During general anesthesia hypnosis can be monitored routinely using EEG derivates like Bispectral index (BIS), entropy,.… However, monitoring analgesia or more precisely the nociceptive-antinociceptive (analgetic) balance during anesthesia proved more difficult. Clinical signs such as heart rate (tachycardia) or blood pressure (hypertension) are usually observed and used to assess nociception and a patient's autonomic unbalance. During general anesthesia administration of opioids like sufentanil and remifentanil decrease the ANS reactivity to noxious stimulation. Recently two variables based on standard anesthesia monitor equipment have been introduced into clinical practice: the Analgesia Nociception Index (ANI), derived from analysis of the small beat-to-beat oscillations of the heart rate during respiration, the heart rate variability (HRV), and the Surgical Pleth Index (SPI), calculated from the plethysmographic amplitude and heart beat interval.
