Myocardial Infarction

Definition

A myocardial infarction, or heart attack, is the death or damage of part of the heart muscle because the supply of blood to the heart muscle is severely reduced or stopped.

Description

Myocardial infarction (MI) is the leading cause of death in the United States. More than 1.5 million Americans suffer a myocardial infarction every year, and nearly half a million die, according to the American Heart Association. Most myocardial infarctions are the end result of years of silent, undetected, progressive coronary artery disease. A myocardial infarction is often the first detected symptom of coronary artery disease. According to the American Heart Association, 63% of women and 48% of men who died suddenly of coronary artery disease had no previous symptoms. Myocardial infarctions are commonly called heart attacks.

A myocardial infarction occurs when one or more of the coronary arteries that supply blood to the heart are completely blocked and blood to the heart muscle is cut off. The blockage is usually caused by atherosclerosis, the build-up of plaque in artery walls, and/or by a blood clot in a coronary artery. Sometimes, a healthy or atherosclerotic coronary artery has a spasm and the blood flow to part of the heart decreases or stops. The result may be a myocardial infarction.

About half of all myocardial infarction patients wait at least two hours before seeking help. This delay dramatically increases the risk of sudden death or disability. The longer the artery remains blocked during a myocardial infarction, the more damage will be done to the heart. If the blood supply is cut off severely, or for longer than 12 hours, muscle cells suffer irreversible injury and die. The patient can die. That is why it is vitally important to teach patients to recognize the signs of a myocardial infarction and seek immediate medical attention at the nearest hospital with 24-hour emergency cardiac care.

About one fifth of all myocardial infarctions are silent, that is, the patient is unaware that the MI has occurred. Although the patient feels no pain, silent myocardial infarctions still damage the heart.

The outcome of a myocardial infarction depends on the location of the blockage, whether the heart rhythm is disturbed, and whether there is collateral circulation to the territory supplied by the acutely occluded coronary artery. Blockages in the left coronary artery are usually more serious than those affecting the right coronary artery. Blockages that produce arrhythmia (irregular heartbeat) can cause sudden death.

Causes and symptoms

Myocardial infarctions are generally caused by severe coronary artery disease. Most myocardial infarctions are caused by blood clots that form on atherosclerotic plaque. This impedes the coronary artery from supplying oxygen-rich blood to part of the heart. A number of major and contributing risk factors increase the likelihood of developing coronary artery disease. Some of these risk factors can be modified, but others cannot. Persons with more risk factors are more likely to develop coronary artery disease.

Major risk factors

Major risk factors significantly increase the likelihood of developing coronary artery disease. Risk factors that cannot be changed include:

• Heredity. People whose parents have coronary artery disease, particularly those who develop it at younger ages, are more likely to be diagnosed with it. African Americans are also at increased risk, due to their higher rate of severe hypertension than caucasians.

• Gender. Men under the age of 60 years of age are more likely to have myocardial infarctions than women of the same age.
• Age. Men over age 45 and women over age 55 are considered at risk. Older adults (those over 65) are more likely to die of a myocardial infarction. Older women are twice as likely to die within a few weeks of a myocardial infarction as men. This increased mortality may be attributable to other co-existing medical problems.

Major risk factors which can be changed are:

• Smoking. Smoking greatly increases both the risk of developing coronary artery disease and resulting mortality. Smokers have two to four times the risk of non-smokers of sudden cardiac death and are more than twice as likely to have a myocardial infarction. They are also more likely to die within an hour of a myocardial infarction. Second-hand smoke may also increase risk.
• High cholesterol. Cholesterol is produced by the body, and obtained from eating animal products such as meat, eggs, milk, and cheese. Age, gender, heredity, and diet affect cholesterol level. Risk of developing coronary artery disease increases as blood cholesterol levels increase. When combined with other factors, the risk is even greater. Total cholesterol of 240 mg/dL or more poses a high risk, and 200-239 mg/dL a borderline high risk. In LDL (low-density lipoprotein) cholesterol, high risk starts at 130-159 mg/dL, depending on other risk factors. Low levels of HDL (high-density lipoprotein) increases the risk of coronary disease; high HDL protects against it.

