In our current study, we analyzed the relationship between diabetic foot ulcers and sympathovagal balance. Only a few studies have evaluated the sympathovagal balance through HRV analysis in patients with diabetic foot . Furthermore, there are few studies that have evaluated surrogate indices of cardiovascular disease, such as endothelial function indices and serum levels of inflammatory adipokines, in a diabetic population with ulcerative lesions of the lower limbs [1,2,3].
Some studies by our own group analyzed endothelial dysfunction and adipose inflammatory markers such as omentine and vaspin in subjects with diabetic foot in comparison with those without foot ulcerations [3, 5]. In this actual study we confirmed our previous findings concerning the lower serum levels of omentin-1 lower and the lower median RHI values of subjects with diabetic foot. We also report the association between diabetic foot and omentin-1 and reactive hyperaemia index (RHI) as marker of endothelial dysfunction.
Nevertheless, to the best of our knowledge, no study has yet correlated endothelial dysfunction, assessed noninvasively by means of the RHI, with dysfunction of the autonomic nervous system, assessed by the analysis of HRV, and serum levels of some inflammatory adipokines in a population of patients with diabetes and foot ulcers.
Aso et al.  evaluated the sympathovagal balance obtained by means of the spectral analysis of HRV in a population with diabetic foot. In the population examined by Aso, the LF/HF ratio, representing the expression of the sympathovagal balance, was not particularly increased, although both the LF (sympathetic activation indices) and the HF (parasympathetic activation indices) were decreased.
Our study showed that patients with diabetic foot show a higher degree of activation of the parasympathetic system, expressed by the increase in HF values, compared to diabetic controls and a lower LF/HF ratio compared to diabetic controls. The values of the LF/HF ratio of patients with DFS indicate a sympathovagal balance trending toward a higher degree of parasympathetic activation , albeit not statistically significant, consistent with the findings reported by Aso .
Takashy et al.  noted that patients with diabetes with neuropathy had significant sympathetic dysfunction. Within the pathogenesis of diabetic ulcers, in addition to the already established role of diabetic micro- and macroangiopathy and the impairment of the somatic nervous system, dysfunction of the autonomic nervous system plays an important role. Sympathetic dysfunction involves sweating, dryness of the skin and a reduction in skin thickness, which leads to susceptibility to infections and the formation of a diabetic ulcer from mechanical-compressive damage. Ahmed et al.  showed that in patients with diabetic foot, the parasympathetic system is compromised early, which is then followed, over time, by permanent sympathetic dysfunction. Our actual results, depicting greater parasympathetic activation, can be explained by a counterbalance of the vagal action against a reduced sympathetic action.
Our study also highlighted a statistically significant negative correlation between the RHI value and HRV indices and the expression of increased parasympathetic activity (RMSDD and HF%) in subjects with diabetic foot and a statistically significant positive correlation with the LF/HF ratio and the expression of the sympathovagal balance. Endothelial dysfunction, due to excessive oxidative stress and a reduction in NO values, involves the inability of the endothelium to dilate following inducible ischemic stimuli. Endothelial dysfunction and reduced sympathetic activity are closely related in the pathogenesis of diabetic ulcers, as endothelial dysfunction and the consequent microangiopathy of the vasa nervorum involve a downregulation of the small autonomic-sympathetic fibers present in the periphery. This association has also been demonstrated in both healthy and hypertensive subjects by Pinter  and Tomiyama , respectively.
Few studies have correlated HRV variables and endothelial dysfunction indices. In a previous study by Bedile Irem Tiftikcioglu et al. , the correlation between HRV and the RHI was analyzed, but this relationship was not statistically significant. This relationship could be explained by the influence exerted by microangiopathy on autonomic neuropathy. Hyperglycemia can cause changes in the intracellular redox state through the depletion of the cellular NADPH pool. Nonenzymatic glycosylation of proteins and macromolecules secondary to chronic hyperglycemia causes a greater tendency toward oxidative stress and high levels of oxidized lipoproteins (LDL in particular). High levels of fatty acids and hyperglycemia have both been shown to cause an increase in the oxidation level of phospholipids and proteins, resulting in an increased prothrombotic tendency, as well as increased platelet aggregation. Numerous prospective studies evaluating endothelial dysfunction in patients with diabetes indicate that it is closely associated with microangiopathy and macroangiopathy .
Our finding concerning a positive correlation between the RHI and the LF/HF ratio is therefore a possible original finding, as well as the negative correlation between the RHI and the parasympathetic function indices. It is possible to assume that the persistent reduction in sympathetic activity in patients with diabetic foot can be compensated for at the expense of greater parasympathetic activity to recover the delicate homeostasis of the sympathovagal balance.
Our results did not highlight any particular difference between the percentages of HF and the LF/HF ratio between patients with diabetic foot and healthy patients. A possible explanation is that a discrete percentage of subjects in the control group suffered from chronic ischemic heart disease with consequent use of beta blockers, which caused an increase in parasympathetic activity.
