What is the recommended first-line analgesic for acute myocardial infarction (heart attack)?

First-Line Analgesic for Acute Myocardial Infarction

Morphine sulfate administered intravenously is the first-line analgesic of choice for pain relief in acute myocardial infarction, except in patients with documented hypersensitivity. 1, 2

Rationale for Morphine Use

• Morphine blocks sympathetic efferent discharge at the central nervous system level, resulting in peripheral venous and arterial dilation, which reduces both preload and afterload, decreasing myocardial oxygen demand 1, 2
• Pain relief with morphine decreases anxiety and circulating catecholamines, potentially reducing associated arrhythmias 1, 2
• Effective analgesia should be promptly administered at the time of diagnosis and not unreasonably delayed to evaluate the results of anti-ischemic therapy 1
• The European Society of Cardiology (ESC) and American College of Cardiology (ACC) guidelines consistently recommend titrated IV opioids, specifically morphine, as the analgesic of choice for acute MI 1

Dosing Protocol

• Initial dose: 2-5 mg morphine sulfate administered intravenously 1, 2
• Administration frequency: Every 5-30 minutes as needed until pain is relieved 1, 2
• Titration: Additional doses of 2 mg can be administered at 5-minute intervals until adequate pain relief 2
• Some patients may require relatively large cumulative doses of 2-3 mg/kg 1

Monitoring and Side Effects

• Common side effects include nausea and vomiting, which can be managed with concurrent administration of antiemetics 1, 2
• Hypotension with bradycardia may occur but is relatively rare and can be managed with leg elevation, fluids, and atropine 1
• Respiratory depression is a risk, especially in patients with chronic lung disease, and can be reversed with naloxone (0.1-0.2 mg IV every 15 minutes) if needed 1
• Recent evidence suggests morphine may attenuate and delay oral anti-platelet agent absorption, potentially affecting reperfusion outcomes 1, 3

Alternative Options

• For patients with documented morphine hypersensitivity, alternative opioids such as hydromorphone or meperidine can be considered 1, 2
• Meperidine has been suggested for inferior wall infarction due to its vagolytic properties, but when equipotent analgesic doses are given, it has no clear advantage over morphine 1
• Buprenorphine may be considered in uncomplicated MI cases where a reduction in systemic vascular resistance is desired 4
• Nitroglycerin can help with pain relief but is not considered a primary analgesic; it should be used as an adjunct therapy for its anti-ischemic effects 1, 5

Important Clinical Considerations

• Pain relief is paramount not only for humane reasons but because pain is associated with sympathetic activation that causes vasoconstriction and increases cardiac workload 1
• Intramuscular injections should be avoided; always administer opioids intravenously 1
• Always have naloxone readily available to reverse potential respiratory depression 1
• Oxygen should be administered to patients with hypoxemia (SaO₂ <90-95%), breathlessness, or acute heart failure, but routine oxygen is not recommended in patients with normal oxygen saturation 1
• A mild tranquilizer (usually a benzodiazepine) should be considered in very anxious patients 1