Syncope (say "SING-kuh-pee") refers to a sudden loss of consciousness that does not last long. Syncope may be the first sign that you have an arrhythmia, and it is a very worrisome symptom for several reasons:

• Fainting can result in a serious injury (for example, if you faint while climbing stairs or driving).
• You faint because your brain did not get enough oxygen to function, which may be a warning sign that you have a serious medical condition.

An arrhythmia can cause syncope in the same way that it causes lightheadedness (presyncope): your heart cannot pump blood effectively during excessively fast or slow heart rates, reducing the amount of blood that reaches your brain. With syncope, however, the arrhythmia causes such a dramatic drop in the blood pressure that the brain does not receive enough blood to keep you awake, and as a result you lose consciousness. In order for an arrhythmia to cause syncope, your heart rate must be extremely fast or extremely slow, or you must have some other heart condition in addition.

How long does syncope last?

It is important to recognize that syncope is transient, meaning that you wake up soon after fainting. Consciousness may return because the arrhythmia spontaneously stops and a normal heart rhythm and blood pressure return. Even if the arrhythmia persists, you may still regain consciousness. When you have an episode of syncope due to an arrhythmia, it typically happens while you are standing or sitting, and the loss of consciousness causes you to fall to the floor. After you are lying down, blood flow returns to your brain, even though your blood pressure may remain low. When adequate blood flow returns to your brain, you will likely wake up.

Which types of arrhythmias can cause syncope?

All arrhythmias can cause syncope under certain conditions. Wolff-Parkinson-White syndrome (WPW) may lead to an arrhythmia, causing syncope. Usually the heart's electrical signal coming from the atrium must be filtered through the atrioventricular (AV) node before reaching the ventricles. The AV node limits how fast the signals get to the ventricle and will block the impulses if they come too fast. When a bypass tract exists, the electrical impulses can reach the ventricles without being filtered by the AV node and cause the ventricles to contract at extremely fast rates. As a result, your heart pumps less efficiently, causing a drop in blood pressure that leads to syncope and even sudden death.