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High-tech device dramatically speeds recovery from hypothermia

Stanford researchers have developed a high-tech solution that dramatically reduces the recovery time for the mild hypothermia that often accompanies major surgery. A device called the Thermo-STAT, which fits over the hand and forearm, also holds promise for the treatment of more severe cases of exposure and has a much broader potential application in manipulating core body temperature to treat a variety of ailments, including chronic disorders like cystic fibrosis, brain damage after strokes and tumors.

One of the most unpleasant and dangerous side-effects of extended surgery is hypothermia. In response to anesthesia, cool temperatures and paper-thin hospital gowns, many if not most of the patients wheeled into the operating room experience a drop in core body temperature. Anesthetics usually take several minutes to wear off, but recovery from this internal cooling can take hours, and patients often endure chills and shivering during this time. Recent studies have shown that even such relatively mild hypothermia increases infection rates and ups the frequency of post-operative heart problems for those at risk.

In the November issue of the Journal of Applied Physiology, Dennis Grahn, senior research scientist in biological sciences; John G. Brock-Utne, professor of anesthesia; H. Craig Heller, the Lorry I. Lokey/Business Wire Professor of Environmental Biology at Stanford; and Donald E. Watenpaugh, research associate in integrative physiology at the University of North Texas Health Science Center in Fort Worth, report on the successful clinical trial of the rewarming technology.

The experimental device consists of an acrylic sleeve and mitt that fits over the hand and seals around the patient's forearm. It contains a water-perfusion blanket to supply heat and is hooked to a vacuum pump to reduce the air pressure inside the sleeve. Its design has been patented by Stanford and licensed by Aquarius Medical Corp. of Scottsdale, Ariz., which has developed and is marketing a commercial version of the rewarmer.

The device relies on the fact that the hand and bottom of the forearm are areas of the body specialized for the transfer of heat. They contain an arrangement of blood vessels specially designed for temperature regulation. Dense networks of arteries and veins under the skin are linked to allow blood to flow directly from one to the other so it can act as a heating or cooling agent.

When the body's internal temperature drops, its automatic response is to shut down the blood flow through these heat exchange blood vessels to prevent more heat from escaping. While this may help a person to survive in some conditions, it also makes efforts to speed the warming process slower and more difficult. The Thermo-STAT reverses this shutdown, called vasoconstriction, by slightly reducing the air pressure within the sleeve. The reduced pressure causes the vessels to open and blood to flow normally.

"This gives us the equivalent of an open heat-pipe that leads directly to the body's interior," says Grahn, who invented the device and serves as a consultant to Aquarius.

The clinical trial reported in the current paper involved 16 hypothermic patients. Ten were equipped with the Thermo-STAT. Two were equipped with the device operating at ambient air pressure. The remaining four were treated using standard techniques: Two were wrapped in warm blankets and two had warm air blown around them. While the standard methods warmed the patients at a rate of about 1.4 degrees Celsius per hour, the new rewarming method, using reduced pressure, produced a rewarming rate of 14 degrees per hour, a ten-fold increase.

"When we started these tests, we expected to get maybe double the rate of rewarming," said Watenpaugh, who worked with Grahn in the early stages of the concept's development. "A ten-fold increase is really exciting."

Grahn's concern about the problem of hypothermia in surgery patients provided the impetus for inventing the rewarmer. Now that it has been developed, it has a number of other potential applications.

One obvious extension is using it to treat people who suffer from severe hypothermia due to winter exposure. Hundreds of thousands of people suffer from hypothermia annually in the United States. The Thermo-STAT is compact, weighing only three to four pounds, and can run on a battery, so it is well suited for use by search-and-rescue teams and paramedics.

In the longer term, Grahn and his colleagues intend to explore the device's ability to manipulate body core temperatures and how this ability can be used to treat other diseases:

• Inducing local hypothermia may be useful in treating chronic disorders such as cystic fibrosis. Cystic fibrosis is caused by a mutation of a protein found in the lungs that prevents the protein from being processed to maturity. Tissue culture studies indicate that the mutated protein functions normally at decreased temperatures. So it may be possible to treat cystic fibrosis by periodically lowering a patient's core body temperature.

• Much of the brain damage caused by a stroke occurs after the initial event. According to animal studies and limited clinical reports, small decreases in brain temperatures can protect the nerves from much of this damage. It may be possible to use the Thermo-STAT in a cooling mode, along with muscle relaxants, to decrease brain temperature rapidly enough to reduce stroke damage.

• Hyperthermia, an elevated core body temperature, could be useful in the treatment of cancer. Studies are currently under way to see if local, regional or whole body hyperthermia induced with both external and internal heating devices can help shrink tumors. Current heating methods are both uncomfortable and carry considerable risk. Adapting the Thermo-STAT could deliver the required heating with less risk to the patient.

Studying the effects of altering core body temperature is a "hot" research area right now, Grahn says. So, researchers are likely to come up with other applications for devices of this sort.


By David F. Salisbury

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