What Is It?
A cardiac arrhythmia is any abnormal heart rate or rhythm.
In normal adults, the heart beats regularly at a rate of 60 to 100 times per minute, and the pulse (felt at the wrist, neck or elsewhere) matches the contractions of the heart's two powerful lower chambers, called the ventricles. The heart's two upper chambers, called the atria, also contract to help fill the ventricles, but this milder contraction occurs just before the ventricles contract, and it is not felt in the pulse. Under normal circumstances, the signal for a heartbeat comes from the heart's sinus node, the natural pacemaker located in the upper portion of the right atrium. From the sinus node, the heartbeat signal travels to the atrioventricular node, or "A-V node," (located between the atria) and through the bundle of His (pronounced HISS - a series of modified heart muscle fibers located between the ventricles) to the muscles of the ventricles. This causes the ventricles to contract and produces a heartbeat.
Cardiac arrhythmias sometimes are classified according to their origin as either ventricular arrhythmias (originating in the ventricles) or supraventricular arrhythmias (originating in heart areas above the ventricles, typically the atria). They also can be classified according to their effect on the heart rate, with bradycardia indicating a heart rate of less than 60 beats per minute and tachycardia indicating a heart rate of more than 100 beats per minute.