Strategies to prevent awareness

Abstract

Anesthetic depth depends on the interaction of two opposing factors: A hypnotic-anesthetic component and a component promoted by nociceptive stimulus. Such stimulus may cause patients to reawaken in the presence of low anesthetic-hypnotic receptor concentration. To minimize this risk, the most appropriate technique for maintaining stable drug concentrations seems to be controlled infusion. Anesthetic depth control also becomes paramount. One of the systems most widely used in the past (albeit experimentally) to assess anesthetic depth and avoid awareness phenomena, has been the evoked potential method. Volatile anesthetic characteristically increase latency and reduce the amplitude of mean latency evoked potentials as a function of drug dosage and concentration. The change in latency and amplitude of mean latency evoked potentials observed with intravenously administered agents, such as thiopentone, etomidate and propofol, is similar to the change observed with volatile anesthetics while, even at high dosages, E.P. morphology is not affected by opiates. Major progress is currently being made in the BIS technique, which not only analyses EEG signal power and frequency, but also considers quadratic interaction between the various EEG components. Currently available results suggest that associating the Total Controlled Infusion technique and BIS is the most reliable way to prevent awareness.