1. Describe how pregnancy changes pulmonary mechanics.

In normal pregnancy, the shape of the thoracic cage is changed. The diaphragm is displaced superiorly (up to 4 cm); the transverse diameter of the chest increases (up to 2 cm); and the overall chest circumference increases 5-7 cm. As a result of these changes, there is an increase in the subcostal angle.

2. How does pregnancy change lung volumes and pulmonary physiology?

Tidal volume increases 30-40% while expiratory reserve volume decreases about 20%. Vital capacity, inspiratory reserve volume, and respiratory rate remain essentially stable. Overall lung volume is decreased (5%) due to elevation of the diaphragm. Residual capacity is also reduced, approximately 20%. Inspiratory capacity increases slightly. Large airway function is preserved as suggested by stable airflow measurements (forced expiratory volume in 1 second and and forced vital capacity).

3. Describe the pulmonary changes in gas exchange associated with pregnancy.

Pregnancy is associated with an increase in oxygen consumption (up to 30%) and increase in minute ventilation (30-40%). Pregnancy is associated with an increase in arterial oxygen levels and a decrease in arterial carbon dioxide (PaCO2) levels. PaCO2 decreases from 40 mmHg (nonpregnant) to 27-32 mmHg (last half of pregnancy), and this important change aids in the transfer of CO2 from the fetal to the maternal circulation. Despite the fall in maternal PaCO2, maternal arterial pH is maintained by an increase in renal excretion of bicarbonate, which leads to a lower maternal serum bicarbonate concentration.

4. What is dyspnea of pregnancy?

Most pregnant women (up to 70%) complain of dsypnea. Progesterone levels are increased in pregnancy, and progesterone appears to stimulate the respiratory center. While the exact etiology is unknown, the combination of increased progesterone levels and decreased PaCO2 may play a role in this sensation. Despite this being a common complaint, it is important to evaluate the patient for pathologic causes of dyspnea.

5. Does pregnancy affect asthma?

Asthma, which affects about 1% of all pregnancies, is the most common obstructive disease encountered during pregnancy. Approximately 10% of those with asthma have exacerbations requiring admission. Recent information suggests that during pregnancy up to 70% of patients with asthma show improvement, approximately 20% show no change, and up to 10% experience worsening symptoms. Evidence suggests that in adolescents with asthma, those who become pregnant experience a higher percentage of exacerbations, and over 50% may require systemic steroids. Noncompliance with treatment and respiratory tract infections are factors associated with asthma flares.

6. Does asthma affect pregnancy?

The evidence concerning an increase in adverse perinatal outcome in patients with asthma is conflicting. A recent case-control study revealed that perinatal mortality is not affected in well-controlled and actively managed asthmatics.

7. How should the pregnant asthmatic be treated?

The goals of therapy in pregnant asthmatics include adequate oxygenation for both mother and fetus and reduction of attacks (including prevention of status asthmaticus). In general, asthmatic patients should follow their peak flow measurements and seek therapy, or make additions to their therapy, when their peak flow falls below 80% of baseline measures. Over the last few years, the treatment of asthma in pregnancy has changed. There is less reliance on the use of aminophylline. Pregnancy usually requires an increase in the dose of aminophylline, and toxicity is exhibited with nausea, vomiting, and even cardiac arrhythmias.

The main therapy for asthma in pregnancy, as well as in those who are not pregnant, is inhaled beta-agonists (bronchodilator), either on a scheduled or as-needed basis. Inhaled steroids can also be used with beta-agonists. Cromolyn (prevents mast cell degranulation) can also be used in pregnancy, although it typically is not used alone. If these agents are not adequate, systemic steroids (those with less mineralocorticoid activity, such as prednisone or methylprednisolone), either orally or parentally, may be used. As for safety in pregnancy, these medications do not appear to increase the risk of fetal malformations. Use of systemic steroids rarely causes fetal adrenal suppression, given that only 10-30% of the maternal dose enters the fetal compartment because of degradation by placental hormones. In general, if a patient has taken systemic steroids during the course of the pregnancy, she should receive intravenous steroids during labor and for up to 24 hours after delivery.

Some prostaglandins can increase airway resistance. Therefore, they should be used judiciously in a pregnant patient with asthma.

