Maternal renal artery Doppler velocimetry in normal and hypertensive pregnancies

Abstract

Background: The measurement of effective renal plasma flow (ERPF) by calculation of p-aminohippurate (PAH) clearance is an invasive, time-consuming procedure. In nonpregnant humans, the clearance of isotopes can be used to measure renal plasma flow, but the use of these radioactive substances during pregnancy is unsuitable. A non-invasive method for repetitive assessment of renal hemodynamics is needed for use in investigations of the kidney in pregnancy. The Doppler ultrasound technique is entirely non-invasive, simple and quick to carry out and provides instant results. The development of duplex Doppler sonography has allowed non-invasive studies of blood flow patterns in deep abdominal and pelvic vessels. The acquisition of the Doppler information combined with color flow mapping ensures accurate localization of the blood vessel and reliable blood flow velocity measurements. Renal artery blood flow velocity waveforms have been studied in normal and hypertensive pregnancies, but the results to date have been contradictory. During normal pregnancy the glomerular filtration rate and renal blood flow increase by 50% and 60-80%, respectively. There are often appreciable changes in kidney function in preeclampsia, reflected in the decrement of renal blood flow and glomerular filtration rate. The histological abnormalities in the kidney in women with preeclampsia and eclampsia are quite well defined and termed 'glomerular capillary endotheliosis.' Thus, renal artery Doppler indices in normal pregnancy and in preeclampsia might be affected concomitantly with the known renal hemodynamic and morphological changes, and might be of use in renal function examinations on a routine basis during pregnancy.

Materials and Methods: Nonpregnant women and women with normal and hypertensive pregnancies were studied by color Doppler ultrasonography, renal blood flow velocities being examined at the level of segmental arteries.

Results and Conclusions: Color flow mapping combined with the pulsed Doppler technique is are liable means of assessing renal blood flow velocity as expressed in dimensionless indices. The most suitable vessels for examination were found to be those in the middle portion of the kidney. The results suggest that of all indices studied the pulsatility index (Pl) might be more accurate for the evaluation of renal blood flow velocity. This study clearly showed that repeated measurements should be performed at least at the same level of the arterial branching in the kidney. Obtaining reliable Doppler indices required five or six cardiac cycles in nonpregnant women but only three cardiac cycles in pregnant women.

The findings demonstrated maternal heart rate to have no influence on renal artery Doppler indices. There were no differences in Doppler indices between the right and left kidney. Renal Doppler indices are significantly higher in pregnant than in nonpregnant women, and there is no significant change in these indices throughout normal pregnancy. Renal artery Doppler indices are lower in women with preeclampsia, PIH and chronic hypertension than in normal pregnant women. The findings suggest that the mechanism of renal autoregulation might be altered in preeclampsia, leaving the glomeruli unprotected from increased blood pressure.

These indices do not distinguish preeclampsia from other hypertensive disorders in pregnancy. Thus, renal artery Doppler velocimetry is of limited value for the evaluation of pregnancies complicated by hypertension. Despite similar central hemodynamic changes and plasma atrial natriuretic peptide (ANP) increases during volume expansion, renal vascular resistance, as estimated by renal artery Pl increased only in healthy pregnant women and not in patients with PIH/preeclampsia. It seems that ANP is partly responsible for the renal hemodynamic changes caused by volume load, and that this response is altered in preeclampsia. Renal artery PI was significantly increased in preeclamptic women after 60 minutes infusion of ANP at low doses. Whether such variables as blood pressure and plasma volume status are determinants of hemodynamic response to ANP remains to be elucidated. Gosling's model seems to be useful for the interpretation of PI in low resistance vascular beds, and might provide new information of kidney pathophysiology. Our results emphasize that renal Doppler indices should be interpreted with caution when assessing renal blood flow in pregnant women, as other hemodynamic factors might influence the results. Blood pressure should always be taken into account.