Abstract Body (Do not enter title and authors here): Background:
Prehospital identification of STEMI reduces time to reperfusion by timely primary PCI or fibrinolytic therapy. We previously reported that 40-50% of prehospital ECG transmissions were so-called ‘masquerading STEMI’ defined as ECGs meeting Glasgow algorithm (GA) criteria for STEMI but not meeting STEMI criteria on adjudication using the 4th Universal Definition of MI (4UDMI). We set out to determine late mortality of patients with these presentations.

Methods:
From June 2010 to October 2021, 2,992 patients at Liverpool Hospital in Sydney met GA criteria for STEMI via prehospital ECG and were classified as confirmed or masquerading STEMI per 4UDMI. Mortality risk between the two groups was compared using a Kaplan-Meier plot and further assessed using the Cox regression model, which was also used to further assess the mortality predictive value of patient baseline characteristics in the masquerading group.

Results
Patients presenting with masquerading STEMI had a hazard ratio (HR) of 2.28 (CI=1.79-2.91, p<0.001) compared with true STEMI for all-cause mortality (figure). Amongst patient characteristics in the masquerading STEMI group, age (HR=1.06[1.05-1.07]), history of stroke (HR=1.42[1.00-2.04]), CKD (HR=1.97[1.47-2.64]) and diabetes (HR=1.49[1.13-1.95]) were positively correlated with increased mortality risk while dyslipidaemia was associated with a lower risk (HR=0.64[0.48-0.86]), potentially confounded by unmeasured statin-therapy.

Conclusions
Patients presenting with masquerading STEMI have a higher risk of mortality compared to true STEMI patients, highlighting need to better charcterise these patients, and develop therapies and/or interventions that may improve outcomes.