Neuromonitoring in hypothermia and in hypothermic hypoxia

Severe accidental hypothermia is often associated with global ischaemia due to cardiac arrest. The purpose of the present study was to evaluate whether rewarming on cardiopulmonary bypass (CPB) should be slow or as fast as possible. Pigs were cooled to 23 degrees C (rectum), subsequently followed by 1 h period of circulatory arrest, whereupon rewarming was started. Pigs were randomly allocated to 3 groups: slowly rewarmed (2 h), rapidly rewarmed (0.5 h) and a control group on CPB maintaining normothermia. EEG was continuously analyzed by means of Anesthesia-Brain-Monitor (ABM-system), which allows simultaneous monitoring of EEG, blood- and intracranial pressures, heart rate and capnogram as trend graphs. The ABM-system thus delivers a continuous on-line printout of all measured variables. Cooling resulted after about 30 min in electrocerebral inactivity (ECI) in all pigs. EEG reappeared in all animals regardless of the considerable long cardiocirculatory arrest, followed by nearly 3 hours of ECI! It would appear tempting to rewarm an accidentally cooled organism as fast as possible. The present study, however, indicate that the brain during rewarming seems to have its own speed for regeneration of the EEG. This speed showed to be slower than the steeply rising temperature during rapid rewarming. Furthermore, a too vigorous rewarming may jeopardize cerebral metabolism; 90 min after the start of rewarming the EEG had reappeared in 5 out of 8 pigs in slow group, but only in 2 out of 7 in rapidly rewarmed animals. In controls the EEG was continuously present throughout the experiments.