Inducing mild hypothermia in people who are having a stroke may lessen brain damage.
Despite advances in the treatment of stroke in the past quarter century, physicians are still searching for ways to improve the outcomes of stroke patients. One promising new treatment for stroke patients is mild hypothermia, which involves cooling patients soon after the onset of a stroke.
Research has shown that controlling fever in stroke patients improves outcomes. And now, animal studies and preliminary research in humans suggest that deliberately lowering the body temperature of patients suffering from a stroke, even if they don’t have a fever, may help minimize brain damage and widen the “window of opportunity” for treatments like tissue-type plasminogen activator (t-PA). So how might this technique benefit stroke patients, and how does it work?
The Evidence for Hypothermia
Investigators first began looking into the benefits of hypothermia in the 1950s, when research revealed that low body temperatures allowed animals to survive with decreased blood flow to the brain during hibernation. Later, in the 1990s, animal studies showed that lowering body temperature decreased the area of the brain that was damaged by a stroke. Hypothermia also appeared to slow or halt the damage that resulted from disrupting oxygen supply to the brain during a stroke.
More recent studies in humans have demonstrated the safety and feasibility of using hypothermia to treat stroke. One study, published in the Journal of Neurological Anesthesiology, looked at the effect of a water-cooled mattress in 18 stroke patients. Doctors were able to keep all but one of the patients’ body temperatures below normal for 24 hours, while using drugs to control shivering and other side effects of cooling. Although 61% of the patients experienced side effects (such as vomiting, low blood pressure, and pneumonia), only 12% died within three months of the stroke, compared with 17% in another study that used t-PA.
Hypothermia -- How Is It Done?
There are numerous ways doctors can slightly lower the body temperature of someone who’s experiencing a stroke.
Surface cooling can be done by rubbing a patient’s skin with ice bags, ice water, or other cool liquids. Cooling blankets, water-cooled mattresses, and cold air are also effective. These techniques can only be used in a hospital. But researchers are testing a cooling helmet that circulates cool air over the top of the head and neck and can be placed on a stroke patient in an ambulance. Similar helmets were used to keep soldiers cool in the desert heat during the 1991 Gulf War, and the military continues to use an updated version of the device.
Intravascular cooling involves cooling the body from the inside. One approach is to infuse cool fluids into the body intravenously. Another method, called the Celsius Control System, was approved by the U.S. Food and Drug Administration in January 2003. With this system, doctors insert a closed catheter into the inferior vena cava (a large vein that passes through the abdomen). A temperature-controlled saline solution is passed though the catheter, and heat exchange occurs when the blood comes in contact with the catheter’s tip. The change in the temperature of the blood affects the temperature of organs throughout the rest of the body, including the brain.
How soon to begin cooling after a stroke, for how long, and to what temperature are still matters of debate. Current evidence seems to indicate that cooling should begin within 6 hours of the stroke , but not more than 12 hours, after stroke onset. The cooling should last for at least 6 hours, experts say, but the best outcomes appear to occur in patients whose body temperatures are lowered for 24 to 48 hours.
Lower temperatures appear more effective for stroke patients, but they are associated with greater side effects. “Mild” hypothermia—a body temperature between 93.2° and 96.8° F—may be best for people with mild or moderate strokes. “Moderate” hypothermia—89.6° to 93.2° F—may be required for those with severe strokes.
How Hypothermia Improves Outcomes
Researchers are not entirely sure why hypothermia may help improve outcomes in stroke patients. Many experts suspect that a major benefit is from the prevention of reperfusion injury, the wave of free-radical formation and brain-cell death that occurs when blood begins flowing again to the deprived area after a stroke. Preventing reperfusion injury also avoids the breakdown of the blood-brain barrier—the layer of cells that block entry of potentially damaging chemicals in the blood into the brain.
Hypothermia can also prevent the accumulation of fluid in the brain (cerebral edema) and can slow cells’ metabolic processes. Further, cooling can be used along with t-PA and may prevent some of its side effects.
Side Effects of Hypothermia
Major side effects of cooling stroke patients are discomfort and shivering, which can be minimized with sedatives and sometimes with drugs that cause temporary paralysis. (These side effects are less likely with intravascular than surface cooling, since with intravascular cooling only the internal organs are cooled and the skin remains warm.)
Other potential side effects of hypothermia include a drop in blood volume, abnormal heart rhythms, too high or too low blood pressure, pneumonia, increased pressure in the skull, longer blood clotting times, kidney failure, and decreased immune function. Generally, these side effects increase with lower temperatures and are reversed when the patient is rewarmed.
The Bottom Line on Hypothermia
To date, no large, randomized, controlled trials have demonstrated that therapeutically induced hypothermia is an effective treatment for people who’ve recently suffered a stroke. Nonetheless, the American Stroke Association reported in 2007 that inducing mild hypothermia after a stroke holds promise. In addition, a number of research groups are currently testing the treatment in randomized trials. If the results of these studies are favorable, cooling may one day become part of standard care for stroke patients.
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