Arrhythmias (or dysrhythmias) are abnormal heart rhythms that cause it to pump abnormally.

When the electrical impulse moves through the heart normally, the heart contracts - about 60 to 100 times a minute. Each contraction represents one heartbeat. The atria contract a fraction of a second before the ventricles so their blood empties into the ventricles before the ventricles contract.

Under some conditions almost all heart tissue is capable of starting a heartbeat, or becoming the pacemaker. An arrhythmia occurs when:

• the heart's natural pacemaker develops an abnormal rate or rhythm.

• the normal conduction pathway from the upper to the lower chambers is interrupted.

• another part of the heart takes over as pacemaker.

Almost everyone knows what a basic ECG tracing (also called EKG) looks like. But what does it mean?

• The first little upward notch of the ECG tracing is called the "P wave." The P wave indicates that the atria (the two upper chambers of the heart) are contracting to pump blood to the ventricles (the two lower chambers of the heart).
  
  
• The next part of the tracing is called the "QRS complex." This part indicates that the ventricles are contracting to pump blood to the body and lungs.
  
  
• The part of the ECG is called the "ST segment." The ST segment indicates the amount of time from the end of the contraction of the ventricles to the beginning of the rest period before the ventricles begin to contract for the next beat.
  
  
• The final curve is called the "T wave." The T wave indicates the resting period of the ventricles as it readies for the next beat.

When your physician studies your ECG, he/she looks at the size and length of each part of the tracing. Variations in size and length of the different parts may be significant.

Atrial fibrillation is an arrhythmia in which the atria, the two upper chambers of the heart, beat rapidly and irregularly. These signals from the atria are sent to the ventricles, the lower, main pumping chambers of the heart, which causes them to also beat in a rapid and irregular fashion. This can lead to symptoms including awareness of the rapid, irregular heart beat, shortness of breath, lightheadness, and sometimes even heart failure. Furthermore, the blood may clot in the upper chambers during atrial fibrillation. If a clot breaks off and travels to an artery in the brain, a stroke may occur. In fact, as we age a greater number of strokes are caused by atrial fibrillation - by the time we are 80 years old, about a third of strokes are caused by this arrhythmia. The best drug to prevent stroke is warfarin (Coumadin). Aspirin is much less effective.