Comparison of bispectral index, 95% spectral edge frequency and approximate entropy of the EEG, with changes in heart rate variability during induction of general anaesthesia

We have compared bispectral index (BIS), 95% spectral edge frequency (SEF) and approximate entropy (ApEn) in 37 patients during induction and recovery from a short general anaesthetic. Heart rate variability (HRV) was also co mpared during induction only. These indices were noted at the start of indu ction, when a syringe held between the thumb and fingertips was dropped, at insertion of a laryngeal mask or tracheal tube (tube insertion), at incisi on, at the end of surgery, on return of the gag reflex and when the patient could follow a verbal command. When indices at the start of induction were compared with those at tube insertion. all four decreased significantly. B IS decreased from a mean of 95.38 (SEM 1.02) to 44.22 (1.05), mean SEF from 20.91 (1.19) to 14.14 (0.70) Hz, mean HRV from 37.1 (7.75) to 17.9 (3.6) b pm(2) and ApEn from 0.90 (0.06) to 0.65 (0.04). Using logistic regression, the indices were compared both individually and in combination as to the po wer of distinguishing awake (at pre-induction) from asleep (at tube inserti on) states. BIS had the best predictive power, with a sensitivity of 97.3%, specificity 94.4%, positive predictive value 94.7% and negative predictive value 97.1%. A combination of the indices conferred no additional predicti ve advantage.