Cardiovascular Physiology of Pregnancy

Abstract

1003P regnancy is a dynamic process associated with significant physiological changes in the cardiovascular system. These changes are mechanisms that the body has adapted to meet the increased metabolic demands of the mother and fetus and to ensure adequate uteroplacental circulation for fetal growth and development. Insufficient hemodynamic changes can result in maternal and fetal morbidity, as seen in preeclampsia and intrauterine growth retardation. In addition, maternal inability to adapt to these physiological changes can expose underlying, previously silent, cardiac pathology, which is why some call pregnancy nature's stress test. Indeed, cardiovascular disease in pregnancy is the leading cause of maternal mortality in North America. 1 We therefore review here the normal cardiovascular physiology of pregnancy to provide clinicians with a basis for understanding how the presence of cardiovascular disease may compromise the mother and fetus and how their decisions about medical care may need adjustment. Maternal Hemodynamic ChangesPregnancy is associated with vasodilation of the systemic vasculature and the maternal kidneys. The systemic vasodilation of pregnancy occurs as early as at 5 weeks and therefore precedes full placentation and the complete development of the uteroplacental circulation.2 In the first trimester, there is a substantial decrease in peripheral vascular resistance, which decreases to a nadir during the middle of the second trimester with a subsequent plateau or slight increase for the remainder of the pregnancy 3 ( Figure 1). The decrease is ≈35% to 40% of baseline. Systemic vascular resistance increases to near-prepregnancy levels postpartum, 4 and by 2 weeks after delivery, maternal hemodynamics have largely returned to nonpregnant levels.5 Increased vascular distensibility, or compliance, has been observed in normal human pregnancy starting in the first trimester.6 Systemic vascular resistance increases to nearprepregnancy levels postpartum. 4 Vasodilation of the kidneys results in a 50% increase in renal plasma flow and glomerular filtration rates by the end of the first trimester. This results in decreases in serum creatinine, urea, and uric acid values. Cardiac OutputCardiac output increases throughout pregnancy.8 Invasive measuring techniques are rarely used during pregnancy, so echocardiography is most commonly used to assess hemodynamics in pregnancy. Cardiac output measurements are usually made with the mother in the left lateral decubitus position to avoid positional variation. The sharpest rise in cardiac output occurs by the beginning of the first trimester, and there is a continued increase into the second trimester.9 After the second trimester, there is debate as to whether cardiac output increases, decreases, or plateaus. By 24 weeks, the increase in cardiac output can be up to 45% in a normal, singleton pregnancy. 10Echocardiography and cardiac magnetic resonance imaging estimates of cardiac output trend similarly in pregnancy. In a comparative study of 34 normal pregna...

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“…The lowest blood pressure occurs in the second trimester, i.e. 5-10/10-15 mmHg below baseline, but the change starts as early as the sixth to eighth week of gestation [2][3][4] .…”

“…Hemodynamic changes involve blood loss of approximately 500 ml. Auto transfusion of blood from the contracted uterus compensates for this blood loss, and the auto transfusion tends to increase cardiac output even further 3,7 .…”