Myocardial Infarction
INTRODUCTION — The heart, like all other organs and tissues in the body, requires a supply of blood. The blood supply to the heart is provided by blood vessels called the coronary arteries. Myocardial infarction, or MI (commonly known as a "heart attack"), is damage or death of an area of heart muscle caused by lack of blood flow through the coronary arteries. For this reason, an MI is sometimes also called a "coronary."
Most often, an MI occurs in people who have coronary heart disease (CHD). In CHD, the coronary arteries become narrowed by fatty deposits called plaques. The fatty plaques inside the coronary arteries restrict blood flow to the heart muscle, which may cause pain or tightness in the chest. This pain or tightness is called angina pectoris.
An infarction myocardial occurs when one of the fatty plaques ruptures. When the plaque ruptures, a blood clot can form that partially or completely blocks the artery. The area of heart muscle fed by that artery no longer receives an adequate supply of blood. This lack of blood supply is called "ischemia." As a result of ischemia, the heart muscle becomes damaged and may die. The death of heart muscle is termed "infarction."
Although symptoms of an acute myocardial infarction can vary, the typical patient will complain of discomfort in the region of the chest. Other symptoms may include shortness of breath; sweating; lightheadedness and dizziness; pain in other areas of the upper body such as the lower jaw, shoulders, or arms; or gastrointestinal symptoms such as nausea, vomiting, or belching. Some patients having an MI will experience few, if any, symptoms.
INITIAL TREATMENT — Patients who come to an emergency department with symptoms suggesting possible MI all receive the same initial treatment, designed to evaluate the problem and provide initial therapy in the event that an myocardial infarction is occurring.
The patient is given oxygen through a nasal cannula (a flexible plastic tube that rests beneath the nose) or by a face mask, and an electrocardiogram (ECG) is taken as quickly as possible. The ECG gives a picture of the flow of electrical activity that causes the heart to beat. Damaged areas usually show an abnormal pattern. The ECG may be repeated several times in the first few minutes to monitor for changes that may be occurring. Blood is drawn and sent to the laboratory for routine tests as well as specific tests that look for substances in the blood that are released by damaged heart tissue (cardiac enzymes or proteins).
An intravenous line (IV) is started so that medicines can be given directly into the veins. Nitroglycerin is given either through the IV or under the tongue in an attempt to relieve the chest pain. Morphine may also be given to help relieve the chest pain and to help ease the patient's anxiety. The patient is given aspirin to chew and swallow to help halt the formation of further blood clots. During this time, the patient is carefully questioned about the onset of symptoms, past medical history, and the presence of risk factors that are known to lead to CHD and MI.
In this early stage, the physician is particularly interested in the ECG patterns that emerge. If the patient's history is suggestive of an myocardial infarction, the characteristics of the ECG will be an important determinant of the next steps in the treatment process; the ECG often yields important information about the location and severity of an MI.
Location of the MI — Depending on which coronary arteries have been obstructed, different areas of the heart can suffer damage during an MI. The main distinctions are:
- Anterior MI
- Inferior MI
- Lateral MI
- Posterior MI
- Right ventricular MI, which is usually associated with an inferior MI
ST elevation versus non-ST elevation MI — The ECG is also used to distinguish between ST elevation and non-ST elevation infarction myocardial. In general, an ST elevation MI is caused by complete obstruction of a coronary artery, and causes damage that involves the full thickness of the heart muscle. In contrast, a non-ST elevation MI is caused by partial obstruction of a coronary artery, and causes damage that does not involve the full thickness of the heart wall. The treatment for these two types of MIs may differ, as described below.
TREATMENT OF NON-ST ELEVATION MI — If the patient's history and ECG suggest a possible non-ST elevation myocardial infarction, the patient is admitted to the hospital. Intravenous heparin (a blood thinner) is given to help prevent further blood clot formation, and a medication called a beta blocker is given intravenously to slow the heart rate and decrease the heart's demand for oxygen.
Following this, two approaches to treatment are possible. The decision is made after a careful consideration of a number of factors.
Intensive medical therapy — The patient's condition is stabilized with intravenous drugs. After several days, the drugs are stopped. If the patient's symptoms do not return, exercise testing is performed. In this test, the patient exercises on a treadmill or bicycle, and the heart's response is examined using a continuous ECG recording and, often, other scanning techniques using a radioactive tracer that demonstrates blood flow to various parts of the heart. The exercise test can indicate whether narrowing or occlusion of one or more coronary arteries is present.
Further treatment decisions are based upon the results of the exercise test. The patient may be discharged on medicines that control symptoms, or the test may indicate that the patient should undergo cardiac catheterization followed by angioplasty, coronary artery stenting, or surgery (explained below).
Early catheterization — In this approach, the patient is taken for a cardiac catheterization within 24 to 36 hours of admission. A small catheter is threaded through an artery, usually in the groin, to the coronary arteries. A dye is injected that allows the arteries to be seen on X-ray.
