Physiopathological Basis of Doppler Indices

Abstract

Doppler ultrasound offers a unique noninvasive technology for assessing circulatory dynamics. Doppler frequency shift waves represent velocity pulse waves at the vascular site of ultrasound interrogation. The Doppler waveform is, therefore, shaped by hemodynamic phenomena both proximal and distal to the point of measurement (Fig. 1). The main technique for analyzing the waveform has been the determination of descriptor indices of the waveform. Most of these indices are virtually angle-independent ratios which are derived from various combinations of the peak systolic, end-diastolic, and average values of the Doppler maximum frequency shift envelope. Most of these indices express wave pulsatility although their formulations are not identical. The indices that are commonly used include resistance index (RI), pulsatility index (PI), and systolic-diastolic (S/D) ratio (Fig. 2). The indices have been used widely for noninvasively investigating regional circulations based on the assumption that they are capable of reflecting downstream flow impedance. Obviously, such an assumption requires experimental verification as the contour of an arterial Doppler wave, being representative of the arterial flow velocity waveform, is expected to be affected not only by peripheral but also by central circulatory dynamics. Relatively recently, a number of studies have sought such experimental verification of the hemodynamic basis of Doppler waveform analysis. Investigations have also been conducted to elucidate the changes in Doppler waveform in relation to homeostatic challenges to the fetus, such as asphyxia or chronic nicotine exposure. These investigations have utilized a variety of experimental approaches using a wide spectrum of models encompassing in vitro circulatory simulation, in vivo models involving fetuses and neonates, and vascular morphological studies on human placentae (Table 1). This paper critically reviews these studies and also presents basic concepts regarding opposition to flow in a pulsatile circulation.