Atherosclerosis is a process, whereby, the artery wall begins to thicken as a result of deposition of calcium, cholesterol, low-density lipoproproteins, and inflammatory cells, such as macrophages. When these products are not sufficiently removed and deposit within the vessel wall, the process of plaque formation and development (atheroma) initiates [16,18,19].

Calcification is an important part of the atherosclerosis process and generally develops within the intimal layer of the vessel wall in patients with kidney disease [16]. Atherosclerotic lesions are present in an intermittent pattern along the length of the artery leading to vascular stenosis and occlusions, and subsequently ischemia [16].

Arteriosclerosis refers to stiffening and loss of elasticity of the arteries. The process typically involves arterial intima and media thickening with or without significant atherosclerotic disease [16,20,21] and is frequently characterized by increased vascular wall thickness and lumen vasodilation (vascular remodeling). The consequences of adverse vascular remodeling, referred to as poor compliance or distensibility, lead to decreased ability of the artery to hemodynamically adapt to the large pulsatile flow that occurs during intermittent ventricular ejection [16,22].

The clinical and functional significance of vascular calcification in CKD and ESRD patients is very significant and important. Vascular calcification can lead to a number, a histologic and structural changes to the artery which impact vessel hemodynamics as well as elasticity [16]. Guerin et al. have reported vascular remodeling, as measured by common carotid artery distensibility and aortic pulse wave velocity, to be proportional to the presence of vascular calcifications [16,17].

In a study by Ibels et al. [23], evaluating arterial histology from uremic patients, several key findings were reported: (1) uremic arteries demonstrated the presence of fibrous neointimal hyperplasia, calcification within the internal elastic lamina, medial elastic fibers, and damage and duplication of the internal elastic lamina [23], suggesting the predominance of vascular calcification within uremic vessels and providing further evidence for the loss of elasticity in CKD and ESRD and (2) the degree of neointimal thickness and vascular arterial calcium concentration correlated with the duration of uremia, suggesting acceleration of vascular arterial changes as a consequence of uremia [23]. These changes to the vasculature from uremia and complications of advanced CKD may potentially impact vascular access maturation and development of future stenosis.

The mechanisms of vascular calcification in CKD and ESRD remain an active area of research. In this section, the mechanisms of vascular calcification will be briefly described. Vascular calcification is the end result from an imbalance between cellular mediators which promote and inhibit mineralization. In addition to proper homeostasis of vascular mineralization, the current research to date has demonstrated that one major mechanism involved in regulation of vascular calcification is vascular smooth muscle cell (VSMC) damage [24]. Normally, VSMCs play an important role in inhibiting calcification, but in pathologic environments such as kidney disease their normal function is compromised and they develop osteogenic phenotypic changes that favor deposition of minerals that leads to calcification [24]. It is also hypothesized that when VSMCs become apoptotic, they release apoptotic bodies which accumulate calcium and initiate the calcification process [24-26]. Furthermore, VSMCs have been reported to bud matrix vesicles from their plasma membranes [24,26,27]. These small membrane bound particles form a microenvironment capable of concentrating calcium and phosphate and allowing for crystal nucleation [24,26,27].

Expression of osteogenic markers such as matrix γ-carboxyglutamic acid protein (MGP) [28,29], osteopontin (OPN) [30], and bone morphogenetic protein-2 (BMP-2) [31], by VSMCs has been described to be associated with vascular calcification in vivo [24]. Temporal gene expression studies from normal and calcified arteries, which include arteries from uremic patients, have reported that VSMCs express core binding factor (-1 (Cbfa1)/Runx2 and Sox9 [24,32,33] and transcription factors for osteoblastic and chondrocytic differentiation at sites of calcification from in vivo studies [24]. When studies were performed to study the temporal expression of these proteins to evaluate the relationship with onset of calcification, the VSMCs in the normal vessel wall expressed constitutive inhibitors of calcification such as MGP [24,33]. However, when examining calcified arteries these inhibitors were found to be down-regulated and expression of VSMC mineralization mediators was up-regulated [24,29].

