Assessment of the pathophysiology of the endothelium and its ability to act as a potential therapeutic target is a growing area of research. Over the past two decades, its role in the initiation, progression, and clinical sequelae of atherosclerosis has been both increasingly studied and recognised (1).
Moreover, endothelial dysfunction has been shown in varied populations to be a predictor of cardiovascular events.
Concomitantly, disturbances in vascular biology provide a pathophysiological basis for identifying individuals at increased risk of future acute cardiovascular events. Together, these findings might lead to widespread prevention of atherosclerosis - in its preclinical stage, which would have a major global health impact.
Nevertheless, endothelial function has, for the time being, been demonstrated to be a marker for risk of cardiovascular events in high risk groups only. Additionally, there is considerable debate regarding what is the most appropriate method for assessing endothelial function. Here we explore methods available. Whether in coronary or peripheral arteries, response to pharmacological agents and/or to hemodynamic provocation tests is assessed as a measure of endothelial function (2). There are invasive and non-invasive techniques exploring various aspects of pathobiology (3).
1) Invasive testing
The invasive technique involves delivery of vasoactive agents via intra-arterial infusion. Response is measured either with high resolution ultrasound or strain gauge pletismography. Intra-arterial examination can be combined with intravascular examination through intravascular infusion of vasoactive stimulus. Response is then measured through intravascular ultrasound.
Acetyl-choline is the vasodilatory agent that is usually used. It activates endothelial cells and stimulates nitric oxide release. Physiologically, in patients with a normal endothelial response, acetyl-choline will cause released access to the vasodilators, and result in dilation of blood vessels and hyperaemia. In patients with endothelial dysfunction, this process is disturbed and results in decreased vasodilation, thus demonstrating the dysfunction. However, in certain patients, a paradoxical vasoconstriction event ensues (4) such that this technique actually carries a risk of coronary ischemia. Moreoever, this technique is invasive and costly, therefore, wide spread use and clinical utility is limited.
2) Non-invasive testing
Atherosclerosis and endothelial dysfunction are diffuse disease processes affecting coronary and peripheral arteries, hence, the physiological basis in assessing endothelium-dependent vasomotion in peripheral vessels. Peripheral vessels are accessible for non-invasive testing and may be considered a “window to the circulatory system”. Numerous methods for evaluating the peripheral arterial system have emerged as alternatives to coronary endothelial function testing.
A - Flow mediated dilatory capability of brachial artery (FMD)
Exploration of the brachial artery's flow mediated dilatory capability is the most commonly used technique. High-resolution vascular ultra-sound investigation of the brachial artery before and during reactive hyperaemia (1) shows vasodilatation from the release of nitric oxide as a response to stress induced during increased blood flow. A lack of vasodilation would suggest decreased release of endogenous vasodilators and therefore, endothelial dysfunction, thus indicating future cardiovascular risk: increased risk for future cardiovascular events and a potential need for revascularisation (5).
Impaired peripheral endothelial function has also independently predicted adverse outcomes in specific patient cohorts, such as heart failure cohorts (6). Further studies indicated that FMD can reveal the effects of treatment of cardiovascular diseases, and an association between improvement of brachial artery FMD with reduction of cardiovascular event was shown (7).
Flow mediated dilatory capability of brachial artery is widely used, however there are technical and interpretational limitations to this method. Further studies are needed to determine whether this methodology is sufficiently reproducible and whether biological variability is low enough to make of assessment of FMD a clinically useful measure of cardiovascular risk.
B - Peripheral arterial tonometry (PAT)
Peripheral arterial tonometry (PAT) evaluates pulse wave amplitude and is linked to endothelial dysfunction: abnormalities in pulse wave amplitude have long been described in peripheral circulation in patients with atherosclerosis (8). Furthermore, peripheral arterial tonometry hyperaemia has been shown to be an adequate surrogate marker to assess changes in vascular function over time and in the Framingham cohort, it was closely linked with cardiovascular risk factor (9). Lastly, studies showed that results correlate with brachial artery as well as coronary artery endothelial function testing (10).
How FMD and PAT differ
Flow mediated dilatory capability of brachial artery and PAT have been shown to be reduced 1) in a dose-dependent fashion in the presence of cardiovascular risk factors, as well as 2) in the presence of atherosclerosis.
However, studies have also shown that FMD is not correlated with PAT findings (11). This is not suprising since FMD of brachial artery and PAT are very different methods for identification of vascular reactivity of different arterial territories: FMD of brachial artery directly registers the dilation capability of the large-conduit artery whereas PAT measures flow response on reactive hyperaemia which is itself related to endothelial function of small (digital) arteries and to the endothelial function of microcirculation.
C- Venous occlusion pletismography
Venous occlusion pletismography measures volume changes induced 1) by mercury strain gauge during hyperaemia in the forearm provoked by vasoactive agents, - including drugs to release nitric oxide or inhibitors of endothelial nitric oxide synthesis or 2) by reactive hyperaemia following forearm occlusion (12).
Measuring the vasoactive response to brachial artery infusion of acetylcholine in patients with known coronary artery disease has been shown to be a prognostic marker for future cardiovascular events (12). Fore-arm endothelial dysfunction as confirmed by occlusion pletismography is also a marker for long-term cardiovascular complications in patients with hypertension (8).
How FMD and Venous occlusion pletismography differ
In contrast to FMD, which measures changes in the brachial artery, occlusion pletismography registers changes in blood flow. These changes are pre-regulated by peripheral resistance from endothelial function of resistance arteries and microcirculation.
D - Circulating markers of endothelial dysfunction
In addition to vascular imaging techniques and measures of blood flow, other testing may provide information regarding endothelial health.
- Cellular adhesion molecules play an important role in vascular function and are expressed on the surface of damaged endothelial cells. They are released into the circulation, and increased levels have been linked to cardiovascular risk and atherosclerosis (13).
- Studies also suggested that cell adhesion molecules (vascular cell adhesion molecule-1 – VCAM-1, endothelial leukocyte adhesion molecule-1 E-selectin, intercellular adhesion molecule-1-ICAM-1) may serve as possible markers of the efficacy of lipid lowering therapy, and to follow the effect of other preventive measures.
- The von Willebrand factor, which has an important role in haemostasis, is elevated in patients with risk factors of atherosclerosis, particularly in hypercholesterolemia and in various atherosclerotic diseases. Studies have shown that elevated levels of the von Willebrand factor indicate endothelial damage and may be a predictor of recurrent cardiovascular events in patients with known cardiovascular disease (14).
- Endothelin-1, an endothelium derived peptide that has powerful vasoconstrictor properties, was shown to be increased in conditions associated with endothelial dysfunction, such as hypercholesterolemia, smoking and atherosclerosis. It has been suggested that the level of circulating endotelin in plasma may be the earliest measurable functional deterioration of the vessel wall in atherogenesis (15).
- Endothelial progenitor cells derived from bone marrow have a role in ongoing endothelial repair and may represent surrogate biologic markers for vascular function and cardiovascular risk.
3) Clinical implications
Knowledge regarding the health of endothelium in a given patient would be useful to identify subjects at risk for cardiovascular events according to the estimated risk they have for atherosclerosis and even irrespective of these risks. It would then be possible to know in which cases to administer intensive medical therapy and elimination of risk factors very early on.
Non-invasive assessment of endothelial function might be a welcome addition to the physician’s diagnostic armamentarium, allowing for detection of subjects who have a higher than expected cardiovascular risk, and should be considered for intensification of therapy (16). Nevertheless which is the ideal technique, -i.e most reliable, producible, affordable, and easy to perform- has yet to be confirmed.