IMU

Coronary Circulation and its RegulationSUFYAN AKRAMMBBS PhD1Lesson outcomesExplain haemodynamic basis of fluctuation in coronary blood flow in the left and right ventricles during a cardiac cycleDescribe neural and chemical control of coronary blood flow Explain the interaction between systemic regulatory mechanisms (sympathetic nervous system) and the local auto-regulatory mechanisms (metabolites) in the control of coronary blood flow23

IntroductionAbout one third of all deaths in industrialized countries of the Western world result from coronary artery disease, and almost all elderly people have at least some impairment of the coronary artery circulationFor this reason, understanding normal and pathological physiology of the coronary circulation is one of the most important subjects in medicine4

Special features of coronary circulationO2 extraction from the blood by the myocardium is near maximum even at rest. There are high levels of oxygen extraction (6575%), compared with the average for the whole body (25%), and during exercise, extraction rates rise to 90%The coronary circulation has the shortest transit time (only 68 seconds) and has a very high density of capillaries, approximately one per myocyte, which are all perfused continuouslyTherefore the only way to supply more O2 when the O2 demand of the myocardium increases is by increasing the coronary blood flow5Blood flow in major organsOrgan% body weight% cardiac output @ restNormal flow (ml/min per 100 g)Maximal flow (ml/min per 100 g)Heart0.5580400Brain21455150Skeletal muscle4018360Skin3410150Kidneys0.5204006006Regulatory Mechanisms7HaemodynamicCoronary capillary blood flow in the ventricle muscle falls to a low value during systole, which is opposite to flow in vascular beds elsewhere in the body. The reason for this is strong compression of the left ventricular muscle around the intramuscular vessels during systolic contraction

During diastole, the cardiac muscle relaxes and no longer obstructs blood flow through the left ventricular muscle capillaries, so blood flows rapidly during all of diastole8HaemodynamicBlood flow through a particular segment of coronary artery depends on the gradient between the aortic pressure and that segment of the coronary artery

The intra-ventricular pressure decreases progressively as it moves away from the point of application, compressing maximally the endocardial arteries and minimally the epicardial arteries

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10Blood flow through the coronary capillaries of the right ventricle also undergoes phasic changes during the cardiac cycle, but because the force of contraction of the right ventricular muscle is far less, the inverse phasic changes are only partial

Note the inverse relationship of coronary blood flow to the aortic pressureNeuralStimulation of the autonomic nerves to the heart can affect coronary blood flow both directly and indirectlyThe direct effects result from action of the nervous transmitter substances acetylcholine from the vagus nerves and norepinephrine and epinephrine from the sympathetic nerves on the coronary vessels themselves. Indirect effects result from secondary changes in coronary blood flow caused by increased or decreased activity of the heart11Neural12Parasympathetic stimulationSympathetic stimulationCardiac Muscle HR, Contractility HR, ContractilityCoronary VesselsDilated (weak effect)Constricted ()Dilated (2)13

Sympathetic1 and 2 Receptors in Ventricles Receptors in Coronaries HR Force of ContractionVasodilation Receptors in Coronaries

VasoconstrictionDirect Effects:Vasoconstriction of Coronary ArteriesIndirect Effects:Vasodilation of Coronary ArteriesHow?NeuralThe indirect effects, which are mostly opposite to the direct effects, play a far more important role in normal control of coronary blood flowThus, sympathetic stimulation increases both heart rate and heart contractility and increases the rate of metabolism of the heart. In turn, the increased metabolism of the heart sets off local blood flow regulatory mechanisms for dilating the coronary vessels, and the blood flow increases approximately in proportion to the metabolic needs of the heart muscle14AutoregulationThe capacity of tissues to regulate their own blood flowWell developed in the:Heart (myocardium)BrainKidneysExercising skeletal musclesTheories of autoregulation:Myogenic theoryMetabolic theory

15Local (Autoregulation)Blood flow through the coronary system is regulated mostly by local arteriolar vasodilation in response to the nutritional needs of cardiac muscleThat is, whenever the vigor of cardiac contraction is increased, the rate of coronary blood flow also increases

Blood flow in the coronary arteries usually is regulated almost exactly in proportion to the need of the cardiac musculature for oxygen16Local (Autoregulation)The exact means by which increased oxygen consumption causes coronary dilation has not been determinedIt is speculated by many research workers that a decrease in the oxygen concentration in the heart causes vasodilator substances to be released from the muscle cellsA substance with great vasodilator propensity is adenosineIn the presence of very low concentrations of oxygen in the muscle cells, a large proportion of the cells ATP degrades to adenosine monophosphate; then small portions of this are further degraded and release adenosine into the tissue fluids of the heart muscle, with resultant increase in local coronary blood flow17Local (Autoregulation)Adenosine is not the only vasodilator product that has been identifiedOther Vasodilator Metabolites (VDMs) include adenosine phosphate compounds, potassium ions, hydrogen ions, carbon dioxide, prostaglandins, and nitric oxide18Overall effectSympathetic activity directly causes coronary vasoconstriction; at the same time, it causes increased heart rate and contractility raising cardiac work and metabolismIncreased production of vasodilator metabolites (VDMs)The effect of VDMs eventually overcomes the direct coronary vasoconstrictor effect of sympathetic nervous systemThus the net effect of sympathetic activity is coronary vasodilation secondary to increased metabolic activity1920Metabolic factors are the major controllers of myocardial blood flow. Whenever the direct effects of nervous stimulation alter the coronary blood flow in the wrong direction, the metabolic control of coronary flow usually overrides the direct sympathetic coronary effects within secondsIschaemic Heart Disease21Ischemic Heart DiseaseIf blood flow to the heart through the coronary vessels is reduced or the oxygenation of the blood is insufficient, then the heart muscle will be damaged. Over two- thirds of deaths from coronary artery disease occur outside hospitals, with few prior symptoms and often no warning. The two most common manifestations of ischaemic heart disease are angina and myocardial infarctionThe major factor in the development of ischaemic heart disease is the narrowing of the coronary arteries by the formation of atheroma22Coronary angiographyCoronary angiography involves the injection of a radiocontrast dye into the left and right coronary arteries. X-ray images are taken from various angles in order to observe fully any narrowings. This information may be useful in assessing the degree to which vessels may be narrowed

23Shows the anatomy of coronary vesselsDoppler ultrasonographyA non-invasive alternative, Doppler ultrasonography, measures the direction and speed of blood flow and has more commonly been used to evaluate the degree of impairment in patients with heart failure. It can also be used to evaluate blood flow in the coronary vessels.

24Shows the flow of blood in coronary vessels25 SufyanAkram@imu.edu.my DID: 03 2731 7480 Ext: 2773 Faculty area 2 @ Level 2Thank you