Coronary slow flow (CSF) is a microvascular disorder characterized by the slow entry of radiopaque contrast agent into distal vascular structures in the presence of normal or near-normal epicardial coronary arteries during coronary angiography [1]. Although there is no clear consensus regarding the pathophysiology of CSF, diffuse atherosclerosis, inflammation, increased platelet aggregability, increased microvascular tone, microvascular and endothelial dysfunction have been suggested to contribute to its pathogenesis [2]. Multiple studies to define the demographic characteristics of patients with CSF have shown that male sex, smoking, and decreased high-density lipoprotein cholesterol (HDL-C) level are more common in these patients [3‐5].

Atherogenic dyslipidemia, which comprises the concurrence of increased serum triglyceride (TG), apolipoprotein B, and small dense low-density lipoprotein cholesterol (LDL-C) levels along with decreased HDL-C level, plays a major role in the genesis of atherosclerotic plaques [6]. Atherogenic dyslipidemia has been implicated in the pathogenesis of endothelial dysfunction, microvascular coronary dysfunction, and atherosclerosis. [7]. Oxidized LDL inhibits endothelium dependent vasodilation by impairing the activity of nitric oxide synthase and contributes to the atherosclerotic plaque formation [8]. HDL protects endothelial cells against the damaging effects of LDL and improves endothelial cell function [9]. Studies have demonstrated that hypertriglyceridemia impairs endothelial function through several mechanisms [10]. TG also induces atherosclerosis via the production of proinflammatory cytokines, coagulation factors, and fibrinogen [11]. The atherogenic index of plasma (AIP) is a relatively novel indicator of atherogenicity calculated as log₁₀ (TG/HDL-C) [12]. Previous studies have demonstrated that, in comparison to simple lipid parameters, the atherogenic coefficient (AC; Non-HDL-C/HDL-C) and Castelli’s risk indices I and II (total cholesterol [TC]/HDL-C and LDL-C/HDL-C, respectively) had stronger correlations with cardiovascular disease and better predictive capability for cardiovascular events [13‐15].

Emerging data increasingly support potential role for endothelial dysfunction and subclinical diffuse atherosclerosis in the etiology of CSF. There is insufficient data in the literature on the relationship between atherogenic indices and CSF. For this reason, the present study was performed to investigate the relationships of lipid profile and atherogenic indices with CSF.

In total, 130 patients with CSF and 130 control participants with NCF were included the study. The mean ages of the two groups were 54.05 ± 9.61 years and 54.82 ± 8.78 years, respectively. Both groups showed male predominance (80% and 56%, respectively). Table 1 shows the demographic and clinical characteristics of the study population. Age, hypertension status, diabetes mellitus status, and dyslipidemia status were similar between groups. The proportion of male sex and current smokers were significantly higher in the CSF group than in the NCF group (p < 0.001). The WBC, NLR, PLR, neutrophil cell count, platelet count, LDL-C, TG, non-HDL-C, AIP, AC, CRI-I, and CRI-II values were significantly higher in the CSF group than in the control group. Furthermore, the HDL-C level and lymphocyte cell count were significantly lower in the CSF group than in the control group. There were no statistically significant differences between the two groups in terms of other laboratory parameters, TC, and LVEF (all, p > 0.05).

The corrected LAD-TFC, LAD-TFC, LCX-TFC, RCA-TFC, and mean TFC values were significantly higher in the CSF group than in the control group (all, p < 0.001). In addition, in 73% of the patients, CSF was observed in the LAD (Table 2).

The multivariate logistic regression analyses demonstrated that AIP (OR, 5.463; 95% CI, 1.357–21.991; p = 0.017), CRI-II (OR, 1.624; 95% CI, 1.138–2.319; p = 0.008), PLR (OR, 1.004; 95% CI, 1.000–1.008; p = 0.049), current smoking (OR, 3.063; 95% CI, 1.664–5.641; p < 0.001), and male gender (OR, 3.464; 95% CI, 1.746–6.875; p < 0.001) were independent predictors of CSF (Table 3).

ROC curve analysis showed that the optimal cutoff value to predict the occurrence of CSF was 0.66 for AIP (sensitivity, 59%; specificity, 73%; area under the receiver operating characteristic curve, 0.695; p < 0.001) (Fig. 2). According to ROC analysis, the optimal cutoff value to predict the occurrence of CSF was 3.27 for CRI-II (sensitivity, 60%; specificity, 79%; area under the receiver operating characteristic curve, 0.726; p < 0.001) (Fig. 3).

