DOI: 10.1097/CCM.0b013e3182a84ea3

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PMID: 24434441

Issn Print: 0090-3493

Publication Date: 2014/02/01


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Therapeutic Hypothermia Is Cool, but Be Aware of the Infection Heat*

Andre C. Kalil; Chad E. Branecki

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Excerpt

Therapeutic hypothermia has been around since antiquity. Hippocrates recommended packing wounded soldiers in snow and ice. Despite tremendous medical advances, the chances for meaningful survival after cardiac arrest are less than 30% due to the “postcardiac arrest syndrome” (1). Therapeutic hypothermia is a medical intervention intended to reduce a patient’s body temperature to 32–34°C, over a 6- to 8-hour period, in the attempt to reduce neurologic injury after a patient’s resuscitation from cardiac arrest, newborn hypoxic ischemic encephalopathy, and possibly stroke. In pivotal clinical trials, targeted temperature management was achieved by surface cooling techniques or invasive core cooling measures. Noninvasive cooling techniques use various cold fluids that circulate through blankets, a torso vest, and leg wraps to achieve the desired temperature. Whereas, invasive techniques use intravascular cooling catheters containing metal coils or balloons filled with cold saline, in combination with cold saline infusions. Sedation, analgesia, and paralysis medications are given to the patient prior to inducing hypothermia to prevent shivering and minimize patient discomfort. The first major clinical trials to demonstrate direct evidence of a benefit of therapeutic hypothermia were published in 2002 and have widely been accepted as the basis for clinical guidelines and treatment protocols (2, 3). Countless research endeavors tried to unlock the exact mechanism of neuroprotection at the cellular level, and a few common themes emerged.
The earliest hypothesis for the effects of hypothermia focused on reducing cellular metabolism. Researchers found that for every 1°C drop in core body temperature, the cellular metabolism is reduced by 5–7%. Therefore, it was thought that by lowering the core temperature one could reduce the harmful effects of ischemia and decrease the body’s oxygen demand (4). Additional theories focused on the relationship of cellular metabolism, glutamate production, and ion homeostasis. In animal models, researchers determined that stores of glucose and adenosine triphosphate are lost within 5 minutes after cardiac arrest. Substrate depletion quickly leads to the loss of transmembrane electrochemical gradients and sequential failure of synapse transmission, axonal conduction, and action potential firing. Consequently, it is not oxygen deprivation itself that is responsible for apoptosis, and cell death, but the inability to make ATP and maintain the cells homeostasis at normal body temperatures. Therapeutic hypothermia may also have neuroprotective effects in preventing reperfusion injury. Reoxygenation initiates chemical cascades producing free radicals that cause lipid peroxidation and other oxidative damage. Various inflammatory immune responses can cause endothelial activation, leukocyte migration, and further tissue injury or susceptibility to infection. Therefore, patients undergoing induction of hypothermia and the rewarming phase must be closely monitored to prevent undesired consequences. Clinicians must be vigilant to avoid overshooting the target temperature as the complications increase in severity, and the risk of adverse events increases at lower temperatures or with rapid fluctuations in the patient’s core body temperature.
Adverse effects of therapeutic hypothermia can be a result of the cooling devices or hypothermia itself. Such events include increased bleeding, infection, deep venous thrombosis, and pulmonary edema (2, 3, 5). In a 2009 study, Nielsen et al (6) conducted an observational study with 986 patients and demonstrated the following adverse effects: pneumonia (41%), hyperglycemia (37%), cardiac arrhythmias (33%), seizures (24%), and electrolyte disturbances (hypophosphatemia, 19%; hypomagnesemia, 18%; and hypokalemia, 18%). These safety issues bring us to this Critical Care Medicine issue, in which Geurts et al (7) performed a systematic review and meta-analysis to evaluate the risk of infections with the use of therapeutic hypothermia.
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