NCLEX Review Guide: Myocardial Infarction

Definition and Mechanism

• Myocardial infarction (MI) is the death of cardiac muscle tissue due to prolonged ischemia, most commonly caused by occlusion of a coronary artery by thrombus formation following rupture of an atherosclerotic plaque. The obstruction prevents adequate blood flow to the myocardium, resulting in tissue hypoxia and eventual necrosis if blood flow is not restored promptly.
• The extent of damage depends on the location of the occlusion, duration of ischemia, presence of collateral circulation, and metabolic demands of the heart at the time of infarction.

Classification of MI

• MIs are classified based on pathological, clinical, and prognostic differences into ST-Elevation MI (STEMI) and Non-ST-Elevation MI (NSTEMI). STEMI results from complete occlusion of a coronary artery causing full-thickness (transmural) infarction, while NSTEMI typically results from partial occlusion causing partial-thickness damage.
• MIs can also be classified anatomically based on the affected heart wall: anterior, inferior, posterior, lateral, or septal, which corresponds to the occluded coronary artery.

Classic Symptoms

• Chest pain/discomfort is the cardinal symptom, typically described as crushing, squeezing, pressure, or heaviness in the center or left side of the chest, often radiating to the left arm, jaw, neck, back, or epigastrium. The pain usually lasts longer than 20 minutes and is not fully relieved by rest or nitroglycerin.
• Associated symptoms include dyspnea, diaphoresis, nausea, vomiting, lightheadedness, fatigue, anxiety, and a sense of impending doom.

Physical Examination Findings

• Physical examination may reveal tachycardia, hypertension or hypotension, tachypnea, diaphoresis, and S3 or S4 heart sounds. Patients may appear anxious, pale, and in distress.
• Additional findings may include signs of heart failure such as jugular venous distention, pulmonary crackles, and peripheral edema, particularly in extensive MI or in patients with pre-existing cardiac dysfunction.

ECG Changes

• The 12-lead ECG is the initial diagnostic tool for suspected MI, with changes evolving through several stages: hyperacute T waves, ST-segment elevation, Q wave development, T wave inversion, and resolution of ST elevation. These changes help localize the infarct and guide treatment decisions.
• Specific ECG patterns correlate with the anatomical location of the infarction: anterior MI (leads V1-V4), inferior MI (leads II, III, aVF), lateral MI (leads I, aVL, V5-V6), and posterior MI (reciprocal changes in V1-V3).

Cardiac Biomarkers

• Cardiac troponins (cTnI and cTnT) are the preferred biomarkers for MI diagnosis due to their high sensitivity and specificity for myocardial injury. Troponin levels rise within 3-12 hours after symptom onset, peak at 24-48 hours, and may remain elevated for 7-14 days.
• Other biomarkers include creatine kinase-MB (CK-MB), which rises within 4-6 hours, peaks at 24 hours, and returns to normal within 48-72 hours. Myoglobin rises early (1-3 hours) but lacks cardiac specificity.

Additional Diagnostic Tests

• Echocardiography can identify wall motion abnormalities, assess ejection fraction, and detect mechanical complications of MI. It is particularly useful when diagnosis is uncertain or complications are suspected.
• Coronary angiography is the definitive test to visualize coronary anatomy, identify the culprit lesion, and guide revascularization strategies. Additional imaging modalities may include cardiac MRI, CT angiography, and nuclear perfusion studies.

Immediate Interventions

• The primary goal in acute MI management is to restore coronary perfusion as quickly as possible to limit myocardial damage. The phrase "time is muscle" emphasizes the importance of rapid intervention.
• Initial management includes oxygen therapy (if SpO2 <94%), pain relief with nitroglycerin and morphine, aspirin administration, and continuous cardiac monitoring.

Initial Management Protocol

1. Provide supplemental oxygen if SpO2 <94% or respiratory distress
2. Administer aspirin 162-325 mg chewed immediately
3. Obtain IV access and draw blood for cardiac biomarkers and basic labs
4. Administer sublingual nitroglycerin for ongoing chest pain
5. Consider morphine 2-4 mg IV if pain persists despite nitroglycerin
6. Obtain 12-lead ECG within 10 minutes of arrival
7. Initiate continuous cardiac monitoring
8. Determine reperfusion strategy based on ECG findings

Reperfusion Strategies

• Primary percutaneous coronary intervention (PCI) is the preferred reperfusion strategy for STEMI if it can be performed within 90 minutes of first medical contact. It involves cardiac catheterization with balloon angioplasty and stent placement to open the occluded artery.
• Fibrinolytic therapy should be administered within 30 minutes of hospital arrival when PCI cannot be performed within the recommended timeframe. Common agents include alteplase, reteplase, and tenecteplase.

