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The authors evaluated a device designed to provide conscious sedation with propofol (propofol–air), or propofol combined with 50% nitrous oxide (N2O; propofol–N2O). An element of this device is the automated responsiveness test (ART), a method for confirming that patients remain conscious. The authors tested the hypotheses that the ART predicts loss of consciousness and that failure to respond to the ART precedes sedation-induced respiratory or hemodynamic toxicity.The protocol consisted of sequential 15-min cycles in 20 volunteers. After a 15-min control period, propofol was infused to an initial target effect-site concentration of 0.0 μg/ml with N2O or 1.5 μg/ml with air. Subsequently, the propofol target effect-site concentration was increased by a designated increment (0.25 and 0.5 μg/ml) and the process repeated. This sequence was continued until loss of consciousness, as defined by an Observer’s Assessment of Alertness/Sedation (OAA/S) score of 10/20 or less, or until an adverse physiologic event was detected.The OAA/S score at which only 50% of the volunteers were able to respond to the ART (P50) during propofol–N2O was 11.1 of 20 (95% confidence interval [CI]: 10.6–11.8); the analogous P50 was 11.8 of 20 (95% CI: 11.4–12.3) with propofol–air. Failure to respond to the ART occurred at a plasma propofol concentration of 0.7 ± 0.6 μg/ml with propofol–N2O and 1.6 ± 0.6 μg/ml with propofol–air, whereas loss of consciousness occurred at 1.2 ± 0.8 μg/ml and 1.9 ± 0.7μg/ml, respectively. There were no false-normal ART responses.The ART can guide individual titration of propofol because failure to respond to responsiveness testing precedes loss of consciousness and is not susceptible to false-normal responses. The use of N2O with propofol for conscious sedation decreases the predictive accuracy of the ART.