Abstract

In vascular diseases, endothelial dysfunction is a systemic pathological state of the endothelium (the inner lining of blood vessels). Along with acting as a semi-permeable membrane, the endothelium is responsible for maintaining a relaxed vascular tone and low levels of oxidative stress by releasing mediators such as nitric oxide (NO), prostacyclin (PGI2) and endothelin (ET-1), and controlling local angiotensin-II activity. This allows the endothelium, specifically in the vessels of the heart, to ensure proper blood flow to and from the heart. In terms of endothelium dysfunction, nitric oxide (causes the widening of vessels; a vasodilator) is important to consider and focus on.

Endothelial dysfunction can happen as a result of many different things, including diabetes. In diabetic patients, circulating platelets can increase endothelial dysfunction by decreasing the production of nitric oxide. It can also result from increased oxidative stress (one of the causes of oxidative stress being platelets disrupting the carotid artery), hypertension, or obesity.

One of the environmental factors that can also lead to the development of endothelial dysfunction is smoking tobacco products. Endothelial dysfunction is a major pathophysiological mechanism that leads towards coronary artery disease, and other atherosclerotic diseases.

Link with atherosclerosis

Endothelial dysfunction is thought to be a key event in the development of atherosclerosis and has been reported to predate clinically obvious vascular pathology by many years. One way to look at endothelial dysfunction is to look at the disturbances in flow-mediated dilation. Depending on how much blood is flowing through an artery at a particular point, the artery must be able to dilate or constrict to allow sufficient blood flow. This is very important at the focal points of circulation where there is lots of blood flowing at one point so it is important for the arteries to dilate. This is why it is known as flow-mediated dilation. Atherosclerosis is characterized by the decreased blood flow to the heart muscles, causing obvious complications to someone's health. If an artery is unable to dilate to the necessary amount to allow sufficient blood flow to the heart because of some issue to this flow-mediated dilation, it can be very problematic. For people who suffer with endothelial dysfunction, their arteries are unable to dilate correctly for many different reasons. One of these reasons can be because of inappropriate effects on the vascular smooth muscle caused by the release of nitric oxide, decreased release of nitric oxide, or problems with the nitric oxide synthase. Often there is an increased concentration of low-density lipoproteins, which act as potent oxidizing agents. This causes an increase in the levels of free oxygen radicals, especially superoxide anion, which react with NO and decrease its availability. Interestingly however, there has been evidence that this can be reversed by the administration of L-arginine. Issues with appropriate blood flow can also happen because of the inappropriate release of vasoconstricting factors or altered signal transduction. For people who have endothelial dysfunction and do not see improvement with different interventions, they are actually at a 7-fold increased risk of developing cardiovascular complications in the future. However, the problem with this assertion in terms of the flow-mediated response indicator of endothelial dysfunction is that a morphological characteristic of atherosclerosis (baseline artery size) is inherent in the calculation of percentage flow-mediated dilation.

Endothelial dysfunction starts with decreased levels of NO, which compromises the vascular bed (either all the vascular beds in the body or in particular areas). This causes endothelial cells to adopt a proinflammatory and proadhesive phenotype, with increased activation of pathways that lead to increased adhesion and increased concentration of adhesion molecules. This helps to create more plaques in the arteries, leading to the pathogenesis of atherosclerosis .

Endothelial dysfunction has been shown to be of prognostic significance in predicting independently vascular events including stroke and myocardial infarction. However, again so has baseline artery size which happens to be part of the calculation of percentage flow-mediated dilation (%FMD). Endothelial function testing might have potential prognostic value for the early detection of cardiovascular disease; clinical trials in the recent years have demonstrated the feasibility of translating this measurement to the clinical practice. However, the baseline artery size component of percentage flow-mediated dilation may also be just as prognostic and is easier to measure reliably than flow-mediated dilation.

