Vasovagal syncope is the most common cause of fainting. It occurs when your body reacts in an exaggerated way to such triggers as the sight of blood or extreme emotional distress. The trigger results in a sudden drop in your heart rate and blood pressure, which reduces blood flow to your brain and causes you to briefly lose consciousness.

Vasovagal syncope is usually harmless and requires no treatment. But your doctor may recommend tests to rule out more-serious causes of fainting, such as heart disorders.

Before a faint due to vasovagal syncope, you may experience some of the following signs and symptoms:

• Pale appearance to your skin
• Lightheadedness
• Nausea
• Feeling of warmth
• A cold, clammy sweat

When to see a doctor
Because fainting can be a sign of a more serious condition, such as heart or brain disorders, you may want to consult your doctor after a fainting spell, especially if you never had one before.

Vasovagal syncope occurs when the part of your nervous system that regulates heart rate and blood pressure malfunctions in response to a trigger, such as the sight of blood. Your heart rate slows, and the blood vessels in your legs widen. This allows blood to pool in your legs, which lowers your blood pressure. This drop in blood pressure and slowed heart rate quickly cause diminished blood flow to your brain, and you faint.

Common triggers for vasovagal syncope include:

• Standing for long periods of time
• Heat exposure
• The sight of blood
• Having blood drawn
• Fear of bodily injury
• Exertion, such as straining on the toilet

Appointments can be brief, so it's a good idea to be prepared to make the best use of the time allotted.

What you can do

• Write down a detailed description of your symptoms, including any triggers that may have precipitated your faint.
• Make a list of all your medications, including vitamins and supplements.
• Write down questions to ask your doctor, including what types of tests you might need and what treatments might be helpful.

What to expect from your doctor
Questions may include:

• What were you doing just before you fainted?
• What signs and symptoms, if any, did you experience before you fainted?
• Have you ever fainted before?
• Have you recently started taking a new medication?
• Have you ever experienced a head injury?
• Has anyone in your family died suddenly from heart problems?

During the physical exam, your doctor will listen to your heart and take your blood pressure. He or she may also massage the main arteries in your neck, to see if that causes you to feel faint.

The diagnosis of vasovagal syncope is often a matter of ruling out other potential causes of your fainting — particularly heart-related problems. These tests may include:

• Electrocardiogram. This test records the electrical signals produced by your heart. It can detect irregular heart rhythms and other cardiac problems that can cause fainting. In some cases, you may need to wear a portable monitor for at least a day or even for a month.
• Echocardiogram. This test uses ultrasound imaging to view the heart and look for conditions, such as valve problems, that can cause fainting.
• Exercise stress test. This test studies heart rhythms during exercise. It's usually conducted while you walk or jog on a treadmill.
• Blood tests. Your doctor may look for conditions, such as anemia, that can cause or contribute to fainting spells.

If there appear to be no heart problems causing your fainting, your doctor may suggest you undergo a tilt table test, which begins with you lying flat on a table. The table then changes position, tilting you upward at various angles, to see if these postural changes affect your heart rhythms or blood pressure.

In most cases of vasovagal syncope, treatment is unnecessary. Your doctor may help you identify your fainting triggers and discuss ways you can avoid them. However, if you experience vasovagal syncope often enough to interfere with your quality of life, your doctor may suggest trying one or more of the following remedies.

Medications

• Blood pressure drugs. Beta blockers such as metoprolol (Lopressor) are designed to treat high blood pressure. However, they are also the type of drug used most often to prevent vasovagal syncope because they block some of the signals that can lead to fainting.
• Antidepressants. Selective serotonin reuptake inhibitors such as paroxetine (Paxil), fluoxetine (Prozac) and sertraline (Zoloft) also have been successful in preventing vasovagal syncope.
• Blood vessel constrictors. Drugs to treat low blood pressure or asthma have sometimes been found helpful in preventing vasovagal syncope.

Therapies
Your doctor may recommend specific techniques to decrease the pooling of blood in the legs. These may include foot exercises, wearing elastic stockings or tensing your leg muscles when standing. Avoid prolonged standing — especially in hot, crowded places — and make sure you drink plenty of fluids.

Surgery
Some people with vasovagal syncope have been helped by the insertion of an electrical pacemaker, which helps regulate the heartbeat.

If you feel like you might faint, lie down and lift up your legs. This allows gravity to keep blood flowing to your brain. If you can't lie down, sit down and put your head between your knees until you feel better.