Large scale studies of survival after cardiac arrest have produced dismal statistics, with survival to hospital discharge of 17.6% when the patient is already in the hospital at the time of the arrest and only 6.1% when the arrest occurs outside the hospital. The development and widespread deployment of portable automatic external defibrillators (AEDS) in public places has increased the number of people who make it to the hospital after cardiac arrest. However, the survivor’s longer term outcome depends in large part on how much brain damage occurs during the arrest and whether or not the restoration of circulation damages the brain further, a phenomenon called reperfusion injury. Because the odds of initial survival have improved, and because lowering the core temperature of the body appears to lessen reperfusion injury, the subject of hypothermia has emerged as a vibrant area of research and therapeutics.
Therapeutic hypothermia is an old idea
Hippocrates (460-370B.C.) recognized the value of cold temperatures in the outcome of soldiers with head injuries and in people suffering from tetanus. In the 1800s, Napoleon’s surgeon used ice to prepare limbs for amputation because it numbed pain and reduced bleeding. Over much of history, miraculous recoveries were reported in victims of cold water submersion. But not until the late 1950s did therapeutic hypothermia become a routine part of some surgical care, when experiments in animals demonstrated its value in protecting the brain during the open-heart surgery. Despite some attempts to cool patients for other problems such as cardiac arrest, strokes and head injuries, the number of problems encountered in during cooling and in the re-warming phase put a damper on the use of the technique. Now, however, we are in the middle of a revival of interest in therapeutic hypothermia.
How does cold help?
Cold protects the brain because the biochemical reactions that sustain life are influenced by temperature. If the heart stops, the brain runs out of fresh supplies for energy production in two minutes. A downward spiral toward brain cell death begins unless blood flow is restored within the next two minutes. When blood flow is restored (reperfusion), a cascade of potentially damaging chemical reactions begins in cells that have been deprived of oxygen. The longer the period of arrested circulation was, the more damaging these reactions are.
The body at different temperatures
Changing the body’s temperature changes the speed and efficiency of its chemical reactions. At temperatures over 105 many processes fail completely. As body temperature falls below normal, chemical reactions slow down. Between 92 and 89.6 the damaging chemical responses that come after blood flow returns are blunted enough to improve outcomes significantly. By 90, pulse and respirations slow and peripheral circulation shuts down. By 86 the patient may still be alive, but looks dead. This level of deep hypothermia is used for some long, difficult cardiac and neurosurgical procedures.
Lowering temperature is now routine, sometimes
Since 2005, the American Heart Association has recommended therapeutic hypothermia as a routine part of patient care after a cardiac arrest in circumstances that depend on the reason for the cardiac arrest, the speed of the resuscitation, and the state of the patient after circulation is restored. Some medical centers are also experimenting with the technique in the treatment of certain types of strokes and head injuries.
How do you lower someone’s core temperature?
How do you cool a body that normally maintains a constant temperature that hovers within a few tenths of a degree of 98.6? Any environment with a temperature less than body temperature provides a gradient for heat loss. Deliberately making someone hypothermic means increasing that temperature gradient. In the presence of a gradient, heat radiates away from the body. Heat is also conducted away when the body is in contact with any colder substance; when the colder substance such as air or water is in motion, heat is lost even faster, by convection. Heat is also drawn away by evaporation of perspiration on the skin’s surface, where the sweat keeps the microclimate humidity at 70% even when you think you are dry. Heat also dissipates when warm moist air is exhaled from the lungs.
Several internal and external ways of changing the temperature gradient exist: ice packs applied to the head, neck, axillae, groin, where large blood vessels are close to the surface; cooling blankets that house cold water circuits; closed catheters through which cold saline circulates inserted into large blood vessels; ice water balloons in the bladder. These methods are directed at the core temperature of the body – the temperature of the internal organs and the brain. They do not cause problems like frostbite, seen commonly with accidental hypothermia, because the ambient temperature is not freezing and the skin is protected from direct exposure to ice packs being used for cooling.
The body resists lowering the temperature
Normally, we protect ourselves from falling temperatures by putting on more clothes and increasing activity, and by shaking and shivering, which produce heat. A patient who has suffered a cardiac arrest will not engage in the normal behavioral responses, but he will shiver and perhaps become agitated, both of which are counterproductive to getting the temperature down. Sedation and even muscle paralysis are therefore necessary for the period of cooling.
Despite problems, therapeutic hypothermia is here to stay
Current therapeutic hypothermia protocols call for maintenance of temperature between 32-34 (89.6-93.2) for 18-24 hours, followed by passive re-warming over the next 24 hours. Overshooting and undershooting of temperature are both common, as are difficulties maintaining electrolyte and sugar balance. Some complications like pneumonia and bleeding problems are more common than in similar patients not being treated with cold temperatures. Much work remains to determine the best timing for induction and maintenance of hypothermia after cardiac arrest, but it is clear that “the sooner the better” is the general rule and that the revival of interest in therapeutic hypothermia is here to stay.
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