Created by Yee, Kwang Chien, medical student at the University of Tasmania
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Coronary artery disease kills millions of people each year. About 1/3 of people with coronary artery disease has the first presentation as death, unfortunately. Althought the management of coronary artery disease has improved over the past decades, the pathogenesis of the disease and the basic coronary arterial flow in the blood vessel are not well understood. This page is aimed at providing some information on those areas and proposed some questions, which remain unsolved by a lot of researches into the field.
As with any other organs in our body, the heart requires blood continuously flowing through its constituent tissue in order to survive. The heart pumps out about 5L of blood every minute. About 5-10% of the output (known as the cardiac output) goes through the coronary vessels. The blood flows through coronary arteries into arterioles and then into capillary beds. It is then channeled through coronary venules and then into coronary veins back into the right atrium. There are, however, certain aspects at which coronary circulation defer from the rest of the circulations in the body. Firstly, the coronary arterial flow occurs during diastole (i.e. during the relaxation phase of the heart cycle) while the perfusion of other organs occurs during systole (i.e. during the contraction phase of the cardiac cycle). The other difference is the oxygen extract ratio of the blood flow. In normal organs, the oxygen saturation of the arterial side is about 96% and on the venous side, the saturation is about 70%. The capillary bed extracts about 25% of the oxygen from the blood that flows through it. Under normal circumstances, however, 70% of oxygen is extracted from the blood flow through coronary capillary bed. The other thing to notice about coronary blood flow is the flow through the coronary vessels are not homogenous. Depending on the heart rate, the flow through subendothelial (i.e vessels that are nearest to the ventricular wall) and subepicardial vessels (vessels that are nearest to the pericardial surface) are not the same.
The right and the left coronary arterial flows are not the same. The left coronary flow will be discussed first. The left coronary arterial and venous flows are out of phase. That means during diastole, the arterial flow is high while the venous flow is low; during systole, however, the arterial flow is low while the venous flow is high. The mechanism for this is still not clear. Certain evidence point towards the contraction of the myocardium causes compression of blood vessels inside w (i.e. increase in extravascular resistance) while other suggest the ventricular pressure plays a major role.
By putting a pressure probe into the coronary artery, we can monitor the arterial flow pattern in the coronary and a typical recording trace shows the following:
wave a -- conincides with atrial contraction
Wave b -- drop is seen when the right ventricular pressure starts to rise (coincides with the start of q wave in ECG).
Wave c -- sudden rise in aortic pressure leads to a spike.
Wave d -- mid-diastolic when coronary arterial flow reduces inspite of increase in pressure.
Wave e -- after closure of the aortic valve, reduction in left ventricular pressure leads to increase in coronary flow rapidly.
Right coronary flow follows the pattern of aortic pressure, with a lower perfusion pressure.
The coronary arterial tree starts from the aortic valve. Each three leaflets of the aortic valve has a semispherical structure called the sinus of Valsalva. One major coronary artery comes of two of the three sinuese to form the left and the right coronary artery.
The left coronary artery arises from the left sinus of Valsava. It perfuses the left ventricular wall and the septa. It courses behind the right ventricular outflow tract for a short distance before it devides into the left anterior descending branch and the left circumflex branch. The left anterior descending (LAD) branch continues its course in the anterior interventricular groove and in the majority of patient, courses around the apex. Along its course, the LAD gives off variable number of septal and diagonal branches. The areas supply by the LAD includes the anterior left ventricular wall, the anterior interventricular wall (by septal branches) and anterolateral wall (by the diagonal branches).
The left circumflex runs in the left atrioventricular groove. It gives off a few obtuse marginal branches. These branches together with the left circumflex supplies the posterior and the lateral wall of the left ventricle.
The right coronary artery arises from the anterior sinus of Valsalva. It follows the right atrioventricular groove to the crux of the heart. The branches that the right coronary gives off include conus branch, sinus node artery, the acute marginal branch , the right ventricular branch, sometimes the posterior descending branch and the posterolateral left ventricular branch.
Three mechanism must be involved in the regulation of the flow through coronary arterial system:
linkage between myocyte metabolism and coronary flow.
myogenic tone
flow dependent dilatation of the coronary vessels.
oxygen consumption and flow
One of the ways to measure the myocyte metabolism is by measuring oxygen consumption and it has been shown that increase oxygen consuption is asscociated with increase coronary artery flow. This relationship, however, is flow dependent.
The coronary artery flow control involves systemic control and local control. Systemic control of coronary artery flow includes neuronal control and hormonal control. Local control of coronary artery flow includes metabolic, myogenic and tissue pressure hypothesis.
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