Elastin is another major structural component of the medial layer of aortic wall. In a retrospective study using the assay for soluble elastin fragments (sELAF) that was developed by the authors,17 Shinohara and colleagues demonstrated that an ELISA measuring sELAF in the serum with the cutoff set at + 3 SD (standard deviation) above the mean of age-adjusted healthy subjects had a sensitivity of 88.9% (16/18) and speciﬁcity of 99.8% (473/474).17 The positive (PPV) and negative predictive values (NPV) were 94.1% and 98.8%, respectively. Based on the NPV, measuring sELAF would appear like an excellent test to rule out an acute aortic dissection. However, since this study was retrospective, data therein need to be conﬁrmed in a prospective conﬁrmatory multicenter study. More importantly, the ELISA for sELAF takes 3 hours to perform, a major drawback for a time sensitive condition such as acute aortic dissection. A positive test would require a follow-up radiologic conﬁrmatory test, which further prolongs the diagnosis. Plasma D-dimer, a degradation product of cross-linked ﬁbrin, has emerged as a promising diagnostic marker with high sensitivity for exclusion of acute aortic dissection. The rise in D-dimer is thought to result from the thrombogenic- ity of the false lumen in AAD and the subsequent activa- tion of the coagulation cascade. As such, D-dimer values are expected to be lower in proximal AAD (type A). In an earlier pooled analysis by Sodeck, a d-dimer 0.1 μg/ml is though to exclude all AAD (sensitivity of 100%).18 In a more recent pooled sensitivity analysis in EM literature, using a commonly used threshold for a positive D-dimer test of 0.5 μg/ml, Marill found that serum d-dimer had a sensitivity of 94%.19 The authors concluded that serum d-dimer can be used in patients with a low likelihood of AAD. Overall, a preponderance of recent studies support drawing D-dimer in the ED given its high sensitivity, noninvasiveness, broad availability and low cost.20–24 Imaging The choice for the diagnostic imaging depends on patient’s stability, local expertise, and availability. The main goals in diagnostic imaging are to rapidly conﬁrm (or exclude) an AAD, classify the type/extent of the AAD, identify and locate the intimal tears, conﬁrm the presence of true/false lumen and whether a thrombus is present, assess any aortic side branch involvement, detect any aortic regurgitation, and discover any extravasation to the pericardium, mediastinum, or thoracic cavity. A chest X-ray can detect abnormalities in 60%–90% of cases of AAD. Abnormalities that may suggest an AAD Vascular Health and Risk Management 2009:5 Current management of type B aortic dissection include an abnormal aortic knob, deviation of the trachea, main stem bronchus, or esophagus, wide mediastinum, pleural cap, or pleural effusion. A normal chest X-ray, however, can not be used to rule out an AAD. In unstable patients with strong suspicion of an AAD, chest X-rays may lead to further delay to deﬁnitive imaging and therapy and should be avoided. Deﬁnitive imaging techniques are essential in the workup of AAD. Data from the 2002 IRAD show that computed tomography angiography (CTA) is used in 63% of cases of suspected AAD, followed transesophageal echocardiography (TEE) in 32%, aortography 4%, and magnetic resonance angiography (MRA) in 1%.25 Compared with the newer imaging techniques, aortography has a number of serious disadvantages, including the use of a heavy dose of IV contrast (1 mg/kg), the risks of an invasive procedure, and the extended time it takes to complete the procedure (up to 2+ hours). As a result, this imaging method is rarely the ﬁrst diagnostic modality in the workup of AAD. Computed tomography angiography, TEE and MRA have similar pooled sensitivity (98%–100%) and speciﬁcity (95%–98%),26 although the pooled positive likelihood ratio appeared to be higher for MRA (positive likelihood ratio, 25.3; 95% conﬁdence interval, 11.1–57.1) than for TEE (14.1; 6.0–33.2) or CTA (13.9; 4.2–46.0). CTA is widely available and relatively rapid, provides visualization of the entire aorta down to iliac arteries, and delineates the involvement of aortic side branches. It is usually the ﬁrst choice for imaging in the work up of AAD. Disadvantages with CTA include the requirement that patients be transported to the CTA suite, the use of potentially nephrotoxic contrast, and the inability to assess aortic insufﬁciency. Overall, CTA of the chest is the preferred ﬁrst diag- nostic imaging method in hemodynamically stable patients (Figure 4). The choice for a second imaging study includes MRA or TEE, depending on clinical condition and local environment. MRA is highly accurate and does not require the use of a contrast dye. It is, however, usually not available on an emergency basis and requires patients to be in MRA suite for an extended period of time. Other issues such as claustrophobia, the use of ventilator, and patient’s use of metal devices (pacemakers, aneurysmal clips) may further complicate its routine use.25 TEE is a viable alterna- tive in patients who are critically ill and/or hemodynamically unstable. The main advantages of a TEE include speed, good sensitivity and speciﬁcity, and the fact that it can be per- formed at the patient’s bedside in the ED. Its main limitations are lack of widespread expertise and 24/7 availability. 59