Faint, black out, swoon, pass out. They're all names for the same thing - a temporary loss of consciousness followed by a fairly rapid and complete recovery. It's frightening when it comes out of the blue, more so when it happens again and again. The technical term, syncope (SIN-kuh-pee), comes from a Greek word that means to cut short or interrupt. What's being interrupted is blood flow to the brain.

Fainting occurs when something interrupts blood flow to the brain. Although usually harmless, fainting can cause injuries and sometimes signals a problem with the heart or circulatory system.

If you faint, it's worth talking with your doctor, especially if there doesn't seem to be an explanation for the faint.

Tensing muscles in the legs and arms can sometimes stave off a faint; standing or tilt training can help some people become less susceptible to the crossed signals that cause fainting.

Going, Going, Gone

It is harder for the heart to pump blood up to the brain than down to the toes. Blood pressure helps overcome the downward tug of gravity and push blood to the head.

We tend to think of blood pressure as a relatively stable entity. It isn't. It changes every time you stand up, sit down, bend over, eat, sneeze, go to the bathroom, get stressed, relax, or heft a bag of groceries. The human body is designed to counter these shifts and quickly bring blood pressure back to its usual point. This requires an intricate feedback system that involves sensors in the aorta and in the carotid arteries leading to the brain.

These sensors stimulate a host of nerve and hormone signals that change how fast the heart beats, how much blood it pumps with each beat, and the contraction and relaxation of blood vessels. But some diseases, some drugs, and sometimes even aging interfere with these near-instantaneous corrections, causing temporary drops in blood flow to the brain.

Unlike most tissues, the brain doesn't store energy. Instead, it requires a constant supply of sugar and oxygen. Halting blood flow for just three to five seconds is enough to put the brain into an energy-conserving shutdown. The brain's signals to nerves and muscles stop, and the person slumps to the ground. Once the body is horizontal, it's easier for the heart to pump blood to the head, and the brain "wakes up" again.

Many things can cause sudden low flow to the brain that leads to fainting. The causes can be grouped loosely into two camps: problems outside the heart (noncardiac syncope), which account for the majority of faints, and problems in the heart (cardiac syncope).

Getting to the Heart of a Faint

Figuring out what caused a fainting spell is the best way to prevent it from happening in the future.

The most important piece of evidence is your story of what happened, especially if it can be corroborated by an eyewitness or two. Your description can suggest a vasovagal reaction (fainting due to standing too long in the heat, getting a shot, etc.), orthostatic hypotension (fainting when you stand up), or something more serious. A physical exam is in order no matter how straightforward the story.

An electrocardiogram (ECG) and echocardiogram are essential tests for evaluating anyone who has fainted. The echocardiogram can spot hidden problems in the left ventricle or other forms of structural heart disease that can cause fainting. The ECG can detect irregularities in the heart's rhythm that could be the culprit.

Unfortunately, capturing erratic heartbeats in the act is like finding the proverbial needle in a haystack. If you have fainted a few times for no apparent reason, and the ECG was "fine," your doctor might ask you to wear a device called a Holter monitor that continuously records the heart's electrical patterns for a 24-hour period.

Longer-term monitoring can be done with a wearable loop recorder or an implanted recorder. These devices constantly monitor the heartbeat but store only the last several minutes. When a fainting spell hits, you press a button to freeze the information in the recorder.

Another diagnostic test is the tilt test, in which your blood pressure and heart rate are measured while lying flat on a moveable table, and then again when the table is tilted so you are almost, but not quite, standing upright.

Noncardiac Causes

The most common reason for fainting arises from crossed wires between the brain and the part of the nervous system (the vagus nerve) that regulates blood pressure and heart rate. In response to some trigger, the blood vessels in your legs relax, making it difficult for blood to return to the heart. Blood begins pooling in the legs.