Vascular characteristics influence the aortic ultrasound Doppler signal: computer and hydraulic model simulations.

There is an increasing demand for non-invasive methods for the assessment of left ventricular function. Ultrasound Doppler methods are promising, and the early systolic flow velocity signal immediately distal to the aortic valve has been used clinically for this purpose. However, the signal is influenced not only by left ventricular ejection but also by systemic vascular characteristics. Their relative contribution to the time-velocity signal has not been analysed in depth previously. A theoretical analysis, based on a three-element Windkessel model, neglecting peripheral outflow in early systole and assuming linear pressure rise, was therefore tested in computer and hydraulic model simulations where peripheral outflow was included. Significant changes in early aortic flow velocity parameters were found when vascular characteristics were altered. As predicted by the theory, with a standardized aortic valve area and aortic pressure change, the simulations confirmed that maximal flow velocity is related to compliance of the aorta and the large arteries, and that maximal acceleration is inversely related to the characteristic impedance of the aorta. Therefore, maximal velocity and acceleration can be used for assessment of left ventricular function only in situations where vascular characteristics can be considered relatively constant or where they can be estimated.