A nonpregnant woman's heart pumps about 4 liters of blood per minute. When she is pregnant, her cardiac output increases by 40 percent to 5.6 liters per minute. Her cardiac output increases by another 40 percent when she is in labor to 7.84 liters per minute. If she takes a beta-adrenergic for preterm labor, her cardiac output increases by yet another 40 percent to 10.98 liters per minute. If she happens to be pregnant with twins, her cardiac output increases by another 18 percent to 12.95 liters per minute. Thus, a woman pregnant with twins, who is experiencing preterm labor and taking a beta-adrenergic for preterm labor has a cardiac output more than *300 percent* higher than than her normal nonpregnant state.
Source: Hankins, GDV, "Complications of Beta-Sympathomimetic Tocolytic Agents," in Critical Care Obstetrics, 2nd ed., Oxford, 1991: pp. 231-232.
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