Pulmonary angiography remains the gold standard for accurately diagnosing PE. The disadvantages include patient discomfort, cost, and complications. Pulmonary angiography has excellent interobserver reliability: the PIOPED study found that review of study angiograms by another radiologist reached the same diagnosis in 96 percent of cases.8 Complications of pulmonary angiography include (1) fatalities in 0.5 percent, (2) major nonfatal complications such as renal failure, significant hematoma, or respiratory distress in 17 percent, and (3) minor complications such as angina, urticaria, or bronchospasm in 5 percent. 8
A pulmonary angiogram is performed by a flexible catheter advanced from a peripheral large vein (typically the femoral vein) through the right atrium and ventricle, and into the pulmonary artery. Selective injections of contrast media are made into the lobar pulmonary arteries by using the results of the V/Q Scan to guide which lobar arteries should be inspected first.
A PE is identified by either an abrupt cutoff of dye with a meniscus or contrast media traveling around an intraluminal thrombus. Narrowing or luminal irregularities can be caused by other processes and are not considered diagnostic for PE. Pulmonary angiography can identify emboli out to the subsegmental divisions, beyond the reliability of either V/Q scanning or dynamic computed tomography (CT). COMPUTED TOMOGRAPHY
CT provides sharp cross-sectional anatomic displays, which enable identification of fluid collections and distinction between soft tissue structures. It eliminates overlap of tissues and thus delineates mediastinal structures very well. A recent upgrade in technology has allowed the scans to be completed more quickly and has the added advantage of being able to evaluate the vasculature by using dynamic and helical techniques.
In the ED, CT scans are useful in assessing the mediastinum to exclude a mediastinal hematoma or aortic injury after blunt trauma, in assessing acute chest pain to exclude the diagnosis of aortic dissection, or in investigating larger pulmonary emboli in unstable patients.
The disadvantages of chest CT include cost, radiation exposure, and adverse reactions to intravascular contrast media. Transporting unstable patients is never easy, but can usually be done safely with monitoring and appropriate personnel accompanying the patient.
Blunt trauma to the chest and abdomen often requires further evaluation by CT. With chest radiographs of supine patients, pneumothorax may be difficult to visualize and be seen only on the upper cuts of the abdominal CT scan.1 The abdominal CT is done with windows through the lower chest and delineates the air-lung interface very well, identifying even a small pneumothorax.
Aortic injury in the setting of decelerating trauma is suspected by findings on the initial AP chest radiograph of supine patients. Criteria found suggestive of aortic injury include mediastinal widening (greater than 8 cm at the T4 level), deviation of the nasogastric tube or endotracheal tube to the right, loss of the aortic knob contour, apical capping, or fractures of any of the first three ribs. 910 However, the most sensitive finding for aortic injury was the subjective impression of mediastinal widening by the physician viewing the radiograph.9 When these findings are noted on the chest radiograph, emergent aortography has been used to accurately visualize the aorta and define any injury. However, using sensitive criteria to avoid missing this catastrophic injury means that about 90 percent of aortograms performed to evaluate patients for potential aortic injury are normal. To reduce the incidence of normal studies, some clinicians will attempt to sit the patient up and perform an upright AP or PA chest radiograph before deciding whether there are findings of mediastinal injury. 11 Not all patients can be sat upright, however, and a decision has to be made regarding another imaging study. Two studies have come into clinical practice: transesophageal echocardiography (TEE) and dynamic CT scan.
Dynamic CT scanning with intravenous contrast can be useful, especially in stable patients with possible aortic injury. 1 l3 and 1i The major advantages of CT are that it is less invasive than aortography and, while on the CT table, the patient can have multiple parts of the body imaged. The major disadvantage is the logistical difficulty in transporting patients who are unstable or on a ventilator to the CT suite. Intravascular contrast media is used with dynamic CT, so a history of a serious reaction remains a contraindication. With appropriate equipment and physicians who can perform and properly interpret the studies, aortography, TEE, or dynamic CT scan are acceptable tests to evaluate aortic injury after blunt trauma.
Dynamic CT scan of the chest may also be helpful in patients with signs and symptoms of aortic dissection: an accurate diagnosis of dissection will be obtained in more than 90 to 94 percent of such patients.1516 Dynamic CT scan can accurately identify dissection by visualizing an intimal flap with opacification of the true and false lumens. In patients with type B (descending aorta) dissection, no other study will be necessary. A disadvantage of dynamic CT scan is that, in type A (ascending aorta) dissection, aortography will still be required to assess the aortic valve and major arterial vessels prior to surgery.
Dynamic CT scan has recently been used to diagnose PE. Currently, the V/Q scan remains the initial imaging modality used, but, with added experience, the dynamic CT scan may replace it. Recent articles report 86 to 91 percent sensitivity and 78 to 95 percent specificity for contrast-enhanced spiral CT in detecting PE out to the segmental divisions of the pulmonary arteries.1 1 and 19 Although dynamic CT can occasionally visualize subsegmental arteries, reliability and interpretation are problematic. Patients must be able to hold their breath, typically for 20 to 24 s, as the CT scan table is moved through the gantry, although scans can be done in 10 to 12 s with thicker image slices. Breath holding may be difficult for dyspneic patients. 19 Another variation is contrast-enhanced electron-beam CT, which has the advantage of a 100-ms scanning time, no need for a breath-holding maneuver, and minimal respiratory or cardiac motion artifact. 20 As radiologists become more familiar with these tests and gain more expertise, the CT scan may become the important noninvasive test for the diagnosis of PE.
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