Noninvasive evaluation of left ventricular afterload: part 1: pressure and flow measurements and basic principles of wave conduction and reflection.

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 and cannot be expressed as a single numeric measure or described in terms of pressure alone. Left ventricular afterload is best described in terms of pressure-flow relations. High-fidelity arterial applanation tonometry can be used to record time-resolved central pressure noninvasively, whereas contemporary noninvasive imaging techniques, such as Doppler echocardiography and phase-contrast MRI, allow for accurate assessments of aortic flow. Central pressure and flow can be analyzed using simplified biomechanical models to characterize various components of afterload, with great potential for mechanistic understanding of the role of central hemodynamics in cardiovascular disease and the effects of therapeutic interventions. In the first part of this tutorial, we review noninvasive techniques for central pressure and flow measurements and basic concepts of wave conduction and reflection as they relate to the interpretation of central pressure-flow relations. Conceptual descriptions of various models and methods are emphasized over mathematical ones. Our review is aimed at helping researchers and clinicians apply and interpret results obtained from analyses of left ventricular afterload in clinical and epidemiological settings.