Despite this new tool, AAD remains a difficult diagnosis in the emergency department
Acute Thoracic Aortic Dissection (AAD) is rare (3.5/100,000 pts/year)  and mimics other very common conditions (3/1000 ED patients with chest or back pain) . But, once diagnosed, in-hospital mortality is reported to be up to 27%. The low incidence of the condition and lack of substantial research on ED diagnosis, are responsible for the lack of ED population-based clinical decision rules or low-risk stratification parameters.
In early 2015, ACEP released its clinical policy on the evaluation and management of adults with ‘suspected’ non-traumatic AAD . The key word here is of course ‘suspected’—and that word is exactly our diagnostic dilemma: in those patients with acute chest (or abdominal or back) pain, when should we suspect AAD as the cause? By highlighting key elements of the ACEP guidelines, we will focus on the inherent strengths and limitations of current clinical information. Click here to read the guidelines.
Q1: In adults with suspected non-traumatic acute AAD, are there clinical decision rules that identify a group of patients at very low risk for the diagnosis of AAD ?
- A. No, there are no prospectively validated ED clinical decision rules to identify very-low risk patients. There is a tool (ADD Risk Score)  derived retrospectively from the IRAD registry  which identifies high-risk patients.
- ACEP Level C recommendation: The decision to pursue workup for acute nontraumatic aortic dissection should be at the discretion of the treating physician .
The ADD score (combined with ECG and CXR findings) does not absolutely exclude the diagnosis of AAD (5% miss rate) [4,5]
Presenting clinical features of ADD can be variable. Atypical presentations are associated with diagnostic delays . Physical exam findings are helpful when present, but are often absent. In fact, AAD patients with the absence of pulse deficits exhibited high rates of missed or delayed diagnosis [6,7]. And even when ‘classic ‘symptoms are present, provider suspicion for AAD can be low. In a recent small prehospital study, AAD was suspected by EMS personnel in less than 1/5 of cases despite 89% of patients reporting acute bouts of intense chest, back or abdominal pain , and this information was rarely transmitted to the ED physician.
Whether normal, non-diagnostic, or concerning for acute coronary syndrome, ECGs alone are not particularly helpful in identifying patients at low risk for AAD, and about 30% have a normal ECG [9,10].
The absence of a widened mediastinum and abnormal cardiac contour yields a negative likelihood ratio (LR) of 0.3 (95%CI, 0.2-0.4) . For this reason, emergency physicians are more likely to exclude the diagnosis of AAD when a widened mediastinum is absent on CXR (OR, 33.16; 95% CI 5.74 to 191.49) .
Q2: In adult patients with suspected acute non-traumatic AAD, is a negative serum D-dimer sufficient to identify a group of patients at very low risk for the diagnosis of thoracic aortic dissection ?
- A. MAYBE.
- ACEP Level C recommendation: Do not rely on D-dimer alone to exclude the diagnosis of aortic dissection .
The guidelines do not champion the use of a D-dimer level alone to exclude the diagnosis of AAD because it is inadequately sensitive, non-specific, and could arguably result in harm due to unnecessary testing . Soon after the 2015 guidelines were released, a meta-analysis by Asha and Miers investigated D-dimer as a “rule-out test” for AAD. Using a D-dimer cut-off of 0.5 µg/mL, this analysis demonstrated a post-test probability for AAD of 0.3% in a low-risk population (sensitivity 98% [95% CI 96.3% to 99.1% and negative LR 0.05 (95% CI 0.03 to 0.09) . These findings suggest a potential role for D-dimer in ruling out AAD in low-risk patients.
Q3. In adults with suspected non-traumatic AAD, does CTA exclude the diagnosis of AAD?
- A. YES.
- ACEP Level B recommendation: CTA has accuracy similar to that of TEE and MRA .
CTA is widely available and fast. The 2014 guidelines on the diagnosis and treatment of aortic diseases by the European Society of Cardiology recommends CTA as an initial imaging investigation for both stable and unstable patients in whom AAD is suspected .
Q4. In adults with suspected non-traumatic AAD, does an abnormal bedside TTE establish the diagnosis of aortic dissection ?
- A. MAYBE.
- ACEP Level B recommendation: Do not rely on an abnormal bedside TTE result to definitively establish the diagnosis AAD . ACEP Level C (consensus) recommendation: Obtain immediate surgical consultation or transfer to a higher level of care if a TTE suggests AAD .
Q5. Does targeted heart rate and blood pressure lowering reduce morbidity or mortality ?
