Venous thromboembolism

Investigation of suspected pulmonary embolism in pregnancy does not differ from investigation in the non-pregnant patient. The originating thrombosis is most frequently located in the veins of the calf or in the ileofemoral segment of the deep venous system. Signs and symptoms indicative of deep venous thrombosis or pulmonary embolism are unreliable during pregnancy, since swelling of the legs, dyspnea, and tachypnea are common, particularly near term.

Non-invasive studies, including impedance plethysmography and duplex Doppler ultrasonography, should be the initial diagnostic step for deep vein thrombosis. However, the use of these techniques has not been extensively validated in pregnant women and their sensitivity is far less satisfactory in detecting thrombi below the knee. If performed in late pregnancy, these procedures should be carried out in a lateral position to avoid false-positive results due to venous compression by the gravid uterus. Magnetic resonance imaging and venography, which is held to be the gold standard for diagnosis, should be restricted to clinically suggestive patients with equivocal non-invasive studies.

The ventilation-perfusion scan is the primary screening tool for the diagnosis of pulmonary embolism. It is self-supporting for clinical decisions when the results are interpreted as normal or as indicating a high probability of embolism. Electrocardiography, echocardiography, chest radiography, and arterial blood gas analysis may support the diagnosis or suggest other conditions (Table 1). A moderate hypoxemia is common in the supine position during late pregnancy owing to the decrease in functional residual capacity mediated by the mechanical effects of the gravid uterus on the diaphragm.

Equivocal cases in whom suspicion of embolism arises and a ventilation-perfusion scan shows a low or intermediate probability of pulmonary embolism should undergo pulmonary angiography. In these patients, negative non-invasive studies for detecting deep vein thrombosis should not obviate the need for angiography since they are likely to miss emboli that originate in the pelvic veins. False-negative rates of up to 57 per cent have been reported in this population.

The diagnosis of venous thromboembolism should be aggressively investigated in suspected cases because of its major immediate and future therapeutic implications. Moreover, the effects of radiation on fetal development should be of little concern, since the estimated exposure of the fetus to radiation during the combination of the aforementioned procedures is less than 5000 Gy, far less than the lowest dose associated with a teratogenic effect on the human fetus.

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