Based on the assumption that a classification system is a very critical subject and
may
significantly improve the prediction of individual responses to treatment and related
diseases, we proposed 16 years ago a classification for endothelial dysfunction
including etiological, functional, and evolutionary aspects (Figure 1).
1
Figure 1
Proposal of an endothelial dysfunction classification. Modified from Evora et
al.
1
Since our first publication, we wrote that a proposition for an endothelial dysfunction
classification might deserve criticism because it could still be seen as unsuitable
and
pretentious. The first question is of a philosophical nature because the present
concepts on endothelial function and dysfunction might eventually change dynamically
over time. The classification could also be interpreted as a premature reductionism,
sounding like an "end of the question" proposal. The lack of clinical applications
could
be the cause of the low interest in an endothelial dysfunction classification. This
editorial aims to explore the differences among the three classification axes and
the
practical and clinical implications of each proposed category. Aspects relevant to
the
etiology of the dysfunctions, in addition to treatment directions, are also
considered.
The dysfunction in the endothelial cell precedes the organic cellular dysfunction
in most
cardiovascular diseases and characterizes the primary endothelial dysfunction
(etiological classification).
2
The
endothelial dysfunction may be primary (or genetically inherited). This implies a
need
for the development of diagnostic methods applied to early detection and primary
prevention of endothelial dysfunction as a useful measure to halt the development
of
cardiovascular diseases. Treatment in these cases is aimed at preventing cardiovascular
risk factors through lifestyle modifications, such as diet and weight control, physical
exercise, and smoking cessation.
3
From
this point of view, endothelial dysfunction should be considered a public health
problem. A secondary (or phenotypic) endothelial dysfunction may occur when endothelial
cells lose their ability to produce nitric oxide (NO) and increase the expression
of
vasoconstrictor, proinflammatory, and prothrombotic factors, configuring a
proatherosclerotic scenario. Such phenotypic alterations contribute to the formation,
progression, and rupture of atherosclerotic lesions, and are commonly found in
hypertension, coronary artery disease, and diabetes.
4
In this type of endothelial dysfunction, pharmacological
treatment shows consistent results in terms of restoring the endothelial function.
For
example, antihypertensive medications to control blood pressure, statin treatment
to
reduce LDL cholesterol levels, and antidiabetics to reduce blood glucose
levels.
5
Studies in the 1990s definitively established the role of the endothelium in all
cardiovascular diseases. Such diseases are associated with endothelial dysfunction
due
to impaired release of endothelium-derived relaxing factors and, consequently, a risk
of
spasm and thrombosis (atherosclerotic or nonatherosclerotic obstructive coronary
disease, hypertension, diabetes, dyslipidemia, atherosclerosis, Raynaud's phenomenon,
and heart failure, among others).
6,7
Therapeutic interventions have been
developed for this type of endothelial dysfunction (vasotonic), which is characterized
by functional impairment, aiming to improve the endothelial function and prevent its
dysfunction in asymptomatic individuals and in patients with coronary artery disease.
Beta-blockers, statins, angiotensin-receptor antagonists, angiotensin-converting enzyme
inhibitors, antioxidants, and insulin sensitizers show benefits in these cases. Other
substances, such as L-arginine, tetrahydrobiopterin, and folic acid, are also under
investigation for their contribution to improving the endothelial function.
8-10
The vasoplegic endothelial dysfunction classification includes the characteristic
situations of severe vasoplegias, many of which are time resistant to the action of
vasoconstrictive amines. This type of dysfunction is characterized by an excessive
production of vasorelaxant substances produced by the endothelium, especially NO,
and
include, for instance, vasoplegias during and after cardiopulmonary bypass, sepsis,
and
anaphylactoid and anaphylactic reactions.
11
The vasoplegic syndrome has a multifactorial genesis and, in the
case of patients undergoing cardiac surgery, occurs mainly due to exposure of the
body
to nonphysiological materials and the use of heparin/protamine,
12
triggering an inflammatory response
syndrome. During this process, there is complement activation, cytokine release,
leukocyte activation, and expression of adhesion molecules, as well as a production
of
oxygen free radicals, arachidonic acid metabolites, platelet activity factor, NO,
and
endothelin. The consequences of the inflammatory response syndrome may lead to
dysfunction of multiple organs and systems, such as the one that occurs in septic
shock.
The decrease in systemic vascular resistance observed in vasoplegic syndromes is
associated with excessive NO production and may be reversed by NO synthase (NOS)
inhibitors and methylene blue.
13
The term "vasoplegic endothelial dysfunction" was created as part of the proposed
classification and deserves some comments. Searching the MEDLINE database using quoted
terms, we found: "endothelium dysfunction" (37,640 papers), "endothelial dysfunction"
(69,115 papers), "vasoplegic endothelial dysfunction" (12 papers), "vasoplegia" (206
papers), and "vasoplegic syndrome" (243 papers). Assuming that the excessive release
of
NO is, in fact, an endothelial dysfunction, this terminology would be unified to the
search of distributive shock (sepsis, anaphylaxis), anaphylactoid reactions, and
vasoplegias related to cardiopulmonary bypass. In this manner, this issue demands
special attention from the scientific community, at least in terms of unifying the
terminology.
1
Endothelial dysfunction may be reversible or partially reversible in such cases,
according to the prognostic or evolutionary classification. Endothelial dysfunction
should be considered in hypertensive postmenopausal women presenting with abnormal
endothelium-dependent vascular function. However, a significant improvement in
endothelial function may be reached after 6 months of antihypertensive therapy. These
changes may identify patients with a more favorable prognosis.
14
Dysfunction of the coronary or peripheral vascular
endothelium is an independent predictor of cardiovascular events and provides valuable
prognostic information. In such cases, modification of risk factors and drug treatment
(statins and angiotensin-converting enzyme inhibitors) may improve the endothelial
function and prognosis.
15
Most risk
factors related to atherosclerosis and cardiovascular morbidity and mortality have
been
found to be associated with the endothelium.
14
These risk factors include hyperlipidemia, hypertension,
diabetes, and smoking, which may be reversed by pharmacological or nonpharmacological
treatment. In other words, it is possible to improve endothelial dysfunction using
medical treatment and exercise, even without completely reversing it.
16,17
Irreversible endothelial dysfunction usually occurs during the progression of
cardiovascular diseases and sepsis.
We have been using the proposed classification since 2000
1
as a didactic model, carefully emphasizing eventual
biases concerning its misinterpretation. However, the current usefulness of an
endothelial dysfunction classification still remains "an open discussion".
Semiquantitative measurements of endothelial dysfunction may potentially amend the
assessment of the proposed categories. We hoped that the classification system would
be
used to improve and uniformly diagnose patients, in addition to providing a route
for
collaborative studies on endothelial dysfunction across academic centers. However,
as
already mentioned, the lack of clinical applications could be the cause for the low
interest in an endothelial dysfunction classification. Perhaps the development of
biomarkers may strengthen the clinical reasoning of cardiovascular diseases from the
point of view of endothelial dysfunction.
17-19