• Hypertension (high blood pressure ). High blood pressure makes the heart work harder, and over time, weakens it. It increases the risk of myocardial infarction, stroke, kidney failure, and congestive heart failure. Blood pressure of 140 over 90 or above is considered high. As the numbers increase, high blood pressure progresses from Stage One (mild) to Stage Four (very severe). When hypertension is combined with obesity, smoking, high cholesterol, or diabetes, the risk of myocardial infarction or stroke increases several times.
• Sedentary lifestyle and lack of physical activity. Inactivity increases the risk of coronary artery disease. Even modest physical activity is beneficial if done regularly.

Contributing risk factors

Contributing risk factors have been linked to coronary artery disease, but their significance and prevalence are not known yet. Contributing risk factors are:

• Diabetes mellitus. The risk of developing coronary artery disease is seriously increased for diabetics. More than 80% of diabetics die of some type of heart or blood vessel disease.
• Obesity. Excess weight increases the strain on the heart muscle and increases the risk of developing coronary artery disease, even if no other risk factors are present. Obesity increases both blood pressure and blood cholesterol, and can lead to diabetes.
• Stress and anger. Stress and anger can produce physiological changes that contribute to the development of coronary artery disease. Stress, the mental and physical reaction to life's irritations and challenges, increases heart rate and blood pressure, and can injure the lining of the arteries. Evidence shows that anger increases the risk of dying from heart disease and more than doubles the risk of having a myocardial infarction right after an episode of anger.

More than 60% of myocardial infarction patients experience symptoms before the myocardial infarction occurs. These symptoms may occur days or weeks before the myocardial infarction. Sometimes, people do not recognize the symptoms of a myocardial infarction or deny that they are having symptoms. Common symptoms include:

• Uncomfortable pressure, fullness, heaviness, squeezing, or pain in the center of the chest. The sensation lasts more than a few minutes, or may go away and return.
• Pain that spreads to the shoulders, neck, left arm, or jaw.
• Chest discomfort accompanied by lightheadedness, fainting, sweating, nausea, or shortness of breath.

All of these symptoms do not necessarily occur with every myocardial infarction. Sometimes, symptoms disappear and then reappear. Individuals with any of these symptoms should immediately call an emergency rescue service or be driven to the nearest hospital with a 24-hour cardiac care unit, whichever is quicker.

Diagnosis

Experienced emergency care personnel confirm the diagnosis of MI, by taking a thorough history, checking heart rate and blood pressure, performing an electrocardiogram, and drawing a blood sample. The electrocardiogram shows which of the coronary arteries is blocked. The blood test detects the leak of enzymes or other biochemical markers from damaged cells in the heart muscle. In clinical practice, timely treatment is based on the patient history, physical examination, and ECG findings.

Treatment

Treatment is initiated in the emergency department with thrombolytic agents, aspirin, oxygen, and beta-blockers. Oxygen is used to ease the heart's work load or to help patients breathe easier. If oxygen is administered within hours of the myocardial infarction, it also may help limit damage to the heart. Subsequent treatment includes close monitoring, nitrates and morphine if needed, electric shock, drug therapy, re-vascularization procedures, coronary angioplasty, and coronary artery bypass surgery.

Patients with complications such as arrhythmias, congestive heart failure, and hypertension or hypotension require additional treatment. A defibrillator may be used to restore a normal rhythm. A temporary pacemaker may be inserted to correct a bradyarrhythmia (slow heart rate). ACE inhibitors may be used to treat congestive heart failure.

