• Chest pain is the main symptom of acute myocardial infarction. A precocious analgesic treatment is justified by patient's comfort and unfavorable hemodynamic consequences of persistent pain. Morphine is the painkiller historically prescribed in this situation. Morphine has never been evaluated vs placebo and is strongly suspected to decrease oral anti-platelet efficacy. Then, morphine has been downgraded, in the 2017 European guidelines (European Society of Cardiology - ESC) from I to IIa. To find alternative treatment is required.
• The methoxyflurane is an anesthetic gas used in emergency setting for about twenty years. It is now commonly used in France. Its analgesic properties have been demonstrated. Its main advantages are its maneuverability as it is delivered by inhalation, i.e. without (before) any venous access and self-administered by the patient. Tolerability is good. It could be an excellent alternative to morphine.

Inclusion Criteria:

• Patient age ≥ 18 years
• Patient managed in pre-hospital setting for a ST elevation myocardial infarction (STEMI) : Chest pain < 12 hours with moderate to severe pain (VAS > 6/10) or STEMI on ECG according to 2017 ESC guidelines

Exclusion Criteria:

• Previous analgesic treatment for this episode of chest pain
• Hypersensitivity to morphine, methoxyflurane, any fluorinated anesthetic or any of the excipients listed in SmPC,
• Decompensated respiratory failure (in the absence of artificial ventilation),
• Severe hepatocellular insufficiency (with encephalopathy),
• Acute head trauma and intracranial hypertension in the absence of controlled ventilation,
• Uncontrolled epilepsy,
• Treatment with buprenorphine, nalbuphine and pentazocine, naltrexone, nalmefene or sodium oxybate,
• Breastfeeding, in case of initiation or continuation after birth of a long-term treatment.
• Known malignant hyperthermia or genetic predisposition of the patient.
• History of serious adverse effects of the patient or his family after administration of inhaled anesthetics.
• History of signs of liver damage after use of methoxyflurane or after anesthesia with a halogenated hydrocarbon.
• Clinically significant renal impairment.
• Known renal failure with creatinine clearance below 30 ml/min or undergoing extracorporeal renal replacement therapy.
• Altered level of consciousness due to any cause, including head trauma, drug or alcohol use.
• Clinical evidence of cardiovascular instability (PAS <90 mm Hg).
• Clinical evidence of respiratory depression.
• Incapacity to self-assess pain intensity
• Incapacity to methoxyflurane self-administration
• Known pregnancy, breastfeeding, minors or incapacity (curatorship or guardianship)
• Participation in another interventional study involving human participants or being in the exclusion period at the end of a previous study involving human participants
• Absence of a Social Security