Assoc. Prof. MUDr. Markéta Bébarová, Ph.D. Department of Physiology, Faculty of Medicine, Masaryk University Coronary Circulation •This presentation includes only the most important terms and facts. Its content by itself is not a sufficient source of information required to pass the Physiology exam. Coronary Circulation •a. cor. sinistra 85% of the blood flow (the frontal part of septum, the conductive system, majority of the left ventricle) •a. cor. dextra (the right ventricle, the posterior part of septum and usually also the posterior part of the left ventricle) Ganong´s Review od Medical Physiology, 23rd edition •placing of coronary arteries and capillaries in the cardiac walls •O2 diffusion directly from the blood situated in the cardiac cavities Large branches of the coronary arteries are placed subepicardially, submerged in the myocardium, reaching the subendocardial layers, where they end up in a very dense capillary network with a minimum number of collaterals. Capillaries are practically all open even at rest. A millimeter layer just below the endocardium is oxygenated by blood directly from the heart cavities. Coronary Circulation Guyton and Hall. Textbook of Medical Physiology, 11th edition •epicardial coronary arteries supply most of the muscle •intramuscular arteries (smaller) penetrate the muscle •plexus of subendocardial arteries •During systole, blood flow through the plexus of subendocardial arteries is reduced (compression of intramuscular arteries) – compensated through extra vessels in the plexus (sensitivity to coronary ischemia). Coronary Circulation •a. cor. sinistra 85% of the blood flow (the frontal part of septum, the conductive system, majority of the left ventricle) •a. cor. dextra (the right ventricle, the posterior part of septum and usually also the posterior part of the left ventricle) Ganong´s Review od Medical Physiology, 23rd edition •coronary angiography •placing of coronary arteries and capillaries in the cardiac walls •O2 diffusion directly from the blood situated in the cardiac cavities The coronary circulation can be mapped by the coronary angiography … Coronary Circulation http://pochp.mp.pl/aktualnosci/show.html?id=55102 The coronary circulation can be mapped by the coronary angiography - a special catheter is introduced into the opening of the coronary artery by means of an arterial approach and the circulation is visualized by injection of a contrast agent; stenosis, extent and localization, can be detected. It is performed before surgery (bypass) or immediately before the coronaroplasty with a balloon catheter with possible stent insertion) or to exclude ischemic etiology of dilated cardiomyopathy. Coronary Circulation Ganong´s Review od Medical Physiology, 23rd edition •during the systole, vessels situated intramurally are pressed by the contracting myocardium •left vs. right ventricle •high heart rate The intramural pressure in the left ventricular wall is so pronounced that the flow almost ceases during the systole, and even reverses at the beginning of the systole. Therefore, the ratio of systole to diastole length is very significant. Thus, at high pulse rates, not only the ventricular filling is critically reduced, but also the left coronary artery flow time reduces. In the right ventricle, the flow changes during the heart cycle are also apparent, but they are much smaller due to the significantly lower contraction (lower afterload). Coronary Circulation •orificia of the coronary arteries ejection isovolumic relaxation Considering the location of openings of the coronary arteries, the blood flow passes by the openings during the ejection phase. After the blood pressure in the aorta exceeds the pressure in the left ventricle, the blood flow changes its direction for a while, the valve closes and blood flows directly to the openings. This flow steeply increases during the diastole, namely during its first phase. Coronary Circulation •O2 extraction is almost maximal already at rest, capillaries are open •The only possibility how to increase O2 supply (for example during exercise) is the coronary vasodilation! During physical activity, the cardiac output increases up to 5 times, which significantly increases myocardial oxygen consumption. However, O2 extraction is almost maximal at rest and the capillaries are open, so increased flow must be done by coronary vasodilation! Coronary Circulation •Control of coronary blood flow 1)reduction/interruption of the blood flow or increased demands (exercise, increased blood pressure) hyperaemia (reactive or active) based on the metabolic vasodilation mediators: ¯pO2, pCO2, ¯pH, [K+]e, adenosine, bradykinin, prostaglandins, NO Coronary Circulation •Control of coronary blood flow 2)the neural regulation of the vessel diameter – secondary impact a)indirect effects b)direct effects (mostly opposite) X Indirect influence plays a much more important role and causes mostly opposite changes than direct effects. Coronary Circulation •Control of coronary blood flow 2)the neural regulation of the vessel diameter – secondary impact a)indirect effects sympathetic system (NE, E) parasympathetic system (ACH) ↑ HR + contractility ® rate of cardiac metabolism ® increased O2 consumption ® activation of local vasodilating mechanisms opposite changes ® vasoconstriction Coronary Circulation •Control of coronary blood flow 2)the neural regulation of the vessel diameter – secondary impact a)indirect effects b)direct effects sympathetic system (NE, E) parasympathetic system (ACH) epicardial vessels – mostly α-rec. ® vasoconstriction vasodilation, but not significant (only few fibers) intramural vessels – mostly β-rec. ® vasodilation vasospastic myocardial ischemia Sympathetic vasoconstriction is small. Sympathetic-induced vasodilatation works in harmony with the metabolic autoregulation. Coronary Circulation •Control of coronary blood flow 2)the neural regulation of the vessel diameter – secondary impact a)indirect effects b)direct effects Whenever the direct effects alter the coronary blood flow in the wrong direction, the metabolic control overrides them within seconds! To sum up, the regulation of the coronary artery flow is mainly metabolic, less nervous through the beta receptors. Coronary Circulation •the resting blood flow: 225 ml/min (4-5% of CO) •at physical exertion: -cardiac output increased 4-7fold -higher afterload cardiac work may increase 6-9fold -coronary blood flow increases only 3-4fold! -efficiency of the cardiac utilization of energy has to increase to make up for the relative deficiency of coronary blood supply Cardiac Muscle Metabolism •at rest: 70% of energy – fatty acids •anaerobic/ischemic conditions: anaerobic glycolysis high glucose consumption + high quantities of formed lactic acid (one of causes of the ischemic pain + ¯pH) lost adenosine replaced by new synthesis of adenine, but very slowly (2% per hour) •severe ischemia: degradation of ATP to ADP, AMP and, finally, to adenosine ® loss of adenosine into circulation through sarcolemma ® vasodilation Major cause of death of cardiomyocytes during ischemia is the adenosine deprival! (30 min of severe ischemia may cause irreversible changes and cell death) Carbohydrates play a smaller role in the resting heart metabolism than in other tissues. Coronary Reserve •ability of coronary vessels to adapt blood flow to the actual cardiac work (ergometry) •the maximal blood flow / the resting blood flow •reduction of the coronary reserve: -relative coronary insufficiency (too high resting demands, high resting blood flow cannot be sufficiently increased) -absolute coronary insufficiency (~ coronary heart disease) (the stenotic arteriosclerotic process) Reduced coronary reserve is a limiting factor of the cardiac output, thus, also of the effort of organism! Examination of the coronary reserve – measurement of the coronary blood flow before and after the exercise ergometry. As in other reserves (respiratory, cardiac), no unit, it expresses how many times the parameter can be increased at maximum load! Behind the stenosis, vasodilation is present already at rest (to ensure sufficient blood flow). During exercise, the coronary reserve is inadequate (a part is already depleted at rest) and myocardial ischemia is manifested.