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CARDIOVASCULAR ADAPTATION

Changes in Hemodynamic Values

Cardiac Output = Heart Rate × Stroke Volume

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.

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

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).

Table 1-1.Central Hemodynamic Changes at 36 to 38 Weeks Gestational Agea

Change in Intravascular Volume

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).

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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