Cardiopulmonary bypass (CPB) is a necessary and common procedure to support the patient’s circulation during cardiac surgery. Although previous studies 1 2 reported that CPB does not increase the postoperative morbidity and mortality in patients undergoing coronary artery bypass graft surgery, it was demonstrated that the incidence of some postoperative complications for these patients remains high. Neurological dysfunction is one of the most commonly reported postoperative complications in patients undergoing cardiac surgery. 3 4 Several factors including cerebral anoxia, embolism, excessive excitatory neurotransmitter release and systemic inflammatory response have been demonstrated to contribute to postoperative neurological dysfunction. 5 However, at present, there is no definitive clinical evidence regarding cerebral protection for patients undergoing cardiac surgery with CPB. 6 Previous studies on animals support the hypothesis that anaesthetics can produce cerebral protection. 7–9 Many recent studies have found that anaesthetic agents may be neuroprotective and may provide cerebral protection to surgery patients. 10 11 However, clinical studies show that the relative effects of inhalation anaesthesia or total intravenous anaesthesia (TIVA) on neuroprotection in cardiac surgery with CPB remain controversial and much debated. 12–14 Therefore, which option provides better cerebral protection to patients undergoing cardiac surgery with CPB is unknown. As inhalation anaesthesia and TIVA are the most commonly used strategies for general anaesthesia, it is important to clarify this issue. Moreover, as it is difficult to include patients in neurological dysfunction studies for cardiac surgery with CPB, the sample size of these previous studies was generally small. For these reasons, it is necessary to systematically review the available literature and perform a meta-analysis to compare the neuroprotective effects of inhalation anaesthesia and TIVA.

The weighted mean differences (WMD) of outcomes in randomised controlled trials (RCTs) and their 95% CI were presented. Heterogeneity across studies was tested by the p value and the I 2 statistic, which is a qkuantitative measure of inconsistency. 16 A random-effects model was used to analyse the summary estimate when the p value was <0.1 or the I 2 value was >50%. Otherwise, a fixed-effects model was applied. In the meta-analysis, potential publication bias was detected by Egger test. Publication bias was assumed existed if the p<0.05.

Study selection was completed by two independent reviewers by screening abstracts and titles of all included papers from the literature search. All the relevant papers were retrieved according to the inclusion criteria. Then based on the abstracts and titles, the second screening of full texts was performed to check if there was an ambiguous decision. Only randomised controlled trials were included in the analysis. Disagreements were resolved through consensus or by a third reviewer. According to the primary criteria for randomised and controlled trials, quality assessment was performed by two reviewers.

In the included studies, S100B levels in serum were detected before CPB (pre-CPB), after CPB (post-CPB) and 24 hours postoperatively. And the primary outcomes were protein S100B levels in serum post-CPB and 24 hours postoperatively. The secondary outcomes included mini-mental state examination (MMSE) scores assessed preoperatively and 24 hours postoperatively, the jugular bulb venous oxygen saturation (SjvO 2 ), arteriovenous oxygen content difference (D(a-v)O 2 ) and cerebral oxygen extraction ratio (O 2 ER) were tested at cooling and rewarming during CPB.

This meta-analysis was restricted to published studies that investigated the cerebral protective effects of anaesthetics in patients with CPB. The PubMed database, EMBASE, MEDLINE, Science Direct/Elsevier, Cochrane Library and China National Knowledge Infrastructure were searched by two independent reviewers up to August 2016, without restrictions on language or study type. The search terms combined text words and medical subject headings (MeSH) terms. For example, the search terms for CPB were: ‘cardiopulmonary bypass’ and ‘heart lung bypass’. Those for TIVA were: ‘propofol’, ‘disoprofol’, ‘etomidate’, ‘midazolam’, ‘sodium pentothal’, ‘thiopental’ and ‘ketamine’, while those for inhalation anaesthesia were ‘halothane’, ‘sevoflurane’, ‘isoflurane’, ‘desflurane’, ‘enflurane’ and ‘methoxyflurane’. (The MEDLINE search strategy is provided in the (online supplementary appendix ) , and the finalised MEDLINE search strategy will be adapted to the syntax and subject headings specifications of the other databases.) All relevant articles and abstracts were retrieved. In addition, references cited within relevant reviews were retrieved manually and only full articles were searched in this case.

