Anatomy and physiology of coronary blood flow

Authors: Heinrich R Schelbert

Publish Date: 2010/06/03

Volume: 17, Issue: 4, Pages: 545-554

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Abstract

Regional myocardial blood flow can now be measured noninvasively in units of milliliters blood per minute per gram myocardium These noninvasive measurements are not confined to a specific imaging modality but are available with MRI CT and PET although thus far most investigations of the coronary circulation in humans have employed PET flow measurements Flow estimates with these different imaging modalities were found in animal experiments to correlate well with invasive flow estimates by the arterial blood samplingmicrosphere technique widely considered as the “gold standard” of flow measurements111 In these comparison studies noninvasivelyderived estimates corresponded linearly with invasivelymeasured myocardial blood flows over a wide flow range ie from as low as 03 mL/minute/g to as high as 56 mL/minute/g When tested in the human heart noninvasive flow estimates correlated well with those obtained through invasivelyobtained indices of flow and its changes by wellestablished alternate measurement approaches1216 Development and commercial availability of userfriendly software programs have facilitated the use of these new measurement capabilities so that they are now readily available in the clinical environment for the study of patients with cardiovascular diseaseAs several investigations have indicated measurements of myocardial blood flow at rest contain only limited diagnostic information For example even in patients with advanced cardiovascular disease such as nonischemic dilated or ischemic cardiomyopathy hypertrophic cardiomyopathy or coronary artery disease resting myocardial blood flows frequently are similar to those in normal individuals1719 It is the response of myocardial blood flow to specificallytargeted pharmacological or physiological interventions that can uncover the presence of functional or structural diseaserelated alterations of the coronary circulation This then underscores the need for closely examining these targets and how they relate to anatomical and functional determinants of coronary blood flow and by inference to myocardial blood flow and their alterations in cardiovascular disease Local and systemic mechanisms regulate the complex interactions between flow and anatomy in order to meet the heart’s energy needs A comprehensive description of the coronary circulatory function and its control exceeds the scope of this review so that the interested reader is referred to detailed reviews on this topic2021 This review then seeks to describe in broad strokes the major functional and anatomical determinants of coronary blood flow as they pertain to noninvasive measured myocardial blood flowsFlow across the myocardium largely depends on the pressure gradient between the aortic root the “coronary driving pressure” and the right atrium Under normal conditions the driving pressure is fully maintained along the epicardial conduit vessels with little if any pressure loss in the distal epicardial arteries However intracoronary pressures decline along the microvasculature with most of the pressure dissipating in the 300100 μm diameter vessels until reaching a pressure of 2030 mmHg still adequate to ascertain a gradient across the capillaries Additional determinants of the resistance to flow include extravascular resistive forces that are directly related to the left ventricular systolic pressure the contractile state of the myocardium and the heart rateAutoregulatory mechanisms coordinate the interaction between intracoronary driving pressure and microvascular resistance in order to maintain adequate flow across the capillaries for substrate delivery and removal Through this mechanism also defined as “coronary autoregulation” decreases in driving pressure are compensated for by decreases in resistance and conversely increases in driving pressure by increases in resistance so that flow remains constant for a given cardiac workload This regulatory mechanism operates within the range of physiologic arterial pressures but fails during hypotension when flows become strongly dependent on the driving pressure22Relationship of myocardial blood flow to cardiac work as reflected by the rate pressure product Myocardial blood flow was measured with N13 ammonia at baseline and again during intravenous dobutamine infusion Regional myocardial blood flows in myocardial territories supplied by nondiseased vessels are shown in green In contrast regional myocardial blood flows in territories subtended by diseased coronary vessels are shown in red Adopted from Krivokapich et al34Major determinants of the resistance to flow include the intravascular pressure the velocity of flow the length of the vessel and importantly its diameter Applying the HagenPoisseuil equation resistance to flow depends on the fourth power of the vessel diameter High velocity flows in the epicardial conduit vessels lead to greater shearstress on the endothelial cells prompting an endotheliumdependent flowmediated dilation of the conduit vessels and thus a resetting of their luminal crosssectional area in order to fully accommodate high velocity flows This flowmediated conduit vessel dilation when measured on quantitative coronary angiography serves as an index of endothelial function Characteristically the crosssectional area of the vessel increases by about 20 to 30 in response to pharmacologically stimulated hyperemia2627 As a result of the flowmediated vasodilation intracoronary pressures are fully maintained along the large epicardial coronary arteries as demonstrated with intracoronary pressure transducers28

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