Pulmonary Embolism
The clinical diagnosis of pulmonary emboli is unreliable, being both insensitive and nonspecific. Many cases are clinically silent with only 30% having the clinical features of a DVT and only 70% demonstrating a DVT on venography. Individuals with a massive pulmonary embolus, who are compromised for more than 60% of pulmonary circulation, are considered critically ill. Right heart failure may progress to cardiovascular collapse with hypertension, coma and death. A sub-massive pulmonary embolus presents with tachycardia, tachypnea and signs of pulmonary infarction with consolidation, rales, hemoptysis, pleuritic chest pain, pleural friction rub, pleural effusion and fever. In most cases individuals often present with a few nonspecific clinical findings and the major clinical complaints of malaise and fever.
Nuclear ventilation / perfusion scans are often used to diagnose a PE. A normal perfusion scan usually excludes a PE but can be found in a minority of individuals with a PE. Perfusion defects are non-specific; about one third of those with defects have a PE. The probability that a perfusion defect is a PE increases with the size, shape and number of defects as well as the presence of a normal ventilation scan. Mismatched perfusion defects (normal ventilation scan), which are segmental in size or larger are “high probability” defects and are associated with an approximately 80% prevalence of PE. Three or more mismatched defects are associated with a prevalence of approximately 90%. Individuals should be treated if presenting with a positive ventilation/perfusion scan and high clinical suspicion of a PE.
Pulmonary angiography is the definitive test for diagnosis of PE (Gill & Nahum 2000). It involves percutaneous catheterization and injection of contrast dye into a pulmonary artery branch (Gill & Nahum 2000). It is an expensive test and is associated with a significant risk of complications (e.g. hemorrhage, embolus, nerve injury). Relative contraindications include significant bleeding risk, allergy to contrast medium, and renal insufficiency (Gill & Nahum 2000). It is associated with a mortality rate of up to 0.5% (Newman 1989; Stein et al. 1992). Pulmonary angiography is most commonly used when ventilation-perfusion scanning is non-diagnostic but clinical suspicion remains high (Tapson et al. 1999). A negative pulmonary angiogram excludes clinically relevant PE (Gill & Nahum 2000; Tapson et al. 1999).
A spiral Computed Tomograpy (CT) scan is a quick, less expensive CT scan which can scan the entire thorax in one breath-hold. It has a sensitivity ranging 64-93% with a specificity of 89-100%. Its accuracy increases with the size of the embolism. It directly visualizes the clot and has the added benefit of diagnosing other disease states in the differential diagnosis (e.g. lung cancer, vascular remodelling, and pleural effusion). Spiral CT is a useful adjunct to the majority of ventilation perfusion scans that have non-diagnostic results that require further testing (PIOPED Investigators 1990).
