---
title: Acute Myocardial Infarction Protocol
description: A comprehensive clinical reference for the diagnosis and management of acute myocardial infarction.
keywords: [acute coronary syndrome, STEMI, NSTEMI, troponin, thrombolytics]
category: clinical
---

## Diagnostic Criteria

The diagnosis of acute myocardial infarction (AMI) is established by the presence of acute myocardial injury, as detected by elevated cardiac troponin values, in the setting of acute myocardial ischemia. The 4th Universal Definition of Myocardial Infarction provides the following criteria:

1. Detection of a rise and/or fall of cardiac troponin values with at least one value above the 99th percentile upper reference limit, and at least one of the following:
   - Symptoms of myocardial ischemia
   - New ischemic ECG changes
   - Development of pathological Q waves
   - Imaging evidence of new loss of viable myocardium or new regional wall motion abnormality
   - Identification of an intracoronary thrombus by angiography or autopsy

2. Cardiac death with symptoms suggestive of myocardial ischemia and presumed new ischemic ECG changes or new LBBB, but death occurred before cardiac biomarkers were obtained or before cardiac biomarker values would be increased.

Patients are further classified as having either ST-elevation myocardial infarction (STEMI) or non-ST-elevation myocardial infarction (NSTEMI) based on the presence or absence of ST-segment elevation on the ECG.

## Initial Management

The primary goals of initial management are to quickly restore coronary blood flow, minimize myocardial damage, and provide supportive care. Key elements include:

1. Oxygen therapy: Titrate to maintain SpO2 ≥ 94%.
2. Aspirin: Administer 162-325 mg chewable aspirin.
3. Antianginal therapy:
   - Nitroglycerin: 0.4 mg sublingual, may repeat every 5 minutes as needed for chest pain.
   - Morphine sulfate: 2-4 mg IV, may repeat every 5-15 minutes as needed for chest pain.
4. Anticoagulation:
   - STEMI: Administer unfractionated heparin 60 U/kg bolus (maximum 4000 U), then 12 U/kg/hr infusion (maximum 1000 U/hr).
   - NSTEMI: Administer enoxaparin 1 mg/kg subcutaneously every 12 hours or unfractionated heparin.
5. Antiplatelet therapy:
   - STEMI: Ticagrelor 180 mg loading dose, then 90 mg twice daily.
   - NSTEMI: Ticagrelor 180 mg loading dose, then 90 mg twice daily, or clopidogrel 600 mg loading dose, then 75 mg daily.

## Reperfusion Therapy

The choice and timing of reperfusion therapy depends on the type of myocardial infarction, time from symptom onset, and patient characteristics.

1. STEMI:
   - If symptom onset is < 12 hours and there are no contraindications, perform primary percutaneous coronary intervention (PCI) as soon as possible, with a target door-to-balloon time of ≤ 90 minutes.
   - If PCI is not available within 120 minutes, administer fibrinolytic therapy (e.g., alteplase 100 mg IV over 90 minutes) unless contraindicated.

2. NSTEMI:
   - Perform an early invasive strategy (coronary angiography with intent to perform revascularization) within 24 hours for high-risk patients (GRACE score > 140, hemodynamic instability, recurrent angina, etc.).
   - For lower-risk patients, an initial conservative approach with medical therapy is reasonable, followed by angiography if the patient develops high-risk features.

## Risk Stratification and Monitoring

Patients with AMI should be closely monitored for complications and risk-stratified to guide management.

1. Vital signs: Monitor heart rate, blood pressure, respiratory rate, and oxygen saturation.
2. Cardiac biomarkers:
   - Troponin I or T: Obtain baseline and serial measurements every 3-6 hours.
   - CK-MB: Obtain baseline and serial measurements every 6-8 hours.
3. Electrocardiogram: Obtain serial 12-lead ECGs, especially if the patient develops new symptoms or hemodynamic instability.
4. Echocardiography: Perform to assess left ventricular function and detect complications (e.g., wall motion abnormalities, ventricular septal defect, mitral regurgitation).
5. Risk stratification:
   - GRACE risk score: Assess in-hospital and 6-month mortality risk.
   - TIMI risk score: Assess short-term (14-day) mortality and ischemic events.

## Complications and Management

Patients with AMI are at risk for several life-threatening complications that require prompt recognition and management.

1. Cardiogenic shock:
   - Defined as systolic BP < 90 mmHg with evidence of end-organ hypoperfusion.
   - Initiate vasopressor and inotropic support (e.g., norepinephrine, dopamine, dobutamine).
   - Consider mechanical circulatory support (e.g., intra-aortic balloon pump, extracorporeal membrane oxygenation).
2. Ventricular arrhythmias:
   - Ventricular fibrillation or pulseless ventricular tachycardia: Immediate defibrillation.
   - Sustained ventricular tachycardia: Administer amiodarone 300 mg IV bolus, then 900 mg IV over 24 hours.
3. Mechanical complications:
   - Acute mitral regurgitation: Vasodilators (e.g., nitroprusside) and inotropic support.
   - Ventricular septal defect: Urgent surgical repair.
   - Free wall rupture: Emergent surgical repair.

## Discharge and Follow-up

Patients should be evaluated for discharge readiness and provided with appropriate follow-up care.

1. Discharge criteria:
   - Stable vital signs and no evidence of ongoing ischemia or arrhythmias.
   - Adequate pain control with oral medications.
   - Ability to perform self-care and ambulate independently.
2. Discharge medications:
   - Aspirin, P2Y12 inhibitor (ticagrelor, clopidogrel), statin, ACE inhibitor or ARB, beta-blocker.
   - Consider aldosterone antagonist if left ventricular ejection fraction < 40%.
3. Follow-up:
   - Cardiac rehabilitation program.
   - Cardiology follow-up within 1-2 weeks.
   - Assess for secondary prevention measures (lifestyle modifications, risk factor control).

In summary, the acute management of myocardial infarction requires a comprehensive, evidence-based approach to rapidly restore coronary blood flow, minimize myocardial damage, and manage potential complications. Ongoing monitoring, risk stratification, and appropriate follow-up care are essential to optimize patient outcomes.