygen supply and demand in the local brain
B-rSO2Clinical application
Research has shown that preoperative low B-rSO2 can predict the short-term and long-term prognosis of cardiac surgery patients and is an independent risk factor for poor prognosis [1]. Heringlake et al. [2] divided 306 patients undergoing transcatheter aortic valve replacement surgery into a general anesthesia group (n=155) and a local anesthesia group under conscious sedation (n=151) based on the anesthesia method. The correlation between B-rSO2 and 1-year postoperative mortality was evaluated, and the results showed that B-rS02 less than 54% was an independent predictor of 1-year postoperative mortality, independent of anesthesia type. Research on cardiac surgery shows that most patients with low B-rS02 have an increased preoperative risk stratification, therefore, lower B-rSO2 may be one of the risk predictors. Compared to the intraoperative trend of rSO2, B-rSO2 may have greater potential for predicting postoperative neurological adverse outcomes. Schoen et al. found that a B-rSO2 level below 59.9% is an independent predictor of postoperative delirium in cardiac surgery patients, and the high incidence of postoperative delirium is not related to intraoperative rSO2 changes. Lei et al. [6] found that the incidence of postoperative delirium in patients with B-rSO2 below 50% was four times higher than that in the group with higher baseline values. Higher B-rSO2 has a protective effect on postoperative delirium, and restoring intraoperative rSO2 (not less than 75% of B-rSO2) cannot reduce the incidence of delirium after cardiac surgery.
B-rSO2 is the average rSO2 in the state of inhaling air, and the ability of patients with lower B-rSO2 to respond to oxygen inhalation has also become one of the research topics.A study selected patients with B-rSO2 less than 51% and found that those who did not respond to oxygen therapy had a significantly increased risk of postoperative death and complications. Patients with elevated rSO2 after oxygen therapy had a better prognosis [1].Seppelt et al. [7] also reached a similar conclusion, suggesting that low B-rSO2 without response to oxygen therapy is negatively correlated with 1-year postoperative survival rate and length of hospital stay, and can serve as a substitute indicator for impaired cardiorespiratory function.Therefore, if B-rSO2 is used for hazard stratification, the changes in rSO2 during oxygen inhalation are worth considering and exploring.
B-rSO2influencing factors
1. Age
Lian et al. [9] included 120 patients aged ≤ 80 years who underwent elective surgery and were divided into infant group, toddler group, preschool group, school-age group, adult group, and elderly group according to age. rS02 was (70.41+4.66)%, (72.43+3.81)%, (70.77+3.27)%, (70.62+2.20)%, (69.76+6.02)%, (62.69+3.14)%, respectively.
The results showed that compared with the other 5 groups, the elderly group had lower B-rSO2 and there was no significant difference in B-rS02 between the other 5 groups. B-rS02 gradually decreased with age in 1616 cardiac surgery patients [8]. There are many studies on the relationship between age and B-rS02, and the general view is that B-rSO decreases in older adults. The possible mechanism of B-rSO reduction in elderly patients: ① Age related decreases in cerebral blood volume, cerebral blood flow, cerebral metabolic rate, and central excitation induced hemoglobin oxygenation in brain tissue; The elderly's cerebral atrophy gradually worsens, accompanied by cerebral vascular degeneration, resulting in low B-rSO2 levels. ② The myelin sheath of the central nervous system gradually prolongs with age, the intracranial near-infrared light path lengthens, and energy attenuation increases, leading to a decrease in rS02 measurement values. This may be the main factor leading to lower B-rS02 levels in elderly people; ③ As age increases, the structure of hemoglobin changes, with a decrease in the amount of oxygenated hemoglobin and a gradual increase in the amount of deoxygenated hemoglobin. This leads to a decrease in the oxygen carrying capacity of hemoglobin in the brain, thereby reducing B-rSO2.
2. Gender
The impact of gender on B-S02 is still controversial, but mainstream research suggests that female B-S02 is lower. In a large-scale retrospective study of cardiac surgery patients, Su et al. [11] observed that B-rS02 levels were lower in female patients than in male patients. Robu et al. [10] found that after eliminating confounding factors such as hemoglobin, female patients still had significantly lower levels of B-rS02. The conclusions regarding gender induced changes in B-rS02 in different studies targeting healthy volunteers are not entirely consistent, and whether these differences are due to hormonal or other factors needs to be further evaluated in future trials.
