Respiratory System

To accommodate the increased oxygen demand and requirement for carbon dioxide elimination, pregnancy is associated with an increase in the respiratory minute volume and work of breathing. Because of difficulties in performing clinical research on pregnant women, few investigations of respiratory changes in pregnancy have been conducted. Unfortunately, many of the findings quoted in the literature are inconsistent and often based on older techniques that were applied to a very limited number of subjects.^[19]

The most impressive change in maternal lung dynamics is a decrease in functional residual capacity (FRC), which at term may have changed by as much as 20% of prepregnancy values. Minute ventilation increases by 45%, primarily as a result of an increase in tidal volume because the respiratory rate is essentially unchanged. Hormonal changes and an increase in the rate of carbon dioxide production are responsible for the increase in ventilation. Progesterone sensitizes the respiratory center to carbon dioxide. PaCO₂ falls to approximately 30 mm Hg by the 12th week of gestation, and it remains at this level for the remainder of pregnancy. Tidal volume increases by 50%, with half of this increase occurring during the first trimester. The parturient's breathing pattern changes; it becomes more diaphragmatic as pregnancy progresses because of the effects of the gravid uterus and limitation of thoracic cage movement. Closing capacity (CC), however, remains unchanged. The resulting decrease in the FRC/CC ratio causes faster small-airway closure when lung volume is reduced; thus, parturients can desaturate at a much faster rate than nonpregnant women can. The rapid development of hypoxia as a result of decreased FRC, increased oxygen consumption, and airway closure may be minimized by administration of 100% oxygen for 3 to 5 minutes before the induction of anesthesia. In an emergency setting, four maximal capacity breaths with 100% oxygen should be sufficient.

Other changes in the respiratory tract and oropharynx during pregnancy may have profound anesthetic implications. Capillary engorgement of the mucosa and edema of the oropharynx, larynx, and trachea may result in a difficult intubation. Any manipulation of the upper airway such as suctioning, insertion of airways, or laryngoscopy may cause edema, bleeding, and upper airway trauma. Because of the particularly friable mucosa of the nasopharynx, instrumentation of the nose should be avoided if at all possible. When performing intubation of a pregnant patient, a smaller than usual endotracheal tube (size 6.0 to 7.0) should be used and repeated attempts at laryngoscopy minimized. Management of a difficult airway will be discussed later in the chapter.