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The common saying among surgeons, "frostbite in January, amputate in July," reflects the grim prospects of people whose extremities are burned by extreme cold.

But a new study suggests that clot-busting drugs used to treat strokes and heart attacks can sharply reduce the number of amputations among victims of frostbite.

"The preservation of limb ... is perhaps the greatest benefit conferred by the use of tPA in frostbite injury," said Kevin Bruen, a pediatric surgeon at the University of Utah in Salt Lake City.

He said the anti-clotting agent, tissue plasminogen activator (tPA), likely works by minimizing the injury to blood vessels as frozen skin is warmed again.

Paradoxically, the greatest harm comes not from the freezing of limbs, but their thawing. As frozen tissue is warmed, the immune system responds with inflammation and clotting that blocks small blood vessels, cutting off the blood supply and leading to progressive gangrene. The process can be excruciatingly painful.

Because tPA reverses clotting, blood flow can be restored to limbs before permanent damage is done, Dr. Bruen said.

The research, published in the Archives of Surgery, involved 32 patients who were treated for severe frostbite in the burn unit of the University of Utah Medical Center.

Of those patients, six were administered the clot-busting drugs within 24 hours.

In that group, the incidence of amputation was 10 per cent, compared with 41 per cent among patients who did not receive tPA.

Specifically, six of 59 frostbitten fingers and toes were amputated among those who received tPA, compared with 97 of 234 among those who did not receive clot-busters.

While the number of patients studied was small, Dr. Bruen said the research is important because it points to the first significant advance in the treatment of frostbite in more than 25 years.

"Today, traditional therapy consists of tissue rewarming, prolonged watchful waiting and often delayed amputation," Dr. Bruen said.

A number of other treatments have been tried, such as hyperbaric oxygen (breathing in pure oxygen while in a sealed pressurized chamber), epidural pain medication and various anticoagulants, but the benefits have proved marginal.

The researcher stressed, however, that clot-busting therapy should be limited to people with severe frostbite, specifically those with abnormal blood flow as demonstrated by an angiogram. Dr. Bruen said tPA should not be used for superficial frostbite or in patients who have suffered trauma.