Over the years I’ve seen and evaluated many syncopal episodes, or fainting spells.

In one, a 62-year-old college professor collapsed without warning while delivering a lecture. Several students rushed to his aid, by which time he was conscious again. An ambulance was called and he was taken to a hospital.

In another case, a 48-year-old woman forgot to take her morning blood pressure pill and skipped lunch while Christmas shopping. She finally took her medication in the evening before heading out to a cocktail party, where she downed a martini.

The next thing she knew, she was flat on her back. Despite her protests, she was taken to a hospital.

And a 23-year-old medical student’s vision blurred as she began dissecting a cadaver in her first-year anatomy class. She suddenly felt giddy, lightheaded, and sick to her stomach, and broke out in a cold sweat.

Seconds later, she passed out. Within seconds she awoke, was helped to her feet after a few minutes, and took the rest of the day off.

The 101 on Syncope

All three had a syncopal episode, each for a different reason but with the same common denominator: a momentary lack of oxygen-carrying blood to the brain, with a loss of consciousness. By falling to the floor and assuming the recumbent position, blood flow to the brain was restored.

A true faint—as opposed to other causes of sudden loss of consciousness, such as head trauma, seizures, and strokes—usually lasts for only seconds to minutes. In the rare instances when attaining that recumbent position is impossible, a faint may be longer, and permanent brain damage can occur.

Otherwise, the main downside of fainting is the potential for physical injury from falling.

Fainting is surprisingly common, with suggested prevalence rates as high as 40 percent. In a study of about 2,000 people, one in five had a fainting episode at some point. It has been estimated that up to 3 percent of emergency-room admissions are due to syncope.

Determining the Cause

Pinpointing the reason for a fainting spell can be difficult. About one-third of syncopal episodes are labeled “of unknown cause.”

And though attempts have been made to establish a “syncope risk score” (to differentiate the more serious causes from the less hazardous or easily explainable), that distinction may not be evident at the time of an ER evaluation.

We do know that the likelihood of a serious underlying cause—and cardiovascular disease predominates—increases with age. We’ve also found that the need for extensive and expensive testing and hospital admission can be reduced by a carefully focused history and physical exam, plus an electrocardiogram (EKG).

As for My Patients . . .

While the professor was in the ambulance, an EKG showed a complete heart block—that his heart’s upper chambers were not “talking” to the lower chambers. His pulse rate was only 28 beats per minute.

Within 2 hours, a cardiac pacemaker had been implanted in his chest. He was back in action several days later.

The martini drinker’s EKG was normal in the ER, but her blood pressure was 80/50. Normal for her, on blood pressure medication, was 140/80. The combination of alcohol and medication (potent blood vessel dilators) had sent her blood pressure into a tailspin that ended in a faint.

In the ER, she was given intravenous fluids and was monitored for a few hours. She learned the hard way that blood pressure drugs and alcohol don’t mix.

The medical student was a victim of her own overactive vagus nerve, which, when stimulated by an upsetting trigger, slows the heart and lowers blood pressure, causing a faint.

Her presyncopal symptoms were typical: blurred vision, nausea, palpitations, and cold sweats. She decided surgery wasn’t in her future.

Feel Like You Might Pass Out?

If you’re one of the fortunate fainters with presyncopal symptoms, try to avoid triggers such as severe pain; the sight of blood or the needle prick involved in taking blood; hot, stuffy rooms; standing for long periods; scary movies; and heights.

And if you feel those telltale symptoms coming on, immediately lower yourself to the floor to avoid getting hurt if you should collapse. Don’t try to get up too soon; recurrent fainting is common.

Editor's note: This article also appeared in the August 2017 issue of Consumer Reports on Health.