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Diagram of a myocardial infarction (2) of the tip of the anterior wall of the heart (an apical infarct) after occlusion (1) of a branch of the left coronary artery (LCA). In the diagram, RCA is the right coronary artery.
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Myocardial infarction (MI) or acute myocardial infarction (AMI), is the medical term for an event commonly known as a heart attack. It happens when blood stops flowing properly to part of the heart and the heart muscle is injured due to not getting enough oxygen. Usually this is because one of the coronary arteries that supplies blood to the heart develops a blockage due to an unstable buildup of white blood cells, cholesterol and fat. The event is called "acute" if it is sudden and serious.
A person having an acute myocardial infarction usually has sudden chest pain that sometimes travels to the left arm and the left side of the neck. The chest pain is referred to as "retrosternal" because it is felt behind the breastbone, also called the sternum. Additionally, the person often suffers from shortness of breath, sweating, nausea, vomiting, abnormal heartbeats, and anxiety. The anxiety is often described as a "sense of impending doom." Women experience less of these symptoms than men, but usually have shortness of breath, weakness, a feeling of indigestion, and fatigue. In many cases, in some estimates as high as 64 percent, the person does not have chest pain or other symptoms. These are called "silent" myocardial infarctions.
Immediate treatments for a suspected acute myocardial infarction include oxygen to help with breathing, aspirin, which prevents the blood from clotting and forming further blockages, and nitroglycerin to treat chest pain.
There are many ways a doctor can test a person to determine if they have had a myocardial infarction, such as electrocardiograms (ECGs) that trace the electrical signals in the heart, echocardiography that maps the heart by the sounds that it makes, internal images taken by magnetic resonance imaging (MRI), and testing the blood for substances associated with damage to the heart muscle. Common blood tests are for creatine kinase (CK-MB) and troponin, both of which are released by damaged muscles such as the heart.
ECG testing is used to differentiate between two types of myocardial infarctions based on the shape of the tracing. When the ST section of the tracing is higher than the baseline it is called an ST-elevation myocardial infarction (STEMI) which usually requires more aggressive treatment. STEMI is treated by restoring circulation to the heart, called reperfusion therapy, and typical methods are angioplasty where the arteries are pushed opened and thrombolysis where the blockage is removed using medications. Non-ST elevation myocardial infarction (NSTEMI) may be managed with medication, although angioplasty may be required if the person is considered to be at high risk. People who have multiple blockages of their coronary arteries, particularly if they also have diabetes, may also be treated with bypass surgery (CABG).
Ischemic heart disease, which includes myocardial infarction, angina and heart failure when it happens after myocardial infarction, was the leading cause of death for both men and women worldwide in 2011. Important risk factors are previous cardiovascular disease, older age, tobacco smoking, high blood levels of certain lipids (low-density lipoprotein cholesterol, triglycerides) and low levels of high density lipoprotein (HDL) cholesterol, diabetes, high blood pressure, lack of physical activity and obesity, chronic kidney disease, excessive alcohol consumption, the use of illicit drugs (such as cocaine and amphetamines), and chronic high stress levels.
There are two basic types of acute myocardial infarction based on pathology:
In the clinical context, a myocardial infarction can be further subclassified into a ST elevation MI (STEMI) versus a non-ST elevation MI (non-STEMI) based on ECG changes. The phrase heart attack is sometimes used incorrectly to describe sudden cardiac death, which may or may not be the result of acute myocardial infarction. A heart attack is different from, but can be the cause of cardiac arrest, which is the stopping of the heartbeat, and cardiac arrhythmia, an abnormal heartbeat. It is also distinct from heart failure, in which the pumping action of the heart is impaired; however severe myocardial infarction may lead to heart failure. A 2007 consensus document classifies myocardial infarction into five main types:
The onset of symptoms in myocardial infarction (MI) is usually gradual, over several minutes, and rarely instantaneous. Chest pain is the most common symptom of acute myocardial infarction and is often described as a sensation of tightness, pressure, or squeezing. Chest pain due to ischemia (a lack of blood and hence oxygen supply) of the heart muscle is termed angina pectoris. Pain radiates most often to the left arm, but may also radiate to the lower jaw, neck, right arm, back, and epigastrium, where it may mimic heartburn. Levine's sign, in which the patient localizes the chest pain by clenching their fist over the sternum, has classically been thought to be predictive of cardiac chest pain, although a prospective observational study showed that it had a poor positive predictive value.
