What is the immediate management for an inferior wall myocardial infarction (MI)?

Immediate Management of Inferior Wall Myocardial Infarction

Administer aspirin 160-325 mg immediately upon arrival to the emergency department, as this reduces 35-day mortality by 21% when combined with reperfusion therapy, and proceed urgently with reperfusion strategy while monitoring closely for right ventricular involvement, which occurs in up to 50% of inferior MIs and dramatically alters management. 1, 2

Initial Stabilization and Pharmacotherapy

Aspirin Administration

• Give aspirin 160-325 mg immediately upon arrival, before ECG confirmation 1
• Use non-enteric-coated formulation for more rapid buccal absorption 3
• After initial loading dose, continue aspirin 75-162 mg daily indefinitely, as maintenance doses in this range are as effective as higher doses with lower bleeding risk 3

Oxygen and Analgesia

• Administer oxygen via nasal prongs only if oxygen saturation is less than 90%—avoid routine oxygen in non-hypoxemic patients as hyperoxia may increase myocardial injury 1
• Provide morphine sulfate or meperidine for adequate analgesia of chest pain 1

Nitrate Therapy - Critical Caveat for Inferior MI

• Avoid nitroglycerin completely if right ventricular infarction is suspected or confirmed, as it can cause profound hypotension by reducing preload 1, 2
• If RV involvement is ruled out, give sublingual nitroglycerin unless systolic blood pressure is less than 90 mmHg or heart rate is less than 50 or greater than 100 bpm 1
• For ongoing ischemia without RV involvement, use intravenous nitroglycerin for 24-48 hours in patients with heart failure, large anterior MI, persistent ischemia, or hypertension 1

Critical Early Assessment for Right Ventricular Involvement

Diagnostic Approach

• Obtain right-sided ECG lead V4R immediately upon presentation—ST-segment elevation ≥1mm in V4R is the single most predictive ECG finding for RV infarction (88% sensitivity, 78% specificity) 2
• This must be done early as ST elevation in V4R can resolve within 10 hours 2
• Look for the clinical triad of hypotension, clear lung fields, and elevated jugular venous pressure (specific but only 25% sensitive) 2
• Distended neck veins or Kussmaul's sign may indicate RV involvement 2

Management Algorithm for RV Infarction

If RV infarction is confirmed:

1. Maintain RV preload through aggressive volume loading with IV normal saline for hypotension—avoid nitrates and diuretics which reduce preload 2

2. Maintain AV synchrony through:

   • IV atropine 0.3-0.5 mg (repeated up to 1.5-2.0 mg total) for symptomatic bradycardia 2
   • AV sequential pacing for symptomatic high-degree heart block unresponsive to atropine 2
   • Prompt cardioversion for hemodynamically significant supraventricular tachycardia 2

3. Provide inotropic support with dobutamine if cardiac output fails to increase after volume loading 2

4. Consider intra-aortic balloon pump for persistent shock 2

Reperfusion Strategy

Primary PCI (Preferred)

• Primary PCI is the preferred reperfusion strategy when it can be performed within 120 minutes of STEMI diagnosis by experienced operators 1
• For inferior MI with RV involvement, early identification is crucial as it represents a high-risk subgroup requiring priority reperfusion 2

Fibrinolytic Therapy (If PCI Unavailable)

• If primary PCI cannot be performed within 120 minutes, initiate fibrinolytic therapy immediately if presenting within 12 hours of symptom onset and no contraindications exist 1
• Use fibrin-specific agents such as tenecteplase, alteplase, or reteplase 1
• Reduce tenecteplase dose by 50% in patients aged 75 years or older to minimize stroke risk 1
• After fibrinolysis, transfer for angiography between 3-24 hours in stable patients, or immediately if cardiogenic shock, severe heart failure, or failed reperfusion occurs 3

Anticoagulation

Administer one of the following 3:

• Unfractionated heparin: IV bolus of 60 U/kg (maximum 4000 U) followed by infusion of 12 U/kg/h (maximum 1000 U) initially, adjusted to maintain aPTT at 1.5-2.0 times control for 48 hours or until revascularization 3

• Enoxaparin: If age <75 years: 30 mg IV bolus, followed in 15 minutes by 1 mg/kg subcutaneously every 12 hours (maximum 100 mg for first 2 doses); if age ≥75 years: no bolus, 0.75 mg/kg subcutaneously every 12 hours (maximum 75 mg for first 2 doses) 3

• Fondaparinux: Initial dose 2.5 mg IV, then 2.5 mg subcutaneously daily (contraindicated if creatinine clearance <30 mL/min) 3

Additional Pharmacotherapy Within First 24 Hours

Beta-Blocker Therapy

• Administer early intravenous beta-blocker therapy followed by oral therapy if no contraindications exist 4
• Metoprolol dosing: Three IV bolus injections of 5 mg each at approximately 2-minute intervals during hemodynamic monitoring; if tolerated, begin oral metoprolol 50 mg every 6 hours 15 minutes after last IV dose, continued for 48 hours, then 100 mg twice daily 5
• Avoid in patients who are hemodynamically unstable 6

ACE Inhibitor Therapy

• Initiate ACE inhibitors within the first 24 hours in patients with heart failure, LVEF <40%, diabetes, or anterior infarction 1
• Lisinopril should be started under close medical supervision in patients with systolic blood pressure below 100 mmHg 6
• Avoid in hemodynamically unstable patients after acute MI 6

Statin Therapy

• Start high-intensity statin therapy as soon as possible and maintain long-term 1

Monitoring and Common Pitfalls

Critical Monitoring

• Establish continuous cardiac monitoring immediately with emergency resuscitation equipment readily available 4
• Monitor for conduction disturbances—sinus bradycardia is common in the first hour of inferior MI 2
• Perform echocardiography to evaluate LV function and detect RV involvement 4

Common Pitfalls to Avoid

• Never administer nitrates in RV infarction—this can cause profound hypotension 1, 2
• Do not routinely combine fibrinolysis with planned immediate PCI, as this increases mortality 1
• Do not give thrombolytics to patients with non-ST-elevation MI 1
• Do not delay aspirin administration waiting for ECG confirmation 1
• Do not use routine oxygen in non-hypoxemic patients 1
• Do not miss early recording of lead V4R, as ST elevation can resolve within 10 hours 2
• Avoid volume depletion, which may mask signs of RV involvement 2