Excerpt from Myocardial Infarction

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Background

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 myocardium.

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 diagnosis.

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.

Pathophysiology

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 injuries. For additional information, see Medscape's article "New Definition of 'MI' Poised for World Domination".

Frequency

United States

MI is a leading cause of morbidity and mortality in the United States. Approximately 1.3 million cases of nonfatal MI are reported each year, for an annual incidence rate of approximately 600 cases per 100,000 people. The proportion of patients diagnosed with NSTEMI compared with STEMI has progressively increased.

International

Cardiovascular diseases account for 12 million deaths annually worldwide. MI continues to be a significant problem in industrialized countries and is becoming an increasingly significant problem in developing countries.

Mortality/Morbidity

Approximately 500,000-700,000 deaths are caused by ischemic heart disease annually in the United States.

One third of patients who experience STEMI die within 24 hours of the onset of ischemia, and many of the survivors experience significant morbidity. For many patients, the first manifestation of coronary artery disease is sudden death likely from malignant ventricular dysrhythmia.

• More than one half of deaths occur in the prehospital setting.
• In-hospital fatalities account for 10% of all deaths. An additional 10% of deaths occur in the first year postinfarction.
• A steady decline has occurred in the mortality rate from STEMI over the last several decades. This appears to be due to a combination of a fall in the incidence of MI (replaced in part by an increase in the incidence of unstable angina) and a reduction in the case-fatality rate once an MI has occurred.

Sex

A male predilection exists in persons aged 40-70 years. Evidence exists that women more often have MIs without atypical symptoms. The atypical presentation in women might explain the sometimes delayed diagnosis of MIs in women.

In persons older than 70 years, no sex predilection exists.

Age

MI most frequently occurs in persons older than 45 years.

Certain subpopulations younger than 45 years are at risk, particularly cocaine users, persons with type 1 diabetes mellitus, patients with hypercholesterolemia, and those with a positive family history for early coronary disease. A positive family history includes any first-degree male relative aged 45 years or younger or any first-degree female relative aged 55 years or younger who experienced a myocardial infarction. In younger patients, the diagnosis may be hampered if a high index of suspicion is not maintained.

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