Along with symptoms of chest pain, the presence of ST segment elevations on ECG is the criterion usually used in practice to identify patients with acute coronary artery occlusion and is essential for the direct and acute referral of patients for primary PCI. However, ECG does not always reflect changes in the posterior wall of the heart, often equivalent to the left circumflex coronary artery's (LCX) supply area, resulting in an underrepresentation of LCX as culprit artery in STEMI populations. There is a general concern that some patients with genuine acute occlusion of LCX may present without ST segment elevation and be denied reperfusion therapy, resulting in larger infarction and worse outcome.

The aim of this trial is to implement record of posterior ECG leads (V7, V8 and V9) in addition to the standard 12-lead ECG in the pre-hospital setting and to evaluate the clinical impact of this implementation: In comparison to a control cohort of STEMI patients diagnosed with a pre-hospital standard 12-lead ECG prior study start, the investigators hypothesize that introducing V7-V9 leads as a clinical routine in the pre-hospital setting will identify patients with STEMI involving LCX and with a non-diagnostic standard 12-lead ECG.

Inclusion Criteria: Intervention group:

• All STEMI patients referred directly from the pre-hospital setting, treated with primary PCI and with available standard 12-lead ECG and ECG with V7-V9 leads.

Exclusion Criteria:

• STEMI patients treated with primary PCI and no ECG with V7-V9 leads.

The intervention group will contain STEMI patients referred directly from the pre-hospital setting to PCI-center for primary PCI during study periode. In these patients, both standard 12-lead ECG and ECG with V7-V9 leads will be recorded and tele-transmitted to the PCI-center.