Abstract 80: Mild Hypothermia Induced Rapidly in Human Sized Animals
There are numerous current reports consistent with application of mild hypothermia (32–34° C) following resuscitation from cardiac arrest. However the time course of this application in humans has been reported to take in excess of one hour to reach a core temperature of 33° C. Recent studies have also shown that core temperature reduction in humans could occur rapidly using immersion in large volumes of stirred ice water. However, such a procedure would be impractical in a clinical setting. Therefore we report here on the ability to rapidly cool human sized animals at rates mimicking those for humans immersed in ice water but without using a large volume of water. Six domestic swine weighing > 60 kg each were anesthetized using propofol and buprinorphine and instrumented with arterial pressure monitoring and ECG, and thermocouple sensors in the pulmonary artery (PA), carotid artery, tympanic membrane and esophagus. With an initial PA temperature at 37 °C, the swine underwent a cooling sequence. This was performed with a flexible surround suit system that provided for a thin 0.5 cm layer of circulating ice water in direct skin contact held between 0.5° and 1.5° C. A pumping system was used to circulate the water volume of 20 liters at a rate of 15 liters per minute. The newly developed system had an averaged PA temperature cooling rate of 18.6(±13.7)° C/hr. The averaged time to decrease the temperature by 3° C was 13.0 (±3.0) min Temperature decrease observed at the other sites was equally rapid. The strongest temperature correlation was between the PA and the esophagus (r=.96). Rapid cooling is likely to be beneficial as it can quickly slow metabolic processes which appear to cause further brain injury in cardiac arrest survivors. The system used in this work was able to rapidly cool the core temperature of large human size animals more quickly than any other practical method reported for use in emergency situations. The results are similar to those reported in previous human studies on ice water immersion: 21° (±8.4) C/hr vs. the 18.6° (±13.7) C/hr reported here). In addition, the rapid PA temperature changes observed in this setting may be accurately reflected by an esophagus sensor. Furthermore, because of the size of the animal tested, similar cooling rates may be achieved in humans.