One-third of all people will experience fainting in their lifetime.

Corbis

One morning as I looked out at the busy waiting room of my internal medicine practice, I watched a new patient, a trim, healthy-looking young woman, check in. Neatly dressed and in no obvious distress, she turned from the reception counter—and without warning collapsed onto the carpeted floor. No cry. No clutching of her chest or head. No weak-kneed stagger as she tried to make it to an empty chair. She just went down on the spot.

I ran around the counter out to the waiting room, unwrapping my stethoscope from my neck as I went. “A-B-C,” I thought to myself: airway, breathing, and circulation, the first three things you check when someone collapses like this. Within seconds I was kneeling next to the woman. My fingers found a regular pulse alongside her windpipe, and I saw that she was breathing without difficulty. Relieved this was not a cardiac or 
respiratory arrest, I released my own breath and watched as she opened her eyes. “Are you all right?” I asked.

She focused on me and said yes. But her wispy voice suggested otherwise, so I observed her closely in case she started to pass out again. We were both oblivious to the audience of other patients glued to their seats watching us, each one silently deciding that his or her cough or rash could wait.


A few moments later, the young woman sat up, alert. The whole event lasted no more than a minute. Her abrupt loss of consciousness followed by rapid recovery led me to the tentative conclusion that she had just experienced an episode of syncope, popularly known as a faint. My nurse and I helped our young fainter to her feet and walked her to an exam room.

The culprit in nearly all cases of syncope is an interruption in blood flow to the brain. The most common type, accounting for nearly half of all cases, is the vasovagal faint, during which the vagus nerve (a cranial nerve that helps regulate heart rate and blood pressure) sends signals that dramatically slow the heart. The signals can be provoked in susceptible people by distress they feel at the sight of blood or when they experience some other upsetting or threatening event. When your heart beats too slowly, it can’t sustain the blood pressure needed to feed oxygen to the brain, and down you go. Although we don’t know why some people are more susceptible than others, it occurs more often in females.

Cardiac rhythm disorders account for another 10 percent of all faints. Originating within the heart itself, unrelated to brain signals or emotional distress, these arrhythmias can disrupt blood flow. Whether causing the heart to beat much too quickly or far too slowly, the common denominator in these abnormal rhythms is low cardiac output, resulting in insufficient blood flow to the brain. As we walked back to the exam room, I asked my new patient if she had a history of heart problems or if she had experienced palpitations (skipping or racing of her heartbeat) prior to fainting. Her answer to both questions was no.

Yet another 10 percent of faints are caused by orthostatic hypotension, a precipitous drop in blood pressure that occurs when a person stands up. We all experience a mild version of this whenever we rise too quickly and feel slightly woozy. Normally we recover rapidly because evolution has equipped our bodies with a physiological control mechanism that prevents all our blood from draining into our ankles when we are standing. But if this mechanism doesn’t function properly, blood 
deserts our heads and follows gravity into our feet. Neuro­logical disorders like Parkinson’s disease—along with certain drugs, including some anti-hypertension medications—can blunt the responsiveness of this system. Orthostatic hypotension also occurs if the circulatory system is not fully expanded, which might happen when you are dehydrated or have had serious blood loss.

Finally, in nearly 2 percent of cases the cause of syncope remains undiagnosed.