Assessment of microvascular alterations in patients with angina pectoris and no evidence of significant coronary artery disease

Atherosclerosis is currently the leading cause of death and disability in the
developed world, often causing myocardial and cerebral infarction, kidney
failure and peripheral arterial occlusive disease. Atherosclerosis is
considered to be a chronic inflammatory response of the arterial wall,
initiated by injury of the endothelium. The cause of this injury is unknown,
but several factors, like hyperlipidemia, hypertension, smoking and toxins have
been indicated to play a role in its pathogenesis. This injury of the
endothelium causes an increased migration of leukocytes and lipids into the
intima, hereby inducing chronic inflammation, endothelial dysfunction and
eventually formation of atherosclerosis.
Severe atherosclerotic obstruction of the coronary arteries causes ischemia of
the myocardium and subsequent angina pectoris. However, a considerable number
of patients with symptoms of chest pain during exercise have no relevant
coronary artery stenosis. Between 10 and 20% of patients with typical angina
chest pain have normal angiograms. In addition, percutaneous coronary
intervention (PCI) of a non-significant coronary artery stenosis did not
improve symptoms and outcome. In these patients the angina has been related to
coronary microvascular dysfunction.

Magnetic Resonance perfusion imaging
MR perfusion imaging is a non-invasive method to quantify myocardial perfusion.
Perfusion abnormalities can be detected in patients with coronary artery
disease after pharmacologic stress with adenosine infusion. Normal tissue will
respond to the vasodilator stimulus with further increase in myocardial
perfusion, whereas a severe stenosis will have no perfusion reserve in rest and
will therefore not be able to increase perfusion during stress. Several studies
have demonstrated that MR perfusion imaging during intravenous administration
of adenosine can detect subendocardial hypoperfusion in patients with typical
angina but no significant coronary stenosis.This result implies that
microvascular disease can cause myocardial ischemia and subsequent angina
pectoris in patients without significant coronary artery disease at coronary
angiography.

Non-invasive glycocalyx measurements
The endothelial glycocalyx, a ~0.5µm thick, hydrated layer consisting of
proteoglycans, glycosaminoglycans, and associated plasma proteins, forms the
interface between the flowing blood and the endothelium. The glycocalyx plays a
pivotal role in orchestrating microvascular homeostasis. Degradation of the
glycocalyx in the microvasculature has been associated with an impaired
shear-dependent NO production, an increased endothelial adhesion of leucocytes
and platelets, and an increased water and protein permeability of the vessel
wall. We propose that assessment of glycocalyx loss as a reflection of
microvascular dysfunction, can be useful in patients with angina.
A new, non-invasive method to measure local glycocalyx dimensions in individual
microvascular blood vessels is orthogonal polarization spectral (OPS) or
sidestream dark field (SDF) imaging of the sublingual circulation. This method
has been validated. Previous data using intravital imaging in experimental
animals showed that glycocalyx loss is associated with a reduced microvascular
blood volume recruitment during vasodilator administration. It was observed
that a healthy glycocalyx provides a protective coating to the vascular
endothelial lining that is not accessible for flowing red blood cells, unless
the glycocalyx is - transiently - exposed to a vasodilator such as
nitro-glycerine (NTG). During exposure to NTG, the glycocalyx barrier function
is transiently reduced in order to facilitate capillary perfusion and exchange.
Thus, in addition to the measurement of glycocalyx dimension at rest
(baseline), it is also essential to determine its response to NTG exposure.

In the current study, we want to determine and correlate the results of both
perfusion capacity (with MR imaging) and sublingual glycocalyx dimensions in a)
patients with proven obstructive coronary artery disease and b) patients with
angina but without significant coronary lesions (suspected to have
microvascular disease). Furthermore, we want to compare the glycocalyx
measurements in these two patients populations to sublingual glycocalyx
dimensions in healthy controls. Since glycocalyx loss is found in the presence
of cardiovascular risk factors and appears to be an early marker of
microvascular dysfunction, we hypothesize that patients with angina but without
significant coronary artery lesions are characterized by a compromised
glycocalyx and are prone to coronary perfusion defects during adenosine
administration. To address this hypothesis, we want to investigate MR-derived
coronary perfusion together with sublingual glycocalyx dimensions in these
patients and compare them with patients with proven coronary artery disease as
well as healthy controls.

