What are the considerations for using nitroglycerin (nitroglycerin) infusion in a patient with an old inferior wall myocardial infarction (MI)?

Nitroglycerin Infusion in Old Inferior Wall MI

Nitroglycerin infusion can be used in patients with old inferior wall MI, but requires careful titration with close hemodynamic monitoring due to the risk of profound hypotension, particularly if there is concurrent right ventricular involvement. 1

Critical Risk Assessment Before Administration

The primary concern with nitroglycerin in inferior wall MI is the frequent association with right ventricular (RV) infarction, which creates a unique hemodynamic vulnerability:

• Patients with inferior MI and RV involvement are critically dependent on adequate right ventricular preload to maintain cardiac output and can experience profound hypotension during nitrate administration. 1
• In one study, 75% (15/20) of patients with inferior MI who developed marked hypotension (>30 mmHg drop with symptoms) after nitroglycerin had electrocardiographic evidence of RV involvement. 2
• Conversely, 90% (18/20) of inferior MI patients without hypotension after nitrates had no RV involvement. 2

Pre-Administration Evaluation

Before initiating nitroglycerin, assess for:

• Right-sided ECG to evaluate for RV infarction (≥1 mm ST elevation in ≥2 right precordial leads) - this is essential in inferior STEMI. 3, 2
• Baseline blood pressure must be ≥90 mmHg systolic; nitroglycerin is contraindicated if SBP <90 mmHg or ≥30 mmHg below baseline. 1, 3
• Assess for bradycardia or tachycardia, which are relative contraindications, especially with hypotension. 1, 3

When Nitroglycerin IS Indicated Despite Inferior MI

The ACC/AHA guidelines clearly state that nitroglycerin should be used with "extreme caution, if at all" in suspected RV infarction, but this does not constitute an absolute contraindication in all inferior MI cases. 1

Appropriate indications include:

• Congestive heart failure or pulmonary edema complicating the MI - benefit is well established. 1
• Ongoing ischemic chest pain unresponsive to sublingual nitroglycerin and beta-blockers. 4
• Hypertension in the setting of MI. 4
• Patients with old (not acute) inferior MI without evidence of RV involvement or current hemodynamic compromise. 1

Safe Administration Protocol

Initial Dosing with Non-Absorbing Tubing

• Start with 5 mcg/min via infusion pump (not 25 mcg/min, which was based on studies using PVC tubing that absorbs nitroglycerin). 4
• Increase by 5 mcg/min every 3-5 minutes until response is noted. 4
• If no response at 20 mcg/min, increase by 10 mcg/min increments, then 20 mcg/min if needed. 4
• Once partial response occurs, reduce dose increments and lengthen intervals between increases. 4

Titration End Points

• Control of clinical symptoms (pain, heart failure). 1
• Decrease mean arterial pressure by 10% in normotensive patients or 30% in hypertensive patients. 1, 5
• Never allow systolic blood pressure to fall below 90 mmHg. 1, 5
• Heart rate increase should not exceed 10 beats/min (generally not >110 beats/min). 1
• Decrease pulmonary artery end-diastolic pressure by 10-30% if monitoring available. 1

Monitoring Requirements

• Establish IV access immediately before first dose. 3
• Careful and frequent vital sign observation for several minutes after initial dose. 1, 5
• Continuous blood pressure and heart rate monitoring during titration. 1, 5
• Consider invasive hemodynamic monitoring if high doses required, blood pressure instability occurs, or uncertainty about left ventricular filling pressure exists. 1

Management of Hypotension

If hypotension develops during nitroglycerin infusion:

• Immediately discontinue nitroglycerin. 1, 5, 6
• Elevate legs to increase venous return. 1, 5, 6
• Administer rapid IV fluid bolus. 1, 5, 6
• Consider atropine if associated bradycardia is present. 1, 5, 6

Important Caveats

The "Old" MI Context

The question specifies an "old" inferior wall MI, which fundamentally changes the risk profile:

• The acute phase concerns about RV infarction and preload dependence are most critical during the first hours to days after MI. 1, 2
• In chronic/old MI, the hemodynamic vulnerability is substantially reduced unless there is residual RV dysfunction or ongoing ischemia. 1
• However, if the patient has chronic RV dysfunction from the old inferior MI, the same precautions apply. 1

Contradictory Evidence on Hypotension Risk

One recent prehospital study found no difference in hypotension rates between inferior STEMI (8.2%) and non-inferior STEMI (8.9%) after nitroglycerin administration. 7 However, this study:

• Used computer-interpreted ECGs without right-sided leads to assess RV involvement. 7
• Concluded that computer interpretation of inferior STEMI alone cannot predict hypotension risk. 7
• This reinforces that the critical factor is RV involvement, not simply inferior location. 2, 7

Dosing Considerations

• Doses >200 mcg/min are associated with increased hypotension risk; consider alternative vasodilator (calcium channel blocker) at this point. 1
• Prolonged infusion causes nitrate tolerance; effectiveness usually returns after 12 hours off nitroglycerin. 1, 6
• Some patients with normal left ventricular filling pressures may be hypersensitive and respond fully to doses as small as 5 mcg/min. 4

Additional Contraindications

• Recent phosphodiesterase-5 inhibitor use (sildenafil/vardenafil within 24 hours, tadalafil within 48 hours) creates risk of life-threatening hypotension. 5, 6
• Severe anemia and increased intracranial pressure are contraindications. 6
• Nitroglycerin should not be mixed with any other medication and interferes with heparin anticoagulation. 4