ABNORMAL RENAL ARTERY WAVEFORM

IUGR

• Higher than normal in some fetuses (1), but may be normal.
• There is a direct correlation between blood oxygen and increased renal artery PI (2). Animal models have demonstrated that prolonged hypoxemia results in abnormal blood flow to the kidneys (3). In humans, the worst fetal outcome occurs in fetuses with IUGR complicated by abnormal renal artery PI's and oligohydramnios.

AMNIOTIC FLUID DYNAMICS

• After 20 weeks gestation, the fetus becomes the primary source of amniotic fluid production and elimination.
• Renal perfusion may play an important role in the oligohydramnios - polyhydramnios feature of monochorionic diamniotic twins (1). Polyhydramnios may be due to increased renal perfusion and urine output, whereas oligohydramnios may be due to decreased renal perfusion and output.
• Some workers have found that the PI remains normal in polyhydramnios (1), suggesting that renal perfusion is not the only factor that can influence amniotic fluid volume. Tubular reabsorption may play an important role in amniotic fluid volume.

REFERENCES

1. Mari G, Kirshon B, Abuhamad F. Fetal renal artery flow velocity waveforms in normal pregnancies and pregnancies complicated by polyhydramnios and oligohydramnios. Obstet Gynecol 1993;81:560-564.
2. Vyas S, Nicolaides KH, Campbell S. Renal flow velocity waveforms in normal and hypoxemic fetuses. Am J Obstet Gynecol 1989;161:168-172.
3. Bocking AD, Gagnon R, White SE et.al. Circulatory response to prolonged hypoxemia in sheep. Am J Obstet Gynecol 1988;159:1418-1424.