Pollock Julia E., et.al. Anesthesiology 2000; 93:728-34.

Reviewed by: R. Prasad, MD

Conclusion:
Spinal anesthesia with 50mg lidocaine in unsedated volunteers is associated with progressive sedation by self-sedation and Observer's Assessment of Alertness/Sedation Scale (OAA/S). Effect not related to block height - effect peaks (60-70min) as block is resolving. These scores did not correlate with BIS scores.

Details:
Methods
-Phase I: Sounds like a pilot study. 12 volunteers had SAB with 50mg hyperbaric lido. BIS measured at baseline (i.e., after 10min in dark room). Periodically thereafter, recorded BIS, OAA/S, block height (sensory, motor). Self-sedation score at end of study.
-Phase II: Power analaysis: needed 6pts per group for 80% power to detect BIS difference of 5 with 5% beta error. Double blind randomized version of same protocol, except did not check motor block level (n=10; 1 excluded from analysis completely, 1 partly - BIS while hypotensive not examined). Control = spinal with "sham injection" (no injection? saline injection?) (n=6).
Results
-Phase I: Statistically significant decrease in BIS, with maximal decreases in BIS noted at 30 and 70 min. But no correlation in BIS or sedation scores with spinal level.
-Phase II: OAAS and self-sedation scores differed btwn groups (lower in spinal group), but BIS similar.

Comments:
-So what? Already knew that neuraxial anesthesia decreases sedation requirements, probably due to decrease in activity of reticular activating system.
-Nevertheless, an interesting study:
1. Peak effect occurred after block starting to resolve...perhaps there is an alternative mechanism for late sedation.
2. BIS may not be particularly useful in unsedated patients.
-Problems with study:
1. very small numbers (despite power analysis)
2. can you really be double-blinded for a spinal block?
3. re: power analysis. Is a difference of 5 in the BIS worth looking at?