- Author: Vera A De Palo, MD; Chief Editor: Harris Gellman, MD more...
The arterial blood gas on room air demonstrates hypoxemia (PaO2 < 80 mm Hg) and an elevated alveolar-arterial oxygen gradient. Acid-base status may demonstrate a respiratory alkalosis.
Enzyme-linked immunoassay (ELISA) can be used to quantify the presence of D-dimer, which is a specific degradation product of cross-linked fibrin. This is an important marker of the activation of fibrinolysis. It can be elevated in pneumonia, cancer, sepsis, and surgery.
A plasma D-dimer level of higher than 500 ng/mL has been shown to have a sensitivity of 97% and a specificity of 45%. The value of D-dimer is in its negative predictive value. A plasma D-dimer level of less than 500 ng/mL essentially excludes PE.
Elevated troponins are associated with adverse prognosis in acute PE. Elevated natriuretic peptides, brain natriuretic peptide (BNP), and N-terminal pro-BNP have been shown to be predictive of adverse short-term outcomes in acute PE and can be predictive of mortality.[22, 23] Measurement of both troponin and BNP are important for risk stratification in patients with PE.
Chest radiographic findings most often are normal. Radiographs may, however, reveal an enlarged right descending pulmonary artery, decreased pulmonary vascularity (Westermark sign), a wedge-shaped infiltrate, or an elevation of the hemidiaphragm (Hampton hump). If infarction occurs, a pleural effusion may be present.
Helical (Spiral) Computed Tomography
Helical (spiral) computed tomography (CT) allows for the imaging of pulmonary vessels by way of intravenous contrast material as the patient moves through a gantry at a constant rate and the radiography source rotates. PE is diagnosed by filling defects, which are either central or adherent to the wall. The CT scan below shows a filling defect.
The advantage of helical CT scanning is that it is minimally invasive and allows concurrent visualization of the parenchyma, pleura, and mediastinum. When looking at the main, lobar, and segmental veins, the sensitivity of helical scanning is about 93%. Its positive predictive value is approximately 95%.
The limitations of helical CT scanning include the need for contrast and for a higher dose of radiation than that required by some other diagnostic modalities. Obliquely or horizontally oriented vessels like those of the segmental branches of the right middle lobe and lingula are poorly visualized. The scan is technically inadequate or inconclusive in approximately 1-10% of the cases.
Results from Doppler ultrasonography can indicate the presence of thrombus within a vein. A normal vein is free of internal echoes and can be compressed. In acute DVT, however, internal echoes are present and the vessel is not compressible.
Duplex scanning of the venous system uses Doppler flow assessment combined with B-mode ultrasonography. The advantage of color flow Doppler ultrasonography is the ability to determine motion and the direction of flow.
Echocardiography can demonstrate signs of right-sided heart strain. Right ventricular dilatation, right ventricular hypokinesis, or tricuspid regurgitation may be present. Interventricular septum bulging into the left ventricle may be present, and the size of the left ventricle may be reduced. Echocardiography can also be used to identify signs of impending heart failure.
Pulmonary angiography has long been the diagnostic criterion standard. Angiography allows for the visualization of the pulmonary vasculature using contrast material, and in the event of PE, it evidences the cutoff of a vein and a lack of flow to the affected area.
It is an invasive procedure that requires the administration of intravenous contrast material, and it is more expensive than other procedures.
Pulmonary angiography leads to increased morbidity in approximately 2-5% of patients; this is related to bleeding and to complications from the use of intravenous contrast material. Mortality occurs in less than 1% of patients in whom this procedure is performed.
Contrast venography is an invasive technique that can provide direct proof of thrombus by demonstrating a filling defect with the aid of contrast medium through the deep venous system. However, it can cause iatrogenic venous thrombosis, tissue sloughing from contrast extravasation, and an allergic contrast reaction.
Ventilation-perfusion scanning is a common screening technique. This modality provides a probability estimate for PE by evaluation of the size and the number of defects in the perfusion of the lung compared with the areas of ventilation.
The diagnosis of PE is easily made with this modality when the probability estimate is high for PE. With a normal scan finding, the possibility of PE is excluded. However, the test results are nondiagnostic in about 66% of cases. The image below compares normal ventilation findings with a perfusion defect.
Impedance plethysmography may detect impaired venous emptying of the leg by assessing the volume response to temporary occlusion of the venous system. Emptying is assessed by the rapidity of volume decrease. Slow emptying indicates obstruction.
Impedance plethysmography is a noninvasive method of assessment. Sensitivity and specificity have been reported to be between 92% and 95%. However, it is of limited value when DVT is asymptomatic or distal or when findings are nonocclusive.
Conditions leading to poor forward blood flow, hypotension, or vein compression can be responsible for false-positive results.
Electrocardiography is of greatest value in ruling out myocardial infarction. Sinus tachycardia often is present, and right axis deviation, right bundle branch block, and deeply inverted T waves in V1 -V3 may be found. An S1 Q3 T3 pattern may be seen.
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