Coronary circulation Flashcards
Where does the right coronary artery arise and where does it supply?
The right coronary artery arises from the aortic root behind the right coronary cusp of the aortic valve and typically delivers blood to right atrium and ventricle
Where does the left coronary artery arise and where does it supply?
The left coronary main-stem arises from behind the left coronary cusp of the aortic valve and divides into the left anterior descending artery and the circumflex artery. It supplies mainly blood for the left atrium and ventricle
In what percentage of humans is blood flow right dominant?
Blood flow is right dominant in 50% of humans and left dominant in 20%. It is equal in both coronary arteries in 30%.
Why is it important to know whether a patient is right, left or equally dominant?
If a patient who is left dominant has disease in the right coronary artery, this is usually not of major importance. However, if the patient is right dominant, then this is important and intervention may be required if the lesion limits blood flow.
What is the venous flow within the coronary circulation?
The venous blood returns predominantly through the coronary sinus and the anterior cardiac veins into the right atrium (Fig 1).
A small amount of blood drains directly into the heart chambers via arteriosinusoidal vessels, thebesian veins and arterioluminal vessels.
The drainage directly into the left ventricle contributes to the physiological shunt.
Why is coronary venous drainage into the left ventricle so important?
Blood in the left ventricle contributes to the physiological shunt. Therefore, when this volume increases, the shunt also increases.
What is the usual oxygen consumption of the myocardium at rest?
Under resting conditions, the oxygen consumption of the myocardium is around 8-10 ml/100 g. The coronary sinus PO2 is relatively constant (2.4-2.7 kPa) at a saturation of 25-40%. Thus the oxygen extraction at rest is about 60% and cannot increase much.
As cardiac work increases, the oxygen consumption of the myocardium increases severalfold, so the higher oxygen demand can only be met by increased coronary blood flow. There is a close linear relationship between oxygen consumption and coronary blood flow
What happens to the oxygen consumption when the cardiac work is due to increased pressure and volume?
However, when cardiac work is the product of volume and pressure, oxygen consumption does not correlate well because the oxygen demand is much higher if pressure work is the major fraction for a given cardiac output. This explains why the oxygen consumption of the right ventricle is only one seventh that of the left ventricle: cardiac output is the same but pulmonary vascular resistance (PVR) is much less than systemic vascular resistance (SVR).
What happens to cardiac efficiency in patients with hypertension?
Cardiac efficiency is considerably reduced in situations of increased ventricular pressure, such as in hypertension or aortic stenosis.
Describe the phasic flow of the coronary blood supply?
The blood flow in coronary arteries depends on the pressure difference between aortic pressure and extravascular myocardial pressure.
Whereas epicardial vessels are mainly unaffected by tissue pressure, the subendocardial blood flow fluctuates with changing myocardial pressures during the cardiac cycle. This results in a typical phasic flow pattern (Fig 1) 1.
In low pressure areas of the heart, like the right ventricle and the atria, blood flow is not significantly reduced during systole. However the flow to the left ventricle is markedly reduced and even briefly reversed due to the high intramyocardial pressure during systole. Therefore, the left ventricle is mainly supplied with blood during diastole.
What happens to the coronary blood flow during tachycardia?
In patients with tachycardia, the time of restricted blood flow increases as the diastole is shortened. This is counteracted by vasodilatation due to higher metabolic activity of the myocardium.
How is coronary vascular tone regulated?
Even though the exact interactions are not yet fully understood, various mechanisms for the regulation of the coronary vascular tone are proposed and can be divided into three systems:
Vascular endothelium
Local metabolism
Neural and neurohumeral factors
How is an increased oxygen demand supplied to the myocardium?
Because the oxygen extraction from coronary arterial blood is already close to maximum during resting conditions, an increased oxygen demand can practically only be supplied by altering the blood flow. This is mainly caused by changes in vascular tone of coronary resistance vessels.
↑ metabolic activity → ↓ coronary vascular resistance
The coronary circulation also shows autoregulation in response to changes of aortic pressure within a range of 40-160 mmHg.
↑ aortic pressure → ↑ coronary vascular resistance
An abrupt change of the perfusion pressure in cannulated coronary arteries, excluding alteration of cardiac work, is initially followed by a change of flow in the same direction, indicated by the closed circles on Fig 1 1. The flow then tends to return towards the baseline, indicated here by the open circles.
What is the main mediator of vasodilation for the coronary circulation?
With intact endothelium, vasodilatation is mainly mediated by nitric oxide (NO) (Fig 1a), which acts as an endothelium derived relaxing factor (EDRF). NO diffuses into vascular smooth muscle cells and acts only locally because of its very short half-life.
What stimulates the liberation of NO?endothelin, which acts via the endothelin receptor type A (ETA receptor) on smooth muscle cells and is released by thrombin, noradrenaline, adrenaline, vasopressin, hypoxia and shear stress.
Endothelin
Acetylcholine
Adenosine
Bradykinin
Shear stress
Hypoxia
