Pregnancy: Hemodynamic changes

Cardiac output increases throughout gestation, about 35-40% during the first trimester and then to 50% above baseline during the second and third trimesters. Cardiac output peaks during and immediately post-delivery, returning to near baseline by 2 weeks postpartum. Immediately post-delivery, cardiac output can reach 2-2.5 times pre-pregnancy values. Initial increases in cardiac output start at 4-5 weeks gestation with an increasing heart rate, 15-25% by the end of the first trimester and slow increase throughout the remainder of gestation. The remainder of increase in cardiac output derives from increased stroke volume, about 20% by the end of the first trimester and 25% at the end of the second trimester.

Concomitantly, there is a slight decrease in blood pressure during pregnancy. The nadir is during the second trimester at about 10-20%. Systemic vascular resistance (SVR) is decreased throughout (nadir of 35-40%) and remains below baseline until delivery. Hormonal changes such as increased estrogen and relaxin are thought to lead to increased endogenous nitric oxide. Additionally, arterial compliance remains relatively preserved leading to shift of blood volume into venous circulation.

Due to this physiologic increase in cardiac output and cardiac stroke work, there is physiologic left ventricular hypertrophy. Left ventricular volumes are increased both in longitudinal and transverse dimensions (increased LVEDV of 30-40% at term) as well as wall thickness. Classically, left ventricular ejection fraction (LVEF) was thought to be increased, but some recent echocardiographic studies track a similar increase in LVESV of 30-40% with preservation of LVEF. These enlarged cardiac chambers lead to increase in annular dimensions and nearly 95% of parturients experience pulmonic and tricuspid insufficiency at term.

Blood volumes increase throughout pregnancy, initially with expansion of plasma volumes during the first trimester. Subsequent plasma and RBC mass increases reach up to 50% above pre-pregnancy values for total circulating blood volumes at term.