The influence of race on B-rS02 is related to skin pigmentation. A retrospective study of a large sample found that B-rS02 levels in African Americans were lower than those in Caucasians, which is related to the absorption of some near-infrared spectra by skin pigments. This is manifested as deeper pigmentation leading to lower B-rSO levels and lighter pigmentation leading to larger B-rS02 levels [11].1. Cardiovascular disease and B-rS02Research has found a certain correlation between changes in cerebral oxygen saturation and alterations in cardiac function. And B-rS02 is one of the effective parameters reflecting overall cardiovascular dysfunction [16]. Heringlake et al. [2] measured B-rS02 in 1178 patients undergoing extracorporeal circulation bypass surgery and found that it was correlated with cardiovascular function indicators such as N-terminal pro brain natriuretic peptide (NTprBNP) and high-sensitivity troponin T left ventricular ejection fraction (LVEF). A B-rS02 value less than 50% was an independent risk factor for 1-year postoperative mortality. Skhirtladze et al. [7] found in their study of patients planning to receive defibrillators for cardioversion that patients with severely reduced LVEF had a significant decrease in rS02; With successful electrical cardioversion and improved cardiac function, rS02 significantly increased. Retrospective studies have shown that preoperative LVEF in cardiac surgery patients is significantly correlated with B-rS02 before anesthesia induction [18]. Kharraziha et al. [19] found that compared to patients with normal heart function, patients with heart failure have lower levels of B-rS02; As the treatment progresses, B-rS02 significantly increases with the improvement of cardiac function [20]. These studies provide a theoretical reference for B-rS02 to become a non-invasive monitoring indicator for guiding the treatment of heart failure patients.The possible mechanism by which cardiovascular disease affects B-rS02: ① Cardiac dysfunction leads to a decrease in cardiac output, resulting in a decrease in mean cerebral arterial pressure. ② The retention of water and sodium will lead to the increase of systemic venous pressure, especially the increase of superior vena cava pressure, which will lead to the increase of intracranial venous pressure, which will lead to the increase of intracranial pressure. The combined effect of two factors leads to a decrease in cerebral arteriovenous pressure gradient, resulting in a significant reduction in effective perfusion pressure and cerebral blood flow in the brain tissue. B-rS02 is also significantly reduced.
2. Neurological disorders and B-rS02
Research has found a certain correlation between cognitive function status and B-rS02. Murayama et al. [21] included 113 adult subjects [aged (72.3 ± 12.0) years] with cognitive status between normal and dementia, and evaluated the relationship between resting prefrontal cortex oxygen saturation and cognitive function. The B-rS02 level in cognitively impaired individuals was significantly lower than that in cognitively normal individuals, and B-rS02 was positively correlated with cognitive function. Hallacoglu et al. [22] measured the cerebral hemodynamic parameters of a diet induced vascular cognitive impairment rat model under anesthesia. Compared with the healthy control group, B-rS02 in cognitive impairment rats significantly decreased after 10 weeks of feeding [(51 ± 4)% vs (62+2)%]. In contrast, the arterial oxygen saturation of the two groups was the same, and there was no difference in hemoglobin content and red blood cell count, indicating that NIRS technology can recognize cerebral hypoxia in the early stage of cognitive impairment, and the decrease in B-rS02 is not related to red blood cell oxygen carrying capacity. The decrease in oxygen supply caused by cerebral microvascular disease may be one of the main mechanisms. Alzheimer's disease (AD) is the most common disease that causes cognitive impairment and disability in the elderly, with early AD patients showing a decrease in B-rSO2 [23]. Viola et al. [24] found that with the progress of hyperbaric oxygen therapy, cognitive function in AD patients improves while B-rS02 also significantly increases. The above clinical studies provide new ideas for the auxiliary diagnosis and treatment of B-rS0 in AD patients. RS02 has attracted widespread attention in the diagnosis and treatment of epilepsy [25]. Current research has shown that rS02 changes to varying degrees before, during, and after epileptic seizures [26-27], but the impact of epilepsy on B-rS02 has not been reported and further research is needed in the future.With the development of diabetes, the structure and function of cerebral microvessels may change. Studies have found that diabetes can cause intracranial arteriolar stenosis and impairment of systolic and diastolic functions [28], and decrease of cerebrovascular autonomic regulation [29]. Damage to cerebral vascular structure and disruption of cerebral blood flow stability can lead to insufficient microcirculation perfusion, which may be manifested as a decrease in rS02 [30].In early studies, central venous oxygen saturation (Scv02) was often used to assess cerebral oxygenation in patients with diabetes. However, Scv02 mainly reflects the overall oxygen balance [31]. Sudy et al. [33] divided 139 heart surgery patients into groups according to whether they had a history of diabetes or not, and studied the correlation between Scv02 and B-rS02. The results showed that Scv02 of diabetes patients was within the normal range before anesthesia induction, while B-rS02 was significantly reduced [(60.4+8.1)% vs (67.2 ± 7.9)%, P<0.05]; There was a good correlation between Scv02 and B-rS02 in non diabetes patients (r=0.52, P<0.001), but there was no significant correlation between B-rS02 and SevO2 in diabetes patients. Compared with Scv02, B-rS02 is conducive to early detection of cerebral perfusion insufficiency in diabetes patients. Chen Zhuoya et al. [4] observed that B-rS02 in the left and right prefrontal lobes of diabetes patients decreased, and it was significantly related to the course of disease and the occurrence of complications. Whether B-rS02 can reflect the severity