Normal physiologic changes during pregnancy can exacerbate symptoms of underlying cardiac disease even in previously asymptomatic individuals. Maternal blood volume rises progressively until the end of the sixth or seventh month. Stroke volume increases over the same time course as a result of the volume change and an increase in LVEF. The latter reflects predominantly a decline in peripheral resistance due to vasodilation and the low-resistance shunting through the placenta. The heart rate then rises in the third trimester to further increase the cardiac output as the stroke volume maximizes out. Overall, cardiac output increases by 30–50%, peaking (and remaining constant) at about the 14th week; systolic BP tends to rise slightly or remain unchanged, but diastolic pressure falls significantly as afterload on the ventricle declines. Venocaval compression of the inferior vena cava from the gravid uterus can lead to reduced venous return and a lower cardiac output in the supine position.
High cardiac output causes alterations in the cardiac examination. A third heart sound is prominent and normal, and a pulmonary flow murmur is common. A mammary souffle may be heard over the breasts. ECG changes include rate-related decreases in PR and QT intervals, a leftward axis shift, inferior Q waves due to the more horizontal position of the heart, and nonspecific ST–T wave changes. Normal echocardiographic findings include slightly increased chamber sizes, functional valvular regurgitation, and occasionally small pericardial effusions.