Any irregularity in your heart's natural rhythm is called an
arrhythmia. Almost everyone's heart skips a beat now and again, and these mild
palpitations are usually harmless.
Electrical impulses from the heart muscle (the myocardium)
cause the heart to beat (contract). This electrical signal begins in the
Sinoatrial Node, also called the SA node or the sinus node. The SA node is
located at the top of the heart's upper-right chamber (the right atrium). The
SA node is sometimes called the heart's "natural pacemaker." When an
electrical impulse is released from the SA node, it causes the heart's upper
chambers to contract.
Sick sinus syndrome is a type of arrhythmia. These are a group
of signs and symptoms that tells doctors the SA node is not working properly.
The SA node usually sends electrical impulses at a certain rate, but if the SA
node is not working properly, the heart may beat too fast, too slow, or both.