Shortness of breath (dyspnea) occurs when the damage to the heart limits the output of the left ventricle, causing left ventricular failure and consequent pulmonary edema. Other symptoms include diaphoresis (an excessive form of sweating), weakness, light-headedness, nausea, vomiting, and palpitations. These symptoms are likely induced by a massive surge of catecholamines from the sympathetic nervous system which occurs in response to pain and the hemodynamic abnormalities that result from cardiac dysfunction. Loss of consciousness (due to inadequate cerebral perfusion and cardiogenic shock) and sudden death (frequently due to the development of ventricular fibrillation) can occur in myocardial infarctions.
Female, elderly, and diabetic patients report atypical symptoms more frequently than their male and younger counterparts. Women also report more numerous symptoms compared with men (2.6 on average vs 1.8 symptoms in men). The most common symptoms of MI in women include dyspnea (shortness of breath), weakness, and fatigue. Fatigue, sleep disturbances, and dyspnea have been reported as frequently occurring symptoms that may manifest as long as one month before the actual clinically manifested ischemic event. In women, chest pain may be less predictive of coronary ischemia than in men.
At least one-fourth of all myocardial infarctions are silent, without chest pain or other symptoms. These cases can be discovered later on electrocardiograms, using blood enzyme tests or at autopsy without a prior history of related complaints. Estimates of the prevalence of silent myocardial infarctions vary between 22 and 64%. A silent course is more common in the elderly, in patients with diabetes mellitus and after heart transplantation, probably because the donor heart is not fully innervated by the nervous system of the recipient. In people with diabetes, differences in pain threshold, autonomic neuropathy, and psychological factors have been cited as possible explanations for the lack of symptoms.
Any group of symptoms compatible with a sudden interruption of the blood flow to the heart are called an acute coronary syndrome.
The differential diagnosis includes other catastrophic causes of chest pain, such as pulmonary embolism, aortic dissection, pericardial effusion causing cardiac tamponade, tension pneumothorax, and esophageal rupture. Other non-catastrophic differentials include gastroesophageal reflux and Tietze's syndrome.
Heart attack rates are higher in association with intense exertion, be it psychological stress or physical exertion, especially if the exertion is more intense than the individual usually performs. The period of intense exercise and subsequent recovery is associated with about a 6-fold higher myocardial infarction rate (compared with other more relaxed time frames) for people who are very physically fit. For those in poor physical condition, the rate differential is over 35-fold higher. One observed mechanism for this phenomenon is increased pulse pressure, which increases stretching of the arterial walls. This stretching results in significant shear stress on atheromas, which results in debris breaking loose from these deposits. This debris floats through the blood vessels, eventually clogging the major coronary arteries.
Acute severe infection, such as pneumonia, can trigger myocardial infarction. A more controversial link is that between Chlamydophila pneumoniae infection and atherosclerosis. While this intracellular organism has been demonstrated in atherosclerotic plaques, evidence is inconclusive as to whether it can be considered a causative factor. Treatment with antibiotics in patients with proven atherosclerosis has not demonstrated a decreased risk of heart attacks or other coronary vascular diseases.
There is an association of an increased incidence of a heart attack in the morning hours, more specifically around 9 a.m. Some investigators have noticed that the ability of platelets to aggregate varies according to a circadian rhythm, although they have not proven causation.
Myocardial infarction results from atherosclerosis. Smoking appears to be the cause of about 36% of coronary artery disease and obesity 20%. Lack of exercise has been linked to 7-12% of cases. Job stress appear to play a minor role accounting for about 3% of cases.
