Background and Goal of Study: Dexmedetomidine used as an adjuvant to local anesthetics may prolong the duration of peripheral nerve blocks. We aim to determine whether dexmedetomidine has a peripheral mechanism of action when controlling for possible systemic effects.
Materials and methods: We conducted a randomized paired trial in healthy volunteers. All participants received bilateral adductor canal blocks with 20 ml ropivacaine 0.5% + 1 ml dexmedetomidine 100µg/ml in one leg and 20 ml ropivacaine 0.5% + 1 ml saline in the other leg in a blinded fashion. The primary outcome measure was duration of nerve block assessed by temperature sensation (alcohol swab). Secondary outcome measures were duration of nerve block assessed by: pinprick, pain during tonic heat stimulation, warmth detection threshold and heat pain detection threshold. The latter three sensory modalities were measured using the Modular Sensory Analyzer Thermal Stimulator (Thermotest, Somedic A/B, Hörby, Sweden). We performed an intention to treat analysis using a paired T-test.
Results and discussion: All 21 participants completed the trial. Mean duration of nerve block assessed by temperature sensation in the leg receiving ropivacaine + dexmedetomidine was 22.2 h (95% CI: 20.9 to 23.5) compared to 20.0 h (95% CI: 18.8 to 21.2) in the leg receiving ropivacaine + placebo, with a mean difference of 2.1 h (95% CI: 1.1 to 3.3), P= 0.001. The duration of sensory block was also significantly longer in the leg receiving dexmedetomidine when assessed by: pinprick, pain during tonic heat stimulation, warmth detection threshold, but not heat pain detection threshold.
By performing simultaneous bilateral adductor canal blocks with ropivacaine + dexmedetomidine in one leg and ropivacaine alone in the other leg, any systemic contribution of Dexmedetomidine, by absorption and systemic redistribution, will equally have affected each nerve block. Hence, the observed difference in duration of the two sensory nerve blocks is attributable to a peripheral effect.
Conclusion(s): Dexmedetomidine used as an adjuvant to ropivacaine prolongs the duration of adductor canal block by a peripheral mechanism in healthy volunteers, but not necessarily to a clinically relevant extent.

Does dexmedetomidine have a perineural mechanism of action, when used as an adjuvant to ropivacaine?