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Myocardial Infarction

Practice Essentials

Myocardial infarction (MI) (ie, heart attack) is the irreversible death (necrosis) of heart muscle secondary to prolonged lack of oxygen supply (ischemia). Approximately 1.5 million cases of MI occur annually in the United States. See the images below.

Acute myocardial infarction, reperfusion type. In

Acute myocardial infarction, reperfusion type. In this case, the infarct is diffusely hemorrhagic. There is a rupture track through the center of this posterior left ventricular transmural infarct. The mechanism of death was hemopericardium.

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Acute anterior myocardial infarction.

Acute anterior myocardial infarction.

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See Are You Missing Subtle MI Clues on ECGs? Test Your Skills, a Critical Images slideshow, to help identify a variety of electrocardiographic abnormalities.

Signs and symptoms

Patients with typical MI may have the following symptoms in the days or even weeks preceding the event (although typical STEMI may occur suddenly, without warning):

Fatigue

Chest discomfort

Malaise

Typical chest pain in acute MI has the following characteristics:

Intense and unremitting for 30-60 minutes

Substernal, and often radiates up to the neck, shoulder, and jaw, and down the left arm

Usually described as a substernal pressure sensation that also may be characterized as squeezing, aching, burning, or even sharp

In some patients, the symptom is epigastric, with a feeling of indigestion or of fullness and gas

The patient’s vital signs may demonstrate the following in MI:

The patient’s heart rate is often increased (tachycardic) secondary to a high sympathoadrenal discharge

The pulse may be irregular because of ventricular ectopy, an accelerated idioventricular rhythm, ventricular tachycardia, atrial fibrillation or flutter, or other supraventricular arrhythmias; bradyarrhythmias may be present

In general, the patient’s blood pressure is initially elevated because of peripheral arterial vasoconstriction resulting from an adrenergic response to pain and ventricular dysfunction

However, with right ventricular MI or severe left ventricular dysfunction, hypotension and cardiogenic shock can be seen

The respiratory rate may be increased in response to pulmonary congestion or anxiety

Coughing, wheezing, and the production of frothy sputum may occur

See Clinical Presentation for more detail.

Diagnosis

Laboratory studies

Laboratory tests used in the diagnosis of MI include the following:

Cardiac biomarkers/enzymes: The American College of Cardiology/American Heart Association (ACC/AHA) and the European Society of Cardiology (ESC) guidelines recommend that cardiac biomarkers should be measured at presentation in patients with suspected MI, and that the only biomarker that is recommended to be used for the diagnosis of acute MI at this time is cardiac troponin due to its superior sensitivity and accuracy.

Troponin levels: Troponin is a contractile protein that normally is not found in serum; it is released only when myocardial necrosis occurs

Complete blood cell count

Comprehensive metabolic panel

Lipid profile

Electrocardiography

The ECG is the most important tool in the initial evaluation and triage of patients in whom an acute coronary syndrome (ACS), such as MI, is suspected. It is confirmatory of the diagnosis in approximately 80% of cases.

Cardiac imaging

For individuals with highly probable or confirmed acute MI, coronary angiography can be used to definitively diagnose or rule out coronary artery disease.

See Workup for more detail.

Management

Prehospital care

For patients with chest pain, prehospital care includes the following:

Intravenous access, supplemental oxygen if SaO2 is less than 90%, pulse oximetry

Immediate administration of nonenteric-coated chewable aspirin

Nitroglycerin for active chest pain, given sublingually or by spray

Telemetry and prehospital ECG, if available

Emergency department and inpatient care

Initial stabilization of patients with suspected MI and ongoing acute chest pain should include administration of sublingual nitroglycerin if patients have no contraindications to it.

The American Heart Association (AHA) recommends the initiation of beta blockers to all patients with STEMI (unless beta blockers are contraindicated).

If STEMI is present and the patient is within 90 minutes of a PCI-capable facility, the patient should undergo emergent coronary angiography and primary PCI. If the patient is longer than 120 minutes from a PCI-capable facility, fibrinolysis should be considered.

Although patients presenting without ST-segment elevation (non-STE-ACS) are not candidates for immediate administration of thrombolytic agents, they should receive anti-ischemic therapy and may be candidates for PCI urgently or during admission.

Coronary care units have reduced early mortality rates from acute MI by approximately 50% by providing immediate defibrillation and by facilitating the implementation of beneficial interventions. These interventions include the administration of intravenous (IV) medications and therapy designed to do the following:

Limit the extent of MI

Salvage jeopardized ischemic myocardium

Recanalize infarct-related arteries

See Treatment and Medication for more detail.

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