[Influence of positioning on the quality of unilateral spinal anesthesia]

Unilateral spinal anaesthesia ("hemi-spinal") is theoretically associated with the advantages of fewer cardiovascular effects and longer duration of action while offering high density motor block of the extremity affected.

In a prospective, randomised study 60 patients received 2 mls of 4 % hyperbaric mepivacaine intrathecally. Group I (n = 30) was returned into the supine position after 5 minutes in the lateral position, Group II (n = 30) was kept in the lateral position for 15 minutes. Spread and subsequently offset of sensory and motor block were assessed separately for each side at predetermined time intervals pre-, intra-, and postoperatively.

Both groups were identical regarding their morphometric parameters. In Group I (5 minutes), the mean cephalad spread of sensory block of the initially dependent side was T6 after 15 mins and T4 after 25 mins. On the initially non-dependent side, the sensory block reached to T5 after 25 mins. In Group II (15 mins), the initial mean difference was equivalent to two segments (T7/T9). After 25 minutes there was no difference to the results in Group I.

Unilateral spinal anaesthesia ("hemi-spinal") is an attractive concept, supposed to be associated with the above mentioned advantages and the additional benefit of a partially maintained sensation of one limb. The results demonstrated in this study seriously question this theoretical concept. The most likely reason for the lack of inter-lateral differences in sensory block is the high concentration of hyperbaric local anaesthetic in the cerebrospinal fluid on the dependent side, which, on turning the patient to the supine position, will transit to the previously non-dependent side subsequently leading to a significant block.