Page 18 - Coronary hemodynamics in acute myocardial infarction - Matthijs Bax

Chapter 1
defined as the ability of vascular smooth muscle cells to constrict as response to an increase in transmural force, such as perfusion pressure.52 This myogenic response, as behavior of small arteries and arterioles is believed to represent efforts of the vessel to minimize wall stress, supported mathematically by Laplace’s law (wall stress = pressure x radius / wall thickness). Recently Goodwill et al. published a comprehensive review about the quite complex regulation of coronary blood flow describing multiple mechanisms including extravascular compressive forces (tissue pressure), coronary perfusion pressure, myogenic, local metabolic, endothelial as well as neural and hormonal influences.53
The coronary autoregulation aims to ensure that the blood and thus oxygen supply matches the myocardial oxygen demand. Coronary venous pO2 is maintained constant even in exercise.54 Several control mechanisms regulate coronary blood flow to maintain this stable venous pO2. Various regulatory mechanisms play a role at different locations in the coronary microvascular tree. Regulation of the intravascular pressure in the microcirculation necessary to prevent myocardial ischemia reflects the regulation of the microvascular resistance. This coronary autoregulation is triggered by several stimuli such as flow generating shear stress which triggers endothelium-dependent vasodilatation;55 distension pressure affecting stretch receptors on vascular smooth muscle cells (VSMC) leading to chemical reactions influencing VSMC tone and hence, intravascular pressure and resistance to flow; metabolic factors as carbon dioxide, reactive oxygen species and other metabolic messengers as well as the modulating influence of the autonomic nervous system and adrenergic effects contribute to the continuously adapting microvascular resistance.56
The discovery of the cardiac microvascular pericyte and adventitial pericyte-like progenitor cell and their functions is of great importance to understand their control in physiological processes as blood flow, regulation of the coagulatory process and vessel permeability. 57-59 Dysfunction of pericytes influences the pathogenesis of cardiovascular disease e.g., myocardial edema, vascular remodeling and post-ischemic no-reflow. 60, 61
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