Spectral analysis of the EEG during halothane anaesthesia: Input-output relations

Abstract

1. 1. The “Halothane-brain compartment” system was investigated in dogs. The input was the inspired concentration of Halothane. The output was the intensity of EEG spectral components. The EEG was analysed by a hybrid system (analogue filters and digital integration in a small computer). For the anaesthesia only a mixture of Halothane and oxygen was used. Temperature, CO2 and pH were kept constant.

2. 2. Input-output relations were determined by recording responses to step and sinusoidal changes of concentration. The most sensitive output parameters were the intensities of the 2–6 c/sec and 14–20 c/sec components.

3. 3. The rise and decay times of the responses to positive and negative steps were in the range 120–400 sec and 100–400 sec (5 dogs), respectively.

4. 4. The sinusoidal average responses showed some distortion, particularly at low frequencies of modulation (T = 11 min), indicating the existence of non-linearities. Evidence for the phenomena of adaptation and saturation was also found. Attenuation of response amplitude and increase in phase shift were found as frequency of modulation was increased, as in a low pass filter. With the data available it was not possible to determine the order of the linear approximation to the system with sufficient accuracy.

5. 5. A dose-response linear relation within the range of Halothane in inspired air between 1.0 and 2.5% was obtained for one dog.

6. 6. It is proposed that the quantitative data obtained in this study provide a basis for constructing a reliable monitoring system of Halothane anaesthesia.