Pulmonary Embolus

Valerie L Ward, MD
Benjamin B Faitelson, MD

October 25, 1996

Presentation

Imaging Findings

An anteroposterior upright chest radiograph demonstrates clear lungs without evidence of a parenchymal or pleural abnormality.

The ventilation-perfusion (V/Q) scans show no perfusion of the apical and posterior segments of the right upper lobe (arrows). Multiple small- to moderate-sized segmental perfusion defects are visible in the remainder of the right lung and in the left lung. Ventilation in both lungs is normal.

Pulmonary angiography reveals an abrupt termination and occlusion (arrow) of the ascending branch of the right pulmonary artery, the truncus anterior. Filling defects (arrow) are present within the arteries supplying the basal segments of the right lower lobe.

Differential Diagnosis

Diagnosis

Discussion

A vascular ultrasound is usually performed because the most common source of PE is deep vein thrombosis of the lower extremity. Other sources include iliac vein thrombosis, inferior vena cava thrombosis, and upper extremity thrombosis. Risk factors for the formation of deep thrombosis include prolonged immobilization, recent surgery (as in this postoperative patient), pregnancy, obesity, trauma, oral contraceptive use, and previous acute thrombosis. Accurate diagnosis of deep vein thrombosis is important because 50% of untreated popliteal vein (and more cephalad veins) disease will result in PE. Fewer than 10% of untreated calf vein thrombi result in embolism. In this patient, an ultrasound of the lower extremities was without evidence of thrombus. Lower extremity ultrasound is negative in 30% of patients with a proven PE, so a normal ultrasound does not rule out this diagnosis.

This patient's next diagnostic test was a ventilation-perfusion (V/Q) scan. V/Q scanning is performed by administering 3.0 mCi of technetium-99m-macroaggregated albumin (^Tc99m-MAA) intravenously. The ^Tc99m-MAA occludes a small percentage of precapillary pulmonary arterioles and capillaries. Perfusion images are obtained on a gamma camera in multiple projections. Subsequently, 20 mCi of xenon-133 gas is inhaled by the patient. Ventilation images are obtained in the specific projection that demonstrates the most perfusion abnormalities. The diagnosis of PE is made by identifying a "V/Q mismatch." That is, a lung segment or subsegment that is not perfused but is ventilated. The absence of perfusion is due to an occlusive thromboembolus within the vessel supplying that particular lung segment.

The PIOPED criteria are used to classify the probablity of PE based on V/Q scan findings. A high probability of PE (>80%) corresponds to two or more large mismatched segmental defects. An intermediate probability of PE (20-80%) is defined as one moderate segmental defect or one large and one moderate segmental defect, or a V/Q pattern of abnormality that is difficult to characterize. A low probability of PE (<20%) is defined as a small subsegmental defect, nonsegmental defects, matched defects, or any defect with a substantially larger chest radiograph abnormality. A normal V/Q scan is without a perfusion or ventilation defect. This patient's V/Q scan had two large segmental defects in the right upper lobe and other segmental defects in both lungs, hence this scan indicated a high probability of PE.

In this recently postoperative patient, pulmonary angiography was performed to confirm the diagnosis of PE. The patient's right groin was prepared and draped using sterile technique. The Seldinger technique was used to percutaneously puncture the right common femoral vein with an 18 gauge single-wall needle. A 7 French pigtail catheter was placed into the inferior vena cava over a standard J-guide wire. The wire was removed. A high iodine (350 mg/ml) nonionic contrast medium was infused into the catheter and an inferior venacavagram showed that the cava was without thrombus. The catheter was directed through the right atrium, right ventricle, pulmonary outflow tract, and main pulmonary artery. Pressure recordings revealed a mildly elevated main pulmonary artery pressure (37/15 mmHg, mean of 17 mmHg). The normal pulmonary artery pressure was 20-25/8-12 with a mean pressure of 15. Since this patient's V/Q scan showed predominantly right lung perfusion defects, selective digital subtraction angiography of the right lung was performed. The right pulmonary arteriogram images showed abrupt cutoff of the truncus anterior by occlusive thrombus. There were also intraluminal filling defects within basilar arteries to the right lower lobe. These findings provided definitive diagnosis of PE.

Patients with PE incur a high risk of mortality with obstruction of greater than 50% of the pulmonary arterial bed. Such obstruction is followed by a rapid rise in pulmonary vascular resistance and arterial pressure, which result in acute right ventricular strain. Right ventricular strain leads to cor pulmonale and then left heart failure. Systemic hypotension ensues and death occurs. Therefore, a filter was placed in the infrarenal inferior vena cava of this patient to prevent further embolization from the deep veins of the lower extremities to the pulmonary vasculature.

Few patients present with the classic symptoms of PE: dyspnea, pleuritic chest pain and hemoptysis. Electrocardiogram changes and arterial blood gas measurements are often nondiagnostic. Therefore, ventilation/perfusion studies and angiography are the standards for diagnosis of PE.

References

2. Dyer R. Handbook of vascular and interventional radiology. New York: Churchill Livingstone, 1993.

3. Johnsrude IS, Jackson DC, Dunnick NR. A practical approach to angiography. Boston: Little, Brown, 1987.

4. Thrall JH, Ziessman HA. Nuclear medicine: the requisites. St Louis: Mosby-Year Book, 1995.

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