Cardiovascular Changes During Pregnancy
Pregnancy is associated with a 20% decrease in systemic vascular resistance (SVR), which likely triggers the renin–angiotensin–aldosterone system to retain sodium and increase plasma volume. This increase in plasma volume exceeds a simultaneous increase in red blood cell mass, resulting in a lower hemoglobin level than in the nonpregnant state. Plasma volume increases 50% over pre-pregnant values by 32 weeks gestation (Table 1). SVR decreases to 30% below pre-pregnancy values by 20 weeks gestation; this change is reflected in a decrease in mean arterial pressure (MAP) and an increase in heart rate (HR) of 15 beats/min. Overall arterial compliance increases from pregnancy onset until second trimester, then decreases from the second trimester through the remainder of the pregnancy. Cardiac output (CO) increases by 30 to 60% until approximately 32–36 weeks gestation.[11–15] Throughout pregnancy, pulmonary vascular resistance decreases, pulmonary blood flow increases, and pulmonary mean arterial pressures remain unchanged. With supine positioning after 20 weeks gestation, the gravid uterus can compress the vena cava, reducing cardiac output, and compress the aorta proximal to the uterine artery further compromising uterine blood flow. A more in-depth review of the hemodynamic changes of pregnancy has been previously published.
Expert Rev of Obstet Gynecol. 2012;7(1):59-75. © 2012 Expert Reviews Ltd.