Another interesting result that emerged from our study is the high LF/HF ratio in patients with diabetes compared to DFS and healthy controls. Some studies evaluated HRV in patients with diabetes with or without microvascular complications [22,23,24]. In most cases, the LF/HF ratio appears to be decreased, especially in patients with diabetic neuropathy [22,23,24]. Min-Young Chun et al.  assessed that worsening of autonomic neuropathy is positively correlated with the consensual reduction in the LF/HF ratio. Eckberg et al.  suggested that in the pathogenesis of diabetic neuropathy, there is early and early dysfunction of the parasympathetic system, which is followed by a reduction in the sympathetic system. Me Ahmed et al.  proposed a similar pathogenesis within the context of diabetic neuropathy complicated by diabetic foot. Our diabetic cohort examined did not show numerous cases of diabetic neuropathy; thus, it is possible that the LF/HF ratio is increased, indicating an imbalance toward sympathetic activation, precisely due to the early initial parasympathetic dysfunction that characterizes diabetes and that can lead to a rather common microvascular complication such as diabetic neuropathy. Our results show that patients with diabetic foot show significantly lower RHI values and therefore a greater degree of endothelial dysfunction than patients with diabetes without ulcers and healthy controls. Previously, Siasos et al.  provided the first evidence regarding the correlation of diabetic ulcerative lesions with endothelial dysfunction. In a recent study by our own group , we reported that patients with diabetic foot had a higher degree of endothelial dysfunction, expressed as lower RHI values, compared to patients with diabetes and a control group of healthy patients. A greater degree of arterial stiffness was also found through the analysis of PWV and a greater degree of “mild cognitive impairment”, as assessed by the MMSE scores.
Diabetic foot syndrome (DFS) represents a micro and macrovascular complication of diabetes. Endothelium-dependent vasodilation is markedly impaired in the arteries of patients with hypertension, diabetes, ventricular hypertrophy and other cardiovascular risk factors [28,29,30,31,32]. Therefore, both the micro- and macrovascular complications of diabetes are well represented by the evaluation of the indices of endothelium-dependent vasodilation, such as the RH-PAT index.
Our results showing significantly lower RHI levels in a cohort of patients with diabetic foot compared with a population of patients with diabetes and healthy controls are therefore in line with our previous studies [3, 5] and demonstrate that endothelial dysfunction evaluated with a noninvasive method can be considered a surrogate cardiovascular risk marker.
In a recent cross-sectional observational study , conducted on the cohort of patients in the ADELAHYDE has been reported the association of arterial changes (arterial hypertrophy and stiffness, endothelial dysfunction) with cognition indexes Artery thickness and stiffness as well as endothelial function should be measured simultaneously and may represent an additional target for the prevention of memory impairment and WMHs.
In a very recent study by our group , worse cognitive performance, a higher prevalence of hyperintense lesions of the white matter on brain MRI, increased PWV values and reduced serum levels of omentin-1 were reported in patients with DFS compared to a diabetic population without ulcers, a healthy population with vascular ulcers and a control population. These patients also had low RHI levels compared to a healthy population with or without mention of a vascular ulcer and comparable RHI values compared to patients with diabetes.
In our study, we have also reported how patients with diabetic foot, compared to diabetic controls without ulcers and healthy controls, have significantly lower serum levels of omentin-1. Recently, in agreement with the results of our group , Yamawaki et al. showed how omentin has a vasodilating effect on isolated blood vessels, increasing the production of endothelium-derived NO . The negative association between omentin, circulating IL-6 and C reactive protein was also shown to be related to endothelial dysfunction . The inflammatory state associated with obesity-induced metabolic disorders  could be the cause of the dysfunction of endothelial cells in the blood vessels of visceral abdominal tissues, decreasing the expression and production of omentin. Proinflammatory cytokines (TNF-α and IL-6) proportionally indirectly correlate with circulating omentin concentrations . Furthermore, reduced concentrations of omentin in the synovial fluid of patients with rheumatoid arthritis have been described . Inflammation has been widely characterized as an important contributing factor to vascular endothelial dysfunction . Cytokines could induce vasoconstriction by reducing the expression of endothelial NO synthase, reducing the bioavailability of NO and inducing the synthesis of endothelin-1 [41, 42]. Reduced expression of omentin in omental fat endothelial cells in patients with visceral obesity may reflect the dysfunction of these cells caused by an obesity-associated proinflammatory state and oxidative stress.
Thus, the concentration of circulating omentin in patients with IGT could be a useful biomarker of endothelial function.
Vaspine is an inflammatory adipokine produced by visceral and subcutaneous human adipose tissue  and appears to act on the adipoinsulin axis and to be associated with insulin resistance, especially in patients with type 2 diabetes mellitus . Although vaspine has shown its ability to improve glucose tolerance and insulin sensitivity in mice, in humans, it seems to be positively associated with obesity-related diseases . Some evidence shows that vaspine levels can change according to the progression of diabetic disease; in particular, they increase at the onset of the disease and decrease later on .
Our results showing non significant differences between the three groups of vaspine serum levels and the significantly lower serum levels of omentin-1 in patients with diabetic foot compared to diabetic and healthy controls can be explained by the different pathogenetic roles of these two adipokines in the context of microvascular foot damage. A recent study  reported a significant correlation between serum vaspine levels and leptin concentrations, indicating that serum vaspine levels are a reflection of the amount of human adipose tissue and that vaspine may play a compensatory role in insulin resistance, the main pathogenesis of diseases related to obesity. Our diabetic foot patients, compared to diabetic and healthy controls, had comparable BMI values and more or less equal body weights, which may be sufficient to explain why serum vaspine concentrations did not vary among the three groups.