Induced Moderate Hypothermia After Cardiac Arrest - American ...

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MCKEAN AACN Advanced Critical Care hypothermia on dogs after cardiac arrest demonstrated positive outcomes that included improved neurological status and survival outcomes. 3 This led the medical community to explore induced hypothermia following cardiac arrest once again as a possible intervention for humans. 4 It was not until 2 landmark studies were published in 2002 that induced hypothermia was considered best practice for patients following cardiac arrest. 2 Supportive Research Bernard and colleagues 7 compared the use of induced hypothermia with standard treatment for patients following VF arrest who remained comatose after return of spontaneous circulation (ROSC). Patients were randomly assigned to hypothermia or normothermia (standard care). The patients assigned to hypothermia were cooled to 33C within 2 hours after ROSC and were kept at that temperature for 12 hours. Of the 77 patients enrolled in the trial, 49% treated with hypothermia were discharged home or to a rehabilitation facility as compared with 26% of the patients treated with standard care. 7 The odds ratio for a good outcome with hypothermia compared with normothermia was 5.25 when baseline differences in age and time from collapse to ROSC were considered. 7 The Hypothermia After Cardiac Arrest Study Group 8 studied patients presenting in the emergency department with VF or nonperfusing ventricular tachycardia with ROSC. The patients randomly selected to receive hypothermia were cooled and maintained at 32C to 34C for 24 hours. Fifty-five percent of patients who received hypothermia had a favorable outcome, indicating that the patient was able to live independently and work at least part time. Only 39% of patients who were maintained at normothermia had similar favorable outcomes. 8 Mortality at 6 months was 41% in the hypothermia group as compared with 55% in those who did not receive hypothermia. 8 Several other studies have shown benefits related to induced mild hypothermia following cardiac arrest. Hachimi-Idrissi and colleagues 9 conducted a study inducing hypothermia by using a helmet device containing a solution of aqueous glycerol around the head and neck to cool the patient to 34°C. Patients who had cardiac arrest from pulseless electrical activity or asystole of presumed cardiac origin were considered for this trial. Study results revealed patients who received hypothermia had a significantly higher central venous oxygen saturation and significantly lower arterial lactate concentrations and oxygen extraction ratio. 9 Another study conducted by Oddo and colleagues 10 included patients who experienced out-of-hospital cardiac arrest from VF, asystole, and pulseless electrical activity. A good outcome, defined as Glasgow–Pittsburgh Cerebral Performance Category 1 or 2, was seen in 55.8% of the patients who received hypothermia treatment as compared with 25.6% treated with standard treatment for patients with cardiac arrest due to VF. 10 In October 2002, the International Liaison Committee on Resuscitation (ILCOR) made the recommendation, based on the above evidence, that all unconscious adult patients with ROSC following out-of-hospital cardiac arrest due to VF should be cooled to 32°C to 34°C for 12 to 24 hours. ILCOR also stated that other rhythms that cause cardiac arrest and in-hospital cardiac arrests may benefit from hypothermia. 11 In 2005, the American Heart Association (AHA) included these recommendations in the postresuscitation support guidelines. 1 Effects of Cerebral Ischemia and Induced Hypothermia The brain has a small amount of oxygen stores. When cerebral perfusion and oxygen delivery stop during cardiac arrest, the oxygen stores are depleted within 20 seconds. 6 If a patient was connected to continuous electroencephalography (EEG) during cardiac arrest, clinicians would see isoelectric lines after 20 seconds, indicating no brain activity present. 4 After oxygen is depleted, the brain turns to anaerobic metabolism to sustain function. Glucose and adenosine triphosphate levels are depleted after 5 minutes if return of blood flow is not achieved. 6 This causes ion pumps that use adenosine triphosphate to fail, allowing for electrolyte imbalance, including potassium, sodium, and calcium, resulting in cellular edema and cell death. 12 After ROSC, it would be assumed that once oxygen supply was returned to the brain, cell death would stop. However, it is believed that reperfusion initiates chemical processes that lead to inflammation and continued injury in the brain. This is known as reperfusion injury. Reperfusion injury is thought to include the release of free radicals, nitric oxide, catecholamines, cytokines, and calcium shifts, which all lead to mitochondrial damage and 344
