The results from our systematic review and meta-analysis suggested that the incidence of POCD - DCR following the use of TIVA may be lower compared to inhalational anesthesia in the first 30 postoperative days. Even though we have succeeded in including a high total number of subjects in our review and meta-analysis, the heterogeneity in definitions of POCD - DCR, different psychometric tests used and its cuff-off values, among other factors, limit the reach of our conclusions. This is reflected in our heterogeneity analysis (I2 = 85%). Our study suggests that the concept of POCD should be redifined into a more objective definition. Moreover, it would be of interest to evaluate a more basic cognitive domain, such as attention, in all awake and alert individuals, since it is well known from the literature that attention plays a pivotal role to the functions of all other cognitive domains. It is reasonable to assume that specific cognitive deficits, such as memory, executive function, among others, may reflect a subjacent attention impairment. Future research should focus on objective attention measurements prior to other specific cognitive domains. This would allow a reduction in heterogeneity on POCD research.
The potential benefits of propofol and TIVA in POCD might be mediated through its positive effects in diminishing the inflammatory cascade. Evidence has shown that propofol has anti-inflammatory properties compared to inhalation agents (23) as in vivo study results have shown lower levels of circulating cytokines and other mediators of inflammation in animals injected with propofol (11). Inflammation has been associated with POCD in many different studies. An increase in various cytokines, including IL-6, TNF-α, IL- 8, and IL-10, have been correlated with postoperative cognitive impairment (24). Recently, a meta-analysis was conducted assessing the association between various inflammatory biomarkers and POCD and concluded that higher postoperative C-reactive protein (n = 11 studies) and IL-6 (n = 17 studies) were associated with POCD (25). However, the possible role of the anesthetics in the inflammatory cascade is still yet to be clarified, with some evidence favoring the use of inhalational agents, such as sevoflurane, specifically in ischemia-reperfusion cell models (26)
The population enrolled in our work has a high median age, consequentially to the fact that most of the research on POCD involves older individuals. Only two of the included studies admitted patients younger than 60 years old (4, 6). Since the number of younger individuals was too small, we were not able to perform a stratification analysis by age. Even though all included studies relied on comparisons of the results of psychometric tests postoperative with the preoperative evaluation, only five studies used healthy controls not submitted to surgery or anesthesia in its study design (3, 5, 19, 21 – 22), to make sure the cognitive decline was not consequence of the advanced age itself.
One limitation that must be stressed refers to the fact that propofol was used in both groups in all included studies, at least as a single bolus agent at the induction phase of anesthesia. It’s uncertain if a single dose of propofol might exert any potential beneficial effects on POCD, consequentially to its potential effects at the inflammatory cascade, even considering that in TIVA group propofol is used in a continuous infusion through all the duration of the procedure. Maybe future studies in POCD should consider using another induction agent in the inhalational group, at the study design phase.
Regarding our secondary aim, we decided to proceed only with a systematic review of the literature, considering the few studies included and, consequently, the small number of subjects, given that the recommendations for testing between 30 days and 12 months postoperatively are relatively recent. Further studies, considering this testing period, are necessary in the future.
Most of these psychometric tests aim at a specific domain of cognitive function (Supplementary Table 1). Many of these tests have been validated in different clinical scenarios, including the postoperative period (27–34). It seems reasonable to hypothesize that different psychometric tests, targeting different cognitive domains, might differ in their ability to diagnose POCD. In addition, little attention has been spent on which specific tests and cognitive domains would be most likely altered in the postoperative period. So far, we have scarce evidence of which cognitive domains are more susceptible to POCD, with few data pointing towards the attention domain and executive function as potentially more affected in the postoperative period (35). As mentioned earlier, the attention domain plays a pivotal role in cognition since its proper function is essential to the functioning of all other domains. However, all the studies included in this review established the diagnosis of POCD accepting any altered domain as equally valid. This should, as well, be an important topic for future research.
Only two authors in our review reported which specific tests showed a significant difference in their postoperative assessment. One study reported that tests most frequently altered were the Semantic Verbal Fluency and the Letter Number Sequence Test, which measures the executive function and speed and visual space working memory cognitive domains (4). Another author reported that the COWAT, the Stroop Neuropsychological Screening, the Clock Test, the Three Word-Three Shapes, the Babcock Story Recall, the Instrumental Activities Daily Living (IADLS), and the TMT-B as the tests showed a difference in their postoperative assessment (7).
Additionally, the application of psychometric tests for diagnosis of POCD that relies on a cut-off, such as one SD from the mean, or similar, could be insensitive for detecting minor but significant changes in cognitive status in the postoperative period. We hypothesize that the use of a test more focused on the attention domain, a pre-requisite for the proper function of all the other cognitive domains, applied as a continuous variable measured over time could potentially be more sensitive in detecting subtle changes in the cognitive function perioperatively. This should be an additional relevant topic for future research.
It should be emphasized that ten out of the 12 studies we included in our present review titrated the level of anesthesia in both groups with the use of EEG-derived monitors, such as the BIS, all studies targeting a value between 40 and 60. The use of these devices might potentially lead to improved titration of anesthesia (3). Therefore, our results cannot be explained as consequence of monitoring the level of consciousness on a particular group.
In conclusion, TIVA might be associated with a lower incidence of POCD, compared with inhalational anesthesia, at least in the first 30 postoperative days. However, future studies investigating POCD, should focus on assessments of attention because the validity of testing all other cognitive subdomains (e.g., memory, executive functions, etc.) relies on its integrity. This could also potentially reduce heterogeneity on POCD research.