RESUMO
Left ejection fraction (LVEF)--resulting from the difference between end-diastolic volume (EDV) and end-systolic volume (ESV), divided by EDV--is a poor index of left ventricular (LV) systolic performance due to its dependency on load conditions, inotropic state and LV remodelling. The characteristic impedance of the ascending aorta (Zc) integrates factors opposing LV ejection during the early ejection period when arterial wave reflection can be neglected. Zc is related to the pressure wave velocity (C) and the cross-sectional area of the aorta. The aim is to demonstrate that LV performance and geometry are closely related to the physical properties of the arterial system. LV pressure-volume loops were obtained from simultaneous measurements of LV (or aortic) pressure and LV volume. The slope Ees (also called LV end-systolic elastance) of the ESP-ESV relationship was assessed. Aortic diameters, pressure and flow measurements were synchronized to evaluate C, aortic forward and backward pressure waves, the elasticity of the aorta (Ep) and thereby Zc. In contrast to LVEF, LV end-systolic elastance (Ees), which reflects the stiffness of the chamber at maximal myofilament activation, is relatively insensitive to load conditions and may be considered as an index of ventricular chamber contractility. For a given Ees value, the end-systolic pressure (ESP) determines the LV end-systolic volume. Ees is determined by cardiac myocytes contractility and density, and thereby concentric remodelling. A tight correlation between Zc and the degree of LV concentric remodelling was found in hypertensive and in normal subjects. Zc was found to increase throughout the full lifespan and also with hypertension. Both Zc and wave reflections determine aortic input impedance estimated from the aortic pressure-flow relationship. Increased arterial stiffness resulted in increasing C and overlap of forward and backward waves and thereby in greater pulse pressure and ESP and a greater difference between ESP and diastolic pressure. Ees is an accurate index of LV systolic performance. Besides the inotropic state of myofibers, Ees depends on the concentric remodelling and thereby on the characteristic impedance of the aorta.
