Background
With the advancement of medical technology, an increasing number of older patients have gained access to surgical treatment in recent decades. To date, approximately half of the older population is estimated to have undergone at least one surgery. Compared to younger patients, the older population has been reported to be much more prone to developing perioperative complications following surgery and anesthesia due to a higher prevalence of comorbidities and increased perioperative risk [
1,
2]. Postoperative cognitive dysfunction (POCD), a common complication of surgery and anesthesia, has been recognized as a new-onset cognitive impairment after any type of surgical intervention, including cardiac and non-cardiac surgery, especially in the geriatric population, which may persist for a few days, months, or even years [
3,
4]. POCD is broadly characterized by a series of alterations in neurocognitive conditions and behavior, including impaired memory, poor comprehension, and reduced attention [
5]. Previous studies have revealed that the incidence of POCD in older individuals varies from 16.7 to 89% one week after surgery [
6‐
9]. It is well established that POCD is associated with a range of negative outcomes, including prolonged hospitalization, changes in mood and personality, reduced quality of life, heavy burden on the family and society, and increased mortality [
10,
11].
Regional cerebral oxygen saturation (rSO
2) monitoring, a non-invasive method to monitor cerebral perfusion and ischemia that is measured by near-infrared spectroscopy (NIRS), has played an essential role in guiding or optimizing perioperative management [
12]. Several studies have revealed that a lower level of rSO
2 during surgery is strongly associated with an increased risk of POCD [
13‐
15]. However, other studies failed to reveal the potential association between intraoperative rSO
2 values and the incidence of POCD [
16‐
18], which raises the question of the clinical validity of rSO
2 monitoring in preventing POCD, especially in older surgical patients. Furthermore, no consensus exists regarding the role of rSO
2 monitoring-based management in the prevention of POCD. Additionally, the quality of evidence on this topic is relatively poor among currently published meta-analyses [
19‐
22]. Moreover, the clinical value of rSO
2 between cardiac and non-cardiac surgical patients has not yet been established.
Considering that the quality of life among older patients is impaired by long-standing POCD and that the conflicting results vary from existing studies, we conducted this systematic review and meta-analysis to gather the existing literature and explore the association between rSO2 monitoring and early POCD during hospitalization in older patients. We hypothesized that rSO2-based perioperative management has a predictive value for the incidence of POCD and other common adverse events in this high-risk population.
Discussion
To our knowledge, this is the first meta-analysis to specifically explore the effects of rSO2 monitoring on the occurrence of POCD and other adverse postoperative complications in older patients, which was assessed based on data from 377 patients from six RCTs. The incidence of POCD ranges from 17 to 89%, with an overall prevalence of 47% in this meta-analysis. The results of our analysis demonstrated that rSO2-guided intervention could remarkably decrease the incidence of POCD and shorten LOS among older patients undergoing non-cardiac surgery. However, our pooled results did not show that the incidence of postoperative cardiovascular or surgical complications was affected by the use of intraoperative cerebral oximetry.
POCD is characterized by a deterioration in cognitive performance after surgery, which is particularly prevalent in older patients. To date, there is no consensus on neuropsychological tests specifically used for POCD [
31]. It is usually detected with different meticulous neuropsychological tests, such as MMSE, MoCA, and neuropsychological test battery, which were all cited in our present meta-analysis [
32]. These cognitive tests present different sensitivity, specificity, test duration and covered domains [
33]. The substantial heterogeneity in methodology subsequently limits comparability and affects consistency of findings. MMSE is a commonly used test for POCD, and a follow-up measurement within seven days postoperatively seems to be broadly accepted [
34]. Notably, however, MMSE lacks the sensitivity and specificity in capturing subtle cognitive deficits [
31,
33]. Compared with MMSE, neuropsychological test batteries are more sensitive and specific but often complicated and time consuming. Furthermore, these test batteries are often delivered by trained staffs, so they are difficult to be popularized and applied in perioperative settings [
35]. We believe that strong efforts are necessary to explore precise and applicable assessment methods for POCD.
POCD is more frequent and lasts longer in older patients following surgery under anesthesia, which may be mainly due to degenerative changes in the structure of the brain and a progressive decline in reserve function [
36]. It is generally accepted that advanced age, especially pre-existing cognitive impairment, is associated with a high incidence of POCD [
37,
38]. Episodes of cerebral ischemia and hypoxia have been regarded as the most closely related to POCD among distinct etiological factors [
39‐
41]. Compared with younger patients, older patients are more predisposed to perioperative ischemia-induced brain injury, which is partly due to their reduced physiologic cerebrovascular reserve induced by atherosclerosis, hypertension, diabetes, smoking, etc. [
42,
43]. Moreover, brain white matter lesions, which are frequently produced by chronic cerebral hypoperfusion in the older population, have been demonstrated to exacerbate the risk of POCD [
44]. The basic value of rSO
2 is lower in older patients [
45]. In summary, poorer cognitive outcomes following surgery under anesthesia might be a consequence of more frequent and severe cerebral hypoxemia and hypoperfusion in older patients.
