Optimal dose of intrathecal isobaric bupivacaine in total knee arthroplasty.

Purpose: Early mobilization is an important aspect of fast-track protocols and intrathecal bupivacaine is often used in primary total knee arthroplasty (TKA). Although the optimal dose is not known, conventional doses leave patients unable to mobilize for two to four hours. The dose of an intrathecally administered local anesthetic should therefore be optimized to achieve immediate postoperative mobilization. This study determined the median effective dose (ED) of intrathecal bupivacaine for primary unilateral TKA.

Methods: Between April 2016 and February 2017 all patients who qualified for unilateral primary TKA were eligible for inclusion. In this dose-finding study, the up-and-down method by Dixon and Massey was used, which is a sequential allocation model. Patients received a dose of isobaric bupivacaine according to the outcome of the preceding patient with an initial starting dose of 5 mg. The dose was increased or decreased by steps of 0.5 mg, depending on the outcome of the preceding patient. During surgery, patients were closely monitored for indications of pain. Time points of regaining motor and sensory functions were determined.

Results: Twenty-five patients were included. Mean (SD) age was 70.1 (8.8) yr old, median [IQR] body mass index was 29.5 [27.3-30.9 kg·m], and 48% were female. In 11 patients the dose was inadequate; of these, nine patients needed additional anesthesia during surgery, and in four of these nine patients a conversion to general anesthesia was required. The median ED was 3.5 (95% confidence interval [CI], 3.1 to 4.0) mg of intrathecal bupivacaine. The calculated ED50 was 3.4 (95% CI, 2.7 to 4.0) mg; the calculated ED95 was 5 (95% CI, 3.7 to 8.0) mg.

Conclusion: In this small study with tight control over operative duration, the median effective dosage of intrathecal isobaric bupivacaine for primary unilateral TKA was 3.5 mg and the ED95 was 5 mg. Reduction of conventional dosages of intrathecal bupivacaine is feasible at centres using fast-track arthroplasty protocols.