Final Jeopardy
These hemodynamic changes are expected to in the second trimester of pregnancy
What is an increase in CO and decrease in SVR?
Pregnancy requires significant physiologic and hemodynamic changes to maintain uteroplacental circulation and support the increased metabolic needs of the mother and fetus. In the first trimester, CO increases and SVR decreases as early as 5 weeks after conception. CO progressively increases throughout the second trimester and peaks by the beginning of the third trimester.
The changes in CO result from: 1) increased heart rate throughout pregnancy (mean 10-30 bpm); 2) increased preload due to a rise in blood volume; and 3) decreased afterload (i.e., SVR). Interestingly, the reduction in SVR and pulmonary vascular resistance maintain the preconception central venous pressure throughout pregnancy despite the increased preload. Systolic and diastolic blood pressure decrease early in pregnancy but rise again toward the latter stages of the third trimester. It is important to note that the reduction in SVR is vital to maintaining a low-resistance, high-flow circuit to maintain adequate uteroplacental circulation. Proposed mechanisms for the fall in SVR include a reduction in aortic stiffness and increased production of prostacyclin and nitric oxide.