Minimum effective local anaesthetic dose of isobaric levobupivacaine and ropivacaine administered via a spinal catheter for hip replacement surgery.

BACKGROUND: Continuous spinal anaesthesia with spinal catheters allows incremental dosing of local anaesthetic and, consequently, less haemodynamic changes. However, little is known about the required doses. Therefore, we designed a study to assess the minimum effective local anaesthetic dose (MLAD) of levobupivacaine and ropivacaine in this context.

METHODS: Forty-one patients undergoing hip replacement surgery were randomly allocated to one of the two local anaesthetic groups in a double-blind manner. The initial dose of local anaesthetic was determined by the response of the previous patient: the effective dose resulted in a 1 mg decrease in the dose of levobupivacaine or ropivacaine, and an ineffective dose resulted in a 1 mg increase. The MLAD was calculated by the Dixon up-and-down method.

RESULTS: The MLAD of levobupivacaine was 11.7 mg (95% CI, 11.1-12.4) and that of ropivacaine 12.8 mg (95% CI, 12.2-13.4).

CONCLUSIONS: These doses are significantly smaller than doses reported before for single-shot spinal anaesthesia. Continuous spinal anaesthesia allows the use of relatively small doses of local anaesthetic.