Description

Acute myocardial infarction (AMI) is the rapid development of myocardial necrosis resulting from a sustained and complete absence of blood flow to a portion of the myocardium. ST-segment elevation myocardial infarction (STEMI) occurs when coronary blood flow ceases, usually following complete atherothrombotic occlusion of a large coronary artery, resulting in transmural ischemia. This is accompanied by release of serum cardiac biomarkers and ST-segment elevation on an electrocardiogram (ECG).

Epidemiology

Incidence
There are >650,000 cases of AMI reported annually in the United States. Early revascularization and AMI management has improved mortality, with a 30-day survival of 95%.

Prevalence
Atherosclerotic heart disease is the leading cause of morbidity and mortality in the United States. ~7.5 million people in the United States are affected by AMI. Prevalence increases with age and is higher in men (5.5%) compared to women (2.9%).

Etiology and Pathophysiology

• Atherosclerotic coronary artery disease (CAD): Atherosclerotic lesions can be fibrotic, calcified, or lipid laden. Thin-capped atheromas are more likely to rupture, causing atherothrombotic occlusion.
• Nonatherosclerotic causes:
  • Embolism from either infective vegetations, or thrombi originating within the right atrium across the foramen ovale (“paradoxical”), from the left atrium or from within the left ventricle
  • Spontaneous coronary artery dissection: prevalent in fibromuscular dysplasia (FMD) and in young women
  • Mechanical or iatrogenic obstruction: chest trauma, dissection of the aorta and/or coronary arteries
  • Coronary artery spasm from increased vasomotor tone; anginal variant
  • Arteritis and other etiologies: hematologic causes (disseminated intravascular coagulation [DIC], severe anemia), aortic stenosis, cocaine, IV drug use, severe burns, prolonged hypotension

Risk Factors

Advancing age, hypertension, tobacco use, diabetes mellitus, dyslipidemia, family history of premature onset of CAD, sedentary lifestyle

General Prevention

Smoking cessation/abstinence; healthy diet; weight loss/control; regular physical activity and exercise; control of hypertension, hyperlipidemia, and diabetes mellitus

Commonly Associated Conditions

Abdominal aortic aneurysm, cerebrovascular disease, atherosclerotic peripheral vascular disease

History

• Symptoms:
  • Classically, sudden onset of chest heaviness/tightness, with or without exertion, lasting minutes to hours
  • Pain/discomfort radiating to neck, jaw, interscapular area, upper extremities, and/or epigastrium
  • Patients with inferior MI may present primarily with abdominal discomfort.
• Previous history of myocardial ischemia (stable or unstable angina, AMI, coronary bypass surgery, or percutaneous coronary intervention [PCI])
• Assess risk factors for CAD, history of bleeding, noncardiac surgery, and family history of premature CAD.
• Medications: Ask if there is a recent use of phosphodiesterase type 5 inhibitors (if recent use, avoid concomitant nitrates).
• Tobacco, alcohol, and/or drug abuse (especially cocaine)

Physical Exam

• General: restlessness, agitation, hypothermia, fever
• Neurologic: dizziness, syncope, fatigue, disorientation (especially in the elderly)
• Cardiovascular (CV): dysrhythmia, hypotension, widened pulse pressure, S3 and S4, jugular venous distention (JVD)
• Respiratory: dyspnea, tachypnea, crackles, rales
• GI: abdominal pain, nausea, vomiting, hiccups
• Skin: cool skin, pallor, diaphoresis

General Considerations

• Elderly patients may have an atypical presentation, including silent or unrecognized MI. They may often present with syncope, weakness, shortness of breath, unexplained nausea, epigastric pain, altered mental status, or delirium.
• Women or patients with diabetes mellitus may present with typical and/or “atypical” symptoms such as fatigue, dyspnea, and malaise.

Differential Diagnosis

Unstable angina, aortic dissection, pulmonary embolism (PE), perforating gastric ulcer, pericarditis, dysrhythmias, gastroesophageal reflux disease (GERD), esophageal spasm, biliary/pancreatic pain, hyperventilation syndrome, anxiety/panic

Diagnostic Tests & Interpretation

Initial Tests (lab, imaging)

• 12-lead ECG:
  • ≥1 mm ST elevation in a regional pattern, involving at least two contiguous leads, with or without abnormal Q waves
  • STEMI of posterior wall: ST depression ± tall R waves in V₁V₂
  • Absence of Q waves represents partial or transient occlusion or early infarction.
  • Consider right-sided and posterior chest leads if inferior MI pattern (examine V₃R, V₄R, V₇V₉).
  • In the setting of ventricular pacing or a prior left bundle branch block (LBBB), the Sgarbossa criteria or BARCELONA algorithm (1) may be helpful.
• 2-Dimensional transthoracic echocardiography is useful in evaluating regional wall motion in MI, left ventricular function, mechanical complications, and mural thrombus.
• Once diagnosis is suspected, emergent coronary angiography with PCI is preferred.