Drugs to stabilize the patient and limit damage to the heart include thrombolytics, aspirin, anticoagulants, painkillers, and tranquilizers, beta-blockers, ACE inhibitors, nitrates, anti-arrhthythmics (rhythm stabilizing) drugs, and diuretics. Thrombolytics, used to limit damage to the heart, work only if given within six to 12 hours of the onset (when the chest pain began) of the myocardial infarction. Thrombolytic drugs act by dissolving the blood clot that is blocking the acutely occluded coronary artery. They increase the likelihood of survival when given as soon as possible after the myocardial infarction. Thrombolytics given within a few hours after a myocardial infarction are the most effective. Injected intravenously, these include acylated plasminogen streptokinase activator complex (APSAC) or anistreplase (Eminase), recombinant tis sue-type plasminogen activator (r-tPA, Retevase, or Activase), and streptokinase (Streptase, Kabikinase). Thrombolytics may only be given if they are not con traindicated by disorders such as active bleeding, trauma or surgery within the preceding two weeks, blood pressure greater than 200/120 mm Hg, and pregnancy.

To prevent additional myocardial infarctions, aspirin and heparin, an anticoagulant, often follow the thrombolytic drug. These prevent new blood clots from forming and existing blood clots from growing. Anticoagulant drugs help prevent the blood from clotting. The most common anticoagulants are heparin and warfarin. Heparin is given intravenously while the patient is in the hospital. Aspirin helps to prevent the dissolved blood clots from reforming.

To relieve pain, a nitroglycerin tablet taken under the tongue or given intravenously. If the pain continues, morphine sulfate may be prescribed. Tranquilizers such as diazepam (Valium) or alprazolam (Ativan) may be prescribed to lessen the anxiety and emotional stress associated with myocardial infarction.

To limit the size of the myocardial infarction and prevent another, beta-blockers are often administered intravenously right after the myocardial infarction. These can also help prevent potentially fatal ventricular fibrillation. Beta-blockers include atenolol (Tenormin), metoprolol (Lopressor), nadolol, pindolol (Visken), propranolol (Inderal), and timolol (Blocadren).

Nitrates, a type of vasodilator, may also be given right after a myocardial infarction to help improve the delivery of blood to the heart and ease chest pain and heart failure symptoms. Nitrates include isosorbide mononitrate (Imdur), isosorbide dinitrate (Isordil, Sorbitrate), and nitroglycerin (Nitrostat).

When a myocardial infarction causes an abnormal heartbeat, arrhythmia drugs may be given to restore the heart's normal rhythm. These include amiodarone (Cordarone), atropine, bretylium, disopyramide (Norpace), lidocaine (Xylocaine), procainamide (Procan), propafenone (Rythmol), propranolol (Inderal), quinidine, and sotalol (Betapace). Angiotensin-converting enzyme (ACE) inhibitors reduce the resistance against which the heart beats and are used to manage and prevent heart failure.

They are used to treat myocardial infarction patients whose hearts do not pump well or who have symptoms of heart failure. Taken orally, they include Altace, Capoten, Lotensin, Monopril, Prinivil, Vasotec, and Zestril. Angiotensin receptor blockers, such as losartan (Cozaar) may substitute. Diuretics can help get rid of excess fluids that sometimes accumulate when the heart is not pumping effectively. Usually taken orally, they cause the body to dispose of fluids through urination. Common diuretics include: bumetanide (Bumex), chlorthalidone (Hygroton), chlorothiazide (Diuril), furosemide (Lasix), hydrochlorothiazide (Hydrodiuril, Esidrix), spironolactone (Aldactone), and triamterene (Dyrenium).

Percutaneous transluminal coronary angioplasty, a type of catheter-based intervention, and coronary artery bypass surgery are invasive revascularization procedures that open blocked coronary arteries and improve blood flow. They are usually performed only on patients for whom clot-dissolving drugs do not work, or who have poor exercise stress tests, poor left ventricular function, or ischemia. Generally, angioplasty is performed before coronary artery bypass surgery.

