Introduction

The heart is the human body's hardest working organ. Throughout life it continuously pumps blood enriched with oxygen and vital nutrients through a network of arteries to all parts of the body's tissues.

The external structures of the heart include the ventricles, atria, arteries, and veins. Arteries carry blood away from the heart while veins carry blood into the heart. The vessels colored blue indicate the transport of blood with relatively low content of oxygen and high content of carbon dioxide. The vessels colored red indicate the transport of blood with relatively high content of oxygen and low content of carbon dioxide.

In order to perform the difficult task of pumping blood to the rest of the body, the heart muscle itself needs a plentiful supply of oxygen-rich blood, which is provided through a network of coronary arteries. These arteries carry oxygen-rich blood to the heart's muscular walls (the myocardium).

Click the icon to see an image of the anterior heart arteries.

If blood flow to the myocardium is interrupted, an injury known as an infarct occurs. This is also known as myocardial infarction or, more commonly, a heart attack.

Click the icon to see an animation about coronary artery disease.

The Process of Atherosclerosis

Coronary artery disease is the end result of a complex process called atherosclerosis (commonly called "hardening of the arteries"). This causes blockage of arteries (ischemia ) and prevents oxygen-rich blood from reaching the heart. There are many steps in the process leading to atherosclerosis, some not fully understood.

Click the icon to see an image of atherosclerosis.

Increasingly, however, researchers are studying the interactions between cholesterol and processes known as oxidation and the inflammatory response.

Cholesterol and Lipoproteins. The story begins with cholesterol and sphere-shaped bodies called lipoproteins that transport cholesterol.

• Cholesterol is a white, crystalline substance that is found in all animal cells and in animal-based foods. It is critical for many functions, but under certain conditions cholesterol can have harmful effects.
• The lipoproteins that transport cholesterol are referred to by their size. The most commonly known are low-density lipoproteins (LDL) and high density lipoproteins (HDL). LDL is often referred to as the "bad" cholesterol and HDL as the "good" cholesterol.

Click the icon to see an image of cholesterol inside an artery.

Oxidation. The damaging process called oxidation is an important trigger in the atherosclerosis story.

• Oxidation is a chemical process in the body caused by the release of unstable particles known as oxygen-free radicals. It is one of the normal processes in the body, but under certain conditions (such as exposure to cigarette smoke or other environment stresses) these free radicals are overproduced.
• In excess amounts, they can be very dangerous, causing damaging inflammation and even affecting genetic material in cells.
• In heart disease, free radicals are released in artery linings and oxidize low-density lipoproteins (LDL). The oxidized LDL is the basis for cholesterol build-up on the artery walls and damage leading to heart disease.

Inflammatory Response. For the arteries to harden there must be a persistent reaction in the body that causes ongoing harm. Researchers now believe that this reaction is an immune process known as the inflammatory response. The following is one theory about how the inflammatory response contributes to heart disease:

• The injuries to the arteries during oxidation signal the immune system to release white blood cells (particularly those called neutrophils and macrophages) at the site. These factors initiate the inflammatory response.
• Macrophages literally "eat" foreign debris, in this case oxidized LDL cholesterol.
• The process converts LDL cholesterol into foamy material that attaches to the smooth muscle cells of the arteries. The cholesterol becomes mushy and accumulates on artery walls.
• Over time the cholesterol dries and forms a hard plaque, which causes further injury to the walls of the arteries.
• In response to this additional harm, the immune system releases other factors called cytokines. These are powerful inflammatory molecules that attract more white blood cells and perpetuate the whole cycle, causing persistent injury to the arteries.

Click the icon to see an image of atherosclerosis.

Evidence is growing that the inflammatory response may be present not only in local plaques in single arteries but also throughout the arteries leading to the heart.

Blockage in the Arteries. Eventually these calcified (hardened) arteries become narrower (a condition known as stenosis).

• As this narrowing and hardening process continues, blood flow slows and prevents sufficient oxygen-rich blood from reaching the heart.
• Such oxygen deprivation in vital cells is called ischemia. When it affects the coronary arteries, it causes injury to the tissues of the heart.
• Injured inner vessel walls also fail to produce enough nitric oxide, a substance critical for maintaining blood vessel elasticity. (Nitric oxide has complex effects and may increase inflammation in the arteries.)
• These narrow and inelastic arteries not only slow down blood flow but also become vulnerable to injury and tears.

Click the icon to see an image of coronary artery blockage

The End Result: Heart Attack. Heart attack can occur as a result of one or two effects of atherosclerosis.

(1) If the artery becomes completely blocked and ischemia becomes so extensive that oxygen-bearing tissues around the heart die.

