Sudden cardiac arrest (SCA) is a devastating event for its victims and their families, with only a small number of survivors. Less than half of the 25 to 50 percent of people whose hearts can be started again survive to leave the hospital because prolonged time without blood flow to the brain often leads to irreversible brain damage.
Medical researchers have placed significant emphasis on increasing SCA survival by reducing brain injury. One relatively new, promising treatment is the use of therapeutic hypothermia for comatose survivors of cardiac arrest. Physicians have known for years that brain injury can sometimes be prevented by decreasing a patient’s core body temperature. Cardiac surgeons, for example, have used hypothermia during open-heart surgical procedures to reduce the body’s metabolism and to minimize damage to vital organs during prolonged periods of little or no blood flow.
The potential benefit of using hypothermia for treating cardiac arrest patients was suggested by reports of occasional drowning victims surviving with minimal brain injury after up to 45 minutes submerged under icy water. Hypothermia slows the body’s metabolism, reducing the cascade of undesirable events that can cause permanent brain damage associated with SCA. The reason that cooling is helpful for some patients is that once the patient is resuscitated and circulation resumes, reperfusion to vital organs occurs. This sets in motion the production of free radicals, which can lead to cell damage or death. Scientists believe that mild therapeutic hypothermia suppresses these chemical reactions and preserves cell health.
Since the mid-1950s, researchers have studied the effects of this cooling therapy in models of animals that survived cardiac arrest, reporting varying degrees of success. Cooling was achieved using ice packs, water blankets, cold air, and other methods. In 2002, the New England Journal of Medicine reported two successful clinical research studies in which patients who survived SCA were cooled to subnormal temperatures of about 91.4°F (33°C). The patient survival rate in each study increased, and the improvement in neurological outcomes was significantly better than among patients in the group who maintained normal temperatures.
As a result of this work and subsequent studies verifying these findings, the American Heart Association in 2003 and again in 2005 issued guidelines recommending cooling comatose survivors of cardiac arrest caused by certain irregular heart rhythms known as ventricular tachycardia (VT) and ventricular fibrillation (VF). Although therapeutic hypothermia will not work for every SCA patient treated, the good news is that this treatment offers more hope for improving brain function than in the past. Physicians believe, although the evidence is not strong, that the earlier the treatment is started the more effective it will be.
The main risks of using hypothermia are infection and bleeding. Decreasing the body’s temperature slows down white blood cells that fight infection and also slows down the processes for developing a blood clot.
If you know someone who has remained comatose after surviving SCA, he may be a candidate for therapeutic hypothermia. Check with the physicians who are caring for the patient to see if the hospital is currently using this therapy and if your family member meets the criteria to be cooled. The therapy is not without risk and requires skilled implementation of techniques for managing the critically ill patients whose body temperatures have been lowered. Members of the critical care team must learn these techniques before using them clinically. Techniques can include ice packs, cooling mattresses, cooling blankets, catheters inserted into large blood vessels, and ice-cold IV saline.
When a patient undergoes therapeutic hypothermia, it is somewhat startling to feel how cold s/he can be to the touch. This is normal and is only temporary. The patient’s temperature will be reduced to about 91°F (33°C), approximately 7° F (4°C) lower than normal.
The patient will be on a respirator, heavily sedated and unable to move. The therapy typically will last for a maximum of 36 hours: 12 to 24 hours of cooling and up to 12 hours to rewarm slowly back to a normal body temperature of 98.6° (37°C).
During the cooling process, the patient will require frequent blood samples to make sure s/he is tolerating the cooling procedure well. Sometimes, additional medications can be given to help control blood pressure and heart rate.
If your family member or friend is treated with therapeutic hypothermia, you will not know anything about his/her level of consciousness until the therapy is terminated. Sometimes, it can take days before the doctor knows whether the therapy is successful.
Always discuss your concerns with the nurses and physicians taking care of your family member.
Key Research Studies
Two prospective randomized trials (see below) compared mild hypothermia (32-34 degrees Celcius with normothermia in comatose SCA survivors. The first, conducted in five European countries, demonstrated that cooling for 24 hours decreased the likelihood of death and increased the likelihood of good neurological recovery. The second, conducted in four hospitals in Melbourne, Australia, showed that cooling patients for 12 hours increased the chances for good neurological recovery.
Mild hypothermia to improve the neurologic outcome after cardiac arrest: The Hypothermia After Cardiac Arrest Group (HACA). N Engl J Med. 2002;346:549-556.
In a randomized trial of 273 survivors of sudden cardiac arrest, 75 of 136 patients (55%) treated with hypothermia had a favorable neurologic outcome as compared with 54 of 137 patients (39%) who maintained normothermia. Mortality in the hypothermia group was 41% compared with 55% in the normothermia group.
Treatment of comatose survivors of out-of-hospital cardiac arrest with induced hypothermia. Bernard SA, Gray TW, Buist MD, et al. NEJM 2002; 346:557-63.
Of the 77 survivors of sudden cardiac arrest who were randomized, 43 patients treated in the hypothermia group (49%) survived with a good outcome compared to 34 (26%) in the normothermia group.