Open Reimplementation of the BIS Algorithms for Depth of Anesthesia

Abstract

BACKGROUND: BIS (a brand of processed electroencephalogram [EEG] depth-of-anesthesia monitor) scores have become interwoven into clinical anesthesia care and research. Yet, the algorithms used by such monitors remain proprietary. We do not actually know what we are measuring. If we knew, we could better understand the clinical prognostic significance of deviations in the score and make greater research advances in closed-loop control or avoiding postoperative cognitive dysfunction or juvenile neurological injury. In previous work, an A-2000 BIS monitor was forensically disassembled and its algorithms (the BIS Engine) retrieved as machine code. Development of an emulator allowed BIS scores to be calculated from arbitrary EEG data for the first time. We now address the fundamental questions of how these algorithms function and what they represent physiologically. METHODS: EEG data were obtained during induction, maintenance, and emergence from 12 patients receiving customary anesthetic management for orthopedic, general, vascular, and neurosurgical procedures. These data were used to trigger the closely monitored execution of the various parts of the BIS Engine, allowing it to be reimplemented in a high-level language as an algorithm entitled ibis. Ibis was then rewritten for concision and physiological clarity to produce a novel completely clear-box depth-of-anesthesia algorithm titled openibis. RESULTS: The output of the ibis algorithm is functionally indistinguishable from the native BIS A-2000, with r = 0.9970 (0.9970-0.9971) and Bland-Altman mean difference between methods of -0.25 ± 2.6 on a unitless 0 to 100 depth-of-anesthesia scale. This precision exceeds the performance of any earlier attempt to reimplement the function of the BIS algorithms. The openibis algorithm also matches the output of the native algorithm very closely (r = 0.9395 [0.9390-0.9400], Bland-Altman 2.62 ± 12.0) in only 64 lines of readable code whose function can be unambiguously related to observable features in the EEG signal. The operation of the openibis algorithm is described in an intuitive, graphical form. CONCLUSIONS: The openibis algorithm finally provides definitive answers about the BIS: the reliance of the most important signal components on the low-gamma waveband and how these components are weighted against each other. Reverse engineering allows these conclusions to be reached with a clarity and precision that cannot be obtained by other means. These results contradict previous review articles that were believed to be authoritative: the BIS score does not appear to depend on a bispectral index at all. These results put clinical anesthesia research using depth-of-anesthesia scores on a firm footing by elucidating their physiological basis and enabling comparison to other animal models for mechanistic research.

On the Importance of Transparency About the Internal Operation of Medical Devices

“…We read with interest the recent article by Dr Connor, “Open Reimplementation of the BIS Algorithms for Depth of Anesthesia.” 1 This is the third article by the author on the elucidation of the internal operation of the bispectral index (BIS) monitor, 2,3 a device that the vendor describes as providing “direct measurement of the patient’s level of consciousness and insight into the effects of anesthesia on the brain.” 4 In this recent article, Dr Connor, using reverse engineering, found, and aptly demonstrated, that the BIS score is calculated through an algorithm that relies on electroencephalogram wavebands in frequencies that lie within the low range of γ (30–47 and 40–47 Hz), rather than, as it has been claimed and believed for 20 years, through an algorithm that calculates a spectral index that would rely on electroencephalogram wavebands within the various classical frequencies of β, α, θ, and δ. In his concluding remarks, Dr Connor correctly calls for openness by the manufacturer about the device’s internal operations that produce the data that are presented to the clinician.…”

EEG-Parameter-Guided Anesthesia for Prevention of Emergence Delirium in Children

“…At present, the correlation between ED and anesthetic depth in previous studies mainly focused on numeric proprietary indices such as the BIS (Medtronic, Minnesota, Minnesota), state entropy (SE), response entropy (RE), and the Narcotrend index (MonitorTechnik, Bad Bramstedt, Germany) [ 24 , 25 , 26 , 27 ]. As the calculation algorithms of these monitoring indices are based on adult EEG characteristics, the current indices may not suitable for children undergoing general anesthesia [ 27 , 28 , 29 ].…”

In Response

“…A score for very light anesthesia is calculated, but it is no more complicated than those I already described. 4 It is a power ratio between the overlapping EEG bands of 2–30 Hz versus 11–20 Hz, normalized onto a sigmoid response whose base is at a BIS of 74. No bispectral component is involved.…”

“…Furthermore, the sedation score takes precedence whenever it is lower than this very light anesthesia score. 4 Thus, the BIS usually calculates this value internally but discards it when producing the final result; I excluded it from my simplest, clearest reimplementation openibis with little consequence. 4 Dr Rampil concedes that “as an external consultant, I never had access to the commercial, delivered computer code.” Well, I have seen the machine code in the A-2000 BIS monitor.…”

“…We both observe that >3000 PubMed citations reference the BIS monitor. 2,4 I wrote, “If the device had not had such an immense academic and clinical impact, there would be no merit in attempting to reverse engineer it. Meanwhile, the questions of what constitutes unconsciousness, and how it should be practically measured, remain absolutely central to the specialty of anesthesiology.” 2 Dr Rampil says that “clearly … there has been no impediment to research” but misses the point of what scientific research could have been done.…”