is a medical treatment method which clinically uses the advantages of a lower body temperature. Therapeutic hypothermia is the only way to treat revived (i.e. resuscitated) cardiac arrest
victims. The new treatment method is able to reduce mortality of successfully resuscitated cardiac arrest victims by 35 percent, while it significantly increases the chance of a good neurologic outcome by 39 percent. Every sixth time a cardiac arrest patient is treated with therapeutic hypothermia, physicians can rescue one life. As cardiac arrest is the most common cause of death in the industrialized world, therapeutic hypothermia has the power to save an enormous number of patients.
In addition, there exists a growing body of evidence that hypothermia increases survival chances and quality for patients suffering from other ischemic insults such as stroke or generalized brain trauma. Hypothermia is also believed to be effective in fever management and in many other medical areas.
Hypothermia has been applied therapeutically since antiquity. The Greek physician Hippocrates
, the namesake of the Hippocratic Oath
and arguably the world’s first modern doctor, advocated the packing of wounded soldiers in snow and ice. Napoleonic surgeon Baron Dominque Larrey recorded that officers, who were kept closer to the fire, survived less often than the minimally pampered infantrymen. In modern times the first medical article concerning hypothermia was published in 1945.This study focused on the effects of hypothermia on patients suffering from severe head injury. In the 1950’s hypothermia received its first medical application, being used in intracerebal aneurysm surgery to create a bloodless field. Most of the early research focused on the applications of deep hypothermia, defined as a body temperature between 20-25 °C (68-77 °F). Such an extreme drop in body temperature brings with it a whole host of side effects, side effects which made the use of deep hypothermia impractical in most clinical situations.
This period also saw sporadic investigation of more mild forms of hypothermia, with mild hypothermia being define as a body temperature between 32-34°C (89.6-93.2°F). In the 1950’s Doctor Rosomoff demonstrated in dogs the positive effects of mild hypothermia after brain ischemia and traumatic brain injury. In the 1980’s further animal studies indicated the ability of mild hypothermia to act as a general neuroprotectant following a blockage of blood flow to the brain. This animal data was supported by two landmark human studies published simultaneously in 2002 by the New England Journal of Medicine. Both studies, one occurring in Europe and the other in Australia, demonstrated the positive effects of mild hypothermia applied following cardiac arrest. Responding to this research, in 2003 the American Heart Association (AHA) and the International Liaison Committee on Resuscitation (ILCOR) endorsed the use of therapeutic hypothermia following cardiac arrest. Currently, a growing percentage of hospitals around the world incorporate the AHA/ILCOR guidelines and include hypothermic therapies in their standard package of care for patients suffering from cardiac arrest. Some researchers go so far as to contend that hypothermia represents a better neuroprotectant following a blockage of blood to the brain than any known drug.
Types of ischemic events
The types of medical events hypothermic therapies may effectively treat fall into three primary categories: cardiac arrest, ischemic stroke
, and neurogenic fever
following brain trauma.
The data concerning hypothermia’s neuroprotectant qualities following cardiac arrest can be best summarized by two studies published in the New England Journal Medicine. The first of these studies conducted in Europe focused on people who were resuscitated 5-15 minutes after collapse. Patients participating in this study experienced spontaneous return of circulation (ROSC) after an average of 105 minutes. Subjects were then cooled over a 24 hour period, with a target temperature of 32-34°C (89.6-93.2°F). 55% of the 137 patients in the hypothermia group experienced favorable outcomes, compared with only 39% in the group that received standard care following resuscitation
. Death rates in the hypothermia group were 14% lower, meaning that for every 7 patients treated one life was saved. Notably, complications between the two groups did not differ substantially. This data was supported by another similarly run study that took place simultaneously in Australia. In this study 49% of the patients treated with hypothermia following cardiac arrest experienced good outcomes, compared to only 26% of those who received standard care.
Most of the data concerning hypothermia’s effectiveness in treating stroke is limited to animal studies. These studies have focused primarily on ischemic as opposed to hemorrhagic stroke, as hypothermia is associated with a lower clotting threshold. In these animal studies, hypothermia represented an effective all purpose neuroprotectant.This promising data has lead to the initiation of human studies. Unfortunately, at the time of this article’s publishing, no results have yet been returned. In terms of feasibility, however, the use of hypothermia to control intracranial pressure
(ICP) after an ischemic stroke was found to be both safe and practical. In 2008, long-term hypothermia induced by low-dose hydrogen sulfide
, a weak, reversible inhibitor of oxidative phosphorylation
, was shown to reduce the extent of brain damage caused by ischemic stroke
The effects of elevated body temperature following cardiac arrest, stroke, and brain trauma are surprisingly pronounced. According to one study, elevated body temperature correlated strongly with an extended stay in the ICU
for patients suffering from either brain ischemia or brain trauma. Another paper stated that those suffering from either brain trauma or brain ischemia that entered the ICU with a fever had a 14% higher mortality rate than normothermic patients. It appears that the ischemic or traumatized brain is particularly susceptible to the damaging influence of elevated temperature. Combating fever through the use of temperature dampening devices represents a critical aspect of neurological care.
