Elizabeth A. Reid, MD Chilly Treatment: Therapeutic Hypothermia and Cardiac Arrest


of the heart to pump blood be- cause of either a heart attack or a primary heart rhythm disrup- tion called ventricular fibrillation— have produced dismal statistics. Of those individuals who suffer cardiac arrest outside the hospi- tal, only 6.1 percent survive. Of those individuals who have heart attacks while in the hospital, only 17.6 percent live to be dis- charged. The development and widespread deployment of por- table automatic external defib- rillators in public places has in- creased the number of people who make it to the hospital after cardiac arrest. However, a car- diac-arrest survivor’s longer-term outcome depends in large part on how much brain damage oc- curs during the arrest and whether or not the restoration of circulation damages the brain fur- ther, a phenomenon called re- perfusion injury.


L


Because hypothermia, which occurs when the body’s tempera- ture falls below 95°F, appears to lessen reperfusion injury, the use of therapeutic hypothermia in the treatment of patients with car- diac arrest has emerged as a vibrant area of research. In sev- eral studies, body temperatures below 95°F improved the six- month outcome of cardiac-arrest survivors, with 49 to 55 percent


10


ARGE-SCALE studies of survival after cardiac ar- rest—the sudden failure


PHOTO: TARO YAMADA/CORBIS


Physicians have been aware of the therapeutic effects of hypothermia for more than two thousand years.


of cooled patients achieving a favorable neurologic outcome, as opposed to similar results in only 26 to 39 percent of survivors who were treated at normal body tem- peratures. Physicians have been aware


of the therapeutic effects of hy- pothermia for more than two thousand years. Hippocrates, the Greek “Father of Medicine,” who lived from about 460 to 370 BC, recognized the beneficial effects of cold temperatures on the out- comes of soldiers with head in- juries and in people suffering from tetanus. More recently, in the early 1800s, the surgeon general of Napoleon’s army, Baron Larrey, used ice to pre- pare limbs for amputation be- cause it numbed pain and re- duced bleeding. But not until the late 1950s did therapeutic hypo- thermia become a routine part of some surgical care, when ex- periments with the procedure in animals demonstrated its value in protecting the brain during open-heart


surgery. Despite


some attempts to cool patients for other problems, such as car- diac arrest, strokes, and head injuries, the number of problems encountered during the cooling and the rewarming phases of the procedure put a damper on the use of the technique. Now, how- ever, we are in the middle of a revival of interest in therapeutic hypothermia.


Protecting the Brain through Cooling If


the heart stops, the brain


runs out of fresh supplies of oxy- gen for energy production in two minutes. A downward spiral to- ward brain cell death begins un- less blood flow is restored within the next two minutes. Unfortu- nately, when blood flow is re- stored (reperfusion), a cascade of potentially damaging chemical reactions begins in cells that have been deprived of oxygen. The longer the heart was stopped, the more damaging these reac- tions are.


Cold helps protect the brain


from reperfusion injury because the biochemical reactions that sustain life are influenced by tem- perature. Changing the body’s temperature changes the speed and efficiency of its chemical re- actions and can help limit


the


damage done when blood flow to the brain is restored after car- diac arrest. At temperatures over 105°F many of the body’s pro- cesses fail completely, but as body temperature falls below nor- mal, chemical reactions slow down. When the body reaches temperatures between 89.6°F and 92°F, the damaging chemi- cal responses that come after blood flow returns to the brain after cardiac arrest are blunted enough to improve outcomes sig- nificantly. When the body’s tem- perature is lowered to 90°F, pulse and respirations slow and peripheral circulation shuts down. By the time the body reaches 86°F the patient may be alive but looks dead. This level of deep therapeutic hypothermia is used for some long, difficult cardiac and neurosurgical procedures.


Cooling the Body


Since 2005, the American Heart Association has recom- mended that therapeutic hypoth- ermia be used under certain cir- cumstances as a routine part of a patient’s care after cardiac ar- rest. Some medical centers are (Continued on page 56)


D E C E M B E R 2 0 1 1 / J A N U A R Y 2 0 1 2


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