Armed with a strong clinical suspicion and screening tests which do not point to other obvious causes, the examiner must confirm the diagnosis of pulmonary embolus. At this time, if there are no contraindications, heparinization should be initiated to prevent further propagation of thrombus and worsening of the clinical condition. Once a baseline partial thromboplastin time is obtained, a heparin bolus of 100 to 150 units/kg is given, followed by continuous intravenous infusion of 10 units/kg/h. Provided that there are no identified active bleeding sites or disorders, heparinization is generally safe, even in the postoperative patient. A perfusion lung scan, or ventilation-perfusion scan, should then be obtained. This test is the most accurate non-invasive test available to make the diagnosis of pulmonary embolus

(PIOPED InyestigatojrsJ.9.9.0).

The perfusion lung scan determines regions of decreased pulmonary arterial perfusion, taking into consideration the fact that several pathological states, including pulmonary embolus, will produce this abnormality. An intravenous injectate of a radionuclide, such as technetium-99m, is delivered prior to nuclear imaging. Areas of capillary trapping of radioactive aggregates (albumin- or microsphere-bound radionuclide) are visualized and denote the distribution of pulmonary blood flow. This perfusion photoscan can be compared with images produced by the ventilation portion of the test. Xenon gas inhalation can be used to increase the sensitivity of a ventilation-perfusion scan, although, if the perfusion phase is performed first and is positive, a ventilation scan may be unnecessary. However, there are several instances where positive perfusion defects correlate with areas of radiographic consolidation or cases of sub-segmental defects, when further confirmation by pulmonary angiography may be necessary.

In 1990, the Prospective Investigation of Pulmonary Embolism Diagnosis (PIOPED) results were published in an attempt to validate the role of perfusion lung scans in the diagnosis of pulmonary embolus. The study was a multicenter trial involving over 900 patients which compared ventilation-perfusion scan results with pulmonary angiography, long held as the gold standard in the diagnosis of pulmonary embolus. Scans were categorized as normal or as a high, intermediate, low, or very low probability of pulmonary embolism. When findings were correlated with pulmonary angiograms, perfusion lung scans were found to have an overall sensitivity of 98 per cent, although the specificity was only 10 per cent. There was also a 4 per cent false-negative rate. Patients with high-probability scans were found to have angiographic evidence of pulmonary embolus in 88 per cent of cases, while intermediate- and low-probability scans were positive for embolus in 33 per cent and 12 per cent of cases respectively. In this trial, clinical acumen was predictive in only 20 per cent of cases.

Non-invasive studies are frequently inconclusive in diagnosing pulmonary embolism ( Table,,,?.). Pulmonary angiography should be performed to confirm the diagnosis in cases where treatment may be urgently needed or anticoagulation is contraindicated. This invasive study carries a morbidity of 1 to 4 per cent and a mortality below 1 per cent. Findings on pulmonary angiography positive for pulmonary embolism include intravascular filling defects or complete vessel occlusion. Other less specific findings suggesting the diagnosis include delayed vessel filling, asymmetric filling, oligemia, or a prolonged arterial phase. Defects which do not correspond to perfusion scan findings may be artifacts on the angiogram. In patients with inconclusive non-invasive studies and in whom pulmonary angiography is deemed dangerous, as in patients with recent myocardial infarction or arrhythmias, the decision to treat empirically must be based on the clinical situation. Techniques such as digital subtraction and selective lobar or segmental injections may reduce the morbidity of the procedure.

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Table 3 Indications for pulmonary angiography

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