###### Abstract
The bidirectional mechanisms responsible for the comorbidity of depression and CAD are complex and multifactorial [12; 13; 14; 15; 16]. Various pathophysiological triggers, including neuroendocrine dysregulation, activation of the inflammatory response, increased platelet activation and aggregation, oxidative stress, and endothelial dysfunction, may all be involved in the relationship between depressive symptoms and CAD [5; 12; 13; 14; 15; 16; 17; 18]. Individuals with depressive disorders tend to have elevated inflammatory markers and pro-inflammatory cytokines that undermine their response to conventional antidepressant therapy [14; 19; 20; 21; 22]. Several studies indicate a bidirectional relationship between depression and inflammatory response [23; 24]. However, although the depression and elevated inflammatory markers frequently covary, it is still unclear how they relate to each other, and to the onset of CAD [25]. Some reports indicate that depressive symptoms may precede and augment inflammatory processes in clinical entities such as coronary artery disease while others suggest that elevation of inflammatory biomarkers precedes depression [25; 26; 27; 28; 29]. Another possibility is that both depression and inflammation may be consequences of another underlying pathologic process [25]. Individuals with depressive disorders tend to have elevated levels of inflammatory markers and pro-inflammatory cytokines that also undermine their response to conventional antidepressant therapy [24]. Among them the C-reactive protein (CRP), a marker of acute phase response, has been used most extensively in clinical practice as a biomarker of systemic inflammation and is routinely measured across medical centers and research laboratories [30; 31]. Levels of CRP are used to detect and follow disease in many fields of medicine and its kinetics after major surgery procedure is usually related to inflammatory response to infection [32]. The increased concentrations of CRP have been found in a subset of patients with depression compared with the non-depressed control group, often preceding the onset of illness [27; 33; 34; 35; 36; 37]. CRP is also one of the elevated inflammatory markers in depression that shows reduced variability thus supporting greater homogeneity in terms of corresponding inflammatory phenotype [38]. Several studies have also shown the correlation between peripheral CRP levels and the increased risk of CAD [37; 39; 40; 41; 42; 43; 44]. The relationship between preoperative depression and increased hospital length of stay following CABG procedures was also reported [45; 46; 47; 48; 49]. However, more than half of the CABG patients are not routinely assessed and treated for depressive syndromes [45; 46]. In addition, only a few studies have assessed the relationship between depression and CRP proteins in CABG patients [29; 48; 50]. In addition, little is known about the causal relationship between these two covariates in predicting CABG recovery. Furthermore, only one study to date showed that the effect of preoperative depression symptoms on postoperative hospital stay of CABG patients may be mediated by the post-surgery inflammation and elevated CRP levels [48]. Specifically, this association was partially mediated by a greater change in CRP from baseline preoperative values to 4-8 days post-CABG surgery. Based on these reports, the present study aimed to assess the relationship between preoperative depressive symptoms with postoperative hospital length of stay (LOS) of patients undergoing elective CABG surgery and to examine the extent to which this association could be influenced by preoperative and postoperative CRP values. Specifically, we hypothesized that the presence of preoperative depression symptoms measured using the Beck Depression Inventory (BDI-II) would be associated with the length of postoperative hospital stay and that the effect of preoperative depression score on the length of stay would be mediated by the higher values of preoperative and postoperative CRP levels.