Sensitivity analysis for the current meta-analysis was also performed. We omitted one study in each turn, and calculated the combined WMD for the remaining studies. The results showed that no single study significantly changed the combined results in the overall meta-analysis, indicating that the results were reliable and statistically stable ( figures 7 and 8 ).

Summary estimate for S100B levels post-CPB and 24 hours postoperatively was analysed in a random-effects model because of the heterogeneity (I 2 =96% and I 2 =99%, respectively). Based on six studies from 230 patients, S100B levels assessed at the end of CPB and 24 hours postoperatively in the inhalation anaesthesia group were significantly lower than those in the TIVA group (WMD (95% CI): −0.41 (–0.81 to –0.01), −0.32 (−0.59 to −0.05), respectively, figure 2 ). Based on three studies from 110 patients, postoperative MMSE scores of the inhalation anaesthesia group were significantly higher than those of the TIVA group (WMD (95% CI): 1.87 (0.82 to 2.92)), figure 3 ]. A significant heterogeneity was detected (I 2 =77%), and thus summary estimate was analysed in a random-effects model.

‘Inhalation anaesthesia’ was defined as a group receiving a volatile agent like isoflurane, sevoflurane or desflurane. In the included studies, patients in the ‘volatile anaesthesia’ group had not received propofol, thiopental or ketamine during the surgery and CPB. The patients in the ‘TIVA’ group had received only intravenous anaesthetics, but not volatile agents. These studies involved 549 patients, including 272 patients with inhalation anaesthesia and 277 patients with TIVA ( table 1 ). Patients’ age ranges in ‘inhalation anaesthesia’ and ‘TIVA’ groups were 44–75 years and 43–74 years, respectively. The mean age of patients was unavailable for three studies. 17–19 All the articles had reported exclusion/inclusion criteria. 17–29 Of these, seven studies had used isoflurane versus TIVA, 17 19–21 23 24 27 four studies had used sevoflurane versus TIVA 18 22 25 26 and two studies had used desflurane versus TIVA 28 29 in patients.

A total of 1485 studies were retrieved. Of these, 1148 remained after duplicate articles were eliminated. After screening titles and abstracts, 445 studies were potentially eligible. Based on the exclusion criteria, 13 studies were ultimately selected ( figure 1 ). All reviewers agreed to include all 13 papers. Although all of these RCTs were considered to have a low risk of bias, nine studies included no details on the method of random sequence generation and allocation. 17–25 Only one study provided the details about the blinding of the data collection. 26

Discussion

In our study, 13 published articles were included to determine the difference in the extent of cerebral protection provided by inhalation anaesthesia and TIVA during cardiac surgery with CPB. Eight out of the 13 studies suggested that inhalation anaesthesia might be superior to TIVA in terms of their cerebroprotective effect after CPB.18 20–22 25–27 29 However, the results reported in other five studies were the opposite.17 19 23 24 28 These results underline the existing debate on which anaesthetic approach is better for the patients. However, in the current systematic review and meta-analysis, the results of primary and secondary outcomes showed that inhalation anaesthesia might be superior to TIVA during cardiac surgery with CPB.

S100B is mainly expressed in the astrocytes, and blood S100B level is commonly used as an outcome parameter for evaluating the postoperative neurological dysfunction.30 Its level in the blood has been shown to increase in patients after ischaemic stroke and brain trauma.31 Serum S100B has also been detected after cardiac surgery complicated by neurological injury in adults; thus, it has the potential to serve as an early marker of brain damage.32 33 In this meta-analysis, the serum level of S100B after CPB in the inhalation anaesthesia group was found to be significantly lower than that in the TIVA group (p<0.05),18 25–27 29 suggesting that inhalation anaesthetics provide better cerebral protection than TIVA against brain damage.

