Resistance and pulsatility Doppler indices: how accurately do they reflect changes in renal vascular resistance

Doppler indices such as the resistance (RI) and pulsatility indices (PI) are generally thought to indicate the resistance of the vascular bed supplied by the artery under investigation. However, this association remains uncertain; some studies have demonstrated an excellent correlation between alterations in vascular resistance and changes in Doppler indices, while others have shown no consistent change. In order to assess the validity of these two indices in estimating alterations in renal vascular resistance, 12 female Large White pigs, 45 weeks old, were entered into a double blind study. In nine of the pigs both kidneys received a single dose of 9.8 Gy 60Co gamma-rays; the remaining three served as sham-irradiated controls. Renal haemodynamics were measured using 131I-hippuran renography and Duplex Doppler prior to and at 8, and 12 weeks after kidney (sham-)irradiation. The sham-irradiated animals exhibited no significant changes in any of the parameters. In contrast, the animals in which both kidneys were irradiated exhibited a significant decrease (p < 0.001) in renal blood flow (RBF) 8 and 12 weeks after irradiation. In addition, both PI and RI were increased following kidney irradiation; however, this increase was not significant until 12 weeks after irradiation (p < 0.01). This suggests that the reduction in RBF was not simply a reflection of an increased vascular resistance and/or that PI and RI do not truly represent alterations in vascular resistance. Further studies are required to confirm the applicability of PI and RI in the assessment of renal haemodynamics.