Risk factors for myocardial infarction include:
Inflammation is known to be an important step in the process of atherosclerotic plaque formation. C-reactive protein (CRP) is a sensitive but non-specific marker for inflammation. Elevated CRP blood levels, especially measured with high-sensitivity assays, can predict the risk of MI, as well as stroke and development of diabetes. Moreover, some drugs for MI might also reduce CRP levels. The use of high-sensitivity CRP assays as a means of screening the general population is advised against, but it may be used optionally at the physician's discretion in patients who already present with other risk factors or known coronary artery disease. Whether CRP plays a direct role in atherosclerosis remains uncertain. Inflammation in periodontal disease may be linked to coronary heart disease, and, since periodontitis is very common, this could have great consequences for public health. Serological studies measuring antibody levels against typical periodontitis-causing bacteria found that such antibodies were more present in subjects with coronary heart disease. Periodontitis tends to increase blood levels of CRP, fibrinogen and cytokines; thus, periodontitis may mediate its effect on MI risk via other risk factors. Preclinical research suggests that periodontal bacteria can promote aggregation of platelets and promote the formation of foam cells. A role for specific periodontal bacteria has been suggested but remains to be established. There is some evidence that influenza may trigger an acute myocardial infarction.
Baldness, hair greying, a diagonal earlobe crease (Frank's sign) and possibly other skin features have been suggested as independent risk factors for MI. Their role remains controversial; a common denominator of these signs and the risk of MI is supposed, possibly genetic.
Calcium deposition is another part of atherosclerotic plaque formation. Calcium deposits in the coronary arteries can be detected with CT scans. Several studies have shown that coronary calcium can provide predictive information beyond that of classical risk factors.
Many of these risk factors are modifiable, so many heart attacks can be prevented by maintaining a healthier lifestyle. Physical activity, for example, is associated with a lower risk profile. Non-modifiable risk factors include age, sex, and family history of an early heart attack, which is thought of as reflecting a genetic predisposition. To understand epidemiological study results, it is important to note that many factors associated with MI mediate their risk via other factors. For example, the effect of education is partially based on its effect on income and marital status.
Acute myocardial infarction refers to two subtypes of acute coronary syndrome, namely non-ST-elevated myocardial infarction and ST-elevated myocardial infarction, which are most frequently (but not always) a manifestation of coronary artery disease. The most common triggering event is the disruption of an atherosclerotic plaque in an epicardial coronary artery, which leads to a clotting cascade, sometimes resulting in total occlusion of the artery. Atherosclerosis is the gradual buildup of cholesterol and fibrous tissue in plaques in the wall of arteries (in this case, the coronary arteries), typically over decades. Blood stream column irregularities visible on angiography reflect artery lumen narrowing as a result of decades of advancing atherosclerosis. Plaques can become unstable, rupture, and additionally promote a thrombus (blood clot) that occludes the artery; this can occur in minutes. When a severe enough plaque rupture occurs in the coronary vasculature, it leads to myocardial infarction (necrosis of downstream myocardium).
If impaired blood flow to the heart lasts long enough, it triggers a process called the ischemic cascade; the heart cells in the territory of the occluded coronary artery die (chiefly through necrosis) and do not grow back. A collagen scar forms in its place. Recent studies indicate that another form of cell death called apoptosis also plays a role in the process of tissue damage subsequent to myocardial infarction. As a result, the patient's heart will be permanently damaged. This myocardial scarring also puts the patient at risk for potentially life threatening arrhythmias, and may result in the formation of a ventricular aneurysm that can rupture with catastrophic consequences.
Injured heart tissue conducts electrical impulses more slowly than normal heart tissue. The difference in conduction velocity between injured and uninjured tissue can trigger re-entry or a feedback loop that is believed to be the cause of many lethal arrhythmias. The most serious of these arrhythmias is ventricular fibrillation (V-Fib/VF), an extremely fast and chaotic heart rhythm that is the leading cause of sudden cardiac death. Another life-threatening arrhythmia is ventricular tachycardia (V-Tach/VT), which may or may not cause sudden cardiac death. However, ventricular tachycardia usually results in rapid heart rates that prevent the heart from pumping blood effectively. Cardiac output and blood pressure may fall to dangerous levels, which can lead to further coronary ischemia and extension of the infarct.
