Why are nitrates contraindicated in patients with posterior myocardial infarction (MI)?

Nitrates in Posterior Myocardial Infarction: Contraindication Explained

Nitrates are contraindicated in posterior myocardial infarction because they can cause profound hypotension due to reduced right ventricular preload in patients with right ventricular involvement, which commonly accompanies posterior MI. 1

Pathophysiological Mechanism

Posterior myocardial infarction typically involves the right ventricle due to its anatomical proximity and shared blood supply. The contraindication for nitrates in this setting is based on several key physiological factors:

1. Right Ventricular Dependence on Preload: Patients with posterior MI frequently have right ventricular infarction, making them especially dependent on adequate RV preload to maintain cardiac output 1

2. Nitrate-Induced Venodilation: Nitrates primarily cause:

   • Peripheral vasodilation, particularly in the splanchnic and mesenteric circulations
   • Reduction in both right and left ventricular preload
   • Decreased venous return to the heart 1

3. Hemodynamic Consequences: When nitrates are administered to patients with posterior MI:

   • The reduced preload severely compromises an already damaged right ventricle
   • This leads to inadequate filling of the left ventricle
   • Resulting in dramatic drops in cardiac output and blood pressure 1, 2

Clinical Implications

The consequences of administering nitrates in posterior MI can be severe:

• Profound Hypotension: Can be life-threatening and difficult to reverse 2
• Reflex Tachycardia: May worsen myocardial ischemia by increasing oxygen demand 1
• Decreased Coronary Perfusion: Further compromises already ischemic myocardium
• Potential for Mortality: Severe hypotension or even death may occur when nitrates are used in patients with acute inferior myocardial infarction associated with right ventricular dysfunction or infarction 2

Diagnostic Considerations

Before administering nitrates in any MI, clinicians should:

• Obtain a right-sided ECG when inferior or posterior MI is suspected 1
• Assess for signs of right ventricular involvement (elevated JVP, clear lung fields with hypotension)
• Consider echocardiography to evaluate right ventricular function

Alternative Management Approaches

For patients with posterior MI requiring anti-ischemic therapy:

• Morphine Sulfate: Can be used for pain relief and mild preload reduction (1-5 mg IV) with careful blood pressure monitoring 1
• Beta-Blockers: Consider if no contraindications exist 3
• Fluid Management: May require volume expansion to maintain adequate preload in RV infarction

Common Pitfalls to Avoid

1. Failure to Recognize RV Involvement: Right-sided ECG leads should be obtained in all inferior/posterior MIs
2. Automatic Nitrate Administration: Avoid reflexively giving nitrates to all chest pain patients without considering MI location
3. Inadequate Monitoring: If nitrates must be used (benefit outweighs risk in select cases), start with very low doses and monitor blood pressure continuously
4. Concomitant Medications: Particularly dangerous when combined with phosphodiesterase-5 inhibitors (sildenafil, tadalafil, vardenafil) or ACE inhibitors 1, 2

By understanding these mechanisms and contraindications, clinicians can avoid potentially dangerous hypotensive episodes in patients with posterior myocardial infarction and provide safer, more appropriate treatment options.