Follow-Up Tests & Special Considerations
Serum biomarkers

• Troponin I and T (cTnI, cTnT) rise 3 to 6 hours after onset of ischemic symptoms.
• Elevations in cTnI persist for 7 to 10 days, whereas cTnT elevations persist for 10 to 14 days after MI.
• High-sensitivity troponin is highly sensitive and provides faster recognition of AMI.
• Myoglobin fraction of creatine kinase-MB (CK-MB) adds little diagnostic value in assessment of possible AMI to troponin testing.

Pregnancy Considerations
Pregnant patients presenting with STEMI will need discussion of risks and benefits of invasive coronary angiography with radiation exposure to fetus. Management should otherwise be the same as in nonpregnant patients.

Diagnostic Procedures/Other

• Portable chest x-ray; transthoracic echocardiography; chest computed tomography angiography (CTA) scan may occasionally be of value acutely in equivocal presentations to evaluate for alternative diagnoses (aortic dissection, PE, ventricular aneurysm).
• Coronary angiography is the definitive test.

General Measures

Following emergent revascularization, admit the patient to the coronary care unit (CCU) or a telemetry unit with continuous ECG monitoring and bed rest and use:

• Antiarrhythmics as needed for unstable dysrhythmia
• Dual antiplatelet therapy (DAPT) with continuation of aspirin 81 mg/day with clopidogrel 75 mg/day or prasugrel 10 mg/day or ticagrelor 90 mg twice daily

Medication

Medication recommendations are based on the 2013 American College of Cardiology (ACC)/American Heart Association (AHA) guideline (2)[C] and 2017 European Society of Cardiology (ESC) guideline (3)[C].