Percutaneous transluminal coronary angioplasty, usually called coronary angioplasty, is a non-surgical procedure in which a catheter (a tiny plastic tube) tipped with a balloon is threaded from the femoral or brachial artery (blood vessel in the thigh or arm) into the blocked artery. The balloon is inflated and compresses the plaque to enlarge the blood vessel and open the blocked artery. The balloon is then deflated and the catheter is removed. Coronary angioplasty is performed by a cardiologist in a hospital and generally requires a two-day stay. It is successful about 90% of the time. For one third of patients, the artery restenoses (narrows again) within six months after the procedure. The procedure may be repeated. It is less invasive and less expensive than coronary artery bypass surgery.

In coronary artery bypass surgery, called bypass surgery, a vein taken from the patient's leg, or the internal mammary artery, may be used to reestablish blood flow beyond the coronary artery blockage. The healthy vein or artery then supplies oxygen-rich blood to the heart. Bypass surgery is major surgery appropriate for patients with blockages in two or three major coronary arteries or severely narrowed left main coronary arteries, as well as those who have not responded to other less invasive treatments. It is performed under general anesthesia using a heart-lung machine to support the patient while the healthy vein is attached to the coronary artery. About 70% of patients who have bypass surgery experience full relief from angina; about 20% experience partial relief. Long term symptoms recur in only about three or four percent of patients per year. Five years after bypass surgery, survival expectancy is 90%, at 10 years it is about 80%, at 15 years it is about 55%, and at 20 years it is about 40%.

There are three additional catheter-based interventions for unblocking coronary arteries that are currently being performed. During atherectomy, the surgeon shaves off and removes strips of plaque from the blocked artery. Laser angioplasty uses a catheter with a laser tip inserted into the vessel to burn or break down the plaque. Insertion of a metal coil called a stent also may be implanted permanently to keep a blocked artery open.

Prognosis

The sequelae (aftermath) of a myocardial infarction is often severe. Two-thirds of myocardial infarction patients never recover fully. Within one year,27% of men and 44% of women die. Within six years, 23% of men and 31% of women have another myocardial infarction, 13% of men and 6% of women experience sudden death, and about 20% have heart failure. People who survive a myocardial infarction have a chance of sudden death that is four to six times greater than others and a chance of illness and death that is two to nine times greater. Older women are more likely than men to die within a few weeks of a myocardial infarction.

Health care team roles

Nurses, ECG technicians, laboratory technologists and other allied health professionals have important roles in the diagnosis of acute myocardial infarction as well as institution of timely treatment. Nurses and other practitioners involved in triage or screening in the emergency department must accurately assess patients with chest pain or other indications of myocardial infarction.

ECG technicians and laboratory technologists are responsible for performing the diagnostic tests, ECG and blood chemistries, to confirm the diagnosis of myocardial infarction. In the emergency department and on the hospital floor, nurses and allied health professionals are responsible for closely monitoring patients to prevent complications following myocardial infarction. During the hospitalization, nurses, dieticians, respiratory and physical therapists collaborate to plan a cardiac rehabilitation program and provide patient and family education.

Patient education

Nurses, physical therapists, and dieticians work together to educate patients and their families. Patients are taught to recognize and accurately describe symptoms such as pain, pressure, or heaviness in the chest, arm, or jaw. Patients are advised to report any changes in the intensity or quality of their pain to nurses or other health care professionals while in the hospital. When necessary, they are counseled by nursing or pharmacy technicians about the use of sublingual (under the tongue) nitroglycerin to relieve chest pain. They are instructed to seek medical attention immediately should serious symptoms return after they have been discharged.

Along with instruction about medication, follow-up care, and the importance of participating in cardiac rehabilitation, patients are informed about ways to reduce their risk of having another myocardial infarction or other cardiac disorders. This education is tailored to the individual patient's needs. It may include referral to a smoking cessation program; nutritional counseling to reduce dietary fat and sodium and achieve a desirable body weight; and recommendations to increase physical activity. Patient education also addresses treatment of any coexisting illnesses such as diabetes, and instruction about ways to more effectively manage stress and anger.

