The answer to the question – what is a heart attack – is clear-cut, yet involved. Understanding it involves some basic knowledge of the heart and rest of the cardiovascular system. It includes some of the structures of the heart, the function of the heart and what goes wrong when a heart attack occurs.
What is a heart attack?
A heart attack is permanent damage and death of a part of the heart’s muscle, resulting from the cutting off of the flow of oxygenated (rich in oxygen) blood through an artery supplying it. The medical terminology for the cutting off of the flow of oxygenated blood to tissue is ischemia. An infarct is a local area of dead tissue caused by ischemia. Since cardiac means pertaining to the heart and myocardial means pertaining to muscle of the heart, a myocardial infarct is a local area of dead cardiac muscle resulting from ischemia. Since infarction is the formation of an infarct, myocardial infarction (MI for short) is the medical term for a heart attack.
Insight into what is a heart attack requires some knowledge of the basic structure and functions of the heart and blood vessels. Heart attack facts pertaining to cause, signs, symptoms and prevention heighten that insight. An understanding of the surprising relationship between the amount of blockage in a heart artery and the risk of sudden death from a heart attack is yet more advanced insight.
The heart is a hollow muscular organ which pumps and receives blood. The left side of the organ pumps blood out. The right side receives blood. Each side consists of a small chamber known as the atrium and a larger chamber known as the ventricle. The atria are just above the ventricles. Flaps of tissue known as valves separate the atria from the ventricles. If healthy, these valves permit blood to flow only one way from the atria into the ventricles. The valve between the left atrium and the right ventricle is called the mitral valve because it has two leaflets. The valve between the right atrium and the right ventricle is called the tricuspid valve because it has three leaflets. A muscular wall known as the septum separates the chambers of the right side of the heart from the chambers of the left side of the heart.
The cardiovascular system
The cardiovascular system is the circulatory system which consists of the heart and blood vessels. It transports blood to and from tissues and vital organs. Arteries are the vessels that transport blood from the heart to cells, tissues and organs. Veins return blood from cells, tissues and organs to the heart.
The left ventricle pumps oxygenated blood into the aorta (the largest artery of the body). The aorta divides into smaller arteries which in turn subdivide into even smaller ones. The process continues like the branching of a tree until the smallest branches known as capillaries reach the tissues. Oxygen, nutrients and other substances necessary for the life and function of tissue cells are delivered to the cells via the capillaries. Carbon dioxide, a waste product of cells, is absorbed from the tissues by the capillaries.
The blood then flows back to the right side of the heart, beginning in the smallest veins which join the capillaries end-to-end. The flow continues as veins merge into larger and larger ones in a manner opposite the branching of the arteries. When the blood reaches the largest vein of the body, it then empties into the right atrium of the heart. Blood returning from the upper portion of the body flows into the right atrium through the superior vena cava (the largest vein of the body which connects with the right atrium from above). Blood returning from the lower portion of the body flows into the right atrium through the inferior vena cava (the largest vein of the body which connects with the right atrium from below).
After blood flows into the right atrium it then passes through the tricuspid valve into the right ventricle. The right ventricle pumps blood through the lungs where it is replenished with oxygen and cleansed of carbon dioxide. The blood that has been processed by the lungs then flows through the pulmonary veins which transport it into the left atrium. That blood flows from the left atrium and to the left ventricle for recycling through the body.
Arteries of the heart
Coronary means pertaining to blood flow in the heart. Therefore, the vessels which deliver oxygen to cardiac muscle are called coronary arteries. Similarly, the array of arteries, veins and capillaries in cardiac muscle comprise the coronary circulation. Blood flow through the coronary arteries begins in small openings in the aorta near its junction with the left ventricle. The openings are the left and right ostia.
Blood flows from the left ostium into the left main coronary artery (the largest of all the arteries). It divides and forms the left anterior descending (LAD) and circumflex arteries. Blood flows through the right ostium into the largest artery of the right side of the heart known as the right coronary artery. Each of these three major arteries divides into smaller branches much like those in other parts of the body. Between these three main arteries and their branches and sub- branches, blood rich in oxygen is supplied to all portions of the heart muscle.
The importance of oxygen in blood
Oxygen is the substance in the blood stream which cells can do without for the shortest period of time before they begin to die. For cardiac cells that period of time is between two and five minutes. Therefore, the absence of sufficient oxygen in cardiac tissue for between two to five minutes results in a heart attack. The disruption which causes the lack of oxygen can be in any of the three major coronary arteries or their branches.
What causes a heart attack?
Although factors and conditions which lead to heart attacks are fairly well understood by doctors and researchers, there is no universally correct answer to the question – what causes a heart attack. One reason is experts are divided in their opinions. Secondly, if the answer existed far fewer people would experience MIs, because more would be prevented. Thirdly, the answer to the question would include knowledge of why MIs occur. Fourthly, the reason for an MI varies between individuals.