First Line

• Supplemental oxygen 2 to 4 L/min for patients with oxygen saturation <90% or respiratory distress
• Nitroglycerin (NTG) sublingual 0.4 mg q5min for a total of 3 doses, followed by NTG IV if ongoing pain and/or hypertension and/or management of pulmonary congestion if no contraindications exist such as systolic <90 mm Hg or >30 mm Hg below baseline, right ventricle (RV) infarct, use of sildenafil or vardenafil within 24 hours or within 48 hours of tadalafil
• Morphine sulfate 4 to 8 mg IV with 2 to 8 mg IV repeated at 5- to 15-minute intervals to relieve pain, anxiety, or pulmonary congestion
• Antiplatelet agents:
  • Aspirin (ASA), non–enteric-coated, initial dose of 162 to 325 mg chewed
  • A loading dose of a P2Y12 inhibitor is recommended for patients with STEMI for whom PCI is planned. Prasugrel or ticagrelor is preferred.
    • Prasugrel 60 mg loading dose; prasugrel is contraindicated in patients with previous stroke/transient ischemic attack. Do not recommend in patients aged >75 years or in patients with low body weight (<60 kg).
    • Ticagrelor 180 mg loading dose; ticagrelor may cause transient dyspnea.
    • Clopidogrel 600 mg loading dose should be given if neither prasugrel nor ticagrelor is available.
    • Cangrelor may be considered in patients not pretreated with oral P2Y12 receptor inhibitors at the time of PCI or in those who are unable to take oral agents.
    • Standard recommendation for duration of DAPT is 6 to 12 months after PCI, depending on bleeding and ischemia risks.
• Anticoagulation therapy:
  • Unfractionated heparin (UFH) 70- to 100-U/kg IV bolus or
  • Enoxaparin 0.5-mg/kg IV bolus or
  • Bivalirudin 0.75-mg/kg IV bolus and then 1.75-mg/kg/hr infusion for up to 4 hours after procedure
• PCI versus fibrinolysis: The goal is to keep total ischemic time within 120 minutes. Door to needle time should be within 30 minutes or door to balloon time within 90 minutes.
  • Coronary reperfusion therapy
    • Primary PCI (balloon angioplasty, coronary stents) in the following:
      • Symptom onset of ≤12 hours
      • Symptom onset of ≤12 hours and contraindication to fibrinolytic therapy irrespective of time delay
      • Cardiogenic shock or acute severe heart failure (HF) irrespective of time delay from onset of MI
      • Evidence of ongoing ischemia 12 to 24 hours after symptom onset
  • Procedural considerations: Radial access is recommended over femoral access. PCI of infarct-related artery (IRA) is indicated.
  • Fibrinolysis
    • If presenting to a hospital without PCI capability and cannot be transferred to a PCI-capable facility to undergo PCI within 120 minutes of first medical contact
    • If no contraindications, administer within 12 to 24 hours of symptom onset, if there is evidence of ongoing ischemia.
      • Alteplase (tPA): 15-mg IV bolus, followed by 0.75 mg/kg (up to 50 mg) IV over 30 minutes and then 0.5 mg/kg (up to 35 mg) over 60 minutes; maximum of 100 mg over 90 minutes
      • Reteplase (rPA): 10 units IV bolus; give second bolus 30 minutes apart.
      • Tenecteplase (TNK-tPA): 30- to 50-mg (based on weight) IV bolus; recommend reducing to half dose in patients ≥75 years of age.
    • Adjunctive antiplatelet therapy with fibrinolysis
      • Aspirin: 162- to 325-mg loading dose followed by 81 mg/day indefinitely
      • Clopidogrel (300-mg loading dose for patients <75 years of age, 75-mg dose for patients >75 years of age); clopidogrel 75 mg/day should be continued for at least 14 days and up to 1 year.
    • Adjunctive anticoagulation therapy with fibrinolysis: Use anticoagulants (UFH, enoxaparin, or fondaparinux) as ancillary therapy to reperfusion therapy for a minimum of 48 hours and preferably duration of admission (up to 8 days) or until revascularization, if performed.
• Glycoprotein IIb/IIIa receptor antagonists at the time of primary PCI in selected patients if there is no reflow or thrombotic complications (abciximab, eptifibatide, or tirofiban)
• ACE inhibitors should be initiated orally within 24 hours of STEMI in patients with anterior infarction, HF, diabetes, or ejection fraction (EF) ≤0.40 unless contraindicated.
• High-intensity statin therapy should be started as early as possible.
• Mineralocorticoid receptor antagonist (spironolactone, eplerenone) is recommended in patients with EF <40% and HF or diabetes, who are already receiving an ACE inhibitor and a β-blocker (BB), if there is no renal failure or hyperkalemia.

Second Line
Long-acting nondihydropyridine calcium channel blocker (CCB) when BB is ineffective or contraindicated and EF is normal; do not use immediate-release nifedipine.

Issues For Referral

Transfer high-risk patients who receive fibrinolytic therapy as primary reperfusion therapy at a non–PCI-capable facility to a PCI-capable facility as soon as possible.

Surgery/Other Procedures

Urgent coronary artery bypass graft (CABG) surgery is indicated in patients with STEMI and coronary anatomy not amenable to PCI who have ongoing or recurrent ischemia, cardiogenic shock, severe HF, or other high-risk features.

Admission, Inpatient, and Nursing Considerations

All STEMI patients should be admitted to a CCU or an intensive cardiac care unit for evaluation and treatment.

Follow-up Recommendations

STEMI patients should follow up with a cardiologist 1 to 2 weeks postdischarge, every 3 months for the 1st year and then yearly. Emphasize medication adherence and encourage smoking cessation. Strongly consider referral to an exercise-based cardiac rehabilitation program.

Diet

Low-fat/healthy-fat diet: reduced intake of saturated fats (to <7% of total calories), eliminate trans-fatty acids (to <1% of total calories); Mediterranean diet is healthy.

Patient Education

May resume sexual activity ≥1 weeks after uncomplicated MI or 6 to 8 weeks after CABG; smoking cessation and low-fat diet

Complications

• Advanced age, diabetes, delayed or unsuccessful revascularization, reduced left ventricular systolic function, and evidence of congestive HF are all associated with worse prognosis.
• HF, myocardial wall rupture, left ventricular aneurysm, pericarditis, dysrhythmias, acute mitral regurgitation, and depression (common)

ICD-10

• I21.3 ST elevation (STEMI) myocardial infarction of unspecified site
• I24.9 Acute ischemic heart disease, unspecified
• I25.10 Athscl heart disease of native coronary artery w/o ang pctrs

SNOMED

• 394659003 Acute coronary syndrome (disorder)
• 401303003 Acute ST segment elevation myocardial infarction (disorder)
• 443502000 Atherosclerosis of coronary artery (disorder)