Clinical relevance and future applications
A substantial number of patients suffer from angina pectoris that cannot be
explained by obstruction of epicardial coronary arteries (i.e. no obstructive
coronary artery disease present on angiography). In these patients it is
important to test whether the angina can be related to coronary microvascular
dysfunction. Identification of angina caused by coronary microvascular
dysfunction is important because of the associated adverse prognosis, including
an increased risk of adverse cardiovascular events such as myocardial
infarction, congestive heart failure, and sudden cardiac death. The current
standard diagnostic tests to detect coronary artery disease, such as invasive
or non-invasive coronary angiography, depict only epicardial coronary arteries
and are not able to assess microvascular disease. As a result, a large patient
population with angina-like symptoms, positive exercise ECG but without
obstructive coronary artery disease on coronary angiogram currently remains
undiagnosed and untreated. In these patients, diagnostic tests for
microvascular dysfunction are lacking, let alone optimal treatment. MR
perfusion imaging can be used to quantify myocardial perfusion and possibly
identify myocardial ischemia due to microvascular disease. This non-invasive
imaging technique therefore has the potential to diagnose microvascular
dysfunction and evaluate new therapeutic strategies in patients without
relevant coronary artery stenosis.
In addition, MRI-derived myocardial perfusion imaging will be related to
sublingual glycocalyx dimensional estimates. Since glycocalyx loss appears to
be an early marker for microvascular dysfunction, we propose that clinical
sublingual glycocalyx imaging will suffice in the near future to diagnose
individuals that are at risk for developing angina. Furthermore, previous
research conducted in animals demonstrated that it is possible to
pharmacologically 1) protect the glycocalyx against free radicals, 2) inhibit
the proteolytic enzymes that break down the glycocalyx and 3) stimulate the
production of the glycocalyx. A recent study reported that the glycocalyx
perturbation and increased vascular permeability that is associated with
diabetes can be partially restored by sulodexide administration. These
developments can lead to new screening methods for patients with angina
pectoris but without significant coronary artery disease and potentially also
lead to the development of new therapeutic interventions for patients with
(micro)vascular disease.

Objective 1
a) To image myocardial perfusion during resting conditions and adenosine
infusion in a) patients with proven coronary artery disease, and b) patients
with angina but without significant coronary artery lesions.
b) To determine whether patients with stable angina but without significant
coronary lesions are associated with (subendocardial) perfusion deficits on MR
perfusion imaging.

Objective 2
a) To estimate the glycocalyx dimensions by sublingual OPS measurements, before
and after sublingual NTG administration in a) patients with proven coronary
artery disease, b) patients with angina but without significant coronary artery
lesions and c) healthy controls.
b) To correlate the MRI-derived coronary perfusion capacity to sublingual OPS
measurements in a) patients with proven coronary artery disease and b) patients
with angina but without significant coronary artery lesions.
c) To compare the measurements of glycocalyx dimensions at rest (baseline) and
after NTG exposure in a) patients with proven coronary artery disease, b)
patients with angina but without significant coronary artery lesions, and c)
healthy controls.

The study will be an observational case control study. The investigations can
be performed in random order.

• OPS glycocalyx measurements (approximately 30 minutes) will be performed in
all three study groups.
OPS imaging of the sublingual microcirculation
Sublingual administration of NTG (1 spray dose)
Second OPS imaging of the sublingual microcirculation

• MRI perfusion scan (approximately 60 minutes) will only be performed in the
two patient groups
Preparations
MR imaging (40 minutes)
Closure of visit

MR stress perfusion imaging is currently a standard, safe, non-invasive
diagnostic tool for patients with (suspected) coronary artery disease. Detailed
imaging of myocardial perfusion in patients with and without significant
coronary artery disease can lead to better insights into (micro)vascular
disease and optimal treatment for patients.
The OPS imaging of the sublingual microcirculation is also a safe and painless
test. However, subjects are restricted by a fasting period from midnight to
their visit.