8. How should status asthmaticus be treated in pregnancy?

Status asthmaticus is a severe asthma exacerbation in which oxygenation becomes difficult despite therapy. Patients with this complication require immediate attention, and in general care is the same as in nonpregnant patients with status asthmaticus. Therapy includes humidified oxygen (30-40%), nebulized beta-agonists, subcutaneous catecholamines (either epinephrine or terbutaline), and intravenous steroids if the patient does not respond to subcutaneous catecholamines. If the patient does not respond with these measures, and it is difficult to maintain adequate oxygenation, intubation becomes necessary. During treatment, fetal monitoring should be used (if the patient is more than 24 weeks pregnant), as well as assessment of maternal oxygenation to guide effectiveness of therapy.

9. Does pregnancy increase the risk for pulmonary embolism?

Compared to nonpregnant women, the risk of thromboembolism in pregnancy is increased approximately fivefold. Between 0.5 and 3 of every 1000 pregnancies are complicated by symptomatic venous thrombosis. If untreated, up to 25% develop a pulmonary embolism, of which up to 15% are fatal. Pulmonary embolism remains a leading cause of maternal mortality in the United States and worldwide. Pregnancy increases the risk for thrombosis because of an increase in many clotting factors. Up to 50% of women who develop a thrombosis have an acquired or congenital thrombophilia (5% carry the factor V Leiden mutation, and 2% carry the prothrombin gene mutation G20210A).

10. How is a thromboembolic disease diagnosed?

Patients can present with tachypnea, tachycardia, shortness of breath, and/or chest pain. Physical exam may reveal a pleural rub, and there may be apparent ECG abnormalities. Chest x-ray often is unremarkable, and there usually is a decrease in PO2 on an arterial blood gas. Studies used to make the diagnosis include impedence plethysmography (positive predictive value 83%), Doppler ultrasound with compression (positive predictive value 93%), and even magnetic resonance imaging. To diagnosis a pulmonary embolism, a ventilation and perfusion (V/Q) scan (radiation dose to the fetus of less than 0.05 rads) can be used if the chest x-ray is normal. If necessary, selective pulmonary angiography should be performed. In experienced hands and with abdominal shielding, the radiation exposure to the fetus approaches that of a V/Q scan.

Key points: pulmonary disease in pregnancy

  • Pregnancy changes pulmonary physiology due both to physical changes in maternal body (upward displacement of diaphragm) and necessary changes in gas exchange (increased oxygen consumption, decreased carbon dioxide).
  • Treatment of asthma in pregnancy is essentially the same as in nonpregnant women.
  • Pulmonary embolism is the leading cause of maternal mortality in the U.S.
  • Tuberculosis infection in pregnancy should be treated with multi-agent therapy, but treatment of a newly positive PPD with negative chest x-ray may be delayed until after delivery.
  • Most women with sarcoidosis have improvement of symptoms during pregnancy but may relapse postpartum.
  • Amniotic fluid embolism is a rapid unpredictable event that is treated with supportive care of respiratory and hematologic abnormalities.

11. How should thromboembolic disease in pregnancy be treated?

Anticoagulation is the recommended treatment for thromboembolic disease. As in nonpregnant patients, therapy is initiated with heparin. However, because warfarin crosses the placenta and causes embryo toxicity and malformations, heparin is continued after initial therapeutic anticoagulation. Anticoagulation is continued throughout the pregnancy and for up to 3 months postpartum. Heparin is associated with decreased bone mineralization, and therefore patients should be treated with additional calcium supplementation. Furthermore, some patients can experience heparin-induced thrombocytopenia.

Experience is accumulating on the use of low-molecular-weight heparin in pregnancy. It is associated with fewer side effects than heparin and can be used with once or twice a day dosing. However, it is more expensive and should be considered when a patient has a complication with heparin.

12. What organisms are associated with pneumonia in pregnancy?

Streptococcus pneumoniae is the most common organism associated with bacterial pneumonia in pregnancy. Other causative bacterial organisms include Haemophilus influenzae and Klebsiella pneumoniae. Other causative agents include influenza A, varicella, and mycoplasma. Pregnant women with pneumonia, especially those with other medical comorbidities, are at greater risk for intubation, other maternal complications, and preterm delivery. Pregnant patients with varicella pneumonia have a significantly increased risk of mortality and of adverse perinatal outcome compared to nonpregnant patients with varicella pneumonia.

