Nearly six million pregnancies are reported in the United States every year. A number of characteristics of the pregnant population appear to be changing over time. For example, women are becoming pregnant later in their reproductive lives. In addition, with advances in medicine and assisted reproductive technology, women with chronic medical conditions are now able to become pregnant. Consequently, pregnant women are now older and have more comorbid conditions than in past generations. Given the physiologic changes that occur during gestation, pregnancy can be an important stressor to chronic medical conditions. Notably, the health of the pregnancy may be a window into the mother’s long-term health.
Labor and delivery constitute a major hemodynamic and respiratory challenge; hence, women with both acute and chronic conditions need to be managed by a multidisciplinary healthcare team, which may include a pulmonologist. This multidisciplinary team should address medical and obstetrical concerns, along with risks and benefits of anesthesia and analgesia, thereby ensuring appropriate management and anticipation of potential complications.
This chapter addresses core pulmonary and cardiovascular physiologic concepts before consideration of important, common clinical entities related to diagnosis and management in pregnancy.
The Physiology of Pregnancy
While the normal physiologic changes of pregnancy are extensive, several areas are of central importance and are considered below. These include changes in respiratory and cardiovascular physiology, normal physiologic developments during labor and delivery, and determinants of fetal oxygenation and ventilation.
During pregnancy, the respiratory system undergoes changes which range anatomically from the nasopharynx to the lungs.
The effects of increasing levels of estrogen on the nasal mucosa include edema, hyperemia, and glandular hypersecretion. The result is gestational rhinitis, which typically occurs in the last few weeks of pregnancy.
The subcostal angle of the rib cage and the circumference of the chest wall increase due to the effects of relaxin.1 The diaphragm moves cranially about 4 to 5 cm.2 These changes occur to accommodate the growing uterus. Diaphragm excursion does not decrease in pregnancy, despite the changes in chest wall configuration. However, due to the higher resting position of the diaphragm, the decreased downward pull of the abdomen, and the aforementioned chest wall changes, functional residual capacity (FRC) decreases by 20% to 30% by late gestation3 and declines further in the supine position (Table 97-1).4 The reduction in FRC may be somewhat attenuated when measured using body plethysmography,5 likely due to the effect of early airway closure during tidal breathing and air trapping.6 Inspiratory capacity increases and, therefore total lung capacity (TLC) is not significantly changed (Fig. 97-1).7
Changes in lung volumes in pregnancy. (Reproduced with permission from Hegewald MJ, Crapo RO. Respiratory physiology in pregnancy. Clin Chest Med. 2011;32(1):1–13.)