This study was performed to investigate the relationships of CSF with traditional lipid parameters and atherogenic indices (e.g., AIP, AC, CRI-I, and CRI-II) that have been associated with increased cardiovascular risk. Atherogenic indices were higher and HDL-C values were lower in patients with CSF, compared with the control group. AIP, CRI-II, current smoking, and male sex were found to be an independent risk factor for CSF. In addition, LDL-C and non-HDL-C values were significantly higher in the CSF group compared to the control group.

In CSF, washout of the contrast agent is prolonged in the absence of any spasm, thrombus, dissection, and any stenosis that causes significant occlusion in epicardial coronary arteries. Although the incidence of CSF in diagnostic coronary angiography is not rare, its pathogenesis has not yet been well elucidated. Various mechanisms have been proposed for its etiology, including subclinical diffuse atherosclerosis. Cin et al. [18] examined the coronary arteries of 19 patients with CSF using fractional flow reserve and intravascular ultrasonography. They noted extensive calcification and diffuse intimal thickening along the vessel walls and atheroma plaques that did not cause lumen narrowing in patients with CSF, as well as diffuse atherosclerosis in the microvascular system and epicardial coronary arteries. Pekdemir et al. [19] reported diffuse intimal thickening and calcifications along epicardial arteries during coronary angiography examinations of patients with CSF. Ding et al. [20] reported that lipoprotein-associated phospholipase A2, which plays a role in inflammation and atherosclerosis in the vessel walls, was significantly and independently associated with the presence of CSF. Considering the data obtained from these studies, it may be reasonable to conclude that diffuse coronary atherosclerosis plays a role in the etiopathogenesis of CSF.

Increases in the levels of plasma very-low-density lipoprotein and small dense LDL-C, and reduced clearance of apolipoprotein B-containing particles from plasma have been identified in patients with atherogenic dyslipidemia [21]. Compared with other LDL subfractions, small dense LDL particles are more atherogenic because they are more susceptible to oxidative stress and can pass through the subendothelial space more easily due to their small diameter. Accordingly, they can stay in circulation longer and have less affinity for LDL receptors [22]. With regard to the origin of small dense LDL formation, Berneis et al. [23] proposed that TG-rich lipoproteins (e.g., very-low-density lipoprotein 1) are converted into small dense LDL after delipidation by hepatic lipase and lipoprotein lipase enzymes. The Framingham Heart Study showed that small dense LDL level is directly correlated with serum TG level and inversely correlated with serum HDL-C level in patients with metabolic syndrome [24]. There is increasing evidence that both predominance and elevated levels of small dense LDL-C play important roles in the initiation and progression of atherosclerosis, as well as increased risk of cardiovascular disease [22, 25, 26]. In a recent meta-analysis of 21 studies, Liou et al. [27] reported positive associations of small dense LDL level and cholesterol content of small dense LDL with the risk of coronary heart disease. These findings are supported by an increasing body of evidence in favor of the causal link between small dense LDL and coronary heart disease. However, because the test to measure small dense LDL is complex and costly, its measurement is unlikely to be applicable in routine clinical practice [28]. It was suggested that the TG/HDL-C ratio could be used as an indicator of LDL subfraction [29]. In addition, TG/HDL-C ratio increases systemic and vascular inflammatory processes by decreasing endothelial protective mechanisms, which leads to progressive coronary atherosclerosis [30].

AIP is regarded as an indirect indicator of small dense LDL-C [12]. Wang et al. [31] described a strong correlation between AIP and syntax score in patients with coronary heart disease. In a prospective observational study of women > 60 years old, a negative correlation between HDL-C concentration and AIP was founded, but observed a positive correlation between all-cause deaths and AIP, after adjusting for age, smoking, and statin therapy [32]. In a study conducted in 1059 patients with a history of acute coronary syndrome before the age of 35 years, the presence and severity of acute coronary syndrome were found to be independently associated with AIP, and these relationships were stronger than those of simple lipid parameters (i.e., TC, TG and LDL-C) [33]. TG/HDL-C ratio, AIP and CRI indices have predicted cardiovascular events better than traditional lipid profiles such as LDL-C and non-HDL-C [34].

To our knowledge, there have been no studies regarding the relationship between CSF and AIP. In the present study, we identified a positive correlation between AIP and TIMI frame count, which is regarded as an indicator of coronary flow reserve. The predictability of TIMI frame count by AIP supports the role of diffuse atherosclerosis in the pathophysiology of CSF. The results of the present study showed that AIP provided a reference for CSF severity. In addition, LDL-C and non-HDL-C, which are defined as the main indicator of atherogenic particles by current guidelines, were higher in the CSF group than in the control group. However, these lipid parameters were not found as predictive variables in the regression analysis.

