For parents worried about a child sick with the flu, deciding when to head to the emergency room can be difficult.

Unlike the typical seasonal flu, which is generally most dangerous to infants and the elderly, the H1N1 strain has hit children, teenagers and young adults unusually hard and with little warning.

The U.S. Centers for Disease Control and Prevention estimated earlier this month that more children have died from the H1N1 flu than people over 65, about 540 children as of mid-October compared to 440 seniors. And the agency recently reported that flu-related pediatric deaths were continuing to rise. An estimated 2,900 adults between 18 and 64 have died. Most years, 90% of people who die of the flu are 65 or older, officials said.

Life-threatening cases, however, remain unusual. As concern grows about the danger of H1N1, doctors also are seeing an uptick in what they call the "worried well," parents who seek emergency care for perfectly healthy children.

So when should you take your child to the emergency room? Doctors say parents and guardians should assess how sick a child is in part based on experience.

"Is there something really different about your child that's different from the seven or eight viral infections your kid gets every year? Those are the changes to look out for," said Dr. Mark Morocco, associate residency director for emergency medicine at UCLA.

Warning signs include significant difficulty breathing; inability to drink fluids or urinate for more than six hours; change in the color of the mouth or lips; or unusual behavioral changes, such as a crying child who cannot be consoled, or a child who doesn't wake up or walk or talk normally.

If any of those symptoms show up in children, parents should take them to the emergency room, Morocco said, noting that "respiratory infections are often things that are the most life-threatening in children."

Lung inflammation is particularly dangerous to infants and young children because their airways are smaller. According to the California Department of Public Health, the flu virus replicates in the airways and lungs, causing them to swell. The inflammation makes it difficult for the lungs to work, reducing the body's ability to take oxygen into the bloodstream.

In California, the most common causes of deaths associated with H1N1 flu have been viral pneumonia and acute respiratory distress syndrome, state health officials wrote in a recent report in the Journal of the American Medical Assn. Experts are telling clinicians to treat the H1N1 strain differently than the seasonal flu.

In a Journal of the American Medical Assn. editorial published earlier this month, former CDC director Julie Louise Gerberding wrote that patients who have a five- or six-day history of flu-like illness and whose ability to breathe is worsening "appear to be at risk for rapid deterioration" and should be treated with antiviral drugs and admitted to the hospital.

"Clinicians should not be falsely reassured by previous good health, young age and absence of major comorbidities because these characteristics do not exclude the potential for respiratory failure and death," Gerberding wrote.

The CDC has also warned that some physicians are not prescribing antiviral drugs to H1N1 patients, pointing to studies that show that about 25% of hospitalized patients with lab-confirmed H1N1 did not receive Tamiflu or similar drugs.

Even among those who did get antiviral drugs, medication was often delayed for one or two days after they were admitted to a hospital, the CDC said. California health officials have also said that antiviral medication can reduce mortality even when given late, which is defined as more than 48 hours after symptoms begin.

Although most people who are hospitalized or have died from H1N1 have underlying medical conditions, a significant proportion of H1N1 victims are otherwise healthy.

"What's surprising about this flu is . . . we're seeing patients between the ages of 10 and 47 with no underlying medical problems that are getting into trouble. And that's scary for us, because it's hard to know who is going to get in trouble," said Dr. Gail Carruthers, director of the pediatric emergency department at Long Beach Memorial Medical Center and Miller Children's Hospital.

Sometimes, patients will report flu-like symptoms for as little as three hours or as long as two weeks, then quickly become significantly worse. Their lungs begin to fail and fill up with fluid, requiring intensive care.

Carruthers recalled two recent patients, a teenager and a middle-aged person, whose lungs began failing even though they had no underlying medical conditions.

"It's almost like watching them drown," Carruthers said. "They feel like they can't get any air." But, Carruthers added, "if you don't feel short of breath, and you have a dry cough, you're probably fine staying at home."

ron.lin@latimes.com