Monitoring the nociception-anti-nociception balance

At present, short-acting drugs are used in order to achieve the three components of anaesthesia, that is, analgesia, hypnosis and immobility. Assessment of the 'analgesia' component in daily clinical routine is, in contrast to the other components, still based on very unspecific clinical 'end' points such as movement, tearing, tachycardia or hypertension. Individually tailored analgesia, however, should enable to maintain an individual nociceptive-anti-nociceptive balance and better avoid these unwanted responses to surgical stimulation. During the last decade, a variety of monitoring systems were developed in order to assess the nociceptive balance. These are, among others, based on the frontal electroencephalography (EEG) and electromyography (EMG) response, evaluation of the autonomic state and autonomic reactions, spinal reflex pathways and calculated drug concentrations. The present review gives an overview on the topic of 'analgesia' monitoring, the available monitoring systems and their clinical evaluation. Most of the systems allow a rapid detection of the nociceptive input; nonetheless, the prediction of an autonomic or somatic response has still to be improved. Several studies reported fewer unwanted events, reduced opioid consumption and shorter emergence from anaesthesia, when opioid administration was based upon monitoring of the nociceptive-anti-nociceptive balance. However, research on the mechanisms of pain processing and for better tools to assess the 'analgesia' component has to continue in order to improve our daily practice.