Study selection
The search process was shown in Fig. 1. The literature search yielded a total of 80 articles from all databases. Of these, 32 articles were duplicated searches, so the process of study selection was performed on the remaining 48 articles. Sequentially, 19 articles were considered irrelevant studies after screening their titles and abstracts. The authors reviewed the full text of the remaining 29 relevant studies and excluded 21 studies from the final analysis because 7 studies were intravenous DEX, 10 studies were preclinical experiments and 4 studies were no ESPB was performed in the control group. Finally, 8 RCTs were included in the study [19-26]. Fig 1 represents the preferred reporting items for systematic reviews and meta-analysis (PRISMA) flow diagram, and summarizes the reasons for exclusion of records [13].
Characteristics of included studies
The details of the included RCTs are summarized in Table 1 (On page 31-33). Of the 8 studies, 7 studies were from China [19, 21-26], 1 studies were from Egypt [20]. The level of nerve block localization was T 5 in 5 trials [19-21, 23, 26], T 4+T 6 in 1 trial [22], T 3 in 1 trial [24], L 3 in 1 trial [25]. With the exception of 1 trial that used bupivacaine [20], 7 all trials used another long-acting LAs ropivacaine [19, 21-26]. The DEX dosage was used in combination with LAs was 0.5 μg/kg in 3 studies [20, 22, 26], and 1 μg/kg in 5 studies [19, 21, 23-25]. The types of surgery are open thoracotomy [20, 26], video-assisted thoracoscopic lobectomy [19, 21-23], modified radical mastectomy [24], and posterior lumbar spine surgery [25].
Risk of bias
A low level of overall risk of bias for included 8 trials. All patients were randomized to each group by appropriate methods, and allocation concealment was adequate in most studies. 1 of the studies reviewed lacked sufficient details in allocation concealment, and blinding of outcome assessors, in such case, we were conservative in our risk of bias evaluation by tending to classify trials as having an “unclear risk of bias” [22]. Furthermore, when the “attending anesthesiologist was informed about the grouping of the patients”, we judged the study to be “high risk of bias” [23]. A full risk-of-bias summary for all included studies is shown in Fig. 2.
GRADE
Main results are shown in Table 2 (On page 34-35).
Primary outcome
Time of the first request for rescue analgesia
5 RCTs reported the time of the first request for rescue analgesia. A significant difference was found between the two groups (5 studies [19-23]: MD = 8.4 hours; 95% CI = 4.70 to 12.10; p < 0.00001; I2 = 89%; random-effects model; GRADE = Moderate; Fig. 3). Patients who used LAs mixed with DEX in ESPB had a significant delay in rescue analgesia.
Subgroup analyses were performed according to the dosage of DEX because the heterogeneity was high. Both the 0.5 μg/kg [20, 22] (MD = 4.86 hours; 95% CI = 3.68 to 6.04; p < 0.00001; I2 = 0%; Fig 3) and 1 μg/kg [19, 21, 23] (MD = 11.00 hours; 95% CI = 9.37 to 12.63; p < 0.00001; I2 = 0%; Fig 3) subgroups prolonged the time of the first rescue analgesia, and the prolonged time was more significant in the high dose subgroup. The data heterogeneity within the subgroup is low.
Secondary outcome
Number of PCIA presses
Across all 8 RCTs we reviewed, 4 RCTs reported the number of PCIA presses within 72 hours after the operation. The pooled analysis showed that DEX as adjuvants significantly reduced the number of PCIA presses (4 studies [19, 21, 22, 25]: MD = -4.12; 95% CI = -7.79 to -0.45; p = 0.03; I2 = 93%; random-effects model; GRADE = Moderate; Fig. 4A).
When we examined the sources of heterogeneity, we carried out a sensitivity analysis. We found that the pooled analysis results remained unchanged after excluding the data of Wang YH et al. [25], but the heterogeneity was low (MD = -5.79; 95% CI = -7.24 to -4.35; p < 0.00001; I2 = 0%; random-effects model; Fig. 4B), indicating that this study is the main source of heterogeneity.
Number of remedial analgesia events
The authors found 4 comparisons of the number of remedial analgesia events between the two groups in the post-operative 72 hours. Pooled analysis showed that the number of remedial analgesia events was significantly less in the DEX group (4 studies [19-22]: OR = 0.33; 95% CI = 0.17 to 0.65; p = 0.002; I2 = 11%; random-effects model; GRADE = High; Fig. 5).
Incidence of PONV
The incidence of PONV was reported in 7 studies and had complete data to pooling. The result indicated that DEX as an adjuvant decreased the incidence of PONV significantly (7 studies [19-24, 26]: OR = 0.57; 95% CI = 0.36 to 0.91; p = 0.02; I2 = 0%; random-effects model; GRADE = High; Fig. 6).
Rate of hypotension
4 RCTs comparing between the DEX group and control group reported the rate of hypotension after surgery. The difference was not found to be significant (4 studies [22-24, 26]: OR = 1.01; 95% CI = 0.37 to 2.74; p = 0.99; I2 = 7%; random-effects model; GRADE = High; Fig. 7). In order to merge the research data to be more complete, although there were no hypotension events in the two studies, they were still included in the pooled analysis as the total number of events [24, 26].
Occurrence of arrhythmia
Arrhythmia is a common side effects after surgery. The data included in this meta-analysis include sinus bradycardia and sinus tachycardia. The pooled effect size demonstrated that the occurrence of arrhythmia was no statistically significant difference between the two groups (4 studies [22-24, 26]: OR = 0.76; 95% CI = 0.19 to 3.07; p = 0.70; I2 = 64%; random-effects model; GRADE = Moderate; Fig. 8) . This result is moderately heterogeneous, but due to the inclusion of fewer studies and events, it is unable to carry out subgroup analysis, so the random effect model is used for analysis. The number of arrhythmia events in two studies was 0 and was still included in the pooled analysis to ensure the integrity of the data [24, 26].