What does this number mean and how is it derived?
The resistive index (RI) measures the resistance of renal arterial flow to the kidney. In a normal situation, flow through the renal artery occurs throughout systole and diastole. However during a variety of conditions–transplant rejection, inflammation, obstruction–there will be reduced to possibly even reversed flow through the renal artery during diastole. When this happens, the resistive index–which is calculated by measuring the arterial waveform throughout the cardiac cycle as described in the figure–is elevated. An RI 0.9 is considered elevated.
The differential diagnosis for an elevated RI should include transplant rejection, obstruction, ATN, pyelonephritis, severe hypotension, or an acute vascular event (e.g. renal vein thrombosis).