Evaluating the level of consciousness with brain biophysics methods in anaesthesia

Objective: The purpose of the thesis is evaluating the level of consciousness with brain biophysics methods during general anesthesia.Method: Total of 14 subjects with an age range of 25-46 (mean 36.54) years participated in this study. The participants were patients undergoing a surgery protocol for lumbar disc hernia. The propofol administration was provided with a target controlled infusion (TCI) device (Fresenius Vial Orchestra Base Primea, Le Grand Chemin, France). Marsh model for calculation of brain dosage (effect-site) was used. Propofol dosages of 0.4, 0.8, 1.2, 1.6 ?g/mL were administered. For auditory evoked potentials (AEP) recording, a 40 channel data acquisition system (Scan 4.3, Neuroscan, Nuamps, USA) and embedded microcontroller stimulus unit (EMISU) were used. The auditory stimuli (1500-1600Hz, 85dB SPL) was applied with headphones which had noise cancellation feature (Creative HN- 700, Republic of Singapore). Central (C3, CZ, C4 electrodes) location was analyzed as region of interest. Additionally, the EEG epochs were set to -500ms prestimulus and +1000ms poststimulus period. BIS recording was performed using a bispectral index monitor (BIS Vista, Aspect Medical System Inc., MA, USA) with a sensor. In every five seconds BIS was recorded via USB interface. One-way ANOVA and posthoc Bonferroni correction were used for statistical analysis.Results: The amplitude of N1P2 was found to be 13.9 µV at 0.0 dosage (no-drug) level, 11.33 µV at 0.4 ?g/mL, 6.00 µV at 0.8 ?g/mL, 3.58 µV at 1.2 , and 2.77 µV at 1.6 ?g/mL dosage levels respectively. At no-drug dosage (0.0) level, AEP amplitude was significantly higher than of 0.8 ?g/mL dosage level (p<0.01) and also 1.2-1.6 ?g/mL dosage level (p<0.001). As a parallel findings were seen in the BIS values. The mean BIS value was found to be 94 at 0.0 dosage (no-drug) level, 92.68 at 0.4 ?g/mL, 87.71 at 0.8 ?g/mL, 79.32 at 1.2 ?g/mL, and 75.61 at 1.6 ?g/mL dosage levels respectively (Figure). At no-drug dosage (0.0) level, BIS values were significantly higher than the other dosage level (p<0.001). Beyond 1.2 all subjects reached loss of consciousness.Conclusion: The vital sign monitoring is most commonly used method in anesthesia. Besides that EEG and AEP based methods were used. There are number of AEP studies during anesthesia which have more focus on early and middle latency responses. The anesthesia procedure administrated by means of the N1P2 amplitude has decreased during step by step increasing propofol dosage. In the current study, we tested the suppression effect of propofol on AEP (late latency responses) while monitoring BIS. This way we have obtained a linear decrease of potentials as anesthetics suppression, accompanied by a direct correlation with BIS values. These late responses disappeared reaching a dosage level of 1.6 ?g/mL propofol target controlled infusion. Accordingly, these late auditory responses should not be expected at regular anesthesia levels of typical surgery (>2 ?g/mL propofol). However the present functional correlation between these two parameters may provide a suitable analysis platform to study the transition between consciousness states as well as their functional properties.