Failure of hypothermia as treatment for asphyxiated newborn rabbits.

Cooling is known to prolong survival in newborn animals when used before the onset of asphyxia. It has therefore been advocated as a treatment for birth asphyxia in humans. Since it is not possible to cool a human baby before the onset of birth asphyxia, experiments were designed to test the effect of cooling after asphyxia had already started. Newborn rabbits were asphyxiated in 100% nitrogen and were cooled either quickly (drop of 1 degree C in 45 s) or slowly (drop of 1 degree C in 2 min) at varying intervals after asphyxia had started. When compared with controls, there was an increase in survival only when fast cooling was used early in asphyxia. This fast rate of cooling is impossible to obtain in a human baby weighing from 30 to 60 times more than a newborn rabbit. Further litters of rabbits were asphyxiated in utero. After deliver they were placed in environmental temperatures of either 37 degrees C, 20 degrees C, or 0 degrees C and observed for spontaneous recovery. The animals who were cooled survived less often than those kept at 37 degrees C. The results of these experiments suggest that hypothermia has little to offer in the treatment of birth asphyxia in humans.