NORMAL PHYSIOLOGIC CHANGES DURING PREGNANCY

Profound hemodynamic alterations occur during pregnancy, labor and delivery, and in the post-partum period.

These changes begin during the first five to eight weeks of pregnancy and reach their peak late in the second trimester.

Hemodynamic deterioration may become clinically manifest in the diseased heart at this time.

· Blood volume increases 40 to 50 percent during normal pregnancy, in part due to estrogen-mediated activation of the renin-aldosterone axis. The rise in blood volume is greater than the increase in red blood cell mass, contributing to the fall in hemoglobin concentration (eg, the "anemia of pregnancy").

· Cardiac output rises 30 to 50 percent above baseline. It peaks by the end of the second trimester, after which it reaches a plateau until delivery. The change in cardiac output is mediated by 1) increased preload due to the rise in blood volume, 2) reduced afterload due to a fall in systemic vascular resistance, 3) a rise in the maternal heart rate by 10 to 15 beats per minute.

· Stroke volume increases during the first and second trimesters, but declines in the third trimester due to caval compression by the gravid uterus. The direct effect of pregnancy on cardiac contractility is controversial.

· Blood pressure typically falls, usually reaching a nadir of 10 mm Hg below baseline by the end of the second trimester.

The decline in blood pressure is mediated by a fall in systemic vascular resistance induced by hormonal changes and by the addition of a low-resistance circuit through the uteroplacental bed.

During labor and delivery, hemodynamic fluctuations can be profound. Each uterine contraction results in the displacement of 300 to 500 ml of blood into the general circulation. Stroke volume increases, with a resultant rise in cardiac output by an additional 50 percent with each contraction. Mean systemic pressure also rises, in part due to maternal pain and anxiety. Blood loss during delivery (300 to 400 ml for a vaginal delivery and 500 to 800 ml for a cesarian section) can further compromise the hemodynamic state.

The hemodynamic changes during the post-partum state are mainly due to relief of vena caval compression after delivery. The resultant increase in venous return augments cardiac output and causes a brisk diuresis.

The hemodynamic changes return to the pre-pregnant baseline within 3 to 4 weeks following delivery.

The marked hemodynamic changes associated with pregnancy explain the characteristic signs and symptoms that occur in the pregnant patient. Normal pregnancy is often associated with fatigue, dyspnea, and decreased exercise capacity. Pregnant women usually have peripheral edema and jugular venous distension. Most pregnant women have audible physiologic systolic murmurs, created by augmented blood flow. A physiologic third heart sound (S3), reflecting the volume overloaded state, can often be appreciated.

During normal pregnancy, noninvasive testing of the heart may include an electrocardiogram, a chest radiograph, and an echocardiogram. The electrocardiogram may reveal a leftward shift of the electrical axis, especially during the third trimester when the diaphragm is pushed upwards by the uterus. Routine chest radiographs should be avoided, especially in the first trimester. Echocardiography is an invaluable tool for diagnosis and evaluation of suspected cardiac disease in the pregnant patient. Normal changes attributable to pregnancy include increased left ventricular mass and dimensions.