However, sometimes people are hesitant to call the emergency number because they're not certain their problem is serious enough, or they're unsure of the protocol. Other people may panic and call 911 unnecessarily when a visit to their family doctor or a trip to an urgent care center would suffice.

"What is really a medical emergency is all in the eye of the beholder at the time of their crisis," says Dr. Gary Vilke, an emergency room physician and professor of clinical medicine at the University of California San Diego Medical Center.

Use good sense, advises Frank Hoff, police dispatch administrator for the San Diego Police communications division, which receives 911 calls made from within the city of San Diego.

"Don't call us to find out how to cook your Thanksgiving turkey. Yes, people have really done that," he says. "However, we encourage people to err on the side of caution and call 911 if they suspect an emergency. We don't want an emergency to happen and someone to die because of the fear of calling 911."

Although there's no way to rehearse for a medical emergency, it helps to know what kinds of medical problems justify a call to 911 and what you may be asked or told to do when you make that call.

Emergency medical experts list certain health problems and medical crises that warrant a call to 911.

SUSPECTED HEART ATTACK

Of course, it could just be indigestion, but if you're not sure and you're worried, "absolutely call 911, but don't be embarrassed if it turns out not to be a heart attack" says Vilke, adding that callers should never be made to feel guilty about calling 911 for a pain that turns out not to be an emergency.

"The next time they may not call and that is when it may be the real thing," he says.

While waiting: The patient should chew an aspirin and not exert themselves.

SUSPECTED STROKE.

Don't hesitate to call 911 if the patient experiences unilateral weakness (it usually is not on both sides), visual changes, facial drooping, slurred speech or vertigo.

While waiting: The patient can take an aspirin. Immediate medical attention is critical for stroke treatment, so calling for help as soon as you suspect a stroke is imperative.

SEVERE BLEEDING

Call 911 if it's a big wound and you can't manage the bleeding or the patient feels dizzy or weak.

While waiting: Put direct pressure on the wound. If an extremity is bleeding, elevate it.

"Don't pile a bunch of towels (on the wound) and wrap it up. Just put one towel over it and squeeze as hard as you can to get the bleeding to stop," Vilke says.

SEVERE PAIN

If the pain is chronic, such as back pain, and the patient has experienced it before, it may not be an emergency call.

"However, if you have severe abdominal pain all of a sudden and you're passing out with it or having a headache that feels like a thunderclap to the head, that's different," Vilke says. "If it's a brand new pain and the worst pain of your life and you're so debilitated you can't get off the couch, call 911."

While waiting: The dispatcher will tell you how to get as comfortable as possible.

CHOKING

Call 911.

While waiting: If you are calling for someone who is choking, you will be connected to an emergency medical dispatcher who may give you instructions to perform procedures such as CPR or the Heimlich maneuver while you wait for the response unit to arrive. Make sure the phone is near the patient to save time.

SERIOUS BURN

Although most household burns are not emergencies, it depends on how big or extensive the burn is and your transportation options. If you have a bad burn and don't have someone to drive you to the hospital, call 911, Vilke says.

While waiting: Apply tepid, not cold, water to the burn to help cool the area and keep it moist. Don't apply any over-the-counter first-aid sprays or creams.

"Don't mess with the burned area. Some things can cause more damage. Let the burn center take care of that," Vilke says.

SEVERE ALLERGIC REACTION

Call 911 even if it seems to be a mild reaction.

"Allergic reactions can turn very bad very quickly, so let the paramedics help," Vilke says.

While waiting: Take Benadryl or use an epinephrine pen or auto injector to help improve breathing and reverse swelling.

POISONING

Call 911 if the patient has lost consciousness, is having trouble breathing or is in severe distress.

While waiting: Do not take syrup of ipecac or try to induce vomiting.

SEVERELY ILL CHILD

"Any time parents are significantly concerned about a child's health problem, they should call 911. Even if they think it might be bad, but they're not sure," says Vilke, who notes although the child's illness may not be a real medical emergency, a call to 911 can prevent distraught parents from taking to the roads in search of an emergency room.

"We don't want moms doing 80 miles per hour down city streets because they're concerned about Johnny's health," he says.

While waiting: The dispatcher will tell you how to make the child as comfortable as possible.