Coronary circulation

Dr Prafull Turerao,

1. Coronary circulation Dr Prafull H Turerao . D.Y.Patil Medical College, Kolhapur Tutor, Department of Physiology.

2. Specific L earning Objective Coronary blood vessels Coronary blood flow: characteristic features Measurement of coronary blood flow Regulation of coronary blood flow Factors affecting coronary blood flow Coronary artery disease

3. Physiologic Anatomy of the Coronary Blood Supply T he main coronary arteries lie on the surface of the heart and smaller arteries then penetrate from the surface into the cardiac muscle mass . It is almost entirely through these arteries that the heart receives its nutritive blood supply.

4. Only the inner 1/10 millimeter of the endocardial surface can obtain significant nutrition directly from the blood inside the cardiac chambers , so this source of muscle nutrition is minuscule.

5. CORONARY BLOOD VESSELS Two coronary arteries (right and left) arise from the root of ascending aorta and supply blood to the myocardium.

6. Right coronary artery Right coronary artery supplies blood to the R ight ventricle, T he right atrium, T he posterior part of left ventricle, T he posterior part of interventricular septum A nd major portion of the conducting system of heart including SA node .

7. Left coronary artery Left coronary artery supplies blood mainly to T he anterior part of left ventricle, L eft atrium, A nterior part of the interventricular septum A nd a part of the left branch of bundle of His.

8. Distribution of Blood Supply Predominant supply by the right coronary artery described above is seen in about 50% individuals. In 20% individuals the predominant supply to myocardium is by left coronary artery. In 30% individuals it is the balanced supply, i.e. equal supply by the two arteries.

9. End arteries. Normally , the coronary arteries appear to function as end arteries . However , the presence of an arterial plaque or occlusion allows the anastomoses present between vessels to become functional. That is why they are also known as functional end arteries and not true end arteries.

10. These anastomoses are of two types : Cardiac anastomoses are those which are present between branches of two coronary arteries and between the branches of coronary artery and deep venous system.

11. Extracardiac anastomoses include those present between the branches of coronary arteries and vessels lying near the heart. S uch as vasa vasora of aorta, vasa vasora of pulmonary arteries, intrathoracic arteries, bronchial arteries and phrenic arteries.

12. Coronary Veins Coronary sinus is a wide vein about 2 cm long, which drains most of the venous blood from the myocardium (mainly left ventricle) into the right atrium. Its tributaries are the great cardiac vein, the small cardiac vein, the posterior vein of left ventricle and the oblique vein of left ventricle

13. Anterior cardiac vein draining venous blood mainly from the right ventricle opens directly into the right atrium .

14. Thebesian veins and coronary-luminal vessels (connections between the coronary vessels and the lumen of heart) constitute the deep venous system. These vessels drain only less than 10% of the venous blood from the myocardium directly into the various cardiac chambers , contributing to an anatomic shunt effect.

15. Coronary Blood Flow A continuous flow of blood to the heart is essential to maintain an adequate supply of O2 and nutrients . The resting coronary blood flow in the resting human being averages 70 ml/min/100 g heart weight, or about 225 ml/min, which is about 4 to 5 percent of the total cardiac output.

16. Three to six fold increase in the coronary blood flow may occur during exercise.

17. Oxygen consumption by the myocardium is very high (8 mL/min/100 g at rest). Because of this, even at rest 70–80% of the oxygen is extracted from each unit of the coronary blood as compared to the whole body (average of 25%) oxygen extraction at rest.

18. The increased oxygen demand of the myocardium during exercise is met with by almost total (nearly 100%) extraction of oxygen and by manifold increase in the coronary blood flow

19. Phasic Blood Flow During systole , the tension developed in the left ventricle is so high that it has throttling effect on the branches of the coronary arteries penetrating through them As a result, the average blood flow through the capillaries of left ventricles falls to the extent that during isometric contraction phase, the blood flow to the left ventricle practically ceases, i.e. becomes zero.

20. During diastole, the cardiac muscles relax and blood flow increases. Thus, most of the coronary blood flow (over 70%) occurs during diastole .

21. Subendocardial region of the left ventricle receives no blood supply during systole so this region is particularly vulnerable to ischaemia and is the most common site of myocardial infarction.

22. Minimum diffusion distance between the capillaries and myocardial cells is 20% shorter in the subendocardial region of left ventricle (16.5 μm ) as compared to the epicardial region (20.5 μm ). Myoglobin content (O2 storage pigment) is higher in the subendocardial region than the epicardial region of the left ventricle.

23. This is true in spite of the fact that this region has been provided with following compensatory (protective) mechanisms: Capillary density in subendocardial region of left ventricle is much higher (1100 capillaries/mm2) than the epicardial region (750 capillaries/mm2).