of diabetes needs further research. However, Liu Weiping et al. [35] did not observe the impact of diabetes on B-rS02 in elderly patients, suggesting that a large sample of prospective controlled research is still needed to further explore the impact of diabetes on B-rS02, which also supports that the study of B-rS02 may be affected by mixed factors such as age and more underlying diseases.4. Kidney disease and B-rSO2Research has found that changes in brain structure and function can be observed in the early stages of chronic kidney disease (CKD), and brain abnormalities and cognitive deficits progress with the severity of CKD, ultimately leading to serious central nervous system diseases such as dementia and stroke, and are more common in hemodialysis (HD) patients [36].Research has shown that B-rS02 levels in HD patients are significantly lower than those in healthy individuals, and even lower than those in peritoneal dialysis patients [37]. This may be related to the poorer kidney function in HD patients, as well as the impact of hemodialysis on the central nervous system itself. Acute loss of blood volume and fluid transfer during dialysis may cause cerebral edema and affect cerebral perfusion [38]; HD patients often suffer from metabolic acidosis, which reduces the affinity between hemoglobin and oxygen, leading to microcirculatory hypoxia [39]; In addition, HD patients often experience cerebral atrophy [40] and an increase in cerebrospinal fluid, which reduces the intensity of near-infrared light received by the rS02 detector. This may have altered cerebral perfusion in HD patients and affected the monitoring of rS02. The study by Okawara et al. [41] found that low B-rSO2 is independently correlated with pH value, disease duration, serum albumin, and hemoglobin concentration in HD patients. The pH value affects cerebral blood flow by altering the diastolic and systolic states of cerebral arteries, therefore the correlation between B-rS02 and pH value can be explained by changes in oxygen supply. The exact mechanism by which serum albumin improves cerebral oxygenation is not yet clear. Some scholars have observed a positive correlation between serum albumin and local cerebral blood flow in patients with cirrhosis [42], but further research is still needed. Shindo et al. [43] found that transient compression of arteriovenous fistulas can observe an increase in rS02, iatrogenic changes in vascular physiological structure and peripheral hemodynamics, which may be one of the reasons for B-rS02 damage in HD patients. Further research is needed to investigate the long-term effects of fistula closure on B-rS02.
5. Liver disease and B-rSO2
Early studies suggested that NIRS monitoring of rSO2 was not suitable for patients undergoing liver and gallbladder surgery. Due to hyperbilirubinemia, the absorption of near-infrared light may lead to false low rS02 measurements without damaging brain oxygenation [44]. However, Murphy et al. [45] measured B-rS02 and intraoperative changes in pancreatic surgery patients and found that high bilirubin levels affect B-rS02 measurements, but do not affect anesthesia induction and monitoring of rS02 changes during surgery.6. Respiratory diseases and B-rSO2Respiratory system diseases are often accompanied by hypoxemia and CO2 accumulation, which have complex effects on cerebral hemodynamics. Chronic hypoxemia leads to reactive damage to cerebral blood vessels [46], affecting cerebral perfusion. The protective effect of hypercapnia, which increases cerebral blood flow by dilating cerebral blood vessels, may gradually weaken with continuous hypoxia exposure, and in the long term, a certain degree of cerebral hypoxia may occur, which may be manifested as a decrease in rS02 [47].
Factors such as monitoring equipment and monitoring locationSchneider et al. [4] used NIRS-rSO2 devices produced by different manufacturers for the same newborns, and the results showed that there were significant differences in the measurement results of different rS02 monitoring instruments, with a maximum variability of 12.66%. This may be due to the different processing algorithms and instrument calibration standards used by different manufacturers. Hirasawa et al. [49] found that changes in blood flow to the forehead skin can also affect B-rSO2 and are not affected by the distance between the light source and the detector (15-30mm). Another study found that when the monitoring electrode is placed in the lower part of the forehead, B-rS02 is significantly higher than when the electrode is placed in the upper part of the forehead [50]. In addition, other studies have observed that heat stress-induced hyperventilation and hypocapnia can lead to decreased cerebral blood flow and increased brain metabolic rate, and B-rS02 may be damaged [51-52]In summary, rS02, as a non-invasive, real-time, and convenient monitoring indicator for cerebral oxygen supply and demand balance, has significant importance in guiding clinical practice. B-rS02 is not only a reference standard for the dynamic changes of rS02 during surgery, but also contains information about cardiovascular disease, nervous system disease, diabetes, kidney disease, liver disease, respiratory system disease and other diseases and their severity. Clinical physicians should fully consider the influencing factors of B-rS02, differentiate between physiological, pathological, and equipment factors that cause rS02 reduction, and use rS02 as an auxiliary tool for risk stratification of surgical patients' prognosis to guide the prevention and treatment of serious complications.The Bolian Zhongke MOC series brain tissue oxygen saturation monitor uses non-invasive, real-time continuous monitoring of brain oxygen saturation to promptly detect issues such as cerebral hypoxia, helping medical personnel to intervene early and optimize treatment plans. Its high sensitivity and specificity ensure the accuracy of monitoring data, reduce intraoperative and postoperative complications, and improve patient safety. Widely used in scenarios such as cardiac surgery, neurosurgery, and intensive care, it not only ensures the life and health of patients, but also improves the safety, quality, and medical level of hospitals.
