Maternal Physiology: Respiratory System

Respiratory System

The changes that occur in the respiratory system during pregnancy are necessitated by the increased oxygen demand of the mother and fetus. These changes are primarily medi-ated by progesterone.

ANATOMIC CHANGES

The maternal thorax undergoes several morphologic changes due to pregnancy. The diaphragm is elevated approximately4 cm by late pregnancy due to the enlarging uterus. Additionally, the subcostal angle widens as the chest diam-eter and circumference increase slightly (see Figure 5.1).

FUNCTIONAL CHANGES

Pregnancy is associated with an increase in total body oxygen con-sumption of approximately 50 mL O₂/min, which is 20% greater than nonpregnant levels. Approximately 50% of this increaseis consumed by the gravid uterus and its contents, 30% by the heart and kidneys, 18% by the respiratory muscles, and the remainder by the mammary tissues.

Functional adaptations in the pulmonary system enhance oxygen delivery to the lungs. Figure 5.2 lists res-piratory volumes and capacities associated with pregnancy. The consequence of diaphragmatic elevation is a 20% reduction in the residual volume and functional residual capacity plus a 5% reduction in total lung volume. Although the maternal respiratory rate is essentially unchanged, there is a 30% to 40% increase in tidal volume due to a 5% increase in inspiratory capacity, resulting in a 30% to 40% increase in minute ventilation.

This significant increase in minute ventilation dur-ing pregnancy is associated with important changes in the acid–base equilibrium. Progesterone causes increased central chemoreceptor sensitivity to CO₂, which results in increased ventilation and a reduction in arterial PCO₂. The respiratory alkalosis that results from a decreased arterial PCO₂ in pregnancy is compensated by increased renal excre-tion of bicarbonate, yielding normal pregnancy bicarbon-ate levels, which means that maternal arterial pH is normal.

SYMPTOMS

Although airway conductance and total pulmonary resis-tance are reduced in pregnancy, dyspnea is common in pregnant women. Dyspnea of pregnancy is believed to be a physiologic response to a low arterial PCO₂. Allergy-like symptoms or chronic colds are also common. Mucosal hyperemia associated with pregnancy results in marked nasal stuffiness and an increased amount of nasal secretions.

PHYSICAL FINDINGS

Despite the anatomic and functional changes in the respi-ratory system during pregnancy, no significant changes in the pulmonary examination are apparent.

DIAGNOSTIC TESTS

Arterial blood gas assessment during pregnancy normally shows a compensated respiratory alkalosis.

Arterial PCO₂ levels of 27 to 32 mm Hg and bicarbonate levels of 18 to 31 mEq/L should be considered normal.Maternal arterial pH is maintained at normal levels of 7.40 to 7.45 (see Table 5.1).

During normal pregnancy, chest radiography may demonstrate prominent pulmonary vasculature due to the increased circulating blood volume