The heart muscle, known as the myocardium, constitutes the bulk of the heart wall. It is responsible for the pumping action of the heart which serves to distribute blood throughout the body. The heart is functioning throughout life, although its activity may increase or decrease during different periods of cardiac demand. Normally, the heart beats between 60 to 80 times per minute when the heart muscle contracts and then relaxes repeatedly.

There are four chambers in the heart – two atria and two ventricles. The right side of the heart receives dexoygenated blood from the body in the atrium and then the ventricles pumps it out to the lungs. Here carbon dioxide is discarded and oxygen absorbed. The now oxygenated blood from the lungs then returns to the left side of the heart. It enters the left atrium and then from the left ventricle it is pumped out to the rest of the body via the large artery known as the aorta. Small sinuses at the root of the aorta allows blood to enter the coronary arteries which provide the blood supply to the heart.

Heart muscle is constantly stimulated by the electrical system of the heart.  This ensures that the heart muscle contracts in a specific rhythm to allow blood to be circulated. If any part of this heart muscle malfunctions, there can be serious disturbance to the circulation. Death of the heart muscle is very dangerous as the portion of the wall that is affected will not be able to contract and relax with the rest of the muscles. Therefore blood flow through the heart is severely compromised and depending on the extent of muscle death, it may be fatal.

What is a heart attack?

The medical term for a heart attack is a myocardial infarction or MI. It is the death of a portion of the heart muscle most frequently due to an interruption of the blood supply to the heart wall. Without an adequate blood supply, the heart muscle is deprived of oxygen and nutrients and waste products manufactured by the cells accumulates in the muscle tissue. Depending on the severity, it may cause myocardial ischemia, which is damage but not death of the muscle due to an inadequate blood supply. Alternatively it may cause death of the muscle tissue which is known as a myocardial infarction (MI).

How does a heart attack occur?

A heart attack rarely occurs suddenly with no previous pathology unless there is acute blood loss that is severe. The most common cause is narrowing of the coronary arteries usually due to the build up of fatty plaques in the wall (atherosclerosis). This narrowing is known as coronary artery disease. It leads to ischemic heart disease which simply means that the heart wall is undergoing damage due to an inadequate blood supply (myocardial ischemia). This can persist for months or years. When the cardiac demands increases like during physical activity or stress, the narrowed coronary arteries cannot provide sufficient blood for the heart muscle to sustain itself.

Although coronary artery disease accounts for the majority of heart attack cases, it is not the only cause. When blood supply to the heart muscle is compromised, normal aerobic respiration in the myocytes (muscle cells) ceases. The cells cannot produce sufficient high energy compounds like ATP which is needed in abundance for the muscles to contract. Simultaneously noxious substances like lactic acid builds up in the cells. Initially this causes reversible injury to the cell known as myocardial ischemia.

If very severe, it can lead to death of the muscle cells within minutes, which is irreversible (myocardial infarction). However, in most cases the insufficiency of cardiac blood supply causes damage over several houses which eventually leads to heart muscle death. Therefore early intervention can prevent death of the heart muscle or at least significantly reduce the area affected. The entire heart wall is not at risk of muscle death simultaneously. The portion of the heart wall that is most likely to be affected is known as the area at risk.

Causes of a Myocardial Infarction

Majority of myocardial infarctions are due to narrowing of the coronary arteries by the build up of atheromatous plaques. Usually these plaques enlarge slowly over time and are known as stable plaques. A sudden change in the plaque like a rupture (unstable plaque) triggers the formation of platelet plug and eventually a blood clot (thrombus). This occurs within a very short period of time, usually within minutes, and may completely occlude the affected coronary artery. Hence the term acute myocardial infarction (AMI). Any tissue that is supplied with blood beyond the area of the occlusion is at risk of cell death (area at risk).

The development of these plaques are associated with certain risk factors like hypertension (high blood pressure), hyperlipidemia (high cholesterol and triglycerides in the blood), diabetes mellitus and cigarette smoking.

Other causes of a myocardial infarction are far less common. This may include :

  • Embolus. A blood clot forms elsewhere and travels through the bloodstream to then occlude the coronary artery.
  • Vasospasm. The constriction of the coronary artery due to contraction of the smooth muscle in its walls. This may occur on its own for no known reason, with stress or the use of certain drugs and illicit substances like cocaine.
  • Rare causes of an infarct may include vasculitis, shock, amyloidosis of the vascular wall, blood disorders like sickle cell disease or even surgical complications typically associated with cardiovascular procedures.

Signs and Symptoms of Myocardial Infarction

If the obstruction of the coronary artery is less than 75%, a person may be asymptomatic. Most patients experience recurrent episodes of ischemic cardiac pain, which is another clinical syndrome of ischemic heart disease known as angina pectoris. These episodes are usually triggered by activity and relieved by rest and may persist for months or year.s However, with a myocardial infarction, the chest pain does not ease with rest. Read more on angina pain vs heart attack pain.

The chest pain in a myocardial infarction is typically described as crushing, choking suffocating, burning or aching. The pain is usually not sharp or stabbing. Typically, ischemic cardiac pain occurs at the center of the chest and patients tend to clutch the area with the severe pain. It may radiate up to the neck or jaw, or to the arm and even as far down as the hand more often on the left side.

During a myocardial infarction, there are typically other clinical features like :

  • Nausea with/without vomiting
  • Breathlessness
  • Anxiety
  • Dizziness
  • Fainting spells or loss of consciousness

Article reviewed by Dr. Greg. Last updated on December 3, 2011