In the management of acute coronary syndromes with ST-segment elevation (STEMI), early analgesia reduces the effects of hyperadrenalism which increases the size of myocardial infarction. In order to reduce pain intensity, the recommendations advocate emergency use of morphine. In STEMI patients, other analgesic treatments could provide analgesia that is at least as effective as morphine. The equimolar oxygen/nitrous oxide mixture (MEOPA) is widely used in emergency medicine and has minor secondary effects that are very rapidly reversible when inhalation is discontinued. Used in association with paracetamol, it could be an at least equally effective alternative to the use of morphine.

The investigators wish to compare the use of morphine according to current recommendations with the use of MEOPA associated with intravenous paracetamol in the management of patients with STEMI. The investigators hypothesize that the association of MEOPA and paracetamol, which is easy to use in a pre-hospital setting, will give patients pain relief as effectively as morphine.

This alternative treatment would avoid the use of morphine, whose potentially damaging consequences on myocardial function have been suggested by experimental studies and by an observational study. The physician of the mobile emergency team (SMUR) verifies the inclusion and non- inclusion criteria for the study. The patient must present STEMI defined in accordance with the recommendations and chest pain of intensity ≥ 4 on the NRS. The specific treatment for STEMI will be given before inclusion in the study, with the exception of analgesic treatment. In particular, inclusion in the study must not delay the initiation of strategies of recanalization and reperfusion.

The SMUR physician in charge of the patient will administer the treatment defined by randomization.

After 30 minutes, the patient will be managed in accordance with the recommendations and will be hospitalized, generally in a cardiology intensive care unit. At one month, the clinical research technician will record the patient's vital status and collect the patient's hospital records.

Inclusion Criteria:

• Patient with STEMI < 12 h treated before hospital admission and pain ≥ 4 on the numerical rating scale.

Exclusion Criteria:

• Acute severe hemodynamic, respiratory or neurological failure
• Heart failure: Killip class III and IV
• Known allergy to morphine or nitrous oxide
• Patient who has already received morphine or MEOPA before the arrival of the hospital team during the 4 hours preceding the pre-hospital intervention
• Contraindications to nitrous oxide
• Patient unable to assess pain intensity on the numerical rating scale
• Patient under legal guardianship
• Pregnancy
• Patient transported by air ambulance