Pulmonary embolism (PE) is the blockage of arteries that perfuse the lungs, generally due to clots. Rapid assessment and initiation of anti-coagulation in suspected PE cases is paramount as the risk of recurrent PE in confirmed cases could be as high as 25% within the first 24 hours. Fortunately, diagnosis and management of pulmonary embolism quickly improved at the turn of the 21st century. Between 1995 and 2001, Wells et al developed criteria to assess the likelihood of a pulmonary embolism based on the clinical presentation. At the same time, a new development in computed tomography (CT), termed spiral CT, permitted visualization of pulmonary vasculature. The Prospective Investigation of Pulmonary Embolism Diagnosis II (PIOPED II) study addressed the benefits of CTA.
One issue when attempting to assess the accuracy and usefulness of a new imaging modality is having a strong gold standard with which to compare. Given that the existing gold standard for PE is a pulmonary angiography or digital subtraction pulmonary angiography (DSPA) – a highly invasive, time-consuming study that uses direct arterial catheter access for contrast-enhanced imaging – an alternative method was needed. Using multiple ancillary tests, the authors employed a composite reference diagnosis as a gold standard in addition to DSPA. A diagnosis of PE was given when ventilation-perfusion (V/Q) scanning showed high probability in a patient without a history of PE or when V/Q scans showed moderate probability with positive lower extremity venous ultrasonography. PE was ruled out with low pre-test probability and negative results from V/Q scans or venous ultrasonography. Using these standards, 632 patients were ruled out for PE. 592 received a CT-PA and were followed for 6 months. Of that group, only 2 required anticoagulation, indicating that the composite reference diagnosis was a suitable substitute in situations where DSPA was not necessary.
The results, while impressive on their own, are more indicative when compared to the results from the first PIOPED study, which examined V/Q scans. In the original study, V/Q scans achieved 98% sensitivity but very poor specificity (10%). In contrast, CTA achieved 83% sensitivity and 96% specificity. When venous angiography was included (CTA-CTV), sensitivity improved to 92%. CTA also inherently provided imaging of the whole chest and upper neck, which in certain cases produced alternative diagnoses.
The results of PIOPED II demonstrate that, when used in conjunction with modified Wells criteria, CTA provides high positive and negative predictive values for PE. Effectively, concordant clinical assessment and imaging results can rule-in or rule-out a diagnosis which was not possible with V/Q scans alone. In patients with suspected PE and who do not have contraindications for IV contrast, it offers a shorter clinical algorithm for diagnosis and management by eliminating the uncertainty of moderate probability V/Q scans. Today CTA has become a second gold standard in the diagnosis of PE due to the invasiveness of pulmonary angiograms. Nevertheless, in both algorithms, any inconclusive results must be followed up by DSPA or serial lower extremity ultrasounds. As the authors themselves admit, the benefits of CTA-CTV are largely dependent on the expertise of the radiology staff reading the film. For institutions with this capability, the diagnosis and management of PE is greatly improved with CTA.