Pharmacological Management

• Antiplatelet therapy includes dual antiplatelet therapy (DAPT) with aspirin and a P2Y12 inhibitor (clopidogrel, ticagrelor, or prasugrel). Aspirin is given immediately and continued indefinitely, while P2Y12 inhibitors are typically continued for at least 12 months after acute MI.
• Anticoagulation with unfractionated heparin, low-molecular-weight heparin, or bivalirudin is used during the acute phase. Additional medications include beta-blockers, ACE inhibitors/ARBs, and statins, which form the cornerstone of secondary prevention.

Mechanical Complications

• Ventricular free wall rupture is a catastrophic complication with high mortality, typically occurring 3-5 days post-MI. It presents with sudden onset of severe chest pain, electromechanical dissociation, and rapid progression to cardiogenic shock and death.
• Other mechanical complications include ventricular septal rupture, papillary muscle rupture causing acute mitral regurgitation, and left ventricular aneurysm formation. These complications typically present with sudden hemodynamic deterioration and new murmurs.

Electrical Complications

• Arrhythmias are common following MI and include ventricular tachycardia, ventricular fibrillation, atrial fibrillation, and various degrees of heart block. The specific arrhythmias depend on the infarct location and extent of damage.
• Conduction disturbances are particularly common with inferior MI (AV blocks) and anterior MI (bundle branch blocks). High-degree AV blocks with inferior MI are often transient, while those with anterior MI typically indicate extensive damage and poor prognosis.

Heart Failure and Cardiogenic Shock

• Heart failure following MI results from loss of functioning myocardium, with clinical manifestations ranging from mild dyspnea to pulmonary edema depending on the extent of damage. Killip classification (I-IV) is used to stratify heart failure severity and predict mortality.
• Cardiogenic shock is the most severe form of heart failure, characterized by systolic BP <90 mmHg, signs of tissue hypoperfusion, and cardiac index <2.2 L/min/m². It typically occurs when >40% of the left ventricle is damaged and carries a mortality rate of 50-70% despite treatment.

Assessment and Monitoring

• Nursing assessment should focus on vital signs, cardiac rhythm, pain assessment, hemodynamic parameters, and signs of complications. Continuous cardiac monitoring is essential to detect arrhythmias, and frequent vital sign checks help identify hemodynamic instability.
• Regular assessment for signs of heart failure (crackles, JVD, S3 heart sound), cardiogenic shock (hypotension, altered mental status, oliguria), and mechanical complications (new murmurs) is crucial for early intervention.

Nursing Assessment Protocol

1. Assess vital signs every 15 minutes until stable, then every 1-4 hours
2. Monitor cardiac rhythm continuously for arrhythmias
3. Assess pain using standardized scale; document location, quality, radiation, intensity
4. Auscultate heart and lung sounds every 4 hours
5. Monitor urine output hourly in unstable patients
6. Check peripheral pulses and capillary refill
7. Assess for signs of complications: new murmurs, crackles, JVD
8. Monitor oxygen saturation continuously

Nursing Interventions

• Nursing interventions focus on maintaining cardiac perfusion, managing symptoms, preventing complications, and providing psychological support. Activity restrictions, positioning for comfort, and oxygen administration as needed are essential components of care.
• Medication administration and monitoring for effectiveness and adverse effects are key nursing responsibilities. This includes antiplatelet agents, anticoagulants, beta-blockers, ACE inhibitors, statins, and analgesics.

Patient Education

• Patient education is crucial for secondary prevention and should begin during hospitalization. Topics include medication adherence, risk factor modification (smoking cessation, diet, exercise), recognition of warning signs, and when to seek medical attention.
• Cardiac rehabilitation should be recommended for all MI patients, as it improves outcomes, reduces mortality, and helps patients return to normal activities. Education about the rehabilitation process and its benefits improves participation rates.