Faced with this extensive array of clinical data, many scientists have attempted to explain the cellular processes responsible for the therapeutic effect of hypothermia following a blockage of blood flow to the brain. The earliest explanations for why hypothermia acted as a neuroprotectant focused on the slowing of cellular metabolism
resultant from a drop in body temperature. For every drop in body temperature of a degree Celsius cellular metabolism slows by 5-7%. Because of this reality, most early theorists believed that hypothermia lessened the harmful effects of oxygen deprivation by decreasing the body’s need for oxygen. The initial emphasis on cellular metabolism explains why the early studies almost exclusively focused on the application of deep hypothermia, as these researchers believed that the therapeutic effects of hypothermia correlated directly with the extent of temperature decline.
More recent data shows that even a modest reduction in temperature can function as a neuroprotectant. This seems to suggest that hypothermia works on pathways that extend beyond mere metabolism. So while current researchers do not dismiss the fact that cellular metabolism plays a part in the therapeutic value of hypothermia, most have looked for other pathways in hopes of explaining hypothermia’s ability to act as a neuroprotectant.
One of the most promising explanations centers around the series of reactions that occur following oxygen deprivation, particularly those concerning ion homeostasis. In truth, cell death is not caused by oxygen deprivation directly, but rather the cascade of reactions that oxygen deprivation leads to. Cells need oxygen to create ATP--a molecule used by cells to store energy--and cells need ATP to regulate cellular ion levels. Simply, cells use ATP to fuel both the importation of ions necessary for cellular function and the removal of ions that are harmful to cellular function. Without oxygen, cells cannot manufacture the necessary ATP to regulate ion levels and thus cannot prevent the intercellular environment from approaching the ion concentration of the outside environment. It is not oxygen deprivation itself that precipitates cell death, but rather the disruption of homeostasis resultant from oxygen deprivation that leads to cellular apoptosis.
Getting back to hypothermia, it is notable that even a small drop in temperature encourages cell membrane stability during periods of oxygen deprivation. For this reason, a drop in body temperature helps prevent an influx of unwanted ions during an ischemic insult. By making the cell membrane more impermeable, hypothermia helps prevent the cascade of reactions set off by oxygen deprivation. Even moderate dips in temperature strengthen the cellular membrane, helping to minimize any disruption to the cellular environment. It is by moderating the disruption of homeostasis caused by a blockage of blood flow that many now postulate results in hypothermia’s ability to minimize the trauma resultant from ischemic injuries.
The therapeutic effect of hypothermia does not confine itself to metabolism and membrane stability. Another school of thought focuses on hypothermia’s ability to prevent the injuries that occur after circulation returns to the brain, or what is termed reperfusion injuries. An individual suffering from an ischemic insult continues suffering injuries well after circulation is restored. In rats it has been shown that neurons often die a full 24 hours after blood flow returns. Some theorize that this delayed reaction derives from the various inflammatory immune responses that occur during reperfusion. These inflammatory responses cause intracranial pressure, pressure which leads to cell injury and in some situations cell death. Hypothermia has been shown to help moderate intracranial pressure and therefore to minimize the harmful effect of a patient’s inflammatory immune responses during reperfusion. Beyond this, reperfusion also increases free radical production. Many now suspect it is because hypothermia reduces both intracranial pressure and free radical production that hypothermia improves patient outcome following a blockage of blood flow to the brain.
When discussing therapeutic hypothermia it is important to note the various clinical realities associated with this medical procedure. To start, time moderates hypothermia’s effectiveness as a neuroprotectant. Much of the animal data suggests that the earlier hypothermia is induced the better the subject’s outcome. However, therapeutic hypothermia remains partially effective even when initiated as long as 6 hours after collapse. Patients entering a state of induced hypothermia should also receive extensive monitoring from medical professionals. Clinicians must remain watchful of the negative side-effects associated with hypothermia. These side-effects include: arrhythmia, decreased clotting threshold, increased risk of infection, and electrolyte imbalance. The medical data suggests that these side-effects can be mitigated only if the proper protocols are followed. Importantly, medical professionals must avoid overshooting the target temperature, as hypothermia’s negative side-effects increase in prevalence the lower a patient’s body temperature drops. The accepted medical standards assert that a patient’s temperature should not fall below a threshold of 32°C (89.6°F).
