Automated responsiveness test (ART) predicts loss of consciousness and adverse physiologic responses during propofol conscious sedation

Background: 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. Methods: 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 mug/ml with N2O or 1.5 mug/ml with air. Subsequently, the propofol target effect-site concentration was increased by a designated increment (0.25 and 0.5 mug/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. Results: The OAA/S score at which only 50% of the volunteers were able to respond to the ART (P-50) during propofol-N2O was 11.1 of 20 (95% confidence interval [CI]: 10.6-11.8); the analogous P-50 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 mug/ml with propofol-N2O and 1.6 +/- 0.6 mug/ml with propofol-air, whereas loss of consciousness occurred at 1.2 +/- 0.8 mug/ml and 1.9 +/- 0.7 mug/ml, respectively. There mere no false-normal ART responses. Conclusion: 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.