What is the recommended pain killer for a patient with myocardial infarction (MI), considering their medical history and potential for impaired renal (kidney) or hepatic (liver) function?

Pain Management in Myocardial Infarction

Morphine sulfate administered intravenously in doses of 2 to 4 mg with increments of 2 to 8 mg repeated at 5 to 15-minute intervals is the analgesic of choice for myocardial infarction. 1

Primary Analgesic: Morphine Sulfate

Morphine remains the definitive first-line analgesic for MI pain despite recent retrospective concerns about outcomes. 1 The drug works through multiple beneficial mechanisms beyond simple analgesia:

• Blocks sympathetic efferent discharge centrally, resulting in peripheral venous and arterial dilation that reduces both preload and afterload, thereby decreasing myocardial oxygen demand 1
• Reduces anxiety and circulating catecholamines, which may decrease associated arrhythmias 1
• Particularly beneficial in patients with acute pulmonary edema, as it alleviates work of breathing and favorably affects ventricular loading conditions 1

Dosing Protocol

Start with 2 to 4 mg IV, then titrate with increments of 2 to 8 mg repeated at 5 to 15-minute intervals based on pain relief and hemodynamic response. 1 Relatively large cumulative doses of 2 to 3 mg/kg are occasionally required. 1

Managing Morphine Side Effects

• For respiratory depression: Administer naloxone 0.1 to 0.2 mg IV every 15 minutes, though this will reverse pain relief as well 1
• For excessive bradycardia: Give atropine 0.5 to 1.5 mg IV 1
• For hypotension with inappropriate bradycardia (rare): Use leg elevation, fluids, and atropine 1
• Respiratory depression is rarely encountered in the setting of severe chest pain or pulmonary edema 1

Alternative Opioid Analgesics

If morphine is contraindicated due to documented hypersensitivity:

• Hydromorphone is an acceptable alternative 1
• Meperidine has been historically recommended for inferior wall infarction due to vagolytic properties, but when equipotent analgesic doses are given, it has no clear advantage over morphine 1
• Pentazocine should be avoided as it produces significant increases in systemic and pulmonary arterial pressures, left ventricular filling pressure, and systemic vascular resistance while decreasing ejection fraction—all deleterious in MI 2

Adjunctive Anti-Ischemic Therapy for Pain

Pain in acute MI results from continuing ischemia of living jeopardized myocardium rather than completed necrosis. 1 Therefore, anti-ischemic interventions can reduce pain:

• Intravenous nitroglycerin for ongoing chest pain: Start with 5 to 10 µg/min and increase by 5 to 10 µg/min every 5 to 10 minutes while monitoring blood pressure (never allow systolic BP <90 mm Hg) 1
• Oxygen for hypoxemic patients (oxygen saturation <90%), though routine oxygen in normoxemic patients may increase coronary vascular resistance 1
• Beta-adrenergic blocking agents reduce myocardial oxygen demand 1

Absolutely Contraindicated Analgesics

NSAIDs (both nonselective and COX-2 selective agents) are contraindicated and must be discontinued immediately. 1 These drugs are associated with:

• Increased risk of death 1
• Reinfarction 1
• Cardiac rupture 1
• Hypertension 1
• Heart failure 1
• Renal insufficiency 1

The ExTRACT TIMI-25 trial demonstrated increased risk of death, reinfarction, heart failure, or shock among patients taking NSAIDs within 7 days of enrollment. 1

Special Considerations for Inferior MI with Right Ventricular Involvement

In patients with right ventricular infarction (occurs in up to 50% of inferior MIs):

• Avoid nitrates entirely as they can cause profound hypotension by reducing preload 3, 4
• Morphine remains appropriate for analgesia but use cautiously with careful hemodynamic monitoring 4
• Maintain RV preload with IV normal saline for hypotension rather than relying solely on analgesics to reduce sympathetic drive 3, 4

Critical Timing Consideration

Effective analgesia should be promptly administered at the time of diagnosis and not unreasonably delayed to evaluate the results of anti-ischemic therapy or reperfusion. 1 While rapid pain relief often occurs after early reperfusion with thrombolytic therapy or PCI, waiting for reperfusion to relieve pain is inappropriate. 1

Addressing Recent Controversy

Recent retrospective analyses have raised concerns about morphine potentially delaying antiplatelet drug absorption and worsening outcomes. 5 However, the highest quality prospective data from the CIRCUS trial (969 patients with anterior STEMI followed for 1 year) found no significant difference in major adverse cardiovascular events between patients who received morphine versus those who did not (26.2% versus 22.0%, P=0.15), with similar all-cause mortality (5.3% versus 5.8%, P=0.89) and infarct size. 6 Given this prospective evidence and the continued Class I recommendation from ACC/AHA guidelines, morphine remains the standard of care. 1