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Pulmonary embolism (PE) continues to be a major cause of death in hospitalized patients. The clinical diagnosis for excluding PE is difficult since the classic triad of symptoms-hemoptysis, pleurisy and shortness of breath-occurs only in a small percentage of patients. Blood gas and EKG analysis have poor clinical sensitivity and specificity in this regard.
The conventional diagnostic approach-nuclear medicine ventilation-perfusion (V/Q) scanning-does not have high sensitivity. A normal scan virtually rules out the existence of PE and high-probability scan is virtually diagnostic, but only 25% of patients fall neatly into either of these categories. In fact, only about 40% of patients who turn out to have PE have a high probability V/Q scan. This figure decreases even further in patients with pre-existing cardiopulmonary disease, especially COPD. And while ultrasound of the lower extremities can well delineate deep venous thrombosis (DVT), the majority of patients with PE show no evidence of DVT.
Until recently, pulmonary angiography was the study recommended for patients with inconclusive scintigraphic findings.
Recently, helical CT has demonstrated sensitivity (85%-90%) and specificity (90%-95%) for the central pulmonary vessels. In fact, using the most modern multidetector scanners, helical CT is more sensitive than V/Q scanning and equally specific. Hence, CT is emerging as an important tool in the evaluation of patients with suspected PE and deep venous thrombosis.
The technique continues to undergo refinement. Currently the best option to diagnose PE, helical CT can miss subsegmental emboli which can pose problems in patients with limited cardiopulmonary reserves.
One other advantage of helical CT is that it can make other diagnoses in 70% to 75% of patients who do not prove to have PE and/or discern other findings in those who do. This is important because at least 20% of hospitalized patients who do not have PE are subsequently found to have other conditions that would have remained undiagnosed were it not for their helical CT exam.
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