Fainting (syncope) is a sudden transient loss of consciousness. It most often happens when the blood pressure is too low (hypotension) and the heart does not pump enough oxygen to the brain. Typically, a fainting spell lasts only a few seconds or minutes, and then the person regains consciousness. Fainting is common and a single spell usually is not serious. 

It may be explained by factors such as stress, grief, overheating, dehydration, painful stimuli, exhaustion, or illness. Profound blood loss or fluid loss (severe diarrhea or vomiting) may also cause syncope. Many causes of syncope, however, are not easy to explain and are not serious. However, some are life threatening, especially fainting during exertion. The most serious causes of syncope are related to heart damage or abnormal electrical system disorders that affect the heart's ability to pump blood efficiently. In some cases, fainting is the only warning sign of an abnormal heart rhythm (arrhythmia) that could lead to sudden cardiac arrest and death.

Types of Syncope

  • Cardiovascular syncope - The most dangerous but rare type of fainting is caused by an abnormal heart rhythm (arrhythmia) or by structural damage to the heart.
  • Non-cardiovascular syncope - The most common type (also called vasovagal type).  There are a variety of causes not necessarily related to the heart. This type of syncopy is often related to problems with the autonomic nervous system (ANS), which controls functions such as heart rate and blood pressure. They may occur even in young, otherwise healthy people with normal heart function.

People with underlying heart disease are at higher risk for cardiovascular syncope, although this may be the initial symptom suggesting such a problem.