Palpitations (cont.)

Palpitations are unpleasant sensations of irregular and/or forceful beating of the heart. Some persons with palpitations have no heart disease or abnormal heart rhythms and the reasons for their palpitations are unknown. In others, palpitations result from abnormal heart rhythms (arrhythmias).

• Arrhythmias refer to heartbeat rhythms that are too slow, too rapid, or irregular.
• Rapid arrhythmias (greater than 100 beats per minute) are called tachycardias.
• Slow arrhythmias (slower than 60 beats per minute) are called bradycardias.
• Chaotic heart rhythms are called fibrillations. When affecting the upper chambers of the heart, they are called atrial fibrillation. When affecting the lower chambers of the heart, they are called ventricular fibrillation.
• When a single heartbeat occurs earlier than normal, it is called a premature contraction, and this can cause the sensation of a forceful heartbeat.
• Abnormalities in the atria, the ventricles, and the electrical conducting system [the sinoatrial (SA) node and the atrioventricular (AV) node] of the heart can lead to arrhythmias that cause palpitations.

The atria (right atrium and left atrium) are the upper chambers of the heart.

• The right atrium receives venous blood from the body and pumps it into the right ventricle.
• The left atrium receives oxygenated blood from the lungs and pumps it into the left ventricle.
• The ventricles are the lower chambers of the heart.
• Each heart has two ventricles (right and left ventricles).
• The right ventricle pumps venous blood to the lungs and the left ventricle pumps oxygenated blood to the rest of the body.

The SA node is the pacemaker of the heart and is located in the right atrium. The electrical signals initiated in the SA node are transmitted to the atria and the ventricles to stimulate heart muscle contractions (heartbeats). The AV node is specialized heart tissue which acts as an electrical relay station between the atria and the ventricles. Electrical signals from the SA node and the atria must pass through the AV node to reach the ventricles.

When rapid arrhythmias (tachycardias) and premature contractions occur because of abnormal electrical activity of the atria, they are called atrial tachycardias and premature atrial contractions (PACs). When tachycardias and premature contractions occur because of abnormal electrical activity of the ventricles, they are called ventricular tachycardias and premature ventricular contractions (PVCs).

Slow arrhythmias (bradycardias) can occur because of slowing of the electrical signals initiated by the SA node, a condition called sinus bradycardia. Bradycardias can also result from varying degrees of "heart block," wherein certain medications or diseases in the electrical conduction system of the heart impede the transmission of signals from the atria to the ventricles (see the "Bradycardias" section below).

Premature contractions are isolated heartbeats that occur earlier than expected. The premature contraction is followed by a pause, as the heart electrical system "resets" itself. The contraction following the pause is usually more forceful than normal contractions. The patients frequently perceive these more forceful contractions as palpitations.

For more, please read the Abnormal Heart Rhythms article.

Source: MedicineNet.com
http://www.medicinenet.com/palpitations/article.htm