As reported by Svenmarker et al,34 it is inevitable that S100B contamination will occur due to the pericardial suction blood, which is often retransfused or processed in the cell saver and then retransfused during CPB. However, a strict control of clinical procedures may decrease its potential effect on the difference of S100B detection between the two groups. In the included studies, the use of retransfusion and cell salvage were not mentioned. Therefore, the possible effect of retransfusion and cell salvage should not be neglected, and this is a potential limitation of the current study.

Among the secondary outcomes, the MMSE is one of the most commonly used parameters for the clinical evaluation of cognitive function. Our results show that postoperative MMSE scores of patients in the inhalation anaesthesia group were significantly higher than those in the TIVA group (p<0.05).18 25 29 These results suggest that inhalation anaesthesia is better than TIVA in terms of protecting the postoperative cognitive function of patients undergoing cardiac surgery with CPB. The meta-analysis also showed that the other outcomes such as D(a-v)O 2 , O 2 ER and SjvO 2 were not significantly different for TIVA and inhalation anaesthesia groups. However, we found that in some studies, the cerebral oxygen metabolic rate (CMRO 2 ) in patients receiving inhalation anaesthetics assessed at cooling and rewarming during CPB was consistently lower than that in patients receiving TIVA.20 21 Additionally, the intraoperative cerebral blood flow (CBF) assessed at cooling and rewarming during CPB in the inhalation anaesthesia group was significantly higher than that in the TIVA group.20 21 A low ratio of global cerebral oxygen and adequate cerebral blood supply is an important parameter for evaluating cerebral protection.35 Thus, these results based on CMRO 2 and CBF can strengthen the finding that inhalation anaesthesia may provide better neuroprotection than TIVA.

Experimental data suggest that inhalation anaesthetics’ positive effects may be caused by preconditioning or postconditioning mechanisms,36 37 which attenuate apoptosis and necrosis of cerebral neurons, thereby reducing neurological dysfunction after ischaemia. Moreover, inhalation agents in preserving satisfactory haemodynamics may contribute to the adequate perfusion and oxygenation of other organ systems,38–41 and thus to improve the patients’ recovery and survival after surgery. Because of the neuroprotection that induced by anaesthetic can be long lasting,42 43 all these effects can be expanded well beyond the immediate perioperative period. Additionally, a recent meta-analysis found that in cardiac surgery,44 as compared with TIVA, inhalation anaesthesia was associated with major benefits in outcome, including reduced mortality, as well as a lower incidence of pulmonary and other complications. Therefore, based on previous findings and the current meta-analysis, it is speculated that inhalation anaesthesia has the potential to serve as a preferential anaesthesia strategy for cardiac patients.

Our study has few limitations. First, the sample size of the included studies was relatively small and the total number of cases is very limited. Second, there was heterogeneity in some of our results. As trials were based in different countries and hospitals, we were unable to avoid the effects of race, age, gender and underlying disease(s) of patients in our study. Therefore, findings of the current study were limited by the overall low quality of evidence and the lack of robust data. Third, our study focused on the overall comparison between inhalation anaesthesia and TIVA, and different inhalation (isoflurane, desflurane or sevoflurane) and intravenous (sodium thiopental, propofol and so on) anaesthetics were investigated in the included studies. Because of the limited number of reported clinical trials, limited outcome data could be considered for subgroup analysis. Therefore, further studies with larger sample sizes are needed to demonstrate which anaesthetics are more beneficial for cardiac patients.

In summary, the results of this meta-analysis indicate that the cerebroprotective effect of inhalation anaesthesia is better than that of TIVA in patients undergoing cardiac surgery with CPB. Further high-quality trials with larger sample sizes are warranted to investigate the effect of anaesthetics on cerebral protection.