After a cardiac arrest, should the patient survive the event, devastating neurologic injury can result due to a lack of cerebral oxygenation. Even when cardiac function is restored, brain damage may lead to death or profound neurological deficits. Patients may remain comatose and require continuous mechanical ventilation indefinitely.
Varying degrees of brain damage can occur after a cardiac arrest, depending on the length of time the brain went without oxygen. Even after the initial event, damage to the tissues can continue to occur as a result of reperfusion. After blood flow is restored to the brain, it triggers certain responses such as inflammation, which can continue for hours. This response can cause further damage to the brain.
Therapeutic hypothermia is a medical treatment which involves lowering the patient’s body temperature to decrease neurological damage after cardiac arrest. Studies have indicated that cooling the body as soon as possible after a cardiac arrest slows metabolism and may protect the brain from at least some of the damage caused by reperfusion.
Therapeutic hypothermia may be indicated for adults who are resuscitated after a witnessed cardiac arrest but still remain in a coma. The treatment can be used in both witnessed out-of-hospital and in-hospital cardiac arrests. It is helpful if the arrest was witnessed in order to establish a time frame of the event.
Although success rates vary, according to the Cleveland Clinic Journal of Medicine some patients had improved survival rates and neurological outcomes following a cardiac arrest if they underwent therapeutic hypothermia for 24 hours after the arrest. In 2005, the American Heart Association included therapeutic hypothermia in their guidelines for treatment of post-cardiac arrest patients who are comatose.
The physiological benefits of therapeutic hypothermia include reduced cerebral edema and decreased intracranial pressure. Cooling the body also slows the cerebral metabolic rate and decreases the risk of cell death. All of the above factors may contribute to decreased brain damage and improved neurological function.
According to the American Heart Association guidelines, patients who are deemed appropriate candidates for therapeutic hypothermia should have the procedure started within six hours of the witnessed cardiac arrest. The goal temperature is between 32 and 34 degrees Celsius. Reaching the goal temperature as soon as possible is usually advised. In many cases, the therapeutic temperature can be reached within three to four hours of starting the procedure.
Patients who have uncontrolled arrhythmias or uncontrolled bleeding may not be appropriate candidates. Therapeutic hypothermia may also be contraindicated in patients who have head injuries and sepsis.
Prior to therapeutic hypothermia being induced, it is essential that patients be properly sedated and have neuromuscular blockade on board to prevent shivering. Shivering is a stress response which results in increased oxygen consumption and heart rate which can impede the benefits of the procedure. An accurate measure of temperature is also important. Monitor temperature with an esophageal thermometer or a pulmonary artery catheter.
There are several cooling methods which may be used for therapeutic hypothermia. Both invasive and noninvasive methods can be implemented. Noninvasive methods may include placing ice packs around the head, torso and neck or the use of a cold air mattress. Invasive methods of cooling the body include placing an intravenous catheter and infusing cold saline.
Side Effects and Risks
As with most medical procedures, there are risks and possible side effects with therapeutic hypothermia. Initially, as the body begins to cool, tachycardia may develop. As the temperature drops below 35 degrees Celsius, bradycardia may be an issue. If significant arrhythmias develop the cooling process may need to be discontinued.
Skin damage may also develop, depending on the cooling method used. If cold blankets or ice packs are used, the skin should be checked frequently for injury to the skin due to the cold.
Hypothermia may also cause metabolic disturbances, thus patients must be carefully monitored. For example, disturbances in potassium and magnesium may develop, as well as hyperglycemia. Other possible side effects may include a change in drug metabolism and increased blood pressure.
In most instances, rewarming should begin 24 hours after the cooling process was started. It is important to achieve rewarming slowly to reduce the possibility of adverse effects, such as a drop in blood pressure. Rewarming can be accomplished by different means, such as by removing the cooling devices, warm infusions and heated blankets. The rewarming process should be very gradual over approximately an eight hour period. Increases in temperature should be between 0.3 and 0.5 degrees per hour until a temperature of 36 degrees Celsius is reached.
Massachusetts General Hospital Stroke Services. Hypothermia after Cardiac Arrest. http://www2.massgeneral.org/stopstroke/protocolHypothermia.aspx#warming Accessed July 2014.
Center for Resuscitation Science; Penn Emergency Medicine. Therapeutic Hypothermia. (2011) https://my.americanheart.org/idc/groups/ahamah-public/@wcm/@sop/@scon/documents/downloadable/ucm_427331.pdf Accessed July 2014.
Oommen, Santosh, MD, Hypothermia After Cardiac Arrest Beneficial but Slow to be Adopted. The Cleveland Journal of Clinical of Medicine. Volume 78 (7) http://www.ccjm.org/content/78/7/441.full Accessed July 2014.