Studies from transgenic animal models have demons-trated the importance of a number of proteins that play an important role in vascular calcification [24]. These include murine knockout models for MGP [34], BMP-2 [29,31], SMAD-6 [24,35], ecto-nucleotide pyrophosphate/phosphodi-esterase 1 (ENPP1) [36], Klotho [37], and Fetuin-A [38].

There remain many unanswered questions about the mechanisms underlying vascular calcification and the role of CKD and uremia in accelerating the vascular calcification process. Improving our understanding of the pathophysio-logical regulation of vascular calcification may have therapeutic potential in regards to treating cardiovascular disease in kidney disease and in hemodialysis vascular access.

The leading cause of mortality in patients with CKD and ESRD is still cardiovascular disease [39]. The high risk of elevated cardiovascular events is even present in early stages of CKD [40]. Recent studies from autopsies of coronary artery samples have shed light on this theory. Schwarz et al., in a comparative study of coronary atherosclerotic lesions in autopsy samples from 27 patients with CKD (21 with ESRD) and 27 age- and sex-matched controls, demonstrated that atherosclerotic plaque was more calcified in patients with CKD compared to their control group [41]. A recent study from Nakano et al., from 126 patients reported that the proportion of advanced atherosclerotic lesions is higher at a lower estimated GFR with a higher prevalence of calcified coronary lesions and thickened arterial intima [42]. However, no medial calcification was noted in this study [42]. Nakamura et al., in their study of autopsied coronary arteries from subjects with different levels of CKD (stage I/II, stage III, stage IV/V, stage VD) showed that intimal calcification was present in all groups, but most prominent and severe in the CKD Stage VD group compared to CKD Stage IV/V group [43]. However, medial calcification was seen in a minority of CKD Stage IV/V group [43]. The collective studies above demonstrate that vascular calcification progressively worsens with decline of kidney function. All studies have demonstrated the presence of intimal calcification, with calcification less prominent within the media.

There are several studies that have documented the presence of vascular calcification in upper extremity vessels used to create new vascular access. Wang et al., collected 30 radial artery specimens from ESRD patients at the time of new arteriovenous fistula creation and found that 37% of arteries had calcification (20% mild/moderate and 17% severe calcification) [44] with calcium deposition primarily located in the medial layer of the artery. In a study of 65 patients, Kim et al., reported that micro-calcification from radial and brachial arteries, using von Kossa staining, was present in 55.4% of patients [45]. Calcification was primarily observed in the medial layer (unlike coronary arteries) and not seen in the intimal layer [45]. Venous calcification and vascular access outcomes were not reported in this study. Lee et al., using Von Kossa staining evaluated 67 veins collected at the time of new vascular access creation [46]. Twenty-two of 67 (33%) samples showed the presence of venous calcification [46]. Among the subset of patients with documented venous calcification, 4/22 (18%), 19/22 (86%), 22/22 (100%), and 7/22(32%) had calcification present within the endothelium, intima, media, and adventitia, respectively [46]. This study did not report the association of pre-existing venous calcification with vascular access outcomes. Finally, Allon et al., in 50 CKD patients undergoing AVF placement, reported arterial micro-calcification in 2/3 of arterial samples [8]. Among samples with the presence of arterial micro-calcification, there was a trend towards an association with AVF non-maturation [8].

There are also several studies that have evaluated vascular calcification after vascular access creation. Schlieper et al., found in a study of 212 patients that 49 patients (23%) had presence of vascular access calcification (in AVF and AVG) using plain X-ray [47]. Furthermore, they reported that the presence of vascular access calcification was significantly associated with patient mortality [47]. Finally, Touissant et al., in 28 hemodialysis patients, using computed tomography imaging fistulograms, showed that vascular calcification was predominately within the aorta (75%) and subclavian arteries (89.3%), but minimal vascular calcification was present at the level of the AVF [48].

The studies above, collectively, demonstrate the presence of both calcification in both veins and arteries used to create new vascular access with possible association with mortality when detected after vascular access creation. There is an urgent need for more investigative studies to understand the pathophysiological mechanisms of calcification development in arteries and veins prior to and after vascular access creation.