24. The coronary blood flow shows changes during phases of the cardiac cycle. The blood flow is determined by the balance between pressure head (i.e. aortic pressure) A nd the resistance (i.e. extravascular pressure exerted by the myocardium on the coronary vessels) offered to the blood flow during various phases of cardiac cycle

25. MEASUREMENT OF CORONARY BLOOD FLOW Nitrous oxide method ( Kety method) Principle - Nitrous oxide method is the most common method used for measuring coronary blood flow. It gives almost accurate value and is based on the Fick’s principle

26. Procedure - The individual is made to inhale a mixture of 15% nitrous oxide and air for 10 min . During inhalation of gases, serial samples of arterial and coronary sinus venous blood (through a catheter introduced) are taken at fixed intervals for 10 min.

27. The coronary blood flow (CBF) is then determined from the amount of nitrous oxide taken up per minute (N2O/ min) and the difference of nitrous oxide content of arterial (A) and venous (V) blood, i.e . CBF = N2O taken up/min ( A − V)

28. Radionuclides utilization technique Principle - The radioactive tracers are pumped into cardiac muscle cells by the enzymes Na+–K+ ATPase and equilibrate with the intracellular K+ pool. Distribution of radioactive tracers is directly proportional to myocardial blood flow and this forms the basis of this technique

29. Procedure. Radionuclide such as thallium-201 (201T1) is injected intravenously. After 10 min, the amount of 201T1 taken up by the myocardial cells is then measured with the help of gamma-scintillation camera over the chest. The amount of coronary blood flow is calculated from these values. Areas of ischaemia are detected by their low uptake.

30. Newer methods Coronary angiographic technique Coronary angiography when combined with measurement of 133Xe washout using a multiple crystal scintillation camera. Electromagnetic flowmeter technique This technique is employed in animals to measure the coronary blood flow .

31. REGULATION OF CORONARY BLOOD FLOW Autoregulation . C oronary circulation shows well developed phenomenon of autoregulation 60-200 mmHg

32. Role of local metabolites Metabolic local factors are the most important factors which regulate the coronary blood flow . Direct effect of O2 . It has been proposed that a decrease in the tissue PO2 could also act directly on the arterioles and cause vasodilation . Oxygen Demand as a Major Factor in Local Coronary Blood Flow Regulation

33. Role of adenosine (Berne’s hypothesis ) Adenosine is considered the major factor in production of coronary vasodilation during hypoxic states. In myocardial ischaemia , either due to generalized hypoxia or due to increased myocardial metabolism the intracellular myocardial adenine nucleotides are degraded to adenosine.

34. The adenosine is capable of crossing myocardial cell membrane producing an extremely strong vasodilator response

35. Role of other local metabolites. Hydrogen ions, bradykinin, CO2 and prostaglandins are the other suggested vasodilator substances

36. Nervous control mechanism Autonomic nerves control the coronary blood flow directly as well as indirectly.

37. Direct nervous control Parasympathetic nerve fibres to coronary vessels through vagus are so less that the parasympathetic stimulation has very little direct effect, causing vasodilation

38. Sympathetic nerve fibres extensively innervate the coronary vessels. The transmitters released at their nerve endings are epinephrine and norepinephrine . The coronary vessels contain both α and β receptors. The net result of direct effect of sympathetic stimulation is vasoconstriction.

39. Indirect nervous control Sympathetic stimulation causes increase in the heart rate and increase of force of contraction of the heart. Thus , an increased activity of heart helps conversion of ATP to ADP which by producing coronary vasodilation increases the coronary blood flow.

40. Parasympathetic stimulation causes decreased heart rate and decreased force of contraction of heart. Thus, indirectly the coronary blood flow is reduced.

41. FACTORS AFFECTING CORONARY BLOOD FLOW Mean aortic pressure. This is the force for driving blood into the coronary arteries. Rise in mean aortic pressure increases the blood flow and vice versa . Emotional excitement. During emotional excitement states, such as fright, auditory and olfactory stimuli, the CBF is increased due to increased sympathetic discharge

42. Muscular exercise . Normal CBF at rest is about 70 mL/100 g tissue/min. During exercise, CBF increases about four times because of sympathetic stimulation by the following mechanisms: Increased activity of heart Increased cardiac output (> 5 folds) Increase in mean arterial pressure

43. Hypotension. There occurs reflex increase in noradrenergic discharge during hypotension which produces coronary vasodilation to increase CBF. This effect is observed secondary to the metabolic changes in the myocardium at a time when there occurs vasoconstriction of splanchnic, renal and cutaneous vessels

44. Hormones affecting CBF are : Thyroid hormones increase CBF because of increase in metabolism. Adrenaline and noradrenaline cause increase in CBF indirectly. Acetylcholine may increase CBF by its action on heart similar to parasympathetic stimulation. Pitressin is known to decrease CBF by increasing coronary resistance. Nicotine is reported to increase CBF through the liberation of norepinephrine.

45. Heart rate. When heart rate is increased , stroke volume decreases, therefore, phasic CBF and O2 consumption per beat also decreases . Effect of ions. Potassium ions (K+) in low concentration cause dilatation of coronary vessels increasing CBF, whereas high K+ ion concentration causes constriction of coronary vessels decreasing CBF.

