DEAR DR. DONOHUE: My brother, who’s trim and fit, felt faint on his 71st birthday while attending a tenants meeting. He left the room to get some fresh air and started sweating profusely. Emergency medics took him to the emergency room, where his vital signs were fine, as was an EKG. The doctor told him it was due to the vagus nerve and something called vagaling. The reason I am worried is that 30 years ago, I had similar symptoms and was diagnosed with an arteriovenous malformation. I had brain surgery and a long recovery. I don’t want this to happen to my brother. Please help me understand this. — R.M.

ANSWER: The doctor must have said he suffered from vasovagal syncope. “Syncope” (SING-kuh-pea) is a faint — a temporary loss of consciousness due to a short-lived shutdown of blood flow to the brain. A vasovagal faint (now called neurocardiogenic) is the most common kind of faint. A hot, crowded room, prolonged standing, stress, overwhelming emotions, alcohol, fatigue, hunger and pain are some of the things that trigger vasovagal syncope.

In the circumstances I listed, blood vessels expand and blood pools in the legs. That diminishes the volume of circulating blood. This is the “vaso” of “vasovagal.” For some reason, the vagus nerve kicks in — inappropriately so. That’s the “vagal” of “vasovagal” and of “vagaling.” The vagus nerve slows the heart. With a slow heartbeat and a deficit of circulating blood, the brain is shortchanged of its circulatory demands. A faint occurs. In the horizontal position on the ground, circulation resumes and the person wakes.

Most of the time, a single faint doesn’t indicate a serious or persistent problem. Things like an abnormal heart rhythm, a ministroke and seizures have to be considered.

The description of your brother’s episode doesn’t suggest a bleeding AVM, arteriovenous malformation. It’s a ball of thin-walled arteries and veins, often found in the brain. AVMs are prone to bleeding. The malformation is there from birth, and bleeding, when it happens, usually does so between the ages of 10 and 30. Headache and seizures are its hallmarks.

DEAR DR. DONOHUE: I have had an underactive thyroid gland since the age of 4 months. I am now 53. My doctor wants me to stop my Synthroid pills (thyroid hormone replacement medicine) for two weeks. What can happen in those two weeks when I am not taking my medicine? I am worried. — S.S.

ANSWER: Synthroid has a half-life of seven days. That means after one week, half the medicine is still in your body and still active. A two- to eight-week vacation from this medicine can be taken without causing ill effects. The doctor, I guess, wants to assess your need for treatment and the correct dose required.

For most, thyroid replacement is a lifelong affair.

DEAR DR. DONOHUE: My son is on dialysis and has been for one year. His doctor has him on four blood pressure medicines because his pressure is so high.

Is it normal to be on this many blood pressure medicines? Why aren’t they working?

We know a person taking Lotensin. Would that help him? I am so worried. He is on the transplant list. If his pressure stays high, would they pass him over? — E.M.

ANSWER: Many people with kidney failure develop high blood pressure that is most difficult to control. It often requires a combination of medicines.

Even when these people are on dialysis, the high blood pressure can persist for some.

His pressure doesn’t disqualify him from a transplant. If anything, it shows how much he needs a new kidney.

Lotensin is a fine blood pressure medicine, but I would let his doctors decide which blood pressure medicines should be prescribed. They are experts in this and have taken care of many patients in situations similar to your son’s.

Dr. Donohue regrets that he is unable to answer individual letters, but he will incorporate them in his column whenever possible. Readers may write him or request an order form of available health newsletters at P.O. Box 536475, Orlando, FL 32853-6475. Readers may also order health newsletters from www.rbmamall.com.