What are the loading doses of medications in an impending myocardial infarction (heart attack)?

Loading Doses for Impending Myocardial Infarction

For patients with impending myocardial infarction, immediately administer aspirin 162-325 mg orally (chewable or non-enteric coated) or 250-500 mg IV if oral route is unavailable, followed by clopidogrel 300 mg loading dose (600 mg if PCI planned within hours). 1, 2

Antiplatelet Loading Doses

Aspirin (First-Line, Immediate Administration)

• Loading dose: 150-325 mg orally (chewable/non-enteric coated formulation) 3, 1, 2
• Alternative: 250-500 mg IV if patient cannot take oral medications 3, 2
• Administer as soon as MI diagnosis is deemed probable, before any other interventions 1, 2
• Maintenance: 75-160 mg daily thereafter for life 3

Clopidogrel (Second Antiplatelet Agent)

• Loading dose: 300 mg orally for most patients 3, 2
• Loading dose: 600 mg orally if PCI is planned within hours (achieves more rapid and stronger platelet inhibition) 3
• Reduced dose: 75 mg (no loading dose) if age >75 years 2
• Administer immediately after aspirin, before any reperfusion therapy 2
• Maintenance: 75 mg daily 4

The CURE trial demonstrated that clopidogrel loading (300 mg) followed by 75 mg daily reduced the composite endpoint of CV death, MI, or stroke by 20% (9.3% vs 11.4%, p<0.001) when added to aspirin in acute coronary syndrome patients 4. The CREDO trial suggested that loading doses given at least 6-15 hours before PCI provide greater benefit than no loading dose 3.

Antiarrhythmic/Hemodynamic Loading Doses

For Ventricular Tachycardia (Sustained, Hemodynamically Stable)

Lidocaine:

• Loading: 1.0-1.5 mg/kg IV bolus 3
• Supplemental boluses: 0.5-0.75 mg/kg every 5-10 minutes up to maximum total loading dose of 3 mg/kg 3
• Maintenance infusion: 2-4 mg/min (30-50 µg/kg/min) 3
• Reduce infusion rates in elderly patients and those with CHF or hepatic dysfunction 3

Procainamide:

• Loading: 20-30 mg/min IV infusion, up to 12-17 mg/kg total 3
• Maintenance infusion: 1-4 mg/min 3
• Lower infusion rates required in renal dysfunction 3

Amiodarone:

• Loading: 150 mg IV over 10 minutes 3
• Supplemental boluses of 150 mg may be given over 10-30 minutes for recurrent arrhythmias (maximum 6-8 boluses per 24 hours) 3
• Maintenance: 1.0 mg/min for 6 hours, then 0.5 mg/min 3

For Atrial Fibrillation with Rapid Ventricular Response

Metoprolol:

• Loading: 2.5-5.0 mg IV over 2 minutes 3
• Repeat every 2-5 minutes up to total of 15 mg over 10-15 minutes 3
• Monitor heart rate, blood pressure, and ECG continuously 3
• Stop if systolic BP <100 mmHg or heart rate <50 bpm 3

Atenolol:

• Loading: 2.5-5.0 mg IV over 2 minutes 3
• Repeat to total of 10 mg over 10-15 minutes 3

For Symptomatic Bradycardia

Atropine:

• Loading: Rapid IV bolus of at least 0.5 mg 3
• Repeat up to total dose of 1.5-2.0 mg (0.04 mg/kg) 3
• Most effective for sinus bradycardia occurring within 6 hours of symptom onset 3
• Indicated for symptomatic sinus bradycardia (heart rate <50 bpm with hypotension, ischemia, or ventricular arrhythmia) 3

Critical Timing Considerations

The sequence matters: Aspirin must be given first, followed immediately by clopidogrel, before any reperfusion therapy is initiated 2. When delay to coronary angiography is anticipated, patients should receive clopidogrel as initial therapy rather than waiting for the catheterization laboratory 3. The early separation of event curves in clinical trials demonstrates that benefit begins within hours of loading dose administration 3.

Common Pitfalls to Avoid

• Never use enteric-coated aspirin for loading doses—it has slow onset of action 3
• Do not give atropine for infranodal AV block (associated with anterior MI and wide-complex escape rhythm)—it is ineffective and potentially harmful 3
• Avoid beta-blockers in patients with clinical signs of hypotension or heart failure during the acute phase 3
• Do not reduce lidocaine infusion rates without considering age, CHF, or hepatic dysfunction—toxicity risk increases substantially 3
• Never administer prasugrel loading dose (60 mg) within 24 hours of fibrin-specific thrombolytic or within 48 hours of non-fibrin-specific agent due to bleeding risk 1