Acute Myocardial Infarction

Peter B. Berger, M.D. Duke University School of Medicine

Definition/Key Clinical Features
Differential Diagnosis
Best Tests
Best Therapy
Best Evidence

Definition/Key Clinical Features

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Differential Diagnosis

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Best Tests

Clinical Definition of MI (American College of Cardiology) Physical Exam


  • Elevated ST segment indicates > 90% likelihood of MI and high risk of mortality
  • CK-MB assays: lack specificity, because CK and CK-MB levels require ≥ 3 hr of profound ischemia to rise
  • Myoglobin level: rises more rapidly than CK but also lacks specificity
  • Troponin level: cardiac-specific; elevated level predicts subsequent cardiac events
  • Echocardiography: treatment should not be delayed for echocardiography when symptoms and ECG indicate acute MI; may be useful in patients with left bundle branch block or abnormal ECGs without ST segment elevation whose symptoms are atypical and in whom the diagnosis is uncertain
  • Radionuclide imaging: sensitive and specific for MI
Predischarge Exercise Testing
  • Generally recommended before discharge from the hospital to assess functional capacity and ability to return to activities of daily living and work
  • Contraindications: spontaneous postinfarction angina, congestive heart failure, hypotension, malignant ventricular arrhythmia; perform coronary angiography instead
Coronary Angiography
  • Patients who have received thrombolytic therapy: recommended only for those with hemodynamic instability or those with spontaneous or exercise-induced ischemia following uncomplicated MI
  • Patients who have not received thrombolytic therapy: may be helpful for those with hemodynamic compromise, postinfarction chest pain, or possible multivessel disease or reduced ventricular function

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Best Therapy

Emergent Therapy

  • Evaluate symptomatic patients < 10 min after arrival in ER
  • Avoid delay in treatment
  • Assess heart rate and BP
  • Perform 12-lead ECG
  • Administer oxygen for the first several hours after admission
  • Administer analgesia immediately
  • Administer aspirin as soon as MI is diagnosed, and continue indefinitely
  • Administer sublingual nitroglycerin if BP > 90 mm Hg
  • Admit to hospital with continuous ECG monitoring
  • Admit high-risk patients to CCU
Reperfusion Therapy
  • Best therapy achieves coronary patency most rapidly, depending on the capabilities of the institution
Thrombolytic Therapy
  • Should be administered < 60 min after presentation, < 30 min if possible
  • Indications
    • ST segment elevation in ≥ 2 leads and < 12 hr of chest pain
    • Classic symptoms of infarction and a bundle branch block that precludes detection of ST segment elevation
    • 6–12 hr of chest pain on presentation
  • Contraindications
    • Significant bleeding
    • Risk of intracerebral bleeding (e.g., advanced age, low body weight, hypertension, warfarin use, previous stroke, GI bleeding, recent surgery)
  • Thrombolytic agents
    • Front-loaded, weight-adjusted t-PA and I.V. heparin: moderately superior to other thrombolytic regimens but much more expensive; associated with a greater frequency of intracerebral hemorrhage than streptokinase
    • Streptokinase and I.V. heparin: contraindicated in patients who have recently received a dose of streptokinase
    • Streptokinase and subcutaneous heparin
    • Combination of I.V. t-PA and streptokinase given concurrently with I.V. heparin
    • Reteplase (recombinant t-PA): given as two boluses, together with aspirin and I.V. heparin; as effective as t-PA
    • Combination therapy: a thrombolytic agent plus a glycoprotein IIb/IIIa inhibitor; not currently indicated
Immediate Angioplasty
  • Preferred therapy for acute MI at institutions where it can be performed without delay; associated with lower morbidity and mortality than thrombolytic therapy alone
  • Alternative treatment strategy for patients with ST segment elevation MI initially assessed at a hospital without on-site cardiac surgery facilities
  • Combined use of stents and platelet glycoprotein inhibitors may normalize antegrade blood flow and reduce need for repeat procedures the following year
Coronary Artery Bypass Surgery
  • Achieves reperfusion more slowly than thrombolytic therapy and primary coronary angioplasty
  • Should be reserved for patients in whom primary coronary angioplasty is precluded or has failed and those with a ventricular septal defect, severe mitral regurgitation, or myocardial rupture
Adjunctive Medical Therapy
  • I.V. heparin: recommended for patients with suspected MI who are not treated with thrombolytics; should be discontinued > 24 hr before discharge
  • Low-molecular-weight heparin: may be as effective as unfractionated heparin; optimal dose is unknown; avoid in renal failure
  • Direct thrombin inhibitors: bivalirudin may reduce incidence of reinfarction but has been associated with increased bleeding events
  • Beta blockers: recommended for all patients with acute MI and without contraindications as early as possible, whether or not they receive reperfusion therapy; patients with the largest infarctions benefit the most; continue indefinitely in the absence of contraindications or side effects
  • ACE inhibitors: use in all patients with significant ventricular dysfunction (i.e., ejection fraction < 40%) contraindications; begin within the first 48 hr of infarction and increase cautiously to avoid hypotension
  • I.V. nitroglycerin: indicated in patients with persistent/recurrent chest pain after reperfusion therapy and those who do not receive reperfusion therapy
  • Prophylactic antiarrhythmic therapy: not recommended for patients without malignant ventricular ectopy
  • Calcium channel antagonists: indicated in patients with ischemia that persists despite use of aspirin, beta blockers, nitrate therapy, and I.V. heparin
  • Magnesium: indicated in patients with MI who have torsade de pointes–type ventricular tachycardia and those with magnesium deficiency
Preventive Therapy

Lipid-Lowering Therapy
  • Measure cholesterol < 24 hr after presentation; prescribe HMG-CoA reductase inhibitors for those patients with LDL level ≥ 130 mg/dl
  • Exercise
  • Weight reduction in overweight patients
  • Avoidance of dietary saturated fat and cholesterol
  • Smoking cessation
  • Dose: 81–162 mg
Holter Monitoring
  • When used before discharge, can help identify patients at increased risk for sudden cardiac death
  • Routine antiarrhythmic therapy not recommended
Risk-Factor Modification
  • Treatment of hypertension and hypercholesterolemia
  • Smoking cessation
  • Cardiac rehabilitation
  • Exercise program

Best Evidence

Berger PB, et al: Circulation 100:14, 1999 [PMID 10393675]

Braunwald E, et al: J Am Coll Cardiol 40:1366, 2002 [PMID 12383588]

Keeley EC, et al: Lancet 361:13, 2003 [PMID 12517460]

Myocardial infarction redefined: J Am Coll Cardiol 36:959, 2000 [PMID 10987628]

Rana JS, et al: Am Heart J 147:841, 2004 [PMID 15131540]

March 2006