DEAR DR. DONOHUE: My father died of ventricular fibrillation at age 64. He was thought to be in good health and had seen a doctor regularly. No one can explain why this happened to him so suddenly and unexpectedly. What’s the difference between this and atrial fibrillation? I know several people who have that and have not died from it. – M.A.

ANSWER: “Fibrillation” means that the heart muscle is not contracting; it’s squirming.

Atrial and ventricular fibrillation are two different entities with greatly different implications. In atrial fibrillation, a common heartbeat abnormality, the upper heart chambers – the atria – are fibrillating. That doesn’t impair pumping of the blood too significantly. It does, however, foster clot formation in the atria, and pieces of clot can be taken to the brain via the circulation and cause a stroke. Blood thinners prevent this disaster.

Ventricular fibrillation always leads to death if it is not quickly ended. Here, the lower heart chambers – the ventricles – are fibrillating. These are the heart chambers that pump blood into the body. Heart pumping stops, and without a blood supply, the brain dies. Death comes on quickly.

Heart attacks, heart failure and other heart conditions, which might not have produced any symptoms until the fateful moment arrives, can bring on ventricular fibrillation. Often the event takes place much like your dad’s did — suddenly and unexpectedly.

You have most likely heard of defibrillators, machines that shock the fibrillating heart back into a normal heartbeat and pumping action. They used to be found only in hospitals. Now they are in many public places and quite easy to use. If people survive a brief episode of ventricular fibrillation, they are often given an implantable defibrillator that shocks them the instant this heart rhythm returns.

DEAR DR. DONOHUE: Can flu kill? In the paper I see that it can, and I hear that it can on the radio and TV, but I wonder if this isn’t an overblown statement to frighten people into getting a flu shot. I don’t believe in shots. I never get them. What is your information on this? – S.S.

ANSWER: Flu can kill. Flu, influenza, is a respiratory illness, not a stomach or diarrhea illness. It brings on shaking chills, high fever, severe muscle pain, runny nose, scratchy throat and often a cough. People want to lie in bed until it is over. The usual cause of death, when it happens, is pneumonia due to a bacterial infection that follows on the heels of the viral infection.

In the last decade of the 20th century, flu killed 325,000 Americans. Most of them were over 65. It kills youngsters too. In 2003, 153 American children died from it. The majority of them were infants.

Vaccines, including the flu vaccine, have contributed more to human health than any other single intervention. You should reconsider your position on “shots.”

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.