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CARDIOVASCULAR ADAPTATION

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Changes in Hemodynamic Values

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Cardiac Output = Heart Rate × Stroke Volume

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The maternal cardiovascular adaptation to pregnancy is characterized by a marked increase in intravascular volume, with expansion of both plasma and red blood cell volume. Heart rate, stroke volume, and cardiac output also increase, with a decrease in systemic vascular resistance. These changes begin by about 6 weeks of gestation, during the embryonic period of development.

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The circulation of early pregnancy is characterized by a high flow–low resistance state.1 By 6 weeks, there is an increase in heart rate, with no change in cardiac output. The cardiac output is unchanged as the result of a significant drop in brachial systolic and diastolic blood pressure as well as central systolic pressure along with a drop in peripheral vascular resistance2 and renal vascular resistance. These physiologic changes lead to an increase in renal plasma flow and glomerular filtration rate.3

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Peripheral vasodilation occurs prior to full placentation accompanied by activation of the renin-angiotensin-aldosterone system (RAAS),3 as evidenced by an increase in nitric oxide concentration, plasma renin activity, and plasma aldosterone level. By 8 weeks a significant rise in end-diastolic volume, stroke volume, and cardiac output has been observed.4 At term, cardiac output has increased by 43%, with a 17% increase in pulse rate, 21% decrease in systemic vascular resistance and a 14% decrease in colloid oncotic pressure and a return of blood pressure to prepregnancy level3,5 (Table 1-1).

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Table Graphic Jump Location
Table 1-1.Central Hemodynamic Changes at 36 to 38 Weeks Gestational Agea
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Change in Intravascular Volume

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A 30% increase in blood volume, about 1200 mL, occurs between the 8th and 32nd weeks of pregnancy,6 with the majority of the increase occurring by 24 weeks’ gestation. Only a slight increase is seen between the 24th and 32nd weeks, with a slight decline thereafter. The timing to peak volume and in absolute increase varies by individual; however, percentage changes are consistent (Figures 1-1 and 1-2).

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Figure 1-1.

Change in liters over time in maternal blood volume from 8 weeks’ gestation to 4 weeks’ postpartum. (Adapted from Gemzell CA, Robbe H, Sjostrand T.)6

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