title: Burst-Suppression Ratio on Electrocorticography Depends on Interelectrode Distance.
abstract: BACKGROUND
  With deepening of anesthesia-induced comatose states, the EEG becomes fragmented by increasing periods of suppression. When measured from conventional EEG recordings, the binary burst-suppression signal (BS) appears similar across the scalp. As such, the BS ratio (BSR), quantifying the fraction of time spent in suppression, is clinically considered a global index of brain function in sedation monitoring. Recent studies indicate that BS may be considerably asynchronous when measured with higher spatial resolution such as on electrocorticography. The authors investigated the magnitude of BSR changes with cortical recording interelectrode distance.
  
  
  METHODS
  The authors selected fronto-parietal electrocorticography recordings showing propofol-induced BS recorded via 8-electrode strips (1-cm interelectrode distance) during cortical motor mapping in 31 patients. For 1-minute epochs, bipolar recordings were computed between each electrode pair. The median BSR, burst duration (BD), and bursting frequency were derived for each interelectrode distance.
  
  
  RESULTS
  At 1-cm interelectrode distance, with increasing BSR, BD decreased exponentially. For a BSR between 50% and 80%, BD reached a plateau of 2.1 seconds while the bursting frequency decreased from 14 to 6 bursts per minute. With increasing interelectrode distance, BD increased at a rate of 0.2 seconds per cm. This correlated with a decrease in BSR with distance that reached the rate of -4.4 percentage per centimeters during deepest anesthesia.
  
  
  CONCLUSIONS
  With increasing cortical interelectrode recording distance, burst summation leads to an increasing BD associated with a reduction in BSR. Standardization of interelectrode distance is important for cortical BSR measurements.
authors.affiliation: *Division of Physiology and Neuroscience, Department of Functional Sciences, "Carol Davila" University of Medicine and Pharmacy, Bucharest, Romania; †Department of Neuroscience and Pharmacology, University of Copenhagen, Copenhagen, Denmark; ‡Intraoperative Neurophysiology Unit, Department of Neurology, Massachusetts General Hospital, Harvard Medical School, Boston, Massachusetts; §Department of Pharmacology, University of Oxford, Oxford, United Kingdom; and ‖Department of Neurology, University Hospitals Case Medical Center, Cleveland, Ohio.
authors.forename: Mihai
authors.initials: M
authors.lastname: Moldovan
authors.forename: Alexandru
authors.initials: A
authors.lastname: Calin
authors.forename: Vishakhadatta M
authors.initials: VM
authors.lastname: Kumaraswamy
authors.forename: Diana
authors.initials: D
authors.lastname: Braver
authors.forename: Mirela V
authors.initials: MV
authors.lastname: Simon
mesh_terms: D001931:Brain Mapping / Q000379:methods; D000069280:Electrocorticography / Q000379:methods* / Q000592:standards; D004566:Electrodes; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D012189:Retrospective Studies