Introduction
Acute aortic syndrome (AAS) is a life threatening emergency, accounting for 1/2000 presentations of acute chest/back pain(1). It is a clinical spectrum of diagnoses including aortic dissection, intramural hematoma and penetrating atherosclerotic ulcer(1).  All lead to blood entering the wall of the aorta. This blood separates the aorta blocking blood supply to vital organs such as the brain or heart. (2, 3). Many physicians do not consider AAS in their initial differential diagnosis, which is in part why 25% are not diagnosed until 24 hours after presenting to the emergency department(4). Mortality follows a linear increase with diagnostic delay and can be as high as 2% per hour(5). The misdiagnosis rate for AAS is estimated to be as high as 38%(4, 6-16). 

The best known tool for risk stratifying patients at risk for AAS is the ADD-RS.  This expert consensus tool was developed as part of the 2010 American Heart Association(AHA) guidelines for investigation of AAS, it has been incorporated into the European society of cardiology(ESC) guidelines for AAS in 2014. This tool has performed poorly in the low prevalence population that we are investigating for AAS in Canada(20). A lack of defined inclusion criteria and a low specificity limits the application of this tool in the canadian practice environment. A national survey of emergency physicians in Canada found that a clinical decision rule was needed to risk stratify patients for AAS. They also reported that the two most important variables used in risk stratification was clinical suspsion for AAS and clinical supison for an alternative diagnosis. Using the GRADE adalopment approach a 20 member national multidisciplinary panel adapted the AHA and ESC guidelines updating with current evidence and refining the clinical decision tool for use in the Canadian practice environment. 

Our goal is to implement these guidelines in order to decrease the miss rate of this catastrophic diagnosis. 

Methods
Our primary clinical objective is to reduce the number of missed cases of AAS. 

Design – step wedge cluster randomized control trial

Setting – University, community, urban, rural emergency departments

Participants >18 years old presenting with truncal pain and/or perfusion deficit ( coronary, mesenteric, limb or neurological) and a suspicion for AAS. 

Intervention – 12 hospitals will be randomly assigned to different time points of intervention. Intervention will include education, policy and real time radiology reminders.

Main outcome – Our primary clinical objective is the proportion of AAS cases missed. Our primary process objective is adherence to an evidence based diagnostic algorithm for AAS. Secondary clinical objectives include assessing impact on the use of: 1) D-dimer , 2) CTA, 3) Specialist referral 4) time in the emergency department

Impact on EM
Standardizing risk stratification of patients presenting with symptoms of AAS has the potential to reduce the number of missed cases of this catastrophic diagnosis.