Maternal cardiovascular changes during pregnancy include a 40 to 45 percent increase in circulating blood volume, a 43 percent increase in cardiac output, and a 17 percent increase in resting heart rate.4 Systemic vascular resistance is 20 percent lower. Blood pressure decreases to a nadir during the second trimester. The diastolic decrement (10 to 15 mmHg) is greater than the systolic decrement (5 to 10 mmHg). Hemodynamic measurements should be taken with the patient in the left lateral decubitus position, since body position has a significant influence on hemodynamic values. The left lateral position increases venous return by relieving the pressure of the uterus on the inferior vena cava. The increase in cardiac output is detectable from the first trimester until after delivery when readings are taken with the patient in this position.5
The heart is displaced up and to the left because of elevation of the diaphragm. This displacement produces a larger cardiac silhouette on chest radiograph and a slight left axis deviation on the electrocardiogram. A small, benign pericardial effusion may also contribute to the enlarged cardiac silhouette.
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