Can morphine and isosorbide dinitrate (nitrate) be given to a patient with an inferior wall ST Elevation Myocardial Infarction (STEMI) for pain relief and to reduce myocardial oxygen demand?

Morphine and Nitrates in Inferior Wall STEMI

Direct Answer

Yes, both morphine and isosorbide dinitrate can be given to patients with inferior wall STEMI, but nitrates require careful screening for right ventricular (RV) infarction and hemodynamic stability before administration. 1

Morphine Administration

Morphine sulfate is the analgesic of choice for all STEMI patients, including inferior wall STEMI, and should be administered intravenously at 2-4 mg with increments of 2-8 mg repeated at 5-15 minute intervals. 2, 1

Key Benefits of Morphine

• Provides pain relief while reducing myocardial oxygen demand through decreased preload and afterload 1
• Alleviates work of breathing and reduces anxiety, particularly beneficial in patients with acute pulmonary edema 2, 1
• Blocks sympathetic efferent discharge centrally and reduces circulating catecholamines 1

Safety Profile in Inferior STEMI

• A 2018 prospective study of 969 anterior STEMI patients found no significant increase in major adverse cardiovascular events at 1 year with morphine use (26.2% vs 22.0%, p=0.15), and no difference in all-cause mortality (5.3% vs 5.8%, p=0.89) 3
• While this study focused on anterior STEMI, the safety profile supports morphine use across STEMI locations 3

Managing Morphine Side Effects

• For excessive bradycardia: administer atropine 0.5-1.5 mg IV 2, 1
• For respiratory depression: give naloxone 0.1-0.2 mg IV every 15 minutes (though this reverses analgesia) 2, 1
• For hypotension with bradycardia: use leg elevation, fluids, and atropine 1

Isosorbide Dinitrate (Nitrate) Administration

Nitrates can be used in inferior wall STEMI but require mandatory screening for right ventricular infarction and specific contraindications before administration. 2

Absolute Contraindications to Nitrates

• Hypotension (systolic BP <90 mmHg) 2
• Right ventricular infarction 2, 4
• Marked bradycardia or tachycardia 2
• Phosphodiesterase inhibitor use within 24-48 hours 2

Critical Caveat: Right Ventricular Infarction Risk

Up to 60% of inferior wall STEMI patients develop hypotension, and profound hypotension is frequently precipitated by nitroglycerin administration in those with RV involvement. 4

• In a 1989 study, 15 of 20 patients (75%) who developed marked hypotension (>30 mmHg drop in systolic BP with symptoms) after nitrate administration had RV involvement 4
• Conversely, 18 of 20 patients (90%) without hypotension after nitrates had no RV involvement 4
• Among 28 patients with documented RV involvement, 20 (71%) developed hypotension in response to nitrates 4

Screening for RV Infarction

Before administering nitrates to any inferior STEMI patient, obtain right-sided precordial leads (V3R-V4R) to assess for ST-segment elevation ≥1 mm in at least 2 right precordial leads, which indicates RV involvement. 4

Reassuring Data on Computer-Interpreted Inferior STEMI

• A 2016 prehospital study of 805 STEMI patients found that hypotension occurred in 8.2% of inferior STEMIs vs 8.9% of non-inferior STEMIs after NTG (p=0.73) 5
• However, this study used computer interpretation alone without right-sided ECG assessment, which cannot reliably identify RV infarction 5
• The key distinction is that computer interpretation of inferior STEMI alone is insufficient—you must actively screen for RV involvement with right-sided leads 4, 5

When Nitrates Are Appropriate

• Intravenous nitroglycerin may be useful for inferior STEMI patients with hypertension or heart failure (without RV involvement) 2
• Can be used for ongoing chest pain starting at 5-10 µg/min, increasing by 5-10 µg/min every 5-10 minutes while monitoring blood pressure 1
• A 2008 study showed that intravenous isosorbide dinitrate combined with aspirin and clopidogrel in transient STEMI led to symptom resolution within 1.2±0.8 hours 6

No Role for Oral Nitrates

There is no role for routine use of oral nitrates in the convalescent phase of STEMI. 2

Clinical Algorithm for Inferior Wall STEMI

1. Immediately administer morphine sulfate 2-4 mg IV for pain relief without delay 1
2. Before considering nitrates, obtain right-sided ECG leads (V3R-V4R) to screen for RV infarction 4
3. Check blood pressure and heart rate to identify contraindications 2
4. If RV infarction is present (ST elevation ≥1 mm in ≥2 right precordial leads), nitrates are contraindicated 2, 4
5. If no RV involvement and BP is adequate, nitrates can be administered cautiously with close hemodynamic monitoring 2, 5

Medications to Absolutely Avoid

NSAIDs and COX-2 inhibitors are contraindicated in all STEMI patients and must be discontinued immediately, as they are associated with increased risk of death, reinfarction, cardiac rupture, hypertension, heart failure, and renal insufficiency. 2, 1