As shown here, the heart has four chambers. When the heart is at rest, the chambers fill with blood. With each heartbeat, the heart squeezes blood out into the body.

A normal heart beats 60 to 100 times each minute.

When you exercise, get excited, or experience stress, your body needs more oxygen. Your heart has to beat faster to keep up with this demand. When you are resting or sleeping your heart rate may be slower.

Your heart has its own electrical system. Special tissues generate electrical signals that travel along pathways through the heart every time it beats.

Another name for a heart attack is myocardial infarction - "MI" for short. A heart attack occurs when the blood supply to part of the heart muscle itself - the myocardium - is severely reduced or stopped. This usually occurs when one of the arteries that supply blood to the heart muscle (coronary arteries) is blocked by an obstruction. The blockage is usually from the buildup of plaque (deposits of fat-like substances) due to atherosclerosis.

A heart attack is often caused by a blood clot forming in a coronary artery. This often results from reaction to rupture or tearing of a plaque that has formed. Such an event is sometimes called a coronary thrombosis or coronary occlusion. A myocardial infarction is the damaging or death of an area of the heart muscle resulting from a reduced blood supply to that area.

If the blood supply is cut off severely or for a long time, muscle cells suffer permanent injury and die. Disability or death can result, depending on how much heart muscle is damaged.

Every 34 seconds, someone dies from heart and blood vessel diseases, America's number one killer. Since most of those deaths are from heart attack - about 490,000 each year - it is important to learn all you can about heart attack. Do not ignore heart problems. It is a matter of life and death!

When a heart attack occurs, it is critical to recognize the signals and respond immediately. About half of all heart attack patients wait two hours or longer before deciding to get help. This reduces their chance of survival. It also lessens the chance of preserving heart muscle, which raises the risk of disability for those who survive.

The importance of time cannot be overemphasized. When a coronary artery is blocked, the heart muscle does not die instantly. Instead, damage increases the longer the coronary artery remains blocked. If a patient gets to an emergency room fast enough, a technique called reperfusion therapy may be done using drugs, often referred to as 'clot busters' (thrombosis), balloon angioplasty, or surgery. The sooner any of the reperfusion techniques occur, the more likely the patient will have a good result.

Get help fast if you feel any of these signs!

The American Heart Association says the body likely will send one or more of these common or "classic" warning signals of a heart attack:

• Uncomfortable heavy feeling, pressure, pain, or squeezing in the center of your chest that last more than a few minutes.
• Pain that goes to your shouldrs, neck or arms.
• Discomfort in your chest along with lightheadedness, fainting, sweating, nausea or shortness of breath.

Should any of these symptoms occur, call 911.

An arrhthmia is any heart rhythm that is "abnormal". It may considered abnormal if it is too fast, too slow or starts somewhere in the heart other than the Sinus node, the heart's normal pacemaker. Sometimes a heart's natural rhythm is interrupted or becomes irregular. It may be that the heart's natural pacemaker sends out electrical impulses too slowly. Sometimes medicines like beta-blockers cause the heart rate to be too slow.

Or, the electrical impulses may be blocked along the pathway through the heart, resulting in a condition called "heart block" which also may cause the heart rate to be too slow. The diagram above demonstrates the location of the specialized tissue that starts and directs the electrical impulses responsible for your heart rhythm.

The most common medical condition requiring a cardiac pacemaker is called "bradycardia", (pronounced bray-dee-car-dee-ah) meaning a heart rate that is less than 60 beats per minute. Bradycardia does not necessarily cause any symptoms. However dizziness, extreme fatigue, shortness of breath, or fainting spells may result from bradycardia. By restoring a heart rate to its appropriate rate, a pacemaker can relieve these symptoms.

Rhythm disorders that involve the Sinus (SA) node are classified under the broad term sick sinus syndrome. If the Sinue node, your heart's natural pacemaker, loses the ability to initiate a heartbeat or increase the heart rate, the heart may not be able to respond effectively to the body's changing circulation demands.

The electrical signal from the SA node must pass through the AV node and continue on through the conduction pathways in the ventricles. The impulse may become slowed, irregular, or stopped at the AV node. This condition is called heart block because the electrical impulse is blocked from proceeding normally from the atria to the ventricles. Heart block is described as first, second, or third degree. How slow the ventricular rate becomes depends on the degree of heart block.