If blockages or narrowings are found, a procedure known as percutaneous transluminal coronary angioplasty (PTCA) may be done. A tiny balloon is threaded into the narrowed coronary artery. The balloon is then inflated, which helps open up the narrowed artery. A stent (an expandable tube) may be placed in the artery to prevent the narrowing from recurring.
In some cases, the X-rays reveal that the blockages cannot be opened using PTCA. In these instances, coronary artery bypass graft surgery (CABG) may be an option. During the CABG operation (often pronounced "cabbage"), a blood vessel is taken from the leg or the chest wall and used as a detour around the blocked coronary artery.
TREATMENT OF ST ELEVATION MI — If the ECG indicates that an ST elevation MI is in progress, the patient is given a medication called a beta blocker intravenously to decrease the heart's demand for oxygen. All of these patients then receive immediate therapy to open the blocked coronary artery. This is called "reperfusion therapy." The more quickly this therapy takes place, the better the chance of saving areas of the heart that might otherwise be damaged. In general, the best outcome occurs when the artery is opened within four hours of the symptoms, preferably within 90 to 120 minutes.
Reperfusion therapy can occur in one of two ways. The preferred method is PTCA (percutaneous transluminal coronary angioplasty), which is explained above. The patient is taken to the catheterization lab urgently for this procedure.
However, not every hospital is equipped to do PTCA. Transfer to another hospital that does perform PTCA is not recommended unless it can be accomplished in a very short time frame. The total time from when the patient first arrived at the initial hospital to when the PTCA is begun at the second hospital must be one hour or less. An acceptable alternative treatment in some patients with ST elevation acute myocardial infarction is the use of intravenous drugs that dissolve blood clots. The clot-dissolving drugs, called thrombolytic or fibrinolytic agents, should be given within 30 minutes of the patient's arrival at the hospital. Thrombolytic drugs cannot be given to certain patients, including those who have active bleeding, a high blood pressure reading, recent trauma, or a history of stroke. They are also not recommended in the treatment of non-ST elevation infarction myocardial.
In some patients, urgent bypass surgery is indicated in the early hours of an ST elevation MI. Patients in this group may include those whose coronary artery plaques are not suitable for PTCA, patients who have severe forms of CHD, or patients who have suffered serious complications of myocardial infarction.
TREATMENT OF THE COMPLICATIONS OF MI — Some of the most common complications of MI and a brief description of the treatment options in each case are presented here.
Ongoing ischemia — Early treatments may fail to halt the ischemic process and larger areas of the heart may become damaged. In patients with non-ST elevation MI who are undergoing intensive medical therapy (as opposed to early catheterization), ongoing ischemia would be an indication for a change in approach to immediate catheterization followed by PCI or surgery. In a patient with any type of MI, if catheterization with PCI or surgery is not possible (because it is not available or the patient cannot tolerate the procedure), additional medicine can be used to try to control the ischemia.
Abnormal heart rhythms — The heart has an electrical system, found within the muscle of the heart, that is responsible for stimulating the heart to beat. The damaged heart muscle can cause disturbances in the way the electrical impulses travel within the heart muscle. An erratic heart beat and/or abnormal heart rhythms (arrhythmia) can result. Some of these abnormal rhythms can be dangerous, and can lead to poor heart function. Abnormal rhythms can also result in sudden cardiac death (a cardiac arrest). Drugs can be used to control many types of heart rhythm disturbances. In other cases, a pacemaker is used. In still others, an electrical current is applied to the heart (cardioversion or defibrillation) in an attempt to correct the problem.
Poorly functioning left ventricle — If the heart's main pumping chamber, the left ventricle, is seriously damaged, its ability to function can be impaired.
- Heart failure refers to a condition in which the ventricle is stiff or weak, meaning it has difficulty pumping blood in the normal way.
- Low blood pressure can occur, risking the lack of adequate blood flow to the rest of the body.
- Cardiogenic shock can develop, meaning that the heart's pumping function is so severely weakened that the body's tissues are suffering from lack of blood flow.
Left ventricular aneurysm — This refers to an area of the left ventricle that has become thin and scarred, often bulging out and moving in an ineffective way during contraction. A left ventricular aneurysm can lead to a poorly functioning left ventricle, described above. If the aneurysm is severe, surgery can be done in an attempt to remove the aneurysm and repair the area. In less severe cases, medications that improve ventricular function may be effective.
Problems with rupture of a valve or a heart wall — An infarction myocardial can cause rupture of the muscles that hold one of the heart valves in place, the mitral valve. Rupture of these muscles prevents the valve from functioning normally, leading to a backward leakage of blood (regurgitation) through the valve.
A rupture of the wall (ventricular septum) separating the left and right heart chambers can also occur (ventricular septal defect), as can rupture of the outer wall of the ventricle. In some cases of mechanical problems, medicines can be used to stabilize the patient prior to surgical care. In other cases, immediate surgery is needed to repair the defective area.
Information Obtained From National Institute Of Health