AHA 2005 Guidelines
Endorsed in the AHA guidelines for emergency cardiac care, therapeutic hypothermia (TH) decreases the body's metabolic demands and acts as a neuroprotective treatment.
Maintaining a core body temperature of 32-34° C for 12-24 hours in those who remain comatose after a ventricular fibrillation cardiac arrest is recommended.9 Utilizing TH for non-fibrillatory cardiac arrest also has been suggested, but is not yet well studied with randomized trials.10,11
In TH, rapid cooling is desired to minimize the effects of shivering, electrolyte, glucose abnormalities and hypovolemia from fluid shifts. Various devices are available to achieve cooling, from intravascular catheters to external wraps and blankets. These devices work by different methods, such as cold fluid infusions and convection. Advantages and disadvantages of various modes are outlined in a more recent consensus statement from the AHA, but optimal methods to achieve and maintain hypothermia goals have not been identified.12
Monitoring of body temperature should be maintained with an indwelling bladder, rectal or core thermometer probe. Initiation of TH early, within 8 hours of presentation, has been considered optimal, but is often not achieved. Additionally, the time required to meet cooling temperature goals has not been clearly defined.
Treatment initiation begins with coordination and collaboration in the ED with medical, nursing, respiratory and neurologic assessment and continues in the ICU. Protocols to guide therapy, which address inclusive and exclusive criteria, serial labs, antiarrhythmics and rewarming times, should be developed specific to each institution. Once the desired temperature is reached, duration of TH should be outlined.13
While considered beneficial, TH may produce complications. The body's natural response to cold is shivering. Short-term neuromuscular blockade may be needed to prevent shivering, as it increases oxygen and metabolic demands. Use of sedation and analgesia should accompany any paralytic drug therapy. Placing mittens or socks on the hands and feet may help control shivering and protect sensitive extremities.
Airway protection and pulmonary function must be maintained with mechanical ventilation during initiation and treatment with TH. Monitoring for arrhythmias, electrolyte, glucose abnormalities and coagulopathies should be addressed within the hypothermia protocol. Use of an insulin protocol to maintain appropriate glucose levels with frequent monitoring may avert complications from hyperglycemia. Bradycardia may be common during the cooling period, but blood pressure often is maintained due to peripheral vasoconstriction. Metabolism and clearance of medications such as sedatives and analgesics used during TH may be prolonged thus requiring consideration for dosing adjustments during therapy.10
Aspiration pneumonia, as a result of cardiac arrest, may be exacerbated by hypothermia therapy and inactivity during the cooling period. Coagulopathies from cooling therapy may complicate treatment and should be monitored. Prevention of ventilator-associated pneumonia should be instituted with frequent oral care, endotracheal suctioning and head-of-bed elevation to at least 30 degrees while maintaining mean arterial pressure. Spinal cord injuries may require reverse Trendelenberg positioning to prevent any further neurological insult.
Rewarming the patient can be a dangerous time during TH intervention. Active rewarming, which utilizes a warming device or blanket, can induce arrhythmias and hypotension. Hypotension due to peripheral vasodilitation and intravascular fluid shifts may occur, requiring the addition of vasoactive medications to maintain mean arterial pressure.
Warming too rapidly may cause a rebound hyperthermia, cerebral edema and an increase in intracranial pressure. Rebound hyperthermia from rapid rewarming should be avoided. Hyperthermia can cause an increase in oxygen and metabolic demands. In patients who do not have an intact cerebral autoregulatory system, these changes may have devastating effects. Therefore, passive rewarming is recommended. Passive rewarming may require regulating or removing the cooling device and allowing the patient to warm via ambient air and warmed blankets.
During TH, continuous temperature monitoring is vital. Rewarming to a normal body temperature should occur passively at a rate of 0.25-0.5° C generally over a period of 4-12 hours.10,14 A general rule to rewarm at 0.5° C an hour is common and should be reflected in the TH protocol. Skin integrity should be monitored throughout the use of TH.
Benefits of Therapeutic Hypothermia
Research has demonstrated a survival rate of as high as 50 percent with favorable neurologic outcomes utilizing TH.7 One meta-analysis review established that to obtain an outcome of one neurologically intact survivor, six patients must be treated.15
One Finnish study evaluated heart rate variability (HRV) during hypothermia use. HRV examines the beat-to-beat fluctuations in rate during sinus rhythm. Variations are thought to arise from changes in the sympathetic and parasympathetic effects on the heart. HRV data from ECGs obtained 24 hours post arrest can provide prognostic data for survival. Low HRV is associated with poor survival rates in a variety of patient populations.16
Measurements in a control and TH randomized group demonstrated improved outcomes with hypothermia with an increased HRV, suggesting a preserved autonomic influence on the heart.16
Despite strong evidence for the use of TH, less than 26 percent of physicians and hospitals utilize this practice.3 Barriers to use of TH include time considerations, device availability, limited protocols and clinician lack of knowledge of the process. To improve the chain of survival, considerations for out-of-hospital TH intervention by trained EMS personnel to be implemented within 1 hour of arrest time may be seen in the future. In fact, TH may soon be considered one of the main components in the chain of survival and post-resuscitative care.6