There are some clinically important changes in respiratory anatomy and physiology during pregnancy.
The upper respiratory tract becomes edematous, predisposing pregnant women to nasal stuffiness, nosebleeds, and postnasal drip.
The chest cavity widens at the base and the diaphragm rises about 4 cm from its pre-pregnancy position. Despite these changes, there is no change in the basic airway mechanics.
Pregnancy represents a state of mild hyperventilation:
- The respiratory rate remains unchanged, but
- The tidal volume increases 40%, leading to an increase minute ventilation.
- This increase in minute ventilation leads to an increase in PO2 from 100 to 110, and a decrease in PCO2 from 40 to 30. More oxygen is being brought in and more CO2 is being blown off.
- The decrease carbon dioxide causes a mild respiratory alkalosis, which in turn is tempered by an increased renal excretion of bicarbonate, leaving the arterial pH only slightly alkalotic, between 7.40 and 7.45.
Most cases of reactive airway disease or asthma occurring during pregnancy are pre-existing. New onset asthma is occasionally seen, but is not common. When it does occur, the diagnosis is made on the basis of history, physical, and response to empiric therapy, since methacholine testing is not recommended during pregnancy.
In broad strokes, about a third of asthmatics will get worse during pregnancy, about a third will get better, and the remaining third will remain about the same as they were prior to pregnancy.
Should it get worse, the worsening is usually seen early in the 3rd trimester. Should it get better, the improvement is generally gradual throughout pregnancy. Acute exacerbations are uncommon during labor and delivery.
Asthma is associated with significant maternal and fetal problems, including:
- Increased perinatal mortality
- Low birth weight
- Fetal growth restriction
- Placental abruption
Management consists of three goals:
- Prevention of acute exacerbations
- Early intervention to thwart acute exacerbations
- Monitoring of the mother and fetus for complications
Prevention includes reducing or eliminating such asthmatic triggers as smoking, environmental factors and allergens. In addition, we frequently use medium-dose inhaled glucocorticoids on a daily basis to keep the airway quiet. Some patients may also require a daily, long acting beta agonist, such as Solu-Medrol, to suppress airway reactivity.
Early intervention means encouraging the patient to identify worsening of her condition with prompt intervention with an inhaled beta agonist, usually albuterol. Fever may indicate a superimposed respiratory infection requiring antibiotics and should not be ignored.
Monitoring of the mother includes monthly visits and twice daily outpatient measurements of Peak Expiratory Flow Rate. PEFR meters are inexpensive and each pregnant asthmatic patient should have her own at home.
Monitoring of the pregnancy will include weekly non-stress testing of the fetus, and serial ultrasound measurements of fetal growth.
During labor and delivery, some commonly-used medications should be avoided because of their tendency to cause bronchoconstriction:
- Prostaglandin F2-alpha
Other medications pose no significant threat to the pregnant asthmatic and some have a bronchodilating effect. Safe drugs for use in labor and delivery include:
- Prostaglandin E2
- Epidural anesthesia
- Magnesium Sulfate
Dilaudid, an opioid narcotic during labor, has been used with good success during labor with many asthmatic patients, but causes a mild histamine release, which theoretically could provoke or aggravate a reactive airway. If used in this setting, airway monitoring should be particularly thorough.
Pulmonary infections occurring during pregnancy need special attention to prevent hypoxemia, and reduce fever. Maternal fever accelerates fetal enzyme systems significantly increasing fetal oxygen requirements.
Appropriate antibiotics, combined with antipyretics, nasal decongestants, and oxygen supplementation as needed to maintain oxygen saturation above 95% may be needed in the case of bacterial airway infections.
Seasonal influenza poses a greater risk to the pregnant woman than to her non-pregnant friends. For that reason, we usually give oseltamivir (Tamiflu) 75 mg bid to those women felt clinically to have the flu, even prior to laboratory confirmation.