13. How is tuberculosis diagnosed in pregnancy?

Mycobacterium tuberculosis is an acid-fast bacillus that causes the pulmonary infection tuberculosis. Recently, tuberculosis has been increasing in the U.S., especially in urban areas, due to increased immigration from developing nations. Patients at risk should be screened by skin testing with a purified protein derivative (PPD). Most women with tuberculosis are asymptomatic, and active disease will develop in less than 10% of those with a positive PPD who are not immunocompromised. Patients who have received vaccination with bacillus Calmette-Guérin will have a positive PPD. If the diagnosis is suspected, a chest x-ray with abdominal shielding should be performed. Definitive diagnosis is made by sputum culture for M. tuberculosis or specific staining. Culture is extremely important given the increase in resistant strains of M. tuberculosis.

14. Describe the treatment of tuberculosis in pregnancy.

Pregnancy does not affect the course of tuberculosis, and adequate treatment does not appear to adversely affect pregnancy outcome. If the diagnosis is confirmed, treatment should be initiated during pregnancy with isoniazid and rifampin. Isoniazid treatment should be given in conjunction with pyridoxine to decrease the risk of peripheral neuropathy. Furthermore, liver function tests should be followed, as liver transaminases will elevate in up to 20% of patients.

Prophylaxis for women with a newly positive PPD during pregnancy is controversial. Treatment may be delayed until the postpartum period. The risk of congenital tuberculosis is small, and perinatal infections can be reduced with treatment of the newborn for the duration of maternal treatment.

15. What is sarcoidosis? Is it a contraindication to pregnancy?

Sarcoidosis is a noncaseating granulomatous disease of unknown etiology. It is diagnosed most commonly in the third and fourth decade of life and can affect the lungs, heart, lymph nodes, skin, central nervous system, liver, and eyes. It is usually diagnosed on routine chest x-ray in asymptomatic patients and requires biopsy to make the diagnosis.

Sarcoidosis occurs in approximately 0.05% of all pregnancies, and it does not appear to adversely affect pregnancy or be affected by pregnancy. Most pregnant women with sarcoidosis experience improvement in their symptoms, but relapses can occur in the postpartum period. When patients with sarcoidosis become pregnant, they should be screened for hepatic and renal involvement. Steroids are used for progression of pulmonary complications related to sarcoidosis.

16. What is cystic fibrosis? Can women with cystic fibrosis consider pregnancy?

Cystic fibrosis is an autosomal recessive disease that affects mucous glands in the respiratory, digestive, and reproductive tracts. Old data concerning pregnancy in women with cystic fibrosis reported a high maternal mortality rate and perinatal mortality rate. However, with careful management by a team including a perinatologist, a pulmonologist, a cardiologist, and a nutritionist, successful pregnancy can be realized in women with cystic fibrosis.

Women with cystic fibrosis are at greater risk for pulmonary infections and must have their pulmonary and cardiac status assessed constantly throughout gestation for evidence of deterioration. Given pancreatic dysfunction and malabsorption in these patients, special attention needs to be given to their nutritional status. There is a greater risk for intrauterine growth restriction.

17. What is an amniotic fluid embolism?

Fortunately, amniotic fluid embolism is a rare complication of pregnancy. It is an unpredictable event with no clear risk factors that cannot be prevented. While it most commonly occurs during labor, it can occur at any time during gestation (including after a second-trimester abortion) and up to 48 hours postpartum. Amniotic fluid embolism presents with cardiovascular collapse, and a significant portion of patients have seizures at the time of presentation. Cardiovasular compromise probably arises from pulmonary vasculature obstruction, and with resulting inflammatory reactions, disseminated intravascular coagulation can develop.

18. Describe the treatment of amniotic fluid embolism.

Care is supportive, with initial attention paid to stabilize maternal oxygenation and hemodynamic function. Anemia and coagulation abnormalities are corrected with appropriate transfusion. The fetal status should be assessed, as the timing and manner of fetus delivery could compromise maternal resuscitation. Maternal mortality is approximately 50%. Even if the mother survives, neurologic status may be adversely affected from the initial hypoxia, hypotension, and seizure activity.

Diagnosis can be confirmed at autopsy with the finding of fetal squamous cells in the pulmonary vasculature. However, this finding is neither specific nor sensitive.

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