Respiratory Physiology in Pregnancy
Section snippets
Upper airway changes in pregnancy
There are significant changes to the mucosa of the nasopharynx and oropharynx during pregnancy. The mucosal changes in the upper airway include hyperemia, edema, leakage of plasma into the stroma, glandular hypersecretion, increased phagocytic activity, and increased mucopolysaccharide content.2, 3 All of these result in nasal congestion often called rhinitis of pregnancy. The clinical definition of rhinitis of pregnancy is “nasal congestion present during the last 6 or more weeks of pregnancy
Chest wall changes in pregnancy
The thorax undergoes significant structural changes in pregnancy: The subcostal angle of the rib cage and the circumference of the lower chest wall increase and the diaphragm moves up.15, 16, 17, 18 These changes are necessary to accommodate the enlarging uterus and increasing maternal weight, but the changes occur early in pregnancy before the uterus is significantly enlarged.15, 18 Hormonal changes rather than the mechanical effects of the enlarging uterus cause relaxation of the ligamentous
Respiratory muscle function
There is no significant change in respiratory muscle strength during pregnancy despite the cephalad displacement of the diaphragm and changes in the chest wall configuration. Maximal inspiratory and expiratory mouth pressures and maximum transdiaphragmatic pressure, measured as gastric pressure minus esophageal pressure, in late pregnancy and after delivery show no significant changes.15, 20 Despite the upward displacement of the diaphragm by the gravid uterus, diaphragm excursion actually
Static Lung Function
Static lung function stays the same in pregnancy except for decreases in functional residual capacity (FRC) and its components: expiratory reserve volume (ERV) and residual volume (RV). FRC depends on 2 opposing forces: the elastic recoil of the lungs and the outward and downward pull of the chest wall and abdominal contents. A reduction in FRC in pregnancy is expected given the 4-cm elevation of the diaphragm, decreased downward pull of the abdomen, and changes in chest wall configuration that
Ventilation and gas exchange
There is a significant increase in resting minute ventilation (VE) during pregnancy. At term, VE is increased by 20% to 50% compared with nonpregnant values.15, 23, 25, 28, 52, 53, 54, 55 The increase in VE is associated with a 30% to 50% (from approximately 450 to 650 mL) increase in tidal volume with no change or only a small increase (1–2 breaths per minute) in respiratory rate. While VE increases in all studies, the time course of the increase is variable. Some studies reveal a progressive
Cardiovascular changes in pregnancy
Significant cardiovascular changes during the course of pregnancy affect respiratory physiology. These changes include increased plasma volume, increased cardiac output, and reduced vascular resistance. The adaptations begin early in pregnancy, and are critical in meeting the increasing metabolic demands of the mother and fetus and in tolerating the acute blood loss that occurs with childbirth. The cardiovascular changes with pregnancy are listed in Table 2.
Exercise physiology in pregnancy
Moderate aerobic exercise during pregnancy appears to be safe for the mother and fetus, and may improve some pregnancy outcomes such as gestational diabetes and preeclampsia.88, 89, 90, 91 Most studies addressing the physiologic response to exercise in pregnancy used submaximal, constant load exercise protocols to avoid potential risk to the fetus.52, 56, 67, 69, 89, 92, 93
At moderate intensity of exercise, pregnant women respond differently to nonpregnant women. The increase in Vo2 for a given
Respiratory physiology in labor, delivery, and postpartum
Hyperventilation, beyond the usual pregnancy-mediated increase in minute ventilation, is common during labor and delivery. Several factors interact to influence minute ventilation during labor and delivery. Pain, anxiety, and coached breathing techniques increase minute ventilation whereas narcotic analgesics have the opposite effect. The result is a wide variation in minute ventilation and breathing patterns, as illustrated by a study of 25 patients during labor that found tidal volumes
Cardiovascular physiology in labor, delivery, and postpartum
Labor, delivery and the postpartum period are associated with significant cardiovascular changes. Cardiac output increases 10% to 15% above late pregnancy levels during early labor and by 50% during the second stage of labor.87, 96 The increase in cardiac output is caused by an increase in both heart rate and stroke volume. Factors contributing to the hemodynamic changes during labor include pain and anxiety with resultant increases in circulating catecholamines, and uterine contractions with
Summary
Significant anatomic and physiologic adaptations involving the respiratory and cardiac systems occur during pregnancy and are necessary to meet the increased metabolic demands of both the mother and the fetus. The prominent respiratory changes include: mechanical alterations to the chest wall and diaphragm to accommodate the enlarging uterus; a reduction in FRC and its components ERV and RV, with little or no change in TLC; and an increase in minute ventilation, resulting in reduced Paco2 and
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