Electrophysiological monitoring and mild hypothermia

Hypothermia affects cerebral metabolism including a variety of neurotransmitter systems. Neurophysiologic changes during hypothermia are characterized by decreases in the membrane resting potential and amplitude. The duration of the action potential is prolonged and nerve conduction velocity is decreased. Nonsynaptic responses are reduced to a lesser extent than synaptic responses. Mild hypothermia to 33.5 degrees C produces only minimal changes in the EEG with small shifts in frequencies to theta- and beta-activity. Electro-cerebral silence occurs at a temperature below 22-25 degrees C. Evoked potentials (EP) of all modalities are affected by hypothermia to a similar degrees. Amplitudes are decreased and latencies are prolonged. Cortical EP components are more profoundly affected than early, subcortical potentials. EP may be reliably recorded at temperatures > 25 degrees C. Changes in EEG and EP are not specific for hypothermia. In addition, a hysteresis between cooling and rewarming has to be taken into account for the interpretation of temperature effects on brain electrical activity. The interactive effects of anesthetics and temperature may preclude EEG and EP measure for a graded quantification of hypothermia.