title: Hypothermia and the approximate entropy of the electroencephalogram.
abstract: BACKGROUND
  The electroencephalogram is commonly used to monitor the brain during hypothermic cardiopulmonary bypass and circulatory arrest. No quantitative relationship between the electroencephalogram and temperature has been elucidated, even though the qualitative changes are well known. This study was undertaken to define a dose-response relationship for hypothermia and the approximate entropy of the electroencephalogram.
  
  
  METHODS
  The electroencephalogram was recorded during cooling and rewarming in 14 patients undergoing hypothermic cardiopulmonary bypass and circulatory arrest. Data were digitized at 128 Hz, and approximate entropy was calculated from 8-s intervals. The dose-response relationship was derived using sigmoidal curve-fitting techniques, and statistical analysis was performed using analysis of variance techniques.
  
  
  RESULTS
  The approximate entropy of the electroencephalogram changed in a sigmoidal fashion during cooling and rewarming. The midpoint of the curve averaged 24.7 degrees C during cooling and 28 degrees C (not significant) during rewarming. The temperature corresponding to 5% entropy (T 0.05 ) was 18.7 degrees C. The temperature corresponding to 95% entropy (T 0.95 ) was 31.3 degrees C during cooling and 38.2 degrees C during rewarming ( P < 0.02).
  
  
  CONCLUSIONS
  Approximate entropy is a suitable analysis technique to quantify the electroencephalographic changes that occur with cooling and rewarming. It demonstrates a delay in recovery that is of the same magnitude as that seen with conventional interpretation of the analog electroencephalogram and extends these observations over a greater range of temperatures.
authors.affiliation: Department of Anesthesia, Robert Wood Johnson School of Medicine, University of Pennsylvania, Philadelphia, Pennsilvania 19104-4283, USA. levyw@uphs.upenn.edu
authors.forename: Warren J
authors.initials: WJ
authors.lastname: Levy
authors.forename: Enrique
authors.initials: E
authors.lastname: Pantin
authors.forename: Sachin
authors.initials: S
authors.lastname: Mehta
authors.forename: Michael
authors.initials: M
authors.lastname: McGarvey
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000704:Analysis of Variance; D001831:Body Temperature / Q000502:physiology; D004569:Electroencephalography; D019277:Entropy; D005260:Female; D006801:Humans; D007036:Hypothermia, Induced; D008297:Male; D008875:Middle Aged; D017694:Rewarming