Nitric Oxide bioavailability reduction in Endothelial Dysfunction

Nitric oxide (NO) is very important for endothelial functioning because aside from being a vasodilator, it also suppresses platelet aggregation, inflammation, oxidative stress, vascular smooth muscle cell migration and proliferation, and leukocyte adhesion [9]. Atherosclerosis and other cardiovascular diseases are characterized by things like increased platelet aggregation and leukocyte adhesion (two factors that help in plaque formation. Therefore, it is evident why nitric oxide reduction is considered the hallmark of endothelial dysfunction

A key and quantifiable feature of endothelial dysfunction is the inability of arteries and arterioles to dilate fully in response to an appropriate stimulus that stimulates release of vasodilators from the endothelium like NO. Endothelial dysfunction is commonly associated with decreased NO bioavailability, which is due to impaired NO production by the endothelium and/or increased inactivation of NO by reactive oxygen species.

This can be tested by a variety of methods including iontophoresis of acetylcholine, direct administration of various vasoactive agents to segments of blood vessels, localized heating of the skin and temporary arterial occlusion by inflating a blood pressure cuff to high pressures. Testing can also take place in the coronary arteries themselves but this is invasive and not normally conducted unless there is a clinical reason for intra-coronary catheterization. When using this method, there is intra-arterial administration of substances like acetylcholine (i.e. a vasodilatory agent). Any changes in the diameter of the arteries is measured using angiography and any potential changes in the rate of blood flow. Using this method, studies have actually shown that when there is the release of vasodilatory agents, patients with endothelial dysfunction actually have constriction rather than dilation .

Of all the current tests employed in the research setting, flow-mediated dilation is the most widely used non-invasive test for assessing endothelial function. This technique measures endothelial function by inducing reactive hyperemia via temporary arterial occlusion and measuring the resultant relative increase in blood vessel diameter via ultrasound. Measurement of endothelial function by peripheral arterial tonometry is also mediated by a NO response. As people with endothelial dysfunction have low NO bioavailability, their blood vessels have a decreased capacity to dilate in response to certain stimuli, compared to those with normal endothelial function. In order to properly perform a test for endothelial dysfunction, patients must avoid having certain medications and food at least 12 hours prior to the test; temperature must be controlled (at room temperature) [2], and ideally should be performed at the same time in the same patient due to circadian rhythms.

Another non-invasive method to measure nitric oxide levels is known as reactive hyperemia-peripheral arterial tonometry (RH-PAT). This method involves adding peripheral arterial tonometry probes to each finger and measuring the pulse wave amplitude. One the reasons why this method is becoming more popular than using flow mediated dilation is because the contralateral arm can be used as an internal control. Having data from the control allows for scientists to correct any changes that may have occurred during the administration of the test .

NO has the following physiological effects that contribute to the inhibition of atherosclerosis: 1) NO is released and produces vasodilation after shear stress in the vessel; the vasodilation NO mediated-response in turn decreases the shear stress. If the shear stress is chronically induced it leads to the upregulation of and release of inflammatory cytokines 2) NO decreases LDL oxidation; 3) NO reduces platelet aggregation to the endothelium 4) NO inhibits smooth muscle cell proliferation 5) NO prevents leukocyte adhesion and infiltration into the vessel.

One of the possible mechanisms that lead to the decrease in nitric oxide levels is the activation of the protein phosphatase 2 (PP2A). In many patients, especially those who have endothelial dysfunction associated with obesity, there is lots of ceramide in the body. Ceramide activates the PP2A, causing it to co-localize with endothelial nitric oxide synthase (eNOS). By phosphorylating and inactivating eNOS, there is a decrease in the concentrations of NO in the body, inducing endothelial dysfunction.

Testing & Diagnosis

The gold standard for measuring endothelial function is angiography with acetylcholine injection. Previously, this was not done outside of research because of the invasive and complex nature of the procedure. As mentioned above, the use of acetylcholine injections to test vasodilation is now safely used for procedures where arterial catheterization is employed (this method is less frequently used though, so overall acetylcholine is not used very often in this way).

