Cardiopulmonary Physiology in Pregnancy
- Profound changes occur in the cardiovascular system early in pregnancy. By the
early second trimester, circulating blood volume increases 40-50%. This is due to an
increase in both the circulation red cell mass and an even larger increase in the plasma
- The larger increase in plasma volume leads to a dilutional anemia and a decrease
in the serum colloid oncotic pressure.
- These changes increase the susceptibility of pregnant patients to the development
of pulmonary edema.
- The cardiac output also increases by about 30-45% by the early second trimester.
In patients with underlying cardiac disease this further worsens the tendency toward
- Gas exchange is also affected by pregnancy. Minute ventilation is increased
during pregnancy (primarily an increase in tidal volume with a normal respiratory rate)
for 2 reasons. First, oxygen consumption and carbon dioxide production increase 20-30% by
the third trimester and up to 100% during labor, necessitating increased minute
ventilation to maintain normal acid base status. In addition, progesterone directly
stimulates the central respiratory center causing a further increase in minute
ventilation. The net effect is a mild chronic respiratory alkalosis with a decrease in the
arterial PaCO2, a slight increase in the PaO2 (alveolar gas equation), a slightly elevated
pH and a slightly decreased HCO3 (renal compensation).
- FRC decreases substantially during pregnancy due to increased pressure from the
gravid abdomen. This results in an increased susceptibility to atelectasis especially in
the supine position. This may lead to mild arterial hypoxemia if blood gases are measured