1: Physiologic Changes in Pregnancy

CARDIOVASCULAR ADAPTATION

Changes in Hemodynamic Values

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 level^3,5 (Table 1-1).

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).