Pressurized oxygen is best at countering carbon monoxide exposure. (Into the Tank).

Carbon monoxide poisoning sends roughly 40,000 people to hospitals every year in the United States. Although doctors routinely treat such patients with oxygen, the medical community still hasn't reached a consensus on the optimum dose or best delivery method.

Scientists report in the Oct. 3 New England Journal of Medicine that breathing pressurized, or hyperbaric, oxygen limits long-term brain damage from carbon monoxide poisoning better than simply inhaling oxygen at normal atmospheric pressure from a mask, the most common therapy.

To deliver hyperbanic oxygen, physicians place a patient in a sealed chamber containing 100 percent oxygen pressurized to 2 to 3 atmospheres, that is, double to triple the air pressure at sea level. Treatment usually lasts a few hours. The pressure in the tank feels akin to that beneath 6 feet of water in a pool, and hyperbaric treatment carries a slight risk of ear discomfort and convulsion. Past studies failed to show a therapeutic difference between hyperbaric and unpressurized oxygen therapy.

In the new study, Lindell K. Weaver of LDS Hospital in Salt Lake City and his colleagues tracked the progress of 152 patients admired for carbon monoxide poisoning in the 1990s. The researchers assigned half those people to receive hyperbaric oxygen in three stints of roughly 2 hours each. The other patients were treated in a tank but received oxygen at 1 atmosphere for 2 hours--a typical emergency room dose--and then just normal air during two more 2-hour sessions.

At 6 weeks, 6 months, and 12 months after treatment, patients completed tests, such as recalling a story, designed to measure brain function. Those who had received hyperbaric oxygen scored significantly higher during all three testing sessions than the others did.

The study included more patients than did past comparisons, kept patients and scientists unaware of which participants were receiving hyperbaric oxygen, and used precise follow-up measurements of brain, damage, says Stephen R. Thom of the University of Pennsylvania School of Medicine in Philadelphia. "It's a very strong study," he says.

When a person inhales carbon monoxide, it forms carboxyhemoglobin, a destructive form of the molecule that shuttles oxygen to cells. Oxygen breathed through a mask at normal pressure for a few hours usually lowers blood carboxyhemoglobin concentrations and combats the nausea, headache, and loss of consciousness that victims experience. But the new study shows that hyperbaric oxygen should be the standard treatment for seriously poisoned people to prevent delayed effects such as memory lapses, confusion, and language problems, Weaver says.