Coronary artery disease: the forerunner of a heart attack
Despite the general uncertainty as to what causes a heart attack, it is well-known that most occur in a setting of coronary artery disease. Coronary artery disease (CAD) is the result of atherosclerosis, the buildup of waxy cholesterol plaque in arteries. Over time, plaque hardens and causes stenosis (narrowing) of arteries. Depending on the age of plaque, it may also contain calcium and fibrous tissue in addition to cholesterol.
Risk factors for atherosclerosis and coronary artery disease
It is not known exactly what triggers atherosclerosis, but it is believed that some type of injury to the lining of the vessel is involved. There is proof that elevated blood cholesterol, cigarette smoking, diabetes, poorly controlled high blood pressure, stress, and sedentary lifestyle are risk factors which predisposed to it.
Obesity is also a risk factor for CAD, but the link between the two is not as clear-cut because of variable research results and recent research findings showing that the relationship between body fat and heart disease has more to do with the location of the fat than the amount. It showed that atherosclerosis occurs more frequently in obese individuals with increased visceral fat but not in obese individuals without increased fat between and around the organs within the abdomen. It also showed that obese individuals with fat primarily in the buttock and hip regions are protected from developing CAD.
What happens during a heart attack
Probably the closest one can come to explaining what causes a heart attack, is to describe the events immediately preceding and during an MI. The narrowing of an artery due to the buildup of plaque does not directly cause a myocardial infarction. Rather, the triggering event is rupture of the plaque or crack formation within it which stimulates the formation of a blood clot in the artery. If the clot is large enough it can cause occlusion (total blockage of blood flow) at the site where it forms. A piece of clot can also break off and cause occlusion in a smaller segment of the vessel or one of its branches further downstream.
The location and size of an infarct depends on which artery or branch becomes occluded. Occlusion of the left main coronary artery can cause irreversible injury to muscle of the front, side, back, and/or bottom of the left ventricle, left atrium, or front of the septum. Occlusion of the circumflex branch of the left main coronary artery or one of its branches can cause infarction of the left atrium and/or the side or back of the left ventricle. Total blockage of the LAD branch of the left main artery can cause death of cardiac muscle in the front and/or bottom of the left ventricle and/or in the front of the septum. Occlusion of the right coronary artery or one of his branches can cause a myocardial infarction of the right atrium, right ventricle, the bottom of either or both ventricles, and/or the back of the septum.
Symptoms of a heart attack
Severe chest pain, nausea, vomiting, excessive sweating, lightheadedness, anxiety and shortness of breath are classic symptoms of a heart attack. The chest pain is behind the mid to lower portion of the breastbone. It is described as squeezing, pressing, crushing, or tight. According to textbooks the pain radiates from the breastbone into the left shoulder, left arm, left side of the neck and/or left jaw. Patients might report that the pain is similar to that experienced with angina except it is more severe and is not relieved by placing nitroglycerin under the tongue.
Many victims of a heart attack don’t present with all of the classic symptoms. They may have only some of those symptoms, atypical symptoms, vague symptoms, or no symptoms at all. Chest pain may be atypical in its description or location. It may not be in the chest at all, but instead can be located in areas such as the back of the throat, a tooth, the head, the upper back between the shoulder blades, or elsewhere. Some patients experience silent MIs, which are heart attacks without any chest pain. Silent MIs occur most commonly in the elderly and in diabetics.
Signs of a heart attack
Signs of a heart attack are physical findings observable by others including medical personnel. They are also test results which confirm that irreversible damage and death of cardiac muscle tissue has occurred. Signs of a heart attack observable by medical or nonmedical personnel can observe are sweating, increased rate of breathing, vomiting and fainting. Physical findings which a medical professional would detect are an irregular pulse or heart rhythm, a rapid or slow heart rate, low blood pressure, dry skin, anxiousness, fluid in the lung tissue, extra heart sounds, distended veins in the neck, cyanosis (bluish discoloration of the skin) edema (swelling of the limbs) and sudden death.
An abnormal pulse or heart rhythm on electrocardiogram can be observed if the cardiac damage has caused disturbances in the electrical activity within the muscle. This disturbance is known as an arrhythmia. The disturbance can be one which causes the heart to beat too fast or too slow, or one that causes the heart rhythm to be irregular. If the arrhythmia causes a severe drop in the blood pressure fainting or sudden death can occur.
Rales (crackling sounds in the lungs heard through a stethoscope placed against the chest wall) are a sign of pulmonary edema (fluid buildup in the lung tissue) associated with congestive heart failure. Congestive heart failure (CHF) is a state of blood and pressure build up in the vessels and/or tissues of the lung because of pump weakness of the left ventricle caused by the muscle damage from the heart attack. If the weakness of the left ventricle is severe enough blood and pressure might back up into the right side of the heart as well. Swelling of veins in the neck and swelling of the legs, ankles, and/or feet are additional signs of heart failure. Cyanosis is a sign of poor oxygen exchange in the lungs due to the congested state.
