What is Myocardial Infarction?
Myocardial infarction (MI) (ie, heart attack) is the irreversible necrosis of heart muscle secondary to prolonged ischemia. Approximately 1.5 million cases of MI occur annually in the United States.
Signs and symptoms of Myocardial Infarction
Patients with typical myocardial infarction may have the following prodromal symptoms in the days preceding the event (although typical STEMI may occur suddenly, without warning):
- Chest discomfort
Typical chest pain in acute myocardial infarction has the following characteristics:
- Intense and unremitting for 30-60 minutes
- Retrosternal and often radiates up to the neck, shoulder, and jaw and down to the ulnar aspect of 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 myocardial infarction:
- The patient’s heart rate is often increased secondary to 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 myocardial infarction or severe left ventricular dysfunction, hypotension is seen
- The respiratory rate may be increased in response to pulmonary congestion or anxiety
- Coughing, wheezing, and the production of frothy sputum may occur
- Fever is usually present within 24-48 hours, with the temperature curve generally parallel to the time course of elevations of creatine kinase (CK) levels in the blood. Body temperature may occasionally exceed 102°F
Myocardial Infarction Diagnosis
Laboratory tests used in the diagnosis of myocardial infarction include the following:
- Cardiac biomarkers/enzymes: The American College of Cardiology/American Heart Association (ACC/AHA) guidelines on unstable angina/NSTEMI (non–ST-segment elevation myocardial infarction) recommend that in patients with suspected myocardial infarction, cardiac biomarkers should be measured at presentation
- Troponin levels: Troponin is a contractile protein that normally is not found in serum; it is released only when myocardial necrosis occurs
- Creatine kinase (CK) levels: CK-MB levels increase within 3-12 hours of the onset of chest pain, reach peak values within 24 hours, and return to baseline after 48-72 hours
- Myoglobin levels: Myoglobin is released more rapidly from infarcted myocardium than is troponin; urine myoglobin levels rise within 1-4 hours from the onset of chest pain
- Complete blood count
- Chemistry profile
- Lipid profile
- C-reactive protein and other inflammation markers
The ECG is the most important tool in the initial evaluation and triage of patients in whom an acute coronary syndrome (ACS), such as myocardial infarction, is suspected. It is confirmatory of the diagnosis in approximately 80% of cases.
For individuals with highly probable or confirmed ACS, a coronary angiogram can be used to definitively diagnose or rule out coronary artery disease.
See Workup for more detail.
For patients with chest pain, prehospital care includes the following:
- Intravenous access, supplemental oxygen, pulse oximetry
- Immediate administration of aspirin en route
- 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 myocardial infarction 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, the decision must be made quickly as to whether the patient should be treated with thrombolysis or with primary percutaneous coronary intervention (PCI)
Although patients presenting with no ST-segment elevation are not candidates for immediate thrombolytics, they should receive anti-ischemic therapy and may be candidates for PCI urgently or during admission.
Critical care units have reduced early mortality rates from acute myocardial infarction by approximately 50% by providing immediate defibrillation and by facilitating the implementation of beneficial interventions. These interventions include the administration of IV medications and therapy designed to do the following:
- Limit the extent of myocardial infarction
- Salvage jeopardized ischemic myocardium
- Recanalize infarct-related arteries