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 se dation with propofol (propofol-air), or propofol combined with 50% nitrous oxide (N2O; propofol-N2O). An element of this device is the automated respo nsiveness test (ART), a method for confirming that patients remain consciou s, The authors tested the hypotheses that the ART predicts loss of consciou sness and that failure to respond to the ART precedes sedation-induced resp iratory or hemodynamic toxicity. Methods: The protocol consisted of sequential 15-min cycles in 20 volunteer s. 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. S ubsequently, the propofol target effect-site concentration was increased by a designated increment (0.25 and 0.5 mug/ml) and the process repeated. Thi s sequence was continued until loss of consciousness, as defined by an Obse rver'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 r espond 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 plasm a 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 fail ure 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.