Medical professionals must attempt to minimize a patient's shivering response. For human beings temperature represents one of our most tightly regulated parameters. When body temperature drops below a certain threshold--typically around 36 °C (96.8 °F) --patients will begin to shiver. It appears that regardless of technique patients begin to shiver when temperature drops below this threshold. This behavior hinders the ability of medical professionals to induce hypothermia. For this reason, hypothermia should be induced in conjunction with pharmaceuticals that prevent this reaction. The drugs most commonly employed are Desflurane
. Finally, clinicians should rewarm patients slowly and steadily in order to avoid unhealthy spikes in intracranial pressure. A patients rewarming should occur at a rate of .5-1 °C an hour in order to avoid injury. In fact, most deaths caused by therapeutic hypothermia occurred during the rewarming phase of the procedure, deaths easily avoided by slow and precise rewarming.
The medical methods through which hypothermia is induced break down into two categories: internal and external.
Cooling catheters control body temperature from the inside of the body, out toward the surface. The process works by inserting a balloon catheter into the femoral (leg), jugular (neck) or subclavian (shoulder) vein. Once the catheter is inserted, a computer-controlled system that is connected to the catheter sends cold saline solution circulating through the balloons that surround the catheter. The patient’s body temperature decreases as the blood circulating through the vein passes over the catheter. The external system automatically adjusts the saline solution temperature running through the catheter until the body reaches a precise, pre-determined temperature, and the system continuously monitors the patient’s core temperature and works to maintain the desired “target” temperature. Once the treating physician decides to raise the patient’s body temperature back to normal, the same central system sends warm saline into the catheter, raising the body temperature at a slow, steady rate.
Catheter-based temperature management has been shown to provide faster, more precise and more efficient cooling compared to all external methods, especially conventional. Another important attribute associated with catheter-based cooling is that it requires minimal effort from the nursing staff after the catheter is inserted because patients are not at potential risks for thermo burn skin necroses and overcooling that could result from external cooling, allowing them to focus on other important areas of patient’s care.
A number of studies in critically ill patients have demonstrated that therapeutic hypothermia via catheter is safe and effective in the treatment of a wide variety of patient populations. Since cooling via catheter involves the insertion of a central venous catheter into a major vein, the potential exists for complications related to use of any central venous catheter. However, the potential for this type of complication is relative since most patients treated with therapeutic hypothermia are critically ill or injured and, therefore, must have a central venous access for other medical treatment uses.
Water blankets are a technology where cold water pushes through a blanket using positive pressure. To lower temperature with optimal speed, medical professionals must cover 80% of a patient’s surface area with water blankets. These blankets are typically augmented by ice packs or cold fans in order to achieve more rapid temperature decline. This technique of temperature management dates back to the 1950’s and represents perhaps the most well studied means of controlling body temperature. Water blankets lower a patient’s temperature exclusively by cooling a patient’s skin and accordingly require no insertion of anything into the patient’s body. Furthermore, nursing professionals can administer water blankets without a supervising M.D.
However, water blankets possess several undesirable qualities. First off, they are particularly susceptible to leaking and for this reason represent a serious electrical hazard. Also, water blankets are labor intensive and require near constant monitoring. Water blankets were not designed with sophisticated temperature management in mind and applying water blankets for this purpose requires large amounts of effort on the part of hospital staff. In addition to the labor required, water blankets lower body temperature at a slower rate than other cooling alternatives. Moreover, water blankets often imprecisely cool patients. Water blankets tend to overshoot the target temperature and cool patients to levels below 32 °C (89.6 °F). Hypothermia’s negative side-effects increase in prevalence the lower a patient’s body temperature drops, making unnecessary fluctuations in temperature detrimental to patient health. Likewise, due to their imprecision, water blankets often rewarm patients at too quick a rate. This leads to spikes in intracranial pressure, spikes that can cause serious brain injury and in some instances patient mortality.
The use of hypothermia for therapeutic purposes represents a technique with increasing levels of application. Virtually unknown a few decades ago, it is now part of the standard package of care for the thousands of individuals suffering from cardiac arrest every year. Yet with human studies concerning hypothermia’s effectiveness in treating ischemic stroke and brain trauma in the process of being completed, it appears that the current therapeutic uses of hypothermia represent only the tip of the iceberg.