Noninvasive Evaluation of Left Ventricular Afterload

Abstract

Abstract-The mechanical load imposed by the systemic circulation to the left ventricle is an important determinant of normal and abnormal cardiovascular function. Left ventricular afterload is determined by complex time-varying phenomena, which affect pressure and flow patterns generated by the pumping ventricle. Left ventricular afterload is best described in terms of pressure-flow relations, allowing for quantification of various components of load using simplified biomechanical models of the circulation, with great potential for mechanistic understanding of the role of central hemodynamics in cardiovascular disease and the effects of therapeutic interventions. In the second part of this tutorial, we review analytic methods used to characterize left ventricular afterload, including analyses of central arterial pressure-flow relations and windkessel modeling (pressure-volume relations). Conceptual descriptions of various models and methods are emphasized over mathematical ones. Our review is aimed at helping researchers and clinicians obtain and interpret results from analyses of left ventricular afterload in clinical and epidemiological settings. (Hypertension. 2010;56:563-570.)Key Words: afterload Ⅲ noninvasive Ⅲ input impedance Ⅲ arterial load T he mechanical load imposed by the systemic circulation to the left ventricle (LV) is an important determinant of normal and abnormal cardiovascular function. With the availability of noninvasive methods to accurately measure central pressure and flow, as well as computational tools for their analysis, there is great potential for their application in order to gain further mechanistic understanding of the role of central hemodynamics in cardiovascular disease. In the first part of this tutorial, we reviewed noninvasive methods to measure aortic pressure and flow, introduced basic concepts of analyses of pressure/flow relations in the time and frequency domains based on simple models of wave conduction and reflection, and briefly discussed wave reflections in the arterial tree. In this article, we review modeling of the arterial system as a windkessel, techniques for analyses of human arterial pressure-flow relations and considerations for interpreting hemodynamic indices in the context of research studies and individual hemodynamic assessments. The Arterial WindkesselIn addition to considering the arterial tree as a system of tubes with wave travel and reflection (or as a transmission line network), one might address it in terms of "windkessel" model components. Frank proposed the original windkessel model as a resistance and compliance (C) pair (2-element windkessel), meant to represent small vessel resistance and large artery compliance, respectively. 1 In the arterial system, total resistance to flow originates predominantly from small arteries, whereas most of the summed compliance is provided by large arteries. This is not a perfect distinction, because large arteries impose some resistance and smaller vessels provide some compliance, whereas the model only measu...

Association of Systemic Arterial Properties With Right Ventricular Morphology: The Multi‐Ethnic Study of Atherosclerosis (MESA)‐Right Ventricle Study

“…TAC decreases as vascular stiffness increases and is largely dependent on the properties of large‐ and medium‐sized conduit arteries 20, 21. Lower TAC (more vascular stiffness) has been previously shown to be associated with lower LV mass and higher LV wall‐to‐cavity volume ratio in MESA 19.…”

“…TAC, as estimated by the ratio of stroke volume to arterial pulse pressure, is a surrogate measure of pulsatile arterial load 20, 21. TAC decreases as vascular stiffness increases and is largely dependent on the properties of large‐ and medium‐sized conduit arteries 20, 21.…”

“…Parameters of pulsatile load include TAC20, 21 and AI,22, 23 which encompass vascular elastance and the wave reflection physiology. The SVR quantifies the resistive load 24, 25.…”

Hemodynamic Patterns Identified by Impedance Cardiography Predict Mortality in the General Population: The PREVENCION Study

“…The resistive component of arterial load can be computed simply as the ratio of mean arterial pressure/cardiac output. Pulsatile load is complex, time varying, and best assessed with detailed modeling of aortic pressure‐flow relations 2, 18, 19. A readily available index of pulsatile arterial load, mainly related to TAC, is the ratio of SV/PP.…”

“…Blood pressure (BP) is determined by interactions between the heart and arterial properties. Mean arterial pressure is the product of cardiac output and systemic vascular resistance (SVR), whereas pulse pressure results from the interaction between the stroke volume generated by the left ventricle (LV) and the biophysical properties of conduit arteries 1, 2, 3. It follows that patients with identical BP may have substantially different hemodynamic patterns.…”