Prevention

Many myocardial infarctions can be prevented through a healthy lifestyle, which can reduce the risk of developing coronary artery disease. For patients who have already had a myocardial infarction, a healthy lifestyle and carefully following doctor's orders can prevent another myocardial infarction. A heart healthy lifestyle includes a low-fat diet, regular exercise, maintaining a healthy weight, no smoking, moderate drinking, no illegal drugs, controlling hyper tension, and managing stress.

A healthy diet includes a variety of foods that are low in fat (especially saturated fat), low in cholesterol, and high in fiber; plenty of fruits and vegetables; and limited sodium. Some foods are low in fat but high in cholesterol, and some are low in cholesterol but high in fat. Saturated fat raises cholesterol, and, in excessive amounts, it increases the amount of the proteins in blood that form blood clots. Polyunsaturated and monounsaturated fats are relatively good for the heart. Fat should comprise no more than 30% of total daily calories.

Cholesterol, a waxy, lipid-like substance, comes from eating foods such as meat, eggs, and other animal products. It is also produced in the liver. Soluble fiber can help lower cholesterol. Patients should be advised to limit cholesterol to about 300 mg per day. Many lipid-lowering drugs reduce LDL-cholesterol by an average of 25-30% when combined with a low-fat, low-cholesterol diet. Fruits and vegetables are rich in fiber, vitamins, and minerals. They are also low calorie and nearly fat free. Vitamin C and beta-carotene, found in many fruits and vegetables, keep LDL cholesterol from turning into a form that damages coronary arteries. Excess sodium can increase the risk of high blood pressure. Many processed foods contain large amounts of sodium. Patients should be advised to limit daily intake to about 2,400 mg—about the amount in a teaspoon of salt.

The Food Guide Pyramid developed by the U.S. Departments of Agriculture and Health and Human Services provides easy to follow guidelines for daily heart-healthy eating: six to 11 servings of bread, cereal, rice, and pasta; three to five servings of vegetables; two to four servings of fruit; two to three servings of milk, yogurt, and cheese; and two to three servings of meat, poultry, fish, dry beans, eggs, and nuts. Fats, oils, and sweets should be used sparingly.

Regular aerobic exercise can lower blood pressure, help control weight, and increase HDL ("highly desirable") cholesterol. It may keep the blood vessels more flexible. Moderate intensity aerobic exercise lasting about 30 minutes four or more times per week is recommended for maximum heart health, according to the Centers for Disease Control and Prevention and the American College of Sports Medicine. Three 10-minute exercise periods are also beneficial. Aerobic exercise—activities such as walking, jogging, and cycling—uses the large muscle groups and forces the body to use oxygen more efficiently. It can also include everyday activities such as active gardening, climbing stairs, or brisk housework.

Maintaining a desirable body weight is vital for heart health. More than half of American adults are overweight as defined by a body mass index (BMI) greater than 25. The percentage of obese adults (BMI greater than 30) is nearly 25%, a 50% increase over the past 20 years. People who are 20% or more over their ideal body weight have an increased risk of developing coronary artery disease. Losing weight can help reduce total and LDL cholesterol, reduce triglycerides, and boost relative levels of HDL cholesterol. It may also reduce blood pressure.

Smoking has many adverse effects on the heart. It increases the heart rate, constricts major arteries, and can create irregular heartbeats. It also raises blood pressure, contributes to the development of plaque, increases the formation of blood clots, and causes blood platelets to cluster and impede blood flow. Quitting can repair heart damage caused by smoking—even heavy smokers can return to heart health. Several studies have shown that ex-smokers face the same risk of heart disease as non-smokers within five to 10 years of quitting.

Patients should be counseled to drink alcohol in moderation. Modest consumption of alcohol may actually protect against coronary artery disease. This is believed to be because alcohol raises HDL cholesterol levels. The American Heart Association defines moderate consumption as one ounce of alcohol per day—roughly one cocktail, one 8-ounce glass of wine, or two 12-ounce glasses of beer. In some people, how ever, moderate drinking can increase risk factors for heart disease, such as raising blood pressure. Excessive drinking is always bad for heart health. It usually raises blood pressure, and can poison the heart and cause abnormal heart rhythms or even heart failure. Illegal drugs, like cocaine, can seriously harm the heart and should never be used.

