CHICAGO - Once swine flu patients are sick enough to need hospital care, they decline very fast, requiring ventilators and advanced treatments that quickly strain scarce hospital resources, several teams reported on Monday.

Writing in the Journal of the American Medical Association they paint a picture of how younger, previously healthy people quickly developed severe respiratory failure, forcing doctors to use extreme measures to save them.

"The fact that they develop this very rapid, very severe respiratory failure means hospitals need to be prepared to manage these patients at any time of the day or night," said Doug White of the University of Pittsburgh School of Medicine, who wrote a commentary in the journal.

He said although the outbreaks in Canada and Mexico were relatively mild, they consumed a vast amounts of life-saving therapies.

If the second wave of H1N1 now spreading across the United States is more severe or broad-based, it may mean some patients will not get the critical life support they need, White said in a telephone interview.

"Saying no will mean they will die," he said.

'MAXED OUT'

"This is one of the most difficult conditions I've ever had to treat," said Dr. Anand Kumar of St. Boniface Hospital in Winnipeg, Manitoba, a hotspot of infection during the first wave of the swine flu in May and June.

Kumar said he normally sees only a few patients a year who become severely ill from an infection.

"In the case of Winnipeg, we saw 40 people on ventilators struggling for their lives simultaneously. It's a bizarre and somewhat frightening experience," he said in a telephone interview.

"At one point, 50 percent of the available ICU (intensive care unit) beds in the entire city were filled with H1N1 patients," Kumar said. "We basically maxed out our capacity."

While 95 percent of the patients he studied had some underlying risk factor, most of these were very common, such as asthma, smoking, obesity or high blood pressure.

"If you'd asked them, they would have said, 'Yes, I'm a pretty healthy person.' That is surprising to me," he said.

Researchers studying the outbreak in Mexico between March and June 2009 had similar experiences.

They saw critical illness mostly among younger people, who developed severe respiratory failure. About 40 percent of them died.

To help some of the very sickest patients who were struggling to breathe even with the assistance of a mechanical ventilator, researchers in Australia and New Zealand tried a type of life support called ECMO that adds oxygen to blood and circulates it throughout the body.

"Despite their illness severity and the prolonged use of life support, most of these patients survived," Dr. Andrew Davies of the Alfred Hospital in Melbourne and colleagues wrote.

Kumar said doctors in Winnipeg used the system for some of their patients who were struggling despite being on ventilators.

"In about 15 to 20 percent of our patients, had to use salvage therapies - basically, stuff that is unproven. But you use them because you've got nothing left," Kumar said.

Kumar said most people who get H1N1 will not have severe disease, which he said only occurs in about 1 in 1,000 patients. "The problem is, if you get half of your population with H1N1, that can turn into a lot of really sick people."