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The coronary microcirculation comprises several anatomically and functionally different coronary vascular compartments with a small diameter (<500 µm) that play a crucial role in the regulation of myocardial perfusion.
On this page:
- location: epicardium, myocardium, endocardium
- blood supply: epicardial coronary arteries
- innervation: sympathetic and parasympathetic nervous systems
The coronary microcirculation consists of the following vascular components 1-5:
- small coronary arteries or pre-arterioles (<300-500 µm*)
- coronary arterioles (<150-200 µm*)
- capillaries (<10 µm*)
- coronary venules (10-50 µm*)
- small coronary veins (50-300 µm*)
*Vessel diameter reflect approximate values and vary between publications in the literature 1-5. The coronary artery-arteriolar transition is gradual usually in the range of 100-400 µm and without abrupt transitions 3.
The small coronary arteries or pre-arterioles form the epicardial component of the coronary microcirculation. The arterioles constitute the functional intramyocardial component of the microcirculation.
The main function of the coronary microcirculation include the following 2:
- regulation of myocardial blood flow
- modulation of peripheral vascular resistance
Myocardial blood flow is modulated by the arterial diameters and tone of pre-arterioles and arterioles. The tone is high under resting conditions 2. A decrease in tone and increase in arteriolar diameter allows for a rise in myocardial blood flow and thus can deal with increased metabolic requirements of the myocardium 2.
The coronary pre-arterioles respond to wall-shear stress and maintain the perfusion pressure of arterioles and account for up to 25% of the coronary vascular resistance.
The coronary arterioles are responsible for matching blood supply and oxygen consumption of the myocardium and for up to 50% of the coronary vascular resistance. The tone of the smallest arterioles hereby responds to myocardial metabolic activity leading to vasodilation and intraluminal pressure reduction in the setting of increased metabolism thus increasing flow upstream in the larger arterioles through endothelium-dependent vasodilatation mechanisms 2.
There is a high heterogeneity of local perfusion and coronary microcirculation within different regions of the heart also within transmural layers 2.
Increased metabolic requirements of the myocardium can be solved with a diameter increase of the arterioles and thus to increase myocardial blood flow 2.
The coronary microcirculation receives arterial blood from the epicardial coronary arteries and their branches.
Venous drainage is via the cardiac veins and their tributaries including the venae cordis minimae 3.
Coronary flow is mainly modulated by non-neural mechanisms. However, α- and β-adrenergic receptors have an effect on vascular endothelial and smooth muscle cells, which results in increased stimulation of β2-adrenergic receptors with increased sympathetic activity 1,2.
Up to date, no real-world practical imaging technique allows direct visualization of the coronary microcirculation and thus evaluation is based on functional assessment 2.
Coronary vasodilation and microvascular resistance can be assessed with coronary flow reserve (CFR) and the index of microvascular resistance (IMR). An impaired coronary flow reserve (CFR <2.0-2.5) and increased index of microvascular resistance (IMR ≥25 units) indicates coronary microvascular dysfunction 1,2.
Cardiac MRI can be used to assess coronary flow reserve 1-5. Stress-induced perfusion defects in the absence of significant coronary stenosis might indicate coronary microvascular dysfunction.
PET-CT and PET-MR can quantify myocardial blood flow [mL/g/min] assessed after intravenous injection of different flow tracers as N-13-ammonia, rubidium-82 or O-15-water 1,2.
Impairment of the coronary microcirculation is called coronary microvascular dysfunction (CMD) that can lead to myocardial ischemia and/or myocardial infarction, which in the setting of non-obstructive coronary artery disease is referred to as 1,2:
- ischemia with non-obstructive coronary arteries (INOCA)
- myocardial infarction with non-obstructive coronary arteries (MINOCA)
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