Physiological Basis of Clinically Used Coronary Hemodynamic Indices

Abstract

Abstract-In deriving clinically used hemodynamic indices such as fractional flow reserve and coronary flow velocity reserve, simplified models of the coronary circulation are used. In particular, myocardial resistance is assumed to be independent of factors such as heart contraction and driving pressure. These simplifying assumptions are not always justified. In this review we focus on distensibility of resistance vessels, the shape of coronary pressure-flow lines, and the influence of collateral flow on these lines. We show that (1) the coronary system is intrinsically nonlinear because resistance vessels at maximal vasodilation change diameter with pressure and cardiac function; (2) the assumption of collateral flow is not needed to explain the difference between pressure-derived and flow-derived fractional flow reserve; and (3) collateral flow plays a role only at low distal pressures. We conclude that traditional hemodynamic indices are valuable for clinical decision making but that clinical studies of coronary physiology will benefit greatly from combined measurements of coronary flow or velocity and pressure. Key Words: blood flow velocity Ⅲ blood pressure Ⅲ collateral circulation Ⅲ coronary disease Ⅲ hemodynamics C oronary physiology has a rich history, founded on numerous animal and theoretical models, and significant milestones were reached as new measuring techniques were developed. Recent progress has been made by applying techniques to measure intracoronary flow, flow velocity, and pressure to aid in clinical decision making, thereby advancing our understanding of human coronary physiology beyond what could be extrapolated from animal studies. One unresolved issue that has arisen from these studies, however, concerns conflicting interpretations of coronary microvascular resistance, a quantity with crucial relevance for clinical decision making. [1][2][3][4] There are 2 conflicting interpretations of coronary pressure-flow lines during hyperemia: (1) coronary pressureflow relations are straight and, in the absence of collateral flow, intercept the pressure axis at venous pressure (Pv); or (2) coronary pressure-flow relations are straight at physiological pressures and, when linearly extrapolated, intercept the pressure axis at a value well above venous pressure (extrapolated zero flow pressure [PzfE]); at lower pressures, however, they curve toward the pressure axis, intercepting it at a lower pressure (actual zero flow pressure [Pzf]) that is still higher than Pv.The purpose of this article is to review the physiological literature with respect to coronary pressure-flow relations as relevant to myocardial microvascular resistance. This key issue relates to important assumptions underlying the frequently used model of myocardial fractional flow reserve (FFR myo ). We conclude with a synopsis of physiological studies demonstrating the curved nature of pressure-flow relations and how this shape relates to the pressure dependence of minimal coronary microvascular resistance.This focused review of coronar...

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