Entropy of the electroencephalogram as applied in the M-Entropy S/5TM Module (GE Healthcare) during increases in nitrous oxide and constant sevoflurane concentrations

  • Francois Jacobus Smith University of Pretoria
  • Sandra Spijkerman University of Pretoria
  • Piet J Becker Medical research Council, University of Pretoria
  • Johan Francois Coetzee Department of Anaesthesiology and Critical Care, Faculty of Health Sciences, Stellenbosch University
Keywords: monitoring, depth of anaesthesia, electroencephalography, entropy, anaesthetics volatile, sevoflurane, anaesthetics gases, nitrous oxide

Abstract

Background: It has been suggested that spectral entropy of the electroencephalogram as applied in the M-Entropy S/5TM Module (GE Healthcare) does not detect the effects of nitrous oxide (N2O). The aim of this study was to investigate the effect on entropy by graded increases in N2O concentrations in the presence of a constant concentration of sevoflurane, in the absence of surgical stimulation. Method: This single-blind, randomised study was conducted at an altitude of approximately 1 400 m. Patients received sevoflurane 2% (1.7% at sea level) and N2O, at end-tidal concentrations of 0%, 10%, 20%, 30%, 40%, 50%, 60% or 70% (equivalent to 8.5%, 17%, 25.5%, 34%, 42.6%, 51.1% and 59.6% at sea level). Entropy was measured before, during and after N2O administration. The absolute changes and ratios of entropy relative to the baseline were calculated. Between- and within-group comparisons were made using analysis of variance and covariance. Results: None of the entropy variables differed significantly within and between groups before and after N2O administration. Within-group analysis revealed that entropy during N2O administration was significantly lower than before or after N2O administration (P < 0.007). While a minor clinical but statistically significant linear relationship was observed between increasing N2O concentration and decreasing entropy from N2O 0% to 60%, a steeper and clinically important decrease (relative change > 20%) was noted at N2O > 60% (> 51% at sea level). Conclusions: The M-Entropy Module S/5TM responds to increasing concentrations of N2O in the presence of 2% (1.7% at sea level) sevoflurane, in the absence of surgical stimulation. There is a linear relationship between increasing N2O concentrations and decreasing entropy with a steep and clinically important decrease at N2O > 60% (> 51% at sea level). The influence of ambient pressure on the partial pressures, which determine the effects of anaesthetic agents, must be taken into account.

Author Biographies

Francois Jacobus Smith, University of Pretoria
BSc(Pharm), MBChB, MMed(Anaes), FCA(SA), MD Professor Department of Anaesthesiology School of Medicine Faculty of Health Sciences University of Pretoria Pretoria South Africa
Sandra Spijkerman, University of Pretoria
MBChB, DA(SA)MMed(Anaes), FCA(SA) Dr Department of Anaesthesiology School of Medicine Faculty of Health Sciences University of Pretoria Pretoria South Africa
Piet J Becker, Medical research Council, University of Pretoria
MSc, PhD Professor Biostatistics Unit, Medical research Council and Unit for Clinical Epidemiology School of Medicine Faculty of Health Sciences University of Pretoria Pretoria South Africa
Johan Francois Coetzee, Department of Anaesthesiology and Critical Care, Faculty of Health Sciences, Stellenbosch University
BSc, MBShB, MMed (Anes), FCA (SA), Dip. Dat. (UNISA), BSc, PhD Professor Department of Anaesthesiology and Critical Care, Faculty of Health Science, Stellenbosch University
Published
2010-08-12
Section
Original Research