Pulmonary embolism
Updates to Article Attributes
Pulmonary embolism (PE) refers to embolic occlusion of the pulmonary arterial system. The majority of cases result from thrombotic occlusion, and therefore the condition is frequently termed pulmonary thrombo-embolism which is what this article mainly covers.
Other embolic sources include:
- air embolism
- carbon dioxide embolism
- other gas embolism (e.g. nitrogen, helium)
- fat embolism
- tumour embolism: comprised of tumour thrombus
- hydatid embolism
- talc embolism
- iodinated oil embolism
- metallic
- amniotic fluid embolism
- cement embolism: comprised of polymethyl methacrylate (PMMA)
- catheter embolism
- septic embolism
Clinical presentation
Clinical signs and symptoms are non-specific 14. Dyspnoea, chest pain, and haemoptysis have been described as a classic triad in pulmonary embolism. The ECG may show an S1Q3T3 pattern. Patients may have deep vein thrombosis (usually from the lower limbs) which are the most common source of the PE.
Pre‐test probability scores are intended to replace empirical assessment of patients with suspected pulmonary embolism:
Pathology
Risk factors
- primary hypercoagulable states
- recent surgery
- pregnancy
- prolonged bed rest/immobility
- malignancy
- oral contraceptive use
- known or previous DVT
- presence of certain venous aneurysms
- e.g. popliteal venous aneurysm 15
The right ventricular failure due to pressure overload is considered the primary cause of death in severe PE 14.
Markers
D-dimer (ELISA) is commonly used as a screening test in patients with a low and moderate probability clinical assessment, on these patients:
- normal D-dimer has almost 100% negative predictive value (virtually excludes PE): no further testing is required
- raised D-dimer is seen with PE but has many other causes and is, therefore, non-specific: it indicates the need for further testing if pulmonary embolism is suspected 4
On patients with a high probability clinical assessment, a D-dimer test is not helpful because a negative D-dimer result does not exclude pulmonary embolism in more than 15%. Patients are treated with anticoagulants while awaiting the outcome of diagnostic tests 4.
Radiographic features
Depends to some extent on whether it is acute or chronic. Overall, there is a predilection for the lower lobes.
Plain radiograph
A chest x-ray is neither sensitive nor specific for a pulmonary embolism. It is used to assess for differential diagnostic possibilities such as pneumonia and pneumothorax rather than for the direct diagnosis of PE.
Described chest radiographic signs include:
- Fleischner sign: enlarged pulmonary artery (20%)
- Hampton hump: peripheral wedge of airspace opacity and implies lung infarction (20%)
- Westermark sign: regional oligaemia and highest positive predictive value (10%)
- pleural effusion (35%)
- knuckle sign 11
- Palla sign 17: enlarged right descending pulmonary artery
- Chang sign 18: dilated right descending pulmonary artery with sudden cut-off
Sensitivity and specificity of chest x-ray signs 1:
-
Westermark sign
- sensitivity: ~14%
- specificity: ~92%
- positive predictive value: ~38%
- negative predictive value: ~76%
- vascular redistribution
- sensitivity: ~10%
- specificity: ~87%
- positive predictive value: ~21%
- negative predictive value: ~74%
-
Hampton hump
- sensitivity: ~22%
- specificity: ~82%
- positive predictive value: ~29%
- negative predictive value: ~76%
-
pleural effusion
- sensitivity: ~36%
- specificity: ~70%
- positive predictive value: ~28%
- negative predictive value: ~76%
-
elevated diaphragm
- sensitivity: ~20%
- specificity: ~85%
- PPV: ~30%
- NPV: ~76%
CT
Acute pulmonary emboli
CT pulmonary angiography (CTPA) will show filling defects within the pulmonary vasculature with acute pulmonary emboli. When observed in the axial plane this has been described as the polo mint sign. The central filling defect from the thrombus is surrounded by a thin rim of contrast, appearing like the popular sweet, the polo mint 9. Emboli may be occlusive or non-occlusive, the latter as seen with a thin stream of contrast adjacent to the embolus. Typically the embolus makes an acute angle with the vessel, in contrast to chronic emboli. The affected vessel may also enlarge 9.
