Deep hypothermic circulatory arrest: I. Effects of cooling on electroencephalogram and evoked potentials

Background. Deep hypothermia is an important cerebral protectant and is cri tical in procedures requiring circulatory arrest. The purpose of this study was to determine the factors that influence the neurophysiologic changes d uring cooling before circulatory arrest, in particular the occurrence of el ectrocerebral silence. Methods. In 109 patients undergoing hypothermic circulatory arrest with neu rophysiologic monitoring, five electrophysiologic events were selected for detailed study. Results. The mean nasopharyngeal temperature when periodic complexes appear ed in the electroencephalogram after cooling was 29.6 degreesC +/- 3 degree sC, electroencephalogram burst-suppression appeared at 24.4 degreesC +/- 4 degreesC, and electrocerebral silence appeared at 17.8 degreesC +/- 4 degre esC. The N20-P22 complex of the somatosensory evoked response disappeared a t 21.4 degreesC +/- 4 degreesC, and the somatosensory evoked response N13 w ave disappeared at 17.3 degreesC +/- 4 degreesC. The temperatures of these various events were not significantly affected by any patient-specific or s urgical variables, although the time to cool to electrocerebral silence was prolonged by high hemoglobin concentrations, low arterial partial pressure of carbon dioxide, and by slow cooling rates. Only 60% of patients demonst rated electrocerebral silence by either a nasopharyngeal temperature of 18 degreesC or a cooling time of 30 minutes. Conclusions. With the high degree of interpatient variability in these neur ophysiologic measures, the only absolute predictors of electrocerebral sile nce were nasopharyngeal temperature below 12.5 degreesC and cooling longer than 50 minutes. (Ann Thorac Surg 2001;71:14-21) (C) 2001 by The Society of Thoracic Surgeons.