Self-Assessment Questions

20. Maternal adaptations to pregnancy: I

• One of your pregnant patients, M.S., comes to clinic for some blood tests as she has been feeling acutely short of breath for the past week. You opt to take an arterial blood gas to check her blood oxygen status. Assuming she only has a mild respiratory infection, what do you expect her results to be? Why do they appear like this during pregnancy?

  Show Answer

  Correct answer:
  If M.S. only has a mild respiratory infection, we can expect that her ABG values will be normal for a pregnant woman. This means she will have a mild respiratory alkalosis, with slightly decreased bicarbonate levels. These changes are due to the effects of elevated progesterone. The decreased bicarbonate levels shift the oxygen dissociation curve to the left for maternal blood, increasing the affinity of maternal hemoglobin for oxygen. As fetal hemoglobin binds oxygen at a lower partial pressure, it is more apt to extract oxygen from maternal blood. These effects ensure that the fetus obtains enough oxygen from maternal blood, although at a slight cost to the mother. Maternal tissues are slightly less perfused due to the left shift in the oxygen dissociation curve.