Determination of analgesia during anaesthesia - Rational for the use of somatosensory evoked potentials - a survey

The hypnotic activity during anaesthesia can be monitored by means of the e lectroencephalogram either by using the native EEG or one of the descriptor s being derived from the different frequency bands (alpha, beta, theta, del ta). However, measurement of analgesia is not straight-forward, as cardiova scular parameters. Involuntary movements, stress hormone levels, sweating o r lacrimation only serve as indirect indicators of insufficient analgesia. During early use of evoked potentials (EPs) in pain measurment it has been noted that amplitudes in the EP closely correlate with increasing pain sens ations, using either electrical or laser stimulation of the median nerve at the upper extremity, the forearm, the finger or the tooth pulp. As analges ics simultaneously attenuate respectively pain sensations and amplitude hei ght, sensory EPs may provide a tool to measure the individual state of anal gesia in patients during anaesthesia, Proof of this hypothesis was brought forward by the effect of different mu- selective opioids (alfentanil, fentanyl and sufentanil) on the EP in a cani ne paradigm. All opioids depressed the late peak of the EP in a dose-relate d fashion which was reversible by naloxone, reflecting receptor specificity . In contrast, kappa-selective opioids (nalbuphin, bremazocine and tifluado m) predominantly induced a dose-related increase of latency of amplitude of the EP. As these effects could only be reversed by a kappa-specific antago nist, differences of results are discussed in the light of different opioid receptor distribution in the various CNS structures being related in the m ediation of pain. Clinical data seem to corroborate the close interrelation of increase in no xious sensory nervous afferents and amplitude height of the late (> 100 ms) EP, both during intubation and traction at the mesenterium, with a simulta neous increase in cardiovascular parameters. Routine measurement of somatosensory EPs in the clinical setting is increas ingly used in vascular surgery in order to monitor the nervous tissue at ri sk. Such a device may useful similar to the assessment of muscle relaxation . By "twitching the brain", EPs reflect the degree of receptor occupancy al ong the sensory tract within areas of the nervous system responsible to con vey painful sensations.