Abstract Body (Do not enter title and authors here): Introduction: Acute coronary syndrome (ACS) is traditionally classified into ST-elevation myocardial infarction (STEMI) and non-ST-elevation ACS (NSTE-ACS) based primarily on electrocardiogram (ECG) findings. However, this binary framework fails to identify a substantial subset of occlusion myocardial infarctions (OMIs) that present without classic ST-elevations, resulting in delayed diagnosis and treatment.

Hypothesis: We hypothesize that (1) a significant percentage of NSTE-ACS patients have undiagnosed OMIs among patients transported by ambulance, and (2) the delayed recognition of OMI in NSTE-ACS patients is associated with worse clinical outcomes, including prolonged time to reperfusion and higher rates of adverse cardiovascular events.

Approach: This retrospective, single-center study analyzed data from patients transported by ambulance between 2013 and 2017. Inclusion criteria were ACS diagnosis, ambulance transport, prehospital ECG acquisition and interpretation, and percutaneous coronary intervention (PCI). OMI was defined as the presence of at least one culprit artery on angiography; non-OMI cases were those without any culprit artery identified. Door-to-balloon times were analyzed between OMI and non-OMI groups. Outcomes included adverse cardiovascular events (e.g., death). Statistical analyses used t-tests and z-tests to identify sex-based differences.

Results: Among 764 ACS patients (277 NSTE-ACS, 487 STEMI), 24.5% of NSTE-ACS cases had angiographic-confirmed OMI despite lacking ST-elevation on ECG. NSTE-ACS with OMI patients experienced significantly longer door-to-balloon times compared with STEMI with OMI and had lower overall catheterization rates (46.6% vs. 65.7%; p < 0.0001). Compared with STEMI, NSTE-ACS was associated with higher rates of return to the ED (29.6% vs. 19.0%; p = 0.024), rehospitalization (22.7% vs. 11.9%; p = 0.0001), and new-onset heart failure (27.1% vs. 14.8%; p < 0.0001), whereas STEMI had higher in-hospital mortality (1.1% vs. 6.2%; p = 0.0009).

Conclusion: Our findings highlight that a substantial subset of NSTE-ACS patients harbor undetected OMIs and experience longer reperfusion delays, which contribute to higher rates of adverse clinical outcomes compared with STEMI. Enhanced diagnostic strategies in the prehospital period for NSTE-ACS are therefore essential to reduce morbidity and mortality in this high risk population.