A noninvasive method to measure endothelial dysfunction is % Flow Mediated Dilation (FMD) as measured by Brachial Artery Ultrasound Imaging (BAUI). Current measurements of endothelial function via FMD vary due to technical and physiological factors. For example, FMD is largely affected by hormones, especially for women. FMD values can differ for the same woman if she is in different phases of her menstrual cycle during the time of measurement. When using this technique on people who suffer from things like heart failure, renal failure, or hypertension, their increased sympathetic tone can often falsify the results. Furthermore, a negative correlation between percent flow mediated dilation and baseline artery size is recognised as a fundamental scaling problem, leading to biased estimates of endothelial function. For research on FMD an ANCOVA approach to adjusting FMD for variation in baseline diameter is more appropriate. Another challenge of FMD is variability across centers and the requirement of highly qualified technicians to perform the procedure.

A non-invasive, FDA-approved device for measuring endothelial function that works by measuring Reactive Hyperemia Index (RHI) is Itamar Medical's EndoPAT™. It has shown an 80% sensitivity and 86% specificity to diagnose coronary artery disease when compared against the gold standard, acetylcholine angiogram. This results suggests that this peripheral test reflects the physiology of the coronary endothelium. Endopat has been tested in several clinical trials at multiple centers (including major cohort studies such as the Framingham Heart Study, the Heart SCORE study, and the Gutenberg Health Study). The results from clinical trials have shown that EndoPAT™ is useful for risk evaluation, stratification and prognosis of getting major cardiovascular events (MACE).

Since NO maintains low tone and high compliance of the small arteries at rest a reduction of age-dependent small artery compliance is a marker for endothelial dysfunction that is associated with both functional and structural changes in the microcirculation that are predictive of subsequent morbid events Small artery compliance or stiffness can be assessed simply and at rest and can be distinguished from large artery stiffness by use of pulsewave analysis with the CV Profilor.

Endothelial Dysfunction and Stents

Stent implantation has been correlated with impaired endothelial function in several studies. According to Mischie et al., sirolimus eluting stent implantation induces a higher rate of endothelial dysfunction compared to bare metal stents. This is problematic because stents have been used to treat many diseases related to endothelial dysfunction, including coronary artery disease. Sirolimus eluting stents were previously used because they showed very low rates of in-stent restenosis but further investigation showed that they often impair endothelial dysfunction in humans and worsen conditions. Therefore, now the commonly used drug is iopromide-paclitaxel because it showed low rates of in-stent restenosis and thrombosis and it does not worsen the person's health condition.

Prevention and treatment

Endothelial function can be improved significantly by exercise, smoke cessation, weight loss in overweight or obese persons, and improved diet. Treatment of hypertension and hypercholesterolemia are also critical; the major pharmacological interventions to improve endothelial function in those set of patients are statins(HMGCoA-reductase inhibitor), and renin angiotensin system inhibitors, (such as ACE inhibitors and angiotensin II receptor antagonists).

Some studies have found the consumption of flavonoid-rich fruit and vegetables, potassium

and arginine supplementation to restore impaired endothelial function. A positive relationship exists between the consumption of trans fat (commonly found in hydrogenated products such as margarine) and the development of endothelial dysfunction.

New third-generation β-blockers and 5-phosphodiesterase inhibitors may affect endothelial function. New non-invasive strategies that measure endothelial function will prove critical to assess which set of patients are improving their endothelial function. Statins have major pleiotropic anti-inflammatory and anti-hypertensive effects besides the cholesterol reduction effect. This immunomodulatory effects of statins may explain why some patients improve their endothelial function with those drugs. Another anti-inflammatory drug that has shown to be effective for treating endothelial dysfunction in patients who also have coronary artery disease is the ipeptidyl peptidase 4 (DPP4) inhibitor.

Epidemiology

The epidemiology of endothelial dysfunction is unknown, as %FMD varies with baseline artery diameter. This can make cross-sectional comparisons of %FMD difficult. Endothelial dysfunction was found in approximately half of women with chest pain, in the absence of overt blockages in large coronary arteries. This endothelial dysfunction cannot be predicted by typical risk factors for atherosclerosis (e.g., obesity, cholesterol, smoking) and hormones.