Renal Hemodynamics in Pregnancy

Abstract

In normal pregnant women, plasma volume begins to rise after six weeks of amenorrhea [1] and attains the maximum increment toward the end of the second trimester. This increase is sustained to term [2]. The maximum amount by which plasma volume increases is extremely variable, ranging from 0.6 to 2.0 liters in different women, and is chiefly related to the size of the product of conception [1]. In pregnancy, the red cell mass also increases, but to a lesser extent than plasma volume, leading to a fall in hematocrit [3]. The increase in blood volume is associated with a rise in cardiac output (CO) [4]. Opinions differ as to whether CO is increased throughout gestation [5] or declines in late pregnancy [6]. Recently, serial measurements of CO in normal pregnant women have been performed by noninvasive technique [7]. These studies have confirmed a 20% increase in CO at week 15. CO rises further to a peak increment of 40% at weeks 25–28 of gestation, and then declines to near postpartum levels in the final weeks. In early pregnancy, the increase in CO is mainly due to an increased stroke volume (SV). As pregnancy advances, the rise in SV diminishes and increased heart rate becomes significantly contributory in maintaining the elevated CO. Echocardiographic studies have shown that the high-output hemodynamic condition in pregnancy is associated with a significant increase in left ventricular mass [8].