Pulmonary unction changes are more pronounced in asthmatics compared with healthy women (Zairina, 2015). Asthma manifestations range from mild wheezing to severe bronchoconstriction, which obstructs airways and decreases airflow. his lowers the forced expiratory volume in 1 second/forced vital capacity (FEVJFVC) ratio and the peak expiratory flow (PEF). The work ofbreathing progressively increases, and patients note chest tightness, wheezing, or breathlessness. Subsequent alterations in oxygenation primarily reflect ventilation-perfusion mismatching because the distribution of airway narrowing is uneven.
Varied manifestations of asthma have led to a simple classiication that considers severity, onset, and duration of symptoms (Table 51-1). With persistent or worsening bronchial obstruction, clinical stages progress as shown in Figure
Hypoxia initially is mitigated by hyperventilation, which maintains arterial P02 within a normal range but lowers Peo2' creating respiratory alkalosis. As airway narrowing worsens, ventilation-perusion defects increase, and arterial hypoxemia ensues. With severe obstruction, ventilation becomes impaired as fatigue causes early CO2 retention. Because ofhyperventilation, this may only be seen initially as an arterial Peo2 returning to the normal range. With continuing obstruction, respiratory failure follows from fatigue.
Although these changes are generally reversible and well tolerated by the healthy nonpregnant individual, even early asthma stages may be dangerous for the pregnant woman and her fetus. This is because smaller functional residual capacity and increased pulmonary shunting render the woman more susceptible to hypoxia and hypoxemia.
Effects of Pregnancy on Asthma
Pregnancy has an unpredictable efect on underlying asthma. In their review of six prospective studies of more than 2000 gravidas, Gluck and Gluck (2006) reported that approximatelyaa third each improved, remained unchanged, or clearly
TABLE 51-1. Classification of Asthma Severity
FIGURE 51-1 Clinical stages of asthma. FEV1 = forced expiratory volume in 1 second.
worsened. Exacerbations are more common with severe disease (Ali, 2013). In a study by Schatz and associates (2003), baseline severity correlated with asthma morbidity during pregnancy. With mild disease, 13 percent of women had an exacerbation and 2.3 percent required admission; with moderate disease, these numbers were 26 and 7 percent; and for severe asthma, 52 and 27 percent. Others have reported similar observations (Charlton, 2013; Hendler, 2006). Finally, morbidity rates are disproportionately increased in black compared with white women.
Up to 20 percent of women with mild or moderate asthma have been reported to have an intrapartum exacerbation (Schatz, 2003). Conversely, Wendel and associates (1996) reported exacerbations at the time of delivery in only 1 percent of women.
Mabie and coworkers (1992) reported an 18-fold increased exacerbation risk following cesarean versus vaginal delivery.
Women with asthma have had improved pregnancy outcomes during the past 20 years. he incidence of spontaneous abortion in women with asthma may be slightly increased (Blais, 2013).