Confusing results for long-lasting single-injection nerve blocks

A new liposomal bupivacaine product shows promise in providing long-lasting nerve block with a single injection, but authors also report some “confusing results” in Anesthesia & Analgesia.

A single injection of liposomal bupivacaine produces femoral nerve block lasting longer than 24 hours in volunteers, according to a study by Dr Brian Ilfeld and colleagues of University of California San Diego. But the results aren’t straightforward, with variable responses and a hard-to-explain “inverse relationship” between dose and response.

 

Evidence of lasting nerve block

The researchers evaluated a liposomal bupivacaine preparation designed to release local anaesthetic over 96 hours after injection.

Fourteen healthy volunteers received single-injection blocks of the femoral nerve in the upper thigh using different doses of bupivacaine. The results showed significant and lasting responses in terms of both sensory and peripheral nerve block. However, the relationship between dose and response was counterintuitive, with lower doses of liposomal bupivacaine producing greater responses. “This inverse relationship is biologically implausible and most likely due to the limited sample size and the subjective nature of the measurement instruments,” Dr Ilfeld and colleagues write.

 

More work needed

Femoral nerve blocks are useful in controlling leg pain in several situations, such as during total knee replacement surgery. With currently available local anaesthetics, single-injection nerve blocks provide a maximum duration of effect between eight and 24 hours.

“An alternative approach is to encapsulate a long-acting local anaesthetic within microspheres or liposomes,” Dr Ilfeld and coauthors explain. Such ‘depo-local’ formulations could be given in a single injection, and release adequate levels of local anaesthetic to maintain effective nerve block over time. Preliminary studies showed promising results with liposomal bupivacaine for this purpose.

Although the results are ‘suggestive’, Dr Ilfeld and coauthors emphasise that larger trials will be needed.

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