Neural and neurohumoral factors affecting coronary blood flow, metabolic factors, coronary steal syndrome, factors contributing to vascular tone, collateral circulation, cerebral blood flow

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Biomedical Science
BMS 420
Charles Miller

12 October Acetylcholine is a probe for stimulating EC (endothelial cell) production of “NO” VD (vasodilatory) response to Ach – normal VC (vasoconstriction) response to Ach – abnormal Flow reversal can occur during isovolumetric contraction phase Left coronary vs right coronary blood flow Ventricular fibrillation – myocardial oxygen consumption drops Higher partial pressure of O 2n coronary sinus Neural and neurohumoral factors also affect coronary blood flow Cardiac sympathetic nerve simulation → vasoconstriction BUT the increased metabolites which result cause vasodilation. Likewise with β adrenergic blockage, vasoconstriction will occur. α and β adrenergic receptor activation will cause vasoconstriction and vasodilation respectively. Stimulation of the vagus nerve will cause vasodilation (intracoronary administration of Ach causes dilation too) but since the metabolism (heart rate/inotropy) is also less, vasoconstriction overrides the tendency to vasodilate. Metabolic Factors Coronary blood flow increases fairly linearly with myocardial metabolism (↑ SV; ↑ afterload; ↑ HR) Normally, O 2xtraction is 50 – 75% vs. 25% for general circulation A decrease in the supply:demand ratio will cause vasodilator generation and the reverse happens with an increase in ratio. Myocardial oxygen balance Arterial O 2ontent, coronary blood flow → myocardial O supply 2 Myocardial metabolic rate → myocardial O demand2 Vasodilator metabolite → coronary blood flow In diseased coronaries, more O requ2red than is supplied by flow (oxygen deficit) Coronary Steal Syndrome What is it? Some coronary vessels cannot dilate more (abnormal) Some normal vessels also present Vasodilator given Normal vessels present dilate further Decrease in blood flow to coronary vessels that are diseased (more blood going to normal vessels) Major factors contributing to vascular tone (and hence coronary blood flow) ↑ myocardial contractile activity → ↑ metabolism →↓ myocardial PO 2 ↓ myocardial PO / 2 metabolism → K ATP channels, NO, adenosine → vasodilation ↑ metabolism → other metabolic factors, PO , PCO , H , K → vasodilation or 2 2 vasoconstriction Systolic compression → vasoconstriction Myogenic mechanism → vasodilation or vasoconstriction Vagus nerves → vasodilation Sympathetic nerves α-receptors → vasoconstriction β-receptors → vasodilation Collateral Circulation Normally there are few communications among the coronary arteries but there are some among the smaller vessels. After an ischemic event some slight increase in this flow
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