The validity of Doppler-derived spectral analysis in the presence of multiple stenoses

It has been suggested that mathematical analysis of ultrasonic Doppler spectral waveforms allows non-invasive quantification of arterial stenoses proximal to the Doppler probe. Less emphasis has been placed on the influence of distal stenoses or distance to the ultrasonic probe. A bench-top electromechanical model with multiple stenoses was constructed to evaluate the effects on spectral waveforms of variations in the severity of proximal and distal stenosis, length of stenoses, distance between stenoses, and distance between ultrasonic probe and stenoses. The following analytic indices, descriptive of spectral waveforms, were evaluated: pulsatility index, spectral broadening index, Laplace damping factor, mode frequency, mean frequency, arithmetic mean frequency, maximum envelope frequency and minimum envelope frequency. All these indices, except the mode frequency, were influenced by the degree of distal as well as proximal stenosis. They were also affected by the distance between the Doppler probe and the upstream and downstream stenoses. The length of stenoses did not influence the flow waveform. Of the model variables, the distal stenosis had the greatest influence on the calculated indices. However, none of the analytic techniques could detect a stenosis smaller than a 70% reduction in cross-sectional area. It was concluded that Doppler-derived spectral waveforms between sequential stenoses are inevitably influenced by events other than the degree of proximal stenosis alone. Therefore, ultrasonic spectral waveforms obtained at a single point of the arterial tree in patients with multi-segment arterial disease should be interpreted with caution.