Meta-analysis of the effect of the pringle maneuver on long-term oncological outcomes following liver resection

Hepatic pedicle clamping reduces intraoperative blood loss and the need for transfusion, but its long-term effect on survival and recurrence remains controversial. The aim of this meta-analysis was to evaluate the effect of the Pringle maneuver (PM) on long-term oncological outcomes in patients with primary or metastatic liver malignancies who underwent liver resection. Literature was searched in the Cochrane Central Register of Controlled Trials (CENTRAL), Medline (via PubMed), and Web of Science databases. Survival was measured as the survival rate or as a continuous endpoint. Pooled estimates were represented as odds ratios (ORs) using the Mantel–Haenszel test with a random-effects model. The literature search retrieved 435 studies. One RCT and 18 NRS, including 7480 patients who underwent liver resection with the PM (4309 cases) or without the PM (3171 cases) were included. The PM did not decrease the 1-year overall survival rate (OR 0.86; 95% CI 0.67–1.09; P = 0.22) or the 3- and 5-year overall survival rates. The PM did not decrease the 1-year recurrence-free survival rate (OR 1.06; 95% CI 0.75–1.50; P = 0.75) or the 3- and 5-year recurrence-free survival rates. There is no evidence that the Pringle maneuver has a negative effect on recurrence-free or overall survival rates.

Liver resection remains the only curative treatment for hepatic malignancies, and can improve long-term survival 1 . Improvements in surgical techniques, better selection of patients, and improved perioperative care have increased the number of hepatectomies performed worldwide each year 1,2 . There is growing evidence that excessive blood loss during hepatectomy and the subsequent need for blood transfusions may contribute to a poor outcome for non-cirrhotic and cirrhotic liver resections 1,2 . Perioperative blood transfusion has been associated with recurrence and poorer long-term survival due to an immune response dysfunction 3 .
Vascular occlusion techniques have been used by some surgeons during hepatic resection to minimize intraoperative blood loss, especially in large tumors or tumors that are adjacent to major vessels 4,5 . Pringle described a technique whereby transient hepatic inflow was occluded by clamping the portal triad. Portal clamping in the Pringle maneuver (PM) has been modified several times in form of intermittent portal clamping 6,7 and selective portal clamping 8 . These modifications can control intraoperative blood loss and decrease the need for transfusion. Some surgeons believe that this reduction in the rate of blood transfusions can improve long-term oncological outcomes. On the other hand, some argue that the PM may increase the risk of ischemia-reperfusion injury to the liver, which may impair hepatocyte function 4,6,7 .
The present systematic review and meta-analysis aimed to evaluate the effect of the PM on long-term oncological outcomes in patients with primary or metastatic liver malignancies who underwent liver resection.

Recurrence-free survival rate.
One-year recurrence-free survival rate. One-year RFS rates were reported for 6758 patients from 17 studies (4223 patients were in the PM group and 2744 patients in the non-PM group). The recurrence of malignant hepatic lesions was reported in 1023 cases (24.2%) in the PM group and in 742 cases (27%) in the non-PM group. Meta-analysis indicated that the PM did not decrease 1-year RFS rate (OR 1.06; 95% CI 0.75-1.50; P = 0.75; Fig. 3A) using a random-effects model. There was considerable heterogeneity among the studies (I 2 = 84%; P < 0.00001).
Three-year recurrence-free survival rate. Recurrence of malignant lesions during the first 3 years after hepatectomy was reported in 6138 cases from 15 studies. Of these, recurrence was reported in 2037 patients (54.6%) in the PM group and in 1233 patients (51.1%) in the non-PM group. Meta-analysis revealed no significant dif-  www.nature.com/scientificreports/ ference in 3-year RFS rate between the groups (OR 0.99; 95% CI 0.74-1.34; P = 0.97) using the random-effects model (Fig. 3B). The studies that reported 3-year RFS rates were not homogeneous (I 2 = 81%; P < 0.00001).
Five-year recurrence-free survival rate. A total of 14 studies with 3781 patients in the PM group and 2591 patients in the non-PM group reported 5-year recurrence. As is seen in Fig. 3C, recurrence was reported in 2521 patients (66.67%) in the PM group and in 1862 patients (71.86%) in the non-PM group. The meta-analysis showed that 5-year RFS rate is not significantly different between the two groups (OR 0.82; 95% CI 0.65-1.04; P = 0.1) using the random-effects model (Fig. 3C). The studies that reported 5-year RFS rates were not homogeneous (I 2 = 67%; P = 0.0002).
Overall survival rates.

