Influence of the injection site (L2/3 or L3/4) and the posture of the vertebral column on selective spinal anesthesia for ambulatory knee arthroscopy.

BACKGROUND We tested the hypothesis that selective spinal anesthesia for ambulatory knee arthroscopy can be accomplished with a small dose of bupivacaine at the L3/4 interspace with or without a head-down tilt of 5 degrees when the patients were in the lateral decubitus position. METHODS In this double-blind study, 123 patients were randomly allocated to receive spinal anesthesia with 4 mg of hyperbaric bupivacaine inserted at either the L2/3 interspace, while the vertebral column was kept horizontal (L2/3 group), or the L3/4 level, with the vertebral column horizontal (L3/4H) or tilted 5 degrees head-down (L3/4T). At 7 min, an additional head down tilt was used in all groups if the sensory block was inadequate. RESULTS In the L3/4T group the sensory block (Th8) reached a significantly higher level 30 min after spinal injection, compared with both the L2/3 (Th10) and L3/4H (Th11) groups. In the L3/4H group, 39% of the patients needed an additional tilt for 3 min at 7 min, compared with 10% (P=0.004) in the L3/4T group. Sacral block developed later and recovered faster (P<0.05) in the L3/4T group compared to the L3/4H group. Home-readiness was achieved equally fast in all groups. CONCLUSION When producing selective spinal anesthesia, the posture of the vertebral column is a major determinant of both sensory and motor segments to be blocked. A 4-mg dose of hyperbaric bupivacaine at the L3/4 interspace with a 5 degrees head-down tilt of the vertebral column for 6 min is recommended for knee arthroscopy.