A decrease in systemic vascular resistance accompanies the increase in cardiac output. Blood pressure during pregnancy is often notable for a rise in pulse pressure due to an unchanged systolic pressure accompanied by a decrease in diastolic pressure. Perhaps the most striking change in respiratory physiology during pregnancy is an increase in resting minute ventilation, which rises by nearly 50 percent at term. Increased levels of progesterone during pregnancy are thought to be responsible for the rise in ventilation above that explained by the enhanced metabolic requirements. As a result of the progesterone-induced increase in alveolar ventilation, arterial PCO2 falls to a plateau of 27 to 32 mmHg during pregnancy.
Maternal oxygenation is preserved during pregnancy.
Interpretation of the arterial PO2 must take into account the corresponding level of PCO2, which is generally accomplished most easily by calculation of the alveolar-arterial oxygen difference. This symptom commonly starts during the first or second trimester, before it can be explained by an increase in abdominal girth. The mechanism of dyspnea during normal pregnancy is not entirely clear. It is likely that progesterone-induced hyperventilation is at least partially responsible, perhaps due to the increase in ventilation above the level needed to meet the rise in metabolic demand. When a pregnant woman complains of dyspnea, distinguishing between underlying disease and progesterone-induced hyperventilation can be a difficult diagnostic problem.