High blood pressure, one of the most common and serious risk factors for coronary artery disease, can be effectively controlled through lifestyle changes and medication. Patients with moderate hypertension may be able to control it through lifestyle changes such as reducing sodium and fat, exercising regularly, managing stress, quitting smoking, and drinking alcohol in moderation. When these changes are ineffective, and for those with severe hypertension, there are eight types of drugs that provide effective treatment.

Stress management means controlling mental and physical reactions to life's irritations and challenges. Techniques for controlling stress include taking life more slowly, spending time with family and friends, thinking positively, getting enough sleep, exercising, and practicing relaxation techniques.

Daily aspirin therapy has been proven to help prevent blood clots associated with atherosclerosis. It can also prevent myocardial infarctions from recur ring, prevent myocardial infarctions from being fatal, and reduce the risk of strokes.

KEY TERMS

Angina— Chest pain that occurs when diseased blood vessels restrict the flow of blood to the heart. Angina is often the first symptom of coronary artery disease.

Atherosclerosis— A process in which the walls of the coronary arteries thicken due to the accumulation of plaque in the blood vessels. Atherosclerosis is the cause of coronary artery disease.

Coronary arteries— The two arteries that provide blood to the heart. The coronary arteries surround the heart like a crown, coming out of the aorta, arching down over the top of the heart, and dividing into two branches. These are the arteries where coronary artery disease occurs.

Plaque— A deposit of fatty and other substances that accumulate in the lining of the artery wall.

Resources

BOOKS

Ahya, Shubhada N, Kellie Flood, and Subramanian Paranjothi. The Washington Manual of Medical Therapeutics, 30th ed. Philadelphia: Lippincott Williams & Wilkins, 2001, pp. 105-116.

American Heart Association. Guide to Myocardial infarction Treatment, Recovery, Prevention. New York: Time Books, 1996.

DeBakey, Michael E., and Antonio M. Gotto Jr. The New Living Heart. Holbrook, MA: Adams Media Corporation, 1997.

PERIODICALS

"Drugs or Angioplasty After a Myocardial Infarction?" In Harvard Health Letter 22, no. 10 (August 1997): 8.

Marble, Michelle. "FDA Urged to Expand Uses for Aspirin, Benefits for Women." In Women's Health Weekly (February 10, 1997).

"More on Anger and Heart Disease." Harvard Heart Letter (May 1997): 6-7.

ORGANIZATIONS

American Heart Association. National Center. 7272 Greenville Avenue, Dallas, TX 75231-4596. (214)373-6300. 〈http://www.medsearch.com/pf/profiles/amerh/〉.

National Heart, Lung, and Blood Institute Information Center. P.O. Box 30105, Bethesda, MD 20824-0105. 〈http://www.nhlbi.gov/nhlbi/nhbli.htm〉.

Texas Heart Institute Heart Information Service. P.O. Box 20345, Houston, TX 77225-0345. 1-800-292-2221. 〈http://www.tmc.edu/thi/his.html〉.

myocardial infarction (MI) (my-oh-kar-di-ăl) n. death of a segment of heart muscle, which follows interruption of its blood supply (see coronary thrombosis). The patient experiences a ‘heart attack’: sudden severe chest pain, which may spread to the arms and throat. The main danger is that of ventricular fibrillation, which accounts for most of the fatalities. Other complications include heart failure, rupture of the heart, phlebothrombosis, pulmonary embolism, pericarditis, shock, mitral regurgitation, and perforation of the septum between the ventricles.
www.bhf.org.uk/living_with_heart_conditions/understanding_your_condition/types_of_heart_conditions/heart_attack.aspx Details of MI from the British Heart Foundation

my·o·car·di·al in·farc·tion • n. another term for heart attack.