33. cardiovascular 5-1-08-09


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33. cardiovascular 5-1-08-09

  3. 3. CORONARY ARTERIES <ul><li>Coronary arteries are superficial vessels. The major coronary vessels travel in the epicardium of the heart and subdivide, sending penetrating branches through the myocardium. </li></ul><ul><li>The penetrating branches subdivide into arcades that distribute blood to the myocardium. </li></ul>
  5. 5. CORONARY ARTERIES <ul><li>Normally, the coronary arteries appear to function as end arteries. However, the presence of an arterial plaque or occlusion allows anastomoses between vessels to become functional. </li></ul>
  6. 6. CORONARY VEINS <ul><li>Left ventricular venous drainage occurs primarily through the coronary sinus. The Thebesian veins and coronary-luminal vessels (connections between the coronary vessels and the lumen of the heart) also return small amounts of blood to the left ventricle. </li></ul><ul><li>Right ventricular venous drainage occurs through the multiple anterior coronary veins. The coronary-luminal connections carry a larger proportion of the flow in the right ventricle than in the left ventricle. </li></ul>
  7. 7. CORONARY FLOW PATTERN <ul><li>A continuous FLOW OF BLOOD to the heart is essential to maintain an adequate supply of O 2 and nutrients. </li></ul><ul><li>NORMAL CORONARY BLOOD FLOW represents approximately 5% of the resting cardiac output (250 ml/min), or approximately 60-80 ml blood/100 g tissue/min. </li></ul><ul><li>During ventricular systole, myocardial wall tension increases and COMPRESSES THE PENETRATING VESSELS, thus increasing the resistance to flow. </li></ul><ul><li>This extravascular compression produces a complex CORONARY FLOW PATTERN. Maximal flow in the left coronary vessels occurs during isovolumic relaxation while the arterial pressure is still relatively high and the myocardium is relaxed. </li></ul>
  8. 8. FACTORS INFLUENCING CORONARY FLOW <ul><li>Coronary blood flow is characterized by a supply-demand relationship. The heart metabolizes all substrates in approximate proportion to their vascular concentration. </li></ul>
  9. 9. FACTORS INFLUENCING CORONARY FLOW <ul><li>Myocardial O 2 consumption averages 6-8 ml O 2 /100 g of tissue/min. Normally, hemoglobin releases approximately 50% of its arterial O 2 content to the myocardium (hemoglobin releases about 25% of its O 2 content for the body as a whole). </li></ul><ul><li>The coronary venous PO 2 is approximately 25-30 mm Hg under resting conditions and decreases during exercise or stress (the cardiac O 2 consumption may increase five- to sixfold in well-trained athletes). </li></ul>
  10. 10. FACTORS INFLUENCING CORONARY FLOW <ul><li>ADENOSINE is a major factor in production of coronary vasodilation during hypoxic states. Adenosine is derived from the degradation of adenosine monophosphate (by 5'-nucleotidase). Release of adenosine into the myocardium produces an extremely strong vasodilator response. </li></ul>
  11. 11. FACTORS INFLUENCING CORONARY FLOW <ul><li>SYMPATHETIC STIMULATION increases the cardiac rate and contractility. The resultant increase in myocardial metabolic activity leads to coronary vasodilation. </li></ul><ul><li>If vasodilation is not adequate, the breakdown of adenosine triphosphate (ATP) will lead to the release of ADENOSINE to enhance vasodilation. </li></ul>
  12. 12. FACTORS INFLUENCING CORONARY FLOW <ul><li>The coronary vessels contain both alpha- and beta-receptors. </li></ul><ul><li>The alpha vasoconstrictor activity is rather weak, allowing the vasodilator “beta- response” to predominate. </li></ul>
  13. 13. MYOCARDIAL OXYGEN CONSUMPTION AND CORONARY BLOOD FLOW <ul><li>In contrast to skeletal muscle, increased myocardial oxygen requirements can be met only by increasing the supply. </li></ul><ul><li>Additional oxygen extraction from the blood is limited. While the oxygen content of venous blood draining resting skeletal muscle is 15 Vol%, coronary sinus blood contains only about 5 Vol%. </li></ul><ul><li>Any increase in cardiac work that increases oxygen consumption, causes an increase in coronary blood flow. </li></ul>
  14. 14. MYOCARDIAL OXYGEN CONSUMPTION AND CORONARY BLOOD FLOW <ul><li>The type of cardiac work also plays a role in determining oxygen consumption. </li></ul><ul><li>Increases in cardiac output (induced by increasing preload) at constant arterial blood pressure (afterload). </li></ul><ul><li>Pressure work, require considerably less oxygen than increases in cardiac work involving increased afterload with constant preload (pressure work). </li></ul><ul><li>Stresses such as aortic stenosis and hypertension are much more energy-costly than equal increases in stroke work associated with exercise . </li></ul>
  15. 15. INTERRELATIONSHIP BETWEEN AORTIC PERFUSION PRESSURE AND THE CORONARY BLOOD FLOW <ul><li>Between 60 to 150 mm Hg of BP (mean pressure) in the aorta, the coronary flow practically remains unchanged. </li></ul><ul><li>This is due to autoregulation of the coronary circulation, which survives even when the nerve supply to the coronary vessels are paralysed or removed. </li></ul>
  16. 16. CLINICAL NOTE: TACHYCARDIA AND CORONARY FLOW <ul><li>During tachycardia, the diastolic period of a cardiac cycle is shortened but the systolic period is not. </li></ul><ul><li>The left coronary artery is filled principally in the diastole. Conclusion is, all other things remaining constant tachycardia causes myocardial ischemia (hypoxia). </li></ul><ul><li>However, the rise of cardiac metabolism which occurs during tachycardia (by virtue of O 2 lack), causes coronary vasodilatation, which cancels the harmful effects of tachycardia. </li></ul><ul><li>Despite this, in elderly people or people with CHD, tachycardia may be dangerous. </li></ul>
  17. 17. CLINICAL NOTE: BLOOD PRESSURE AND CORONARY FLOW <ul><li>Conditions characterized by very low diastolic pressure, (e.g. aortic incompetence) are bad for coronary filling. </li></ul><ul><li>In severe cardiovascular shock, where the mean BP falls less than 60 mm Hg, a vicious cycle may be produced. </li></ul><ul><li>Thus, severe myocardial infarction -> severe cardiovascular shock and severe fall of BP -> insufficient coronary filling due to the low BP -> further infarction. Therefore the duty of the attending physician is to cut the vicious cycle off as quickly as possible. </li></ul><ul><li>Vicious cycle - one vice leading to further vices and ultimately causing repetitions of the first vice and thus is an example of + ve feed back. </li></ul>
  18. 18. I thank all of you for your patience!