Somatosensory evoked potentials for closed-loop control of anaesthetic depth using propofol in the urethane-anaesthetized rat

Primary somatosensory cortical mass responses have been shown to exhibit do se-dependent changes in latency when general anaesthetics are administered. Here we describe a system in which the latency of evoked responses was mea sured automatically in real time in five animals. Latency changes were used to operate a closed-loop control of propofol delivery by intravenous infus ion. The system attempted to induce and maintain a 1 ms increase in evoked response latency; this was reversed when infusion was discontinued. Allowin g for the rapid and large biological fluctuations in the evoked response, t his was achieved successfully. The system maintained a mean increase in lat ency of 1.27 (SD 0.42) ms. The mean statistical dispersion index of data ob tained during the controlled period was 1.23 (0.3); in an ideal controllabl e system it approximates to 1. Such a system may provide a means for the au tomatic delivery of anaesthetics.