Analysis of maternal circulation and renal function in physiologic pregnancies; parallel examinations of the changes in the cardiac output and the glomerular filtration rate.

OBJECTIVE

The aim of the study was to examine the temporal relations between the parameters of the maternal hemodynamics and homeostasis in different phases of pregnancy and the postpartum period.

METHODS

Eleven healthy pregnant women were involved in the study. The value of the peripheric vasodilatation was measured by the ratio I/A of the maternal sphygmogram (I: incisure point, A: amplitude of the carotid pressure curve). The change of the cardiac output was determined by the ejection angle composed from maternal carotid sphygmogram and its first electrical derivate (dP/dT). The glomerular filtration rate (GFR) was measured with the in vitro radiofluorescent method.

RESULTS

The primary peripheral vasodilatation and GFR increase occur early in the first trimester of a normal pregnancy, start to decrease at the end of the third trimester, and return to the normal value in the postpartum period. The increase of GFR precedes the increase of cardiac output. The cardiac output increases progressively from the first trimester and starts to decrease in the third trimester of a normal pregnancy. The extracellular volume increases at the beginning of first trimester progressively until the end of pregnancy and returns to the normal value in the postpartum period.

DISCUSSION

The increase of GFR and the cardiac output during pregnancy are contributed to volume-establishment and to the cessation of the special "underfilled" condition of the maternal circulation in the normal pregnancy. We suppose that the maximal increase of GFR precedes the maximal increase of the cardiac output because the GFR increase is caused by maximal renal vasodilatation. The increase of cardiac output is caused in the first trimester by the shunt effect of the enhanced renal blood flow of maternal kidneys, and in the second and third trimester by the shunt effect of the feto-placental unit and the shunt effect of the maternal kidneys.