Background We tested the joint hypotheses that both perineural and systemic dexmedetomidine prolong the duration of an ulnar nerve block (UNB) compared with ropivacaine alone and that systemic dexmedetomidine is noninferior compared with perineural dexmedetomidine in block prolongation.

Methods We performed bilateral UNBs in 22 healthy volunteers on two separate days. On the first day, each arm was randomized to either 4 mL ropivacaine 5 mg/mL+1 mL dexmedetomidine 100 µg/mL (Perineural) or 4 mL ropivacaine 5 mg/mL+1 mL saline (Systemic). On the subsequent treatment day, each arm was randomized to 1 mL of saline plus 4 mL of ropivacaine at either 7.5 mg/mL(HiRopi) or 5 mg/mL (NoDex). The primary outcome measure was the duration of sensory block assessed by mechanical discrimination.

Results Mean sensory block duration was longer in both the Perineural (14.4 hours, 95% CI 13.1 to 15.6) and Systemic treatments (9.2 hours, 95% CI 8.6 to 9.8) compared with the NoDex treatment (7.1 hours, 95% CI 6.6 to 7.6) (p<0.0001 for both). Systemic dexmedetomidine was inferior (not noninferior) compared with perineural dexmedetomidine, as the 95% CI of the difference (mean difference 5.2 hour, 95% CI 4.2 to 6.1) exceeded the noninferiority limit of 3.6 hour. Onset time did not differ among the groups. The other test modalities demonstrated similar block durations as the primary outcome.

Conclusions Adding dexmedetomidine perineurally to ropivacaine doubles the duration of an UNB. Systemic dexmedetomidine also prolongs the duration of UNB, but has less of an effect compared with the perineural route.

Trial registration number NCT03222323.