The cardiac defibrillator is a device that was specifically designed to terminate these potentially fatal arrhythmias. The device works by delivering an electrical shock to the patient in order to depolarize a critical mass of the heart muscle, in effect "rebooting" the heart. This therapy is time dependent, and the odds of successful defibrillation decline rapidly after the onset of cardiopulmonary arrest.
Medical societies recommend that the physician confirm that a patient is at high risk for myocardial infarction before conducting imaging tests to make a diagnosis. Patients who have a normal ECG and who are able to exercise, for example, do not merit routine imaging. Imaging tests such as stress radionuclide myocardial perfusion imaging or stress echocardiography can confirm a diagnosis when a patient's history, physical exam, ECG and cardiac biomarkers suggest the likelihood of a problem.
The diagnosis of myocardial infarction can be made after assessing patient's complaints and physical status. ECG changes, coronary angiogram and levels of cardiac markers help to confirm the diagnosis. ECG gives valuable clues to identify the site of myocardial damage while coronary angiogram allows visualization of narrowing or obstructions in the heart vessels. At autopsy, a pathologist can diagnose a myocardial infarction based on anatomopathological findings.
A chest radiograph and routine blood tests may indicate complications or precipitating causes and are often performed upon arrival to an emergency department. New regional wall motion abnormalities on an echocardiogram are also suggestive of a myocardial infarction. Echo may be performed in equivocal cases by the on-call cardiologist. In stable patients whose symptoms have resolved by the time of evaluation, Technetium (99mTc) sestamibi (i.e. a "MIBI scan") or thallium-201 chloride can be used in nuclear medicine to visualize areas of reduced blood flow in conjunction with physiologic or pharmacologic stress. Thallium may also be used to determine viability of tissue, distinguishing whether non-functional myocardium is actually dead or merely in a state of hibernation or of being stunned.
WHO criteria formulated in 1979 have classically been used to diagnose MI; a patient is diagnosed with myocardial infarction if two (probable) or three (definite) of the following criteria are satisfied:
The WHO criteria were refined in 2000 to give more prominence to cardiac biomarkers. According to the new guidelines, a cardiac troponin rise accompanied by either typical symptoms, pathological Q waves, ST elevation or depression, or coronary intervention is diagnostic of MI.
The risk of a recurrent myocardial infarction decreases with strict blood pressure management and lifestyle changes, chiefly smoking cessation, regular exercise, a sensible diet for those with heart disease, and limitation of alcohol intake. People are usually commenced on several long-term medications post-MI, with the aim of preventing secondary cardiovascular events such as further myocardial infarctions, congestive heart failure or cerebrovascular accident (CVA). Unless contraindicated, such medications may include:
An MI requires immediate medical attention. Treatment attempts to salvage as much myocardium as possible and to prevent further complications, hence the phrase "time is muscle". Oxygen, aspirin, and nitroglycerin may be administered. Morphine was classically used if nitroglycerin was not effective; however, it may increase mortality in the setting of NSTEMI. Reviews of high flow oxygen in myocardial infarction found increased mortality and infarct size, calling into question the recommendation about its routine use. Other analgesics such as nitrous oxide are of unknown benefit.
Percutaneous coronary intervention (PCI) is the treatment of choice for STEMI if it can be performed in a timely manner. If PCI cannot be performed within 90 to 120 minutes then fibrinolysis, preferably within 30 minutes, is recommended. If after fibrinolysis there is significant cardiogenic shock, continued severe chest pain, or less than a 50% improvement in ST elevation after 90 minutes then rescue PCI is indicated emergently. After PCI people are generally placed on dual antiplatelet therapy for at least a year (which is generally aspirin and clopidogrel).
