Pulmonary embolism (PE) is a huge problem with as many as 5% of all inpatient deaths being related to PE. Predisposing causes include bed rest, recent surgery, venous insufficiency, recent fracture and malignancy.
Major emboli usually originate in pelvic and lower leg veins although upper limb veins, cardiac chambers and central catheters are also potential sites. The clinical picture is often non-specific.
The chest X-ray is usually abnormal and signs include atelectasis, pleural effusion, elevated hemidiaphragm, prominent pulmonary artery and cardiomegaly. Focal oligaemia is uncommon and the chest X-ray is usually non-specific and, therefore, unhelpful. A rare but specific sign of PE is the Hampton's hump (Fig. 1.46). This is haemorrhagic infarction of the lung usually seen as a peripheral ill defined wedge-shaped opacity with convex borders.
Ventilation perfusion scintigraphy has been the method used in most centres to diagnose PE for some time. This involves obtaining perfusion 'Q' images of the lungs and ventilation 'V' images and comparing the two studies. Isotope labelled 'microspheres' are injected and a small proportion of these wedge in the pulmonary circulation where their position is imaged with a gamma camera. The ventilation part of the study is made during inhalation of a radioactive gas. These V/Q scans are classified as normal, high, intermediate or low probability of PE. In classic PE several large perfusion defects are identified but ventilation is normal - there is ventilation perfusion mismatch high probability of PE (see Fig. 1.45).
With other chest pathologies such as collapse and consolidation, both the perfusion and the ventilation images are abnormal - matched ventilation perfusion defects low probability of PE.
The management of patients with normal or high probability V/Q studies is straight forward, but patients with intermediate probability scans still have a 30% incidence of PE.
Pulmonary angiography (see Fig. 1.47) will demonstrate filling defects in the pulmonary circulation and is considered a reliable although invasive test for PE.
CT pulmonary angiography (see Fig. 1.48) will reliably identify clot in lobar and segmental vessels with greater than 90% sensitivity and specificity. The significance of subsegmental clots (missed on CT) is unknown. The great advantage of CT is that it will identify unsuspected pathology (e.g. lung carcinoma) which is the true cause of clinical symptoms.
Fig. 1.48 Pulmonary embolus CT pulmonary arteriogram. Filling defects/blood clot is present in the pulmonary arteries bilaterally.
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