- A. MAYBE- to PR ≤ 60 and BP ≤120 systolic.
- ACEP Level C recommendation: Decrease blood pressure and pulse if elevated. However, there are no specific targets that seem to reduce morbidity and mortality .
It is widely accepted that blood pressure and pulse control are important in the early management of AAD but precise targets are not known. The ACEP clinical policy chronicles a body of evidence that describes the role of shear force and pulsatile flow in the progression of aortic dissection. Based on some of these older studies, pulse rate control is felt to be important in the early management of AAD and expert consensus calls for a target heart rate of 60 beats per minute. A target systolic blood pressure of <120 mm Hg is also recommended . Uncontrolled hypertension is a significant “treatable” risk factor for AAD. Furthermore, patients in whom Type A dissections were immediately fatal had higher premorbid blood pressures (mean pre-event systolic blood pressure 151.2 vs 137.9; p<0.001) .
Taking a closer look
Feasibility of emergency physician-performed bedside US for AAD: Several reports of bedside ultrasound (US) identification of AAD in the ED have been recently described [15-19]. In these case reports, ED-physician bedside US evaluation of the aorta (abdominal and thoracic) resulted in a strong suspicion for, or the diagnosis of, AAD. Confirmatory radiographic testing was obtained and prompt surgical consultation was established in each case. The most common US finding in these cases was mobile intimal flaps in the abdominal aorta and/or a dilated aortic root [15-19]. In a recent retrospective pilot study, focused cardiac US performed by emergency physicians demonstrated a sensitivity of 0.77 (95% CI = 0.58 to 0.98) and a specificity of 0.95 (95% CI = 0.84 to 0.99) for the detection of aortic dilatation and aneurysm . A leading-edge to leading-edge method in the parasternal long-axis orientation was used. Using CTA measurements as the reference standard, a cut-off value of 40 mm defined aortic dilation. The ascending aortic diameter was measured at the sinuses of Valsalva, sinotubular junction and the largest visible part of the proximal ascending aorta . In a recent small prospective study, strong inter-rater reliability (intraclass correlation coefficient 0.80) was achieved between a sports medicine physician and a cardiac sonographer in measuring the aortic roots of young athletes . This further supports the feasibility of non-cardiologist/non-sonographers performing point-of-care (POC) ultrasound assessments for AAD.
Reference values for AAD: These cases suggest a potential role for emergency POC US for AAD screening, especially in unstable patients. As such, the development of standards for aortic assessment by bedside US is needed. Currently, a wide range of reference values and ultrasonographic approaches are described. But their accuracy and utility are limited by several factors. Aortic root size varies with age, gender and body surface area . Furthermore, variability in aortic root diameter measurements has been demonstrated when different measurement guidelines are used . The 2005 American Society of Echocardiogaphy (ASE) guidelines recommend a parasternal long-axis acoustic window to acquire 2-dimensional views and measurement from leading-edge to leading-edge during diastole at four levels; aortic annulus, sinuses of Valsalva, sinotubular junction, ascending aorta . The 2010 ASE pediatric guidelines recommend measurement from inner edge to inner edge during systole .
Recent efforts have been made to establish widely applicable reference values and nomograms for aortic dilatation. Cut-off values for the definition of aortic root dilatation have been studied, and absolute dimensions of >39mm in women and >39mm in men have been proposed. However, given aortic root diameter increases with age, body size and male gender, inappropriate or inflexible application of diagnostic criteria without regard to patient characteristics can lead to under-diagnosis of aortic root dilatation . If emergency physician-performed US is to become an effective screening tool for AAD, the assessment method and calculation for aortic root measurement should be standardized [22, 27]. Additional study is also needed to determine the feasibility of its application in the emergency clinical arena.
Despite considerable efforts to identify reliable clinical predictors of AAD, emergency physicians are at a distinct disadvantage in establishing the diagnosis. For now, suspicion for AAD must be combined with diagnostic information to generate a pre-test probability needed to guide further diagnostic decisions. Adequate sensitivity and logistical convenience makes CTA the most important and reliable diagnostic modality for ruling out the disease quickly. Over-testing and its inherent harm may be unavoidable since the price of a missed AAD is high. An exciting prospect in the quest to improve AAD investigation is the potential role for bedside emergency physician-performed TTE. If this technique is refined and standardized for wide application, medical resources could be focused more precisely to identify low- and high- risk patients.
Tell us what you think about ACEP’s clinical policy in the comments.
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