The prognosis post myocardial infarction varies greatly, depending on a person's health, the extent of the heart damage and the treatment given. For the period 2005–2008 in the United States, the median mortality at 30 days was 16.6% with a range from 10.9% to 24.9% depending on the hospital. Using variables available in the emergency room, people with a higher risk of adverse outcome can be identified. One study found that 0.4% of patients with a low-risk profile died after 90 days, whereas in high-risk people it was 21.1%.
Some of the more reproduced risk-stratifying factors include: age, hemodynamic parameters (such as heart failure, cardiac arrest on admission, systolic blood pressure, or Killip class of two or greater), ST-segment deviation, diabetes, serum creatinine, peripheral vascular disease and elevation of cardiac markers. Assessment of left ventricular ejection fraction may increase the predictive power. The prognostic importance of Q-waves is debated. Prognosis is significantly worsened if a mechanical complication such as papillary muscle or myocardial free wall rupture occurs. Morbidity and mortality from myocardial infarction has improved over the years due to better treatment.
Complications may occur immediately following the heart attack (in the acute phase), or may need time to develop (a chronic problem). Acute complications may include heart failure if the damaged heart is no longer able to adequately pump blood around the body; aneurysm or rupture of the myocardium; mitral regurgitation, in particular if the infarction causes dysfunction of the papillary muscle; Dressler's syndrome; and arrhythmias, such as ventricular fibrillation, ventricular tachycardia, atrial fibrillation and heart block. Longer-term complications include heart failure, atrial fibrillation, and the increased risk of a second myocardial infarction.
Myocardial infarction is a common presentation of ischemic heart disease/coronary artery disease. The World Health Organization estimated in 2004, that 12.2% of worldwide deaths were from ischemic heart disease; with it being the leading cause of death in high or middle income countries and second only to lower respiratory infections in lower income countries. Worldwide more than 3 million people have STEMIs and 4 million have NSTEMIs a year.
Rates of death from ischemic heart disease have slowed or declined in most high income countries, although cardiovascular disease still accounted for 1 in 3 of all deaths in the USA in 2008. In contrast, ischemic heart disease is becoming a more common cause of death in the developing world. For example in India, ischemic heart disease had become the leading cause of death by 2004 accounting for 1.46 million deaths (14% of total deaths) and deaths due to ischemic heart disease were expected to double during 1985–2015. Globally it is predicted that disability adjusted life years (DALYs) lost to ischemic heart disease will account for 5.5% of total DALYs in 2030, making it the second most important cause of disability (after unipolar depressive disorder), as well as the leading cause of death by this date.
At common law, in general a myocardial infarction is a disease, but may sometimes be an injury. This can create coverage issues in administration of no-fault insurance schemes such as workers' compensation. In general, a heart attack is not covered; however, it may be a work-related injury if it results, for example, from unusual emotional stress or unusual exertion. In addition, in some jurisdictions, heart attacks suffered by persons in particular occupations such as police officers may be classified as line-of-duty injuries by statute or policy. In some countries or states, a person having suffered from a myocardial infarction may be prevented from participating in activity that puts other people's lives at risk, for example driving a car or flying an airplane.
Patients who receive stem cell treatment by coronary artery injections of stem cells derived from their own bone marrow after a myocardial infarction (MI) show improvements in left ventricular ejection fraction and end-diastolic volume not seen with placebo. The larger the initial infarct size, the greater the effect of the infusion. Clinical trials of progenitor cell infusion as a treatment approach to ST elevation MI are proceeding.
There are currently three biomaterial and tissue engineering approaches for the treatment of post-MI conditions, but these are in an even earlier stage of medical research. Many questions and issues must be addressed before they can be applied to patients. The first involves polymeric left ventricular restraints in the prevention of heart failure. The second utilizes in vitro engineered cardiac tissue, which is subsequently implanted in vivo. The final approach entails injecting cells and/or a scaffold into the myocardium to create in situ engineered cardiac tissue.
|at=(help). doi:10.1097/MCC.0b013e328357f07b. PMID 22889871.
|at=(help). doi:10.1038/nrcardio.2012.156. PMID 23165072.
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