Hypothermia and recovery from respiratory arrest in a neonatal rat in vitro brain stem preparation.

This study was designed to examine the possibility that respiratory arrest during hypothermia occurs at the level of premotor or motor neurons rather than at the level of the central rhythm generator itself. Specifically, we sought to determine the consequences of hypothermic cooling until respiratory arrest, and subsequent rewarming, on neurons in the pre-Bötzinger Complex, as an indication of the output of the entire rhythmogenic network; and from cervical spinal (phrenic) ventral roots, as an indication of motor neuron output, in an in vitro neonatal rat brain stem-spinal cord preparation. We found that hypothermia led to a slowing of the respiratory rhythm with little or no decrease in the magnitude of phrenic motor output or the field potential of pre-Bötzinger Complex neurons. Ultimate arrest occurred abruptly and simultaneously in recordings from both sites, indicating that the arrest was due to failure of the central rhythm-generating network, primarily due to removal of a conditional excitation. On being rewarmed, the motor output recorded at both sites was usually fractionated, initially suggesting that changes occurred in network synchronization either during cooling or during reactivation following hypothermic arrest.