Relationship between the resistive index and vascular compliance and resistance.

PURPOSE To evaluate the dependence of the resistive index (RI) on not only vascular resistance but also vascular compliance. MATERIALS AND METHODS An in vitro model that made use of a pulsatile pump, blood-mimicking fluid, and variable compliance and resistance was used to investigate the relationship between the RI and both vascular compliance and resistance. RESULTS In the absence of vascular compliance, the RI was independent of vascular resistance. With vascular compliance, the RI was dependent on vascular resistance and increased with increasing resistance. The higher the compliance, the more the RI was affected by resistance. CONCLUSION The RI is misnamed and should actually be called the "impedance index" because resistance and compliance interact to alter the Doppler arterial waveform. A greater understanding of this relationship may enable future studies that take both resistance and compliance into account to better detect pathologic conditions.