Arrhythmias are uncoordinated or irregular contractions of the heart muscle caused by a problem in the electrical system of the heart. Some arrhythmias are benign and will not affect your overall health or cause symptoms. Others may be more serious and require treatment.

It may help to first understand how the heart should work. The heart is made up of four chambers:

• Two upper chambers (atria/atrium) —Receive blood from the body and empty the blood to the lower chambers.
• Two lower chambers (ventricles) —Receive blood from the upper chambers and pump blood to the lungs or out to the body. The left ventricle is the strongest muscle/chamber in the heart and is responsible for pumping the blood to the entire body.

The contraction of these four chambers needs to be coordinated to be able to efficiently move blood throughout the body. The atria contract first to push blood into and help fill the ventricles. Then the ventricles contract to push blood to the body or lungs. The nerves of the heart stimulate and coordinate the heart muscles in an organized manner:

• An electrical signal begins in a nerve bundle, in the atria, called the sinoatrial (SA) node. The SA node, also known as the pacemaker of the heart, stimulates the heart to beat at 60-100 times per minute.
• The SA node sends electrical signals to the muscles of the atria which makes them contract. At the same time, the SA node sends the signal down the heart through a nerve pathway.
• The signal travels from the SA node to the atrioventricular (AV) node, which sits between the ventricles and the atria. The signal slows here so that the ventricles have time to fill.
• The AV node then sends signals to the muscles of the ventricles to make them contract.

Arrhythmias may affect the timing of the contractions between the atria and ventricles or the speed and strength of contractions of four different chambers. Arrhythmias can be:

• Bradycardia —heartbeat less that 60 beats per minute
• Tachycardia —heartbeat more than 100 beats per minute (Temporary tachycardia is a normal reaction to physical activity to help supply more oxygen to working muscles. Tachycardia that is not in response to physical activity can cause problems or be a sign of problems.)
• Extra beats
• Skipped beats

Heart diseases & disorders. Heart Rhythm Society website. Available at:
http://www.hrsonline.org/Patient-Resources/Heart-Diseases-Disorders#electrical
Accessed March 18, 2014.

What is an arrhythmia? National Heart, Lung, and Blood Institute website. Available at:
http://www.nhlbi.nih.gov/health/health-topics/topics/arr
Updated July 1, 2011. Accessed March 18, 2014.