Background: Using a push-button device, we investigated whether visual or auditory response time would increase with increasing sedation, and assessed the responsiveness score of the Observer's Assessment of Alertness/Sedation (OAA/S) scale at the point of first loss of response to visual or auditory stimulation.
Methods: In experiment 1 we applied visual and auditory stimulation to 19 patients as the propofol target plasma concentration (CPT) was increased to determine whether the visual or auditory response would be lost first. Thirty patients were each then infused with propofol, starting at a CPT of 0.3 µg ml−1 and increasing by increments of 0.2 µg ml−1, during which visual (experiment 2) or auditory (experiment 3) stimulation was applied when the effect-site concentration (CE) of propofol reached CPT. Visual response time (VRT), auditory response time (ART), CE and total amounts of propofol, and OAA/S score at the first loss of visual/auditory response were measured.
Results: Visual response disappeared earlier than auditory response in 84.2% of the patients. Visual response time and ART were linearly prolonged as the CE of propofol increased. The CE and total amounts of propofol at the first loss of visual response were 1.2 ± 0.4 µg ml−1 and 57.9 ± 16.7 mg, compared with 1.4 ± 0.5 µg ml−1 and 71.6 ± 26.1 mg, respectively, at the first loss of auditory response. The median (range) OAA/S scores at the first loss of visual and auditory response were 4 (3–4) and 3 (2–4), respectively.
Conclusion: VRT and ART were linearly prolonged with increasing sedation. Visual response may be useful in monitoring conscious sedation.