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Introduction

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Acute and chronic pulmonary disorders are frequently encountered during pregnancy. Chronic asthma or an acute exacerbation is the most common and affects up to 4 percent of pregnant women. These disorders along with community-acquired pneumonia accounted for almost 10 percent of nonobstetrical hospitalizations in one managed care plan (Gazmararian, 2002). Pneumonia is also a frequent complication requiring readmission postpartum (Belfort, 2010). These and other pulmonary disorders are superimposed on several important pregnancy-induced changes of ventilatory physiology. For example, pregnant women, especially those in the last trimester, do not appear to tolerate severe acute pneumonitis as evidenced by the disparate number of maternal deaths during the 1918 and 1957 influenza pandemics.

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The important and sometimes marked changes in the respiratory system induced by pregnancy are reviewed in Chapter 4 (Respiratory Tract), and values for associated tests can be found in the Appendix (Serum and Blood Constituents). Lung volumes and capacities that are measured directly to assess pulmonary pathophysiology may be significantly altered. In turn, these change gas concentrations and acid-base values in blood. Some of the physiological alterations induced by pregnancy were summarized by Wise and associates (2006):

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  1. Vital capacity and inspiratory capacity increase by approximately 20 percent by late pregnancy.

  2. Expiratory reserve volume decreases from 1300 mL to approximately 1100 mL.

  3. Tidal volume increases approximately 40 percent as a result of respiratory stimulation by progesterone.

  4. Minute ventilation increases 30 to 40 percent due to increased tidal volume. As a result, arterial po2 increases from 100 to 105 mm Hg.

  5. Increasing metabolic demands cause a 30-percent increase in carbon dioxide production, but because of its concomitantly increased diffusion capacity along with hyperventilation, the arterial pco2 decreases from 40 to 32 mm Hg.

  6. Residual volume decreases approximately 20 percent from 1500 mL to approximately 1200 mL.

  7. Chest wall compliance is reduced by a third by the expanding uterus and increased abdominal pressure, which causes a 10- to 25-percent decrease in functional residual capacity—the sum of expiratory reserve and residual volumes.

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The end result of these pregnancy-induced changes is substantively increased ventilation due to deeper but not more frequent breathing. These are thought to be stimulated by basal oxygen consumption as it increases incrementally from 20 to 40 mL/min in the second half of pregnancy.

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Asthma

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Asthma is seen frequently in young women and therefore often complicates pregnancy. Asthma prevalence increased steadily in many countries beginning in the mid-1970s but may have plateaued in the United States, with a prevalence in adults of approximately 10 percent (Barnes, 2012; Centers for Disease Control and Prevention, 2010c). The estimated asthma prevalence during pregnancy ranges between 4 and 8 percent, and this appears to be increasing (Kwon, 2006; Namazy, 2005). Finally, evidence is accruing that fetal as well as neonatal environmental ...

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