CRI-II (LDL-C/HDL-C ratio) represents the proportion or relationship between the atherogenic and antiatherogenic lipoproteins. CRI-II was a more precise predictor for cardiovascular events than classic lipid parameters (i.e., TC, TG and LDL-C) used independently [13, 35]. Fujihara et al. [36] demonstrated that CRI-II was an independent predictor of coronary artery stenosis and vulnerable coronary plaque. Bleda et al. [37] showed that improvement of nitrite plasma levels, a marker of endothelial function, was associated with decreased total cholesterol/HDL-C values after statin treatment in patients with peripheral artery disease. Kıs et al. [38] reported that HDL-C/LDL-C ratio was associated with endothelial functions in patients with coronary artery disease. They showed that as the HDL-C/LDL-C ratio increased, flow-mediated dilatation increased. It has been suggested that while nitric oxide and endothelial progenitor cells were increased, LDL-C and inflammation markers (i.g., high sensitivity C-reactive protein, endothelin-1and interleukin-6) were decreased after atorvastatin treatment in patients with CSF [39]. In a study involving 54 patients with CSF, Kalaycı et al. [40] reported that TG/HDL-C ratio, CRI-I and II values were higher in the CSF group than in the control group. The authors stated that age, smoking and TG predict CSF. In the present study, CRI-II was an independent predictor of CSF which was different than Kalaycı et al. [40] investigation.

CSF has been reported to be more common in young male smokers. In our study, both smoking and male sex were independent risk factors for CSF. These results are consistent with previous studies. In their cross sectional study, Sanghvi et al. [41] reported that history of tobacco use was 45.5% and male sex was 62.5% in the CSF group. Furthermore, current smoking was an independent risk factor for CSF. In another study performed by Rao V et al. [4] in an Indian population, 66% of patients were males and 68% of patients were smokers in subject with CSF. In a prospective study involving 39 patients with CSF, Arbel et al. [3] reported that current smoking was the most significant variable related to CSF. Smoking association with CSF could be explained as follows: endothelial dysfunction, impaired endothelium-dependent coronary vasodilatation, increased microvascular resistance, increased oxidized LDL, and increase in mediators leading to atherosclerosis [3, 5, 42, 43].

There have been multiple investigations of the roles of inflammation and biomarkers that reflect the inflammatory state (e.g., white blood cells [overall and subtypes], acute phase reactants, adhesion molecules, and cytokines) in coronary artery disease, as well as their relationships to adverse events [44‐47]. Aksan et al. [44] proposed that the inflammatory biomarker neutrophil gelatinase-associated lipocalin may be a useful marker and predictor for CSF. Turhan et al. [46] reported that the levels of plasma soluble adhesion molecules, such as intercellular adhesion molecule-1, vascular cell adhesion molecule-1, and E-selectin were elevated in patients with CSF. In addition, the biomarkers PLR [48] and NLR [49] are reportedly elevated in patients with CSF, compared with controls. These parameters have been significantly and independently associated with CSF. Our results were consistent with those of previous studies, in which NLR and PLR indices were higher in patients with CSF than in controls. PLR was only identified as predictive variable in regression analysis. Large platelets are more thrombotic, contain more dense granules, and are metabolically and enzymatically more active, compared with smaller platelets [50]. Their dense granules contain substances that are crucial mediators of coagulation, thrombosis, inflammation, and atherosclerosis [51]. In the present study, platelet values were higher in the CSF group, but there were no significant differences between the two groups in terms of MPV values.

This study had several limitations. First, this was a single-center, retrospective observational study with a small sample size. Because of its retrospective design, inflammatory markers related to atherosclerosis (e.g., high-sensitivity C-reactive protein, interleukin-6, and adhesion molecules) were not studied, and a more detailed evaluation of the relationship between CSF and inflammation could not be performed. Apolipoprotein B and small dense LDL-C, which reflect the total atherogenic particle load better than LDL-C, were also not measured. Unfortunately, imaging modalities (e.g., intravascular ultrasonography or optical coherence tomography) could not be used, although these would have better demonstrated a potential relationship between CSF and subclinical diffuse atherosclerosis. Endothelial function tests such as pulse wave analysis and flow-mediated dilatation could not be performed. In addition, selection bias may have occurred during selection of the control group due to the retrospective study design, and some individuals with NCF in the control group may have had undetected microvascular dysfunction.

The results of this study indicated that AIP and CR II are independent predictors of CSF. Prospective studies in larger cohorts of patients may elucidate the role of atherogenic dyslipidemia in the pathophysiology of CSF. The results may also provide additional support for the importance of lipid-lowering therapy in the management of patients with CSF.