Background and Goal of Study: Standard monitoring based on parameters of the cardiovascular system and respiration provides information for assessing the hypnotic component of anaesthesia. The present investigation evaluates the ability of a fuzzy based indicator combining different standard parameters and permutation entropy (PeEn)  of heart rate variability (HRV) to indicate “depth of anaesthesia”.
Materials and Methods: After ethics committee approval, 263 adult patients undergoing surgery under general anaesthesia were included . Standard parameters and EEG were continuously recorded and stored together with relevant patient data. During anaesthesia induction, Tunstall's isolated forearm technique was performed to detect loss of consciousness (LOC). After skin incision, the anaesthetic concentration was increased until EEG burst suppression (BS). At the end of surgery, drugs were discontinued and return of consciousness (ROC) was recorded.
Data were analyzed during consciousness (before LOC, after ROC), general anaesthesia and BS. A fuzzy inference based on a Takagi Sugeno Kang model  was used for data driven combination of standard parameters: heart rate (HR), blood pressure (BP), PeEn (m=5, 0.15-0.45Hz bandwidth) of HRV together with anaesthetic and oxygen concentration.
Prediction probability (PK) including 95% bootstrap confidence intervals was investigated to indicate the indicators ability to separate anaesthetic levels (threefold cross validation).
Results and Discussion: The fuzzy model leads to a PK of 0.85 (0.83-0.87) for separation of consciousness, general anaesthesia and BS.
This is significantly higher than the best included parameter BP with PK of 0.76 (0.73-0.79).
Conclusion(s): In order to assess “depth of anaesthesia” the single standard parameters, which provide partial information about the hypnotic components, have to be combined into a single indicator. Results show that anaesthetic conditions can be revealed through such an indicator. In particular, inclusion of HRV permutation entropy may provide additional information related to non linear heart dynamics. Because of time delay of standard parameters, this indicator might be inadequate to reflect rapid changes. However, it could be integrated in standard anaesthesia monitoring to provide reliable trend information of the hypnotic component.
 Phys Rev Lett 2002; 88: 174102
 Anesthesiology 2006; 105: A1553
 IEEE Trans Syst Man Cybern 1993; Vol. 23, No. 3