Abstract Body (Do not enter title and authors here): Background
Timely presentation of patients with ST-segment elevation myocardial infarction (STEMI) is critical for reducing mortality and improving clinical outcomes. Prehospital delays with a median symptom-to-call time over 2.5 hours, is a major barrier. To address this, a novel credit card-sized device (HeartBeam, Santa Clara, CA, USA) that records 3D, 3-lead ECG – similar to Frank's XYZ heart vectors – is being evaluated. Unlike most ambulatory ECGs that record only coplanar signals, this 3D, 3-lead device provides a more complete view of the heart’s electrical activity, enabling ischemia detection.

Objective
To assess the diagnostic accuracy of the non-coplanar 3-lead ECG compared to simultaneously recorded standard 12-lead ECG for the detection of acute ischemia.

Methods
66 consecutive patients with angiographically confirmed coronary artery disease suitable for percutaneous coronary intervention (PCI) were enrolled. Transient coronary artery occlusion using a balloon catheter during the PCI procedure (target duration 90 seconds), served as a model for myocardial ischemia. Since this was a proof-of-principle study, a standard 12-lead ECG with four additional electrodes placed in the positions corresponding to the 3D, 3-lead ECG system was used. Both 12-lead and 3-lead signals were recorded simultaneously throughout the procedure. Cardiologist readers evaluating both ECG types, were not asked to apply ST-segment criteria for acute ischemia. Instead, they provided a binary expert judgement on presence of acute ischemia (Yes/No). The study was conducted at Clinical Center of Serbia, Belgrade.

Results
A total of 120 set of recordings were made during balloon occlusions, along with 66 negative pre-occlusion recordings. The 3-lead ECG demonstrated sensitivity and specificity of approx. 75% and 69%, respectively, compared to 83% and 66% for the 12-lead ECG. When the 3-lead ECG was overlaid with an asymptomatic baseline (as in the product version), sensitivity was 73% and specificity increased to 96%. Additional statistical results are shown in the figure.

Conclusion
The study results demonstrate that the 3D 3-lead ECG detected acute ischemia with performance comparable to standard 12-lead ECG. With the baseline overlaid, the 3D 3-lead system achieved higher accuracy than the standard 12-lead ECG in detecting ischemia. This user-friendly, always-with-the-patient system may help reduce symptom-to-door time for STEMI patients.