(2) If the plaque itself develops fissures or tears. Blood platelets adhere to the site to seal off the plaque, and a blood clot (thrombus) forms. A heart attack can then occur if the formed blood clot completely blocks the passage of oxygen-rich blood to the heart.

Click the icon to see an image of the developmental process of atherosclerosis.

Angina

Angina is the primary symptom of coronary artery disease and, in severe cases, of a heart attack. It is typically experienced as chest pain and occurs when the heart muscle does not get as much blood (hence as much oxygen) as it needs for a given level of work (ischemia). Angina is usually referred to as one of two states:

Click the icon to see an image about angina.

• Stable Angina (which is predictable).
• Unstable Angina (which is less predictable and a sign of a more serious situation).

Angina itself is not a disease. Much evidence indicates that onset of angina less than 48 hours before a heart attack may be protective, possibly by conditioning the heart to resist the damage resulting from the attack. Angina may be experienced in different ways and can be mild, moderate, or severe.

Click the icon to see an image of angina.

Specific factors are typically considered in determining whether symptoms indicate angina:

• Quality of the pain. Angina pain is typically described by patients as squeezing, heavy, suffocating, or griplike. It is rarely described as stabbing or burning. Changing one's position or breathing in and out does not affect the pain. The intensity of the pain does not always relate to the severity of the medical problem. Some people may feel a crushing pain from mild ischemia, while others might experience only mild discomfort from severe ischemia. In some cases, the patient experiences shortness of breath, fatigue, or palpitations instead of pain. In others, the ischemia is entirely asymptomatic ("silent ischemia").
• Duration. A typical angina attack lasts minutes. If it is more fleeting or lasts for hours, it is probably not angina.
• Location. Pain is usually in the chest under the breast bone. It often radiates to the neck, jaw, or left shoulder and arm. Less commonly, patients report symptoms that radiate to the right arm or back.
• Triggers of Angina. Angina is usually triggered by physical exertion, emotional stress, or exposure to cold.
• Factors that Relieve Angina. Angina is usually relieved by rest or by taking nitroglycerine under the tongue.

Stable Angina. Stable angina is predictable chest pain. Although less serious than unstable angina, it can be extremely painful. It is usually relieved by rest and responds well to medical treatment (typically nitroglycerin). Any event that increases oxygen demand can cause an angina attack. Some typical triggers include:

• Exercise
• Cold weather
• Emotional tension
• Large meals

Angina attacks can occur at any time during the day, but most occur between 6 AM and noon.

Unstable Angina and Acute Coronary Syndrome. Unstable angina is a much more serious situation and is often an intermediate stage between stable angina and a heart attack, in which an artery leading to the heart (a coronary artery) becomes completely blocked. A patient is usually diagnosed with unstable angina under one or more of the following conditions:

• Pain awakens a patient or occurs during rest.
• A patient who has never experienced angina has severe or moderate pain during mild exertion (walking two level blocks or climbing one flight of stairs).
• Stable angina has progressed in severity and frequency within a 2-month period, and medications are less effective in relieving its pain.

Unstable angina is now usually discussed as part of a condition called acute coronary syndrome (ACS). ACS also includes people with a condition called NSTEMI (non ST-segment elevation myocardial infarction) -- also referred to as non-Q wave heart attack. With NSTEMI, the blood tests suggest a developing heart attack. These conditions are less severe than heart attacks but may develop into full-blown attacks without aggressive treatment. [For additional information on this syndrome see In-Depth Report #12: Heart attack and acute coronary syndrome.]

Prinzmetal's Angina. A third type of angina, called variant or Prinzmetal's angina, is caused by a spasm of a coronary artery. It almost always occurs when the patient is at rest. About two-thirds of people with it have severe atherosclerosis in at least one major blood vessel. Irregular heartbeats are common, but the pain is generally relieved immediately with standard treatment.

Click the icon to see an image of a coronary artery spasm.

Silent Ischemia. Some people with severe coronary artery disease do not experience angina pain. This condition is known as silent ischemia, which some experts attribute to abnormal processing of heart pain by the brain. This is a dangerous condition because patients have no warning signs of heart disease. Some studies suggest that people with silent ischemia experience higher complication and mortality rates than those with angina pain. (Angina pain may actually protect the heart by conditioning it before a heart attack.)

Syndrome X. Syndrome X is a condition that occurs when patients have atypical angina chest pain. Their electrocardiograms are abnormal during a stress test, but they have no signs of blocked arteries. It is more likely to occur in women. Although it unclear what causes this condition, imaging tests suggest that Syndrome X may also be caused by ischemia, as is angina.

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Review Date: 4/12/2006
Reviewed By: Harvey Simon, M.D., Editor-in-Chief, Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital