- Correlation of approximate entropy, bispectral index, and spectral edge frequency 95 (SEF95) with clinical signs of "anesthetic depth" during coadministration of propofol and remifentanil. Bruhn, J., Bouillon, T.W., Radulescu, L., Hoeft, A., Bertaccini, E., Shafer, S.L. Anesthesiology (2003)