46. Metabolic factors. Increased metabolism of the heart increases O2 consumption leading to relative hypoxia. Hypoxia causes vasodilation due to direct effect and also due to release of adenosine leading to increased CBF . Temperature . Hyperthermia increases metabolism and so causes increase in the CBF, while hypothermia decreases metabolic rate and thus decreases CBF as well.

47. CORONARY ARTERY DISEASE Coronary artery disease (CAD) also known as ischaemic heart disease results due to the insufficient coronary blood flow. It is a condition associated with development of atherosclerosis in the coronary arteries, which supply the heart muscles (myocardium). With atherosclerosis, the arterial wall is hardened and its lumen becomes narrow due to plaque formation

48. Angina pectoris Definition . Angina pectoris refers to a transient form of myocardial ischaemia, especially occurring during increased Oxygen demand (e.g. during exercise) in patients with coronary artery disease having about 60–70% narrowing of coronary arteries. Superadded thrombus formation causing incomplete coronary occlusion results in an unstable angina.

49. Characteristic features. Typically , the angina is described as a feeling of uncomfortable pressure, fullness, squeezing or pain in the substernal region, which may be localized or may be referred to the inner border of left arm, neck or jaw . Pain occurs due to accumulation of anoxic myocardial metabolites and factor P which stimulates pain nerve endings.

50. Myocardial infarction Myocardial infarction (MI) or acute myocardial infarction (AMI), commonly known as a ‘heart attack’ refers to a degree of myocardial ischaemia (due to interruption of blood supply) that causes irreversible changes ( necrosis i.e. cell death or infarction) in the myocardium .

51. Signs and symptoms Sudden severe chest pain is a classical symptom of MI. Pain lasts for more than 30 min and typically may radiate to left arm and left side of neck. Pain occurs due to the anoxic metabolites and necrotic tissue products. Associated symptoms with pain, often complained by patients are shortness of breath , nausea, vomiting, palpitation, sweating and anxiety (often described as a sense of impending doom).

52. Approximately 25% of all myocardial infarction are ‘silent’ i.e. without chest pai n or other symptoms. Silent MI usually occurs in diabetics with associated autonomic neuropathy in elderly.

53. Diagnosis of MI is made by triad of: Typical signs and symptoms associated with ECG changes seen on serial tracings and Changes in serum levels of certain enzymes and proteins (cardiac biomarkers ). ECG changes in myocardial infarction are very important to diagnose, localize the area of infarction and to know the duration of infarction. Typical ECG changes (hallmark) seen in MI include: Elevation of ST segment in the leads overlying the infarct area and Depression of ST segments in the reciprocal leads.

54. “Coronary steal" syndrome Value of Rest in Treating Myocardial Infarction The degree of cardiac cellular death is determined by the degree of ischemia and the workload on the heart muscle. When the workload is greatly increased, such as during exercise, in severe emotional strain, or as a result of fatigue , the heart needs increased oxygen and other nutrients for sustaining its life.

55. Collateral Circulation Lifesaving Value of Collateral Circulation in the Heart The degree of damage to the heart muscle caused either by slowly developing atherosclerotic constriction of the coronary arteries or by sudden coronary occlusion is determined to a great extent by the degree of collateral circulation that has already developed or that can open within minutes after the occlusion

56. Furthermore, anastomotic blood vessels that supply blood to ischemic areas of the heart must also still supply the areas of the heart that they normally supply . When the heart becomes excessively active, the vessels of the normal musculature become greatly dilated . This allows most of the blood flowing into the coronary vessels to flow through the normal muscle tissue, thus leaving little blood to flow through the small anastomotic channels into the ischemic area so that the ischemic condition worsens. This condition is called the "coronary steal" syndrome.

57. Treatment Treatment with Drugs Several vasodilator drugs, when administered during an acute anginal attack, can often give immediate relief from the pain. Commonly used short-acting vasodilators are nitroglycerin and other nitrate drugs.

58. Coronary Angioplasty In this procedure a small balloon-tipped catheter, about 1 millimeter in diameter, is passed under radiographic guidance into the coronary system and pushed through the partially occluded artery until the balloon portion of the catheter straddles the partially occluded point. Then the balloon is inflated with high pressure, which markedly stretches the diseased artery .

59. Surgical treatment A surgical procedure was developed in the 1960s, called aortic-coronary bypass , for removing a section of a subcutaneous vein from an arm or leg and then grafting this vein from the root of the aorta to the side of a peripheral coronary artery beyond the atherosclerotic blockage point. One to five such grafts are usually performed, each of which supplies a peripheral coronary artery beyond a block.

60. References Textbook of Medical Physiology – Guyton And Hall 13th Edition Textbook of Physiology – A K Jain 6th Edition Medical Physiology For Undergraduate Students – Indu Khurana 1st Edition Images – Net source

61. Thank You.