Nursing Pharmacology Chapter 9: Antianginal Drugs
Coronary blood flow regulation:
Coronary blood flow is controlled by the myocardial oxygen demand and modulated by varying coronary vascular resistance considerably.
The myocardium extracts a fixed and high percentage of oxygen.
In the absence of atherosclerotic disease, intramyocardial resistance arterioles can significantly dilate.
By regulation of smooth muscle tone, intramyocardial arterioles (resistance vessels) a balance is maintained between coronary blood flow and myocardial oxygen requirement.
In healthy individuals, the large epicardial vessels are conductance vessels.
(Selwyn, A.P. and Braunwald, E Ischemic Heart Disease in Harrison's Principles of Internal Medicine (Isselbacher et al., eds) McGraw-Hill, Inc., New York, 1994, p 1077)
Control of coronary blood flow involves:
Nervous system regulation.
More important of these two mechanism is local myocardial metabolism, i.e. local arterial vasodilation is regulated by myocardial requirements.
Normally, increases in coronary blood flow, even in a denervated heart, occurs in response to increased myocardial contractility and rate.
Production of local vasodilator products may be responsible.
Candidates for these vasodilatory substances include:
Autonomic nervous system activity can affect coronary vasculature tone.
On the parasympathetic (vagal) side: there are so few fibers terminating on the coronary vasculature that the vagal dilating effects are minimal.
Coronary vascular bed has both alpha and beta-adrenoceptors.
α-adrenergic receptor activation produces constriction, mainly in epicardial capacitance vessels when these receptors are mostly found.
Coronary vessel vasodilation is mediated by β-adrenoceptors which are mainly localized in intramuscular arteries.
Sympathetic activation probably produces more constriction than dilation and in individuals with accentuated responses to α-receptor activation may be susceptible to vasospastic myocardial ischemia
Guyton, A.C. and Hall, J.E. in Textbook of Medical Physiology, W.B. Saunders & Co., Philadephia, 1984, p 258-259.