Discussion
Intraoperative bleeding is one of the most common and life-threatening complications during liver surgery, and has been associated with increased long-term morbidity and mortality 26 . In addition, intraoperative hemorrhage increases the rate of blood transfusions, which have a negative impact on long-term postoperative outcomes by reducing the patient's immune defense 26,27 . Excessive bleeding and blood transfusion also reduce patient survival 26,27 . Excessive intraoperative bleeding and vascular occlusion are both associated with an increased risk of postoperative surgical complications and unfavorable clinical outcomes. Therefore, the optimal approach to liver resection is to perform surgery without hepatic vascular occlusion while minimizing blood loss and the need for blood transfusion.
Despite several strategies to reduce intraoperative bleeding, the PM remains the most commonly used technique because it was shown to reduce blood loss with high efficacy in initial randomized trials 10,26 . However, some studies have not confirmed these initial findings and have even suggested a higher risk of ischemia-reperfusion injury for healthy liver tissue 28,29 . Furthermore, an increased rate of postoperative complications has been shown in patients who undergo PM during hepatectomies in some studies 30 . To prevent liver injuries, portal pedicle clamping was modified in the PM to an intermittent approach 31 . Despite this modification, the overall efficacy of www.nature.com/scientificreports/ the PM remains controversial 32,33 . Whether the PM promotes liver injury remains a topic of debate. Furthermore, how the PM affects recurrence and survival in patients with malignant lesions who underwent hepatectomy is not well understood. Although some studies have suggested that prolonged PM increases recurrence 1,20 , others have demonstrated no effect 15,19,34 . For instance, Al-Saeedi et al. revealed that a PM of less than 20 min did not increase the recurrence rate after 3 years 9 . Recent studies showed that the PM has no significant positive impacts on clinical outcomes after minor liver surgeries 13,32 . However, major liver resections, which have more intraoperative www.nature.com/scientificreports/ blood loss, probably benefit more from the PM. To address this controversy, we performed a meta-analysis to compare the long-term oncological outcomes of hepatectomy with and without a PM. The PM, regardless of whether it is complete or intermittent, was shown to be an independent risk factor for cancer recurrence in one study 13 . However, other studies have reported no negative impact of the PM on patient survival and disease recurrence 17,18 . In a recent randomized-controlled trial, the intermittent PM did not affect disease-free survival after hepatectomy, but did improve the OS rate 10 . The positive effect of the intermittent PM was particularly promising in patients with hepatic disorders such as cirrhosis 10 . In the present analysis, we observed no significant differences in 1-, 3-, and 5-year overall and recurrence-free survival between the PM and non-PM groups. Furthermore, subgroup analysis revealed no significant effects of tumor type (i.e., primary or metastatic) on 1-, 3-, and 5-year survival between the PM and non-PM groups. This is in accordance with previous findings from large patient cohorts and clinical trials. www.nature.com/scientificreports/ The PM was shown to be a risk factor for disease recurrence in several studies. It has been hypothesized that ischemia during portal pedicle clamping causes microvascular damage by breaking adhesions between tumor cells and endothelial cells 35 . The hepatic ischemia-perfusion cycle might increase the expression of E-selectin, which plays a crucial role in cancer cell metastasis 36,37 . However, we found no significant increase in disease recurrence following hepaectomy with the PM, indicating that the PM is not associated with disease recurrence after hepatectomy.
During reperfusion, liver parenchymal cells are thought to be injured by cytokines and radical oxygen species, which are produced by active Kupffer cells 38 . However, a meta-analysis reported no significant patient benefits of hemihepatic vascular occlusion over complete hepatic vascular occlusion, despite a lower rate of liver injury 39 . This suggests that significant hepatic injury is not caused by the PM, and that the potential benefits outweigh the potential disadvantages. In addition, of enrolled studies in this meta-analysis, four studies (2335 cases) reported the number of patients with steatosis, and no significant difference was observed in means of fatty liver distribution among patients with and without PM a. However, included studies failed to provide more detailed data on clinical or oncological impacts of liver texture characteristics (e.g. macrovesicular or microvesicular liver steatosis, or liver fibrosis) on outcomes of the pringle maneuver, which prohibited us from carrying out subgroup analyses.
A study by Fagenson et al. reported that patients undergoing minor liver resection and cases with metastatic disease had a worse outcome when PM was performed 40 . This finding is in similar line with our previously published report. Our results showed that PM is useful in patients who underwent extended liver resection, but this surgical maneuver may not be beneficial in minor hepatectomies 9 . It can be derived that PM is associated with encouraging early perioperative outcomes without worsening the long-term survival among well-selected patients. On this basis, it cannot be denied that the selection of patients undergoing PM plays a principal role in increasing of safety and efficacy of PM.
There are some limitations to the present study. The main weakness is the variability in PM techniques, underlying liver disease, tumor stage status, and preoperative liver function between the included studies. Due to lack of subgroup results regarding the underlying liver disease, especially liver cirrhosis, it was not possible to assess the impact of PM in cirrhotic patients. In addition, although several studies have compared the PM with non-PM techniques, the number of RCTs is low, and most studies have a retrospective design, which can have a selection bias because PM enable surgeons to perform more aggressive hepatectomy in patients with more advance tumors with worse prognosis. We have added to study from the same center in our meta-analysis 6,13 ; the first study was performed between January 2007 and December 2010 13 and the second study was performed between January 2010 and December 2012 6 . These two studies may include overlapping patients in 2010 which can create some bias in present meta-analysis.
In conclusion, the present study shows that the PM is a suitable surgical technique for managing intraoperative bleeding during liver resection, and does not increase tumor recurrence and long-term mortality. We believe that the PM is a useful and acceptable aopproach to major or extended liver resection. However, further studies in large patient cohorts and randomized trials are needed to comprehensively evaluate the advantages and disadvantages of this procedure.

