Electroencephalographic Power-Spectral Changes Coincident with Onset of Carbon Dioxide Narcosis in Rhesus Monkey

SUMMARY Rhesus monkeys were exposed to continually increasing chamber CO2 concentrations to determine narcotic levels of environmental CO2 (barometric pressure was 656 mm. of Hg; chamber N2 was replaced by CO2 at 7.5%, 15%, or 30%/hr.). Due to the inaccessibility of the monkeys in an environmental control chamber, the criterion for narcosis was loss of all signs of body movement in response to loud noise. Transition from barely responsive to unresponsive occurred at CO2 concentration of 16.8 ± 1.6%. The uncertainty of the relationship of loss of overt response to loud noise and onset of CO2-induced narcosis led to cortical electroencephalography as a validating method. Examination of electroencephalographic (eeg) power-spectrums as CO2 increased demonstrated that a close relationship existed between the power loss at the 10- to 14-Hz. range and loss of consciousness. As narcotic concentrations of CO2 were approached, 10- to 14Hz. power markedly decreased. When narcotic concentrations were exceeded, further 10- to 14-Hz. power loss was minimal. This distinctive change in eeg power lent strong support to the contention that loss of overt response to loud noise and onset of narcosis were related events and, conversely, that the transition point between degraded consciousness and CO2 narcosis could be determined by examination of eeg power-spectrums.