Will adding dexmedetomidine prolong your femoral nerve block?

 

Citius, Altius, Fortius. – The Olympic Motto

Abdulatif et al. The effects of perineural dexmedetomidine on the pharmacodynamic profile of femoral nerve block: a dose-finding randomised, controlled, double-blind study. Anaesthesia. 2016 Oct;71(10):1177-85. doi: 10.1111/anae.13603.

This is a randomised controlled trial from the University of Cairo that looked at the pharmaco-dynamic effects of adding various doses of the alpha-2 agonist dexmedetomidine to plain bupivacaine for femoral nerve blocks in patients undergoing elective arthroscopic anterior cruciate ligament repair.

The primary outcome was the duration of the sensory block. Sixty-two patients were randomized into one of four groups – a control group that had a block with bupivacaine only and three treatment groups who received the same dose of bupivacaine with 25mcg, 50mcg or 75mcg of dexmedatomidine added. A single operator performed all the blocks after which the participants received a standardised GA and underwent their operation.

The 50mcg and 75mcg groups showed significant prolongation of the sensory block demonstrated by both pin-prick testing and time taken for first request of analgesia. The morphine requirement in these groups was also significantly reduced (albeit from an already fairly low level) compared to the control group. However, motor block was also prolonged, which could cause problems with post-op mobilisation/rehabilitation and could prolong hospital stay. Although the study was not powered to look at adverse effects, patients receiving 75mcg of dexmedatomidine were significantly more likely to experience at least one episode of hypotension during the study period. No complications related to the block itself (eg neuropraxia, vascular puncture, infection) were reported in this study.

On discussion at journal club, no major issues were identified with the study design. We felt that appropriate statistical analyses were carried out and the paper was reasonably clearly written.

This study adds to evidence from previous studies in neuraxial and brachial plexus blocks that perineural dexmedatomidine seems to prolong the duration of nerve blocks. However, this is an off-license use of dexmedatomidine and good safety data on this type of use is not yet available. This paper was not powered to look at safety. Additionally, the significant prolongation of the motor block has potentially deleterious effects to the early mobilisation of patients and may increase the risk of falls.

Should sufficient safety data be acquired by further studies, perineural dexmedatomidine may well be helpful in those cases where a prolonged peripheral nerve block is deemed advantageous, but in the particular operation in this paper, it’s hard to believe that the small absolute reductions in total morphine requirement outweigh the potential harms of the prolonged motor block.

Summary by Dr Tom Tarrell

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