As a continuous and non-invasive technology, NIRS can penetrate the brain at a depth of 3–4 cm below the skin and estimate oxygenation in detected brain tissue [
12,
46,
47]. Several trials have demonstrated a close association between perioperative rSO
2 value and postoperative cognitive outcomes in older patients, which may offer a unique opportunity to elucidate the neuropathological mechanisms of POCD [
13,
28,
48]. We arrived at a similar conclusion in the present meta-analysis. However, other studies failed to reveal such an association [
29,
30]. Zorrilla-Vaca et al. included 15 RCTs comprising 2,057 patients in a meta-analysis to estimate the effects of intraoperative rSO
2-based management on clinical outcomes, which suggested that the use of rSO
2 monitoring was related to a reduction in the occurrence of POCD, but the heterogeneity within the included studies was high [
49]. This result was similar to those reported by Ding et al. [
19] and Chen et al. [
20]. However, the findings of a Cochrane review suggested that the effects of rSO
2 monitoring on POCD were uncertain owing to the low quality of evidence and high heterogeneity among the included studies [
22]. We speculated that the inconsistent results were probably due to the principles of rSO
2 monitoring and differences among the included participants. On the one hand, rSO
2 measurement may interfere with the increased distance between the skin and brain tissue, such as in the case of cortical atrophy in older patients. On the other hand, the rSO
2 value reflects mixed arterial and venous saturation in localized areas of the frontal lobes, but not the whole brain. Our meta-analysis showed that rSO
2-guided intervention could reduce the incidence of POCD in older patients undergoing non-cardiac surgery rather than cardiac surgery. Cardiac surgery with cardiopulmonary bypass has been shown to induce microthromboembolic event-related cerebral microvascular dysfunction [
50‐
52]. If cerebral microemboli do not occur in the frontal cortex, false-negative NIRS results will be recorded, which means that intraoperative rSO
2 values may remain normal in cases of severe cerebral ischemia in other brain regions. The findings of Rummel et al. support our hypothesis, which suggests that the rSO
2 value remains normal even in severe hemispheric stroke because the anterior cerebral artery can be supplied by the contralateral side [
53]. In non-cardiac surgery, systemic hypotension or anemia may be responsible for a reduction in global cerebral blood flow and oxygen supply, which can be effectively reflected by cerebral desaturation in the frontal lobe in older patients. Therefore, using NIRS to manage anesthesia during major non-cardiac surgery may help alleviate global cerebral ischemia and hypoxia and decrease the risk of POCD.
LOS, an important and practical indicator, is commonly used to assess overall healthcare utilization. In the present meta-analysis, we found that the LOS of older non-cardiac surgical patients who did not receive intraoperative rSO
2 monitoring was significantly prolonged and was accompanied by an increased incidence of POCD. A recently published prospective study revealed that older surgical patients with POCD are prone to need a prolonged LOS, which suggests the potential consequence of POCD, although clinically subtle, has a noticeable adverse impact on healthcare system expenditure [
10]. Therefore, we should actively apply appropriate strategies, such as intraoperative rSO
2 monitoring, to prevent the occurrence of POCD in a high-risk population. Meanwhile, there may be one possible cause for our failure to show a prophylactic effect of intraoperative rSO
2 monitoring against postoperative cardiovascular or surgical complications in older surgical patients. Compared to the brain, other vital organs and surgical incisions are more tolerant to ischemia/hypoxia-induced injury. When cerebral desaturation leads to neurological damage, other tissues may not suffer from ischemia/hypoxia-related dysfunctions.
The current meta-analysis has several potential limitations. First, the population we focused on was patients older than 60 years, which may limit the generalizability of the results. Second, only two literatures on cardiac surgery were included in our meta-analysis, which makes the analysis less convincing. The present study provided preliminary results owing to the small sample size, which requires further large RCTs to clarify the neuroprotective effects of rSO2 in older surgical patients. Third, diverse neuropsychological tests were applied, including MMSE and MoCA, which also influenced our results. Finally, well-defined reference rSO2 values and clinically relevant thresholds for cerebral desaturation must be explored and subsequently established in future studies.
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