Widerstandsindex

Widerstandsindex is a term used in German-language medical literature to describe a Doppler ultrasound parameter that quantifies downstream vascular resistance. It is more commonly referred to in English as the Resistive Index (RI). The index is derived from spectral Doppler ultrasound waveforms and provides a dimensionless, unitless value that helps assess blood-flow resistance in an artery.

The standard definition is RI = (PSV − EDV) / PSV, where PSV is the peak systolic velocity and EDV is the end-diastolic velocity within the same arterial waveform. The measurement is typically obtained in vessels such as the renal arteries, but it can also be applied to other arteries including hepatic, splenic, or cerebral vessels. RI values range from 0 to less than 1; higher values indicate greater downstream resistance and reduced diastolic flow.

Clinical interpretation depends on the vessel studied and the broader clinical context. An elevated RI can suggest increased vascular resistance due to renal parenchymal disease, acute injury, obstruction, or transplant-related changes, among other causes. However, RI should not be used in isolation to diagnose a condition; it is interpreted alongside the overall ultrasound findings and patient status.

Measurement requires proper technique: a suitable Doppler angle, correct sample-volume placement, and consistent acquisition conditions. RI can be influenced by heart rate, age-related vascular changes, and machine settings, so careful standardized assessment is essential. In practice, Widerstandsindex serves as one of several tools to evaluate vascular resistance and renal or organ perfusion in clinical decision-making.