VOLUME 20 • NUMBER 4 • OCTOBER–DECEMBER 2009 INDUCED MODERATE HYPOTHERMIA AFTER CARDIAC ARREST cell death. This process may last as long as 24 to 48 hours. 2,6,12,13 In low perfusion states, the blood–brain barrier is disrupted, leading to worsening cerebral edema. 13 The complete benefits of induced hypothermia are not well understood. The cerebral metabolic rate is decreased by 6% to 7% for every 1°C decrease in the body temperature. 5 Decreasing the cerebral metabolic rate decreases cerebral oxygen consumption. 2 Hypothermia helps to stabilize the influx of calcium and glutamate by slowing the neuroexcitatory processes, thereby reducing the disruptions in the blood–brain barrier and preventing premature cell death. 13 Hypothermia is also thought to decrease many of the chemical reactions that occur during reperfusion, such as free radical production. 2 Temperatures less than 35°C lead to decreased neutrophil and macrophage functions. This reduces the inflammatory response that is initiated after ischemia. 13 Hypothermia Methods Several methods may be used to induce and maintain hypothermia. The methods may be classified as noninvasive and invasive. Noninvasive Induced Hypothermia Noninvasive methods include traditional interventions such as using ice packs, fans, alcohol baths, and cooling blankets not attached to automatic temperature control modules. These methods are extremely labor intensive and require manual control of the patient’s temperature. The nurse must closely monitor the patient’s temperature and regulate the intervention to achieve and maintain the target temperature. In some instances this may require staffing to be 1:1. Also, the literature does not provide strong evidence for best practice using these methods. No specific guidelines are available for the noninvasive induction of hypothermia, such as “place 4 bags of ice to the axillary and groin areas and when the patient’s temperature reaches 34.5C remove one bag.” These methods are based more on the nurse’s judgment, without any specific evidence for achieving best outcomes. A much higher risk of unintentional overcooling or warming too quickly with these traditional methods has been shown. Rewarming is a definite problem with these techniques. It has also been shown that noninvasive methods may take a prolonged time to reach the target temperature or it may not be achieved at all. However, these methods are more readily available at most institutions. The equipment cost may also be lower although the cost for the 1:1 nurse–patient ratio may be more. 13,14 Improvements and advances have been observed in some of the noninvasive methods that help decrease labor intensity and risks involved with induced hypothermia. The Arctic Sun by Medivance (Louisville, CO) (Figure 1) uses gel pads placed on the patient’s skin to cover approximately 40% of the patient’s body. The pads may even be pulled back and reapplied as many times as needed, allowing access for patient care. Water circulates through the pads using a negative pressure system, which minimizes the risks of leaks as compared with other devices. The pads and a patient temperature source, such as a temperature-sensing urinary catheter, are connected to a console that automatically adjusts water temperature to achieve and maintain target temperature. It also provides a way for controlled warming to help prevent rebound hyperthermia. 13,15 Invasive Induced Hypothermia Invasive methods include use of iced (4°C) intravenous fluids and the use of intravascular catheters. A few studies show that use of iced intravenous fluids may be effective in inducing hypothermia. 16,17 In a study by Bernard and colleagues, 16 lactated Ringer solution was stored in the emergency department blood refrigerator with a controlled temperature of 4°C until needed. When a patient was identified for the study, 30 mL/kg of the lactated Ringer solution was infused over 30 minutes by using a pressure bag. Ice packs were then placed to maintain temperature at 33C for 12 hours. 16 Kliegel and colleagues 17 infused 2000 mL of iced lactated Ringer solution when patients met criteria for their study. As soon as possible, an intravascular catheter was placed and connected to a cooling device to maintain the temperature at 33C for 24 hours. 17 Iced intravenous fluids may help to induce hypothermia quickly in the emergency department and has the potential for being started out in the field by paramedics. Machines such as the CoolGard 3000 or the newer version Thermogard XP (Figure 2) by Alsius (Irvine, CA) use an automatic temperature-control module similar to that used by the Arctic Sun. These devices by Alsius use a closed-loop central venous catheter usually 345
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