Myocardial Infarction

Introduction
Background

Myocardial infarction (MI) is the irreversible necrosis of heart muscle secondary to prolonged ischemia. This usually results from an imbalance of oxygen supply and demand. The appearance of cardiac enzymes in the circulation generally indicates myocardial necrosis. Myocardial infarction is considered, more appropriately, part of a spectrum referred to as acute coronary syndromes (ACSs), which also includes unstable angina and non–ST-elevation MI (NSTEMI). Patients with ischemic discomfort may or may not have ST-segment elevation. Most of those with ST-segment elevation will develop Q waves. Those without ST elevations will ultimately be diagnosed with unstable angina or NSTEMI based on the presence of cardiac enzymes.

Myocardial infarction may lead to impairment of systolic function or diastolic function and to increased predisposition to arrhythmias and other long-term complications.

Pathophysiology

Atherosclerosis is the disease primarily responsible for most acute coronary syndrome cases. Approximately 90% of myocardial infarctions result from an acute thrombus that obstructs an atherosclerotic coronary artery. Plaque rupture is considered to be the major trigger of coronary thrombosis. Following plaque rupture, platelet activation and aggregation, coagulation pathway activation, and endothelial vasoconstriction occur and lead to coronary thrombosis and occlusion.

Consider nonatherosclerotic causes of acute myocardial infarctions in younger patients or if no evidence of atherosclerosis is noted. Such causes include coronary emboli from sources such as an infected cardiac valve through a patent foramen ovale (PFO), coronary occlusion secondary to vasculitis, primary coronary vasospasm (variant angina), cocaine use, or other factors leading to mismatch of oxygen supply and demand, as may occur with a significant gastrointestinal bleed.

Frequency
United States

Approximately 1.5 million cases of myocardial infarction occur each year.
International

Cardiovascular diseases cause 12 million deaths throughout the world each year, according to the third monitoring report of the World Health Organization, 1991-93. They cause half of all deaths in several developed countries and are one of the main causes of death in many developing countries; they are the major cause of death in adults everywhere.
Mortality/Morbidity
Cardiovascular disease is the leading cause of death in the United States; approximately 500,000-700,000 deaths related to the coronary artery occur each year.
Ischemic heart disease is the leading cause of death worldwide.
Approximately 6.3 million deaths due to heart disease occurred in 1990 worldwide, which represents 29% of all deaths. The prevalence of coronary artery disease (CAD) is increasing rapidly in nonindustrialized countries.
Race

Cardiovascular disease is the leading cause of morbidity and mortality among African American, Hispanic, and white populations in the United States.
Sex
A male predominance in incidence exists up to approximately age 70 years, when the sexes converge to equal incidence.
Premenopausal women appear to be somewhat protected from atherosclerosis, possibly owing to the effects of estrogen.
Age
Incidence increases with age.
Most patients who develop an acute myocardial infarction are older than 60 years. Elderly people also tend to have higher rates of morbidity and mortality from their infarcts.
Clinical
History

Symptoms of myocardial infarction include the following:

Chest pain
This is usually described as a substernal pressure sensation that also may be described as squeezing, aching, burning, or even sharp pain.
Prolonged chest discomfort lasting longer than 30 minutes is most compatible with infarction.
Radiation to the left arm or neck is common.
The sensation is precipitated by exertion and relieved by rest and nitroglycerin.
Chest pain may be associated with nausea, vomiting, diaphoresis, dyspnea, fatigue, or palpitations.
Atypical chest pain is common, especially in patients with diabetes and in elderly patients. However, any patient may present with atypical symptoms. These symptoms are considered the anginal equivalent for that patient.
Shortness of breath
Shortness of breath may be the patient's anginal equivalent or a symptom of heart failure.
It is due to elevated end-diastolic pressures secondary to ischemia, which may then lead to elevated pulmonary pressures.
Atypical presentations
20% of patients are asymptomatic or have atypical symptoms.
Atypical presentations are common and frequently lead to misdiagnoses.
A patient may, for example, present with abdominal discomfort or jaw pain as his or her anginal equivalent.
An elderly patient may present with altered mental status.
Low threshold should be maintained when evaluating high- and moderate-risk patients, as their anginal equivalents may mimic other presentations.
Women tend to present more commonly with atypical symptoms such as sharp pain, fatigue, weakness, and other nonspecific complaints.

Physical

Physical examination findings for myocardial infarction can vary; one patient may be comfortable in bed, with normal examination results, while another may be in severe pain with significant respiratory distress requiring ventilatory support.

Low-grade fever may be present.
Hypotension or hypertension can be observed depending on the extent of the myocardial infarction.
Fourth heart sound (S 4 ) may be heard in patients with ischemia. With ischemia, diastolic dysfunction is the first physiologically measurable effect and this can then cause a stiff ventricle and an audible S 4 .
Dyskinetic cardiac bulge (in anterior wall myocardial infarction) can occasionally be palpated.
Systolic murmur can be heard if mitral regurgitation (MR) or ventricular septal defect (VSD) develops.
Other findings include cool, clammy skin and diaphoresis.
Signs of congestive heart failure (CHF) may be found, including the following:
Third heart sound (S 3 ) gallop
Pulmonary rales
Lower extremity edema
Elevated jugular venous pressure
Causes
Atherosclerosis with occlusive or partially occlusive thrombus formation
Nonmodifiable risk factors for atherosclerosis
Age
Sex
Family history of premature coronary heart disease
Modifiable risk factors for atherosclerosis
Smoking or other tobacco use
Diabetes mellitus
Hypertension
Dyslipidemia
Obesity
New and other risk factors for atherosclerosis
Elevated homocysteine levels
Male pattern baldness
Sedentary lifestyle and/or lack of exercise
Psychosocial stress
Presence of peripheral vascular disease
Poor oral hygiene
Nonatherosclerotic causes
Vasculitis
Coronary emboli
Congenital coronary anomalies
Coronary trauma
Coronary spasm
Drug use (cocaine)
Factors that increase oxygen requirement, such as heavy exertion, fever, or hyperthyroidism
Factors that decrease oxygen delivery, such as hypoxemia of severe anemia

http://emedicine.medscape.com/article/155919-overview