33. cardiovascular 5-1-08-09
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CARDIO-VASCULARSYSTEM:
BLOOD FLOWIN SPECIAL AREAS (part – I)
SPECIALCIRCULAIONS
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CORONARY CIRCULATION
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CORONARY ARTERIES
• Coronary arteries are superficial vessels. The major coronary vessels travel in the epicardium of the heart and subdivide, sending penetrating branches through the myocardium.
• The penetrating branches subdivide into arcades that distribute blood to the myocardium.
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NORMAL CORONARY BLOOD FLOW
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CORONARY ARTERIES
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.
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CORONARY VEINS
• 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.
• 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.
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CORONARY FLOW PATTERN• A continuous FLOW OF BLOOD to the heart is
essential to maintain an adequate supply of O2 and nutrients.
• 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.
• During ventricular systole, myocardial wall tension increases and COMPRESSES THE PENETRATING VESSELS, thus increasing the resistance to flow.
• 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.
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FACTORS INFLUENCING CORONARY FLOW
Coronary blood flow is characterized by a supply-demand relationship. The heart metabolizes all substrates in approximate proportion to their vascular concentration.
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FACTORS INFLUENCING CORONARY FLOW
• Myocardial O2 consumption averages 6-8 ml O2 /100 g of tissue/min. Normally, hemoglobin releases approximately 50% of its arterial O2 content to the myocardium (hemoglobin releases about 25% of its O2 content for the body as a whole).
• The coronary venous PO2 is approximately 25-30 mm Hg under resting conditions and decreases during exercise or stress (the cardiac O2 consumption may increase five- to sixfold in well-trained athletes).
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FACTORS INFLUENCING CORONARY FLOW
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.
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FACTORS INFLUENCING CORONARY FLOW
• SYMPATHETIC STIMULATION increases the cardiac rate and contractility. The resultant increase in myocardial metabolic activity leads to coronary vasodilation.
• If vasodilation is not adequate, the breakdown of adenosine triphosphate (ATP) will lead to the release of ADENOSINE to enhance vasodilation.
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FACTORS INFLUENCING CORONARY FLOW
• The coronary vessels contain both alpha- and beta-receptors.
• The alpha vasoconstrictor activity is rather weak, allowing the vasodilator “beta- response” to predominate.
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MYOCARDIAL OXYGEN CONSUMPTION AND CORONARY BLOOD FLOW
• In contrast to skeletal muscle, increased myocardial oxygen requirements can be met only by increasing the supply.
• 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%.
• Any increase in cardiac work that increases oxygen consumption, causes an increase in coronary blood flow.
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MYOCARDIAL OXYGEN CONSUMPTION AND CORONARY BLOOD FLOW
• The type of cardiac work also plays a role in determining oxygen consumption.
• Increases in cardiac output (induced by increasing preload) at constant arterial blood pressure (afterload).
• Pressure work, require considerably less oxygen than increases in cardiac work involving increased afterload with constant preload (pressure work).
• Stresses such as aortic stenosis and hypertension are much more energy-costly than equal increases in stroke work associated with exercise.
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INTERRELATIONSHIP BETWEEN AORTIC PERFUSION PRESSURE
AND THE CORONARY BLOOD FLOW• Between 60 to 150 mm
Hg of BP (mean pressure) in the aorta, the coronary flow practically remains unchanged.
• This is due to autoregulation of the coronary circulation, which survives even when the nerve supply to the coronary vessels are paralysed or removed.
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CLINICAL NOTE: TACHYCARDIA AND CORONARY FLOW
• During tachycardia, the diastolic period of a cardiac cycle is shortened but the systolic period is not.
• The left coronary artery is filled principally in the diastole. Conclusion is, all other things remaining constant tachycardia causes myocardial ischemia (hypoxia).
• However, the rise of cardiac metabolism which occurs during tachycardia (by virtue of O2 lack), causes coronary vasodilatation, which cancels the harmful effects of tachycardia.
• Despite this, in elderly people or people with CHD, tachycardia may be dangerous.
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CLINICAL NOTE: BLOOD PRESSURE AND CORONARY FLOW
• Conditions characterized by very low diastolic pressure, (e.g. aortic incompetence) are bad for coronary filling.
• In severe cardiovascular shock, where the mean BP falls less than 60 mm Hg, a vicious cycle may be produced.
• 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.
• 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.
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