Myocardial infarction (MI) is the rapid development
of myocardial necrosis caused by a critical imbalance
between oxygen supply and demand of the myocardium. This
usually results from plaque rupture with thrombus
formation in a coronary vessel, resulting in an acute
reduction of blood supply to a portion of the
Although the clinical presentation of a patient is a
key component in the overall evaluation of the patient
with MI, many events are either "silent" or are
clinically unrecognized, evidencing that patients,
families, and health care providers often do not
recognize symptoms of a MI. The appearance of cardiac
markers in the circulation generally indicates
myocardial necrosis and is a useful adjunct to
Cardiac markers help to categorize MI, which is
considered part of a spectrum referred to as acute
coronary syndrome that includes ST-elevation MI (STEMI),
non–ST-elevation MI (NSTEMI), and unstable angina. This
categorization is valuable because patients with
ischemic discomfort may or may not have ST-segment
elevations on their electrocardiogram. Those without ST
elevations may ultimately be diagnosed with NSTEMI or
with unstable angina based on the presence or absence of
cardiac enzymes. Additionally, therapeutic decisions,
such as administering an intravenous thrombolytic or
performing percutaneous coronary intervention (PCI), are
often made based on this categorization
The most common cause of MI is narrowing of the
epicardial blood vessels due to atheromatous plaques.
Plaque rupture with subsequent exposure of the basement
membrane results in platelet aggregation, thrombus
formation, fibrin accumulation, hemorrhage into the
plaque, and varying degrees of vasospasm. This can
result in partial or complete occlusion of the vessel
and subsequent myocardial ischemia. Total occlusion of
the vessel for more than 4-6 hours results in
irreversible myocardial necrosis, but reperfusion within
this period can salvage the myocardium and reduce
morbidity and mortality.
Nonatherosclerotic causes of MI include coronary
vasospasm as seen in variant (Prinzmetal) angina and in
patients using cocaine and amphetamines; coronary emboli
from sources such as an infected heart valve; occlusion
of the coronaries due to vasculitis; or other causes
leading to mismatch of oxygen supply and demand, such as
acute anemia from GI bleeding. MI induced by chest
trauma has also been reported, usually following severe
chest trauma such as motor vehicle accidents and sports
The history is critical in making the diagnosis of MI
and sometimes may provide the only clues that lead to
the diagnosis in the initial phases of the patient
- Chest pain, usually across the anterior precordium
is typically described as tightness, pressure, or
- Pain may radiate to the jaw, neck, arms, back, and
epigastrium. The left arm is more frequently affected;
however, a patient may experience pain in both arms.
- Dyspnea, which may accompany chest pain or occur
as an isolated complaint, indicates poor ventricular
compliance in the setting of acute ischemia. Dyspnea
may be the patient's anginal equivalent, and, in an
elderly person or a patient with diabetes, it may be
the only complaint.
- Nausea, abdominal pain, or both often are present
in infarcts involving the inferior or posterior wall.
- Lightheadedness with or without syncope
- Nausea with or without vomiting
- Elderly patients and those with diabetes may have
particularly subtle presentations and may complain of
fatigue, syncope, or weakness. The elderly may also
present with only altered mental status. Those with
preexisting altered mental status or dementia may have
no recollection of recent symptoms and may have no
- As many as half of MIs are clinically silent in
that they do not cause the classic symptoms described
above and consequently go unrecognized by the patient.
A high index of suspicion should be maintained for MI
especially when evaluating women, patients with
diabetes, older patients, patients with dementia, and
those with a history of heart failure. Patients with a
permanent pacemaker in place may confound recognition
of STEMI by 12-lead ECG due to the presence of paced
The physical examination can often be
- Patients with ongoing symptoms usually lie quietly
in bed and appear pale and diaphoretic.
- Hypertension may precipitate MI, or it may reflect
elevated catecholamine levels due to anxiety, pain, or
- Hypotension may indicate ventricular dysfunction
due to ischemia. Hypotension in the setting of MI
usually indicates a large infarct secondary to either
decreased global cardiac contractility or a right
- Acute valvular dysfunction may be present.
Valvular dysfunction usually results from infarction
that involves the papillary muscle. Mitral
regurgitation due to papillary muscle ischemia or
necrosis may be present.
- Rales may represent congestive heart failure.
- Neck vein distention may represent pump failure.
With right ventricular failure, cannon jugular venous
a waves may be noted.
- Third heart sound (S3) may be present.
- A fourth heart sound is a common finding in
patients with poor ventricular compliance that is due
to preexisting heart disease or hypertension.
- Dysrhythmias may present as an irregular heartbeat
- Low-grade fever is not uncommon.
The most frequent cause of MI is rupture of an
atherosclerotic plaque within a coronary artery with
subsequent arterial spasm and thrombus formation.
Other causes include the following:
- Coronary artery vasospasm
- Ventricular hypertrophy (eg, left ventricular
hypertrophy [LVH], idiopathic hypertrophic subaortic
stenosis [IHSS], underlying valve disease)
- Hypoxia due to carbon monoxide poisoning or acute
pulmonary disorders (Infarcts due to pulmonary disease
usually occur when demand on the myocardium
dramatically increases relative to the available blood
- Coronary artery emboli, secondary to cholesterol,
air, or the products of sepsis
- Cocaine, amphetamines, and ephedrine
- Coronary anomalies, including aneurysms of the
- Increased afterload or inotropic effects, which
increase the demand on the myocardium
- Aortic dissection, with retrograde involvement of
the coronary arteries
- Although rare, pediatric coronary artery disease
may be seen with Marfan syndrome, Kawasaki disease,
Takayasu arteritis, progeria, and cystic medial
necrosis (see Myocardial
Infarction in Childhood).
Risk factors for atherosclerotic plaque formation
include the following:
- Male gender
- Hypercholesterolemia and hypertriglyceridemia,
including inherited lipoprotein disorders
- Diabetes mellitus
- Poorly controlled hypertension
- Type A personality
- Family history
- Sedentary lifestyle