Why there is a risk of pulmonary embolism in late pregnancy?

Most pulmonary embolisms in otherwise healthy individuals comes from blood clots formed in the legs, known as deep vein thrombosis (DVT).

Women in late pregnancy are at a greater risk for DVT because their bodies are undergoing changes to protect them during childbirth, including increasing the ability of blood to clot so that blood loss during birth is minimized.

Pulmonary embolism is the leading cause of death of mothers in childbirth in the US.

Signs and symptoms of PE are more problematic because dyspnea and tachypnea are common in pregnancy. In nonpregnant patients, tachypnea, dyspnea, chest pain (pleuritic), apprehension, and crackles are present in at least 50% of patients. Chest radiographs are abnormal in 80% or more of patients with PE, and the findings are nonspecific. Electrocardiogram findings are abnormal in 70% of patients with PE but are nonspecific. Arterial oxygen tension is low in the majority of patients with PE.

Objective diagnostic testing

As with DVT, PE requires objective diagnostic testing to confidently confirm or exclude the diagnosis. This is particularly true in pregnancies because the diagnosis of DVT or PE requires (1) prolonged therapy (<9 mo of heparin during pregnancy), (2) prophylaxis during future pregnancies, and (3) avoidance of oral contraceptive pills.

The first objective diagnostic test should be compression ultrasonography; if it is not available, IPG is adequate. If the findings from noninvasive leg studies are negative, then proceed to ventilation-perfusion lung scanning. Perfusion scanning alone is recommended initially, and the ventilation scan is added when perfusion defects are noted. Pulmonary angiography might be necessary if lung scan findings are of low probability or indeterminate and clinical suspicion remains high.

Several studies show no increased risk of teratogenicity in patients undergoing radiological procedures in the diagnosis of maternal venous thromboembolic disease. A complete and adequate evaluation to document the presence or absence of PE requires less than 0.005 Gy. Obtaining the appropriate diagnostic study in pregnancies is mandatory.

Management

Patients, whether pregnant or not, in whom PE is strongly considered should be treated immediately with intravenous unfractionated heparin unless a high risk or contraindication is present for the use of any anticoagulants. Extensive clinical experience and cohort studies establish heparin as the safest anticoagulant to use during pregnancy because it does not cross the placenta. The initial loading should be 5000-10,000 units. Following loading, an infusion of 18 U/kg should be started. The patient's activated partial thromboplastin time should be monitored and kept in the therapeutic range, which is 1.5-2 times the baseline value.

Although only a relatively modest amount of data have been gathered, low molecular weight heparin, which does not cross the placenta, can be given once a day and does not require monitoring. Low molecular weight heparin has not been shown to increase the risk of bleeding with surgical procedures, including cesarian delivery, in a small number of patients.

Warfarin should be avoided throughout pregnancy. It can result in embryopathy characterized by mental retardation, optic atrophy, cleft lip, cleft palate, and cataracts in addition to hemorrhage. The teratogenic effects are particularly common during the first trimester. Warfarin crosses the placenta; it can cause fetal and neonatal hemorrhage and placental abruption.

Duration of anticoagulation

Patients who sustain a DVT or PE antepartum should receive anticoagulation therapy with heparin throughout the pregnancy. After delivery, warfarin should be started; the heparin can be discontinued once an adequate International Normalized Ratio is achieved. Warfarin should be continued for at least 6 weeks postpartum or until at least 3 months of anticoagulant therapy have been completed.

Complications of treatment

Osteopenia has been reported with unfractionated heparin administered for more than 6 months. No information is available about the beneficial effects of concomitant multivitamins, calcium, or vitamin D supplementation. The problem of osteopenia and osteoporosis might be less with low molecular weight heparin, but providing optimum calcium and vitamin D supplementation is reasonable for all patients receiving long-term heparin administration during pregnancy.

Prophylaxis

In women with a history of VTE during pregnancy, the recurrence rate in a subsequent pregnancy has been estimated to be 4-15%. Women with a known hypercoagulable state are also at increased risk of VTE during pregnancy and should receive prophylaxis. Effective prophylaxis has been demonstrated with unfractionated heparin administered twice daily with an average daily dose of 16,400 IU/d or 225 IU/kg of body weight per 24 hours. The amount of heparin required for adequate prophylaxis might increase through the second and third trimester. Therefore, heparin should be adjusted to midinterval plasma heparin levels of 0.05-0.25 U/mL, measured as factor Xa activity. Low molecular weight heparin is most appropriate for prophylaxis because it requires a single daily dose, does not require laboratory monitoring, and may be associated with a lower risk of osteoporosis and bleeding.