- Physiologic adaptations in the mother occur in response to demands created by pregnancy. These include:
1 support of the fetus (volume, nutritional and oxygen support, clearance of fetal waste)
2 protection of the fetus (from starvation, drugs, toxins)
3 preparation of the uterus for labor
4 protection of the mother from potential cardiovascular injury at delivery.
- Maternal age, ethnicity, genetic factors, and maternal comorbidities affect the ability of the mother to adapt to pregnancy.
- All maternal organ systems are required to adapt to the demands of pregnancy. The quality, degree, and timing of the adaptation vary from one individual to another and from one organ system to another.
Respiratory system (Figure 38.1)
- Respiratory adaptations during pregnancy are designed to optimize maternal and fetal oxygenation, and to facilitate transfer of CO2 waste from the fetus to the mother.
- Many pregnant women report a subjective perception of shortness of breath (dyspnea) in the absence of pathology. The reason for this is unclear.
- The mechanics of respiration change with pregnancy. The ribs flare outward and the level of the diaphragm rises 4 cm.
- During pregnancy, tidal volume increases by 200 mL (40%), resulting in a 100–200 mL (5%) increase in vital capacity and a 200 mL (20%) decrease in the residual volume, thereby leaving less air in the lungs at the end of expiration. The respiratory rate does not change. The end-result is an increase in minute ventilation and a drop in arterial PCO2 (see table below). Arterial PO2 is essentially unchanged. A compensatory decrease in bicarbonate enables the pH to remain unchanged. Pregnancy thus represents a state of compensated respiratory alkalosis.
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