Heart attack diagnosis
Findings on an electrocardiogram (EKG) and blood tests confirm that a heart attack has occurred. The findings on EKG are specific changes in the shapes and positions of the electrical waves seen on the tracing. Those changes indicate if the muscle injury occurred recently or remotely. They will also oftentimes show what part of the heart was injured and whether the damage involves the full thickness of the muscle or just the inner portion.
The findings on blood test are based on the fact that when muscle cells die they release various substances including enzymes and proteins. The measurement of enzymes specific for cardiac muscle, along with EKG, was the main method of confirming a heart attack for many years. But in the late 1990s a more sensitive and specific test was developed. It measures a protein released by injured cardiac muscle cells. The protein is known as troponin. Troponin begins to be detectable 4 hours after an MI and reaches its peak between 12 and 16 hours following the acute event. It remains elevated for up to two weeks following a heart attack.
Heart attack prevention
Heart attack prevention involves preventing the development of plaques and secondary blood clot formation in coronary arteries. Some measures to prevent plaque formation can be self-directed. They are lifestyle changes and other actions which reduce the risk of atherosclerosis. Some of the more important ones are quitting smoking, exercise, a healthy diet and reducing stress. Other preventive measures require joint effort on the part of patients and physicians. They include the control of high blood pressure and diabetes.
Heart attack prevention with medications
Medications that control high blood pressure, high cholesterol and diabetes can prevent heart attacks by controlling those risk factors which in turn reduces the likelihood of plaque formation in arteries. Other medications prevent heart attacks by making unstable plaques stable or by preventing blood clot formation within an artery even if cracking or rupturing of a plaque occurs. Statins are a class of medication which converts unstable plaques into stable ones. They also provide the dual benefit of decreasing plaque formation by lowering cholesterol. Beta blockers also prevent heart attacks by stabilizing plaques. Aspirin prevents clot formation even in the setting of plaque cracking or rupturing.
Heart attack prevention through revascularization
Revascularization means the restoring of blood flow through a blocked or narrowed artery. It can be performed surgically or without surgery. A thoracic surgeon performs revascularization during open heart surgery. An interventional cardiologist restores blood flow through a procedure which does not require surgery.
Surgical revascularization is the placement of a segment of artery or vein from another part of the body around a blocked or narrowed heart artery bypassing it. The operation is called coronary artery bypass graft surgery or CABG for short.
Nonsurgical revascularization involves the placement of a catheter with a balloon on the end of it into a coronary artery and restoring blood flow through it by expanding the balloon. The catheter and balloon are inserted through the skin into one of the arteries of the limbs or the artery just below the collarbone. The catheter and balloon or then advanced into the heart and the occluded artery through the appropriate ostium prior to expanding the balloon at the point of the blockage. The catheter and balloon are then withdrawn after blood flow through the artery has been restored.
Percutaneous means through the skin and transluminal means through the channel of the blood vessel. Therefore, the general term for the procedure is percutaneous coronary intervention (PCI). The term for opening a narrowed artery by expanding the balloon at the end of a catheter is angioplasty. Therefore, the specific term for the procedure is percutaneous transluminal angioplasty (PTCA). Stents are pieces of wire mesh oftentimes placed during PCI to help keep an artery or arteries open following angioplasty.
Heart attack prevention comparing medical treatment with revascularization
Recent research shows that coronary artery bypass graft surgery is better than percutaneous intervention in preventing heart attacks in patients with coronary artery disease involving multiple vessels. Additional research also shows that PCI plus medical therapy is no better than medical therapy alone in preventing heart attack in patients with stable CAD.
Heart attack facts about degree of blockage and sudden cardiac death
Even though there is a clear association between risk factors for atherosclerosis and cardiac disease, the relationship between the degree of artery blockage and the risk of a fatal heart attack is opposite of what one would expect. The primary reason is that thin (unstable) cholesterol plaques are more prone to rupture or crack and cause blood clot formation than thicker (stable) ones. Therefore an artery with a greater degree of narrowing caused by a thicker plaque is less likely to result in a heart attack than an artery with a lesser degree of narrowing.
Also, during the time it takes for a thick plaque to accumulate the body forms new vessels which bypass the blockage resulting from the plaque. Additionally, previously unused collateral arteries (collateral circulation), which are a natural part of the coronary circulatory system begin to be used. The end result is the averting of a heart attack by the continued supply of oxygenated blood to heart muscle.
An individual with a thick cholesterol plaque is more likely to experience recurring episodes of chest pain and thus be under medical treatment. On the other hand, a person with a thin unstable plaque who does not experience cardiac chest pain is unlikely to be under medical treatment for CAD and is thus a candidate for a massive MI caused by clot formation. The only symptom that person might experience is severe first-time chest pain followed by sudden death.
So, contrary to popular belief, you are much more likely to experience a fatal heart attack if one of your coronary arteries is 20% narrowed than if it is 95% blocked. If it were possible, many late joggers (usually male) would attest to this fact.