Thromboembolism in pregnancy

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Chapter 47 Thromboembolism in pregnancy

Incidence. The incidence of venous thromboembolism (VTE) is 1 in 500–2000 pregnancies.

Significance. Pregnancy is associated with a 10-fold increase in risk of VTE compared with the non-pregnant population. VTE remains a leading cause of maternal mortality in developed countries. It results in approximately 1 death per 100,000 pregnancies.

The prevalence of VTE is distributed equally between the three trimesters. Although two-thirds of events occur antenatally, the daily risk is greatest in the postnatal period. The risk of VTE after caesarean section is approximately twice that of vaginal delivery. Deep venous thrombosis (DVT) is more common in the left leg. Pelvic vein thrombosis is more common in pregnancy than in the non-pregnant population.

Predisposing factors

All three components of Virchow’s triad (venous stasis, endothelial injury and a hypercoaguable state) may be present in pregnancy.

Hypercoaguability

Pregnancy is associated with alterations in both thrombotic and fibrinolytic mechanisms. In the first trimester there is an increase in fibrinogen, prothrombin, and factors VII, VIII, IX and X. There is increased platelet aggregation and resistance to activated protein C. There is a reduction in protein S, factor XI and factor XIII. A reduction in fibrinolytic activity occurs due to a decrease in antithrombin levels and an increase in function of fibrinolytic inhibitors PAI-1 and PAI-2. After delivery there is an elevation in factors V, VII and X, and antithrombin levels return to normal.

Venous stasis

Compression of large veins by the gravid uterus and pregnancy-associated changes in venous capacitance result in a reduction in venous return from the lower limbs.

Clinical assessment

The clinical diagnosis of VTE is insensitive and non-specific. Few women with clinically suspected VTE have the diagnosis confirmed when investigations are employed. However, when clinical suspicion exists, treatment should be instituted until the diagnosis can be excluded by objective testing.

Signs and symptoms of VTE include leg pain and swelling, lower abdominal pain, low-grade pyrexia, dyspnoea, chest pain, haemoptysis and collapse.

Diagnosis

Deep venous thrombosis (DVT)

• Doppler ultrasound is the primary study for the diagnosis of DVT.

• Lack of compressibility is 95% sensitive and >95% specific for a proximal leg vein thrombosis. It is less sensitive for calf and pelvic vein thromboses.

• Venography is considered the gold standard investigation for suspected DVT. It is rarely used due to its invasiveness and attendant ionising radiation, and because Doppler ultrasound is almost as reliable.

• Magnetic resonance imaging (MRI) may be a useful modality in the diagnosis of pelvic vein DVT, with a sensitivity approaching 100% in the non-pregnant population. No adverse effects have been documented, though the safety of MRI in pregnancy remains unproven.

• The negative predictive value of a normal D-dimer falls progressively throughout pregnancy, making it less useful at later gestations.

Pulmonary embolism

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