The heart is a muscle that depends on electrical signals from a biological "pacemaker" (located in the atria of the heart) to generate the regular and evenly spaced contractions of the chambers of the heart necessary to pump blood with its vital oxygen and nutrients throughout the body.  Heart disease and natural limitations may cause these signals to become interrupted or chaotic in a number of ways producing arrhythmias.   

One very important variation in these signals is known as atrial fibrillation.  In this disorder, the signals in the atria become chaotic and only at irregular periods succeed in stimulating the heart muscles to full contraction.  This chaotic behavior severely limits the  ability of the heart to pump sufficient blood to meet the body's needs.

Atrial Fibrillation is a common arrrhythmia in small animals (14%  of all canine arrhythmias).  Left untreated it can cause important changes in the functionality of the heart.  Atrial fibrillation may be caused by underlying heart disease, it may occur due to non-cardiac disease or anesthesia or it may be idiopathic in some large breed dogs. 

Treatment requires management of the underlying problem and slowing the heart sufficiently to allow time for the chambers to fill, thus permitting the heart to pump more effectively.  If diagnosed with this disease, your pet may be given medications to reduce the risk of a blood clot caused by the slower movement of blood through the heart chambers.