Acute pulmonary thromboemboli can rarely be detected on non-contrast chest CT as intraluminal hyperdensities 12.
Chronic pulmonary emboli
In contrast to acute pulmonary embolism, chronic thromboemboli are often complete occlusions or non-occlusive filling defects in the periphery of the affected vessel which form obtuse angles with the vessel wall 9. The thrombus may be calcified.
Features noted with chronic pulmonary emboli include:
- webs or bands, intimal irregularities 3
- abrupt narrowing or complete obstruction of the pulmonary arteries 3
- “pouching defects” which are defined as chronic thromboembolism organised in a concave shape that “points” toward the vessel lumen 3
Indirect signs include 7:
- mosaic perfusion
- vascular calcification
- bronchial or systemic collateralisation
Ultrasound
Cardiac echo has an important bedside role in suspected major PE looking for right ventricular wall hypokinesis (McConnell sign), right ventricular dilatation and pulmonary artery hypertension.
Lung ultrasound in the diagnosis of PE is controversial with a poor evidence base.
MRI
It is difficult to obtain technically adequate images for pulmonary embolism patients using MRI. Magnetic resonance pulmonary angiography should be considered only at centres that routinely perform it well and only for patients for whom standard tests are contraindicated. Technically-adequate magnetic resonance angiography has a sensitivity of 78% and a specificity of 99% 13.
Nuclear medicine
A ventilation/perfusion (V/Q) scan will show ventilation-perfusion mismatches. A high probability scan is defined as showing two or more unmatched segmental perfusion defects according to the PIOPED criteria.
Treatment and prognosis
Providing cardiopulmonary support is the initial treatment. Anticoagulation is provided in patients without risk of active bleeding. If the emboli are large or there is a large clot burden, thrombolysis is an option. In some cases, embolectomy or placement of vena caval filters is required.
Complications
- acute emboli
- pulseless electrical activity (PEA) in the context of a large obstructing saddle embolus
- acute or chronic emboli
-
right ventricular dysfunction
- CT features suggestive of right ventricular dysfunction include 8
- abnormal position of the interventricular septum
- inferior vena caval contrast reflux
- RVD (right ventricular diameter): LVD (left ventricular diameter) ratio >1 on reconstructed four chamber views
- RVD: LVD ratio >1 on standard axial views is not considered to be a good predictor of right ventricular dysfunction 8
- CT features suggestive of right ventricular dysfunction include 8
-
right ventricular dysfunction
- subacute-to-chronic emboli
- chronic emboli
Differential diagnosis
- pulmonary artery sarcoma
- pulmonary artery vasculitis e.g. Takayasu arteritis
- misidentification of pulmonary veins for arteries
- arterial bifurcations (a.k.a branch-points) can mimic PE but usually easily identified on multiplanar assessment
- artefact may cause pseudofilling defects and can be caused by
- breathing motion
- beam hardening
- hyperconcentrated contrast in the superior vena cava
- medical devices e.g. catheters, orthopaedic prostheses
- patient's arms in a down position
- patient movement
- transient contrast bolus interruption 16 - due to Valsalva or patent foramen ovale, causing unopacified blood to enter the right ventricle and pulmonary arteries. Scanning in end expiration can reduce or eliminate this artefact.
- chronic emboli may be mistaken for acute emboli
- or vice-versa
-<li>Palla sign <sup>17</sup>: enlarged right descending pulmonary artery</li>-<a title="Chang sign (pulmonary embolism)" href="/articles/chang-sign-pulmonary-embolism">Chang sign</a> <sup>18</sup>: dilated right descending pulmonary artery with sudden cut-off</li>- +<a title="Palla sign" href="/articles/palla-sign">Palla sign</a> <sup>17</sup>: enlarged right descending pulmonary artery</li>
- +<li>
- +<a href="/articles/chang-sign-pulmonary-embolism">Chang sign</a> <sup>18</sup>: dilated right descending pulmonary artery with sudden cut-off</li>
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