Methods
This systematic review and meta-analysis was reported according the Preferred Reporting Items for Systematic Reviews (PRISMA) guidelines 41 .
Eligibility criteria. The research question was formulated according to the PICOS strategy.
• Population: all adult patients who underwent liver resection • Intervention: PM during liver resection • Comparators: no PM • Outcome: overall or recurrence-free survival rates • Study design: all study types methodological designs, including human subjects, except case series with less than ten patients, narrative or systematic reviews, letters, conference abstracts, and study protocols.
Duplicate publications or overlapping cohorts were excluded.
Search strategy. According to Goossen et al. 42 the following databases were searched. www.nature.com/scientificreports/ Outcomes and data items. Recurrence-free survival rate. The recurrence-free survival (RFS) rate was defined as the number of the patients who survived without signs of recurrence after primary liver resection. We measured the RFS after 1, 3, and 5 years.
Overall survival rate. The overall survival (OS) rate was defined as the number of patients who survived after liver resection, regardless of disease recurrence. We measured the OS at 1, 3, and 5 years.
Quality assessment. The Cochrane risk-of-bias tool was used to assess the quality of randomized-controlled trials (RCT) and the ROBINS-I tool was used to assess the quality of non-randomized studies (NRS) 43,44 .
The Cochrane risk-of-bias tool evaluated several items, including bias arising from the randomization process, bias arising from the timing of identification and recruitment of individual participants in relation to the timing of randomization, bias due to deviations from intended interventions, bias due to missing outcome data, and bias in the selection of the reported result. The overall risk of bias was low if the study was judged to be at low risk of bias for all domains. There were some concerns of bias if some concern of bias was detected in at least one domain. The risk of bias was high if the study was judged to be at high risk of bias in at least one domain or if some concerns of bias were detected in multiple domains.
Statistical analysis. Statistical analyses were performed by RevMan version 5.3 (Nordic Cochrane Centre, Cochrane Collaboration, Copenhagen, Denmark). Pooled results were analyzed using the Mantel-Haenszel method. Results were presented as odds ratios (OR) or as survival rates with 95% confidence intervals (CI). Because of clinical heterogeneity between studies, a random-effects model was used. A P value < 0.05 for the Q-test or a I 2 index more than 75% indicated statistical heterogeneity among studies. An I 2 index between 50 and 75% indicated moderate statistical heterogeneity. www.nature.com/scientificreports/ Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/.