Fainting (syncope) is a sudden 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, exhaustion, or illness. Profound blood loss or fluid loss (severe diarrhea) may also cause syncope.
Many causes of syncope, however, are not easy to explain. Some are life threatening. The most serious causes of syncope are related to heart damage or 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.
About 90 percent of people who faint have cardiovascular syncope, the most serious type of fainting disorder