Discussion

ToF is a serious CHD, and early post-operative complications have a long-term impact on the quality of life and survival of children after surgical treatment. ^{Reference Apitz, Webb and Redington1,Reference Yim, Riesenkampff, Caro-Dominguez, Yoo, Seed and Grosse-Wortmann3} The CPB-induced inflammatory response is an important factor leading to post-operative complications in ToF. ^{Reference Bronicki and Hall7} IL-6 is a critical inflammatory cytokine that plays an important role in myocardial fibrosis. ^{Reference Dobaczewski, de Haan and Frangogiannis5} At the same time, myocardial fibrosis is an important factor leading to heart failure. ^{Reference Kong, Christia and Frangogiannis4} Therefore, it is necessary to explore the molecular mechanism of CPB leading to myocardial fibrosis, focusing on IL-6. In our study, we first found that IL-6 was generally up-regulated in the post-CPB group (Fig 2d), suggesting that CPB directly causes high expression levels of IL-6. The WGCNA and PPI network analysis identified six genes that were identified as HUB genes (JUN, FOS, ATF3, EGR1, IL-6, and PTGS2). Amongst these, FOS, ATGF3, EGR1, and IL-6 are known to be closely related to myocardial fibrosis. ^{Reference Yim, Riesenkampff, Caro-Dominguez, Yoo, Seed and Grosse-Wortmann3,Reference Pan, Sun and Shan21–Reference Shen, Xing and Gong23} JUN has been reported to play an important role in Inflammation, ^{Reference Xiao, Wang and Lahusen24} and as the target of non-steroidal anti-inflammatory drugs, PTGS2 plays a critical role in regulating inflammation and pain. ^{Reference Hata and Breyer25–Reference Smyth, Grosser, Wang, Yu and FitzGerald27} One study showed that inhibition of PTGS2 prolonged allograft survival and reduced inflammation and myocardial damage during acute cardiac allograft rejection in a rat model. ^{Reference Ma, Szabolcs and Sun28} Thus, inflammation-related genes appear to be significantly up-regulated post-CPB during ToF surgery. It is known that inflammation is an important inducer of myocardial fibrosis. ^{Reference Leslie, Schwarz, Simpson and Huber29} IL-6 is generally regarded as an inflammatory biomarker, and is commonly used to assess the presence and severity of low-grade inflammation. ^{Reference Baumeister, Akhtar, Ciufolini, Pariante and Mondelli30}

Here, we further explored biological processes associated with CPB. A biological process enrichment analysis shows that genes with a strong correlation with CPB are significantly involved in the response to lipopolysaccharide, molecules of bacterial origin, leukocyte cell−cell adhesion, epithelial cell proliferation, and regulation of epithelial cell proliferation. Furthermore, a molecular function enrichment analysis and GSEA analysis showed neuroactive ligand–receptor interaction and a number of immune-related pathways are enriched in the post-CPB group and the IL-6-high group. One study has shown that neuroactive ligand–receptor interaction is associated with human arrhythmogenic right ventricular cardiomyopathy. ^{Reference Chen, Long, Xu, Wu and Zhang31} Another study has suggested that the myocardial protective effect of sevoflurane is through neuroactive ligand–receptor interactions in patients undergoing coronary artery bypass graft surgery; ^{Reference Wang, Cheng, Zhang and Li32} we, therefore, speculate that neuroactive ligand–receptor interactions are closely related to cardiac function.

It is well known that the MAPK signalling pathway and the T-cell receptor signalling pathway are associated with inflammatory responses. ^{Reference Wang, Chen and Cao33,Reference Rudd34} In addition, activation of the Wnt signalling pathway leads to adverse remodelling after ischaemic injury, whereas inhibition of Wnt signalling improves cardiac function. ^{Reference Zelarayán, Noack and Sekkali35} The ErbB signalling pathway is closely associated with heart failure, and it is a target for drugs to treat heart failure. ^{Reference Lemmens, Doggen and De Keulenaer36} The JAK/STAT signalling pathway is an important cytokine signal transduction pathway, which regulates diverse physiological and pathological processes including cellular inflammation, differentiation, proliferation, apoptosis, and immunity. ^{Reference Wang, Li and Li37} Activation of the JAK/STAT signalling pathway as a result of the up-regulation of IL-6 results in pro-inflammatory responses that promote myocardial fibrosis (Fig 5c and d).

From the PanglaoDB analysis, we found that IL-6, ATF3, EGR1, and PTGS2 are closely related to fibroblasts (Fig 6), which further supports our findings. These data suggest that using drugs during CPB procedures that can prevent a pro-inflammatory response may improve prognosis. In this regard, aprotinin and ulinastatin are common anti-inflammatory drugs that are used in CPB, procedures. Propofol, sevoflurane, and dexmedetomidine have also been found to have anti-inflammatory and myocardial protective effects during CPB. ^{Reference Wang, Cheng, Zhang and Li32,Reference Biccard, Goga and de Beurs38–Reference Mikawa, Akamatsu and Nishina41}

Although we have uncovered several interesting results in this study, it does have significant limitations. First, the sample size is small and so this may affect the validity of the findings, and second, the proposed molecular mechanisms are theoretical and so require further experimental verification.